Cr and Ni equivalents and C contents in 304 stainless steels assessed by sensitization tendency and 16-atom composition formulation

Carbide precipitation and ferrite formation often occur in 304-type austenitic stainless steels due to insufficient austenite stabilities. Based on the 16-atom formula for 304 grades derived from the cluster formula approach, the present work explores the optimal Cr and Ni balance and relevant C contents by considering the sensitization tendency in alloy series (Cr 3.0 Si 0.125 )-(Ni 1.625 Mn 0.25 )-Fe 11 -(Cr 23/6 C) 0.0125∼0.0625 and (Cr 2.7∼3.1 Si 0.125 )-(Ni 1.625 Mn 0.25 )-Fe 11.3∼10.9 -(Cr 23/6 C) 0.0625 , or (17.0∼19.4)Cr-10.7Ni-1.5Mn-0.4Si-(0.02∼0.08)C in wt.%. These alloys are arc-melted and copper-mold cast into 30g ingots. Upon sensitizing (650 °C/1h/furnace cooling), both M 23 C 6 and ferrite are the least present in alloys featuring Cr and Ni equivalents of 19.2∼20.0 and 13.5∼13.9. Under the confinements from the above equivalents and from the ASTM standard, fairly high C contents are necessary for 304-type of steels, (0.05∼0.07)C for 304, (0.05∼0.10)C for 304H, and (0.02∼0.03)C for 304L. Stainless steels complying to the optimal Cr eq and Ni eq ranges satisfy a Ni/(Cr-2) ratio range of 1∼2 in terms of 16-atom formula.