H ILL IN I S UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN PRODUCTION NOTE University of Illinois at Urbana-Champaign Library Large-scale Digitization Project, 2007. A~ ~V 'F 4~ ~~~-ci} '~4~'Y. X ~ 1F~ 1 '7 ~N .J~ " .6~. - J~< ~-~:2 vY' 17/ F '\ "F ~' ~ &~ '~' ~& I ( ~F'/ ~ F "F' V '~ ~ ~'~'k F~ (~F\ 4 'Fl ( ~'\ Fr F ' Ti' a'"' K 3 jF UNIVERSITY OF ILLINOIS ENGINEERING EXPERIMENT STATION BULLETIN SERIES No. 394 AN INVESTIGATION OF CREEP, FRACTURE, AND BENDING OF ARSENICAL LEAD ALLOYS FOR CABLE SHEATHING - SERIES 1949 A REPORT OF AN INVESTIGATION CONDUCTED BY THE ENGINEERING EXPERIMENT STATION UNIVERSITY OF ILLINOIS IN COOPERATION WITH THE UTILITIES RESEARCH COMMISSION BY CURTIS W. DOLLINS RESEARCH ASSISTANT PROFESSOR OF ENGINEERING MATERIALS PUBLISHED BY THE UNIVERSITY OF ILLINOIS PRICE: NINETY CENTS 3060-9-50-44871 OrI".Os ;: PRESS : ABSTRACT The results of a research on the properties of lead and lead-alloy sheathing for underground-power cable are given in this bulletin. Special emphasis is placed on the arsenical-lead alloys which have come into commercial use during the last few years. The data cover creep rates under steady tensile stresses up to 300 p. s. i., time to frac- ture under steady stresses of 400 to 2000 p. s. i., and number of cycles to fracture in slow bending. These properties are important factors in the serviceability of the cable sheath. Small amounts of arsenic in combination with other constituents and with proper production technique are shown to produce a marked improvement in all three of these properties of creep resistance, life to fracture, and ability to withstand bending. This improvement appears to be due to retardation of recrystallization at the stresses and temperatures that are generally encountered by the sheaths in service. At the temperatures and stresses in normal service, the reduction in creep rates from those of commercially pure lead is considerable. However, for some arsenical sheaths that have been produced, at the upper limits of 150 deg F and 200 p. s. i. which might occur for some cables during emergency ampere-loading in service, the creep resist- ances were little or no better than those of copper-bearing lead. In such cases the retardation of recrystallization appears to be offset by recovery which removes the strain hardening and thus lowers the creep resistance. The arsenical-lead alloys have good ductility, as is shown by the elongations to fracture of strip specimens under steady tensile stress. As the rate of strain is decreased with accompanying lengthening of the time to fracture, for a number of tests the ductility decreases until a minimum is reached; then for still lower rates of strain the ductility increases. The larger the grain size the lower the rate of strain or stress at which the minimum occurs. For the alloys tested the mini- mum points are well above the range of stresses encountered by sheaths in service. 4 ABSTRACT (CONCLUDED) The arsenical-lead alloys have outstanding ability to withstand slow bending of the type that occurs in service due to daily expansion and contraction of the cable. The magnitude of the improvement in this property as well as in other properties over the properties of copper-bearing lead has varied -considerably, but some samples of arsenical lead sheaths have shown great improvements in overall prop- erties. At small bending strains, the smaller-grain alloys give the better results; at the larger bending strains, the larger-grain alloys give the better results. Reliable indications of these various properties of cable sheathing of any type are obtainable only from long-time tests. CONTENTS PAGE I. INTRODUCTION 7 1. Previous Research on Lead Sheath Materials 7 2. Scope of Bulletin 7 3. Variable Factors in Tests of Lead and Lead Alloys 8 4. Acknowledgments 9 II. CREEP TESTS OF TENSILE SPECIMENS OF LEAD ALLOYS 12 5. Racks for Creep Tests of Tensile Specimens 12 6. Test Data for Creep Tests of Tensile Specimens 12 7. Results of Creep Tests of Tensile Specimens 12 III. CREEP OF CABLE SHEATHING UNDER INTERNAL PRESSURE 27 8. Sheathing Tested, and Apparatus for Testing Sheathing, Under Long-Continued Internal Pressure 27 9. Correlation of Tension Strip Tests with Creep of Sheath Under Internal Oil Pressure 27 10. Test Results to Failure of Sheaths Subjected to Internal Oil Pressure at 150 deg F 36 11. Effect of Press Stop Marks on the Creep of Sheathing Subjected to Internal Pressure 39 IV. TENSILE STRENGTH AND DUCTILITY OF CABLE SHEATH METAL UNDER STEADY LOAD 42 12. Test Specimens and Testing Machines Used 42 13. Test Data of Long-Time Tensile Tests 42 V. REPEATED-BENDING TESTS ON SPECIMENS CUT FROM CABLE SHEATHING 48 14. Apparatus Used in Bend Tests of Strip Specimens 48 15. Strip Bend Test Results 48 VI. CABLE BEND TESTS 51 16. Repeated-Bending Tests of Full-Size Section of Cable 51 17. Testing Machine and Specimen Used 51 18. Results of Bend Tests on Cables 52 VII. SUMMARY 56 APPENDIX: REFERENCES 60 FIGURES PAGE Creep-Time Diagrams - Long-Time Tests on Tin-Antim n and Arsenical Lead Alloys 13-15 2. Creep-Time Diagrams-Long-Time Tests on Arsenical Lead 16-18 3. McVetty Diagrams for Total Creep of Arsenical Lead Alloys - 5000-hr Tests 20 4. Comparison of Creep of Specimens Under Tensile Stress of 200 p. s. i. at Temperatures of 110 deg F and 150 deg F, Based on Tests to 5000 Hr: McVetty Method 21 5. Comparison of Total Creep in Arsenical Leads for 5000 Hr 23 6. Creep-Time Diagrams of Common Desilverized Lead A, Under Internal Pressure at 78 deg F 29 7. Creep-Time Diagrams of Copper Lead Under 25-p. s. i. Internal Pressure at 78 deg F 30 8. Creep-Time Diagrams of Arsenical Leads Under 40-p. s. i. Internal Pressure at 78 deg F 31 9. Creep-Time Diagrams of Chemical Lead and Copper Lead Under Internal Pressure at 150 deg F 32 10. Creep-Time Diagrams of Arsenical Leads Under 40-p. s. i. Internal Pressure at 150 deg F 33 11. Creep-Time Diagrams of Four Types of Leads - C-302A, Common Desilverized Lead A; C-417, Arsenical Lead; C-296, Chemical Lead; and C-299, Copper Lead - All Under Internal Pressure at 150 deg F 34 12. Deformation Diagrams of Internal Pressure Samples Containing Press Stop Marks 41 13. Strength-Time Diagrams of Arsenical Lead at 78 deg F and 110 deg F 43 14. Strength-Time Diagram of Tin-Antimony Lead at 110 deg F 44 15. Ductility-Time Diagrams of Arsenical Leads at 78 deg F and 110 deg F 45 16. Strain-Life, Stroke-Strain, and Stroke-Life Diagrams of Cable Bend Tests 53 TABLES NO. PAGE 1. Data on Sheathing Tested 8 2. Chemical Composition of Lead and Lead Alloys 10 3. Summary of Creep-Test Results of 10-in. Specimens 24 4. Sheaths Tested Under Internal Oil Pressure 28 5. Summary of Strip Bend Tests at 110 deg F 49 I. INTRODUCTION 1. Previous Research on Lead Sheath Materials For the past twenty years there has been conducted, on a widening basis, a series of investigations into the creep, fracture, and bending of lead and lead alloys used for cable sheathing. Some of the results are given in Bulletins 243, 272, 306, 347, and 378 of the University of Illinois Engineering Experiment Station. The first bulletin dealt only with the creep of sheathing materials. The others included fracture and bending test results, but none contained much informa- tion on arsenical alloys. Since Bulletin 378 was published in July 1948, tests on lead and lead alloys have been reported by Ferguson and Bouton(t, *), Gohn and Ellis(t2, Phelps, Kahn, and Magee(3), and Smith(') in the United States. Professor Greenwood(8,9, 10) and associates have delivered three papers in Australia and in the United States. Not all data obtained on creep and related phenomena appear in the literature; several lead manufacturers and fabricators have installed research apparatus and have carried on extensive tests, report of which has not appeared in print. 2. Scope of Bulletin The investigations previously reported have made it evident that accurate predictions pertaining to creep and fracture of lead sheath- ing in service require tests of exceedingly long duration. Truly indicative tests of the bending resistance of lead sheathing are time- consuming indeed. Data from such tests are reported herein. Creep tests of 10-in. strip specimens of eleven arsenical alloys and of a few other materials studied for the sake of comparison are reported at room temperature (which averages about 78 deg F), 110 deg F, and 150 deg Ft. All tests are past 5000 hr. Internal pressure tests of full-section sheathing at 78 deg and 150 deg are reported, all of which have gone 18,000 hr. A few tests at 150 deg have gone to failure of the sheath. Bending tests of strip specimens at 110 deg and 10-min cycles or 1-hr cycles are reported. The bending resistance of full-section cable sheathing is shown. Tests were made at room temperature (78 deg) with a frequency of one cycle in 6 min. * Parenthesized superscript numerals refer to correspondingly numbered entries in the References. t Hereafter, temperatures are given in degrees Fahrenheit without mention of the scale. 8 ILLINOIS ENGINEERING EXPERIMENT STATION 3. Variable Factors in Tests of Lead and Lead Alloys Most of the specimens tested (see Table 1) were cut from cable sheathings which, by means of the usual factory processes, were ex- truded by various cable manufacturers into commercial lengths of cable. The rates of extrusion are not known; they may vary widely from sheath to sheath, and often within the same sheath. As a result the work hardening of the lead at the press varies, and this affects grain structure and physical properties. Stopping the lead press during extrusion affects this work hardening and interrupts the quenching process. Quenching practices after extrusion vary. Localized impurities sometimes do and sometimes do not occur at critical sections of the specimens. The test results are further in- fluenced by the position of the charge welds with relation to the specimen. The flattening out of the samples and the subsequent machining of the specimens affect the lead alloys differently, and the size and shape of the specimen change somewhat the creep and ductility values obtained. The samples did not have the same aging at room temperature, as the time between extrusion and testing varied. TABLE 1 DATA ON SHEATHING TESTED A number of the metals containing a significant amount of arsenic are called "arsenical leads." The rest of the metals are classified as of the types to which they most nearly conformed as prescribed in Tentative Standard B-29-40T of the American Society for Testing Materials. Classification of metal arsenical lead arsenical lead arsenical lead arsenical lead arsenical lead arsenical lead arsenical lead arsenical lead arsenical lead arsenical lead arsenical lead arsenical lead tin-antimony lead chemical lead copper lead copper lead common desilverized lead A common desilverized lead A Nominal dimensions of sheath Outside diameter, in. 27A2 2'ý12 22764 2ýý4 2'9s4 22;k4 251ý4 2294 22;4 21 914 24564 21%2 2% 27/ 3 3912 Wall thickness, in. 964 44 %4 914 964 9s4 s14 964 914 964 %14 9%4 %4 i%2 %2 Sample descrip- tion C-331 C-389 C-404 C-417 C-430 C-433 C-436 C-441 C445 C-446 C-450 C-451 C-411 C-361 2C 2G Manufac- turer I I I V VII IV VIII VII VIII V IV V VII IV VII VII V I Sheath received Feb. 1944 Jan. 1945 July 1945 May 1946 Mar. 1947 Sept. 1947 Dec. 1947 Dec. 1947 June 1948 July 1948 July 1948 July 1948 Jan. 1946 Aug. 1942 Nov. 1935 Apr. 1936 Oct. 1936 Sept. 1935 BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS These many variables may explain some of the differences among test results of various samples in this investigation and those re- ported by other laboratories. The range in results has been considered representative of that which might be expected in commercially produced sheaths used for power installations. The chemical analysis of the various alloys tested is given in Table 2. 