Impact Toughness and Microstructure of Low-Alloy Steel Weld Produced by Automatic Submerged Arc Welding

The impact toughness, microstructure and break features of low-alloy steel weld samples produced by automatic submerged arc welding have been compared. It has been established that when a notch is applied along the weld axis, fracture occurs along large grains of grain-boundary ferrite 80-140 mu m in size with an unfavorable cleavage planes orientation {001}. The combination of such grains with non-metallic inclusions 3-5 mu m in size facilitates the transcrystalline cleavage initiation. The effect of austenite grain size, crystallographic texture, and intergranular fracture on the impact toughness value is shown. Continuous chains of grain-boundary ferrite at the front of the main crack line up along the notch-parallel boundaries of columnar grains of axial orientation, which increases the tendency to transcrystalline cleavage. At the same time, with the lateral orientation of the columnar grains of the former austenite, the front line of the main crack intersects only certain separate areas of the grain-boundary ferrite. Offset notch results in an increase in both impact toughness and plasticity of weld metal.