Abstract: To meet high power, long service life, high reliability and high precision requirements of nuclear power pump gear box, the deep controlled ion nitriding process experiments were carried out on the 42CrMo steel matrix with medium hardness. The results show that when three-stage ion nitriding process with nitrogen hydrogen mixing atmosphere is adopted and using the optimized total process time of about 60 h, a nitrided layer can be obtained with 0.90 mm thickness and gradeⅠsurface brittleness, and γ′ single-phase, and the maximum residual compressive stress amplitude of the nitrided layer achieves 620 MPa. This process is applied to deep nitriding of the gear ring with modulus of 12 mm and diameter of ø1400 mm for the circulating water pump gearbox for nuclear power plants and successfully passes the bench test, and is serviced in China's nuclear power plants for many years replacing the imported one.

Key words: nuclear power pump, large inner gear ring, deep ion nitriding