| Valve seat inserts should have a good wear resistance because they are used in the valve train area of internal combustion engines. Currently, the most of the valve seats are being manufactured by P/M method having the three steps of sintering, Cu-infiltrating and heat treatment. In this study, the effect of Co contents on microstructures and mechanical properties have been investigated at each steps. On sintering specimens, increase of Co content results in decrease of the hardness because of decrease of the ratio of martensite by decrease of diffusion of Cr-hard particles. On Cu-infiltrating specimens, the decrease of radial crushing strength with the increase of Co is resulted from the residual pores by Kirkendall`s effect. And after heat treatment, the increase of Co results in decrease the ratio of the residual austenite of the matrix because of over-diffusion of the Cr. According to the result of the engine simulation test, matrix can not be hardened without the addition of Co-hard particles, the excess addition of Co-hard particles reduce the wear resistance by the increase of residual pores. Therefore, the appropriate amount of Co-hard particles is 6 wt%(Co 3.6 wt%). |
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