4. Acknowledgments This study has been supported by funds contributed by the Utilities Research Commission, Chicago, of which A. D. Bailey is Chairman, M. S. Oldacre the Director of Research, and H. P. Ruge the Secretary and Assistant Director. The Advisory Committee ap- pointed by the Utilities Research Commission for this study is at present as follows: Herman Halperin (Chairman), Senior Staff Engineer, Common- wealth Edison Company C. E. Betzer, Senior Engineer, Engineering Department, Common- wealth Edison Company C. A. Crawford, Testing Engineer, Commonwealth Edison Company C. A. Jaques, Assistant to Electrical Engineer, Public Service Com- pany of Northern Illinois B. R. Richardson, Electric Superintendent, Western United Gas and Electric Company J. L. Smith, Supervising Engineer, Construction Department, Com- monwealth Edison Company H. W. Oerman, Design Engineer, Public Service Company of North- ern Illinois. Many meetings of the Advisory Committee have been held during the progress of the investigation, and hearty acknowledgment is hereby made of the help of the committee in planning tests, in criti- cizing methods of testing and reporting test results, and in suggesting further lines of research. The work has been carried on as a cooperative research investi- gation by the Utilities Research Commission of Chicago, Illinois, and the Engineering Experiment Station of the University of Illinois. The tests described in this bulletin have been made in the Arthur Newell Talbot Material Testing Laboratory of the University of Illinois. The author is indebted to Professor F. B. Seely and also to ILLINOIS ENGINEERING EXPERIMENT STATION 0C ýC C C'0'CO t - 0CO '0CC tC.11' COC~ '' 0 CO) '- C 00--O) O -C - 0 00li0 00000C C 000 0000C 0 0 C 0 000o -0oo00o00 0 0 0 0o * ^ iCC M ~ CCC. 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C- ^OCC C'0 C 0 ^*' C)1 aSj3 u M ,. 0a CCC C)^ C < 11i).o -*acC So C3 > - CC'C 01 C C/C S -l o CCg S* '6 -CS.) 0 C- C 1CC'C1C 0CCC'0^^ 0 C)'CC- ECO§ 0-CC C)E o 0 r C)°C) C C)' 0 CC +> S S;a 0?' CL|C) CC '05-0 * E- S tNE '0o l'l' M ) r *30*0' 0 H BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS 11 Professor Emeritus H. F. Moore for helpful guidance in this investi- gation. Many of the testing machines and pieces of equipment used were developed under Professor Moore's direction, and most of the testing technique was outlined by him. Acknowledgment is made of the services of six student laboratory assistants - N. B. Blaski, K. T. Upstone, J. R. Alles, W. E. Hensley, G. C. Willing, and R. V. Fostini. ILLINOIS ENGINEERING EXPERIMENT STATION II. CREEP TESTS OF TENSILE SPECIMENS OF LEAD ALLOYS 5. Racks for Creep Tests of Tensile Specimens The creep tests reported herein may be compared directly with those in Bulletins 306(13) and 378(15). The same thermostatically con- trolled racks and 10-in. multiplying extensometers were used in these tests as before. The cathetometers, apparatus, and procedure were the same as those reported in the previous bulletins. Vibration was not enough to affect creep test results on lead alloys seriously. Most tests were conducted on the third floor of a three-story brick building whose walls are self-supporting and whose floors are reinforced concrete supported on structural steel, so that little vibration normally occurs. The racks containing the specimens were suspended on dampened springs. 6. Test Data for Creep Tests of Tensile Specimens The data obtained from creep tests of strips of cable sheathing consist of recorded observations of elapsed time and elongation under steady tensile stress. The data in graphical form show elapsed time plotted as abscissas, and creep (expressed in percent increase in length) plotted as ordinates. Figures la and 2c show the test results to the end of test period of creep tests of 10-in. tensile specimens. 7. Results of Creep Tests of Tensile Specimens The arsenical leads (containing arsenic, tin, bismuth, and in some cases antimony and copper) have in general the same type of creep curves as the other lead alloys. The long-time creep, then, may be divided arbitrarily into three stages: (1) a preliminary stage during which creep progresses rapidly and its rate diminishes; (2) a stage in which the creep rate stays nearly constant for the greater part of the life of the specimen; and (3) a final stage in which the creep rate increases with necking down of the specimen until final fracture occurs. None of the 10-in. tensile strip specimens of arsenical lead have failed as yet. At stresses below 300 p. s. i. a large part of the first stage of creep of the arsenical leads is due to the immediate elongation upon appli- cation of load. This is largely elastic action. However, considerable strain hardening takes place in this period. At the relatively low stresses usually encountered in service much of the deformation that BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS I C-411// 36 % Tin 032% Antimony__ __ I C- 430 .4rsnjca'/ Lea 0.8---- HO°. - Time in Thousanas of Hours Fit. 1. CREEP-TIME DIAGRAMS-IA)ON-TIME TESTS ON TIN-ANTIMONY AND ARSENICAL LEAD ALLOYS: PART A 14 ILLINOIS ENGINEERING EXPERIMENT STATION (j 1~ T/77e in Thousands of Hours FIG. 1. CREEP-TIME DIAGRAMS - LONG-TIME TESTS ON TIN-ANTIMONY AND ARSENICAL LEAD ALLOYS: PART B BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS C 451 Arsenical/ Zea'd ____T50F Tilne in Thousands of Hours 8 9 /0 FIG. 1. CREEP-TIME DIAGRAMS - LONG-TIME TESTS ON TIN-ANTIMONY AND ARSENICAL LEAD ALLOYS: PART C ILLINOIS ENGINEERING EXPERIMENT STATION I I Time in Thousands of Hours FIc. 2. CREEP-TIME DIAGRAMS -LONG-TIME TESTS ON ARSENICAL LEAD: PART A BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS occurs takes place in the first stage of creep. The test results indicate that, for most lead alloys including the arsenicals, the creep generally continues to decrease at a slow and diminishing rate throughout the second stage; but in some cases the trend was toward increasing creep rates. The decrease has been observed in commercially pure lead sheath even after more than ten years on test, and is more pronounced in the age-hardening and strain-hardening lead alloys. In fact, the dividing points between the stages of creep are seldom sharply defined; instead, gradual merging occurs. This makes it 1~ I, Time in Thousands of Hours FIG. 2. CREEP-TIME DIAGRAMS - LONG-TIME TESTS ON ARSENICAL LEAD: PART B ILLINOIS ENGINEERING EXPERIMENT STATION difficult, if not impossible, to predict accurately the total creep for extended periods of time from creep tests of less than 5000 hr in which the stresses are near the range of service stresses. However, for predicting creep for stresses near 200 p. s. i., the assumption of constant creep in the second stage is, when based on tests covering a reasonably long period, very useful in that it permits comparison of various materials and extrapolation of the results to longer periods. It tends to give values of creep for extended periods of time which are on the high side and hence are considered safe for design purposes. Time in Thousands of Hours FIG. 2. CREEP-TIME DIACRAMIS- LONG-TIME TESTS ON ARSENICAL LEAD: PART C BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS In this as in previous bulletins the beginning of the third stage of creep is regarded as marking the effective termination of service life of lead and lead alloys. The extrapolation of the data must there- fore be limited to the second stage. The method proposed by P. G. McVetty gives predictions of creep to 10,000 hr which, if the creep rate (v) at 5000 hr and the corresponding value of eo are used, are in close agreement with the results of creep tests actually continued to 10,000 hr. In McVetty's method the creep rate in the second stage is con- sidered constant, and on the creep-time diagram the straight line representing this constant rate is projected back to the zero axis for time. The intercept of this line with the zero axis is designated eo. Then if v is the creep rate during the second stage, the total elonga- tion or creep for any time (t) up to the beginning of the third stage is = eo + vt. The McVetty method is satisfactory for the arsenical alloys for stresses of 200 p. s. i. and less. But for 300 or more p. s. i. at tempera- tures above 110 deg these alloys have a self-annealing tendency. After a short strain-hardening period these alloys seem to recover, they lose their strain hardening, and their creep rate increases as the test progresses. The arsenical leads are usually very ductile; they continue to creep at this accelerated rate for long periods of time. This condition leads to lower predictions of total creep for 10,000 hr from 5000-hr tests than actually occurs for higher stresses and temperatures. The estimated total creep in 10,000 hr as given by the McVetty method is shown in Fig. 3 for all the arsenical leads not previously reported. Since all tests have been in progress for 5000 hr or more, tests of that duration were used rather than 2000-hr tests as in the previous bulletins. A diagram such as Fig. 3 shows clearly the effects of stress and temperature on the total creep of the various alloys for a given time. Greenwood calls this type of curve a creep-yield curve and sug- gests a criterion for design - 1 percent extension in 1 yr. This would give a very short life for the cables if stressed to the point of 1 per- cent expansion per year. A better design limit would be a total ex- tension that could be accepted during the life of the cable with reasonable assurance that fracture would not occur. This limit might be something like 4 percent for copper-bearing lead and 8 or 10 percent for arsenical-lead alloy. Of course, for some cable installa- tions such large expansions could not be accepted for other reasons, such as void formation in solid-type cable or wedging in the duct of any large cable. 