INVESTIGATION OF MANUFACTURING CONDITIONS OF POWDER METALLURGY VALVE SEAT WITH CONTROL OF MICROSTRUCTURE AND MECHANICAL PROPERTIES

In the present work, tow kinds of steel powder were (Distaloy HP-1 & Ultrapac-LA) selected and subjected to power metallurgy processing. For Ultrapac-LA a heterogeneous microstructure consisting of tempered martensite, nickel-rich ferrite, divorced pearlite and nickel-rich regions surrounding pores is observed. For Distaloy HP-1, in slow cooling rates (0.4˚C.s-1) the amounts of martensite and bainite varies between 50-60% and 30-40%, however, for fast cooling rates (1.2˚C.s-1) the variation is between 70-80% and 10-20%. The machanical properties of the prepared sample were studied with controlled production conditions such as cooling rate and heat treatment. increasing porosity plays an important role in the tensile stress and fatigue atrength. The ultimate tensile strength and fatigue strength were more than 1000 MPa and 400 MPa in this research (for Distaloy HP-1, 7.2 g.cm-3, 0.5% carbon content and fast cooling rates). Macroscopic examination of the fracture surfaces for all speciment revealed that fatigue crack growth and final fracture regions were brittle and without noticeable permanent deformation. The final fracture for all were very similar, final fracture in thses two material conditions revealed brittle macrobehaviour and ductile microbelaviour.