Effect of Cryogenic Treatment on the Microstructure Modification of SKH51 Steel

This research aimed to investigate the microstructure modification mechanism used to improve the hardness and wear resistance of SKH51 steel. The cryogenic treatment (CT), including both shallow cryogenic treatment (SCT) and deep cryogenic treatment (DCT), was used to modify the microstructure of SKH51 steel in this research. The effect of short and long holding time (12 and 36 h) in CT was studied. The microstructures were evaluated by using a light optical microscopy (LOM) and a scanning electron microscopy (SEM). The phase identifications of the matrix, carbides, and a-parameter of the matrix were analyzed by using X-ray diffraction (XRD). The M6C and MC carbides size, aspect ratio, and distribution were analyzed using digimizer image analysis software on the SEM micrographs. Micro-Vickers were employed to evaluate the hardness of the targeted samples. Wear tests were performed by using a 6 mm diameter WC ball as the indenter and 5-N-constant load with a ball-on-disk wear tester. The results suggested that the increase of the secondary carbide was caused by the contraction and expansion phenomena of the matrix’s lattice, forcing the carbon atom out and acting as the carbide nucleation. The influence of holding time in the SCT and DCT regions was different. For the SCT, increasing the holding time increased the volume’s fraction of MC carbide. Conversely, the M6C carbide size grew with increasing holding time in the DCT region, while no significant increase in the number of MC carbide was observed. The cryogenic treatment was found to increase the volume fraction of the MC carbide by up to 10% compared to the conventional heat treatment (CHT) condition in the SCT region (both 12 and 36 h) and DCT with 12 h holding time. Due to the microstructure modification, it was found that the cryogenic treatment can improve material hardness and lead to an increase in the wear resistance of SKH51 by up to 70% compared to the CHT treated material. This was due to the increase in the compressive residual stress, precipitation of the MC, and growth of the M6C primary carbide.