Microstructural Development during Welding of TRIP steels

The Advanced High Strength Steels (AHSS) are promising solutions for the production of lighter automobiles which reduce fuel consumption and increase passenger safety by improving crash-worthiness. Transformation Induced Plasticity Steel (TRIP) are part of the advanced high strength steels which offers a high strength and toughness combination with excellent uniform elongation. However, the higher alloying content of these steel limits their weldability and the thermal cycle of a welding process destroys the carefully designed microstructure which results in inferior mechanical properties of the weld. The aim of this work is to improve our understanding of the response of the material to welding by (i) studying the microstructural evolution during weld thermal cycles of specific joining processes generally employed in automotive manufacturing, (ii) the use of specific heat treatments and (iii) the influence of thermal cycles on the final properties of the welded material. The experimental investigations on the microstructural evolution during welding showed that the presence of strong oxidizing elements such as aluminium and silicon lead to the formation of non-metallic inclusions in the fusion zones. The partitioning of aluminium to the solidifying fusion and grain boundaries in the fusion zone leads to the stabilization of ferrite in aluminium containing TRIP steels. The magnetic, dilatometric and synchrotron X-ray diffraction analyses described the phase transformation kinetics and lattice parameter behaviour during heating and cooling thermal cycles representative of typical welding conditions. Experiments also indicated ambient temperature transformation of austenite after cooling. The mechanical properties of the welded TRIP steels were affected by the distribution of retained austenite in the weld zone and its stability at room temperature after welding. Based on the findings of this research, recommondations were made to the welding industry to avoid inherent metallurgical problems that may arise during welding of TRIP steels in practice