Increase of heat resistance and properties of die steel with control of austenitic transformation during operation

Steels with adjustable austenitic transformation during the exploitation (RAPE) are designed for applied in the temperature range of 800 – 950 °C, which corresponds to the austenitic structure, but at the lower temperature range (630 – 650 °C) tempering fragility of the second kind occurs, which makes it difficult to operate. This is inconvenient when the die tool works in a wide range of temperatures during deformation of various metals and alloys. An improved steel composition (4H4N5M3F2) (RAPE) for press molds for hot pressing of copper and its alloys at temperatures of 630 – 650 °C is proposed. Steel was obtained by electroslag remelting, it was heat treated and the properties were determined. It is shown that the heat resistance of the investigated steel is higher by 2 HRC than the heat resistance of steels grade 4H5MF1S and 3H3M3F, which are used under the same operating conditions. The yield strength and impact strength of steel 4H4N5M4F2 considerably exceeds the characteristics of steel 3H3M3F (high –grade rolled products). The annealing temperature limits, which allow obtaining the necessary technological properties of steel, were established. It is shown that the steel structure consists of two areas: dark (obviously it is released martensite) and light (the area with a lower content of the carbide component). With an increase in tempering temperature from 640 to 660 °C, the hardness (HV) of the dark and light components decreases from 478.6 to 459.8 and from 427.8 to 376.0, respectively. The heat resistance of the investigated steel after quenching and tempering in optimal conditions increased to 650 °C (41 HRC).