Abstract: The present invention provides a boom and arm member of a construction machine excellent in weld zone fatigue strength and a method of improvement of its fatigue strength, that is, a boom and arm member of a construction machine superior in weld zone fatigue strength, the boom and arm member of a construction machine characterized in that toes of full penetrated weld zones of the boom and arm member have vibration strike marks, and a method of improvement of its fatigue strength. Preferably, the residual stress of the bottom surface of the vibration strike marks is a 10 MPa or higher compressive residual stress.

Abstract: The present invention provides high strength thick steel plate superior in crack arrestability high in strength, free of deterioration of HAZ toughness, and free of anisotropy, that steel plate containing, by mass %, C: 0.03 to 0.15%, Si: 0.1 to 0.5%, Mn: 0.5 to 2.0%, P: ?0.02%, S: ?0.01%, Al: 0.001 to 0.1%, Ti: 0.005 to 0.02%, Ni: 0.15 to 2%, and N: 0.001 to 0.008% and having a balance of iron and unavoidable impurities as chemical components, having a microstructure of a ferrite and/or pearlite structure with bainite as a matrix phase, and having an average circle equivalent diameter of crystal grains with a crystal misorientation angle of 15° or more of 15 ?m or less in the regions of 10% of plate thickness from the front and rear surfaces and of 40 ?m or less in the other region including the center part of plate thickness.

Abstract: A method of production of a metallic product with a nanocrystallized surface layer comprising subjecting a surface layer of a metallic product to ultrasonic impact treatment by one or more ultrasonic indenters vibrating in a plurality of directions, then subjecting the surface layer subjected to the ultrasonic impact treatment to heat treatment at a low temperature to cause precipitation of nanocrystals.

Abstract: A method of improvement of toughness of a heat affected zone in a multi-layer welded joint, a fillet welded joint, and a one-pass or several-pass large heat input welded joint of a steel plate is provided, that is, a method of improvement of toughness of a heat affected zone in a welded joint of a steel plate characterized 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 or shot peening by ultrasonic vibration steel balls to thereby make an average of longitudinal axis of crystal grains up to a depth of 2 mm or more 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 the equivalent of the crystal grain size of the steel plate matrix before the welding at a depth of ¼ of a thickness t from the surface of the steel plate.

Abstract: A circularly welded joint featuring excellent fatigue strength obtained by welding the ends of two pieces of steel plates perpendicularly together and used for the welded structures such as buildings, ships, bridges, construction machinery and off-shore structures, a method of producing the circularly welded joints and a welded structure using the circularly welded joints are provided. A circularly welded joint is obtained by welding the ends of two pieces of steel plates perpendicularly together. Between the two pieces of the steel plates, at least the steel plate on the side on which the main stress is exerted is one that suppresses the propagation of cracks due to fatigue and, preferably, one having the compressive residual stress in the surface layer of the steel plate.

Abstract: The present invention provides high strength thick steel plate superior in crack arrestability high in strength, free of deterioration of HAZ toughness, and free of anisotropy, that steel plate containing, by mass %, C: 0.03 to 0.15%, Si: 0.1 to 0.5%, Mn: 0.5 to 2.0%, P: ?0.02%, S: ?0.01%, Al: 0.001 to 0.1%, Ti: 0.005 to 0.02%, Ni: 0.15 to 2%, and N: 0.001 to 0.008% and having a balance of iron and unavoidable impurities as chemical components, having a microstructure of a ferrite and/or pearlite structure with bainite as a matrix phase, and having an average circle equivalent diameter of crystal grains with a crystal misorientation angle of 15° or more of 15 ?m or less in the regions of 10% of plate thickness from the front and rear surfaces and of 40 ?m or less in the other region including the center part of plate thickness.

Abstract: The present invention provides a boom and arm member of a construction machine excellent in weld zone fatigue strength and a method of improvement of its fatigue strength, that is, a boom and arm member of a construction machine superior in weld zone fatigue strength, the boom and arm member of a construction machine characterized in that toes of full penetrated weld zones of the boom and arm member have vibration strike marks, and a method of improvement of its fatigue strength. Preferably, the residual stress of the bottom surface of the vibration strike marks is a 10 MPa or higher compressive residual stress.

Abstract: A circularly welded joint featuring excellent fatigue strength obtained by welding the ends of two pieces of steel plates perpendicularly together and used for the welded structures such as buildings, ships, bridges, construction machinery and off-shore structures, a method of producing the circularly welded joints and a welded structure using the circularly welded joints are provided. A circularly welded joint is obtained by welding the ends of two pieces of steel plates perpendicularly together. Between the two pieces of the steel plates, at least the steel plate on the side on which the main stress is exerted is one that suppresses the propagation of cracks due to fatigue and, preferably, one having the compressive residual stress in the surface layer of the steel plate.

Abstract: A method of improvement of toughness of a heat affected zone in a multi-layer welded joint, a fillet welded joint, and a one-pass or several-pass large heat input welded joint of a steel plate is provided, that is, a method of improvement of toughness of a heat affected zone in a welded joint of a steel plate characterized 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 or shot peening by ultrasonic vibration steel balls to thereby make an average of longitudinal axis of crystal grains up to a depth of 2 mm or more 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 the equivalent of the crystal grain size of the steel plate matrix before the welding at a depth of ¼ of a thickness t from the surface of the steel plate.

Abstract: A method of production of a metallic product with a nanocrystallized surface layer comprising subjecting a surface layer of a metallic product to ultrasonic impact treatment by one or more ultrasonic indenters vibrating in a plurality of directions, then subjecting the surface layer subjected to the ultrasonic impact treatment to heat treatment at a low temperature to cause precipitation of nanocrystals.