Hydrogen Embrittlement

The susceptibility of chromium-plated parts to hydrogen embrittlement is affected by hardening of the steel, grinding, surface defects, pickling, cathodic cleaning, and the depth of plate relative to the thickness and hardness of the part being plated. (It should be noted that the chemistry of the chromium plating solution is not a factor.) Unless the hydrogen absorbed during pickling and cathodic cleaning is removed, subsequent plating further embrittles the part to such an extent that breakage can occur during plating. This effect becomes more prevalent with increasing hardness of steel and on parts of thin cross section.

The thickness of plate on thin steel sections is of importance from the standpoint of notch effect. This was illustrated in an actual production setting. Thin sections of steel, 2.5 by 0.5 mm (0.10 by 0.020 in.) and 25 to 38 mm (1 to 1 —in.) long, were plated with chromium to a thickness of 8 to 13 ^m (0.3 to 0.5 mil). The hardness of the steel was 57 to 59 HRC. These parts were aligned by being bent until permanently set. When the thickness of the chromium plate was increased to range from 15 to 23 ^m (0.6 to 0.9 mil), the parts would break before taking a set. Baking them at 205 °C (400 °F) for 4 days did not relieve this condition. It was necessary to decrease the hardness of the steel to 53 to 55 HRC to prevent breakage of parts with heavier plate.

Stress Relieving before Plating. Surfaces to be chromium plated must be free from stresses induced during machining, grinding, or hardening. Stresses from the hardening operation may be further increased during grinding and result in microcracks. If the hardness of the steel is less than 40 HRC, it is unlikely that any damaging effect will occur as a result of residual stress. Steel with a hardness exceeding 40 HRC should be stress relieved before it is plated by heating at 150 to 230 °C (300 to 450 °F).

Baking after Plating. Steel parts with a hardness above 40 HRC should be baked at a temperature of at least 190 °C (375 °F) for 4 h after plating to ameliorate the effects of hydrogen embrittlement. This treatment should be started as soon as possible, preferably within 15 min after plating. The fatigue strength of parts subjected to alternating stresses is reduced by the baking treatment, so such parts should be shot peened before plating.

The use of shot peening and baking, as related to the hardness of steel to be chromium plated, is described in federal specification QQ-C-320B, amendment 1, as follows:

Plated parts below 40 HRC and subject to static loads or designed for limited life under dynamic loads, or combination thereof, shall not require shot peening prior to plating or baking after plating.

Plated parts below 40 HRC that are designed for unlimited life under dynamic loads shall be shot peened in accordance with military specification MIL-S-13165 before plating. Unless otherwise specified, the shot peening shall be accomplished on all surfaces for which the coating is required and on all immediately adjacent surfaces when they contain notched fillets, or other abrupt changes of section size where stresses will be concentrated.

• Plated parts with a hardness of 40 HRC, or above, and subject to static loads or designed for limited life under dynamic loads, or combinations thereof, shall be baked after plating at 190 ± 14 °C (375 ± 25 °F) for not less than 3 h.

• Plated parts with a hardness of 40 HRC, or above, and designed for unlimited life under dynamic loads, shall be shot peened in accordance with military specification MIL-S-13165 before plating. Unless otherwise specified, the shot peening shall be accomplished on all surfaces for which the coating is required and on all immediately adjacent surfaces when they contain notched fillets, or other abrupt changes of section size where stresses will be concentrated. After plating, parts shall be baked at 190 °C (375 °F) for not less than 3 h.

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