Precipitation of copper particles during pearlite transformation

It is known that e-Cu phase precipitates within fcrrite phase independently on ccmcntitc phase in steel. Therefore, Cu addition should be effective for strengthening carbon steels. In this study, Cu was added to Fc-0.8%C eutcctoid alloy up to 2mass% in order to obtain high strength pcarlitic steel. The effect of Cu on pearlitic transformation behavior and the detail of pcarlitic structure were also investigated. First of all, isothermal heat treatment was performed after austenitizing in the Cu bearing eutectoid steels to form pearlitic structure. Microstructural observations revealed that the pcarlitic transformation is significantly retarded by the addition of Cu, while the morphology of pearlite, such as lamellar spacing and block size, arc hardly influenced by the Cu addition. As a result of chemical analysis for the ccmcntitc phase, it was found that the short range redistribution of Cu atoms had been occurred from ccmcntitc to fcrrite through the intcrfacial diffusion at (pearlite / austcnitc) interface during the pcarlitic transformation. The Cu precipitated at the (fcrrite / austcnite) interfaces during pcarlitic transformation as well as within the ferrite phase after the transformation. This resulted in the age hardening during isothermal heat treatment in the eutectoid steel containing 2mass% Cu. The yield strength and tensile strength of the pearlite of Cu bearing eutectoid steels depended on not only lamellar spacing but also the dispersion of Cu particles precipitated in the ferrite phase. The 0.2% proof stress was given by the addition of precipitation strengthening in fcrrite phase to the strength of Cu-free pearlite. For example, the 2mass% Cu addition increases the 0.2% proof stress of pearlitic steel by approximately 250MPa at a maximum.