Microstructure transformation and mechanical properties of electron beam welded joints of fusion CLAM steel

China low activation martensitic(CLAM) steel for fusion reactor were butt-welded through electron beam welding process,and followed by post-welding heat treatment(PWHT) at 740℃ for 1 h.The microstructure and microhardness distribution of welded joints were analysed.The results showed that the weld seam with good formation and without appearance defects has been achieved.In as-welded condition,the weld metal and fusion zone consisted of a mixture of coarse lath martensite and a little δ ferrite.The coarse and fine grained zone was composed of fine lath martensite.The partially quenched zone mainly consisted of fine lath martensite and ferrite.The softening did not occur across welded joint.After PWHT,the lath martensite in the welded joint was transformed into tempered sorbite,which led to the significant decrease of the hardness of each zone.Among them,the highest hardness emerges at fusion zone was not at fine grained zone,which was just 20% higher than base metal.The PWHT did not make the tempered zone and base metal more soft obviously.