The University of Michigan * Office of Research Administration Ann Arbor, Michigan ELEVATED TEMPERATURE PROPER TIES OF Fe-Mo- B ALLOYS AND TYPE 434 STEEL by T. M.,Qullen J. W. Freeman Project 06742 March 16, 1965. Climax Molybdenum Company of Michigan 14410 Woodrow Wilson Detroit 38, Michigan

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ELEVATED TEMPERATURE PROPERTIES OF Fe-Mo-B ALLOYS AND TYPE 434 STEEL A research program sponsored by the Climax Molybdenum Company of Michigan has been conducted at the University of Michigan. This program had as its objective the determination of the creep-rupture properties of several alloys out to times well in excess of 1000 hours, The materials investigated included Type 434 ferritic stainless steel and several Fe-Mo-B alloys. The rupture properties of Type 434 (l7Cr-lMo) steel were evaluated at 900~, 10500 and 1200'F. The Fe-Mo-B alloys were divided into two types, cobalt-free alloys which were tested at 9000F, and complex alloys which were tested at 1200~F. CONCLUSIONS The rupture strength of the l7Cr-lMo ferritic steel (Type 434) at 1200~F was similar to that of Type 430 (17Cr) steel At 900~F the 17CrlMo alloy underwent an aging reaction during testing which significantly strengthened it. There is little doubt that this strengthening is associated with the so-called 4750C (885' F) embrittlement which can occur in high chromium ferritic steels. The Fe-Mo-B alloys showed significant differences in level of strength with chemical composition. The compositional variations, however, were too numerous to allow the complete separation of the influence of the individual elements. In the cobalt-free Fe-Mo-B alloys, the high aluminum material had the most promising long time properties. In the complex alloys the highest levels of long time strength properties were obtained in vanadium-free alloys containing 5. 7 percent cobalt. 1

EXPERIMENTAL MATERIAL AND PROGRAM Two types of alloy were used in this investigation. The first alloy was Type 434 stainless steel. This material is classified as a ferritic stainless steel having a nominal composition of 17 percent chromium and 1 percent molybdenum, balance iron. Rupture tests on specimens from two different heats of the steel were conducted at 9000, 1050~ and 1200~F. The machined specimens were received from the project sponsor. The second class of alloys tested in this program were Fe-Mo-B materials. The reported chemical compositions of these materials were as follows: Heat Mo Ni Co Zr B Al C V F e 170-3 4.8 2.2 2.0.05.015.07 - - Bal. 170-4 4. 8 2.2 5. 7.05. 015.07 - Bal, 170-5 4. 8 2.2 5. 7.05.048.07 - - Bal, 170-6 4 8 2, 2 5. 7.05.048.07,05 0 40 Bal. 166-2 1.0 2.2 - -. 015.07 - - Bal. 166-4 1. 0 3.0 -.05. 015.07 - - Bal. 166-6 1. 0 3 0 -.05.015 1. 50 - - Bal. 168A 1.0 0. 5 - -.01.07 - 0. 1 Bal. 168B 1.0 1.0 - -.01.07 - 0. 1 Bal. 168C 1.0 1.5 - -.01.07 - 0, 1 Bal. 168D 1. 0 2.0 - -.01.07.05 0. 4 Bal. The above tabulation separates the alloys into two groups by their compositional complexity. Creep-rupture tests were conducted on specimens from the first group at 12000F. The test specimens were furnished by the sponsor. Screening tests were used to select the three most promising alloys from the second group of Fe-Mo-B materials, These three alloys were rupture tested at 9000F. 2

RESULTS AND DISCUSSION Type 434 Steel At the beginning of this investigation it was not recognized that high chromium steels could undergo a transformation in the region of 900"F. This transformation has been referred to in the literature as 4750C (885'F) embrittlement (Ref. 1). Aging at temperatures in the vicinity of 885~F has been shown to significantly increase the room temperature tensile strength of high chromium steels and simultaneously drastically reduce their ductility at ambient temperatures (Refs. 1, 2, 3). The exact nature of the transformation or aging reaction has not been established. With this background in mind the results obtained on the Type 434 steel specimens are more readily understood. The results are shown in Table I. The 10500 and 1200~F rupture data follow expected trends with the exception of the 13, 500 psi test at 1050OF which fractured in a very short time (Fig. 1). The results obtained at 9000F, however, were quite startling until it was recognized that the so-called 4750C embrittlement could occur. The rupture tests which were completed at this temperature were conducted under sufficiently high stresses to cause rupture in very short times (10 hours or less). It is assumed that in these cases the embrittling transformation had not progressed sufficiently far to strengthen the alloy before rupture occurred. If some critical amount of time at 900OF did pass then the transformation so enhanced the strength of the steel as to result in extremely long although undetermined rupture times, The magnitude of the strengthening reaction can be emphasized by examination of the results obtained from specimen 434-10 (Table I). This specimen deformed approximately 11 percent on initial loading and during the first three days of the test. After this time, however, no significant creep was measured for the duration of the test (approximately 2600 hours). Specimen 434-1 was tensile tested at 900SF after being exposed to different stresses ranging from 30, 000 psi to 43, 000 psi, for a total time of 3

