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What is the reason for the heat treatment crack of 45# steel nut?

Apr 18, 2018

45 # steel H10 M30 nuts, quenched and found in the hexagonal cross-section in the circumferential direction of the arc and radial cracks visible to the naked eye, the support surface also has a visible ring crack. This is a common heat treatment crack. Most of the fracture surfaces are dull, partially blackened, and some are shiny and have tear-like strips (due to breaking the fracture). No other defects are found.

Metallographic examination of cracks on both sides of the decarburized layer is not seen, the surface of the crack open cracked most of the black, and then a small part of the break there is a metallic sheen, indicating that the oxidation of the surface is caused by oxidation at high temperature tempering. The crack is cracking along the crystal, the tail is more pointed, and it is lean and rigid.

There is no decarburization on both sides of the crack. There are two possible causes: one is that there is no crack before quenching, cracking occurs during quenching and cooling, and no oxidative decarburization occurs on both sides of the crack; the other is because the cracking of the nut is severe and the crack is high. The sample is a crack that propagates when the crack propagates after the heat treatment.

The author believes that cracks are quench cracking, thermal stress and superposition of the stress caused by the increase of quenching stress, resulting in cracking of the nut. It is recommended that the original quenching process be 830°C. There should be localized superheated microstructure in the microstructure, which is now changed to 800 to 810°C. The use of aqueous quenching and cooling was replaced by rapid quenching oil. Oil quenching media have poorer cooling capacity than water, especially in the range of 300-200°C, which is much lower than water. Therefore, with the quick quench oil cooling ratio than water, both the purpose of quenching can be achieved, and the deformation of the internal thread of the nut can also be reduced.

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