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Creep in Five Steels at Different Temperatures

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Creep in Five Steels at Different Temperatures DEPARTMENT OF COMMERCE BUREAU OF STANDARDS George K. Burgess, Director TECHNOLOGIC PAPERS OF THE BUREAU OF STANDARDS, No. 362 [Part of Vol. 22 CREEP IN FIVE STEELS AT DIFFERENT TEMPERATURES BY H. J. FRENCH, Senior Metallurgist H. C. CROSS, Junior Metallurgist A. A. PETERSON, Assistant Scientific Aid Bureau of Standards January 10, 1928 PRICE, 15 CENTS $1.25 Per Volume on subscription Sold only by the Superintendent of Documents, U. S. Government Printing Office Washington, D. C. UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON 1928 T 362 CREEP IN FIVE STEELS AT DIFFERENT TEMPERATURES By H. J. French, H. C. Cross, and A. A. Peterson ABSTRACT This report describes so-called creep tests in which the elongation of metal specimens is observed with time under a fixed load. Tests were made on a low- carbon structural steel, a high-chromium steel, a chromium-molybdenum struc- tural steel, high-speed steel, and a high chromium-high nickel austenitic steel and correlated with short-time tension tests at corresponding temperatures within the range 70 to 1,350° F. (20 and 730° C.). So-called creep charts are given in which the relations are shown between stress, temperature, elongation, and time for each of the steels. These charts enable the approximate determination of the stress permitting life of different durations with different total elongations. The application of these charts is discussed in some detail. Correlation of creep tests with short-time tension tests show that, when using accurate equipment, the proportional limit was in the range of stresses which could be sustained for long periods with small amounts of deformation. The best resistance to oxidation in air was shown by the high chromium-high nickel steel and the high-chromium steel. These appear to be superior to the high-speed steel, which, however, with the high chromium-high nickel steel showed the best load-carrying ability in the range 1,100 to 1,350° F. (595 to 730° C). The chromium-molybdenum steel was not structurally stable at 1,200° F. (650° C). Oxidation was accompanied by decarburization and grain growth. CONTENTS Page I. Introduction 236 II. Previous investigations 236 1. Creep tests compared to short-time tension tests 236 2. Effect of temperature on creep in steels 237 III. Steels tested 238 IV. Test methods and equipment employed 239 1. Creep tests 239 2. Tension tests 240 V. Experimental results 242 1. Time-elongation curves 242 2. Creep charts 242 3. Correlation of creep tests with short-time tension tests 250 4. Comparison of data from different sources 256 5. Comparisons of the different steels 260 VI. Summary and conclusions 263 VII. Selected bibliography on flow in metals 266 235 236 Technologic Papers the Bureau Standards vol. 22 of of i I. INTRODUCTION The high temperatures and pressures associated with recent devel- opments in power-plant equipment, oil-cracking processes, and the chemical industries have shown the need for additional information on the behavior of metals at high temperatures. Attention has recently been focused in particular upon so-called " creep tests" or the flow in metals subjected to stresses for long periods at elevated temperatures. Within the past five years many investigations have been made in this field, but the information obtained has apparently not satisfied the requirements of designing engineers who insist upon having a more nearly quantitative evaluation of the relations between stress, temperature, elongation, and time for different metals. This report is prepared to permit selection of working stresses for five steels at temperatures between atmospheric and 1,350° F. (732° C). Preliminary results for three of these were previously l reported (8, 24). Comparisons are given of the results of the creep tests with short-time tension tests at corresponding temperatures. II. PREVIOUS INVESTIGATIONS 1. CREEP TESTS COMPARED TO SHORT-TIME TENSION TESTS Examination of published data will create some definite, but not necessarily correct, impressions concerning safe working stresses and creep in metals. It will likewise develop seemingly inconsistent results and show differences in opinion with respect to various phases of the subject. Certain investigators, such as Dickenson (4), are convinced that short-time tension tests are useless and misleading to designing engineers and have turned their attention to creep tests. Others, such as Brearley (4) and Rosenhain (17), are either not wholly convinced that such a view is justified or maintain that useful infor- mation can be secured from carefully made tension tests. It is prob- ably true that most of the published results of tension tests can not safely be used for determining working stresses, but some of the creep tests