Effect of Baking on Spot Welds of a Prestrained Ultra-Low Carbon Bake Hardening Steel Sheet

Abstract

The effect of bake hardening (BH) treatment on the strength of spot-welded joints made on prestrained (PS) sheets of an ultra-low carbon (ULC) BH grade steel has been investigated using tensile-shear specimens. A series of these experiments has been carried out on spot-welds made on as-received and PS sheets; a part of the latter being subjected to BH treatment. Determination of hardness profile along the weldments, characterization of the microstructures, and substructures by optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM), as well as fractographic examinations are some necessary supplementary examinations. The results infer that the strength of the PS sheets increases by BH treatment, but this treatment does not exhibit any significant influence on the strength of the spot-welds made on PS sheets. The hardness profile along HAZ and fusion zone of the spot-welds made on as-received sheets remains almost unaltered for PS specimens or that subjected to BH treatment. The locations of failure around the spot-welds of as-received (AR), PR, and bake hardened specimens are similar in nature, and these failures usually originate from the HAZ-base metal interfacial region.

Keywords:

• Bake hardening
• Cottrell atmosphere
• Prestrain
• Spot weld
• Tensile shear

ACKNOWLEDGMENT

The authors would like to thank Tata Steel Ltd., Jamshedpur, India for their financial support to this work.

Notes

*Prof. G. Mukhopadhyay is on study leave from R&D and Scientific Services, Tata Steel, Jamshedpur 831001, India.