Method for making a valve-seat insert for internal combustion engines

A process for manufacturing a double layered valve-seat insert composed of a valve-seat layer and a base layer, comprising the steps of:

• preparing a mixture of raw materials for the valve-seat layer having a composition consisting essentially of, by weight, 4 to 8% Co, 0.5 to 1.5% Cr, 4 to 8% Mo, 1 to 3% Ni, 0.3 to 1.5% C, 0.2 to 0.6% Ca and the balance of Fe and inevitable impurities, said Co, Cr and Mo being present mainly in a form of a Co-Cr-Mo hard alloy and a Fe-Mo hard alloy dispersed in the Fe matrix of the valve seat layer;
• preparing separately and distinctly from the valve-seat raw material mixture a mixture of raw material for the base layer having a composition consisting essentially of, by weight, 11 to 15% Cr, 0.4 to 2.0% Mo, 0.05 to 0.3% C and the balance of Fe and inevitable impurities;
• pre-compacting the base layer mixture;
• compacting the base layer together with the valve-seat layer mixture to form a double layered green compact;
• sintering the green compact in a neutral or reducing atmosphere to form a double-layered sintered body;
• and
• infiltrating copper into the resulting double-layered sintered body by fusion, the fusion-infiltration of copper being carried out by heating the sintered body together with copper, so arranged that the valve-seat layer is in contact with copper, in a converted gas atmosphere at a temperature ranging from 1100.degree. to 1150.degree. C. to selectively infiltrate copper into the valve-seat layer by fusion.