Low-Temperature Nitridation of 2205 Duplex Stainless Steel
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LOW-TEMPERATURE paraequilibrium carburization and nitridation of austenitic stainless steels have led to steels with remarkably improved properties—increased surface hardness, increased fatigue- and wear resistance, and improved corrosion resistance, particularly in sea water.[1–10] If carbide or nitride formation is prevented, extremely high levels of carbon and nitrogen supersaturation can be realized, owing to the presence of carbide- and nitride-forming elements such as Cr or Mo, which serve to greatly decrease the activity coefficients of carbon and nitrogen.[9,10] Under near-equilibrium conditions, these elements promote carbide and nitride formation. Under paraequilibrium conditions, however, substitutional solute atoms are essentially immobile and precipitation is kinetically suppressed. The approach to paraequilibrium is determined only by the diffusion of the interstitial species. At temperatures typical of these processes (below 450 C), interstitial atoms can diffuse considerable
J.C. DALTON, F. ERNST, and A.H. HEUER are with the Department of Materials Science and Engineering, Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH 44106. Contact e-mail: [email protected] Manuscript submitted May 19, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
distances in industrially relevant times. Surface hardness, fatigue resistance, wear resistance, and corrosion resistance are significantly enhanced, and result from a ‘‘colossal’’ interstitial supersaturation. No significant dimensional or aesthetic changes are found in the interstitially-hardened workpieces. While paraequilibrium carburization and nitridation can improve desirable surface properties of martensitic and duplex stainless steels, the microstructural changes in the martensite or ferrite phases giving rise to these improvements have not been adequately explored. From previous studies on paraequilibrium nitridation or carburization of duplex stainless steels,[11–31] nitride or carbide phases, presumably formed within the d-ferrite, are commonly observed. Global structural characterization techniques, such as X-ray diffractometry (XRD), hint at an isothermal ferrite-to-austenite transformation. However, due to the widely varying processing parameters employed in these prior studies, it is difficult to ascertain what microstructural changes occur and under what conditions. In Reference 11, we reported the response of d-ferrite in 17-7 PH (precipitation-hardened) and 2205 duplex stainless steel to low-temperature gas-phase carburization. For all processing conditions, the d-ferrite grains in both alloys showed an unusual weak contrast in conventional transmission electron microscopy (TEM) bright-field imaging, resembling the appearance of an amorphous phase. Spinodal decomposition of the
d-ferrite into nanometer-scale Fe-rich (aFe ) and Cr-rich (a0Cr ) ferrite domains was observed. The Cr-rich domains were significantly enriched in carbon and were accompanied by an extremely high dislocation density (estimated to be 1016 m2 ). In spite o
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