Hydrogen Embrittlement

If an electrodeposited coating is to be applied to a highly stressed part or a high-strength (over 1100 MPa, or 160 ksi) heat-treated steel part, it is important that the processing not decrease the static or fatigue strength of the part. Hydrogen embrittlement does not affect fatigue life. Coatings having high residual stresses, such as chromium, affect fatigue life; however, this is not the case with cadmium.

Cadmium deposited from a cyanide solution is more likely to produce hydrogen embrittlement than any other commonly plated metal. Heat-treated steels, particularly those plated and used at 35 HRC and above, are susceptible to hydrogen embrittlement. Most susceptible is spring steel that has not been adequately stress relieved after forming. The requirements of Federal Specification QQ-P-416F may be used as a guide for stress relief before plating and hydrogen embrittlement post-treatment (Table 13). Other guidelines vary from these, but the latest revision (F) seems to be the most stringent.

Table 13 Heat treat specifications required to stress relieve cadmium-plated components

Hardness, HRC

Stress relief before plating

Hydrogen embrittlement relief (within 4 h of plating)

Temperature

Minimum time, h

Temperature

Minimum time, h

°C

°F

°C

°F

34-54

175-205

350-400

4

36-45

175-205

350-400

23(a)

46-54

175-205

350-400

23

>55

120-150

250-300

23

120-150

250-300

23

(a) Fasteners and bearings

(a) Fasteners and bearings

Although the thickness of the plated deposit appears to have no direct bearing on hydrogen embrittlement, it is always more difficult to release the hydrogen (by baking) from heavy deposits.

By adhering to the following procedures, hydrogen embrittlement can be minimized or made inconsequential:

• Use mechanical cleaning methods, such as brushing, blasting, and tumbling.

• Wherever possible, avoid the use of strong acid-pickling solutions and extended exposure to acid pickling.

• If pickling is essential to the preparation of medium-strength and high-strength steel parts, bake the parts at 175 to 205 °C (350 to 400 °F) for 3 h after pickling and before plating.

• In plating, use the higher current densities to produce a more porous deposit; 755 A/m2 (70 A/ft2) in a cyanide bath without brighteners has been satisfactory for steel at 46 HRC.

• After plating, bake parts at 175 to 205 °C (350 to 400 °F) for 3 to 24 h. The shorter baking periods are generally adequate for parts with a tensile strength below about 1520 MPa (220 ksi); longer baking periods are recommended for steel of tensile strength above about 1520 MPa (220 ksi) or for lower-strength parts if sharp notches or threads exist. Parts greater than 25 mm (1 in.) thick should also be baked for 24 h. The elapsed time between plating and baking must never exceed 8 h and should be carried out as soon as possible, preferably within 4 h.

• Plate parts to a thickness of about 5 pm (200 pin.), bake for 3 h at 195 °C (385 °F), activate in cyanide, and then complete the plating to the required final thickness.

The applications of shot peening and baking, as related to the hardness of the steel to be plated, are described in Federal Specification QQ-C-320 (Amendment 1) and are summarized in the article "Industrial (Hard) Chromium Plating" in this Volume.

0 0

Post a comment