The purpose of my diploma thesis was to optimise the process of induction hardening K313 tool steel with 5 wt. % chromium, using a single inductor with 400 Hz frequency for the production of forged rolls for cold rolling.
In the tests, I described and analyzed heat treatment based on the results of hardenability tests (sensitivity-to-grain growth) and tempering diagrams. I first performed experiments on samples with dimensions 20 mm × 20 mm × 80 mm, which have been heated in an electric resistance chamber furnace and then hardened with air to room temperature. The results from these tests provide the basic understanding of conventional heat treatment.
The experiments for induction hardening were performed on samples, measuring Φ 240 mm × 600 mm. All tests were performed on the induction machine BIKS 750, which is designed for induction hardening of steel rolls with single or double inductors. We analyzed the microstructure and measured the content of retained austenite on the samples from the hardenability test to determine the optimal austenitisation temperature, which we used to make tempering diagrams. We then measured the hardness and proportion of retained austenite on the samples for the tempering diagrams. The same tests were performed on samples, that were cooled to –130 °C after being hardened. I also described and analyzed mechanical properties (toughness) and microstructure, which is required to control the performance of heat treatment.
From the results we conclude, that the most suitable austenitisation temperatures for K313 steel are between 1000 and 1040 °C. To achieve hardness up to 62 HRC the most suitable technology is without cryogenic treatment, and to reach hardness above 62 HRC it is better to use cryogenic treatment.
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