Hydrogen embrittlement of bolts after electroplating and its solution

Direct conclusion: The bolts of grade 10.9, 12.9, 14.9, and carbon steel heat-treated and hardened bolts produced with alloy steel as raw material, after electroplating (or after pickling only). Hydrogen embrittlement must be removed at the first time, and the method of removing hydrogen embrittlement is: 200 degree oven heating for 3-4 hours to precipitate hydrogen atoms.
Hydrogen embrittlement is usually characterized by a significant decrease in the plasticity of the steel, a sharp increase in brittleness, and a tendency to rupture under static load (often below the σb of the material). The tendency to rupture and damage occurs after a period of time. It is well known that hydrogen has a certain solubility in steel. 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 into the defect, which results in a significant reduction in the surface energy and thus a sharp reduction in the critical stress required to damage the steel.

Causes of hydrogen embrittlement

(0). Electroplating and pickling
Hydrogen seepage during pickling is a rather complex process, i.e. it involves the conjugate step of corrosion, the parallel and series steps of hydrogen adsorption and precipitation on the metal surface and immersion inside the metal, and the deeper problem of stress corrosion. The higher the strength of the workpiece, the higher the risk of hydrogen embrittlement during the plating and pickling process.
(1). Environmental factors
If the steel is in an environment with high hydrogen content, such as water, acid or hydrogen, hydrogen diffuses through adsorption on the steel surface, causing the steel to become brittle. At the same time, hydrogen partial pressure has a significant effect on the rate of hydrogen crack expansion, and increasing hydrogen pressure will increase hydrogen embrittlement sensitivity.
(2). High strength and heat treatment

Generally speaking, the higher the strength of steel, the greater the hydrogen embrittlement sensitivity. Some foreign developed countries explicitly stipulate that “high strength steel is not allowed to be pickled” in order to prevent hydrogen embrittlement.

Heat Treatment

The hydrogen embrittlement of steel is closely related to its microstructure and heat treatment. 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 martensite, and the network distribution of high carbon martensite is the most sensitive.
Anti-hydrogen embrittlement measures
(0). Hydrogen drive (hydrogen removal). Treatment. After pickling workpiece, carry out 200℃×3~4h hydrogen repelling treatment.
(1). Improvement of pickling process. For example, reduce the acid concentration. The pickling process with low concentration has obvious economic and social benefits to reduce acid consumption, improve the environment, and improve the surface quality of the workpiece, and also reduce the degree of hydrogen penetration of the workpiece. Secondly, multi-functional high-efficiency oil and rust remover can also be used. 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 major progress of the steel pickling process. Also can introduce multi-functional slow inhibition. Multi-functional slow inhibition agent with corrosion and fog inhibiting function, not only the pickling speed, and inhibit the function of hydrogen seepage is stronger, high corrosion inhibition rate.
(2). Reduce pickling time. Steel in the pickling solution of the amount of hydrogen permeation pickling time is proportional, do not pickling time is too long.
(3). Stress corrosion first and then pickling (only for some special workpieces). . Concerned about stress corrosion. Stress corrosion cracking is the process of brittle cracking of the material caused by the combined effect of static load tensile stress and specific corrosive environment on the workpiece. After straightening the quenched parts, whether it is positive or negative, where the straightened workpiece must first remove the stress and then pickling, negating the chances of hydrogen brittle cracking or becoming brittle is quite high.
(4). 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, intensifying the tendency of hydrogen embrittlement fracture.

Hydrogen embrittlement test method

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 plating If hydrogen seepage is generated due to 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 Aeronautics has developed a standard for measuring hydrogen embrittlement (HB5067). It is available for reference. The standard provides for the test and identification of the hydrogen embrittlement properties of structural and high-strength steels with tensile strength ≥ 1275 MPa after treatment with electroplating and chemical covering processes, using the delayed damage method.
(1). Principle of the method
Early brittle fracture occurs in structural and high-strength steels under static loads less than the yield strength for a certain time due to hydrogen and stress.
(2). Technical requirements for hydrogen embrittlement specimens
There are four main points.
First, the specimen material. Should be made of the same material as the product parts, heat treatment to the upper limit of tensile strength (hardness and tensile strength have a certain correspondence). .
Second, the shape and size of the specimen. Delayed damage of the specimen shape and size should be in line with the provisions of the following figure, in addition to the dimensional tolerances indicated in the figure, other dimensional tolerances should be in line with the relevant provisions of the national tolerance of form and position.
Third, the manufacture of the specimen. Process the specimen along the material in the direction of the cis-fiber according to the figure, rough machining and then heat treatment to the required tensile strength, and then finish machining to the specified size, notch grinding with a medium-soft fine-grained aluminum oxide 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 requirements of the drawing. Measure the notch root diameter (φ4.5±0.05mm in the drawing) root by root. The dimensions of the notch are recorded and numbered.
Fourth, the specimens should all 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 specimen after stress relieving will not drop.
(3). Test method
The specimen is prepared and plated according to the required plating process. The thickness of the plating layer of the specimen with the plating layer should be not less than 12~18μm. The plating layer should be completed at one time, and no back plating or repeated plating is allowed. After plating, the specimen should be plated as soon as possible (not more than 3h). Perform hydrogen removal. The specification for hydrogen removal shall be in accordance with the specification of the workpiece or the steel plating process.
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 at the root size of the notch before plating. 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 to 5 specimens.
(4). Evaluation of results
Using 6 equal specimens for delayed damage test, 200h under the specified load are not broken, then the steel by this coating process hydrogen embrittlement performance is considered qualified. If one specimen break time is less than 200h, the hydrogen embrittlement performance is considered unqualified.