Effect of inhomogeneous microstructure on the deformation and fracture mechanisms of 316LN stainless steel multi-pass weld joint using small punch test

Abstract

The local microstructural effect on the deformation behavior and fracture mechanisms in different zones of 316LN stainless steel multi-pass weld joint of the nuclear power plant was investigated by using small punch test. The results indicated that microstructure of the weld joint was highly inhomogeneous from region to region. The microstructure of base metal (BM) and heat affected zone (HAZ) was austenitic cellular crystal with twins, while that of as-deposited weld metal (WM) was duplex austenite-ferrite. By inducing multi-pass thermal cycles, at least five distinct morphologies of ferrite were identified in the WM. On the other hand, the deformation and fracture mechanisms in different zones of the multi-pass weld joint were strongly influenced by the inhomogeneous microstructure. The WM showed lower deformability than the BM and HAZ due to the deformability mismatch between ferrite and austenite. In BM and HAZ, the cracks tended to initiate at the slip bands and propagate along the slip bands. However, in WM, the cracks tended to initiate at the ferrite and ferrite-austenite interface and preferentially propagate along the needle-like ferrite. In the fusion zone (FZ), the plastic deformation always first occurred in the HAZ followed by the deformation of the WM near the fusion boundary. And the crack initiated at the region adjacent to the fusion boundary but did not propagate along the fusion boundary. Fractographic analysis demonstrated that the fracture model of all the specimens was ductile fracture except a few quasi-cleavage fracture features were observed in the HAZ.