Microstructure and Properties of Hardened 100CrMnSi6-4 Bearing Steel After Accelerated Carbide Spheroidization and Long-Duration Annealing

摘要

The experimental programme of accelerated carbide spheroidization deals with significant shortening of the time necessary for producing a steel microstructure consisting of ferritic matrix and globular carbides. The conventional way to obtain such structure is long-duration soft annealing after hot forming of the semi-product. Research of accelerated carbide spheroidization showed that it is possible to spheroidise lamellar pearlite by thermomechanical or thermal treatment within several minutes. The thermomechanical treatment consists of forming at temperatures around the A_(c1) temperature, whereas the thermal treatment is based on temperature cycling around the A_(c1) temperature. The bearing production process could be significantly shortened by eliminating the conventional long-duration soft annealing. Furthermore, the semiproducts would be processed one by one by using the accelerated carbide spheroidization process. It is not a batch process like conventional soft annealing, where a large quantity of material is annealed in a furnace at the same time. That allows monitoring and control of technological parameters for each particular piece and tailoring the process to processing small series of various materials. From the morphological point of view, accelerated carbide spheroidization produces microstructures very similar to those resulting from conventional soft annealing, but the carbide particles and the grain size of the matrix are significantly smaller. Finer microstructure results in higher hardness than in conventionally annealed steel. Finer microstructure also leads to more homogeneous and finer structure after final heat treatment-the hardening process. This fact indicates that mechanical properties of the final product depend on the prior structure produced by soft annealing. Finer carbides in the structure enhance the hardness and reduce the risk of crack initiation at the carbide-matrix interface. The microstructures and mechanical properties of the hardened steel after various kinds of soft annealing are compared in the present article.