Thermal stability of manganese-stabilized stainless steels

Section 3 AUSTENI~C STAINLESS STEELS

ELSEVIER

Journal of Nuclear Materi. s 212-215 (1994) 437-441

Thermal stability of manganese-stabilized

stainless steels *

R.L. Klueh, E.A. Kenik Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376,

USA

Abstract Previous work on experimental high-manganese reduced-activation they had improved tensile properties relative to type 316 stainless

austenitic

stainless

steels

demonstrated

that

steel (316 SS) in both the annealed and 20% cold-worked conditions. Seven steels were tested, which included an Fe-20Mn-12Cr-0.2X (in wt%,) base composition, and this composition with various combinations of Ti, W, V, P, and B. Tensile tests have now been completed on these steels after thermal aging to SIOO h at 600°C. Thermal stability varied with composition, but the alloys were as stable or more stable than 316 SS. After aging to SOOOh at 6OO”C,the strength of the annealed steels increased slightly, and the strength of the cold-worked steels decreased. In both conditions, a steel with a combination of all the alloying elements had the best strength after thermal aging. Despite having higher strength than 316 SS after aging, the ductility of the strongest alloy was still as good as that of 316 SS.

1. Introduction Conventional structural alloys used for components of a fusion reactor will become highly radioactive during their service lifetime, making the disposal of the components a difficult nuclear waste problem. This has led to the development of reduced-activation alloys [l]. Such alloys contain only elements in which the induced radioactivity would decay much more rapidly than in conventional alloys. Common alloying elements that must be eliminated from reduced-activation alloys include Ni, MO, Cu, Nb, and N. Nickel-stabilized austenitic stainless steels have long been considered potential structural alloys for fusion reactors. Therefore, a reduced-activation austenitic stainless steel substitute was sought. Several programs to develop such a steel have been initiated [2-71, including one 3’~Ihe Oak Ridge National Laboratory (ORNL) [4,8,9j. The ORNL program sought to develop reduced-

* Research sponsored by the Office of Fusion Energy, US Department of Energy, under contract DE-ACO5-840R21400 with Martin Marietta Energy Systems, Inc. 0022-3115/94/$07.00

activation or fast induced-radioactivity decay (FIRD) austenitic stainless steels using manganese as a replacement for nickel [4,81. The first step was the determination of a “modified Schaeffler diagram” for highmanganese alloys [8]. The modified diagram was used to choose a nominal Fe-20Mn-12Cr-0.25C (all compositions are in wt%) steel as a base composition, and this composition was alloyed with Ti, W, V, P, and B. Seven Fe-Mn-Cr-C steels with various combinations of Ti, W, V, B, and P were examined and shown to have strength and ductility equivalent to or better than type 316 stainless steel (316 SS) [91. After the program began, the high-manganese steels were concluded to pose potential safety problems in a fusion reactor first wall because of high afterheat if cooling became inadequate. The steels no longer have a high priority in the US fusion program. Although the development program has been discontinued, knowledge obtained in the program could be of interest for the fusion program in the future and for other applicatioas. Therefore we intend to publish the information obtained. In this paper, microstructural observations and tensile properties are presented on the seven solute-modified alloys after thermally aging to 5000 h at 600°C.

6 : -5 t Elsevier Science B.V. All rights reserved

SSDI 0022-3115(93)EO305-S

R.L. Klueh, E.A. Kenik/Journal

438

of Nuclear Materials 212-215

2. Experimental procedure Seven experimental alloys, including the base composition, were obtained as 600-g vacuum arc-melted button heats. Table 1 lists alloy compositions and designations. By making elemental additions to the base composition (designated MnCrC), alloys were obtained with Ti (MnCrCTi), W (MnCrCW), a combination of Ti and W (MnCrCTiW), and combinations of these elements with V, B, and P (MnCrCTiBP, MnCrCTiVBP, and MnCrCTiWVBP). Nominal levels of Ti, W, V, B, and P of 0.1, 1, 0.1, 0.005, and 0.03 wt%, respectively, were sought. Type 316 SS was tested for comparison. Details on ingot production, sheet fabrication, heat treatment, and specimen fabrication were given previously [9]. Tensile specimens were aged 500, 1000, 2500, and 5000 h at 600°C in the 20% cold-worked condition and after 20% cold work plus a solution anneal of 1 h at 1050°C. Tensile tests were conducted at 600°C in vacuum on a 120~kN-capacity Instron universal testing machine at a crosshead speed of 8.5 pm/s (a nominal strain rate of 4.2 x lO-4/s). Microstructures were examined by optical and transmission electron microscopy (TEM).

