Fe-(0.5-4)mass%Cu alloys were cooled from γ field under various cooling conditions (water-quenching, air-cooling, furnace-cooling) and phase transformation mechanisms were investigated by means of microstructural examinations and dilatometry. In all of the cooling conditions, hardness of Fe-Cu alloys becomes higher with increasing Cu content. Effect of cooling conditions on hardness tends to be significant in alloys with Cu more than 1 mass%. TEM observation shows that strength of alloys depends not only on a difference of matrix; martensite or ferrite, but also on dispersion of ε-Cu particles. In the case of air-cooling for an Fe-4mass%Cu alloy, the alloy undergoes preferentially γ→α massive transformation and then ε-Cu particles precipitates finely within the massive ferrite matrix. This leads to a large strengthening with a moderate ductility. Strength of Cu bearing steels can easily be controlled by varying cooling condition after a solution-treatment: Steels are soft enough to be deformed and machined after furnace-cooling, but strengthened after resolution-treatment followed by air-cooling.
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Kimura et al. (1997) studied this question.
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