Investigation on microstructure and mechanical properties of ni-based alloy overlayer by ultrasonic impact treatment

Ultrasonic impact treatment (UIT) was performed on the Ni-based alloy overlayer of austenitic stainless steel pipe girth weld. The difference of microstructure, microhardness and maximum shear strength distribution of the joint before and after UIT was studied by autobiographic test and shear punch test. The electrode potential data were tested by a three-electrode electrochemical workstation and tafel curve was drawn. The results show that a grain refinement layer of about 30 um and a transition layer of about 60 um are formed in the over-layer of nickel-based alloy after UIT. Meanwhile, the transition layer has high corrosion sensitivity to oxalic acid solution, and the main precipitates in the overlayer are austenite and M23C6. In 3.5 % NaCl solution, the self-corrosion potential is -432 mV. The average microhardness of the overlayer of nickel-based alloy is 270 HV, which is 18.9 % higher than that of the sample without UIT. In the meantime, the maximum shear strength of nickel-based alloy is 955 MPa, which is 9.6 % higher than the sample without UIT. It is found that the ferrite is mainly lath and skeleton with no obvious change under UIT or not. Meanwhile, an average microhardness is 191 HV, and shear strength is 678 MPa. The microstructure of the welded joint is mainly consisted of austenite phase and ferrite phase, and FA solidification mode has been a major influence in the welding process. At the boundary between overlayer and circumferential welded joint of 304 austenitic stainless steel, reheat cracks in this area are occurred easily. Due to the migration of carbon, local martensite bands will be formed which will lead to large heterogeneity of microhardness.

Automated summary

The study found that after UIT treatment, a grain refinement layer and transition layer were formed in the nickel-based alloy overlay. The microhardness and maximum shear strength of the overlay significantly increased, but the transition layer showed high corrosion sensitivity to oxalic acid solution.