In this paper, a novel method of pressurized metallurgy technology was proposed to improve cast structure of M42 high speed steel (HSS). The effect of solidification pressure (0.1, 1 and 2 MPa) on the cast structure of M42 HSS was investigated by means of experimental analysis and calculation of Thermo-Calc and DICTRA software. Increasing solidification pressure can obviously enhance cooling rate by improving interfacial heat transfer coefficient, which results in some remarkable improvement of the cast structure of M42 HSS. Firstly, the primary/secondary dendrite arm spacing and the average thickness of eutectic ledeburite reduce, which means dendrite structure is refined and eutectic ledeburite more homogeneously distributes with smaller size. Secondly, increasing solidification pressure, the volume fraction of M 6 C carbides decreases obviously and that of M 2 C increases correspondingly. And the morphology of M 2 C carbide changes from larger size lamellar and straight-rod shape into smaller size curved-rod morphology under higher solidification pressure due to larger nucleation number and overgrowth of γ, indicating that carbides are refined and distribute more uniformly. At last, higher solidification pressure is beneficial to reduce the lamellar spacing of M 2 C carbide and make compositions distribute more homogeneously.

This study systematically investigated the influence of high nitrogen (N) addition (0.205 wt.%) on microstructure and mechanical properties of as-cast M42 high speed steel. The results demonstrate that the conventional and high-nitrogen M42 cast ingots are mainly composed of martensite, retained austenite and various precipitates (M 2 C, M 6 C as well as MC in M42 cast ingot or M(C, N) in M42N cast ingot). The addition of N could increase the retained austenite content, trigger the transformation of MC to M(C, N), favor the formation of M 2 C at the expense of M 6 C, and improve the distribution uniformity of M 6 C at the macroscopic scale. Moreover, the addition of N could lead to the reduction of the secondary dendrite arm spacing as well as the decrease of the thickness and area fraction of eutectic carbides, and improve the distribution uniformity of eutectic carbides at the microscopic scale. The M(C, N) particles form directly from the liquid phase prior to the formation of primary austenite, which could act as the heterogeneous nuclei of primary austenite and thus promote the refinement of the as-cast microstructure. The addition of N slightly decreases the macro-hardness and ultimate compression strength of the cast ingot but increases its ductility, which could be ascribed to the increase of retained austenite content and the reduction in the amount of eutectic carbides. Therefore, high N addition can significantly improve the as-cast microstructure of M42 high speed steel, which is promising for the further enhancement of the mechanical property and service life of the final product.KEY WORDS: M42 high speed steel; pressurized metallurgy; nitrogen; as-cast microstructure; precipitates; mechanical property. N i J J J J J D

Hot deformation behavior of high nitrogen martensitic stainless steel 30Cr15Mo1N is investigated by isothermal compression tests at the temperature range of 900-1250 8C and the strain rate range of 0.01-10 s À1 . The results indicate that the activation energy of this alloy (503.5 kJ mol À1 ) is higher than that of conventional martensitic stainless steels. The developed Arrhenius-type constitutive equation considering strain compensation can predict flow stress of 30Cr15Mo1N with good consistency. The critical conditions for initiation of dynamic recrystallization (DRX) are determined based on the strain hardening rate versus flow stress curves. The DRX is inhibited and grain size decrease with increasing strain rate from 0.01 to 1 s À1 . However, the fraction of DRX increased and grain size coarsened when strain rate is further increased to 10 s À1 . Cr-rich M 23 C 6 and M 2 N precipitated at grain boundaries at lower temperatures, which can effectively pin grain boundaries and, thereby retard or even inhibit the process of DRX. In addition, dislocations tangled and piled up around precipitates, thereby hindering dislocation movement. The influence of precipitates on DRX can be eliminated due to their dissolution at higher temperatures.

Hot deformation behavior and microstructure evolution of 2707 hyper duplex stainless steel (HDSS) were investigated through hot compression tests in the temperature range of 900-1250 • C and strain rate range of 0.01-10 s −1 . The results showed that the flow behavior strongly depended on strain rate and temperature, and flow stress increased with increasing strain rate and decreasing temperature. At lower temperatures, many precipitates appeared in ferrite and distributed along the deformation direction, which could restrain processing of discontinuous dynamic recrystallization (DRX) because of pinning grain boundaries. When the temperature increased to 1150 • C, the leading softening behaviors were dynamic recovery (DRV) in ferrite and discontinuous DRX in austenite. When the temperature reached 1250 • C, softening behavior was mainly DRV in ferrite. The increase of strain rate was conducive to the occurrence of discontinuous DRX in austenite. A constitutive equation at peak strain was established and the results indicated that 2707 HDSS had a higher Q value (569.279 kJ·mol −1 ) than other traditional duplex stainless steels due to higher content of Cr, Mo, Ni, and N. Constitutive modeling considering strain was developed to model the hot deformation behavior of 2707 HDSS more accurately, and the correlation coefficient and average absolute relative error were 0.992 and 5.22%, respectively.

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