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Welding in the World

, Volume 63, Issue 1, pp 151–159 | Cite as

Transient softening at the fusion boundary in resistance spot welded ultra-high strengths steel 22MnB5 and its impact on fracture processes

  • Oleksii SherepenkoEmail author
  • Sven Jüttner
Research Paper
  • 105 Downloads

Abstract

This work gives an insight in the transient softening at the fusion boundary of resistance spot welds of hot stamped steel 22MnB5. The development of the softened region was investigated for three different energy input concepts with short, middle, and long welding times; thereby, an experimental technique of interrupted spot welds combined with metallographic investigations and hardness measurements was employed. The observed softening can be explained with the effect of carbon segregation, supported by the diffusional γ → δ phase transformation or delta-ferrite nucleation from the liquid L → δ, which may lead to formation of δ-Fe at the fusion boundary. Subsequent quenching at cooling speeds of about 2500 °C/s after the welding process prevents carbon redistribution and decomposition of δ-Fe. This assumption is in good agreement with the results of carbon measurement at the fusion boundary and metallographic observations. The influence of the softened region on fracture of resistance spot welds was investigated, using cross-tension test and metallographic analysis. The transition of the fracture mode from fracture through the weld nugget for short welding time to fracture along the fusion boundary for long and middle welding times is observed. Marginally higher maximal force and energy absorption by welded joints with long welding times was observed.

Keywords

Spot welding Metallography Microstructure Segregation Martensite Coarse grained heat affected zone 

Notes

Acknowledgements

The authors would like to acknowledge the province Saxony-Anhalt for financial support of the study in the framework of the postgraduates funding and thank all the students and colleagues, who have contributed their effort to the investigations.

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Copyright information

© International Institute of Welding 2018

Authors and Affiliations

  1. 1.Institute of Materials and Joining TechnologyOtto von Guericke University MagdeburgMagdeburgGermany

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