Synchrotron diffraction analysis of retained austenite in welded transformation induced plasticity (TRIP) steels

A quantitative analysis of the retained austenite (RA) fractions in gas tungsten arc welded silicon and aluminium containing transformation induced plasticity steels was carried out by synchrotron X-ray diffraction measurements. The variation in RA transverse to the weld line was measured to study the effect of weld thermal cycles on the stabilisation of austenite in the heat affected zone (HAZ) and the fusion zone (FZ). The results showed that the FZ of silicon based steels contained a higher amount of RA (∼7%) than aluminium based steels, which contained only ∼4%. During the solidification of the weld pool, aluminium was found to partition to solidifying δ-ferrite and to stabilise the soft δ-ferrite grains at the fusion boundaries. Owing to this partitioning, the HAZ was enriched in carbon and the RA content was found to increase with distance from the fusion boundaries. In contrast, this partitioning behaviour was not present in silicon based transformation induced plasticity steels and a lesser amount of RA was found in the coarse grained HAZ than in the FZ.