Analysis of the high temperature creep behavior of a vacuum electron beam welded joint of G115 steel at 630 °C

Vacuum electron beam welding was employed to fabricate a high-quality welded joint of G115 steel, which was then characterized using OM, SEM, and TEM to clarify its microstructure. The creep behavior of the joints was analyzed using a high-temperature creep test. These results show that the joint weld metal had small grains, a relatively coarse microstructure in the heat-affected zone, and a tempered martensitic structure without ferrite δ. Herein, an insufficient polygonization transition was observed in some lath bundles, while many carbide phases, such as M23C6 and MX, were present near the lath boundaries with increasing aggregation and growth and were distributed continuously in the shape of a strip along the crystal face. At 630 °C and different stress levels, increasing the applied stress significantly shortened the creep fracture time and increased the minimum creep rate. Typically, creep fracture follows the Monkman–Grant equation, and the strain accumulation during the steady-state creep stage is negligible but significant during the acceleration stage. Various necking degrees were observed under different applied stresses, and obvious differences in the number and size of dimples occurred during creep specimen fracturing, which was ductile fracture. During creep, the voids aggregated and grew continuously, and local plastic instability occurred when the voids reached critical dimensions, which resulted in the final fracture.