The difference in mechanical properties of bolts is obvious, which is reflected in the difference in microstructure. The structure with good performance is normal and belongs to tempered sorbite. The low-performance ferrite is present in the microstructure. According to the quenching process, the material must be rapidly cooled after the austenite transformation is completed during the heating process. The presence of ferrite in the bolt is due to the quenching and cooling of the bolt after the austenitization is not completed. In fact, the bolt is quenched in the two-phase region, so that the tempered sorbite + ferrite mixed structure is finally obtained. Hardness and strength are reduced.
There are two cases depending on the ferrite morphology retained in the microstructure after quenching. One is that if unmelted ferrite is retained at high temperature due to insufficient heating, its form is blocky and retains the banding distribution before quenching; the second case is that the pre-cooling time in the air is too long after quenching, and then quenching In the human medium, ferrite is precipitated from austenite before quenching the medium. The ferrite form is distributed along the austenite grain boundary.
The ferrite morphology in No. 3 bolt is consistent with the second basic. It can be inferred that the poor performance of some bolts in No. 3 bolt is mainly due to the excessive residence time in the air before quenching the medium, so that the microstructure does not appear. Normal quenching of the structure causes a drop in strength and eventually breaks during assembly.
Continuous cooling transformation C curve, special attention should be paid to the influence of cooling rate on the microstructure during quenching and cooling. If the cooling rate is slow, the cooling curve will hit the nose of the C curve, causing the material to undergo upper bainite transformation, resulting in material properties. Down the river. The microstructure of No. 2 bolt is tempered sorbite, which belongs to normal structure. It can be seen from the high-fold shape of tempered sorbite in Fig. 6. The carbite precipitated carbide is uniformly dispersed in the martensite strip. It reflects its strong toughness in performance.
Conclusions and recommendations (1) No. 1 bolt fracture is mainly due to insufficient cooling rate during quenching, and upper bainite structure with poor mechanical properties in the microstructure. The No. 3 bolt fracture is mainly caused by the excessive residence time in the air during quenching and the partial structure of the tempered Soxite ten-block ferrite in some bolts, causing the strength to decrease and causing the fracture. (2) It is recommended to strictly control the heat treatment process. Ensure that the microstructure of the bolts yields tempered sorbite with good toughness.
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