The hydrogen embrittlement fracture defect of rock drill piston carburizing and quenching can be easily solved by high-frequency annealing equipment

The small rock drill piston is an important part of the rock drill and is made of 20CrMnTi steel. The technical requirements are: the depth of the hardened layer is 1.5-1.8mm, and the surface quenching hardness is 56-60HRC. The heat treatment process uses carburizing heating and kerosene protection, and quenching and cooling. There have been many brittle fracture accidents at the piston ring groove during production.

The fracture occurs at the ring groove wall where the strength of the part is low and the stress concentration is weak. The fracture is parallel to the outer circle and peels off like a shell. Some cracks are found after quenching, and some brittle fracture occurs during processing or use. Observation of the cross section revealed that the crack originated from the inner layer and then expanded outward. The cross section was smooth and it was a brittle fracture.

The fracture specimens were tested for chemical composition, metallographic structure and grain size, and all were normal. The cracks are located in the transition zone of the carburized layer. Metallographic observation found no defects such as oxidative decarburization and abnormal inclusions. This shows that the brittle fracture of the piston ring is not caused by the decrease in strength caused by material defects and structural abnormalities.

Analysis pointed out that the carburizing atmosphere using kerosene as the penetrant contains a large amount of hydrogen. At higher temperatures, the diffusion coefficient of hydrogen is large and it is easy to penetrate into the steel matrix. When the mass fraction of hydrogen in the steel exceeds 1×10-6, it will seriously affect the elongation, area shrinkage and delayed fracture performance of the part material, causing the performance to deteriorate; when a large amount of hydrogen enters the steel, hydrogen embrittlement is likely to occur. If the hydrogen in the steel is reheated and quenched after carburization, the hydrogen will escape from the steel and eliminate its harmful effects. However, the workpiece is protected by drops of kerosene during carburization heating. The workpiece is in a high hydrogen atmosphere, and the hydrogen in the parts cannot escape. out, hydrogen will continue to penetrate. During quenching, the hydrogen in the workpiece cannot dissolve in a large amount of martensite and is suddenly released, which can easily lead to hydrogen embrittlement. At the same time, the transition zone after carburizing and quenching forms a high tensile stress peak due to the phase change structural stress and thermal stress. The two work together to cause hydrogen embrittlement cracks or even fracture. In addition, hydrogen embrittlement fracture has delayed characteristics. During subsequent processing (such as shot peening or grinding), due to the combined effect of external force, hydrogen accumulation and release and internal stress, the parts will undergo delayed fracture after a certain period of time. After inspection, observation and comprehensive analysis, it was found that the brittle fracture at the piston ring was hydrogen-induced brittle fracture.

To this end, the heat treatment process has been improved, that is, high-frequency annealing equipment is used for induction quenching heat treatment. This method effectively avoids the high-temperature penetration of hydrogen into the steel and prevents hydrogen embrittlement.

After many manufacturers adopt new heat treatment processes, the quality of the pistons produced is stable, no hydrogen embrittlement cracks and fractures have been found, and good results have been achieved. What's even better is that this process is suitable for large-scale production and can greatly improve workers' production efficiency.