are likewise misleading (24). Only a few investigators, includ- ing Lynch, Mochel, and McVetty (12), the authors (8, 24), and Pomp and Dahmen (33), have attempted to refine short-time tension- test methods and correlate results with carefully made creep tests. The latter require considerable time, are therefore expensive, and are subject to errors of appreciable magnitude, especially when attempting to determine quantitatively the relations between stress, temperature, creep, and time. i The figures given in parentheses here and throughout the text relate to the reference numbers in the bibliography given at the end of this paper. — French, Cross, Peterson Creep in Five Steels at Different Temperatures 237 Creep tests, unless carefully made and the results reasonably inter- preted, may create a false sense of security on the part of engineers, for even with accurate determinations on a single bar, melt, or lot of steel the question remains what variations from observed numerical values may ordinarily be expected if tests are repeated on other bars, melts, or lots of the same type of steel. Where maximum and minimum values of tensile strength (or elastic properties) are included in specifications for engineering ma- terials for use at atmospheric temperatures, the range is seldom less 2 2 than 10,000 lbs./in. , and a variation of only 3,000 to 5,000 lbs./in. vo£ 22-" \>>^ I j£ o c =\fc__ i Q 7 Id =14 /' =lt 1 / r =10 < J I / / y =t L =i TEMPERATURE Fig. 1. Diagram showing the character of the relations between stress, tem- perature, time, and elongation in ordinary structural steel A, Creep chart. B, Time elongation curves where strain hardening is observed. C, Time elongation curves where strain hardening is not observed. for commercial steels would be considered very small and satisfac- tory for most practical purposes. If, with the same metals and reasonably accurate test methods at high temperatures, only half this total variation is observed, the per cent variation may be enormous, as the " strength" or load-carrying ability may only be a few hundred or thousand pounds per square inch. Under such conditions an approximate determination would offer many advantages, provided it could be secured quickly and could likewise be depended upon. Therefore, attention will be given in the latter portion of this report to methods for a quick and approximate determination of working stresses. 2. EFFECT OF TEMPERATURE ON CREEP IN STEELS The conceptions developed from creep tests previously reported by one of the authors are illustrated in Figure 1. In the upper part of the temperature range considered (b c, fig. 1 A) any appreciable 238 Technologic Papers of the Bureau of Standards [ Vol. 22 deformation initially produced by the applied load is followed by continuous creep which ultimately results in fracture of the speci- men; at lower temperatures {ah, fig. 1 A) initial deformation, even of appreciable magnitude, is not necessarily accompanied by contin- uous creep. Due to the ability of metals to strain harden, initial changes in dimensions may be followed by a practical cessation of creep or at least a decrease to an entirely different order of magnitude. Typical time-elongation curves upon which these conceptions are based are shown in Figure 1 B and C. However, definite information is lacking whether the creep can ever be zero at temperatures above the range in which the metal will strain harden. Even at lower temperatures there is the question whether strain hardening results in an absolute cessation of flow or a decrease in the rate of flow to very low values. It is difficult to obtain a direct answer to these questions, but the latter is, perhaps, of academic interest rather than of practical importance. It is important to know whether the rate of flow can be zero at relatively high temperatures where the metal does not show strain hardening. Table 1, hemical compositions and other details of the steels tested Chemical composition (per cent) Hot Heat treatment Mark finished or condition in Remarks to— which tested C Mn P S Si Cr Mo Ni W V Cu ?A 0.37 0,021 0?8 m 1-inch As hot-rolled by Ordinary boil- plate. supplier. er plate. K3 .39 .51 .015 .029 .19 0.87 0.21 .... .... ... .... lj^-inch 1,625° F., 2 hours A structural al- rod. air; 1,550° F., loy steel, heat \ Al hours oil; treated. 1,300° F., 2 hours oil. A336... .77 .24 .031 .035 .42 3.9 ..... 0.14 13.6 1.9 .... 1-inch As received Low tungsten- rod. from manufac- high vana- turer; proba- dium high- bly annealed speed steel. after hot- working. A340-- .28 .38 .026 .013 .17 20.5 0.98 J^-inch 1,830° F. water; A high-chromi- rod. 1,200° F. um "stain- resisting" steel. A347A .24 .53 .009 .005 2.96 18.05 .005 23.3 .07 lj/s-inch l,450°F.,2hours, rod. furnace cool. A high nickel- high chromi- A347B . .21 ---- 2.96 18.10 23.4 ...do Annealed by um, "austen- supplier; prob- itic" steel.
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