• PDF / 1,579,481 Bytes
• 6 Pages / 593.972 x 792 pts Page_size
• 98 Downloads / 264 Views

DOWNLOAD

REPORT

---

emperature interstitial hardening via carburization and nitridation are effective ways to enhance engineering performance of austenitic stainless steels. At low ‘‘paraequilibrium’’ processing temperatures, the substitutional solutes in the steels are effectively immobilized due to their sluggish diffusion kinetics, thereby attenuating the propensity for carbide or nitride formation.[1,2] The surface hardness, fatigue resistance, and corrosion resistance are significantly enhanced due to the resulting ‘‘colossal’’ interstitial supersaturation.[1–11] Absent carbide or nitride formation, carbide- and nitride-forming elements, such as Cr and Mo, enable colossal interstitial supersaturation by decreasing the activity coefficient of carbon and nitrogen.[1,2] In addition to single-phase austenitic stainless steels, paraequilibrium interstitial hardening has been successfully used

AMIRALI ZANGIABADI and JOHN C. DALTON, Research Assistants, DANQI WANG, Research Scientist, and FRANK ERNST and ARTHUR H. HEUER, Professors, are with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106. Contact mail: heuer@ case.edu Manuscript submitted September 7, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A

for duplex,[6,12,13] martensitic,[14,15] and precipitation-hardening (PH)[16–20] stainless steels. Carburization-induced ferrite-to-austenite and nitridation-induced ferrite-to-austenite phase transformations (and case hardening) were reported for AISI 301 and 2205 stainless steels originally containing 40 vol pct and 50 vol pct ferrite, respectively.[6,13] This paper is concerned with an unusual phenomenon observed in the martensite phases of the 15-5 PH and 17-7 PH stainless steels, and in the d-ferrite phase of the 2205 duplex stainless steel, following paraequilibrium nitridation. In all three steels, the nitrogen supersaturation induced the formation of martensitic austenite, i.e., austenite that formed from parent martensite or parent ferrite via an isothermal martensitic transformation. The so-called reverse martensite-to-austenite phase transformation has been studied extensively in many different steels.[21–28] Previous work has shown that such martensite-to-austenite phase transformations can occur either athermally (i.e., martensitically) or isothermally, depending on the heating rate.[22,23,26] Most importantly, new austenite nucleated at martensite/prior austenite phase boundaries,[22] and the transformation was apparently controlled by interface migration.[24,25] The diffusionless (martensitic) phase transformation to austenite gave rise to lath-shaped plates, while equiaxed austenite was observed when such phase transformation occurred in a diffusive manner during an isothermal hold.[27] Transformation of ferrite-to-austenite by a martensitic reaction has received less attention, but has been observed most convincingly in a 26 pct Cr-5 pct Ni stainless steel.[29] Briefly, this steel was homogenized at 1570 K (1297 °C) for 30 seconds and water quenched, which produced a coarse-