20 ILLINOIS ENGINEERING EXPERIMENT STATION Total Creep in Per Cent in /0000 Hours FIG. 3. MCVETTY DIAGRAMS FOR TOTAL CREEP OF ARSENICAL LEAD ALLOYS- 5000-MR TESTS ev BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS The bar graphs of Fig. 4 show the total creep to be expected in 10,000 hr at 110 and 150 deg for specimens with tensile stress of 200 p. s. i. based on 5000-hr tests. The dark bars indicate the creep in the first 10,000 hr; the light bars the additional creep in each suc- ceeding 10,000 hr. Here again the effect of temperature is apparent on the creep properties of the alloys. The creep properties of the various alloys differ widely: some of them do not have the creep resistance of chemical lead at 150 deg, whereas others are definitely more resistant to creep. As indicated in Fig. 4, the creep resistance of C-411 for 200 p. s. i. at 110 deg is very low. This alloy strain-ages with considerable precipitation at the grain boundaries under low stress conditions. Sheath and eta I Creep Rate and Total Creep in Per Cent Sheth0 1o R.0 .0 6 C-/92 & e--Creep Rate per 0, 000 Hours C-331 i- Total Creep in /0,000 Hourls C-433 C-436 C-44/ C-430 Arsenical Lead C-389 C-417 C-445 (a)-At 11/0 deg. F. C-446 i C- 4SO C-4/so T77-Anlimony Lead C-411// 0 1.0 2.0 30 4Z C-J33I C-433 C- 436 C-44/ C- 430 I Arsenical Lead C-389 C-4/ 7 C-445 (b)-Af /50 deg. F C-446 C-450 C-45/ Chemical Lead C-361 0 1.0 2.0 3.0 44 FIG. 4. COMPARISON OF CREEP OF SPECIMENS UNDER TENSILE STRESS OF 200 p.s.i. AT TEMPERATURES OF 110 DEG F AND 150 DEG F, BASED ON TESTS TO 5000 HR: McVETTY METHOD ILLINOIS ENGINEERING EXPERIMENT STATION The bar graphs of Fig. 5 show the total creep at 110 and 150 deg for the first 5000 hr of test with stresses of 150, 200, and 300 p. s. i. The creep resistance of C-411 at 110 deg is very low for all stresses tested. Since the antimony and tin content for this alloy is higher than for any of the other alloys, it is believed that this is the cause of the low creep resistance of this material. C-331, C-389, and C-445 are nominally the same alloy. The wide range in creep re- sistance suggests that this alloy responds to heat treatment and extrusion conditions. For greater ease in extrapolating the creep of the various al- loys past 10,000 hr the data necessary for the McVetty method are given in Table 3. The minus value of e, indicates that these alloys have a self-annealing tendency for the temperature and stress condition indicated. Arsenic is slightly soluble'6) in lead at the eutectic temperature. Consequently, leads containing only arsenic respond to heat treat- ment and may be age-hardened. Arsenic in any considerable quantity would give an alloy too strong for cable sheathing without the neces- sary ductility needed for this service. The age hardening of the arsenic is controlled in the industry to a usable degree by balanced additions of smaller amounts of tin and bismuth. Quenching some arsenical alloys as they come from the lead press increases their creep resistance. Heating for a week at 120 deg C increases their creep resistance somewhat, and for high stresses at 150 deg F the artificial aging gives excellent creep resistance, but the ductility of the material is lowered somewhat. In order to investigate the effect of heat treatment on the creep properties of the arsenical leads an alloy containing 0.16 As, 0.012 Bi, 0.113 Sn, 0.06 Cu, and 0.029 Sb was tested in the quenched and unaged condition and in the unquenched and unaged condition. This alloy is labeled C-417 in the quenched and unaged condition, and C-446 in the unquenched and unaged condition. Its creep resist- ance was increased for all stresses tested at both temperatures except for 300 p. s. i. at 110 deg F. Its ductility is reduced by quenching and aging but remains high for most lead sheathing alloys. Sample C-445, containing 0.161 percent As, 0.069 percent Bi, and 0.127 percent Cu, was quenched at the lead press and then heated for a week to 120 deg C. This sample had the best creep resistance at 150 deg F of all arsenical leads that have been tested. At 110 deg F, however, it had somewhat less creep resistance than samples C-331 and C-389, which were nominally the same alloy but which were produced at another factory and not heated. Sample C-445 in the BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS My I 'o ~ ~ E ty t ^ ' [6 m o I w --^ I3 (!: () Nm [.' KS EN NI EI' IIsI U m I u/ S/-//-7// 000.9 * I 2 ~J '~ /0101 I m m M ESS W) M m m m I- i l m E [ m m m e 24 ILLINOIS ENGINEERING EXPERIMENT STATION TABLE 3 SUMMARY OF CREEP-TEST RESULTS OF 10-IN. SPECIMENS Material C-417 arsenical lead average grain size diam in in. 0.0020 (0.0051 mm) C-430 arsenical lead average grain size diamn in in. 0.0032 (0.0081 mm) C-433 arsenical lead average grain size diam in in. 0.0019 (0.048 mm) C-436 arsenical lead average grain size diam in in. 0.0038 (0.0097 mm) C-441 arsenical lead average grain size diam in in. 0.0030 (0.0076 mm) C-445 arsenical lead average grain size diam in in. 0.0030 (0.0076 mm) C-446 arsenical lead average grain size diam in in. 0.0019 (0.0048 mm) C-450 arsenical lead average grain size diam in in. 0.0022 (0.005 mm) C-451 arsenical lead average grain size diam in in. 0.0024 (0.0061 mm) Temp, deg F 78 (room) 110 150 110 150 110 150 110 150 110 150 110 150 110 150 110 150 110 150 Stress, p.s.i. 300 200 150 300 200 150 350 300 250 200 150 100 300 200 150 300 200 150 300 200 150 300 200 150 300 200 150 300 200 150 300 200 150 300 200 150 300 200 150 300 200 150 300 200 150 300 200 150 300 200 150 300 200 150 300 200 150 300 200 150 2000-hr eo, % 0.061 0.028 0.018 0.116 0.029 0.029 -0.20 0.02 0.04 0.042 0.020 0.010 0.075 0.070 0.050 0.000 0.090 0.075 0.052 0.048 0.037 -0.16 -0.01 0.04 0.05 0.06 0.04 0.02 0.08 0.07 0.06 0.04 0.03 -0.04 -0.02 0.02 0.084 0.036 0.038 0.06 0.06 0.04 0.12 0.064 0.02 -0.04 0.02 0.03 0.04 0.034 0.028 -0.07 0.02 0.02 0.08 0.04 0.014 0.10 0.05 0.04 total creep, 0.116 0.048 0.0215 0.345 0.060 0.043 2.08 0.585 0.360 0.109 0.050 0.028 0.510 0.240 0.110 1.175 0.525 0.475 0.285 0.164 0.070 1.37 0.60 0.19 0.38 0.15 0.08 0.88 0.45 0.30 0.31 0.09 0.045 0.67 0.25 0.16 0.18 0.052 0.043 0.21 0.10 0.05 0.256 0.08 0.027 0.64 0.19 0.09 0.14 0.053 0.036 0.92 0.26 0.18 0.166 0.057 0.02 0.53 0.16 0.068 5000-hr eo, 0.115 0.046 0.023 0.200 0.060 0.030 -1.000 -0.250 -0.050 0.065 0.035 0.022 0.100 0.060 0.050 0.000 0.090 0.075 0.002 0.048 0.037 -0.62 -0.11 0.04 0.02 0.10 0.035 -0.33 0.08 0.14 0.05 0.06 0.03 0.02 0.00 0.02 0.084 0.038 0.032 0.09 0.05 0.04 0.081 0.067 0.02 -0.34 0.02 0.03 0.032 0.035 0.028 -0.38 0.01 -0.02 0.082 0.043 0.018 0.05 0.03 0.02 total creep, 0.145 0.060 0.025 0.645 0.090 0.050 5.830 1.680 0.905 0.196 0.075 0.042 1.116 0.507 0.225 2.930 1.180 1.100 0.658 0.340 0.112 3.07 1.55 0.40 0.89 0.30 0.14 2.35 1.60 0.58 0.72 0.15 0.065 1.74 0.68 0.40 0.324 0.076 0.053 0.435 0.19 0.08 0.56 0.108 0.038 1.78 0.435 0.19 0.294 0.083 0.049 2.50 0.65 0.42 0.292 0.08 0.03 1.23 0.35 0.12 Total creep in 10,000 hr extrapolated from 2000-hr test 0.336 0.128 0.0355 1.261 0.184 0.099 11.40 2.845 1.64 0.377 0.170 0.050 2.175 0.850 0.300 5.875 2.175 2.000 1.165 0.580 0.165 6.65 3.05 0.75 1.70 0.59 0.24 4.32 1.93 1.22 1.31 0.29 0.105 3.55 1.35 0.72 0.564 0.116 0.063 0.81 0.26 0.09 0.800 0.144 0.055 3.40 0.87 0.33 0.54 0.129 0.068 4.94 1.22 0.82 0.52 0.125 0.044 2.25 0.60 0.18 5000-hr test 0.175 0.088 0.027 1.090 0.120 0.070 13.650 3.860 1.910 0.327 0.115 0.062 2.032 0.894 0.350 5.860 2.180 2.050 1.312 0.584 0.150 7.38 3.32 0.72 1.76 0.50 0.245 5.36 2.12 1.02 1.39 0.24 0.10 3.46 1.36 0.78 0.564 0.114 0.074 0.86 0.33 0.12 1.039 0.149 0.056 4.24 0.85 0.35 0.556 0.131 0.070 5.76 1.29 0.88 0.502 0.117 0.042 2.41 0.67 0.22 actual, by test 0.185 0.069 0.030 1.20 0.14 0.08 13.66 3.495 1.712 0.305 0.102 0.058 0.54 0.105 0.070 0.84 0.33 0.09 0.97 0.143 0.048 3.84 0.79 0.284 0.067 0.486 0.107 0.041 2.50 0.681 0.148 BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS quenched and heat-treated condition had somewhat higher tensile strength and lower ductility in short-time tensile strength tests than samples C-331 and C-389. Arsenic in lead tends to reduce the grain size more than most metals commonly used for sheathing alloys. The arsenical alloys tested had an average grain size diameter of 0.0014 in. (0.0035 mm) to 0.0038 in. (0.0097 mm), whereas the average grain diameter of the chemical leads, antimony lead, and calcium leads was 0.005 in. (0.0127 mm). The creep of lead and lead alloys consists of flow at the grain boundaries and slip within the grains. The alloys with the larger grain size resist flow better than the fine-grain alloys, because there is less grain boundary area available to deform. On the other hand, the large- grain alloys are less resistant to slip, since the small grains offer more slip interference. For a given stress the fine-grain arsenical leads are more resistant to slip at lower temperatures, where the grain bound- aries are relatively more resistant to deformation and hence more re- sistant to creep at the lower temperatures. As the temperature is increased (110 deg to 150 deg) the small grain boundary area of the large-grain alloys gives them more creep resistance, since the bound- ary area is less stable than the grains at the higher temperature. The test results indicate that in alloys where the tin content is higher than the arsenic content the alloys had less resistance to creep. This fact becomes more obvious as the temperature is increased. From these tests the optimum tin content seems to be from about one-half the arsenic content to equal the arsenic content. The best arsenic content seems to be slightly less than 0.2 percent. The copper content should not be over 0.06 percent. Copper in this amount gives good creep resistance to lead, but the ductility is lowered somewhat when copper is used with arsenic. More than 0.03 percent antimony tends to reduce the ductility of the arsenical leads. The conditions of extrusion (temperature and pressure) affect the creep properties of the sheathing. Low temperatures and the resulting high pressures give a fine-grain work-hardened metal which, if free of excessive precipitation of the alloying constituents, is usually high in long-time tensile strength and has good creep resistance to higher stresses. If the extrusion temperature is too low, poor welds occur and the sheathing tends to split at the welds in service. At high extru- sion temperatures, the sheathing does not receive as much cold working as the extrusion pressure is reduced. The grain size tends to be larger, giving better creep resistance at the lower stresses. The quenching process at the press retains more of the alloying elements ILLINOIS ENGINEERING EXPERIMENT STATION in solution. Precipitation at the grain boundaries is reduced, since the alloying materials are more finely dispersed when quenched from a higher temperature. This solution heat treatment places the sheath- ing in a good condition for artificial aging. The alloying elements in the arsenical leads greatly retard re- crystallization(4). No abrupt change in the creep rate is noted in most of the tests observed, as Greenwood and Orr (17) found in fairly pure lead. The self-annealing tendency (as indicated by increasing slope of the creep curves) shown by the arsenical lead for stresses of 300 p. s. i. or more at 150 deg may be due to the retarded recrystalliza- tion in these alloys. The recrystallization of these alloys is spread over a relatively longer time, and no radical change in their creep curves is noted, but rather a gradual increase in the creep rate. The tendency of fine-grain metals to recrystallize at lower temperature may contribute to the relatively higher creep rates of fine-grain arsenical leads at 150 deg. The added cold working of the lower extrusion temperature and resulting higher extrusion pressure may have deformed these materials enough so that only a small amount of creep stressing will cause them to start to recrystallize. Recovery of these alloys at 150 deg reduces the tendency for any general recrystallization, however. The 300-p. s. i. specimens of C-445 and C-446 at 110 deg show a change in slope of the creep curve which may be due to recrystalli- zation. The high stress and low temperature appear to have caused some strain hardening, whereas at 150 deg for these alloys recovery or relaxation prevents strain hardening, and therefore recrystalliza- tion also, and only a gradual increase in the creep rate is found. BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS III. CREEP OF CABLE SHEATHING UNDER INTERNAL PRESSURE 8. Sheathing Tested, and Apparatus for Testing Sheathing, Under Long-Continued Internal Pressure In order to study further the creep resistance of various sheathing materials under internal pressure, the equipment for testing cylin- drical samples of cable sheathing described in Bulletin 306, pages 54-57, was enlarged to accommodate nine samples of cable sheathing at a time. This equipment consists of a rack holding 6-ft samples of cable, sealed at both ends, having constant pressure applied by means of oil reservoirs. A mercury column applies a pressure of 25 p. s. i. to one set of samples at room temperature, and a tank con- taining compressed air applies a pressure of 40 p. s. i. to the other set of samples at room temperature. In the 150-deg tests a hydraulic accumulator is used for the 25-p. s. i. pressure and an air tank for the 40-p. s. i. pressure. In the 150-deg tests, 4-ft samples are used. In both tests, expansion of the sheath is determined periodically by measurement of the outside diameters with micrometer calipers. These measurements are made on small steel studs soldered to the sheath at stations 12 in. apart along the sample and across 2 diame- ters at each station. The 150-deg tests are made in 4 electrically heated ovens, thermostatically controlled, with 3 specimens to the box. Table 4 gives a list of the sheaths tested, their dimensions, and test dates. 