Effect of post welding heat treatment on corrosion behavior of NiCrMo-3 deposited metal

In this paper, the difference of corrosion resistance for NiCrMo-3 deposited metal with as-welded and as-PWHT (post welding heat treatment) at 690 ℃ for 8 h was investigated by scanning electron microscope, transmission electron microscopy and other characterization methods. The results show that the as-welded specimens have better intergranular corrosion resistance than that of as-PWHT specimens. Pitting corrosion and intergranular corrosion occur when the specimens were immersed in HNO₃ solution for different times. In addition to pitting corrosion and intergranular corrosion, partial inter-dendrite corrosion occurs in as-PWHT deposited metal. The increase of the number and size of NbC and Laves phase leads to the increase of corrosion sensitivity of the specimens after heat treatment. The potential difference between precipitated phase NbC, Laves phase and matrix leads to pitting corrosion under the action of corrosive medium. The contents of Ni and Cr elements decrease due to grain boundary precipitates, which increased the intergranular corrosion sensitivity of deposited metal. After heat treatment, the elements redistribution leads to the enrichment of Nb, Mo elements and depletion of Ni, Cr elements, resulting in inter-dendrite corrosion.