High-strength fasteners are often used in important occasions, such as bridges, vehicle engines, and spacecraft. The failure of these fasteners will lead to catastrophic consequences. Preventing failures caused by hydrogen embrittlement and preventing hydrogen embrittlement risks are fundamental issues that need to be considered in the entire fastener supply chain. Due to the complex failure mechanism of hydrogen embrittlement and many influencing factors, the hydrogen embrittlement failure of fasteners has not yet been predicted. However, there are technologies that point out: the description of the theory and mechanism of hydrogen embrittlement damage, the three basic conditions for hydrogen embrittlement failure, and then the hydrogen embrittlement test method and process, surface cleaning, coating (especially electroplating), baking Wait.
Two types of hydrogen embrittlement
1. Internal hydrogen embrittlement IHE
Caused by the hydrogen remaining in the material during pickling or electroplating
——Such as steel
——Hydrogen remaining in the material during pickling or electroplating
2. Environmental hydrogen embrittlement EHE
Caused by external source of hydrogen under stress
——Stress corrosion cracking, hydrogen induced stress corrosion cracking and hydrogen promoted stress corrosion cracking, etc.
——Such as fasteners in service
——Ocean, acidic service environment
Three conditions for hydrogen embrittlement of fasteners
1. With high tensile strength or hardness or surface hardening or cold work hardening
2. Under tensile stress
3. Hydrogen has been absorbed
Stress and hydrogen are the triggering conditions; material sensitivity is the basic element of hydrogen embrittlement, so it is the root cause
The hydrogen embrittlement of fasteners during the manufacturing process and service is due to:
——Cold drawing, cold forming, rolling and machining
——Chemical pickling, cleaning, phosphating
——Quenching and tempering (quenching + high temperature tempering), carburizing, surface hardening
——The influence of hot-dip galvanizing heating and cooling
——Served in the corrosive environment of ocean, chemical industry and acid rain
Fasteners are sensitive to hydrogen embrittlement and related measures
Increasing the hardness of the fastener material is likely to cause internal hydrogen embrittlement IHE. According to the hardness of the fastener quenching and tempering, the technical measures to prevent IHE are shown in the table:
1. When the hardness of electroplated fasteners is lower than 320 HV (8.8 level), there is no special baking to remove hydrogen measures;
2. When the hardness of the electroplated fastener is equal to or higher than 320 HV (10.9 level) and includes 380 HV (12.9 level), after additional process verification and/or product testing, it should be baked to remove hydrogen;
3. Fasteners with complex components and large cold heading deformation ratio (such as carriage bolts and hexagon socket countersunk head screws should be baked to remove hydrogen;
4. At the same time, more attention should be paid to controlling hydrogen embrittlement and minimizing the degree of hydrogen generation during the production process and the process of coating shoes.
Baking is a low-temperature heat treatment that can make hydrogen escape from the part or migrate to the trap position without moving. As the content of interstitial hydrogen atoms (free hydrogen atoms) decreases due to baking, the fracture time and threshold stress are increased.
Baking should be carried out before the purification coating process, or after the passivation coating/sealant/top coating process, but the performance of the coating system should not be impaired.
The three elements of baking effect include:
3. The permeability of the coating