Distortion and residual stress generated by laser beam welding of high strength steel

Abstract

In order to elucidate distortion and residual stress generated by welding of high strength steel (HT780) by laser beam, a series of experiments and analyses were carried out. The angular distortion generated by bead-on-plate welding was V-shaped and its magnitude was about 2 mm. The longitudinal bending distortion was extremely small. On the surface of the plate, the residual stress component in the welding direction was tensile and it was smaller than the yield stress in the weld metal. The residual stress was almost zero outside the weld metal. The phase transformation range in the cooling stage and the temperature dependency of mechanical properties were obtained. The mechanical properties in the phase transformation range in the cooling stage could not be specified due to transformation expansion. Therefore, they were idealized by considering transformation expansion and transformation superplasticity. The validity of the idealized mechanical properties was verified by simulating the experiment by the thermal elastic–plastic analysis. It was elucidated that the welding out-of-plane distortion and tensile residual stress were largely controlled by phase transformation in the cooling stage, although the bead width of laser beam welding was extremely narrow.

Keywords::

• laser beam welding
• high strength steel
• welding distortion
• residual stress
• phase transformation

Acknowledgements

Yasuda Koichi of JFE Steel kindly assisted in the obtaining of physical properties. The study also includes some of the findings made by the PF Study: Basic Research and Development Team for Revolutionary Improvements in the Strength and Performance of Ferrous Metals of the Japan Research and Development Center for Metals.

Notes

Presented at the Autumn National Conference, 2010.