Quenching and Partitioning (Q&P) is a novel steel heat treatment to create microstructures containing martensite and retained austenite. In recent years, there has been a growing interest in applying Q&P treatments to martensitic stainless steels with the aim to create stainless steels with the mechanical properties of Advanced High-Strength Steels (AHSS). The development of Q&P stainless steels for lightweight structural applications could be a game changer for the automotive industry. The application of Q&P stainless steels can increase the service life of a car, reduce maintenance cost and contribute to lowering CO2 emissions. In this thesis, Q&P treatments with varying quenching temperatures were applied to two novel stainless steel alloys with composition 0.2C-0.35Si-0.7Mn-12.5Cr and 0.2C-0.35Si-3.0Mn-12.5Cr. The effect of quenching temperature and Mn content on the microstructure development was investigated using dilatometry, X-ray diffraction (XRD), optical microscopy, electron backscatter diffraction (EBSD), electron probe micro analysis (EPMA) and phase field simulations. It was found that Mn addition lowers the optimal quenching temperature and increases the maximum retained austenite fraction that can be obtained by Q&P treatment. Austenite phase fractions of approximately 0.22 and 0.3 were stabilized in the microstructures of the low-Mn and the high-Mn alloy, respectively. The carbon concentration of retained austenite increases with decreasing quenching temperature but is much lower than expected from the full partitioning assumption. The non-uniform distribution of primary martensite and untransformed austenite as well as carbide precipitation was seen as a means by which austenite enrichment and retention are reduced. Additionally, results from this study indicate that segregation of Mn and Cr affects the local Ms temperature and causes microstructural banding of primary martensite which leads to a non-uniform retained austenite distribution in the final microstructure.