HYDROGEN-ASSISTED STRESS CRACKING OF HIGH-STRENGTH WHEEL BOLTS

Early failures occurred with two sizes of wheel bolts used for attachi ng front wheels and dual rear wheels to heavy truck hubs. Failure resu lted in fracture of the bolts, and was a response to the material and process specification which produced a steel microstructure highly sus ceptible to hydrogen-assisted stress cracking. The microstructure resu lted from an alloy steel that was carburized, heat treated, and then z inc plated. This combination of material and processing produced a hig h-strength SAE Grade 8 bolt (equivalent to an ISO 10.9 grade) with a h ard, brittle case and an anodic zinc coating. A slight misalignment of the wheel bolt coupled with a ball seat mounting design for the wheel nuts created a combined axial and bending stress that exceeded the th reshold for hydrogen-assisted stress cracking. Copyright (C) 1996 Else vier Science Ltd.