Influence of purity and forging temperature on the microstructure of high-purity iron

The influence of purity and forging temperature on the microstructure of high-purity iron has been investigated by using two kinds of high-purity iron; Iron-K of 99.981 mass% purity and Iron-A of 99.993 mass% purity. Both of high-purity irons could be forged to more than 80% reduction in height at room temperature without cracking. The hardness and the microstructure of as-forged irons are not uniform. Therefore, the behavior of recovery and recrystallization of forged irons are not uniform through the specimens. The microstructure of forged iron is strongly affected by the purity and the forging temperature. It is emphasized that giant columnar grains grow in parallel to the forging direction in Iron-A forged at 1263 K, above the alpha-gamma transformation temperature. However, uni-axual grains with random orientation grow though Iron-K forging at 1263 K. These recrystallization behavior relates to the large mobilities of alpha/gamma interface and grain boundaries due to the effect of purification, although the formation mechanism is not obvious.