The influence of interstitial diffusion across the fusion line on the HAZ microstructure and properties in 12% chromium type 1.4003 steels
The 12% chromium type EN 1.4003 ferritic stainless steels are susceptible to grain growth and associated embrittlement in the heat-affected zone during welding. Grain growth can be restricted by increasing the amount of austenite that forms on cooling through the dual-phase (austenite+ferrite) fi eld. This investigation examined ways of locally increasing the interstitial (carbon or nitrogen) content of the heat-affected zone during welding, and studying the effect of such an increase in interstitial content on the microstructure and mechanical properties of the welded joints. Signifi cant changes in heat-affected zone microstructure and mechanical properties were observed on increasing the carbon content of the weld metal through the use of a higher-carbon welding consumable, and on increasing the weld metal nitrogen content through the use of nitrogen-containing shielding gas. This suggests that the weld thermal cycle is suffi ciently long to allow diffusion from the weld metal across the fusion line into the high temperature heat-affected zone, resulting in higher levels of martensite, smaller ferrite grain sizes, higher heat-affected zone hardness values, and improved toughness.