Effects Of Tempering Process On The Microstructure And Properties Of 25CrMo48V Steel

As the demand for petroleum resource across the world keeps increasing, the focus of oil exploitation transfers to the area with more abundant reserves but poorer environment. The exploitation of the ultra-deep petroleum well requests the oil casing should possess more excellent properties. Tempering treatment would significantly influence the microstructure and properties of steels. The toughness of steels increases along with the recovery process within the matrix, and the strength increases accompanied by the secondary hardening during the tempering treatment. The effects of tempering treatment on the evolution of carbide categories, the properties, and microstructure of the tested steel are worth studying.Concerning the problem above, different tempering treatment routes including single step at 580 ? and 650 ?, respectively, and two-step at 580 ? and then 650 ? were implemented on the ultra-high strength 25 CrMo48V steel to find out the relationship between the properties and the microstructure or carbides, which evolved during the tempering process. The main conclusions were achieved as followed:?i? When 25 CrMo48V steel was tempered at 650 ?, the sharply decease of retained internal stress and dislocation density during the recovery of matrix would lead to the improvement of the toughness. The strength was supposed to be improved to the largest degree, due to the thinnest martensite lath acquired by tempering at 580 ?. However, the two-step tempering treatment ensured the lower dislocation density and the small martensite lath size of the 25 CrMo48V steel, which in turn fulfilled the demand of both high strength and high toughness.?ii? M₃C was formed during quenching. The precipitation of secondary hardening phase Mo₂C occurred when the samples were tempered at 580 ?. Both the size of M₂C and M₃C would increase to act as the nucleation site for the more stable M₂₃C₆ phase. During the whole tempering process, the MX carbide did not change with the temperature or the duration at all.?iii? M₂C and M₃C carbides are coexist with a small number of M₂₃C₆ during the two-step tempering treatment. The precipitation of Mo₂C could further improve the strength while the Cr₂₃C₆ Precipitation in a small quantity could hardly influence the corrosion resistance.