Effect of Warm Deformation Parameters and Cooling Rates on the Recrystallization Transformation Microstructure in 40Cr Steel

The effect of warm deformation parameters and cooling rates on transformation of the microstructure of 40Cr steel was investigated using both single- and double-pass compression tests. The compression tests were performed at temperatures ranging from 800 to 700 °C followed by controlled cooling at rates from 0.1 to 5 °C/s. All of the final transformation microstructures were observed by optical microscopy, and they were mainly composed of ferritic and pearlitic microstructures. For further quantitative analysis, a program based on image processing technology was developed to automatically calculate the volume fractions of ferrite and pearlite. After qualitative and quantitative analysis of the final microstructures, the results showed the following. The ferrite volume fraction significantly increased with increasing strain for the same deformation temperature. The strain rate had almost no effect on the ferrite fraction at the lower cooling rates. The ferrite fraction markedly increased with increasing strain rate when the cooling rate was greater than 1 °C/s. The ferrite fractions after single- and double-pass compression were 27-49 and 24-43% for a total strain of 0.7. The cooling rate after deformation had a remarkable effect on the microstructure morphology, and the ferrite volume fraction significantly decreased with increasing cooling rate.