9. Correlation of Tension Strip Tests with Creep of Sheath Under Internal Oil Pressure The expansion of the cable sheathing at various stations along the sheath is plotted in the graphs in Figs. 6-11, along with the average of all the stations. There is considerable scatter in several of the station readings, and some indicate consistently more strain than others. Since stations 1 and 4, or 5 or 6, depending on the length of sheath, are only 6 in. from the wiped seals on the ends of the specimens, a certain amount of end restraint is indicated in some of the tests. For other specimens the end stations show as much creep as do any of the inside stations. Out-of-roundness, dents, and extrusion marks may cause areas of localized stress which would affect the creep resistance greatly at different points in the sheath. Initial out-of-roundness of the sheath samples may affect the dial micrometer gage readings in these tests, indicating a change in shape or roundness and not an actual change in diameter. Charge welds, and resulting grain structures, would also ILLINOIS ENGINEERING EXPERIMENT STATION TABLE 4 SHEATHS TESTED UNDER INTERNAL OIL PRESSURE Thick- Approximate Outside ness Manu- Placed in Sheath chemical diam, (nomi- fac- test rack Remarks composition in. nal), turer in. Tests at Room Temperature T common desil- 3 912 I Jan. 23, 1937 still under test verized lead A x common desil- 3 %4 V Jan. 23, 1937 still under test verized lead A 2C copper lead 2% %4 VII June 15, 1936 still under test 2G copper lead 2% %2 VII Apr. 23, 1936 still under test C-331 arsenical lead 2%z %4 I Oct. 3, 1944 increased to 40 p. s. i. internal pressure Feb. 14, 1945 C-404 arsenical lead 22?4 914 I Jan. 6, 1947 still under test at 40 p. s. i. internal pres- sure C-417 arsenical lead 2% s V June 24, 1946 same as C-404 C-430 arsenical lead 2914 Y VII Mar. 26, 1947 same as C-404 Tests at 150 deg F 2'12 22?4 21i%2 2%ý 2%2 21/12 2% 21%42 21464 22i%2 22%i2 94 944 %4 ý%2 %4 %4 %4 1%2 %14 IV I IV VII I I V VII IV Apr. 12, 1945 Dec. 29, 1943 Feb. 2, 1943 Apr. 12, 1945 Apr. 12, 1945 Apr. 12, 1945 Aug. 23, 1946 June 16, 1947 Jan. 25, 1943 July 24, 1947 July 25, 1947 July 25, 1947 failed June 3, 1947 at 40 p.s.i. placed under25 p. s. i. steady internal pres- sure at 150* F April 10, 1945 same as C-299 still under test failed Aug. 18, 1948 failed Apr. 19, 1948 failed June 30, 1949 failed June 13, 1949 failed Jan. 8, 1948 still under test still under test still under test C-296 C-299 C-296 2G C-331 C-389 C-417 C-430 C-302 Press stop mark samples 1 2 3 chemical lead copper lead chemical lead copper lead arsenical lead arsenical lead arsenical lead arsenical lead common desil- verized lead A arsenical lead arsenical lead arsenical lead BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS give different creep test results for stations near such points. A varying rate of extrusion would affect the amount of work hardening (except of alloys showing elongated grains) and also the quenching rate, with a consequent change in grain size, orientation, and strain hardness retained by the sheath before being tested by internal pres- sure. Uneven die block temperature causes eccentric sheath and un- even work hardening of the lead. Both factors cause erratic creep test results. IA .y 2.L 0 6 ---------1-i-I-I-i LIII'#C '#5 e Hunrs of Days 7Tne in Hundreds of DayIs FIG. 6. CREEP-TIME DIAGRAMS OF COMMON DESILVERIZED LEAD A, UNDER INTERNAL PRESSURE AT 78 DEG F I I I I I I I I I I Circumferenfial/ Stress 243 lb. per sq n.5 0 -- ---- -e tk- __Average 1-- 0 \t C ' 0 __ __ _ _ - - - -< c- - - - - - - - Ave^^nage^ %4. 6 *4 2 f ILLINOIS ENGINEERING EXPERIMENT STATION '1~ S... 1.0 - Tensle Specimen - j- " - /.Z- - - I I I II - 0.8 /4 I "- - Average Creep l.4 L ? . (a)-Sheath 2C / Circunmferential Stress /2 .per sq I 0.2_ _ _ - ^ '1- 48 T/me i7' Hundreds of D0ys FIG. 7. CREEP-TIME DIAGRAMS OF COPPER LEAD UNDER 25-p.s.i. INTERNAL PRESSURE AT 78 DEG F These conditions also affect somewhat the creep test results on strip specimens. The strip specimens for creep tests were cut longi- tudinally from sheath, and only a small part of the sheath diameter was used for these specimens, so that variations in thickness were at a minimum. Only sheathing material of uniform grain structure free from welds was used. Hence, more uniform testing conditions pre- vailed in the strip tests than in the test cylinder, and as a result the data show less scatter. 2 /3 24 30 36 6 J fl .... .. BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS I 1 ()- I 0.2 -- Cfrcumfeernt/a/ Stress /10 303 /b. per sq. in. * Pressure Changed from \ \ "I f- 26o 40 /b. per sy /t. ----- Sta ---n (Tens//e Spec//we, 303/h per s. th. / SeAverage Creep 4 .3 I Se(bl- C-404 I i Circumferentia/ Sfress 3/1 lb. per sqt in. 0./ ain - ^,'"^ T TI-ne In Ha'uareds of Daq,'s FIG. 8. CREEP-TIME DIAGRAMS OF ARSENICAL LEADS UNDER 40-p.s.i. INTERNAL PRESSURE AT 78 DEG F Is ,A 0 2 4 6 8 /0 /Z 14 e6 V ^ .; < i ILLINOIS ENGINEERING EXPERIMENT STATION In order to compare the creep of sheath under internal pressure with that of strip loaded with a tensile stress equal to the hoop or circumferential stress of the sheath, 10-in. specimens were cut from the same sheathing material and loaded in the creep racks. The hoop stress of the sheath was determined by the thin-walled-cylinder V 1 I I I I I I I I i 0 / I nII - Ieo)- rC-e96 Chemeca Leade - - . I---- ^ Inferncal Pressure 40 lb. per scf. in Iq -1 I- C,~cumferen ha'! sSftress 3262 ii,. pet ~ ,,'~'. I~] [ I I-1-F - I J. I . I i Z A e±A _,,. o.i 0 I 2 3 4 - 6 7 Time in Hundreds of l4a/s FIG. 9. CREEP-TIME DIAGRAMS OF CHEMICAL LEAD AND COPPER LEAD UNDER INTERNAL PRESSURE AT 150 DEG F formula S= PD/2t, where P= the internal pressure in pounds per square inch, D = internal sheath diameter in inches, and t = the thick- ness of the sheath in inches. In most cases the strip specimen had a greater total creep than the sheath specimen. At 150 deg the tensile specimens of arsenical lead C-331 and C-417 have practically the same creep resistance as the sheath. At room temperature (78 deg) (a) - C-331 Ai^^;- / ^i ,Sd Crcurnferential Stress 303 b. per sq. in. Station 4 // Diameter Station f / / C|™ Specimen Fa/ed 0 /1 3 4 5 6 7 8 9 /0 I 14 ] - Station -/ 4 (b)- C-389 / 3 Arsenical Lead Circumferentia/ Stress, 345 /b per sq . . / Tape Diameter Creep Tensile Specimen " • 34 S5/b per sq. n."S f'» 1^pecimen Fai/ed1 Time in Hunadreds of Daiys FIG. 10. CREEP-TIME DIAGRAMS OF ARSENICAL LEADS UNDER 40-p. s. i. INTERNAL PRESSURE AT 150 DEG F O / £ 3 4 5 6 7 8 9 /0 // /2 ILLINOIS ENGINEERING EXPERIMENT STATION k. IN ~1.. IN 3 6 9 ,/2 5/- Time in Hundreds of Daays 18 2/ FIG. 11. CREEP-TIME DIAGRAMS OF FOUR TYPES OF LEADS - C-302A, COMMON DESILVERIZED LEAD A; C-417, ARSENICAL LEAD; C-296, CHEMICAL LEAD; AND C-299, COPPER LEAD -ALL UNDER INTERNAL PRESSURE AT 150 DEG F 5 ---- I IIII (a) - 302A I Common Desi/verized Lead 4 --- Ccumferentia/ Stress 2/ Ib. per sq. in. Internal Pressure 25 lb. per sq. in. I / Specimen C 1-1- -- - 0 / 2 3 4 5 6 7 9 /0 Time in Huna'reds of Days i /___--__l (b)--C-417, CArseical Lead S- Circumferential Stress- I313 l. p 5r sq. In. 8 / Faaled at 1037 Dayvs, at /2. 7 Q Expansion. S(c)-C-296, Chemica Lead S-Circumferentia/ Stress C-69 0 Ib. per sq. in. 4 Chemical Lead, / Tensi/e - -Spec/men, - 200 /b per sq i. 6 D-C-299, Copper Le ad - - Circumferential Stress - A y- 192 Ib. p6er 5 i \ × l I Iq iI. I .~; .~ BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS the strip creep specimens of C-417 and 2C have higher creep rates than the corresponding sheath samples at 78 deg, whereas the 150- deg strip specimen of C-430 has a higher creep rate than the sheath. The sheath at 150 deg has the higher creep rate in the case of C-296. The 78-deg tests of C-331 and X have about equal creep rates for sheaths and strip specimens. The comparison between the strip creep test results and the sheath creep test results is not as close for some sheathing materials. If a uniform grain structure and uniform work hardening prevailed throughout the sheath, the biaxial state of stress in the sheath would tend to reduce the creep and expansion necessary for fracture of the sheath to less than that obtained in the strip tests. Unfortunately, some lead sheathing tested received such treatment during fabrica- tion as to give it a duplex grain structure. This treatment was severe enough in some cases to overcome the biaxial stress effect. The extrusion process tends to produce a striated structure in the sheath, giving grains elongated in the direction of extrusion. The cross-section of a longitudinal specimen has a finer grain structure and hence will creep more at relatively low stresses than the sheath which has a somewhat larger structure normal to the circumferential stress. In the strip specimen, more of the grain boundary area has to resist the stress, and as creep at low stresses consists mostly of flow at the grain boundaries more creep is obtained in the strip speci- mens for a given stress. In the sheath the plane normal to the cir- cumferential stress has less grain boundary area and as a result has more resistance to creep. However, the elongation at fracture is greater for the strip specimens because more slip within the grain can take place due to grains being previously elongated in the direc- tion of extension; hence, less slip interference is introduced. The tendency for pure lead to recrystallize and coalesce near room temperature would remove the striated structure acquired in extrusion. The sheathing materials with higher alloying content would retain more of the striated structure, especially those con- taining arsenic, calcium, and silver, which because of an increase in the recrystallization temperature greatly retard recrystallization of lead. The arsenical alloys can retain considerable cold work or strain hardening at the lower temperatures. Under these conditions their creep resistance is fairly high. However, these alloys will tend to recrystallize; as a result they show a rapid creep rate for a time, followed by another period of strain hardening. This phenomenon occurred for the C-331 alloy at 78 deg (Fig. 8) after 1400 days at 303 p.s.i., with an average