At the beginning of World War II, a Spitfire fighter of the Royal Air Force of the United Kingdom crashed due to the fracture of the engine main shaft, resulting in the destruction of the aircraft and the death of the pilot. This incident once shocked the British government and the public.
In 1975, a 15-centimeter stainless steel pipe suddenly burst in a refinery in Chicago, USA, causing an explosion and fire, leading to a long-term shutdown. In the exploitation of the Clark gas field in France, a pipeline burst, causing a fire that lasted for a month.
These catastrophic and malignant accidents occur instantaneously without any prior warning or negotiation, posing a serious threat to people’s production and property safety. At first, scientists were at a loss as to the cause of the accidents, with various theories abounding. However, after long-term observation and research, they finally identified the culprit behind this series of malignant accidents – hydrogen embrittlement.
Unraveling the Mystery of Hydrogen Embrittlement Fracture
Hydrogen embrittlement is typically characterized by a significant decrease in the plasticity of steel and a sharp increase in its brittleness, with a tendency to fracture under static load (often below the material’s σb) after a period of time. It is well known that hydrogen has a certain solubility in steel.
During the steelmaking process, a small amount of hydrogen remains in the steel after the molten steel solidifies. The hydrogen content in the steel produced under normal circumstances is within a very small range. The solubility of hydrogen in steel decreases rapidly with the drop in temperature, and the supersaturated hydrogen will precipitate.
Hydrogen is the element that diffuses the fastest in steel, with the smallest atomic radius, and still has strong diffusion ability in the low-temperature zone. If there is sufficient time for hydrogen to escape from the surface of the steel during cooling or if the hydrogen content in the steel is low, hydrogen embrittlement is less likely to occur.
However, if the cooling rate is fast, the cross-sectional size of the steel part is large, or the hydrogen content in the steel is high, the hydrogen located in the center of the steel part cannot escape in time, and the excess hydrogen will enter some defects in the steel, such as dendrite gaps and pores.
If hydrogen accumulates near the defect, it will generate a strong internal pressure, leading to the initiation and propagation of microcracks. This is because the adsorption of hydrogen atoms on the defect reduces the surface energy significantly, thereby sharply lowering the critical stress required for steel failure.
Generally speaking, hydrogen embrittlement of steel occurs within the temperature range of -50 to 100℃ near room temperature. When the temperature is too low, the diffusion rate of hydrogen is too slow, and the accumulation is small, so no hydrogen embrittlement damage will occur; at high temperatures, hydrogen will be “baked” out of the steel, and hydrogen embrittlement damage is unlikely to occur.
With the development of science, people have discovered a new view on the mechanism of hydrogen embrittlement: hydrogen promotes plastic deformation at the crack tip, and plastic deformation promotes the concentration of hydrogen in this area, thereby reducing the fracture stress value of this area, which promotes the generation of micro-cracks, and the plastic flow also accompanies the crack propagation.
Factors Affecting Hydrogen Embrittlement Fracture of Steel
After long-term research, it has been found that the factors influencing hydrogen embrittlement and fracture of steel mainly include the following three aspects:
(1) Environmental factors
For instance, when steel is in an environment with a high hydrogen content, such as water, acid, or hydrogen gas, hydrogen diffuses by adsorbing onto the surface of steel, causing it to become brittle. Meanwhile, hydrogen partial pressure has a significant impact on the propagation speed of hydrogen cracks. Increasing the hydrogen pressure will enhance the sensitivity to hydrogen embrittlement.
(2) Strength factor
Generally speaking, the higher the strength of steel, the greater its susceptibility to hydrogen embrittlement. Some developed countries abroad have explicitly stipulated that “high-strength steel is not allowed to be pickled” precisely to prevent hydrogen embrittlement.
The chemical composition affects the hydrogen embrittlement fracture of steel through strength. This is because hydrogen and atoms such as S and P segregate at the grain boundaries, weakening the grain boundary bonding force and thus promoting fracture along the grain boundaries first.
(3) Heat Treatment
The hydrogen embrittlement of steel is closely related to its microstructure and heat treatment. Experiments and facts show that the poorer the thermodynamic stability of the microstructure, the greater the sensitivity to hydrogen embrittlement. For instance, the hydrogen embrittlement tendency of pearlite and ferrite is much lower than that of martensite, and the reticular distribution of high-carbon martensite is the most sensitive.
Measures for Preventing Hydrogen Embrittlement in Heat Treatment
In the heat treatment industrial chain, multiple processes require pickling, such as pickling after quenching and before tempering, pickling after tempering and before sandblasting, pickling before steam treatment or nitriding, pickling before surface strengthening such as TiN, and pickling before electroplating, etc. The purpose of pickling varies at different stages. Some are to remove oxide scale, some are to enhance the surface activity of the workpiece, and some are to reduce the size, etc.
