Microstructure and Mechanical Properties of High-Strength Steel with Improved Machinability

A structural steel (grade 40KhGM) with improved machinability was studied. The steel contains an increased amount of sulfur and was modified with calcium during metallurgical production stage. A qualitative analysis of the size of nonmetallic inclusions present in the studied steel was performed in comparison with conventional steel (40KhGMA) with low sulfur and calcium contents. A dilatometric study was carried out to compare the specifics of supercooled austenite transformation and hardenability of steel with improved machinability and standard steels. The corresponding continuous cooling transformation (CCT) diagrams of austenite transformations were plotted. The dependencies of the mechanical properties of the studied steels with various impurity contents on the tempering temperature have been established. According to the results, calcium-treated steel with improved machinability is characterized by a similar set of mechanical properties as conventional low-sulfur steels. This implies that the steel with improved machinability can be successfully used for the production of critical and high-strength parts using automated lines and machines.