Carbide precipitation, coarsening and hardness variations have been investigated for a quenched and tempered Dievar steel by SEM, TEM, STEM, HRTEM, XRD and Rockwell hardness testing. The carbide precipitation sequence has been ascertained as: M8C7 + bainitic/auto-tempered carbides → M8C7 + M2C + M7C3 → M8C7+ M7C3 +M23C6 → M8C7 + M7C3 + M23C6 + M6C. Boundary carbides coarsen obviously faster than matrix ones. Hardness variations originating from the carbide evolution have been qualified. A fine-dispersion of M2C carbides leads to a high hardness tempered at 550°C, whereas the hardness drops drastically for tempering at 600/650°C predominately due to the dissolution of unstable M2C and the coarsening of carbides.
Various surface treatments have been developed on Ti and its alloy to improve their tribological properties. However, for some components, only specific surfaces need to be hardened rather than the whole surfaces. This study develops a duplex heat treatment including selective surface hardening by pre-painting TiO2 powder and thermal oxidation. It is found TiO2 powdered layer can be used as an oxygen reservoir for boost diffusion treatment at a temperature of 800–900°C. The abundance of oxygen provides high potential and accelerates the diffusion process, deeply hardening the surface case. Moreover, the duplex treatment is optimal for selectively hardened surfaces with high load-bearing capacity and much improved tribological properties. Pre-painting the surfaces that need to be hardened is easily operated and has good potential to be applied in industry.
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