The traditional pickling process is cumbersome, has a long flow, is costly, energy-consuming, highly polluting, and has poor working conditions. What is even more terrifying is that it poses a significant threat to the internal quality of steel – hydrogen embrittlement. Therefore, improving the pickling process and taking anti-hydrogen diffusion measures have become issues of concern for generations.
(1) Improvement of Pickling Process
The rust on the surface of steel is mainly composed of iron oxides and hydroxides, etc. The removal of these rusts is mainly accomplished by the synergistic action of acid components and surfactants, etc. The process is roughly dissolution and peeling. To overcome the drawbacks of conventional pickling, the following improvements can be made.
First, reduce the acid concentration. Generally, steel parts use 30% to 35% HCl (mass fraction), which removes the oxide scale quickly but consumes a large amount of acid, generates heavy acid mist, and has a strong over-corrosion effect on the substrate, making it difficult to ensure product quality. If a low-concentration pickling process is used, it can significantly reduce acid consumption, improve the environment, and enhance the surface quality of the workpiece, bringing obvious economic and social benefits.
This process takes advantage of the porosity of the oxide scale. Under the action of wetting agents, the acid solution rapidly penetrates to the interface between the substrate and the oxide scale, where the chemical reaction Fe + 2HCl == 2FeCl2 + H2↑ occurs. The mechanical stripping effect of hydrogen gas is utilized to remove the oxide scale and clean the surface.
Due to the slow reaction of oxides in dilute acid, corrosion inhibitors such as urea have a strong adsorption force on the exposed substrate, preventing over-corrosion, reducing the wasteful consumption of acid, and also reducing the amount of hydrogen absorbed by the workpiece.
Secondly, take advantage of the comprehensive characteristics of the mixed acid solution. In production, hydrochloric acid or sulfuric acid is commonly used to remove rust, but the performance of the two is different. If hydrochloric acid and sulfuric acid are mixed in an appropriate proportion to form a mixed solution, it can combine the functions of both, not only increasing the rust removal speed but also reducing the operating temperature.
Thirdly, use multi-functional and highly efficient oil and rust removers. In recent years, “two-in-one” and other types of oil and rust removers and fast rust removers have emerged and are widely used, which represents a significant advancement in the steel pickling process.
Finally, adopt special pickling processes. Different pickling processes should be adopted for different workpiece shapes, uses, and heat treatment states, meaning that the pickling process should also be personalized.
(2) Measures to Prevent Hydrogen Embrittlement
The hydrogen permeation during pickling is a rather complex process, involving not only the conjugate steps of corrosion but also the parallel and series steps of hydrogen adsorption and desorption on the metal surface and its penetration into the metal interior. It also involves the deep-seated issue of stress corrosion.
Research indicates that direct electrochemical measurement of hydrogen permeation under pickling conditions is a feasible method to study the hydrogen permeation behavior during pickling. To reduce the hydrogen permeation degree of steel parts, the following anti-hydrogen permeation measures can be taken.
First, introduce multi-functional corrosion inhibitors. Multi-functional corrosion inhibitors have both corrosion inhibition and anti-fogging functions. They not only have a fast acid washing speed but also have a strong ability to prevent hydrogen permeation and a high corrosion inhibition rate.
Second, control the pickling conditions. The amount of hydrogen absorbed by steel in the pickling solution is not significantly related to the acidity, but is directly proportional to the pickling temperature and proportional to the square root of the pickling time. It is recommended to use a pickling method with a higher acid concentration and a very short pickling time.
For high-strength steels such as high-speed steel quenched parts, this issue should be paid more attention to. Specific production units should formulate strict processes and control the three key elements of acid concentration, acid solution temperature, and pickling time.
Third, pay attention to the issue of stress corrosion. Stress corrosion cracking refers to the process in which a workpiece, under the combined effect of static tensile stress and a specific corrosive environment, undergoes brittle cracking. For quenched parts that have been straightened, whether they are struck from the front or the back, all straightened parts must be stress-relieved before acid washing.
This significantly reduces the probability of hydrogen embrittlement cracking or embrittlement. Many units have learned profound lessons from this, but it has not received sufficient attention.
Fourth, prevent metal impurities from contaminating the pickling solution. It has been found that when the pickling solution contains metal impurities such as P, As, Sn, Hg, Pb, Zn, and Cd, it will promote an increase in hydrogen absorption and intensify the tendency of hydrogen embrittlement fracture.
Fifth, hydrogen removal treatment. It is best to conduct a hydrogen removal treatment at 180 to 200 degrees Celsius for 3 to 4 hours for all workpieces that have undergone acid washing.
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