1. concept of hydrogen embrittlement
Hydrogen embrittlement refers to the phenomenon that the mechanical properties of metal materials are severely degraded and brittle due to hydrogen absorption or hydrogen permeation during the process of smelting, processing, heat treatment, pickling and electroplating, or when they are used in hydrogen-containing media for a long time. .
Hydrogen embrittlement is not only found in ordinary steel, but also in stainless steel, aluminum alloys, titanium alloys, nickel-based alloys and zirconium alloys.
From the perspective of mechanical properties, hydrogen embrittlement has the following manifestations: hydrogen has little effect on the yield strength and ultimate strength of metal materials, but the elongation and the reduction of area are severely reduced, the fatigue life is significantly shortened, and the impact toughness value is significantly reduced.
Under the continuous action of tensile stress lower than the breaking strength, the material will suddenly be brittle after a period of time.2.Mechanism of dihydrogen embrittlement
The mechanism of hydrogen embrittlement is still controversial in academic circles, but most scholars believe that the following effects are the main reasons for hydrogen embrittlement:
1. During the solidification process of the metal, the hydrogen dissolved in it was not released in time, and diffused to the vicinity of the defect in the metal. At room temperature, the atomic hydrogen combined into molecular hydrogen at the defect and continued to gather, thereby generating a huge internal pressure. Crack the metal.
2. In the hydrocracking furnace in the petroleum industry, the working temperature is 300-500 degrees, and the hydrogen pressure is as high as dozens to hundreds of atmospheric pressures. At this time, hydrogen can penetrate into the steel and react with carbon to generate methane.
Methane bubbles can nucleate and grow in places such as inclusions or grain boundaries in steel, and generate high pressure to cause damage to steel.3. Under stress, hydrogen dissolved in metal may also cause hydrogen embrittlement.
Atoms in metals are arranged periodically according to certain rules, which is called lattice. Hydrogen atoms are generally in the gaps between metal atoms, and the local places where atoms are misaligned in the crystal lattice are called dislocations, and hydrogen atoms tend to gather near dislocations.
When the external force acts on the metal material, the stress distribution inside the material is uneven, and stress concentration will occur in the rapid transition area of the material shape or at the internal defects and microcracks of the material.
Under the stress gradient, the hydrogen atoms diffuse in the lattice or follow the dislocation movement to the stress concentration area. Due to the interaction between hydrogen and metal atoms, the bonding force between metal atoms is weakened, so cracks will initiate and expand in the high hydrogen area, resulting in brittle fracture.
In addition, the enrichment of hydrogen in the stress concentration area promotes the plastic deformation of this area, resulting in the generation and propagation of cracks.
In addition, there are many microcracks in the crystal, and when hydrogen gathers to the cracks, it is adsorbed on the surface of the cracks, reducing the surface energy, so the cracks are easy to expand.
4. Some metals have a greater affinity with hydrogen, and supersaturated hydrogen is easily combined with this metal atom to form a hydride, or under the action of an external force, the high concentration of hydrogen accumulated in the stress concentration area combines with the metal atom to form a hydride .
Hydride is a brittle phase structure, which often becomes the source of fracture under the action of external force, resulting in brittle fracture.
3. Prevention of hydrogen embrittlement
Hydrogen embrittlement has brought risks to the use of metals by humans. Therefore, the purpose of studying hydrogen embrittlement is mainly to prevent hydrogen embrittlement. Since there are many reasons for hydrogen embrittlement, and human understanding is not thorough enough, it is still impossible to completely prevent hydrogen embrittlement. The current measures to prevent hydrogen embrittlement are as follows:
1. Avoid excessive hydrogen brought in – reduce relative humidity during the metal smelting process, and bake various additives and ingot molds to keep them dry.
Dehydrogenation treatment–slow down the cooling rate of the steel ingot to allow enough time for hydrogen to escape, or anneal the steel in a vacuum furnace to remove hydrogen.
2. Appropriate alloying elements are added to the steel to form a diffusely distributed second phase, which serves as an irreversible trap for hydrogen, so that the content of movable hydrogen in the material is relatively reduced, thereby reducing the tendency of hydrogen embrittlement of the material.
3. Develop new hydrogen-resistant steels. The diffusion rate of hydrogen in the body-centered cubic crystal structure is much higher than that in the hexagonal close-packed structure or face-centered cubic structure, so hydrogen-resistant steels often have a face-centered cubic structure. Based on the same phase, plus other strengthening measures, it can meet the requirements of service strength.
4. Take appropriate protective measures – add a corrosion inhibitor to the acid solution or electrolyte during pickling or electroplating, so that a large number of hydrogen atoms generated in the solution combine with each other on the metal surface to form hydrogen molecules and escape directly from the solution. Prevent hydrogen atoms from entering the interior of the metal.