Microstructure and texture of friction stir welds in low carbon steel using prior austenite reconstruction method

ABSTRACT

Friction stir welding (FSW) was performed on thick plates of high-strength low-alloy (HSLA) steel with polycrystalline cubic boron nitride (PCBN) tools. To investigate the mechanism of microstructure formation in the stir zone (SZ), the prior austenite structure undergone through a thermo-mechanical history during the FSW process was investigated. The prior austenite structure was reconstructed by the local crystal orientations of prior austenite calculated from the bainite orientations by using the Kurdjumov–Sachs orientation relationship between the prior austenite and bainite. The reconstructed austenite revealed a heterogeneity in the SZ. The austenite grain size in the advancing side (AS) of SZ was larger than that of the retreating side (RS). Thus, a reheated temperature of AS was considered to be higher than that of RS. The textures of reconstructed austenite were also different in SZ. The rolling texture was observed in RS, but the shear texture was observed in the centre and AS. The heterogeneous microstructure and texture in SZ was considered to be caused by the material flow behaviour in FSW process.

KEYWORDS:

• Friction stir welding
• low-carbon steel
• microstructure
• prior austenite
• texture

Summary

Findings for the austenite structure before transformation are important for the elucidation of the formation behaviour of ferrite structure and this is linked to the future understanding of the microstructure formed in the FSW stir zone of ferrous materials.