From the chemical composition point of view, the material meets the composition requirements of 55CrSi steel. According to the current 55CrSi material heat treatment process of the author's factory, the tempered troostite should be obtained. From the results of inclusion detection, the inclusions are well controlled and meet the requirements of high quality spring steel for inclusions. No large inclusions exist, and no cracks are found to originate from a certain point. It is considered that inclusions are not the cause of crack formation.
From the results observed in the metallographic phase, it can be seen that there is a martensite structure at the spring break. The spring is not broken during the rolling, and after the spring is rolled and tempered by 420e for 40 minutes, the steel wire structure can be considered normal, and at least the martensite and other tissues will not appear before the grinding.
Abnormal tissue such as martensite is only partially present on the grinding surface, indicating that the tissue is produced during grinding. Document <1> shows that in the case of grinding parts with high heat treatment strength, if the processing pressure is too large, cracks are likely to occur in many cases because the temperature rises too fast during processing, and the surface Mars is formed. Body and other mixed tissues and cause cracks.
55CrSi is a kind of high-quality spring steel. The addition of Cr and Si alloy elements makes it have good hardenability. It is completely possible to produce quenched structure during vigorous grinding. To illustrate this problem, the author intercepts a wire of the same material and the same state, and grinds the end face with a grinding wheel. When grinding, use two different forces to grind out two parts of the surface on the end face. One is to use a small force to grind out the metallic (silver-white) surface (A), and the other is to use a larger force to grind out the blue-black. Surface (B), as shown.
Comparing the end face of the sample to the metallographic observation of the surface grinding, as shown, the structure of the A region was found to be a normal tempered troostite, and the structure of the B region was a mixed structure containing martensite and bainite. The reason for the spring break in a spring factory may be that the individual springs are slightly larger in size or improperly assembled during grinding, and they are subject to greater grinding force when the same amount of grinding is applied. Another reason may be grinding during grinding. Caused by improper volume control.
Improper grinding process, possible hazards: (1) If a) Zone A tempered troostite structure @500b) Zone B martensite, bainite mixed structure @500 wire grinding effect on the formation of cracks That is, the spring has serious surface defects, which will directly lead to early fracture; (2) brittle structure such as martensite exists, which is prone to crack when the spring is deformed, affecting fatigue life; (3) martensite and other tissues exist, The hardness is very high, the shot peening cannot be achieved, and the surface cannot form compressive stress, which also affects the fatigue life. The broken spring sample was close to the fracture position, the shot blasting trace was not obvious, and shot peening did not work.
In order to ensure product quality, it is recommended to take measures: (1) strict standard operation; (2) springs with blue-black and oxidized color on the grinding surface should be treated with caution, on the one hand, another tempering treatment should be carried out immediately to improve the organization. Then, blasting treatment is carried out; on the other hand, further inspection is to be carried out to check whether cracks are present.
Conclusion (1) The reason why the spring pressure of a factory is broken is that the spring generates quenching structure and crack during grinding. (2) The spring end face grinding should be standardized to avoid quenching structure or cracks. (3) For springs with a blue-black oxidized color on the ground surface, the spring should be treated with caution. On the one hand, it should be tempered in time, and on the other hand, it should be checked for cracks.
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