关键词: 万向轴, 40CrNiMoA钢, 脆性过载断裂, 调质处理

Abstract: Cardan shaft water cap joint of oil drilling tools fractured at its corner during assembly. A comprehensive analysis was conducted to determine the fracture nature of the joint and analyze the causes of the fracture, including observing the macro and micro fracture characteristics of the 40CrNiMoA steel water cap joint, examining the joint's microstructure, testing its hardness, impact property value and tensile properties. The research results show that the fracture of the water cap joint is a brittle overload fracture and happened during the assembly process, which is attributed to two main reasons. One is that its microstructure after heat treatment has insufficient quenched and tempered depth, resulting in poor toughness. Another is that the poor fit between the joint and the sleeve causes the joint to bear additional bending moment besides torque, leading to fracture under the combined action of bending and torsion loads. To address this issue, two improving measures are proposed. Firstly, optimizing the quenching and cooling process of the joint to maximize the depth of the hardened layer and to enhance the joint's strength and toughness. Secondly, carefully inspect the surface condition of the internal thread of the sleeve before assembly to strictly ensure the surface integrity of the thread.

Key words: cardan shaft, 40CrNiMoA steel, brittle overload fracture, hardening and tempering