Formability of in-situ Austempered Transformation-induced Plasticity Steels After Laser Beam Welding

Conformabilidade de Aços com Plasticidade Induzida por Deformação Após a Solda a Laser Seguida de Austêmpera in-situ

Abstract

Laser welded Transformation-induced plasticity (TRIP) steels are known to generate martensite in the fusion (FZ) and the heat-affected (HAZ) zones. To solve this issue, the present study proposes a high temperature (HT) welding to avoid decay below the martensite start temperature after laser welding. Therefore, an inductive heating has been used to reach 500 °C before laser weld of superposed 1.6mm thick TRIP steel class 750. After welding the temperature was kept constant at 500 °C for more 10 minutes in order to austemper. The microstructures of the welds at high temperature are composed of bainite and residual austenite with an FZ hardness up to 300 HV, compared to 450 HV of the ambient temperature (AT) weld. The HT values of hardness are slightly higher than a traditional post-weld heat treatment (TW), 300 HV compared to 250 HV, because of the tempering kinetics in each case. Erichsen cup indentation tests shown the HT coupons presents better formability compared to the AT or TW conditions. The present contribution highlights a possible solution to the intrinsic brittleness during cold forming of laser welded TRIP 750 steel by applying an inductive in-situ austempering.

Key-words:
Laser welding; Transformation-induced plasticity steels; Post-welding heat treatments