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.