What is hydrogen embrittlement
Dec. 30, 2020
What is hydrogen embrittlement
The phenomenon that metal materials contain hydrogen or work in a hydrogen-containing environment, their plasticity and toughness decrease is called hydrogen embrittlement. The solubility of hydrogen in body-centered cubic metals is very small, but the diffusion rate is extremely high, so it is also most sensitive to hydrogen embrittlement. Hydrogen embrittlement can also occur in face-centered cubic metals, but relatively speaking, it is less sensitive to hydrogen embrittlement. Metallic materials have caused hydrogen in the smelting, pickling, electroplating, welding, heat treatment and other processes. When the material is subjected to external load, the hydrogen embrittlement caused by the existing hydrogen inside is called internal hydrogen embrittlement; material In the service process, the embrittlement caused by the absorption of hydrogen from the environment is called environmental hydrogen embrittlement. When hydrogen embrittlement can be reduced or removed by dehydrogenation treatment, it is called reversible hydrogen embrittlement; if hydrogen has caused permanent damage to the material, the situation that hydrogen embrittlement cannot be eliminated even after dehydration treatment is called irreversible hydrogen brittle. When hydrogen embrittlement occurs, the fracture mode of the material changes from ductile fracture to brittle fracture in most cases.
High-strength steels are very sensitive to internal hydrogen embrittlement and environmental hydrogen embrittlement; while low-strength steels, austenitic steels and nickel-based alloys are not sensitive to internal hydrogen embrittlement, but their environmental hydrogen embrittlement tendencies are relatively large. Mn significantly increases the hydrogen embrittlement tendency of ferrite and martensitic steels, but has relatively little effect on the hydrogen embrittlement of austenitic steels. For low-alloy ultra-high strength steel. The fine lath martensite with uniform carbide particles has the best resistance to hydrogen embrittlement. For medium and low-strength steel s quenched and tempered martensite or bainite structure has the best hydrogen embrittlement resistance. For pearlite steel, its hydrogen embrittlement resistance increases with the decrease of the pearlite layer spacing. Most studies have shown that the sensitivity of spherical pearlite to hydrogen embrittlement and post-hydrogen-induced cracking is smaller than that of sheet-produced pearlite. High-carbon quenched martensite has the greatest hydrogen embrittlement sensitivity. When temper embrittlement does not occur, the As the fire temperature increases, its resistance to hydrogen embrittlement is improved.
In the pre-treatment and electroplating process, the penetration of hydrogen into the plating layer and the metal matrix can also cause hydrogen embrittlement. Electroplating is an electrochemical process. During electroplating, the base metal or workpiece protected by the defendant is used as the cathode, and the plated metal is used as the anode. It is diffused into the electrolyte solution containing the plated metal ions. The reduction reaction is that when metal atoms are deposited, hydrogen ions are reduced to hydrogen atoms. Some of the hydrogen atoms form hydrogen and escape, and the other part penetrates into the coating and the matrix metal lattice to cause hydrogen embrittlement. The change of the flow density of the workpiece before plating, the nitriding rate will have a maximum value.
In addition to reasonable selection of electroplating layer and control of process parameters to reduce hydrogen permeation, the widely used hydrogen removal treatment process is heating and baking. After electroplating, the hydrogen removal treatment of 150~300℃ and holding for 2~4h is the main method to eliminate the embrittlement of electroplating. The specific treatment temperature and time should be determined according to the size, strength, properties of the coating and the length of the plating time. The hydrogen removal treatment is often carried out in an oven.
