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Improvement of Bolt Vacuum Hardening Crack

When the hardness of M24 bolts was tested after vacuum hardening and vacuum tempering, a large number of cracks were found. The material of the cracked bolt is 42CrMo steel, which has good hardenability, and its ideal critical diameter reaches 135 mm, which can be used to manufacture parts with higher strength. This paper analyzes the reasons for bolt cracking.

Bolt vacuum hardening

The bolt is mainly cracked longitudinally, and its macroscopic appearance is shown in the figure. It can be seen from the figure that the cracks start from the surface. The vacuum hardening temperature of the bolts is 860C, oil quenching, the vacuum hardening oil is ordinary oil, and the tempering temperature is 430C.

cracked bolt

Analysis and Discussion on Bolt vacuum hardening Crack

There is no decarburization on both sides of the crack of the bolt, indicating that the bolt did not crack during vacuum hardening and heating, that is, the crack was generated during the vacuum hardening process, and oxidation occurred on both sides of the crack during the tempering process. According to the characteristics of intergranular distribution of cracks, it can be concluded that the cracks are vacuum hardening cracks. The grain size of the bolt is 6.5, and the grain size is not too large, so the factor that the vacuum hardening temperature of the bolt is too high can be ruled out.

The investigation of the vacuum hardening process of the bolts found that the placement of the bolts during the vacuum hardening process was irregular, and some bolts were placed vertically with the threads downward, which was not conducive to uniform cooling, especially likely to prolong the time for the rupture of the steam film at the deformed part. The normal vacuum hardening method is to enter the vacuum hardening agent with the big head of the bolt first, and then the small head. Studies have shown that: for workpieces with steps, the time for the vapor film to disappear is also different if the placement is different.

When checking the vacuum hardening cooling system of the bolt, it was found that the water cooling coil leaked, and water entered the vacuum hardening oil, resulting in a significant increase in its cooling strength. And due to the incompatibility of water and oil, it will inevitably lead to uneven vacuum hardening and cooling of workpieces. In addition, 42CrMo steel has good hardenability, the critical diameter of oil quenching is 50mm, and the effective diameter of cracked bolts is smaller than this value, so the vacuum hardening stress of bolts is very large.

The above analysis shows that the bolt is vacuum quenched and cracked. The main reason for vacuum hardening cracks is that water enters the oil tank, resulting in increased cooling strength of the oil and uneven vacuum hardening cooling. In addition, the disordered placement of the bolts also leads to uneven cooling of the vacuum hardening, resulting in a large vacuum hardening stress, which eventually leads to cracking of the bolts.

Vacuum Heat Treatment Recommendations

(1) The placement of the bolts should be regular to avoid downwards where the steam film is likely to be generated; properly increase the temperature of the vacuum hardening oil and reduce the cooling intensity in the convective cooling stage.

(2) Regularly test the vacuum hardening oil to prevent it from entering water and aging.

(3) Strengthen the maintenance of vacuum hardening oil and replace it regularly.