Patent ID: 7754033

Claim:
A method of improvement of toughness of a heat affected zone in a welded joint of a steel plate, wherein said steel plate has a plate thickness t, characterized by subjecting a surface of a heat affected zone formed by a last pass of a multi-layer welded joint of a steel plate to impacts by an ultrasonic vibration tool using one or more pins having a diameter of 10 to 30 mm with an oscillating amplitude of between 20 to 60 μm to thereby make an average of longitudinal axis of crystal grains to a depth of at least 2 mm from the surface of the steel plate in the microstructure adjacent to a fusion line (FL) of a weld metal and a steel plate matrix in said heat affected zone formed by the last pass equivalent to the crystal grain size of the steel plate matrix before the welding at a depth of ¼ of the thickness t from the surface of the steel plate.