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.

Effect of nitrogen on microstructure and corrosion behaviour of high nitrogen martensitic stainless steels manufactured by pressurized metallurgy was investigated by microscopy, electrochemical and spectroscopy analyses. Results indicated that increasing nitrogen content significantly enhanced the corrosion properties of martensitic stainless steels, while excess nitrogen deteriorated the corrosion resistance. The impacts of increased nitrogen content could be summarized as three aspects: the change of precipitation content and conversion of main precipitates from M23C6 to M2N; the enhanced protection performance of passive film by enrichment of Cr, especially Cr2O3 and CrN; the improved repassivation ability by increased nitrogen content in solid solution.

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The ablative properties of elastomeric insulations are often inadequate for solid rocket motor (SRM) applications. These materials exhibit relatively high erosion rates during the operation of an SRM unless the charred insulation layers are reinforced with suitable fibre fillers. As alternatives to traditional synthetic rubber materials, flexible semi-inorganic rubbers such as polyphosphazene elastomers are now used as state-of-the-art heat-shielding materials. We have successfully managed to prepare a poly(diaryloxyphosphazene) elastomer (PDPP) as well as some insulation materials that are free of any fibrous fillers (with only the addition of inorganic oxides, such as fumed silica and zinc oxide). These polyphosphazene insulations exhibit excellent linear ablation rates (0.08 mm s−1 after a 20 s ablation test) as compared to synthetic organic rubbers. In addition, integrated and rigid charred layers without noticeable swells are formed on the surfaces of the matrices resulting in ‘coral fleece-like’ hollow microtubes, which show better ablative resistance performance than do traditional insulations. The pyrolysis products of PDPP have been characterized by pyrolysis gas chromatography mass spectrometry and the mechanism of its decomposition is also discussed.

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