Abstract: In order to solve the fracture problem of the 40CrNiMoA steel upper joint, the main reasons for the fracture are the stress concentration caused by the small radius of variable diameter R, the over high hardness and the insufficient elongation via chemical composition analysis, metallographic analysis and mechanical property test of the workpiece fracture. The risk of crack fracture in heat treatment and use of the product can be reduced by optimizing the structural design and heat treatment processes. The results show that the mechanical properties of the workpiece material can meet the requirements of both product design and use by increasing the diameter rounded corner to R20, adopting intermediate heat preservation, controlling the heating rate to the quenching temperature of 810 ℃, cooling in slow oil, and subsequently, controlling the heating rate to the tempering temperature of 570 ℃ plus cooling in slow oil to prevent tempering brittleness. After testing in actual working conditions, no fracture accident occurs in the product life cycle.

Key words: 40CrNiMoA steel, upper joint, failure analysis, structural design optimization, process improvement