Failure Analysis of Cylinder Head Bolt Fracture

The fracture positions of the two cylinder head bolt fittings (numbered 1 and 2 respectively) are at the junction of the rod and the head. The bolts are mainly subjected to torsional load during assembly and tightening. At this time, the principal stress is at 45° to the axis, and the shear stress is It is perpendicular to the axis. From the perspective of the fracture, the crack initiation area is at a 45° angle to the axis, which appears as a normal fracture during torsion, which is the result of normal stress. The instantaneous fracture area is perpendicular to the axis, and the area of u200bu200bthe instantaneous fracture area accounts for most of the total fracture area, indicating that the stress during fracture is large or the material strength is insufficient.

2.2　Chemical composition and microstructure analysis

The bolt material grade is 40Cr steel, the hardness requirement is 32～38HRC, the metallographic structure requirement is 1～3 level (JB/T8837-2000). After inspection, the chemical composition (mass fraction) of the No. 1 cylinder head bolt meets the requirements of GB/T3077-1988, see Table 1. The matrix structure of the two bolts is fine tempered sorbite, which is evaluated according to JB/T8837-2000, and the structure is grade 1, which meets the technical requirements of grades 1 to 3. The grain boundary in the edge structure is clearly visible, but its color is obviously lighter than other parts. It is estimated that there is a decarburized layer on the surface, and the No. 2 bolt is more obvious. (Guide: Briefly describe the common sense of fasteners)

Both bolts have residual band structure, and there are non-metallic inclusions in the band structure. The band structure of No. 1 bolt is shown in Figure 2. The sulfide-like inclusions in between are clearly visible, but they are all within the specified range. .

2.3　Micro-hardness test

After magnetic particle inspection, no magnetic traces were found. After testing, the hardness value of No. 1 bolt is 34HRC, and the hardness value of No. 2 bolt is 36HRC and 37HRC, which meets the technical requirements of 32～38HRC. Since the bolt has been quenched and tempered, it seems that there is still no evidence to determine whether there is a decarburized layer at the edge based on the metallographic structure. In order to confirm whether there is a decarburized layer at the edge, a microhardness test was performed on the edge and the center. The edge hardness is lower than the center hardness, further confirming the existence of a decarburized layer at the edge. The reduction of edge carbon content reduces the degree of erosion, so the color is lighter and the grain boundaries are clearly visible. Although the surface of the bolt is machined after quenching and tempering to remove the decarburized layer produced by thermal processing, it is often difficult to remove the decarburized layer by mechanical processing at the junction of the rod and head. The decarburized layer is retained at the junction of the

2.4　Analysis and Discussion

Since the maximum stress is located at the edge of the bolt, and there is decarburization at the edge, the strength of the edge is reduced. Although the bolt allows a certain amount of decarburization layer, the existence of the decarburization layer is always detrimental to the service of the bolt. Since the crack originates from the decarburization, decarburization becomes one of the main factors for the failure of the bolt.

More related hardware stamping parts industry news: