Why does hydrogen embrittlement fracture occur in steel?

Let’s look at a few cases of accidents:

  • At the beginning of World War II, a Spitpie fighter of the Royal Air Force crashed due to a broken engine spindle, destroying the plane, and the incident had shocked the British court.
  • In 1975, an oil refinery in Chicago, the United States, a 15cm stainless steel pipe suddenly ruptured, causing an explosion and fire, resulting in a long-term shutdown.
  • France in the exploitation of the Clark gas field, due to the rupture of the pipe, resulting in a fire that lasted a month.
  • China in the development of a large oil field, also had a blowout due to pipeline rupture, causing heavy losses.
  • U.S. “Polaris” missiles can not be launched due to the rupture of the solid fuel engine casing, the U.S. Air Force F-11 fighter jets in the air suddenly crashed, etc..
  • On the way to the car due to the sudden breakage of the drive shaft and overturned, is cutting on the machine tool suddenly broke and other accidents are numerous.

These catastrophic accidents, instantaneous, no prior warning, fracture without discussion, a serious threat to the safety of people’s production and property. At first, scientists on the cause of the accident, there are many different opinions, at a loss. Later, after long-term observation and research, we finally found out the culprit of this series of malicious accidents – hydrogen embrittlement.

Unraveling the mystery of hydrogen embrittlement fracture

Hydrogen embrittlement is usually manifested as a significant decrease in the plasticity of steel, a sharp increase in brittleness, and a tendency to rupture and damage after a period of time under static load (often lower than the σb of the material). It is well known that hydrogen has a certain solubility in steel.

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During the steelmaking process, traces of hydrogen remain in the steel after it has solidified. Usually the steel is produced with a hydrogen content in a small range. The solubility of hydrogen in steel decreases rapidly with decreasing temperature and supersaturated hydrogen is about to precipitate out.
Hydrogen is the fastest diffusing element in steel, with the smallest atomic radius, and still has a strong diffusion capacity in the low temperature region.
If there is enough time for the hydrogen to escape from the steel surface during cooling or if the hydrogen content in the steel is low, hydrogen embrittlement is less likely to occur.
If the cooling speed is fast, the steel section size is relatively large or the hydrogen content in the steel is high, the hydrogen in the center part of the steel is too late to escape, and the excess hydrogen will enter some defects in the steel, such as dendritic gaps, pores. If the hydrogen accumulation near the defect will generate strong internal pressure and lead to the sprouting and expansion of microcracks. This is due to the adsorption of hydrogen atoms after the defect, so that the surface energy is greatly reduced, which leads to a sharp reduction in the critical stress required for steel damage.
In general, hydrogen embrittlement of steel occurs between -50°C and 100°C. When the temperature is too low, the diffusion of hydrogen is too slow, and the aggregation of less will not precipitate; at high temperature, the hydrogen will be “baked” out of the steel, and the hydrogen embrittlement damage will not occur.
With the development of science, people found a new view of the mechanism of hydrogen embrittlement: hydrogen promotes the plastic deformation of the crack tip area, and the plastic deformation, in turn, promotes the concentration of hydrogen in the area, thus reducing the value of the fracture stress in the area, which promotes the generation of microcracks, and the expansion of cracks is accompanied by plastic rheology.

Factors affecting hydrogen embrittlement fracture of steel

It has been found by long-term research that there are three main factors affecting the hydrogen embrittlement fracture of steel as follows.

Environmental factors

If the steel is in an environment with high hydrogen content, such as water, acid, hydrogen, hydrogen diffusion through adsorption in the steel table, causing the steel to become brittle. Also hydrogen partial pressure has a significant effect on the rate of hydrogen crack expansion, and increasing the hydrogen pressure will increase the hydrogen embrittlement sensitivity.

Strength factor

Generally speaking, the higher the strength of steel, the greater the sensitivity to hydrogen embrittlement. Some foreign developed countries explicitly stipulate that “high strength steel is not allowed to be pickled” in order to prevent hydrogen embrittlement. The chemical composition is through the strength to affect the steel hydrogen embrittlement fracture, because the hydrogen and S, P and other atoms deviated in the grain boundary will cause the grain boundary bonding weakness, thus promoting the first fracture along the grain boundary.

