Study on microstructure and properties of 30CrMo after salt bath nitriding

Abstract

30CrMo steel, a main material of sucker rod, was modified by salt bath nitriding at 580°C and 630°C for 30min, 60min, 90min, 120min and 150min respectively in this study. Characterization of the modified surface layers was made by means of metallographic test, brittleness test, Vickers micro-hardness test, wear test, XRD, SEM and EDS analysis. The result shows that the layer is composed of an oxide film, a loose layer, a white layer and a diffusion layer after salt bath treatment at 580°C, while the expanded austenite layer is formed at 630°C. The layer thickness is increased with the treated temperature and time. The brittleness of the layer at 630°C is generally poorer than that at 580°C. And the white layer is mainly composed of Fe2N and Fe3N through the XRD analysis the layer thickness at 630°C is higher than that at 580°C, and the maximum value of the layer thickness is 0.55mm at 630°C for 120min. The wear resistance was improved by salt bath nitriding and the sample treated at 630°C for 150 minutes had the best abrasion-resistant capability and according to the wear surface, the wear mechanism has changed for different nitriding temperature and time. The content of C, N, O and Fe elements from the surface to the core has changed, whose trend are not the same.