Microstructure and Performance of Aluminized Coatings Prepared by Pack-cementation Promoted by Surface Gradient Crystal Defects on 40Cr Steel

The pack-cementation aluminizing process is considered to be one of the most effective and economical methods to improve the corrosion resistance of steels. In this work, pre-torsion deformation combined with the pack-cementation aluminizing was conducted to prepared Al-coatings on the 40Cr steel. The steels were twisted prior to the aluminizing process, which can introduce high-density crystal defects into the steel surface, and increases the diffusion channels and thereby improving the diffusion rate of Al atoms during the aluminizing process. Microstructure of the samples before and after aluminizing was characterized systematically. The microhardness and corrosion resisting property were measured. The effects of pack-cementation temperatures (600, 650, 700 and 750 ℃) on the microstructure and properties of Al-coating were analyzed. The results show that the pre-torsion deformation introduces high-density crystal defects on the surface of 40Cr, which can improve the diffusion efficiency and increase the thickness of the infiltration layer. The higher the temperature, the thicker the infiltration layer, but the larger the grain size, the stronger the inhomogeneity of the structure and the lower the hardness. From 600 to 700 ℃, the corrosion resistance is gradually enhanced due to the thickening of the coating, but when the temperature reaches 750 ℃, more holes appear in the infiltrated layer, resulting in poor corrosion resistance. In contrast, the 700 ℃ sample is resistant to the best corrosion performance. © 2023 Editorial Office of Chinese Journal of Mechanical Engineering. All rights reserved.