Heat treatment

It has been identified that the hydrogen embrittlement of steel is closely related to its microstructure and heat treatment, and experiments and facts show that the worse the stability of the organization in thermodynamics, the greater the sensitivity to hydrogen embrittlement. For example, pearlite, ferrite organization of hydrogen embrittlement tendency is much lower than that of martensite, and mesh distribution of high carbon martensite is the most sensitive.

Heat treatment anti-hydrogen embrittlement measures

In the heat treatment chain, multiple processes require pickling, such as pickling before tempering after quenching, pickling before sandblasting after tempering, pickling before steam treatment or oxynitriding, pickling before surface strengthening such as TiN, and pickling before electroplating. The purpose of pickling is different at different stages, some are to remove oxide, some are to improve the surface activity of the workpiece, some are to reduce the size, etc. Traditional pickling process is tedious, long process, high cost, high energy consumption, serious pollution, poor labor conditions, etc. What is more terrible is that it is very harmful to the inherent quality of steel – hydrogen embrittlement. For this reason, improving the pickling process and taking anti-hydrogen seepage measures have been a concern for several generations.

Improvement of pickling process

The rust on the steel surface is mainly iron oxides and hydroxides, etc. The removal of these rusts is mainly done by acid components with the synergistic action of surfactants, etc. The process of action is roughly dissolution and flaking. In order to overcome the defects brought by conventional pickling, the following improvements can be made.
First, reduce the acid concentration. General steel parts using 30% to 35% HC1 (mass fraction), the speed of removing the oxide skin is fast, but the consumption is large, the acid mist is heavy, and the over-corrosion of the substrate is also strong, making it difficult to ensure product quality. If using low concentration pickling process has obvious economic and social benefits to reduce acid consumption, improve the environment and improve the surface quality of the workpiece. The process uses the porous nature of the oxide, under the action of the wetting agent so that the acid quickly penetrate the interface between the substrate and the oxide Fe + 2HC1 = 2FeC12 + H2↑ chemical reaction, the use of hydrogen mechanical flaking effect, to remove the oxide clean surface. Due to the slow reaction of oxide in dilute acid and the strong adsorption of corrosion inhibitors such as urea on the exposed substrate, overcorrosion is prevented and the useless consumption of acid is reduced, while the amount of hydrogen seepage from the workpiece is also reduced.
Secondly, the use of mixed acid solution of the comprehensive characteristics. 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 prepared into a mixture in appropriate proportions, the functions of both can be improved, and the operating temperature can be reduced.
Again, the use of multi-functional and efficient oil and rust remover. In recent years, “two in one” and other kinds of oil and rust remover and fast rust remover are more commonly used, which is a big progress of the steel pickling process.
Finally, special pickling process is adopted. For different workpiece shape, use, heat treatment state to take different pickling process, that is to say, the pickling process should also be personalized.

Measures to prevent hydrogen embrittlement

Pickling process of hydrogen permeation is a rather complex process, that is, it involves the conjugate step of corrosion, and involves hydrogen adsorption and precipitation in the metal table and immersion in the metal internal parallel and series steps, and also involves the deep-seated problem of stress corrosion. It is shown that direct electrochemical measurements of hydrogen seepage under pickling conditions are a feasible method to study the hydrogen seepage behavior of the pickling process. In order to reduce the degree of hydrogen seepage from steel parts, some anti-hydrogen seepage measures can be taken as follows.

