Room temperature mechanical properties and microstructure of a low alloyed TRIP-assisted steel subjected to one-step and two-step quenching and partitioning process

The capability of one-step and two-step quenching and partitioning processes (Q&P) to treat the microstructure of a low alloyed TRIP-assisted steel was considered. The treated microstructures primarily consisted of lath martensite, retained austenite, lower bainite, and fresh martensite. Two various types of retained austenite in respect of size and morphologies were observed: blocky and film-like retained austenite. Short holding times were appropriate for carbon enrichment of the retained austenite. However, the volume fraction and carbon concentration of retained austenite vs. partitioning time followed an opposite trends in two step processed microstructures. This was attributed to the competitive processes of bainite or epsilon carbide formation at the specified partitioning temperature. The nano indentation experiments were employed to assess the local mechanical properties of individual phases specifically martensite and enriched retained austenite. The frequent pop-in in load-displacement curves indicated the capability of enriched austenite for martensitic transformation. Owning to the higher diffusion rate, the carbon concentration was relatively higher in two-step processed microstructures. In addition, film like or lamellar morphologies (which hold higher population in partitioned structures at 400 °C) possessed better transformation stability rather than a blocky type, which tends to transform to martensite under higher amount of strain. The evolution of mechanical properties was discussed relying on the variations of volume fraction, morphology and carbon concentration of retained austenite.