Defects caused by quenching the valve cylinder liner using a medium frequency induction heating machine and its preventive measures

The working conditions of the engine valve cylinder liner are at a high temperature, semi-dry lubrication state, and are subject to strong friction from the piston ring. Therefore, we often use a medium frequency induction heating machine for quenching heat treatment. However, due to the influence of many factors during the quenching process, the cylinder liner will produce some quenching defects. Today, we will look at the causes of defects and preventive measures.

1. Quality problems caused by the unqualified casting structure of the cylinder liner

There are many quality problems caused by the unqualified casting structure of cylinder liners, such as:

1) After the cylinder liner was quenched, it was found that longitudinal cracks began to appear from both ends. After inspection, the cross section of the casting was rough, with pores, a large number of inclusions and white shell blocks. This is due to casting defects (shrinkage cavities, porosity, pinholes, supercooled graphite, cementite, etc., which are all casting defects) causing cracks after quenching.

2) The hardness is extremely high after quenching, and there are circumferential cracks on the surface of the cylinder liner. After inspection, the hardness of the as-cast cylinder liner is 255HBW (required 170-241HBW), and the microstructure of the outer ring is basically dendritic supercooled graphite with ledeburite, which is the cause of the cracks.

3) After quenching, it is found that the hardness is not enough. After inspection, the ferrite content in the as-cast structure reached 40%-50%, and the casting hardness ≈162HBW, which did not meet the requirements.

2. Deformation of cylinder liner quenching

The cylinder liner is a thin-walled part, with the thinnest point being about 7.5mm. In order to reduce deformation, when the inner hole is scanned and quenched, the outer circle of the cylinder liner should be cooled by an auxiliary sprayer, and the amount of grinding left on one side is reduced from 1mm to 0.35mm.

3. Quenching cracks in cylinder liner

Quenching cracks in the cylinder liner have occurred in the initial heating zone. This is because when a machine-type generator is used, the temperature at the heating end is likely to be high at the beginning, and cracks are likely to occur after quenching. The cross-section shows that the hardened layer is deeper at the starting end and there is more retained austenite in the quenched structure. The solution is to leave a transition area of ≤15mm at the starting end and reduce the power when starting to heat (using the capacitance reduction method at that time). Through comparative experiments, it was found that the quenching started after reducing the power did not crack, while the quenching started without reducing the power cracked.

There is a solution to everything. We also have ways to solve the defects caused by quenching the cylinder with a medium frequency induction heating machine. Today, we have talked about some methods to solve the defects, I believe it will be helpful to you. But today these are only theoretical knowledge, and we should apply these theoretical knowledge into practice.