Effect of aluminum addition on the austenite for-mation kinetics of CHQ steel

Abstract

CHQ steel components has been widely used in automotive, ship and aerospace engineering, thermal plants, the precision parts such as fastener, pinions, connecting rods, spline sockets bolt and nuts are being manufactured by cold heading operations. Effect of aluminum addition on reheating temperature as well as austenite formation has been observed

mechanism of grain refinement by AlN particles by which it can influence the mechanical properties of steel has been investigated. Grain refinement mechanism by micro-alloying AlN particles has been widely used technique but this process has still not completely universally understood and still not effectively used. Combination of two steels with and without addition of aluminum as micro-alloying constituents has been studied in this work that clearly shows the different $AC_1$ and $AC_3$ temperature in both the steels and further aluminum combines with nitrogen to form AlN particles that are responsible for pinning the prior austenite grain boundaries and hinders grain growth. However, a study of re-austenization behavior using Pb-bath rapid-heating experiments suggested that the addition of Al significantly affects the austenite formation kinetics during inter-critical annealing and results in the formation of much finer reheated austenite grain structure. The reason for this is partly because a much finer pro-eutectoid ferrite network forms during cooling (i.e., hot-rolling) in Al-containing steel because the austenite-to-ferrite transformation temperature is significantly depressed by Al-addition. In particular, the result showed that the pro-eutectoid ferrite/pearlite interface is the most potential nucleation site of austenite during re-austenization and that the refinement of pro-eutectoid ferrite network leads to a significant increase in the nucleation sites of austenite during inter-critical annealing. Thus it is believed that grain refinement of reheated austenite grain structure due to Al addition results not only from the grain growth retardation by AlN particles but also from the intrinsic chemical effect of Al addition on the austenite formation kinetics on reheating.