| Martensitic stainless steel is commonly used in a variety of industrial applications such as steam turbine blade, medical instrument, and measuring appliance, etc. due to its high hardness, better wear and moderate corrosion resistances. However, for those engineering components suffering from deterioration by mechanical or chemical effects, the surface strength and tribological properties of martensitic stainless steel after heat treatment are still deficient and not enough to meet the performance requirement in the harsh environment. In this paper, A Zr-alloyed layer was prepared on the surface of 4Cr13 and 8Cr17 stainless steels by double glow plasma surface alloying technique. Effects of plasma alloying temperature and time on the microstructure, composition and phase structure of the Zr-alloyed layer were studied, and the formation mechanisms of the alloying layer was studied as well. The hardness, nano-indentation behavior and tribological properties of the Zr-alloyed layer were analyzed. At the same time, wear mechanisms of Zr-alloyed layer after different alloying time were also investigated. The main experimental results show as follows:(1) A compact Zr-alloyed layer can be fabricated on the surface of 4Cr13 and 8Cr17 stainless steel by plasma Zr-alloying treatment. The Zr-alloyed layer is mainly composed of the outer Zr-rich layer, rich-ZrC middle layer and Fe-Cr-Zr-C diffusion layer which adheres strongly to the substrate.(2) The thickness of the Zr-alloyed layer formed on the surface of 4Cr13 and 8Cr17 steels increases with alloying temperature and time. With temperature elevated from 900?C to 1000?C, the alloyed thickness gradually increases from 16μm to 23μm for 4Cr13 steel and 17μm to 24μm for 8Cr17 steel. At higher alloying temperature, the diffusion coefficients of atoms increases, resulting in increased thickness of alloying layer. When increasing alloying time from 0.5h to 4h, the alloying depth of the Zr-alloyed 4Cr13 steel increases from 3μm to 24μm, and from 3μm to 27μm for Zr-alloyed 8Cr17 steel. The Zr-alloying thickness of both stainless steel is liner as alloying time, while for the ZrC-rich layer, the thickness vs. alloying time is basically parabola.(3) The hardness of the Zr-rich layer and ZrC-rich layer is about 608HV0.025 and 844HV0.025 for Zr-allyed 4Cr13 steel, 618HV0.025 and 863HV0.025 for Zr-alloyed 8Cr17 steel, which are higher than that of untreated substrate. The high hardness of the Zr-alloyed steels is attributed to the substitution of Zr for Fe and dispersion strengthening of ZrFe2 and ZrC.(4) The values of hardness and elastic modulus obtained by nano-indentation are higher, showing a good mechanical properties of Zr-alloyed 4Cr13 and 8Cr17 surfaces. At the maximum load of 5mN, the maximum indentation depth of the test points, corresponding to the ZrC-rich layer is the lowest, leading to an improved bearing capacity. The closer distribution of elastic modulus in the alloying layer is beneficial for deformation compatibility and bonding strength.(5) The friction coefficient and specific wear rate of Zr-alloyed 4Cr13 and 8Cr17 steels after different alloying times are lower than those of the untreated 4Cr13 and 8Cr17 steels. Especially the improvement of tribological properties after alloying at 950?C/0.5h is significant in that the friction coefficient for both steels is only 0.3 and the specific wear rate of alloyed 4Cr13 and 8Cr17 steels are 1/6 and 1/4 of substrate, respectively, indicating a better combination of wear resistance and antifriction performance. The formation of ZrC and solid solution strengthening of Zr are the main reason for the increase of the wear resistance. |
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