Hydrogen embrittlement of fasteners after heat treatment

The hydrogen embrittlement of fasteners is due to hydrogen atoms entering the material during the early processing. In most cases, fasteners undergo hydrogen embrittlement under static tensile loads. Hydrogen embrittlement is less likely to occur when conducting high-speed material tests, such as ordinary tensile tests. Hydrogen atoms usually diffuse like areas in the material that are subjected to three-way stress. The stress level in the material and the degree of hydrogen accumulation in the system will affect the ratio of hydrogen diffusion to the trap site. The accumulation of hydrogen in the trap position will reduce the fracture stress of the material, resulting in crack formation, crack propagation and failure in the material. The expansion of hydrogen in fasteners subjected to static load can be directly observed by the current delay time of hydrogen embrittlement fracture. Due to the hydrogen embrittlement tendency of the material, the total amount of the material, the hydrogen diffusion ratio and the spin stress level, the hydrogen embrittlement fracture time delay varies greatly, ranging from a few minutes to several days or weeks.

If the fastener has ever been in contact with an environment with hydrogen atoms during processing, it may be hydrogen embrittlement. Any treatment that produces hydrogen during the chemical reaction of steel will allow hydrogen to enter the material, thereby increasing the material's hydrogen embrittlement tendency. Steel fasteners used in the automotive industry will be in direct contact with active hydrogen atoms under chemical conversion coating treatment conditions such as environmental corrosion, cathodic electrolytic oil removal, acid deoxidation, chemical cleaning, blackening, and electroplating. . Since the electroplating process will produce hydrogen, it has the greatest effect on the hydrogen absorption of steel fasteners. The total amount of hydrogen absorbed during the electroplating process largely depends on the efficiency of the electroplating solution. In general, high-efficiency electroplating processes produce less hydrogen than low-efficiency electroplating processes. Factors such as too much or too little electroplating solution loading in the electroplating drum will have a great impact on the efficiency of the electroplating process. (Guide: Surface treatment methods of fasteners)

Other processes that produce hydrogen when interacting with steel, such as pickling, descaling after heat treatment, or pre-plating treatment, also cannot be ignored. John-son’s research describes the effect of immersion in acid on the toughness of steel. The absorption of hydrogen during fastener processing is cumulative. The hydrogen introduced into parts by a single treatment may not be enough to cause hydrogen embrittlement, but the accumulation of hydrogen introduced into parts by multiple processes may cause hydrogen embrittlement.

The adverse effects of hydrogen absorption during electroplating or cleaning may be eliminated or reduced during the heat treatment (usually baking) after electroplating. The severity of hydrogen embrittlement usually depends on the strength level or cold working condition of the fastener. Troiano once gave the relationship between failure time and hydrogen content and baking time. By baking, the accumulation of hydrogen in the material is reduced, and the failure time and lower critical stress level are prolonged and improved. Here, the critical stress level refers to the stress level below which hydrogen embrittlement will not occur, similar to the fatigue limit.

Whether the baking time is sufficient mainly depends on the hardness level of the material, the electroplating process, the type of coating and the thickness of the coating. Fasteners with a lower hardness level after electroplating treatment should generally be baked for less than 4 hours: the same plating, but fasteners with a higher hardness level are generally baked for at least 8 hours. It has been suggested that fasteners with hardness between 3133HRC should be baked for 8 hours; fasteners with hardness between 33-36HRC should be baked for 10 hours: fasteners with hardness between 36-39HRC should be baked 12 hour. Fasteners with hardness between 39-43HRC should be baked for 14 hours. The formulation of the baking process should also consider the hardness level of the fastener and the type of coating. The plating layer can play a role as a hydrogen diffusion barrier to a certain extent, which will hinder the diffusion of hydrogen to the outside of the fastener. Generally speaking, it is easier for hydrogen to diffuse outside through loose coatings like fasteners than it is to diffuse outside through dense coatings. There is even this difference between the zinc coating and the denser cadmium coating. In order to make as much hydrogen diffusion material as possible, it is necessary to take a long baking time. A.W.GrobinJr. It is believed that when the thickness of the coating exceeds 2.5 μm, it is more difficult for hydrogen to diffuse out of the steel. In this case, the galvanized layer becomes an obstacle to hydrogen diffusion. It can be considered that the baking treatment in this case actually redistributes the hydrogen to the various trap positions in the material. The hydrogen embrittlement failure of fasteners has already aroused widespread concern in the automotive industry. This kind of failure happens unexpectedly, which adds a great burden to automobile companies and fastener suppliers, which not only causes them to suffer economic losses, but also poses a threat to the company's user satisfaction and the safety of automobiles.

The prevention of hydrogen embrittlement failure of fasteners has been paid more and more attention in the automobile industry. Fasteners suffering from hydrogen embrittlement can fail early within a few minutes after assembly when the actual stress is much lower than the tensile strength of the material. In the automobile assembly workshop, the hydrogen embrittlement failure of fasteners will greatly reduce the production efficiency. Cars with potential hydrogen embrittlement failure risk must be inspected one by one, and all possible fasteners should be replaced with new and reliable fasteners, and replacing the fasteners will take a lot of time. Replacing fasteners damaged by hydrogen embrittlement will be a big burden for both automobile manufacturers and fastener manufacturers.

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