  • 1. Introduce multi-functional slow inhibition. Multi-functional slow inhibition agent with corrosion and fog inhibition function, not only the pickling speed is fast, and the function of inhibiting hydrogen seepage is stronger, and the corrosion inhibition rate is high.
  • 2. Control pickling conditions. The amount of hydrogen seepage of steel in the pickling solution is not too much related to the acidity, but is proportional to the pickling temperature and proportional to the square root of the pickling time, so it is recommended to use the pickling method with high acid concentration and very short pickling time. High-speed steel quenched parts and other high-strength steel pickling more attention to this issue. Specific production units, should develop a rigorous process, control the concentration of acid, acid temperature, pickling time three major elements.
  • 3. Pay attention to the problem of stress corrosion. Stress corrosion cracking refers to the workpiece is subjected to static load tensile stress and a specific corrosive environment under the joint action, resulting in the process of brittle cracking of the material. After straightening the quenched parts, whether it is positive or negative, where the workpiece straightened must first go to stress and then pickling, negating the chances of hydrogen brittle cracking or become brittle, many units have a profound lesson, but did not pay enough attention to.
  • 4. To prevent metal impurities from contaminating the pickling solution. It has been identified that when the pickling solution contains P, As, Sn, Hg, Pb, Zn, Cd and other metal impurities, it will promote the increase in the amount of hydrogen seepage and intensify the tendency of hydrogen embrittlement fracture.
  • 5. Hydrogen drive treatment. Is where the pickled workpiece, the best 180 – 200 ℃ × 3 – 4h hydrogen drive treatment.

Hydrogen embrittlement test method

If hydrogen seepage is produced by heat treatment or surface treatment, it should be driven away in the shortest possible time so that the component does not fail due to hydrogen embrittlement damage. Hydrogen embrittlement can also be determined by test methods.
The former Ministry of Aviation has developed a standard for measuring hydrogen embrittlement (HB5067) for reference. This standard provides for testing and characterizing the hydrogen embrittlement properties of structural and high-strength steels with tensile strengths ≥ 1275 MPa after treatment with electroplating and chemical covering processes using the delayed damage method.

Method Principle

Structural steel and high-strength steel due to hydrogen and stress, in less than the yield strength of the static load for a certain period of time will occur early brittle fracture.

The technical requirements for hydrogen embrittlement specimens are four main points.

  • 1. The specimen material. Apply the same material as the product parts, heat treatment to the upper limit of the tensile strength (hardness and tensile strength have a certain correspondence).
  • 2. The shape and size of the specimen. Delayed damage to the shape and size of the specimen should be in line with the provisions of the following chart, in addition to the dimensional tolerances indicated in the chart, other dimensional tolerances should be in line with the relevant provisions of the national form tolerances.
  • 3. The manufacture of specimens. The specimen is processed in the direction of the downward fiber of the material according to the figure, roughly processed and then heat-treated to the required tensile strength, then finely processed to the specified size, and the notch is ground with a medium-soft fine-grained alumina wheel. Grinding should ensure that the radius of the root of the notch is rounded and smooth. After grinding, projection check to ensure that the notch size meets the drawing requirements. Measure the size of the notch root diameter (φ4.5±0.05mm in the drawing) root by root and make a record of the number.
  • 4. The specimens should be relieved of grinding stress before capping, and the maximum temperature of stress relieving should be 10℃~20℃ lower than the tempering temperature of the workpiece, while avoiding the tempering brittle zone of the material to ensure that the hardness of the specimens will not drop after stress relieving.

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Notch schematic

Test method

The specimen is prepared and plated according to the required plating process. The thickness of the plating of the notched specimen with plating should be not less than 12-18μm, and the plating should be completed at one time, and no back plating or repeated plating is allowed. After plating, the specimen should be de-hydrogenated as soon as possible (not more than 3h). The specification for hydrogen removal should be in accordance with the workpiece or the plating process specification for the steel.
Delayed damage specimens may be performed on a durable testing machine of appropriate tonnage depending on the total load. The cross-sectional area of the specimen is calculated by the size of the root of the notch before plating when loaded. The number of static loads on the specimen is 75% of the notched tensile strength of the uncoated specimen, and the fracture time is recorded.
The notched tensile strength of the uncoated specimen shall be the average of 3-5 specimens.

Evaluation of results

The steel is considered to be hydrogen brittle by this coating process if it is not broken under the specified load for 200h after a delayed damage test with 6 equal specimens. If one specimen break time is less than 200h, then the hydrogen embrittlement performance is considered unqualified.

SourceChina Pipe Fittings Manufacturer – Yaang Pipe Industry (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel productsincluding Super Duplex Stainless Steel FlangesStainless Steel FlangesStainless Steel Pipe FittingsStainless Steel PipeYaang products are widely used in ShipbuildingNuclear powerMarine engineeringPetroleumChemicalMiningSewage treatmentNatural gas and Pressure vessels and other industries.)

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