Delayed Failure of an Electroplated Steel Bolt Because of Hydrogen Embrittlement
Cadmium-plated high-strength steel bolts were used to facilitate quick disassembly of a vehicle. One bolt was found fractured across the root of a thread after being torqued in place for one week. The bolts were made of 8735 steel heat treated to a tensile strength of 1241 to 1379 MPa (180 to 200 ksi) with a hardness of 39 to 43 HRC, followed by cadmium plating. The bolt that failed and several that did not were examined. It was found that failure of the bolts was the result of time-dependent hydrogen embrittlement. Had the remaining bolts been torqued to the normal stress levels, all would have failed within two weeks. The bolts were baked, as specified by ASTM B 242, at 205 deg C (400 deg F) for 30 min. No further failures occurred. Baking for 30 min is the minimum baking time; however, baking times up to 24 h are recommended for greater safety.