We continue our discussion on liquid-metal embrittlement with a case study involving fasteners. In this example, steel bolts (150 mm long x 1.0 mm diameter) are used to hold a cover on an automotive air-conditioning compressor. Leaks were observed in service, and upon closer inspection, leakage was revealed to be in the contact area of a washer to the bolt flange where the attachment to the cover is made (Figs. 3, 4).
The material in question was an SCM435 (0.33-0.38% C, 0.15-0.35% Si, 0.60-0.90% Mn, ≤ 0.030% P, ≤ 0.030% S, ≤ 0.30% Cu, ≤ 0.25% Ni, 0.90-1.20% Cr, 0.15-0.30% Mo), a chromium/molybdenum steel. The steel was heat treated to achieve a hardness in the range of 35-39 HRC. The bolts were subsequently processed in an exothermic gas atmosphere, which produced a 0.0004-inch (0.01-mm) oxide layer. The application required that the bolt flange be mated to a tin-coated brass washer (Fig. 3) at assembly.