Application of Extra High Purity Austenitic Stainless Steel to Weld Overlay

Stress Corrosion Cracking (SCC) was understood to be the result of a combination of susceptible material, a corrosive environment and tensile stress above a threshold. An Extra High Purity Fe-Cr-Ni austenitic stainless steel (EHP alloy) was developed with conducting the new multiple refined melting technique in order to suppress the total impurities (C, O, N, P, S, B, Si, Mn) less than 100ppm. EHP alloy has great intergranular corrosion resistance. It is considered that intergranular corrosion becomes initiation of SCC. So, we try to apply EHP alloy to weld overlay materials to prevent from occurring SCC. EHP alloy (Fe-25Cr-20Ni, Fe-25Cr-35Ni) was melted by the new technique. The conventional weld metals (Type Y316L and Inconel 82) were also prepared as comparisons. Specimens were machined from the welded metal of each material. Intergranular corrosion tests were performed in boiling 8kmol/m³ HNO₃ solutions containing 1kg/m³ Cr(VI) ions. The intergranular corrosion of conventional weld metals was severer than those of EHP alloys. Crevice Beam Bending tests to evaluate susceptibility of SCC were carried out in high temperature water of 561K with saturated oxygen (32ppm) for 1000h. Though cracks and intergranular corrosion were observed in all specimens, cracks of conventional weld metals were much more than those of EHP alloys. It was confirmed that EHP alloy has also excellent SCC resistance as a weld overlay material.