3. Results and discussion

Microstructures of the unaged steels were presented previously 191.Addition of alloying elements to the MnCrC base composition affected the grain size and the amount of precipitate formed. All steels with titanium had an estimated ASTM grain size number 8, compared to 3 for MnCrC and 4 for MnCrCW. The annealed 316 SS had ASTM grain size 4 [9]. Previous TEM studies of the steels annealed at 1050°C found no precipitate in MnCrC and MnCrCW - the steels without titanium 191. All steels with titaTable 1 Fe-Cr-Mn

nium annealed at 1050°C contained MC precipitates [9]. Most of the MC was in the matrix, although some grain boundary MC was observed in the MnCrCTiBP, MnCrCTiVBP, and MnCrCTiWVBP. Grain boundary precipitates in the latter two steels were mainly M&Z,. Some of the cold-worked steels also contained precipitates; these remained after the intermediate anneals at 1150°C that were used during processing to obtain the 0.76-mm sheet [9]. Thermal aging promoted precipitation. Optical microscopy indicated that precipitation in the solutionannealed steels occurred primarily on grain boundaries. For the cold-worked steels, considerable precipitation occurred in the matrix on slip bands, although grain boundary precipitation was also observed. Foils and carbon extraction replicas of specimens aged 5000 h were examined by TEM. All solution-annealed steels contained extensive grain boundary precipitation. The precipitates in steels without titanium were M,,C,, but steels with titanium also contained coarse Tic at boundaries, some of which may not have completely dissolved during the 1050°C anneal. Little matrix precipitation occurred in steels without titanium, but steels with titanium formed more matrix precipitates - primarily MC. Steels with both vanadium and titanium contained slightly more MC than alloys with titanium and no vanadium. Fig. 1 is an extraction replica from the MnCrCTiVBP and illustrates the grain boundary and matrix precipitate. The most extensive matrix precipitation was observed for Mn~r~i~BP, which, as discussed below, was the strongest steel and showed the greatest resistance to strength loss during thermal aging. The most intergranular precipitation probably formed in the MnCrCW. Matrix precipitation often appeared to lie in parallel straight lines (Fig. 1). This was also observed after the 1050°C anneal and was attributed to grain boundary migration during annealing [9]. Examination of micrographs af the aged steels indicates that lines of precipitate within a given grain often cross and are at angles to the grain boundaries, suggesting that precipi-

alloys tested

Alloy Designation

Composition Cr

Mn

C

MnCrC MnCrff i MKhCW MnCrCTiW MnCrCTiBP MnCrCTiVBP MnCrCTiWVBP 316 SS

11.83 11.73 11.80 11.71 11.85 11.84 11.70 17.28

20.51 20.50 20.46 21.13 20.50 20.82 20.39 1.70

0.24 0.25 0.23 0.25 0.24 0.22 0.25 0.06

a Balance

(1994) 437-441

iron.

b-t%)

a Ti

W 0.11

0.12 0.10 0.10 0.10 < 0.05

0.09 0.83 0.77

1.08

V

P

0.01 0.01 0.01 0.01 0.01 0.10 0.10

0.003 0.003 0.004 0.003 0.034 0.033 0.027 0.037

B

Ni

0.005 0.005 0.005 12.44

R.L. Kkeh, EA. Kenik /~ou~l

of ~~~l~ar Materials 212-215 (1994) 437-441

439

Fig. 2. Extraction replicas from annealed MnCrCTiVBP alloy after thermal aging 5000 h at 600°C showing a M,,C, precipitate coating on a TiC particle. Fig. 1. Extraction replica of the annealed MnCrCTiVBP alloy after thermal aging 5000 h at 600°C.

