Effect of ultra-sonic peening on laser-arc hybrid welded NV E690 steel

Hybrid laser-arc welding (HLAW) was carried out on 690 MPa high-strength NV E690 steel plates of 15 mm thickness using the double-pass welding process. Fine weld joints with good performance were successfully obtained by double-pass HLAW. Ultrasonic peening treatment (UPT) was performed on the welded seams to improve the microstructure and mechanical properties. Refined microstructure and reduced porosity ratio were introduced by UPT. Some porosity closed while some changed from circular into flat. Average residual stress changed from 301 to −426 MPa after UPT. Average micro-hardness changed from 350–375 HV to 420–430 HV. Tensile strength and bending strength were around 830 and 1759 MPa, respectively, close to those before UPT. Low-temperature (−40 °C) absorbed impact energy varied from 32 to 40 J, with 25% improvement. Refined microstructure contributed to the improvement of micro-hardness and mechanical properties. However, due to the limited thickness of the ultrasonic peened layer, the improvement of strength, which depended on the strength of the test material as a whole, was not obvious. Stress corrosion resistance of the ultrasonic peened weld was improved. Mechanism of the microstructure evolution and its relationship with the tensile, bending strength, low-temperature absorbed impact energy and stress corrosion resistance were discussed.