Microstructures and mechanical properties of austenite in melt-quenched Fe-Ni-C alloys.

Abstract

By the rapid quenching technique, metastable alloys with an austenite single phase and a duplex structure of austenite and cementite have been found in Fe-Ni-C ternary system. Their formation ranges are limited to about 1.2-2.0wt%C and 5-30wt%Ni for the austenite single phase and to about 2.0-3.2wt%C and 10-40wt%Ni for the duplex phases. These austenitic alloys have ultra fine grains of about 0.4μm in diameter and exhibit high hardness and strength as well as good elongation. The Vickers hardness, 0.2% proof stress and tensile fracture strength increase with the amounts of carbon and nickel, and the highest values attained are about 590DPN, 1300MPa and 1750MPa, respectively. The elongation increases with decrease in carbon or nickel content and is about 7% for Fe-20wt%Ni-1.8wt%C alloy. These high hardness and strength are due to the grain size refinement, the solid solution hardening by carbon and/or the precipitation hardening of fine M3C carbide. In addition, the changes in the microstructures and mechanical properties during tempering have been examined. It has been observed that there is no change in the austenite structure except for the precipitation of cementite from the austenite phase and the strength and elongation decrease with increasing tempering temperature because of the grain coarsening of the austenite and cementite phases and the reduction in solid solubility of carbon in austenite.