strain of less than 0.10 percent. This finding indicates that this material had severe cold working, probably ILLINOIS ENGINEERING EXPERIMENT STATION extruded at a fairly low temperature with high extrusion pressure. C-404 (Fig. 8), nominally the same alloy, recrystallized near 500 days for a stress of 311 p.s. i. at 78 deg with about equal strain. The inference is that C-404 was probably overstrained at the press, since this material had started to recrystallize. Its grain size was considerably larger than that of C-331 and it completed the re- crystallization process during the earlier test period. Some recovery, however, had taken place during the interval between extrusion and testing; therefore some strain hardening took place during the first 300 days of testing. The room temperature sample ir of common desilverized lead A and the 150-deg sample 2G of copper lead (Figs. 6 and 9) seem to have a long third stage of creep. However, the data indicate that recrystallization has taken place in both these samples. The lower alloy content of r has allowed recrystallization to take place with less strain at a lower temperature than in the case of 2G. The room temperature sample X of common desilverized lead A shows no evi- dence of recrystallization, although it has expanded 3.5 percent in 4500 days with a circumferential stress of 243 p. s. i. This fact would indicate that the sample of ir received considerable work hardening before being placed under test. It had received most of the strain necessary to cause recrystallization, since it began to recrystallize with an expansion of only 0.25 percent in the creep test. 10. Test Results to Failure of Sheaths Subjected to Internal Oil Pressure at 150 deg F Since the manuscript of Bulletin 378 was prepared five sheaths have failed in 150-deg racks. The common desilverized lead A sheath, C-302, failed after 24,000 hr with a circumferential stress of 212 p. s. i. Since this sheath had been removed from service before testing, it had considerable prestraining before testing. A slight recovery was indicated by the slight strain hardening in the first 1000 hr of test, after which it recrystallized over a fairly short time and strain- hardened again. With the prestraining it had received in service, a strain of only 0.2 percent was necessary for recrystallization at 150 deg. This sample was used in the pressure-vacuum tests (see Bulletin 378). Figure 11 shows the expansion of the sheath in percent of change in outside diameter as measured by a displacement method. After failure the average diameter change was 6.484 percent as measured with micrometers. The expansion at the point of failure was 10.428 percent. BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS The chemical lead sample C-296 (Fig. 9) with 40 p. s. i. gage pressure and a circumferential stress of 320 p. s. i., described in Bul- letin 378, failed after 18,000 hr of testing with an average total expansion of 6.15 percent as measured by the dial micrometer gage. The diameter tape gave (from readings taken between the soldered- on studs) an average expansion of 7.12 percent. This fact indicates some creep restraint from the soldered studs. The expansion at the point of failure was 7.91 percent. The creep curve for this sample shows a very uniform rate of creep to failure. The arsenical alloy C-389 sample (Fig. 10) failed after 26,500 hr. The 40 p. s. i. gage pressure gave a circumferential stress of 345 p. s. i. The dial micrometer gage gave a total increase in diameter of 12.13 percent. The diameter tape gave an average of total creep of 13.56 percent, also indicating restraint of creep by the soldered studs. The increase in diameter at the point of failure was 17.85 percent. This sample failed at a seam weld. The tensile strip specimen had a higher creep rate than the sheath sample. The sheath sample had a fairly long third stage of creep. The alloy content seems high enough to retard recrystallization, as indicated by the smooth creep curve, which suggests that recovery tends to remove any appreciable strain hardening at this temperature. The arsenical alloy C-331 (Fig. 10) failed after 29,500 hr with a circumferential stress of 303 p. s. i. produced by a 40-p. s. i. gage pressure. The average total strain as measured by the dial micrometer gage was 25.48 percent, and by the diameter tape was 26.36 percent. The expansion at the point of failure was 32.95 percent. Since the sheath failed at a point only 2 in. from a wiped seal at the station 4 end of the sheath, some end restraint was encountered. The maximum expansion was 37.76 percent; it occurred 2 in. past station 4, or 2 in. from the point of failure. The C-331 specimen had a press stop mark and was further handicapped by the charge weld. The failure occurred at the seam weld. Like the C-389 sample, which contained a charge weld, this specimen was not a representative sample of the alloy being tested. The ductility of the samples was the highest of any sheath tested, and no test of strip specimens from these sheaths, some containing welds, indicates low ductility resulting from welds. However, the welds do reduce the ductility of the strip specimens. The expansion of the sheath near the press stop mark was a minimum of 6.334 percent. The C-430 arsenical lead sample (Fig. 10) failed after 16,900 hr with 40-p. s. i. gage pressure and a circumferential stress of 303 p. s. i., ILLINOIS ENGINEERING EXPERIMENT STATION at 150 deg. The dial micrometer gage gave an average total expan- sion of 6.80 percent. The diameter tape gave an average total ex- pansion of 6.84 percent. The increase in diameter at the point of failure was 7.08 percent. This alloy, like the other arsenical sheath samples, does not have the creep resistance of chemical lead at this stress and temperature. C-430 had a lower creep rate but less ductility than C-331 at this stress and temperature. Since little evidence of precipitation at grain boundaries was found in C-331 and C-389 and considerable evidence was found in C-430 and also in failed tensile specimens of C-411, it is believed that the antimony content in the latter alloys is too high. The sheath sample of C-430 failed with little reduction in area in a little over half the time of C-331 with less than one-fourth the expansion or ductility. The precipitation at the grain boundaries of antimony has weakened this material. The C-430 sheath failed at the bottom weld although the weld looked very sound under the microscope. Upon microexamination the failure was classed as intercrystalline. The arsenical lead sample C-417 failed after 24,900 hr with a circumferential stress of 313 p.s.i. at 150 deg. The total expansion as measured by the displacement method was 12.17 percent. The diameter tape and micrometer measurements gave an average expan- sion of 11.2 percent. The expansion at the point of failure (diameter tape) was 13.25 percent. The failure was a very ductile type, knife- edge, and seemed to be at a charge weld, since the sheath necked down at several points spirally along its length. The charge weld, however, was not pronouncedly visible under the microscope. This material contained 0.029 percent antimony. Since it had good duc- tility and life for this stress and temperature it is believed that this amount of antimony is not injurious. The superior ductility of the arsenical leads, unless they contain too much antimony, allowed them to withstand higher stressing (300 p. s. i.) longer than the copper and chemical leads. Their creep resistance is less at 150 deg. The grain size was extremely fine for the C-331 sample of arsenical lead as received. After failure, the structure was about equal to that of C-417 and C-433, or about 0.002 sq in. average grain size - indicating that this alloy (C-331) received severe cold working short of recrystallization at the lead press. And the creep tests of strips at 78 deg and 110 deg have not gone sufficiently far to strain the lead enough for recrystallization at these temperatures. The alloy BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS content is enough to retard recrystallization at 150 deg, so that no rapid increase in creep rate is experienced although a gradually in- creasing creep rate is noted after the first few thousand hours of test. Recovery in this alloy at 150 deg is sufficient to spread recrystalli- zation over a relatively long period. 11. Effect of Press Stop Marks on the Creep of Sheathing Subjected to Internal Pressure The wide variations in the creep of sheathing near and at press stop marks have indicated the need of further study of this problem. Most reels of large cable will have two or more of these press stop marks, since few presses can handle a slug large enough to sheath a large cable of appreciable length. The press must be stopped to check for eccentricity and one press stop mark will usually result from this procedure also. If dross and oxides are removed or pre- vented from forming, no serious defects are introduced into the sheathing. However, a drastic change in the work hardening of the metal takes place, resulting in non-uniform grain structures and great differences in creep rate. This effect of press stop marks is not confined to the alloys of lead; it is also pronounced in common desilverized lead. If work hardening is severe enough in the resumption of extrusion, recrystal- lization may take place and an area of large grains is produced which has good creep resistance but is low in ductility. Premature failure can occur here, since the sheathing is overstrained in fabrication. To investigate this problem further, three sheaths were extruded in a commercial lead press. The press was stopped for 15 min on each sheath. This placed the press stop mark at the center of each sample. An effort was made to have part of two charges of lead in each sample. The following temperature conditions prevailed during the test: Sample No. 1 Sample No. 2 Sample No. 3 Pouring temperature 480 deg C 485 deg C 488 deg C Cylinder temperature 130 deg C 140 deg C 140 deg C Die Block temperature 215-255 deg C 215-255 deg C 210-242 deg C During the entire extrusion period, including the time the press was stopped, cold water was applied to the sheath at a point 16 in. from the die. Samples 1 and 3 received no further treatment. Sample 2, how- ever, was subjected to an additional temperature program comparable to that given to oil-filled cable when drying and saturating. The .program consisted of keeping this cable in an oven at 125 deg C for 40 ILLINOIS ENGINEERING EXPERIMENT STATION seven days. The sample was unintentionally held at 150 deg F for 5500 hr before internal pressure was applied. The alloy used for these samples contained 0.11 percent arsenic, 0.11 percent tin, 0.05 percent copper, and 0.02 percent bismuth. The 40-p. s. i. gage pressure gave a circumferential stress of 332 p. s. i. for these sheaths. The creep properties of this alloy at 150 deg F are very good. The maximum expansion of any of the samples is 3.45 percent in 19,632 hr. The aging of this alloy at 125 deg C for 7 days has not affected its creep resistance to any appreciable degree. Figure 12 shows the expansion as percent increase in diameter at various points along the samples. BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS e/O/SUt'dXJ ,t~/o~9 J~ EVI 0 E-1 z 0 Q I?4 S. z§ I I 0 C z 0 ILLINOIS ENGINEERING EXPERIMENT STATION IV. TENSILE STRENGTH AND DUCTILITY OF CABLE SHEATH METAL UNDER STEADY LOAD 12. Test Specimens and Testing Machines Used The specimens and testing machines used in the fracture tests reported herein are fully described in Bulletin 347. The specimens were 4% in. long, with a critical section 2 in. long and Y in. wide. The thickness of the specimen is the thickness of the sheath. The critical section is lightly marked off in l-in. gage lengths by use of a special apparatus, in order to determine the maximum elongation after fracture in the 1-in. gage length in which failure occurred. Before the specimens were made, the sheathing was etched to deter- mine the structure and weld area. All specimens reported herein are cut transversely from the sheathing and were free from welds in the active section of the test specimen. The testing machines were operated at room temperature, which averaged 78 deg, and at 110 deg. The specimens were loaded directly with dead weights, and in the tests that failed in less than 168 hr the time was automatically recorded with clocks. 