approximately 4000 hours. The complete history of this specimen is shown in Table II. The ultimate tensile strength of the specimen after the prolonged stressed exposure was 90, 500 psi as compared to the sponsor's reported ultimate strength at 9000F of 43, 800 psi. This specimen after tensile testing is shown in Figure 2a, The striking features of the structure were the numerous twins which formed in the sample. These twins were most pronounced near the fracture. There were, however, a number of twins in the threaded section of the specimen. In order to determine whether these twins formed during the stressed exposure or the tensile testing a second specimen (434-10) was examined. This specimen had been interrupted after exposure for 2690 hours at 900'F under a stress of 41, 000 psi. The microstructure of this specimen did not show any evidence of significant twinning (Fig. 2b). It is therefore assumed that the twinning formed during the tensile test. It is further assumed that it occurred as a result of the embrittling transformation (Ref. 3). The microstructure of this steel shows the presence of numerous rod-shaped inclusions (Figs. 2a and 2b). These inclusions are very probably amorphous. It is considered unlikely that these particles contributed significantly to the strength of the steel. No attempt had been made to estimate the rupture strength of this steel at 900~F, The reason for the scatter in the 1050~F data has not been determined and as a consequence no estimate of the rupture strength has been made at this temperature. At 1200~F, however, the rupture properties of Type 434 steel can be summed up as follows: Stress for rupture in: 50 hours 500 hours 1000 hours 7600 psi 5500 psi 5000 psi These strengths are approximately the same as those reported for Type 430 steel (17Cr) in Reference 4. 4

Fe- Mo- B Alloys Cobalt-Free Materials The cobalt-free Fe-Mo-B alloys were rupture tested at 900~F, except for one heat (166-2) which was inicluded with the complex alloys tested at 1200 "F. A preliminary rupture test was run on each of the six heats of varying chemical composition to determine which of the experimental alloys had the most promising properties. Three heatswere selected and these materials were subjected to additional tests at 900 ~F, The results obtained from these tests and from the preliminary tests are reported in Table III. These data have been plotted in the form of stressrupture time curves, which are shown in Figure 3. The comparative rupture strengths of the alloys are shown in the following tabulation: Stress (psi) for rupture at 900~F Heat 50 hours 500 hours 2000 hours 166-6 59, 000 56, 000 56, 000 168C 56, 000 49, 500 46, 000 168D 114, 000 81, 000 66, 000 This tabulation shows that the short time rupture strength of Heat 168D was the highest of the three heats. The strength of this heat, however, fell off very rapidly at the longer rupture times. This is very apparent in Figure 3, which shows the slope of the stress-rupture time curve to be steep. It is probable that the excellent short time strength of this heat, its poor rupture ductility (Table III) and its high degree of instability are related to the higher vanadium content of the alloy. The alloy with the best long time properties was Heat 166-6. This heat had a very flat stress-rupture time curve (Fig. 3) and an apparent high degree of rupture ductility. Two rupture tests were discontinued after times of 1950 and 3430 hours because of the lack of any indication of impending failure. 5

Minimum creep rates were measured during the tests. These have been plotted as a function of stress in Figure 4, The minimum creep rates measured on the two discontinued specimens from Heat 166-6 were very low, approaching the limits of detectability. These low rates are probably the result of a structural change in the alloy. These rates are much lower than would be expected at such high stresses. Complex Materials Five Fe-Mo-B alloys were creep-rupture tested at 1200~F. Three specimens from each heat were tested with the results shown in Table IV. Stress versus rupture time graphs of the data are plotted in Figure 5. Four of the five heats had rupture curves of approximately equal slope. The rupture curve exhibited by the fifth alloy Heat 166-2, had a much steeper slope, indicating an unstable composition. The minimum creep rate data, plotted as a function of stress in Figure 6, show the same trends as the rupture data. The slopes of the curves of Figure 6 are approximately equal for four of the alloys with the line through the data from Heat 166-2 having a much steeper slope. The rupture data from these five materials have been evaluated in terms of stress for rupture in specific times at 12000F, The time periods selected were 10, 100 and 1000 hours. The results are as follows: Stress (psi) for rupture at 1200~F Heat 10 hours 100 hours 1000 hours 170-3 24, 700 17,600 12,800 170-4 31,000 22, 000 15,500 170-5 30,500 22 000 16,000 170-6 23,000 15,300 10,200 166-2 19,000 6,900 2, 500 This tabulation shows Heats 170-4 and 170-5 to have almost equivalent rupture strengths. Examination of Figure 6 shows their minimum creep rate behavior to also be very similar, The only aim compositional difference in the alloys was in boron level, This indicates that boron vari6

ations in the range from 150 to 480 ppm had only a minor effect on creep or rupture properties at 1200~F. In the complex Fe-Mo-B alloys higher vanadium content (0. 4%) was associated with reduced rupture strength just as was observed in the cobalt-free alloys. A reduction of cobalt level from 5. 7 to 2. 0 percent also resulted in reduced rupture strength in the complex alloys (compare results from Heat 170-4 with results from Heat 170-3). The alloy with the weakest properties at 12000F also had the leanest composition (Heat 166-2). It is quite evident that the absence of cobalt resulted in reduced rupture strength and also reduced rupture ductility.