tates may nucleate on preferential crystallographic planes. In several alloys, some M,,C, was found in the matrix after aging, and it often appeared to contain large amounts of titanium (up to 10%) [lo]. On closer examination, however, this M,sC, was found to have formed on Ti-rich MC particles, resulting in a T&rich core with an outer shell that contained little titanium (Fig. 2). This indicated that pre-existing TiC particles (those not completely dissolved during the solution anneal) served as heterogeneous nucleation sites for the M&, &Of. Cold-worked specimens showed extensive precipitation in both the grain boundaries and matrix, as shown for the MnCrCIiVBP in Fig. 3. Much of the matrix precipitate appeared to be aligned along ctystallographic planes (slip bands), as observed in optical microscopy. Just as for the annealed steels, precipitates consisted of M.& and MC, the latter appearing in steels containing titanium. More detailed information on the composition of precipitates observed after aging 5000 h at 600°C was published previously [lo]. No Laves phase was found in either the annealed or cold-worked steels after aging 5000 h.

the steels were stronger than type 316 SS before and after thermal aging. The manganese steels fell into two categories: those with and those without titanium. Those with titanium were considerably stronger than those without, which included the base composition and the composition with just tungsten added. These latter two steels were only slightly stronger than 316 ss.

3.2. Tensile behavior Thermal aging for 5000 h increased the yield stress for all of the solution-annealed steels (Fig. 4a). All of

Fig. 3. Extraction replica of the cold-worked MnCrCIIVBP alloy after thermal aging 5000 h at 600°C.

440

R.L. Klueh, E.A. Kenik /Journal

of Nuclear Materials 212-215

Although tungsten by itself had little effect on the yield stress of the solution-annealed MnCrC base composition, tungsten in combination with other alloying elements was effective in strengthening the steels. This is seen by comparing the MnCrCTiWVBP and the MnCrCTiVBP steels (Fig. 4a). The steel containing tungsten in combination with all the other selected elements is the strongest alloy in the unaged condition and after aging 5000 h at 600°C. After the 5000 h age, this steel is substantially stronger than the other four titanium-containing steels, which have similar strengths. The ductility of the solution-annealed steels, as measured by total elongation, indicated that the strongest manganese steels had the lowest ductility in both the unaged and aged conditions (Fig. 4b). The MnCrCTiWVBP, which was the strongest steel, had the lowest ductility, while the MnCrC and MnCrCW, the weakest of the high-manganese steels, had the highest ductility. The ductilities of the other five highmanganese steels fell between those extremes, similar to the observations on the yield stress. However, the ductilities of all the aged manganese-stabilized steels

(1994) 437-441

-

MnCrC

7 MnCrCW l MnCrCTiiP 4 MnCrCTi + MnCrCTiW 0 MnCrCTiVBP 350 L MnCrCTiWVBP 0 316 SS 1 0 1000 2000 3000 4000 n

AGING

, 5000

TIME (h)

10

6

6

0

1000

2000

AGING

3000

4000

5000

TIME (h)

Fig. 5. (a) Yield stress and (b) total elongation as a function of aging time for cold-worked high-manganese steels and type 316 stainless steel.

0

1000

2000

AGING 50

3000

4000

5000

TIME (h)

1

z

A MnCrCTi l MnCrCTiW 0 MnCrCTiVBP 0 316 SS - MnCrCTiWVBP 10 I 0

1000

2000

AGING

4000

3000

TIME

I 5000

(h1

Fig. 4. (a) Yield stress and (b) total elongation as a function of aging time for annealed high-manganese steels and type 316 stainless steel.

