Regarding the problem of hydrogen embrittlement during Hard chrome plating

Regarding the problem of hydrogen embrittlement during Hard chrome plating

1. The definition of hydrogen embrittlement

Hydrogen embrittlement is the polymerization of hydrogen dissolved in steel into hydrogen molecules, causing stress concentration, exceeding the strength limit of steel, and forming small cracks in the steel, also known as white spots. Hydrogen embrittlement mainly occurs on low-alloy high-strength steel, stainless steel and elastic parts. The main cause of hydrogen embrittlement is surface treatment. Hydrogen generated during the treatment of parts such as electroplating and nitriding penetrates into the metal and leads to disordered lattice arrangement. , Produce distortion, cause internal stress to increase, make the metal or the coating brittle, and cause the fracture of the parts or the peeling of the coating. In the process, hydrogen embrittlement needs to meet two conditions: the metal has a higher hydrogen content; there is a certain external force.

Due to the characteristics of hysteresis cracking caused by hydrogen embrittlement, the service performance of parts is seriously affected, and there is a greater safety hazard. Therefore, in the design and processing of parts, especially hydraulic components working under high pressure, strict control is required.

2. Applicable scope and procedures of dehydrogenation embrittlement treatment

Not all metals will produce hydrogen embrittlement during surface treatment. According to research data and practice, hydrogen embrittlement mainly occurs in the following materials after surface treatment of parts.
(1) Heat-treated carbon steel and alloy steel parts with tensile strength above 981N/mm2 (or hardness above 38HRC).
(2) Parts of spring steel (including piano wire, oil tempered steel wire, and high-carbon steel wire) with a hardness of 370HV or more.
(3) Martensitic stainless steel products with a tensile strength of 1236N/mm2 or more (or 36HRC or more) subjected to heat treatment.
In the current processing, the main processes that can cause hydrogen embrittlement are pickling, electroplating, nitriding, and electrolysis. When the above processes are processing the surface of the part, hydrogen will be generated and penetrated into the part, resulting in hydrogen embrittlement.

3. Case analysis of hydrogen embrittlement

Hydraulic products generally need to work under high pressure, and some parts bear greater force. When the parts absorb hydrogen, they will break at their weak points. The valve core is hollow. Due to functional requirements, the hollow part needs to be sealed with a one-way valve. , The valve core is working under high pressure, which requires strong impact resistance and toughness at the matching part of the one-way valve. At the same time, in order to meet certain wear resistance requirements, many manufacturers choose this kind of spool surface chrome treatment, and require The plating layer is thick, but due to the low current efficiency of the chromium plating process, it takes a long time for electroplating and severe hydrogen permeation.

4. measures and prevention of hydrogen embrittlement

To remove the hydrogen embrittlement caused by the adsorption of hydrogen on the parts, the drying process is generally selected, and the drying process is generally divided into:
(1) General drying treatment, the drying temperature is about 200℃.
(2) Low temperature drying treatment, the drying temperature is below 180℃.
The drying method is as follows:
(1) Drying treatment is best carried out immediately after the hydrogen adsorption process, and the longest shall not exceed 4h.
(2) The time and temperature of the drying treatment are carried out in accordance with the requirements in the attached table. In principle, the treatment must be completed at one time. After the treatment starts, it cannot be interrupted. It is continuously maintained and completed within the prescribed time.
(3) In the case of re-plating after the plating layer is peeled off, or in the case where another plating layer is applied after the plating, the embrittlement process is more than two times, when the drying treatment time from the initial process to the final process is within 4h, only The drying treatment after the final step is carried out, and the others can be omitted.

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In the design and processing of parts, hydrogen embrittlement should also be prevented in advance. For example, the materials that are prone to hydrogen embrittlement can be properly heated, shot peened or vibrated before surface treatment, so that the hydrogen on the workpiece can escape , The stress is released and hydrogen embrittlement is reduced; on the premise of meeting the performance, try to use processes with less hydrogen permeation, such as surface treatment degreasing, rust removal, etc., and chemical degreasing, cleaning agents or solvents can be used. Oil: When removing rust and scale, use sand blowing to remove rust as much as possible. If pickling is used, it is necessary to add corrosion inhibitors in the pickling solution.

5. concluding remarks

Generally speaking, the greater the strength of the material, the greater the sensitivity to hydrogen embrittlement. In addition to the hardness, the following points should be considered comprehensively:
(1) The use safety factor of parts, the parts with high safety importance should be strengthened to remove hydrogen treatment.
(2) Parts with small cross-sectional areas, such as small springs, thinner springs, etc.
(3) With notched parts that are prone to stress concentration.
Hydrogen embrittlement in the surface treatment process is extremely harmful. The requirement for hydrogen embrittlement is a clear concept that technicians must make when designing and processing. Appropriate measures should be taken according to different parts and usage conditions to achieve the purpose of eliminating hydrogen embrittlement.