Effect of Temperatures and Strain Rates on the Average Size of Grains Refined by Dynamic Recrystallization for as-extruded 42CrMo Steel

In order to study the dynamic recrystallization (DRX) behavior of as-extruded 42CrMo high-strength steel, a series of isothermal upsetting experiments were carried out in a temperature range of 1123 similar to 1348 K and a strain rate range of 0.01 similar to 10 s(-1) on Gleeble 1500. It was found that DRX softening is more recognizable at higher temperatures and lower strain rates, and stress level increases with increasing strain rate and decreasing deformation temperature. The types of flow stress evolution were distinguished by the following three characteristics: WH followed by DRX, WH followed by DRV, and WH followed by no dynamic softening. At a fixed temperature, the average grain size refined by DRX linearly decreases with increasing strain rate in log scale. At a fixed strain rate, the average grain size remains almost constant below 1273 K, while it rapidly increases above 1273 K. At a larger strain rate, data set of grain sizes has a smaller standard deviation to the average size value, and data have tightly grouped. This indicates that as strain rate increases, the microstructure becomes more and more uniform. The relationships between the average grain size and Zener-Hollomon parameter were nonlinearly fitted by the equation D-A = 319.81202 - 13.6114 lnZ + 0.15322 (lnZ)(2). The results from this equation show that the average grain size decreases with increasing Zener-Hollomon parameter. On the plot of DA versus lnZ, the regions corresponding to DRV (lnZ 37.8) and DRX (lnZ 37.8) were clarified clearly.