Softening characteristics in ultra-narrow gap GMA welded joints of ultra-fine grained steel

When a 800 MPa grade ultra-fine grained steel with ferrite grains less than 1 μm and dispersed fine cementite is welded, fine ferrite grains are coarsened resulting in remarkable softening in the heat affected zone (HAZ). The peak temperature at an arbitrary location in HAZ during welding was calculated by heat conduction analysis and the effect of welding thermal history on the microstructure of the UFG steel HAZ was examined by microscopic observation. Softening as a result of ferrite grain coarsening was observed in the region where the peak temperature reaches between 920 and 1300 K for the ultra-fine grained steel with an Ac 1 temperature of 980 K and Ac 3 of 1150 K. The formation of martensite–austenite constituents started as a second phase above the Ac 1 temperature and they curbed HAZ softening in the peak temperature range between 1000 and 1250 K.