Hot Ductility and High Temperature Microstructure of High Purity Iron Alloys

Abstract
The inherent properties of metals are affected by impurity elements, sometimes strongly. There are many brittle phenomena in iron and its alloys due to the harmful effect of trace impurities such as sulphur, phosphorus, hydrogen and so on. On the other hand, a large number of alloying elements also embrittle iron due to the transformation and precipitation of secondary phase. For example, the ductility of Fe-Cr alloy decreases with the increase in chromium content, although the strength and the corrosion resistance increase with chromium content. In Fe-Cr alloy containing high chromium, 475°C embrittlement and σ-phase embrittlement are well known. An Fe-50mass%Cr alloy of conventional purity is extremely brittle due to the formation of σ-phase. However, we found the highly purified alloy is essentially ductile. In the workshop of UHPM-94, the experimental results on the ductility of Fe-50mass%Cr alloy were presented and discussed. In this research, the effect of purification on the hot ductility of high purity Fe-18mass%Cr and Fe-50mass%Cr alloys was investigated by tensile testing at high temperature. It was found that the ductility of Fe-18mass%Cr alloy is remarkably improved by purification, especially by the reduction of interstitial impurities such as carbon and nitrogen. The highly-purified Fe-50mass%Cr alloy has astonishing ductility at the temperature range between room temperature and 1073K. Also in a high purity Fe-50mass%Cr alloy, the formation of the σ-phase was not observed during ageing for 1000h at 973K. These results are also very important for the development of high-performance Fe-Cr alloys and of the manufacturing process. Consequently, purification technology is very useful for progress in metal science.