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What are the principles and causes of screw hydrogen embrittlement?

Update:23-11-2021
Summary:

Hydrogen embrittlement usually manifests as delayed fra […]

Hydrogen embrittlement usually manifests as delayed fracture under stress. There have been galvanized parts such as automobile springs, washers, screws, leaf springs, etc., which fractured within a few hours after assembly, and the fracture ratio reached 50% of 40%. A certain special product cadmium-plated parts had batch cracks and fractures during the use process, and organized a national research and developed a strict hydrogen removal process. In addition, there are some hydrogen embrittlement that does not manifest as delayed fracture. For example, electroplating hangers (steel wire, copper wire) have serious hydrogen penetration due to multiple electroplating and pickling and deplating, and they often occur when they are folded during use. The phenomenon of brittle fracture; some quenched parts (large internal stress) will crack during pickling. These parts have serious hydrogen penetration, cracks occur without external stress, and it is no longer possible to use hydrogen removal to restore the original toughness.

The delayed fracture phenomenon is caused by the diffusion and accumulation of hydrogen inside the parts to the stress-concentrated parts, and there are many metal defects in the stress-concentrated parts (atomic lattice dislocation, holes, etc.). When hydrogen diffuses to these defects, hydrogen atoms become hydrogen molecules, generating huge pressure. This pressure, the residual stress inside the material and the applied stress on the material form a resultant force. When the resultant force exceeds the yield strength of the material, it will lead to Fracture occurred. Since hydrogen embrittlement is related to the diffusion of hydrogen atoms, diffusion takes time, and the speed of diffusion is related to the concentration gradient, temperature and material type. Therefore, hydrogen embrittlement usually manifests as delayed fracture.

The hydrogen atom has a very small atomic radius and is easy to diffuse in metals such as steel and copper, while it is difficult to diffuse hydrogen in cadmium, tin, zinc and their alloys. The cadmium plating layer is difficult to diffuse. The hydrogen generated during cadmium plating originally stays in the plating layer and the metal surface under the plating layer, and it is difficult to diffuse outward, and it is particularly difficult to remove hydrogen. After a period of time, hydrogen diffuses into the metal, especially the hydrogen that enters the defects in the metal, it is difficult to diffuse out. The diffusion rate of hydrogen at room temperature is quite slow, so it needs to be heated immediately to remove hydrogen. Increasing temperature increases the solubility of hydrogen in steel. Excessive temperature will reduce the hardness of the material. Therefore, the temperature selection for stress relief before plating and hydrogen removal after plating must be considered not to reduce the hardness of the material, and it must not be in some steel materials. The brittle tempering temperature does not destroy the performance of the coating itself.
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