Abstract
In view of the requirements for mechanical properties and service life above 650 °C, a high-Mn austenitic hot work die steel, instead of traditional martensitic hot work die steel such as H13, was developed in the present study. The effect of heat treatment on the microstructure and mechanical properties of the newly developed work die steel was studied. The results show that the microstructure of the high-Mn as-cast electroslag remelting (ESR) ingot is composed of γ-Fe, V(C,N), and Mo2C. V(C,N) is an irregular multilateral strip or slice shape with severe angles. Most eutectic Mo2C carbides are lamellar fish-skeleton-like, except for a few that are rod-shaped. With increasing solid solution time and temperature, the increased hardness caused by solid solution strengthening exceeds the effect of decreased hardness caused by grain size growth, but this trend is reversed later. As a result, the hardness of specimens after various solid solution heat treatments increases first and then decreases. The optimal combination of hardness and austenitic grain size can be obtained by soaking for 2 h at 1170 °C. The maximum Rockwell hardness (HRC) is 47.24 HRC, and the corresponding austenite average grain size is 58.4 μm. When the solid solution time is 3 h at 1230 °C, bimodality presented in the histogram of the austenite grain size as a result of further progress in secondary recrystallization. Compared with the single-stage aging, the maximum impact energy of the specimen after two-stage aging heat treatment was reached at 16.2 J and increased by 29.6%, while the hardness decreased by 1–2 HRC. After two-stage aging heat treatment, the hardness of steel reached the requirements of superior grade H13, and the maximum impact energy was 19.6% higher than that of superior grade H13, as specified in NADCA#207-2003.
Highlights
In recent years, with the development of manufacturing industry and advanced metal materials, the working conditions of tools and dies become increasingly severer
Current commercial martensitic die steels such as AISI H13 and AISI H21—which are widely used for hot extrusion dies, forging dies, and casting dies—have a limited working capacity above a working temperature of 650 ◦ C because of the decreasing strength [1]
It would be natural to assume that die materials—which do not possess the mentioned barrier within the service temperature range and have a strength exceeding that of highly heat-resistant martensitic steels at a temperature exceeding 700 ◦ C—should have an austenitic structure or belong to high-temperature alloys
Summary
With the development of manufacturing industry and advanced metal materials, the working conditions of tools and dies become increasingly severer. When the temperature exceeds 650 ◦ C, the strength of austenitic hot work die steel is much higher than that of martensitic type Another effective method for increasing steel quality is alloying with nitrogen. Nitrogen is an effective and economic substitute for some expensive elements: nickel, manganese, molybdenum, and tungsten According to these characteristics, high-nitrogen steels, especially high-nitrogen austenitic stainless steels, have been studied intensively in recent years [5,6,7]. It reported that the effects of nitrogen element increase the impact toughness of steel and high-temperature strength of hot work die steels [8,9,10]. The effect of heat treatment on the microstructures and mechanical properties in nitrogen-microalloyed high-Mn austenitic hot work die steel (HMAS-N) were examined first. An alternative hot work die steel—which is used for dies, needles, and washers—for the hot pressing of copper alloys is expected to serve above 700 ◦ C
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