13. Test Data of Long-Time Tensile Tests The test results of long-time tensile tests of strips cut from cable sheathing are shown in Figs. 13-15. A semi-logarithmic plotting was used, since this gives a more nearly straight-line relationship be- tween tensile stress and hours for fracture than either cartesian or log-log plots. Since the creep of lead is sensitive to many variables - work hardening, strain hardening during testing, segregation, precipitation, etc. - the maximum elongations are plotted and con- nected with straight lines unless a definite trend is indicated, and the semi-log plotting is used for similarity or consistency. There seems to be no definite relationship between short-time tensile strength as indicated by the static tensile test and length of life under long-time tensile stress. Neither is the static tensile test any criterion of creep resistance. In comparing the effect of work hardening on life under long-time tensile stress of a given alloy, the static tensile test seems to be of some value. In comparing different alloy content, however, one can easily be misled by the static ten- sile test. Many alloys of high short-time tensile strength have rela- tively short long-time strength, and some alloys of low short-time strength have relatively high long-time strength. For example, the test results show that the long-time tensile strength of C-430 is low BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS / z C) -3- 4 7 130LIyj N 0/ S~I/~0y N ~d4/~2O.&' 0/ &/flO/7' /_ J .; '? I N t-N ^ ;! ^ s » > » k > 1 I .t ~J < *1.. ~ 1- Q 0 q) ~ 0 I,- Cd) i ILLINOIS ENGINEERING EXPERIMENT STATION as compared with that of C-404. The static tensile strength of C-404 is 2335 p. s. i. and that of C-430 is 2910 p. s. i. when tested at very near the same speed. The effect of alloying lead with some metals is to increase greatly its static and short-time tensile strength. Other metals, however, 30000 1 0006 3/ 000 /00 /0 400 600 1000 1200 1400 Stress in /b. per sq. in. FIG. 14. STRENGTH-TIME DIAGRAM OF TIN-ANTIMONY LEAD AT 110 DEG F tend to lower its long-time tensile strength. Alloys with more than 0.20 percent antimony are usually a little lower in long-time tensile strength, which is undesirable in electrical power cable. Too much tin should be avoided. Alloys containing more tin than arsenic have lowered long-time tensile strength. The test results seem to indicate that the lead can be easily oversaturated with the alloying constituents. Precipitation at the grain boundaries tends to weaken the material. Strain aging then becomes a problem. \ # BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS 45 04 rC i/?,/2V~j 0/ .S'J/~0ft a We ILLINOIS ENGINEERING EXPERIMENT STATION Artificial aging, such as is received when oil-filled cables are dried out - namely 7 days at 125 deg C - greatly increases the short-time tensile strength of the arsenical leads low in copper and antimony. No evidence yet exists that they will over-age to the point of excessive precipitation in long-time tests. Quenching of the ar- senical lead at the press improves both short and long-time tensile strength. A solution heat treatment at the press seems desirable. Quenching the sheathing immediately after extrusion retains more of the alloying element in solution and tends to stop recovery from removing work hardening. The sheaths containing lower alloy con- tent can be extruded at the lowest temperature consistent with good welds and when rapidly quenched retain considerable work hardening, which improves their strength so that it equals that of higher- content alloys. The grain size of lead is affected by the cold working and the alloy content. As the extrusion temperature is lowered or the alloy content increased the grain size is refined. As the alloy content is increased so must be the extrusion temperature, or precipitation at the grain boundaries is encountered. The higher alloy content leads must be extruded at higher temperatures, and therefore their grain size will be larger. The lower content alloys should have a fairly small grain size to be in the best work-hardened condition. However, a smaller amount of straining will cause them to recrystallize, es- pecially at higher temperatures, because there will be less retarding by the alloy content. Recrystallization is an advantage here, be- cause when recrystallization takes place the alloy usually has greater elongation before fracture occurs. Too high an alloy content reduces the ductility of lead. Only 0.06 percent copper will reduce the long-time ductility under low stress conditions. Smaller amounts of antimony are worse in the long- time tests. Although chemical lead, copper lead, and antimonial lead are very ductile in the short-time static tensile test, to a slight ex- tent tin reduces the ductility in the long-time tests. A combination of tin, antimony, and silver in lead, even in small amounts, reduces the ductility in the long-time tests. An alloy, containing 0.36 per- cent tin, 0.32 percent antimony and 0.01 percent silver (C-411) had low long-time ductility. Some of the alloys seem to fail with less elongation as the time for failure is increased, and then for greater times the elongation increases. The grain boundary areas seem to be weaker than the grains. The larger the grain size the lower the stress and hence the longer the time for the elongation to start increasing. When the BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS stresses are high enough to cause slip within the grains, high values of ductility are obtained. As the stresses are lowered less slip is ob- tained and more of the elongation occurs as flow at the grain bound- aries. There is a stress condition such that the minimum amount of flow at the grain boundaries is obtained because the boundary area becomes overstressed at localized points of weakness. At lower stress more grain boundary area is deformed before some point is over- stressed. Deformation within the grain under high stress conditions relieves high localized stresses in the boundaries enough to balance the resolved shear stress of the grain. Then under intermediate stresses some recovery takes place at the testing temperatures, and less slip occurs within the grains; hence, lower values of ductility are recorded. The good ductility of some fine-grain alloys in the long-time test is probably due somewhat to recrystallization. After the critical deformation is finally reached the metal recrystallizes and a large elongation occurs in the third stage of creep. In the short-time test the stresses are too high to get the full elongation resulting from slip after recrystallization, and the ductility is lower than for large-grain alloys. Also, more slip interference may be experienced in the fine-grain leads in the high stress tests. More- over, the severe deformation at the lead press has used up much of the ductility of the fine-grain alloys. ILLINOIS ENGINEERING EXPERIMENT STATION V. REPEATED-BENDING TESTS ON SPECIMENS CUT FROM CABLE SHEATHING 14. Apparatus Used in Bend Tests of Strip Specimens The machines used for these tests are fully described in Bulletin 378. The specimen is a simple beam supported at each end by double pivots and loaded at two symmetrical points along its span by a double and a single pivoted push rod. The machines are mounted in ovens which are heated electrically and are thermostatically con- trolled at 110 deg. The strain is measured by the use of a radius of curvature gage of 2-in. gage length which is described in Appendix B, Bulletin 378. The number of cycles necessary to cause failure is recorded by ratchet counters which cease to record when the speci- men is completely broken in two. Two sets of test data are reported herein. One group of tests was conducted at a frequency of one cycle in 10 min; in the other a 1-hr cycle was used. Because of the great length of time required to complete tests at these low frequencies, only three tests of each of most alloys were made at each frequency. Only alloys that were of special interest were run at one cycle an hour. The total strains at the outer surfaces of the specimens were 0.3, 0.4, and 0.5 percent (-0.15, -0.2, and -0.25 percent strain in each direction). These strains, of course, were accurate only at the start of the test. There was a slight increase in the readings of the curvature gage as the specimen elongated under stress for the first few cycles. Since cable sheathing behaves in this manner in service, the condition was not considered as serious. These values of strains were used because they are considered about equal to those commonly met in service where the cable bends in the manholes due to the daily change in temperature resulting from different electrical loads. The specimens are unpolished, being taken from cable sheathing and having its original thickness and surface irregularities. For this reason and because lead strain-hardens, some scatter is apparent in the test results. These results, however, seem sufficiently accurate to permit classification of the relative resistance of the various materials to bending. 15. Strip Bend Test Results The tendency - pointed out in Bulletins 347 and 378 for higher rates of strain - for the number of cycles necessary to cause frac- ture to decrease as the length of cycle is increased for a given strain is evident in these tests also (Table 5). Since, to produce this amount BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS 49 of strain in each cycle, the stresses are necessarily very high in the specimens, considerable creep takes place. As the rate of strain is decreased or the time per cycle is increased, more creep per cycle takes place, thus greatly decreasing the number of cycles necessary to cause failure. Because the elapsed time for failure is increased TABLE 5 SUMMARY OF STRIP BEND TESTS AT 110 DEG F Cycles for failure Material Total strain, percent 10-min cycles 1-hr cycles chemical lead: C-361 0.5 3 951 2 888 0.4 4 047 3 177 0.3 4 652 4 400 tin-antimony lead: C-411 0.5 7 777 0.4 9 494 0.3 15 613 arsenical lead: C-389 0.5 18 400 0.4 19 732 0.3 30 634 21 343 arsenical lead: C-404 0.5 8 249 7 255 0.4 10 764 8 219 0.3 15 501 8 017 arsenical lead: C-417* 0.5 13 100 0.4 16 543 0.3 30 209 arsenical lead: C-427 0.5 7 029 4 175 0.4 8 936 6 606 0.3 14 379 13 825 arsenical lead: C430 0.5 6 934 3 625 0.4 9 491 4 683 0.3 26 923 8 043 arsenical lead: C-433 0.5 5 792 0.4 7 990 0.3 14 969 arsenical lead: C-436 0.5 6 884 0.4 10 204 0.3 13 609 arsenical lead: C-441 0.5 6 209 0.4 7 232 0.3 14 321 arsenical lead: C-445 0.5 11 814 0.4 16 180 0.3 27 438 arsenical lead: C-446 0.5 11 511 0.4 14 303 0.3 19 369 arsenical lead: C-450 0.5 8 963 0.4 13 583 0.3 31 860 arsenical lead: C-451 0.5 10 329 3 823 0.4 13 198 0.3 15 078 * A duplicate test gave the following results, reading down: 7872, 9158 and 35,724. 