Precipitation of copper in proeutectoid cementite in steel

The morphology and mechanism of Cu precipitation in grain boundary allotriomorphs of proeutectoid cementite forming isothermally inaustenite in two Fe-10Mn-3Cu-0.8C and Fe-lOMn- lCu-0.8C (nominal wt%) alloys are being studied. These particular alloys can be partially decomposed to the proeutectoid and eutectoid phases recognisable in ferritic steels, but possess the advantage that unlike ferritic steels the parent austenite phase is retained after cooling to room temperature thus facilitating studies of the decomposition reactions.A 50 g ingot of each experimental alloy was argon arc melted using high purity materials and homogenised. Samples were rolled, swaged and machined to 3mm diameter rod, solution treated at 1200 °C for 1 hr and quenched in water. Specimens were then solutionised at 1200°C for 10 min and isothermally transformed at 615 °C for 4 hrs. Methods for the preparation of thin foils for TEM are given elsewhere.The heat treatment resulted in the formation of grain boundary allotriomorphs of cementite in austenite (Fig 1). TEM confirmed that the cementite allotriomorphs formed with the Pitsch orientation relationship to one of the austenite grains at the grain boundary (Fig 2c ) consistent with a previous analysis.