The microstructural changes in secondary carbides during spheroidized annealing (SA) and quenching and tempering (QT) processes in 60Cr16MoMA martensitic stainless steel has been comprehensively studied. Using scanning electron microscope, X-ray diffraction, inductively coupled plasma-atomic emission spectrometry, and transmission electron microscopy, a substantial dissolution of submicron-sized M23C6 carbides after QT treatment was observed, leading to an increase in the mean carbides diameter. The highest solubility occurs at a 0.5 μm diameter, and carbide roundness improves during QT. Notably, the transformation of M7C3 carbides into M23C6 carbides was observed in QT samples through a multi-site in-situ nucleation mechanism. Additionally, it has been observed that Nb-rich MC carbides effectively restrict the growth of alloy carbides during heat treatment.