Heat Treatment and Wear of High-Alloyed Irons

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The heat treatment and the wear resistance under unlubricated rolling-and-sliding condition and lubricated rolling condition of high-alloyed irons (the white iron of high-speed steel type) were investigated. The carbide forming elements, 6mass%Cr, 2mass%Mo, 4mass%W and 6mass%V,were added to irons and the carbon content was varied from 1.5mass% to 2.5mass%. 4mass% or 8mass%Co was also added to the 2.0mass%C iron to strengthen the matrix structure. These irons were melted in a high-frequency furnace and cast in a sand mold. Results were summarized as follows.(1) The matrix structure transformed to martensite by quenching of austenitizing temperature of 1373K and cooling rate 5-10K/min, and the secondary temper hardening occurred sufficiently by tempering at 823K.The secondary temper hardening was insufficient in the case of quenching from 1273K.(2) The amount of carbides increased and the as-cast hardness became high with the carbon content of iron. However, the hardness of 2.5%C iron was lower than 2.0%C iron at the secondary temper hardening. This is due to that carbon and alloying elements in the martensite decreased with the carbon of iron.(3) By addition of Co to iron, its quenching properties naturally deteriorated and the hardness of the quenched atructure decreased, but MC type carbide became granular and M_2C,M_6C and M_7C_3 being small : besides, the secondary temper hardening appeared remakably and the retained austenite became stable.(4) The wear resistance under unlubricated rolling-and-sliding was considerably improved by adding 8mass%Co to the iron.(5) The rolling fatigue life of the white iron of high-speed steel type under lubrication was about 10 times as long as high Cr white iron. The white iron became particularly excellent by adding 8mass%Co.(6) The addition of Co improved the wear resistance of the white iron of high-speed steel type. However, a small amount of Co addition was ineffective and Co over 8mass% should be necesarry to improve.