Influence of alloying elements on the microstructure evolution and mechanical properties in quenched and partitioned steels

Quenching and partitioning (Q&P) is one of the most promising heat treatment processes to produce microstructure in steels which contains martensite and high fraction of retained austenite. In the present study, three newly designed steels (A-C) were produced by casting with varying amounts of C, Mn, Si and Al and then subjected to the quenching and partitioning (Q&P) treatment. Microstructural investigation revealed that all the three steels were characterized by two phase microstructure comprising of lath martensite and austenite (interlath film and blocky morphology) as constituents. All the three steels contained almost similar volume fraction of retained austenite in it (0.15-0.18) irrespective of variation in the alloying content. The microstructure of the steel with lowest amounts of C, Si and highest amount of Mn (steel C) contained the finest martensite laths, high fraction of interlath austenite films with high austenite contiguity ratio in comparison with the other two steels (A and B). It also exhibited best strength (882 MPa)-toughness (188 MJ) combination. Austenite fraction analysis on the tensile tested samples suggested that in all three steels a significant percentage (35-50) from the total retained austenite undergo strain induced transformation to martensite. The study suggests that the Q&P steels are very sensitive to small variation in the chemical composition with respect to microstructural features and consequently mechanical properties, although fraction of microstructural constituents does not change significantly.