Improving Toughness of Electron Beam Welds of Heavy Mn-Mo-Ni Steel Plates for Pressure Vessels

Abstract

Electron beam welding melts and solidifies steel plate without using any welding material, unlike the conventional welding. Therefore, the toughness at the weld metal can decrease, depending on the chemical composition of the steel plate. Toughness at the electron beam weld can be increased by turning the microstructure from upper bainite into lower bainite and making the effective grain size finer. The microstructure can be controlled by the addition of alloy elements and optimization of impurity elements. In case the chemical compositions cannot be varied, largely because of the specification for their ranges, and the weld metal microstructure remains as upper bainite even after the application of microstructure control, methods to improve the toughness of electron beam weld itself, regardless of steel grades, becomes necessary. Phenomena peculiar to the electron beam weld are segregation during solidification and intergranular segregation over the dendrite surface. The fracture initiation is accelerated by the microcracks caused by the segregations during solidification. The fracture propagation is promoted by intergranular cracking caused by the intergranular segregation. By reducing these segregations, the fracture initiation and propagation are restrained and toughness increases despite the upper bainite microstructure. This can be achieved by the higher purification of steel. Through the foregoing investigations, ASTM A533 Type B Class 2 steel plate of 100 mm in thickness for electron beam welds has been developed for pressure vessels. Various welding tests as pressure vessels have been conducted, and it becomes clear that the developed steel plate has excellent toughness at the weld superior to those obtainable by conventional welding. The use of this steel greatly reduces the welding period compared to the conventional welding method.