Hot-Dip Aluminizing of Flow-formed AISI 4140 Steel

Cold plastic deformations recognizably cause increasing lattice imperfections such as point defects and dislocations in the structure, which could then have an effect on diffusion characteristics of the material. In order to explore such an effect of flow forming, a flow formed AISI 4140 steel and an annealed 4140 steel were subjected to the HDA process in a molten Al7020 bath at 750°C for 4 min, and a subsequent diffusion annealing was performed at 800°C, and their coating characteristics such as coating thickness and hardness were compared. The results indicated that the coating thickness of the flow formed samples was higher (80°m) than that of the annealed sample (50°m) after the HDA process. Diffusion annealing increased the coating thickness of both samples five times, reaching 400°m and 250°m for the flow formed and the annealed samples, respectively. Comparing the measured thickness of the coatings revealed that flow forming accelerates diffusion during the HDA process, probably due to the defect structure induced by the flow forming. On the other hand, the coating hardness was in between 1000-1100 HV for both samples, implying that the initial condition of the sample does not have a remarkable effect on hardness after the HDA process.