Structure-property correlation in Al-diffusion coated steels

Abstract

Mild steel and stainless steel samples were pack-calorized under varying conditions and the calorized specimens were characterized by optical microscopy, microhardness data and X-ray diffraction. The mechanical, fatigue and creep resistance properties were also determined. The concept of favourable and unfavourable phases has been introduced, based on the Pilling-Bedworth ratio for compatibility with the base. In the Fe-Al system, only FeAl and Fe3Al have been found to satisfy this criterion and are therefore desired. The formation of phases such as FeAl3 and Fe2Al5 is considered undesirable. The process parameters have therefore been varied and optimized to form only favourable phases. It is found that a lower AI activity pack such as Fe-35 wt% Al gives better results than a higher Al activity pack. The optimum temperature and time of calorizing for both mild and stainless steel are found to be 900 °C for 2 to 4 h. Physical characteristics of the pack such as small, uniform size powder of homogeneous composition are also important to give reproducible results. The mechanical properties of selected calorized (900 °C, 2 and 4 h) mild steel specimens using an Fe-62 wt% Al pack show that there is an appreciable rise (more than 100%) in surface hardness and only a small (around 5%) fall in reduction in area while the percentage elongation remains unaltered on calorizing. There is a considerable fall in high-cycle fatigue properties under push-pull vibration while, under a three-point loading high-frequency fatigue test, the probability to failure and crack initiation reveal an improvement.