Analysis of Hydrogen Embrittlement and Head Fracture of High Strength Bolt after Heat Treatment

Release time:

2021-07-06

Analysis of Hydrogen Embrittlement and Head Fracture of High Strength Bolt after Heat Treatment

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In the fastener industry, hydrogen embrittlement and head fracture of 8.8, 10.9 and 12.9 high-strength bolts after heat treatment have occurred many times. The reasons are analyzed as follows:

　　1. On the whole, the deformation of the cold heading process is in line with the technical requirements.

Special attention should be paid to the fact that the boundary between the rod and the head during cold heading production should not be misaligned up and down during overmolding. Under the condition that there is no absolute guarantee, the head material is allowed to move slightly to the rod, the rod material is never allowed to move slightly to the head, and it is not allowed to move slightly back and forth (the rod length is slightly shorter in the front sequence than in the back sequence). Attention should also be paid to the transition with R at the boundary, the pre-order is larger than the post-order.

　2. The main reason is the material selection and improper heat treatment.

First of all, the unit drawing is incorrectly marked: for example, grade 8.8 hexagonal flange bolt, material 35K, technical requirement "heat treatment HRC22-32" (corresponding to "heat treatment HRC22-25"); Another example is grade 10.9 hexagon socket head cap screws, which are made of 35CrMo (or 40Cr) and require "heat treatment HRC32-35" (corresponding to "heat treatment HRC28-32").

The former Ministry of Machinery JB has guiding standards, bolts: grade 8.8 ~ material 35K, heat treatment HRC22-25; Grade 10.9 ~ material 35CrMo, 40Cr, heat treatment HRC28-32; Grade 12.9 ~ material 42CrMo, heat treatment HRC35-38; Grade 8.8 and above are high-strength bolts. Different grades correspond to different materials and heat treatment hardness. Heat treatment of low-level materials to high-level hardness will lead to hidden dangers of heat treatment hydrogen embrittlement and head fracture, 40Cr (or 35CrMo) cannot be used to produce 12.9 grade bolts.

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