Control of Retained Delta Ferrite in Type 410 Stainless Steel

Type 410 stainless steel is used in petro-chemical refineries for its high resistance to halide stress corrosion cracking, sulfide corrosion cracking, and sulfur attack at elevated temperatures. Along with its adequate corrosion resistance 410SS also exhibits low cost and hardenability making it an ideal material for hydro-processing applications. Problems related to meeting toughness and hardness code requirements within the weld metal and heat effected zone (HAZ) have been experienced during fabrication of 410SS welded components. The loss of toughness has been related to excessive amounts of delta ferrite in the weld metal and HAZ. The objective of this study was to quantify the effect of cooling rate and alloying compositions within ASTM and AWS specifications for 410SS on delta ferrite formation. C, Cr, Ni, and Mo, were used as factors in a model-based design of experiment (DOE) within CALPHAD based software DICTRA™ to simulate the effects of composition and cooling rate on delta ferrite formation. Based on the DOE results, a predictive model for quantification of retained delta ferrite in 410SS welds was developed along with evidence for cooling rate effect on retained delta ferrite. Optical metallography was also used to demonstrate possible ferrite content within the 410 composition range.