EFFECT OF TEMPERING TEMPERATURE ON SINTERED PROPERTIES OF SINTER HARDENED PM STEELS

The development of low alloy steel powders specifically designed for s inter hardening applications enables components to reach high sintered strength and apparent hardness in sintering furnaces equipped with ei ther conventional or rapid cooling units. By adjusting the mix formula tion, the sintered properties can be tailored to optimise both strengt h and apparent hardness. As untempered martensite is a fragile structu re, sinter hardened parts are generally given a tempering treatment to restore strength and improve toughness. However, this treatment also affects the apparent hardness of PM parts. A study was carried out to evaluate the effect of tempering temperature on mechanical properties of sinter hardened steels containing 2% copper and either 0.65 or 0.80 % carbon. Testpieces were pressed to 6.8 g cm(-3) and sintered at 1120 degrees C in a production furnace equipped with a conventional coolin g unit. Three tempering temperatures, 150, 175, and 205 degrees C, wer e selected for the test programme. A reduction in apparent hardness of about 10 HRC (85 HV) was observed after tempering at 150 degrees C bu t tensile properties were significantly increased, particularly at hig h carbon concentration.