Phase transformations during aging of a nitrogen-strengthened austenitic stainless steel
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I.
INTRODUCTION
N I T R O N I C 50 is a nitrogen-strengthened austenitic stainless steel 1 having approximately twice the strength of Types 304 and 316 stainless steels over a range in temperature from room temperature to 300 ~ It was developed to have good welding properties, and to exhibit a combination of strength, ductility, and resistance to intergranular corrosion in the as-welded condition. 2'3 Key compositional differences from 304 and 316 stainless steels are higher chromium, nickel, manganese, and nitrogen, and the presence of 0.2 wt pct Nb and 0.2 wt pct V. The Mo content of 2 wt pct is comparable to that of 316 stainless steel. Some work on the microstructural characterization of NITRONIC 50 has been published. Mallick 4 has identified many of the phases precipitating during short-time aging in the temperature range of 600 to 1150 ~ and Ritter et al. 5 investigated equilibrium phases and phase chemistries up to the solidus for this alloy (1370 ~ A very limited study on microstructures in cold-drawn and recrystallized XM-19, the ASTM designation for the alloy, has also appeared. 6 However, since no data are available on the effects of longtime aging on the microstructural stability of NITRONIC 50, it was decided to examine the precipitation sequences in the alloy. A temperature regime consistent with the temperatures experienced in the heat affected zones in welds was chosen.
*NITRONIC is a trademark of Armco Steel Corporation. ANN M. RITTERand MICHAEL F. HENRY are Staff Metallurgists with General Electric Company, CorporateResearch and Development, Schenectady, NY 12301. Manuscript submitted August 3, 1984. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
Two heats of NITRONIC 50 were used in this study, and their compositions are given in Table I. Samples from both heats were solution-annealed at 1121 ~ for 20 minutes and water-quenched. They were then aged at 675 ~ for 2, 24, 336, or 1008 hours. Thin foils from specimens of Heat #356148 were prepared using a 20 pct perchloric acid/ 80 pct methanol electropolishing solution. Specimens from Heat #656596 were metallographically-polished and then severely etched in a solution of 100 ml water/50 ml hydrochloric acid/25 ml hydrogen peroxide. Extraction replicas were made from these etched samples using a two-stage acetylcellulose technique. 7 In addition, samples of precipitate residue from this heat were prepared for X-ray diffraction analysis by electrolytically digesting the matrix in a solution of 7 pct HC1/methanol for a period of several hours. All specimens were examined in a 200 kV scanningtransmission electron microscope equipped with an energy Table I.
Element Cr Ni Mo Fe Mn Nb V C N Si S P
Composition of NITRONIC 50 Heats
#656596 21.4I wt pct 11.92 wt pct 2.09 wt pct 58.25 wt pct 4.90 wt pct 0.17 wt pct 0.17 wtpct 0.055 wt pct 0.26 wt pct 0.48 wt pct 0.066 wt pct 0.024 wt pct
#356148 21.62 wt pct 12.71 wt pct 2.22 wt pct 56.88 wt pct 5.03 wt pct 0.18 wt pct 0.16 wtpct 0.053 wt pct 0.31 wt pct 0.54 wt pct 0.012 wt pct 0.03 wt pct
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