Effect of δ-ferrite on impact properties of supermartensitic stainless steel heat affected zones

The work presented in this paper focuses on the effect of the presence of non-equilibrium δ-ferrite on the impact properties of a supermartensitic stainless steel. To generate homogeneous δ-ferrite containing microstructures the material was, in the first part of the present study, subjected to a series of high temperature furnace heat treatments. Three microstructures possessing variable ferrite content and different grain sizes were generated. Charpy impact results indicate that the presence of 14% δ-ferrite in a martensitic matrix of 60 μm prior austenite grain size, raises the ductile to brittle transition temperature (DBTT) by about 50°C compared with a fully martensitic microstructure of 80 μm. For a similar grain size, reducing the amount of δ-ferrite from 14 to 2% restored the DBTT to a level comparable to that of the tempered parent material. A Gleeble simulator was used to create the range of microstructures found in a weld heat affected zone (HAZ). Testing of the simulated HAZ indicated that toughness was not significantly affected by the presence of δ-ferrite. The DBTT was comparable to that of the parent material and lower than that of the heat treated specimens containing 14% δ-ferrite.