Abstract: Aiming to avoid the corrosion and wear problem of the 42CrMo steel tie rods of large rolling mill used in the coastal areas of Southeast Asia, nitrocarburizing and post-oxidation combined treatment was carried out by using a single heat source pulsed ion nitriding furnace. The research results on the pre-oxidation, nitrocarburizing and post-oxidation processes of the simulated specimen show that, the combined treatment process which meets the product requirements is: firstly pre-oxidation at 350 ℃ for 2 h in air furnace, secondly, nitrocarburizing at 550 ℃ for 24 h (furnace pressure of 260 Pa, NH₃∶CO₂ ratio of 20∶1), thirdly, post-oxidation at 520 ℃ for 2 h (furnace pressure of 300 Pa, NH₃∶O₂ of 6∶1), then turning the power and penetrating agent off and introducing air into the furnace at a speed of 0.5 L/min until the furnace temperature dropping to 300 ℃, finally, quickly and evenly coating the surface of the specimen with anti-rust oil when the specimen cooled to about 150 ℃. Under this combined process, the depth of the infiltration layer reaches 0.39-0.41 mm, the compound layer reaches 10-12 μm, which is thicker than that under the same process without pre-oxidation. After salt spray testing, corrosion spots appear on the specimen only until 70 h, and the corrosion resistance is higher than that only undergoing nitrocarburizing treatment without post-oxidation treatment.

Key words: large rolling mill, tie rod, 42CrMo steel, ion nitrocarburizing, post-oxidation, corrosion resistance