Effect of fast cooling process on microstructure and toughness of heat affected zone in high strength pipeline steel X120

A process for improving the heat affected zone (HAZ) toughness by fast cooling during welding was developed. The holding time at high temperature above Ac₃ and the cooling time from 800 to 500°C were decreased when the welding thermal cycle was experienced a fast cooling process. The microstructures in the HAZ with varied thermal cycles were examined by optical and scanning electron microscopes. The shorten holding time led to thinner HAZ width and finer austenite grains in the fusion line and coarse grained HAZ, while the decreased cooling time from 800 to 500°C resulted in finer bainitic ferrite in the HAZ. Martensite–austenite constituents in the fusion line were reduced in amount and size and predominantly changed into film-like shape. Experimental results indicated that the fast cooling process played an important role in improving the HAZ toughness of pipeline steel X120.