New layering strategy and the gradient microstructure distribution of surface metamorphic layer for 8Cr4Mo4V steel by grinding

As high-alloy steel, 8Cr4Mo4V steel has been widely used in the field of high-speed aerospace bearings in recent years because of its excellent thermal strength, thermal fatigue, corrosion resistance, hardness, and thermal stability. In this article, the heat-treated high-strength tempered martensitic 8Cr4Mo4V steel is taken as the object of study with the investigation of advanced technology, such as X-ray diffractometer, scanning electron microscope, electron backscattered diffraction, etc. An in-depth and comprehensive discussion of the effects of different grinding parameters, such as depth, wheel linear velocity, cooling conditions, etc., on the microstructure of the surface metamorphic layer was carried out, and it was found that the surface metamorphic layer has an obvious microstructure gradient distribution. A new layering strategy for the metamorphic layers is proposed, from the surface to the interior in the following order: the large plastic deformation layer, the no acicular martensite layer, the mixed layer, and the matrix layer. Ultimately, the key to the formation of the large plastic deformation layer is the re-nucleation of surface austenite due to high temperatures, and the wheel assists the process of crystal orientation. Similarly, the mixed layer or the no acicular martensite layer appears as the tempered martensite reaches the phase transition temperature. The manufacturing parameters have a great influence on the gradient distribution of microstructure, and this study is intended to be a practical guide for the grinding of 8Cr4Mo4V steels. Graphical abstract