CHARACTERIZATION OF SECONDARY CARBIDE IN MARTENSITIC STAINLESS STEEL AFTER DEEP-CRYOGENIC TREATMENT PROCESSES

摘要

The carbide in X90CrMoV18 martensitic stainless steel governs both hardness and fracture toughness, which are the key performance of tool steels. Optimizing these opposite properties results in longer tool life by increasing the wear resistance. Deep-Cryogenic Treatment (DCT) has been introduced in this research to fully transform the Retained Austenite (RA) into martensite and/or secondary carbide. The precipitation of M7C3 (Primary Carbide; PC) increases only the hardness while M_(23)C_6 (Secondary Carbide; SC) improves the toughness. Hence, the increase in M23C6/M7C3 carbide ratio will enhance the tool life and the role of DCT in this regard is our research focus. The carbide type, size and shape are observed by Scanning Electron Microscopy (SEM) and Light Optical Microscopy (LOM). The synchrotron Grazing Incident X-Ray Diffraction (GIXRD) is applied for investigating the change in c/a ratio of martensite matrix and carbide identification. The synchrotron X-ray Absorption Spectroscopy (XAS) is applied to the quantification of precipitation. The latter is quantified by XAS with Linear Combination Fit (LCF) of the Extended X-ray Absorption Fine Structure (EXAFS) spectra. A suitable DCT condition increases 4.2 % in the hardness while surface toughness is raised to 150 %. The carbon atoms in martensite matrix are forced out from the lattice resulting in larger amount of SC. The average diameter of the carbide decreases by 15 % after DCT.