Effect of quench cooling rate on residual stress, microstructure and mechanical property of an Fe–6.5Si alloy

The effects of cooling rate on residual stress, microstructure and bending mechanical properties of an Fe–6.5Si alloy were investigated using various quenching mediums. The results show that the surface residual tensile stress increases and the residual compressive stress decreases with an increase in the cooling rate. Compared with as-cast Fe–6.5Si alloy, the maximum residual tensile stress of the oil, water and saltwater quenched Fe–6.5Si alloy are increased by 75%, 111% and 122%, respectively. The number and size of the precipitated phase in the quenched Fe–6.5Si alloy decrease remarkably, but the cooling rate (74–375 °C/s) has little influence on the number and morphology of precipitated phase. With increasing the cooling rate, the degree of order decreases, and the sizes of the ordered phases in the as-cast, oil and salt water quenched Fe–6.5Si alloy are 1–3 μm, 20–200 nm and 5–50 nm, respectively. The oil-quenched Fe–6.5Si alloy exhibits obvious plasticity at room temperature. The average bending fracture deflection increases from 0.60 mm (as-cast specimen) to 1.12 mm (oil-quenched specimen). Since the surface residual tensile stress increases with the cooling rate, the average bending fracture deflection of the water and saltwater quenched Fe–6.5Si alloy are less than that of the oil-quenched Fe–6.5Si alloy, being 0.90 and 0.78 mm, respectively.

► Plasticity of Fe–6.5Si alloy mainly depends on microstructure and residual stress. ► Residual tensile stress of quenched alloy increases with increasing cooling rate. ► Oil quench process can effectively improve the plasticity of as-cast alloy. ► Overhigh quenching rate is unfavorable for improving the plasticity of the alloy.