REFERENCES 1. Lena, A, J., Hawkes, M. F.: 475~C (885~F) Embrittlement in Stainless Steels, Transactions AIME, May 1954, p. 607. 2. Fisher, R. M,, Dulis, E. J., and Carroll, K. G.: Identification of the Precipitate Accompanying 885~F Embrittlement in Chromium Steels, Transactions AIME, May 1953, p. 690. 3, Blackburn, M. J., Nutting, J.: Metallography of an Iron21% Chromium Alloy Subjected to 4750C Embrittlement, Journal of the Iron and Steel Institute, July 1964, J 202 Part 7, p. 610, 4. Elevated Temperature Properties of Chromium Steels, ASTM Special Technical Publication No. 228, p. 95.

TABLE I RUPTURE TEST RESULTS FROM TYPE 434 FERRITIC STAINLESS STEEL Specimen Temp. Stress Rupture Elong. R. A. Code OF psi Time, Hours %o %o Initial Heat 434-2 900 35, 000 >2594. 5a 3.0 4.0 434-3 900 38, 000 >1898. 6a 7. 0 8. 0 434-10 900 41,000 >2690.2a 11.0 12.0 434- 11 900 42,500 10. 2 28, 0 55.0 434-5 1050 19,000 59.7 58. 0 70. 5 434-4 1050 ~15, 000 369 3 59. 0 74. 5 434-6 1050 13, 500 114.9 42. 0 73. 5 434-9 1200 9, 000 16.3+4. 5 76.0 83. 0 434-7 1200 7,000 102. 2 71. 0 82. 0 434-8 1200 5, 000 1001.7 68. 0 81. 5 Heat 32708 2 900 52,500 0. 08 25. 0 58. 5 3 900 48,000 1. 1 32.0 61 5 4 900 40,000 > 578.3a 1.0 0.8 5 900 45, 000 4, 5 36, 0 53. 5 a - Test discontinued

TABLE II EXPOSURE HISTORY OF SPECIMEN 434-1 AT 900OF Stress Time at Stress (psi) (hours) Comment 30, 000 2038, 5 Cooled under load 35, 000 165 Test restarted, after 165 hours stress raised 38, 000 387 Stress then raised 41,000 553 Stress then raised 41,500 233 Stress then raised 42,000 342 Stress then raised 43, 000 403 Test discontinued, cooled under load Specimen tensile tested at 900~F Ultimate Tensile Strength 90, 500 psi 0.2 percent Offset Yield Strength 73, 500 psi Elongation 20% Reduction of Area 35. 5%

TABLE III CREEP-RUPTURE RESULTS AT 900~F FROM COBALT-FREE Fe-Mo-B ALLOYS Specimen Stress Rupture Elong. R. A. Min. Creep Code psi Time, hours %o %o Rate (%o/hr.) 168A 53,000 5. 4 20.0 83. 8 168B 59,000 0.4 21.0 79 5 166-4 57,000 25. 5' 21.0 73. 0 168C 56, 000 54. 7 20. 0 82.0 - 168C-1 50, 000 400. 2 16.0 76. 0 0. 0013 168C-2 45, 500 2372. 9 12.0 44. 0 0. 000224 168D 90, 000 > 223 (Failed in threaded section) 168D-2 100,000 123.4 2,5 6,5 0. 0022 168D-3 95,000 158. 3 1.0 2. 0 0. 00197 168D-4 75, 000 1083. 7 1.0 3. 5 0.00029 168D-5 70,000 1077.2 2.0 1.0 0. 00018 166-6B 59, 000 76. 9 25. 0 74. 0 0. 0606 166-6C 55, 000 >3434 2 (Discontinued) 0. 00001 166-6D 57 000 94.4 18, 0 73. 0 0.,0359 166-6E 56, 000 >1950. 3 (Discontinued) 0. 000012

TABLE IV CREEP-RUPTURE RESULTS AT 1200OF FROM COMPLEX Fe-Mo-B ALLOYS Specimen Stress Rupture Elong. R. A. Min. Creep Code psi Time, hours %o _o Rate (%o/hr. ) 170-3A 21,,000 32.2 56 86 0. 47 170-3B 17,000 112. 3 43 89 0. 0882 170-3C 12,000 1661. 7 39 90 0. 00286 170-4A 26,000 34. 5 37 82 0. 31 170-4B 22,000 26.6 48 88. 5 0. 49 170-4C 15,000 1242.6 55 85 0. 0027 170-5A 26, 000 24. 4 61 84 0. 57 170-5B 22,000 106. 0 55 89 0. 074 170-5C 17,000 692.8 38 85. 5 0. 00547 170-6A 29,,000 2. 8 53 81 5. 775 170-6B 22,,000 13. 0 57 84 1.42 170-6C 11,000 677. 5 83 86 0. 017 166-2A 16. 000 14. 3 10 10. 3 0,23 166-2B 12,000 26.4 8.5 6.5 0.059 166-2C 9,000 53, 4 6 6.2 0. 020

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