Why does hydrogen embrittlement fracture occur in seamless steel pipes?
Hydrogen embrittlement usually manifests as a significant decrease in plasticity of steel, a sharp increase in brittleness, and a tendency to fracture after a period of static load (usually lower than the σb of the material).
Generally speaking, the factors affecting the hydrogen embrittlement fracture of seamless pipes mainly include the following three aspects:
1) Environmental factors
When the steel is in an environment with high hydrogen content such as water, acid, hydrogen, etc., the hydrogen diffuses through adsorption on the surface of the steel, making the steel brittle. At the same time, the partial pressure of hydrogen has a significant effect on the growth rate of hydrogen cracks, and increasing the partial pressure of hydrogen will increase the susceptibility to hydrogen embrittlement.
2) Strength coefficient
In general, the higher the strength of the steel, the greater the susceptibility to hydrogen embrittlement. Some countries expressly stipulate that “high-strength steel is not allowed to be pickled” to prevent hydrogen embrittlement. The chemical composition affects the hydrogen embrittlement fracture of the steel through the strength. This is because the segregation of hydrogen and S, P and other atoms at the grain boundary will weaken the binding force of the grain boundary, thereby promoting the first fracture along the grain boundary.
3) heat treatment
The hydrogen embrittlement of steel is closely related to its microstructure and heat treatment. Experiments and facts have shown that the poorer the thermodynamic stability of the structure, the greater the susceptibility to hydrogen embrittlement. For example, the hydrogen embrittlement tendency of pearlite and ferrite is much lower than that of martensite, and the high-carbon martensite distributed in a network is the most sensitive.
Measures to prevent hydrogen embrittlement
The research shows that the direct electrochemical measurement of hydrogen permeation under pickling conditions is a feasible method to study the hydrogen permeation behavior in pickling process. In order to reduce the degree of hydrogen permeation of steel parts, the following measures can be taken to prevent hydrogen permeation.
1) Introduce multifunctional slow inhibition. The multi-functional corrosion inhibitor has the functions of corrosion inhibition and mist retardation, not only has a fast pickling speed, but also has a strong hydrogen permeation inhibition effect and a high corrosion inhibition rate.
2) Control pickling conditions. The amount of hydrogen permeation of steel in the pickling solution has little to do with the acidity, but is directly proportional to the square root of the pickling temperature and pickling time. A method with higher acid concentration and shorter pickling time is recommended. Pickling of high-strength steel such as high-speed steel quenched parts should pay more attention to this problem. The specific production unit should formulate a strict process flow to control the three elements of acid concentration, acid temperature, and pickling time.
3) Pay attention to the problem of stress corrosion. Stress corrosion cracking refers to the process of brittle cracking of materials under the combined action of static tensile stress and specific corrosion environment. Straightening and quenching, whether it is a frontal strike or a counterattack, all straightened workpieces must be stress-relieved before pickling to deny the possibility of hydrogen embrittlement cracking or embrittlement.
4) Prevent metal impurities from contaminating the pickling solution. Studies have found that when the pickling solution contains P, As, Sn, Hg, Pb, Zn, Cd and other metal impurities, it will promote the increase of hydrogen permeation and aggravate the tendency of hydrogen embrittlement fracture.