Example 10 Baking of Cadmium Plated 6150 Alloy Steel to Eliminate Hydrogen Embrittlement

A spring used in a high-temperature relief valve under intermittent loading had dimensions and specifications as follows: wire size, 8.76 mm (0.345 in.); outside diameter of spring, 50 mm (2 in.); length, 75 mm (3 in.); six coils; 6150 alloy steel at 43 HRC; stress relieved immediately after coiling. The plating sequence was:

1. Alkaline clean.

2. Rinse in cold water.

3. Electroplate with cadmium 8 pm (320 pin.) thick.

4. Rinse in hot water.

5. Relieve hydrogen embrittlement in boiling water 2h.

The spring broke with a shatter fracture typical of that caused by hydrogen embrittlement. The corrective action was to bake the spring at 190 °C (375 °F) for 5 h.

For additional information on this subject, refer to the section "Hydrogen Embrittlement" in this article.

Service Temperature. Cadmium-plated, high-strength steel parts that are subjected to heavy loading should never be used at temperatures above 230 °C (450 °F). Cadmium melts at 320 °C (610 °F); at temperatures approaching 260 °C (500 °F), damage occurs that adversely affects mechanical properties.

Diffused Coatings. The aviation industry has developed an application for cadmium for low-alloy steel jet engine parts. The substrate is first plated with 10 pm (400 pin.) of nickel and then 5 pm (200 pin.) of cadmium. The alloy is diffused at 340 °C (645 °F) for about 1 h. Coverage with nickel must be complete, because cadmium can detrimentally affect the steel substrate when heated above the melting point of cadmium. In this way, an alloy with a very high melting point can be formed. Low-alloy steel parts that operate in jet engines at a temperature of 540 °C (1005 °F) were coated with this diffused alloy. After operating for 1 h at 540 °C (1005 °F), the parts withstood 100 h of salt spray without rusting. Cadmium can also be plated on copper and zinc, as well as on nickel.

Solderability. Although cadmium usually solders well with solders of the 60% tin, 40% lead type, using an inactive rosin flux, its performance may sometimes be unaccountably erratic. Solderability can be improved and made more consistent by predepositing a thin (3 to 4 pm, or 120 to 160 pin.) layer of copper. If the final cadmium deposit is at least 4 pm (160 pin.) thick, the copper coating will not adversely affect corrosion resistance in mild indoor atmospheres. It is important for health and safety reasons to see the section "Toxicity of Cadmium" in this article.

Cadmium on Stainless. Cadmium can be successfully plated over stainless steels and heat-resisting chromium-nickel alloys if the basis metal is first activated and given a light coating of nickel in a nickel chloride-hydrochloric acid bath (U.S. Patent 2,437,409). Composition and operating conditions for this bath are as follows:

Factor

Specification

Nickel chloride

240 g/L (32 oz/gal)

Hydrochloric acid (1.16 sp gr)

120 g/L (16 oz/gal)

Temperature

Room temperature

Current density

55 to 2150 A/m2 (50 to 200 A/ft2)

Time

2 to 4 min

Anodes

Nickel

0 0

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