50 ILLINOIS ENGINEERING EXPERIMENT STATION with the slower stress cycle, stress corrosion becomes more of a factor in decreasing the bending resistance. In most cases the fine-grain alloys have the higher bending re- sistance to this range of strains. Since there is little evidence of slip in the failed specimens and all failures were intercrystalline, the alloys with the most boundary area in which to take the bending deformation had fewer high stressed areas to hasten failure. Scatter is due to the fact that, in some duplicate tests, the structure was duplex, random, in grain size in most sheaths. There is a tendency for the large-grain materials to approach the life of the fine-grain materials as the strain per cycle is reduced. The high alloy content that tended to reduce the creep resistance and ductility in the long-time tensile test does not seem to affect the bending resistance so much at the lower strains. Where the tin con- tent is over 0.14 percent, these tests indicate lower values of bending resistance. Copper alone does not greatly increase the resistance to bending in this range of strains. Arsenic in the range of 0.16 to 0.18 percent seems to be the best amount for bending resistance. BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS VI. CABLE BEND TESTS 16. Repeated-Bending Tests of Full-Size Section of Cable Underground electric power cables expand and contract in length with change in temperature as the electrical load varies, so that at each end of a 400-to-500-ft length there is a daily movement which usually amounts to from 4 to 3% in. This movement produces re- peated bending of the cable in the manholes, and in many cases the life of the cable depends on the ability of the sheath to withstand the bending. A large number of tests have been run in dummy man- hole machines in Chicago by the Commonwealth Edison Company, in Philadelphia by the Philadelphia Electric Company, and by some of the fabricators of cables. Since these machines require specimens several feet long and some test the manhole installation as a whole, a machine was developed and built which would require only a few feet of cable for test samples and which would determine the bending resistance of the sheathing under more closely controlled condi- tions and the effect of the various cable designs upon the bending of the sheathing. 17. Testing Machine and Specimen Used The testing machine employed is fully described in Bulletin 378. It has been enlarged to accommodate two specimens at a time. The cable sample is 32 in. long, is bent to a radius of 24 in., and is sealed at both ends by wiping the conductors and sheathing together so that when the barrel-stave chucks grip the sheathing the conductors carry the bending stress to the sheathing in the critical section. The chucks are mounted in trunnions to allow free pivoting. To further reduce end conditions in the test, the sheathing is cut com- pletely around at a distance of 1 in. from the chucks. Since an air pressure knockoff switch is used to determine failure, rubber sleeves reinforced with friction tape are cemented to the sheath around the cuts. One chuck is fixed to the base of the machine; the other is fastened to a rod that is actuated by an adjustable cam geared to run for these tests one cycle in 6 min. All tests reported herein were conducted on 500,000 circular mil, 3 conductor, 9 to 13 kilovolt cable. The strain in the sheathing is measured by means of a 1-in. strain gage sensitive to 0.0001 in. Small steel gage points are cemented to the top and bottom of the test sample at three 1-in. intervals in the middle or critical section. ILLINOIS ENGINEERING EXPERIMENT STATION 18. Results of Bend Tests on Cables The strains plotted in Fig. 16a are the maximum weighted average strains in the 1-in. gage length in which failure occurred. The weighted average strain is computed by the following method. The strain, measured by a 1-in. strain gage at 1000-cycle intervals during the test, is averaged, and the products of this average strain and the number of cycles between consecutive readings are added. This sum is then divided by the number of cycles necessary for failure, to determine the average strain prevailing during the test. The failures usually occur in the bottom center 1-in. gage length; how- ever, localized bending of the conductors or bending of the sheath at the edges of the metal binder tapes in the cable may cause failure to occur in an outer gage length or on top of the sheath. The failures are intercrystalline, with little bulging at total strains between 0.3 and 0.6 percent. At strains above 0.8 percent some bulging is evident. In considering only the maximum weighted average total strain prevailing during the test it seems that the fine-grain alloys such as C-389 and C-417 have the best resistance to the lower strains. As the total strain is increased these alloys do not seem to have this advantage. Such a finding is in keeping with the fatigue studies made on other metals. The endurance limit is usually higher for fine-grain material than for coarse-grain material. However, when the stress is above the yield point the coarser-grain materials have the best bending resistance. C-433 is fine-grained, as is C-446. These materials did not have the same severe work hardening at the press and quench- ing as the materials named above. Their low strain resistance is not as high, but their high strain resistance is better. C-430 and C-441 are large-grain arsenical lead alloys. Their total alloy content is high, and with their relatively large grains they were probably ex- truded at a fairly high temperature which gave little work hardening. Their resistance to low strain is low; their high strain bending resist- ance is not very good but is probably better than that of C-389 and C-417. Recrystallization in the fine-grain alloys may increase their bending resistance to higher strains. Since no high strain tests were run on C-389 and C-417 this hypothesis has not been established. Because the bending in the manhole depends mainly on the length of cable and the change in temperature under operating con- ditions and slightly on the ratio of outside cable diameter and inside duct diameter, the sheathing has little effect upon this change in length and the resulting bending. The maximum amount of localized strain developed in the various sheathing alloys for a given change in cable length is an important factor. In the cable-bending machine BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS I c: ^ c* i; U) .!. ^ I ^ s U) I p ^si "I * ^ U) ^I Q5 OD ILLINOIS ENGINEERING EXPERIMENT STATION the ratio between the stroke and the prevailing maximum total strain was studied. Values are plotted in Fig. 16b. In this comparison C-430 and C-441 have a good record. It takes a very high stroke to develop very high strain in these sheaths. The fine-grain alloys such as C-389, C-417, C-433, and C-446 are little better than the chemical lead C-361. As mentioned above, the sheaths of the cable bend samples are cut, completely around, 1V2 in. from the chucks. This condition allows part of the stroke to be used in bending the samples locally at the cuts. The sheaths are slightly belled out at the cuts, especially during the early part of the test. Consequently the amount of strain- ing at the critical section of the sample depends on the stiffness of the sheath. Because of more slip interference the fine-grain sheaths under high stress conditions are stiffer, and as a result more strain occurs at their critical sections. Since the sheaths are not cut in service, the large-grain sheaths do not have this advantage to so great an extent. As a result this strain-stroke ratio would not have the significance in practice that is indicated in the cable bend tests. In comparing the life of the samples for a given stroke (Fig. 16c), C-450, C-451, C-430, and C-441 and the chemical lead C-361 seem to be best suited for the larger movements in service at the duct mouths. On the other hand, if the bending in the manhole is controlled by having longer or more gradual approaches from the duct mouth to the joint, the fine-grain alloys such as C-450, C-417, C-389, C-433 and C-446 may be used to advantage. The higher the arsenic content the greater is the bending resist- ance to lower strains. For the larger strains the alloys having more tin give better bending resistance. The alloys C-450 with 0.144 percent arsenic and slightly more tin seem to have the best all-around bending resistance. Antimony in amounts above 0.20 percent tends to reduce the bending resistance for both high and low strains. Copper in amounts above 0.035 per- cent tends to reduce the bending resistance. The effect of bismuth on the bending resistance of cable sheathing is not clearly demon- strated in these tests. Bismuth up to 0.10 percent does not affect the bending resistance to low strain adversely nor does it increase the bending resistance to high strains. The alloys of lower alloying content tend to expand more in the first few cycles; hence a larger stroke is necessary for a given strain in the sheath than in the materials of higher alloy content. However, the alloys that have a higher tin content than that of arsenic creep enough to require a larger stroke to produce the high strains. BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS When these alloys are compared on the basis of life for a given stroke the finer-grain ones have better bending resistance in the short-stroke tests but less bending resistance in the long-stroke tests. The alloy C-450 with a grain size of 0.022 in. seems to have the best all-around bending resistance as indicated in the cable bend tests. The alloys that have good bending resistance tend to have lower creep resistance. Service conditions, then, dictate the type of sheath- ing alloy for any specific installation. In bend tests of cable made in Chicago by the Commonwealth Edison Company it was found that the construction of the cable under the sheath has a large influence on the test results. Sample C-361 is belted cable, which helps toward high test results. Samples C-389 and C-417 are shielded cable with metal binder, which tends to cause lower test results. The others are shielded cable with 20-mil paper binder, for which the test results would be intermediate if the sheaths were identical in bending resistance. If samples C-389 and C-417 had been shielded with 20-mil paper binder, greater life in the high strain tests would be expected. ILLINOIS ENGINEERING EXPERIMENT STATION VII. SUMMARY 1. The investigation reported in this bulletin was made from the viewpoint of the user of lead sheathing for electric power cables. Most of the tests were made on specimens of sheathing or on speci- mens cut from sheathing made of arsenical lead alloys produced at various cable factories, some from commercial production and some from experimental lengths, but all extruded by the factory lead presses. There has been no attempt in this investigation to make alloys of lead for the purpose of studying their properties. 