抄録
Electron beam (EB) welding of conventional high manganese steels containing large amount of nitrogen was difficult due to occurrence of blow holes and voids. Accordingly, a new type of high manganese steel (14% Mn, 0.6% C, 2% Cr, 2% Ni) without nitrogen addition has been developed to prevent occurrence of blow holes and voids.
The weld defects occurred in this steel only when a bottle neck type bead was formed and the bead width was increased. The weld defects were voids, HAZ cracks and weld metal cracks.
In this report, formation mechanisms of the HAZ crack and the weld metal crack in EB welds were investigated.
The steel plates used in the tests were 90 mm in thickness. A high vacuum type welding machine with a maximum output of 42 kW (60 kV) was used throughout the tests. After EB welding, microstructure observation and micro-analysis in cross sections were conducted and the fracture surfaces of HAZ cracks and weld metal cracks were observed by Scanning Electron Microscopy.
Synthetic cracking test of the weld metal crack was conducted to clarify causes of occurrence of the crack.
The results indicated that 1) occurrence of the HAZ crack was explained in relation to grain boundary liquation near the weld bond, 2) the HAZ crack occurred due to grain boundary liquation, 3) it could be explained that grain boundary liquation occurred due to melting of a sulfide of low melting temperature and 4) it could be reasoned that the weld metal crack originated in the HAZ crack, and then, the HAZ crack propagated into the weld metal and formed the dendrite boundary crack.
