The hydrogen embrittlement of fasteners is due to the fact that hydrogen atoms enter the interior of the material during the early processing. In most cases, the fasteners undergo hydrogen embrittlement under conditions of static tensile loading. Hydrogen embrittlement is less likely to occur in high strain rate material tests, such as ordinary tensile tests. Hydrogen atoms generally diffuse into regions of the material that are subjected to three-way stress. The level of stress in the material and the degree of hydrogen accumulation in the system will affect the ratio of hydrogen diffusion to the trap location. The accumulation of hydrogen at the trap location will cause the fracture stress of the material to drop, such that crack formation, crack propagation, and failure occur in the material. The diffusion of hydrogen into the fasteners subjected to static loading can be directly observed by the delay time before hydrogen embrittlement fracture. Due to the hydrogen embrittlement tendency of the material, the total amount of hydrogen in the material, the diffusion ratio of hydrogen, and the level of the applied stress, the delay of the hydrogen embrittlement rupture varies greatly, ranging from a few minutes to several days or weeks.
Hydrogen embrittlement can occur if the fastener has been exposed to an environment with hydrogen ions during processing. Any treatment that produces hydrogen during the chemical or electrochemical reaction of the steel will cause hydrogen to enter the material, thereby increasing the hydrogen embrittlement tendency of the material. Steel fasteners used in the automotive industry will be in direct contact with active hydrogen atoms under conditions of environmental corrosion, cathodic de-oiling, acid descaling, chemical cleaning, blackening and electroplating. . Since the electroplating process will produce hydrogen, it has the greatest effect on the absorption of hydrogen by the steel fasteners. The total amount of hydrogen absorbed during the electroplating process is highly dependent on the efficiency of the plating solution. In general, efficient electroplating produces less hydrogen than inefficient plating. Factors such as excessive or too little plating solution loading in the plating drum will have a significant impact on the efficiency of the plating process.
Other processes that generate hydrogen when working with steel, such as pickling, descaling or pre-plating treatment after heat treatment, are also indispensable. John-son's research is a good description of the effect of immersion in acid on the toughness of steel. The absorption of hydrogen during fastener processing is cumulative. The hydrogen introduced into the part by a single treatment may not be sufficient to cause hydrogen embrittlement, but the accumulation of hydrogen introduced into the part by various treatments may cause hydrogen embrittlement.
The adverse effects of hydrogen absorption during plating or cleaning can be eliminated or mitigated during the heat treatment (usually referred to as baking) after plating. The severity of the hydrogen embrittlement hazard typically depends on the strength level of the fastener and/or the cold working condition. Troiano has given the relationship between the time of failure and the hydrogen content and baking time. By baking, the accumulation of hydrogen in the material is alleviated, and the failure time and lower critical stress levels are prolonged and increased. Here, the critical stress level refers to a stress level below which hydrogen embrittlement does not occur, similar to the fatigue limit.
Whether the baking time is sufficient depends mainly on the hardness level of the material, the plating process, the type of plating and the thickness of the coating. Fasteners with a lower hardness level (≤35HRC) that are plated should generally be baked for at least 4 hours; fasteners with the same plating but higher hardness (≥36HRC) should generally be baked for at least 8 hours. It has been suggested that fasteners with a hardness between 31 and 33 HRC should be baked for 8 hours; fasteners with a hardness between 33 and 36 HRC should be baked for 10 hours; fasteners with a hardness of between 36 and 39 HRC should be baked. Bake for 12 hours. Fasteners with a hardness between 39 and 43 HRC should be baked for 14 hours. The baking process should be developed taking into account both the hardness level of the fastener and the type of coating. The coating can act as a hydrogen diffusion barrier to a certain extent, which will hinder the diffusion of hydrogen out of the fastener. In general, it is easier for hydrogen to diffuse out of the fastener through the loose coating than to diffuse out through the dense coating. This difference exists between the galvanized layer and the denser cadmium plating. In order to diffuse as much hydrogen as possible out of the material, it is necessary to take longer baking times. AWGrobinJr. believes that when the thickness of the coating exceeds 2.5 μm, it will be difficult to diffuse hydrogen out of the steel. In this case, the galvanized layer becomes an obstacle to hydrogen diffusion. It is believed that performing the bake process in this case actually redistributes the hydrogen to the various trap locations in the material.
The automotive industry, in which the hydrogen embrittlement of fasteners has failed, has long attracted widespread attention. This failure has unexpectedly added a significant burden to auto companies and fastener suppliers, not only causing economic losses, but also posing a threat to the company's user satisfaction and the safety of the car.
Preventing hydrogen embrittlement failure of fasteners is gaining increasing attention in the automotive industry. Fasteners that are subject to hydrogen embrittlement hazards can experience early failure within minutes of assembly under conditions where the actual stress is much lower than the tensile strength of the material. In automotive assembly plants, the hydrogen embrittlement failure of fasteners will greatly reduce production efficiency. Cars that are at risk of potential hydrogen embrittlement failures must be inspected one by one and replaced with all new and reliable fasteners, and replacement of fasteners can take a significant amount of time. Replacing hydrogen-brittle-damaged fasteners will be a burden for automakers and fastener manufacturers.
Concerned about surprises
Label: Hydrogen embrittlement after heat treatment of fasteners
Previous: The use of hydraulic hole opener Next: Three major concerns of daily maintenance of packaging machinery
This Mosaic is a pattern made of small regular or irregular piece of glass,which often used as floor and wall decoration. The building material glass mosaic is the best choice of swimming pools,Bathroom wall decoration and commercial place wall decoration.
- Popular design,frost proof and long-lasting durability.
- Different color,size and designs of glass mosaics provide customers with beautiful mosaic pool art collections to beautify and customized the bottom of the swimming pool.
- Resistant to fading,staining and discoloration.
Glass Mosaic,Glass Mosaic Tile,Mosaic Tile Walls,Mosaic Tiles Floor
Foshan Castel IMP&EXP Co.,Ltd , https://www.foshancastel.com