Strategic approaches to enhance quenching and partitioning applicability: optimizing mechanical properties and microstructure of commercial low-silicon 20MnB5 steel

Quenching and partitioning (QP) is a heat treatment designed to induce a multiphase microstructure composed of martensite and retained austenite. This treatment introduces high tensile properties in the material, coupled with enhanced ductility compared to traditional treatments. This enhancement arises from the strain-induced transformation of retained austenite into martensite when subjected to loads. Austenite stabilization at room temperature is achieved through carbon diffusion from martensite to austenite during partitioning. Therefore, the chemical composition of the alloy is typically tailored to promote this phenomenon. Silicon is added to suppress carbide precipitation, while manganese is added to enhance austenite stability. However, in this study, we focus on commercial low-silicon 20MnB5 steel. This grade is a low-alloyed steel commonly used in the heat-treated condition and is potentially influenced by the properties introduced through QP treatment. Multiple quenching and partitioning treatments are designed and executed to investigate the applicability of QP on 20MnB5 steel. Intercritical treatment strategies are employed to enhance the hardenability of the selected alloy, aiming to prevent bainite transformation and increase the effectiveness of quenching and partitioning, increasing the free carbon at disposal for partitioning. XRD analyses are conducted to identify retained austenite in the final specimen, quantify its amount, and observe its morphology and location. Optical microscopy (OM) and scanning electron microscopy (SEM) are used to characterize the introduced multiphase microstructure. Tensile tests are performed to assess the mechanical properties introduced by the treatment. In conclusion, the study demonstrates the applicability of intercritical quenching and partitioning (QP) treatments on 20MnB5 steel. However, in the observed conditions, the stabilization of a fraction of retained austenite is not correlated to the greatest increase in UE, leading to the conclusion that the control of the surrounding microstructure is the primary factor that influences the final properties of the material.