Hydrogen content determination The hydrogen content of the fracture spring gasket was measured, and the average hydrogen content was 17@10-6. The above test results show that the metallurgical quality of the failed part material meets the requirements, and the failure of the gasket due to the unqualified quality of the raw material can be eliminated. The hardness of the failed gasket is slightly higher, and as 65Mn steel, in order to ensure sufficient elasticity, the hardness is generally controlled to the upper limit of the specified value. The hardness value of the fracture member is only higher than the upper limit of 0.51.0HRC, so it does not affect its normal use.
Analysis of hydrogen source of spring washers It can be seen from the processing technology that the hydrogen in the spring washer is mainly derived from the electroplating process, because there is also a hydrogen evolution process at the same time of electroplating; the workpiece must be pickled before electroplating, and the surface of the workpiece is removed in the pickling process. In addition to the chemical reaction between the oxide and the acid, some of the metal reacts with the acid, and the generated hydrogen enters the metal in a large amount. The composition of the pickling solution, the pickling temperature, the pickling time, and the alloy composition all affect the hydrogen content. Therefore, a large amount of hydrogen will be contained in the plating layer and inside the member after plating.
Spring washer fracture analysis The shape of the spring washer fracture shows that the fracture is hydrogen brittle damage, and the hydrogen embrittlement is caused by the hydrogen present in the metal matrix. Hydrogen concentration occurs under the loading stress, resulting in hydrogen embrittlement. From the microscopic analysis, the hydrogen atom has the smallest atomic radius (RH=0.053 nm), so it is easy to enter the internal crystal lattice of the metal, and then under the action of stress, the matrix metal is dislocated, resulting in rapid diffusion and concentration of hydrogen to the region where the stress rises. From the hydrogen atom to the hydrogen molecule, that is, the above reaction of H++eH2HH2↑ at the site of hydrogen accumulation produces a huge volume expansion effect, and the expansion effect at the grain boundary or dislocation plug is formed. crack. The crack propagates rapidly after the crack is generated. The rate of crack propagation varies according to the type of material and the concentration of hydrogen. The highest crack growth rate can reach the sound velocity <1>.
In general, when the hydrogen content in the steel is above (510) @10-6, hydrogen-induced cracking occurs, and the average hydrogen content in the failed gasket is as high as 17@10-6. When the hydrogen in the matrix encounters stress, hydrogen is generated. The aggregation eventually leads to hydrogen embrittlement fracture <3>.
Conclusion The spring washer fracture mechanism belongs to hydrogen embrittlement fracture. The improper removal of hydrogen from the spring washer after plating causes the gasket to be completely dehydrogenated, resulting in hydrogen embrittlement fracture during assembly. According to the failure analysis results of the above 65Mn steel spring washers, it is recommended that: (1) replace the spring washer material. It is understood that the Shanghai Spring Washer Factory was commissioned by the former aerospace industry to solve the hydrogen embrittlement problem of the 65Mn steel spring washer. The factory was in 1996. The development of stainless steel spring washers began in November 1996 and was successful in November 1996. In the same year, it passed the acceptance test of Aerospace Corporation and Shanghai Standard Parts Corporation.
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