2. In tensile-creep tests of arsenical alloys, just as for other lead alloys, at stress of 300 p. s. i. or less continued to fracture there seem to be three fairly well defined stages: (a) a preliminary stage during which creep starts at a relatively high rate and diminishes in rate as the stage proceeds; (b) a stage during which the creep may re- main nearly constant, diminishes gradually at a greatly reduced rate for several thousand hours, or gradually increases for a long period; and (c) a third stage during which the creep rate increases, the specimen tends to "neck down," and final fracture occurs. The creep rate during the second stage, which is the minimum creep rate during the entire test, is usually taken as the index of creep resist- ance of the metal. Low creep rate means high creep resistance. 3. Another index of creep resistance is the total creep (including both the creep in the first stage and also the elastic deformation immediately following loading) up to any given time in the second stage. The McVetty method described in Bulletin 306 is used in determining this index of creep resistance. This is the most accurate method for lead alloys, because of the large amount of creep in the first stage plus the elastic deformation and the consequent strain hardening of most alloys. However, under higher stress and tem- peratures this method gives a slightly lower estimate of the total creep of the arsenical leads than actually occurs, since these materials have self-annealing tendencies. Negative values of eo are obtained. 4. Creep deformation in lead alloys is a result of the combination of two mechanisms: (a) creep within the grains, which occurs as crystographically directed slip, or translation, and (b) plastic flow at the grain boundaries. At higher stresses creep deformation which precedes failure is more a result of slip, whereas with the lower stresses reported in the creep tests, boundary flow becomes the major mechanism of deformation. The alloying constituents tend to form a mechanical lock within the grains, retarding slip due to stresses below certain limits. Failure then takes place at the grain boundaries. BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS When some of the alloying material precipitates out of solution or a surplus is present, and precipitates at the grain boundary, thus weakening the joint between the grains to a strength less than that of the mechanically stiffened grains, failure occurs at the grain boundaries. This phenomenon gives an intercrystalline type of failure common to most lead alloys subjected to a long-continued steady load. The ductility as measured by the elongation after fracture is then lower than in the short-time test, where some of the failure resulted in slip within the grains. 5. The presence of other metals in lead greatly retards recrystal- lization of lead. Arsenic gives the lead an extremely fine grain size and also retards recrystallization. When recrystallization does occur the grains are small in comparison to other alloys of lead. It is believed that the self-annealing tendency of arsenical leads is a slow recrystallization spread over a long period of time. The excellent creep properties of the arsenical lead at lower temperature are due to the ability of these materials to retain considerable work hardening from the lead press. At the higher temperatures some recovery and slow recrystallization take place which increase the creep rate and lead to greater elongations. 6. A graphically tabulated evaluation of the creep resistance of the various alloys tested appears in the present bulletin as Fig. 4. The McVetty method was used to estimate the total creep and the rate of creep for 10,000 hr based on 5000-hr tests at 110 deg and 150 deg of strip specimens loaded with a tensile stress of 200 p. s. i. 7. Results of long-time tensile tests extending past 10,000 hr are shown as strength-time and ductility-time graphs. The tendency for some of these alloys to decrease in ductility as the time for fracture increases at the lower stresses is readily apparent. However, toward the lowest test stresses, which approach those encountered in service, the trend often reverses and the ductility increases. The larger the grain size the lower the stress seems to be at which the reversal in trend occurs. In service this increase in ductility might be offset by a decreasing strength at the grain boundaries. Failure at the low stresses is due mainly to flow at the grain boundaries. Some of the alloying substances, such as antimony in amounts greater than 0.20 percent, tend to precipitate out of the solution into the grain boundaries and to cause cracks to start at the grain boundaries. 8. The extremely fine grained arsenical alloys have their high stress ductility reduced because of the slip interference offered by the fine structure. However, the large amount of boundary area avail- able for flow gives the material good ductility for moderately high ILLINOIS ENGINEERING EXPERIMENT STATION stresses. An example of this is the C-389 alloy, which had its lowest value of ductility for a stress of 1500 p. s. i., 34 percent in 1 in. For higher and lower stresses higher values of ductility were obtained. 9. As previously reported, tin and antimony reduce the creep rate of lead very little for low stresses. In fact, when present in amounts over 0.20 percent they overcome the effect of 0.04 percent copper. Sheath containing 0.10 to 0.20 percent of arsenic and of tin has good creep resistance combined with good ductility. 10. Full-section specimens of arsenical lead sheath under con- stant internal oil pressure have very nearly the same creep char- acteristics as strip specimen cut from sheathing when stressed only slightly above service conditions. Sheaths with tongued welds have expanded 17 to 37 percent before failure, yet have withstood equal stresses for longer times than more creep-resistant materials. 11. Multiple strip bending machines operating at 110 deg at 10- min and 1-hr cycles are used to measure the bending resistance of longitudinal strips cut from sheathing. In the range of strains com- monly met in service the superiority in bending resistance of the arsenical leads is demonstrated. In this test with complete reversal of strain the fine-grain alloys have the greatest bending resistance. Failures are intercrystalline. Aging 7 days at 125 deg C reduced only slightly the bending resistance of an arsenical lead. The 1-hr-cycle tests gave lower cycles for failure than did the 10-min cycle tests. Alloys containing 0.10 to 0.20 percent arsenic and tin have the best bending resistance. Alloys containing over 0.03 percent copper and 0.03 percent antimony have reduced bending resistance unless they contain over 0.10 percent tin and arsenic. 12. The bend tests of full-section cable gave about the same test results as the bending tests of strips. The superior bending resistance of the arsenical alloys was noted here also. The fine-grain alloys were more resistant in the short-stroke tests and the larger-grain alloys more resistant in the longer-stroke tests. The large-grain alloys tend to expand more readily in the first few cycles on the long-stroke tests and as a result are strained less in the remaining cycles. In these tests, as in the strip bend tests, the mechanism of failure is mainly flow at the grain boundaries. 13. The two bend tests stress the material in a somewhat dif- ferent manner. The stress gradient is much more severe in the strip bend test. The fine-grain arsenical alloys are favored in this test, since they have more boundary area in which to flex and distribute the completely reversed strain. In the cable bend tests the large- grain alloys seem best when life for a given stroke is considered. BUL. 394. CREEP, FRACTURE, AND BENDING OF LEAD ALLOYS 59 Their greater short-time ductility enables them to expand more readily in the first few strokes and relieves the strain. Strain har- dening of the load affects the strip bend tests and causes some scat- ter in the data. Since a 2-in. gage length is used in the strip bend test, the localized strain is higher than in the cable bend test where a 1-in. gage length is used. A larger length of material is more highly strained, and more points of high localized strain tend to induce failure. Hence, usually lower values of life for a reported strain are noted in the strip bend tests. 14. A cable sheathing should have the highest ductility consistent with good creep and bending resistance. Many alloys of lead have good creep resistance but lack ductility. Some alloys have good bending resistance to low strains but poor resistance to high strains. The use of short-time tests can lead to poor selection of alloys for this service. Furthermore, the installation and the loading in service determine the alloy most suited for a given set of field conditions. APPENDIX: REFERENCES NO. 1. Ferguson, L., and Bouton, G. M., "The Effect of a Coating of Polybutene on the Fatigue Properties of Lead Alloys." Symposium on Stress-Corrosion Cracking of Metals. Published jointly by the ASTM and AIMME, 1945. 2. Gohn, G. R., and Ellis, W. C., "The Effect of Small Percentage of Silver and Copper on the Creep Characteristic of Extruded Lead," ASTM Proceedings, Vol. 48, 1948, pp. 801-14. 3. Phelps, H. S., Kahn, F., and Magee, W. P., "Influence of Small Percentages of Silver on the Tensile Strength of Extruded Lead Sheathing," ASTM Pro- ceedings, Vol. 48, 1948, pp. 815-24. 4. Beck, P. A., "Recrystallization of Lead," AIME Transactions, October 1939. 5. Smith, A. A., "Creep and Recrystallization of Lead," AIME Transactions, Vol. 143, 1941. 6. Hickernell, L. F., and Snyder, C. J., "F-3 Lead Alloy - An Improved Cable Sheathing," AIEE Transactions, Vol. 65, 1946, p. 563. 7. Smith, A. A., and Howe, H. E., "Creep Properties of Some Rolled Lead-Antimony Alloys," AIME Transactions, Vol. 161, 1945. 8. Greenwood, J. N., and Cole, J. H., "Influence of Various Factors on the Creep of Lead," Metallurgia, Vol. 37, No. 222, April 1948. 9. Greenwood, J. N., and Cole, J. H., "The Influence of Various Factors on the Creep of Lead Alloys," Metallurgia, January 1949. 10. Greenwood, J. N., "The Influence of Vibration on the Creep of Lead," ASTM Proceedings, Vol. 49, 1949. 11. Halperin, Herman, "Lead Ratings of Cable - II," AIEE Transactions, Vol. 61, 1942, p. 930. 12. Halperin, Herman, and Betzer, C. E., "Studies of Cable Movement and Sheath Life," Minutes of 40th Meeting, Trans. and Destr. Comm., EEI, Chicago, May 6, 7, 8, 1946, Appendix II, p. A-64. 13. Moore, H. F., Betty, B. B., and Dollins, C. W., "Investigation of Creep and Fracture of Lead and Lead Alloys for Cable Sheathing," University of Illinois Engineering Experiment Station Bulletin 306 (1938). 14. Moore, H. F., and Dollins, C. W., "Fracture and Ductility of Lead and Lead Alloys for Cable Sheathing," University of Illinois Engineering Experiment Station Bulletin 347 (1943). 15. Dollins, C. W., "An Investigation of Creep, Fracture and Bending of Lead and Lead Alloys for Cable Sheathing -Series 1946," University of Illinois Engi- neering Experiment Station Bulletin 378 (1948). 16. Hofmann, Wilhelm, "Blei und Bleilegierungen," Julius Springer, Berlin, 1941. 17. Greenwood, J. N., and Orr, C. W., "The Influence of Composition on the Properties of Lead," Proceedings Aus. I. M. M. (Inc.), No. 109, 1938. ~ ~ .j ,~, I ~ :~ ? N~ A /~ '~~' -~ ;4~ 9 -~ p~' ~ I ~2~¾ .~- ~ ~' K ~ - V 'K~VY A~ -~A * ~) ?~' -t ~r~; ~,AI ~' ~ I A :u~'. 2~2~ 4A~~ Ti ) &\ '~d >L T~ TN I- 1- A I A,- ~K. 'TN - ;' -~\ 'I-