exceeded that for 316 SS, even though the 316 SS was the weakest steel. Thermal aging of the cold-worked steels to 5000 h caused a decrease in the yield stress of all steels (Fig. 5a). Steels without titanium were again the weakest, and in this condition they were weaker than 316 SS. The other five high-manganese steels were stronger than 316 SS. Although the MnCrCTiW steel was the strongest cold-worked steel in the unaged condition, the MnCrCTiWVBP was strongest after the 5000 h age, just as it was in the solution-annealed condition. The reason for this was that the MnCrCTiWVBP showed little strength decrease after 5000 h at 600°C. Ductility behavior of the cold-worked steels reflected the strength behavior, with the strongest steels having the lowest ductility and vice versa (Fig. 5b). The MnCrCTiWVBP steel had the lowest ductility of the manganese-stabilized steels after the 5000 h age. However, the ductility was comparable to 316 SS, even though the 316 SS had a much lower strength. For both the solution-annealed and cold-worked conditions, the relative ultimate tensile strength behav-

ior was similar to that of the yield stress. The relative uniform elongation behavior was similar to that of the total elongation. The results indicate that the high-manganese steels are quite resistant to thermal aging at 600°C. This is especially true for the MnCrCTiWVBP in both the annealed and cold-worked conditions. The primary concern with these steels is corrosion resistance [ill. One way to improve corrosion resistance is to lower the carbon concentration or increase the chromium ~ncentration. This has been attempted and will be discussed in a future paper.

smallest strength loss in the cold-worked condition. Despite the higher strength, the steel had ductility equivalent to or greater than 316 SS.

Acknowledgements Tensile tests were carried out by N.H. Rouse; TEM specimens were prepared by J.W. Jones; the manuscript was reviewed by D.J. Alexander and J.E. Pawel.

References 4. Summary and conclusions Thermal aging studies were conducted on a series of experimental, high-manganese austenitic stainless steels. An Fe-20Mn-12Cr-0.2X base composition was used, to which various combinations of Ti, W, V, B, and P were added to improve strength. Aging was carried out at 600°C to 5000 h on the steels in the solution-annealed and 20% cold-worked conditions. Results were compared with type 316 SS. The yield stress of all the high-manganese steels in the solution-annealed condition increased with thermal aging, and the strength of the steels before and after aging exceeded that for 316 SS. Despite the greater strength, the ductility of the manganese steels also exceeded that for 316 SS. Cold-worked steels showed a strength decrease with aging time at 600°C. All but two of the experimental cold-worked steels had yield stresses that exceeded that for 316 SS. The ductility of all of the cold-worked high-manganese steels was as good or better than that for 316 SS. In both the solution-annexed and cold-worked conditions, an Fe2OMn-l2Cr-O.25C-lW-O.O3P-O.~5B steel was most stable. During aging, it showed the largest strength increase in the solution-annealed condition and the

(11 R.W. Conn et al., Panel Report on Low Activation Materials for Fusion Applications, UCLA Report, University of California at Los Angeles, PPG-728 (1983). [21 E. Ruedl, D. Rickerby and T. Sasaki, in Fusion Technology 1984, vol. 2 (Pergamon, Oxford, 1984) p. 1029. [3] E. Rued1 and T. Sasaki, J. Nucl. Mater. 122 & 123 (1984) 794. [4] R.L. Klueh and E.E. Bloom, in Optimizing Materials for Nuclear Applications, eds. F.A. Garner, D.S. Gelles and F.W. Wiffen (The Metallurgical Society, Inc., Warrendale, PA, 1985) p. 73. [S] H.R. Brager, F.A. Garner, D.S. Gelles and M.L. Hamilton, 3. Nucl. Mater. 133 & 134 (1985) 907. [6] A.H. Bott, F.B. Pickering and G.J. Butterworth, J. Nucl. Mater. 141-143 (1986) 1088. [7] M. Tamura, J-J. Hayakawa, M. Tanimura, A. Hishinuma and T. Kondo, J. Nucl. Mater. 141-143 (1986) 1067. [S] R.L. Klueh, P.J. Maziasz and E.A. Lee, Mater. Sci. Eng. 102 (1988) 115. [9] R.L. Klueh and P.J. Maziasz, Mater. Sci. Eng. Al27 (1990) 17. [lo] E.A. Kenik, P.J. Maziasz and R.L. Klueh, Proc. 47th Annual Meeting Electron Microscopy Society of America, ed. G.W. Bailey (San Francisco Press, San Francisco, 1989) p. 284. [ll] G.E.C. Bell, P.F. Tortorelli, E.A. Kenik and R.L. Klueh, J. Nucl. Mater. 179-181 (1991f 615.