The Effects of Copper and Sulphur on the Gas Carburizing of Sintered Iron

When gas carburizing a sintered iron component there is a tendency to form a deep, irregular case, owing to the inherent porosity allowing gas to permeate. Creation of a liquid phase during sintering may seal the pores and allow better control of carburizing. Addition of 1–2% copper to the powder mix is effective at relatively high compact densities. One worker has illustrated the greater effectiveness of additions of <1% sulphur. The investigation described here has shown that a better response to case hardening by gas carburizing is obtained when Cu and S are added together than by S addition alone. It appears that S melts, coats pore surfaces and begins to seal capillaries; then the Cu melts, adding bulk, and completes pore sealing. Having examined additions of up to 0·9%S and 6%Cu, the optimum specification is a compact of 6·9 Mg/m³ sintered density with 2%Cu, 0·3%S added to the powder mix. Gas carburization of compacts of this type for 2 h at 850°C, followed by oil quenching, gave a case of 0·5 mm depth and 800 HV_0·2 microhardness near the surface. The experiments were made under laboratory conditions; industrial application may be limited by possible S contamination of the sintering furnace atmosphere.