A Study of the Strengthening and the Production of Sigma Phase in 20–9–3 Austenitic Stainless Steel Weld Metals

ABSTRACTIt is general practice to weld bullet proof steels with 20Cr-9Ni-3Mo Duplex stainless alloy. Little information is available regarding increasing the strength of these weldments or their response to time at temperatures used for heat treatments. This investigation sought to study possible strengthening mechanisms involving silicon additions and the effects of heat treating such welds at 1050°C.A 2% silicon addition gave a 16% increase in strength with approximately equal contributions from stacking fault energy, solute and second phase hardening. However, the ductility was significantly reduced by a high ferrite content. When this δ ferrite content was corrected by cobalt additions a 26% increase in strength was achieved, giving properties very similar to bullet proof steels.A 1050°C heat treatment programme showed rapid sigma phase formation from δ ferrite but the process was reversed on further heating. This phenomenon was attributed to alloy migration as the weld microstructure approached equilibrium, the hardness of the alloys followed the variation in δ ferrite content being at their heightest at the beginning and end of heat treatment.