Mechanical properties of isothermally aged high-nitrogen stainless steel
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The effects of nitride (Cr2N) precipitation on the tensile, impact, and hardness properties of a typical high-nitrogen, low-carbon austenitic stainless steel (SS), nominally Fe-19Cr-5Mn-5Ni-3Mo-0.024C0.69N, were determined. Annealed and cold-rolled (20 pct reduction in thickness) specimens were isothermally aged at 700 ~ and 900 ~ for times ranging from 0.1 to 10 hours. Only grain boundary Cr2N precipitation occurred in annealed materials aged at 700 ~ Precipitation at 900 ~ occurred sequentially at grain boundaries, by cellular precipitation, and, finally, by transgranular precipitation within the matrix. Nitride precipitation had little effect on yield and ultimate strengths but reduced tensile ductility and impact toughness. Embrittlement occurred due to grain boundary separation (700 ~ and 900 ~ and fracture through cellular precipitate regions, initiated at nitrides (900 ~ Prior deformation increased precipitation kinetics and had a controlling influence on nitride morphology, enhancing grain boundary and transgranular Cr2N and retarding cellular precipitation. Nitride structures produced in cold-rolled materials were just as detrimental to material plasticity as those produced in annealed materials, but prior deformation increased the kinetics of embrittlement. Due to strain recovery, the yield and ultimate strengths of cold-rolled materials decreased with aging time and temperature.
I.
INTRODUCTION
HIGH-nitrogen austenitic stainless steels (SSs) are developing into a new class of engineering materials distinctly different from nitrogen- and carbon-alloyed austenitic SSs (i.e., AISI 200 and 300 series alloys) from which they have evolved. Recent advances in melting technologies using high N 2 gas pressures above the melt have made it possible to produce alloys with N concentrations in excess of 1 wt pct.tt,2] Steels containing N levels exceeding those obtainable at atmospheric pressure (approximately >0.4 wt pct) should be considered "high nitrogen."t31 This is a basic distinction between the new alloys being developed and conventionally processed materials which contain levels of N attainable at atmospheric pressure. One primary advantage offered by high-N alloys is high yield and tensile strengths which can exceed those of conventional alloys by up to 200 to 300 pct in the annealed condition.t4,5,6] This strengthening occurs without concomitant losses of toughness and ductility. The effect of interstitial N on yield strength is twofold. The primary effect is matrix solid-solution strengthening and the second is an increase in the effectiveness of grain size strengthening (Hall-Petch strengthening) with increasing N. [4,5] Due to a high strain-hardening potential, cold deformation of highN SSs has produced yield strengths over 2 GPa.t31 Nitrogen also imparts several other desirable properties to austenitic SSs: (1) N is a strong austenite stabilizing element, thereby reducing the Ni required; (2) N is beneficial to the pitting corrosion resistance of SSs, t3,7,81 and (3) aus-
tenite stability with respec
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