1. Causes and hazards of hydrogen embrittlement of fasteners
Fastening and rivet studs, rivet nuts, rivet screws and other fasteners in the manufacturing process (such as: quenching and tempering (quenching + high temperature tempering), cyanidation, carburizing, chemical cleaning, phosphating, electroplating, rolling mill In processes and service environments such as machining and machining (inappropriate lubrication and scorching), due to the reaction of cathodic protection or the reaction of corrosion, hydrogen atoms may enter the matrix of steel or other metals and remain in the matrix. Under stress conditions of yield strength (nominal strength of the alloy), it may result in a decrease or loss of elongation or load carrying capacity, cracks (usually submicroscopic), and sudden breaks during service or storage. Causes severe brittle failure. There are many reasons for hydrogen embrittlement of fasteners, but the electroplating process is one of the key factors.
The brittle fracture of fasteners due to hydrogen embrittlement generally occurs suddenly and is unpredictable, so the consequences of this form of failure are very serious. Especially in the safety performance requirements, it is necessary to reduce the occurrence of hydrogen embrittlement. Therefore, it is an important task to remove hydrogen embrittlement by electroplating fasteners.
2. The situation and characteristics of fasteners prone to hydrogen embrittlement failure
A, high tensile strength or hardening or surface hardening;
B, adsorbing hydrogen atoms;
C. Under tensile stress.
As the hardness of the part increases, the carbon content increases, and the degree of cold work hardening is strengthened, during the pickling and plating process. The solubility of hydrogen and thus the total amount of hydrogen absorbed will also increase, which means that the hydrogen embrittlement sensitivity of the part is stronger. Parts with smaller diameters are more sensitive to hydrogen embrittlement than parts with larger diameters.
3. Measures to reduce hydrogen embrittlement of electroplated fasteners
A. For electroplated fasteners with a hardness greater than or equal to 320 HV, the stress release process should be added before the cleaning process; during the cleaning process, anti-corrosive acid, alkaline or mechanical methods should be used. The time to immerse in the preservative acid is designed to be as small as possible.
B. For fasteners with a hardness exceeding 320 HV, the heat treatment process after cold drawing, cold forming, machining, and grinding shall comply with the provisions of ISO9587D;
C. The intentional introduction of residual stress should be avoided as much as possible. Such as: bolts, screws are milled after heat treatment;
D. Heat-treated or cold-worked hardened fasteners with a hardness exceeding 385 HV or a performance grade of 12.9 and above are not suitable for pickling treatment. Special methods without acid, such as alkaline cleaning and sand blasting, should be used.
E. Heat treatment or cold work hardening of fasteners with a hardness exceeding 365 HV shall be carried out by a large cathode power plating solution plating process.
F. Steel fasteners For the purpose of electroplating, the surface should be specially treated, that is, after a small immersion time, the plating is performed.
G. Select a suitable coating thickness because the increase in plating thickness increases the difficulty of hydrogen release;
4. Measures to remove hydrogen embrittlement
The measure to remove hydrogen embrittlement is actually the drying process, which can be said to reduce the hydrogen embrittlement to a small, heating process at a given temperature and within a specified time. The post-plating drying process is the process of evaporating and irreversibly collecting hydrogen in the steel to release hydrogen atoms. Details of the drying process are given in Appendix A of the standard. The drying process is different depending on the product variety, geometry, material, performance grade or hardness, cleaning process, coating type and plating process. Pay attention to the following points when removing hydrogen embrittlement:
A. It should not be dried at a temperature exceeding the tempering of the parts;
B. The drying process should be carried out immediately after electroplating (within one hour) before the chromate passivation treatment;
C, the drying temperature is reasonable at 200 ° C - 230 ° C, generally using a lower drying temperature and a longer drying time;
D. The drying duration is selected within 2h-24h, generally 8h. It is a typical example of drying duration.