Effects of heat treatment processes on microstructure and impact toughness of weld metal of vacuum electron beam welding on CLAM steel

China low activation martensitic steels used for fusion reactor were welded with vacuum electron beam welding process, and followed by post-welding heat treatment(PWHT), including different normalizing and tempering treatment and high temperature tempering treatment at 740℃ for different hours.The characteristics of microstructure of the weld metal were investigated by optical microscope(OM), scanning electron microscope(SEM) and transmission electron microscope(TEM) before and after PWHT.Also, impact toughness tests of weld metal were performed at room temperature.Experimental results indicate that the microstructure of the weld metal in as-welded condition is composed of coarse lath-shaped martensite with high volume fraction of δ-ferrite.Impact toughness of weld metal in as-welded condition is low.After tempering treatment, δ-ferrite is preserved while the lath-shaped martensite transforms to tempered martensite.Carbides of the weld metal which is M₂₃C₆ type are precipitated on the grain boundaries of δ-ferrite in significant quantities.Unfortunately, δ-ferrite is surrounded by coarse carbides like a chain of islands after tempering treatments, which makes the cohesion between δ-ferrite and prior austenitic grain boundaries weakened.Impact toughness of the weld metal is not improved due to the boundaries of δ-ferrite are apt to become the crack sources when the weld metal is subjected to impact loads.Meanwhile, superior impact toughness is obtained by normalizing and tempering heat treatment for δ-ferrite is eliminated after the treatment and the weld metal of the joint transforms to tempered martensite which is the same to the base metal.