Failure analysis of boron steel 27MnCrB5-2 structural bolts during tightening of railcar wheel-axle

Herein, a premature failure analysis of structural tempered high-strength boron steel 27MnCrB5?2 bolts (0.0006 wt B), used for railcar wheel-axle assembly, is presented. Fractographic analysis revealed a moderate ductile fracture. The applied torque produced elongation of the bolt until final fracture. A scarce presence of precipitates was observed, mainly of Ti(C,N), TiC, and inclusions such as Al2O3 and MnS. The mechanical properties of the boron steel 27MnCrB5?2 bolts were 1112 and 1170 MPa for the yield strength of 0.2% (?0.2%) and ultimate tensile strength (UTS), respectively. However, the calculated Von Mises equivalent stress (916 MPa) was lower than ?0.2% and UTS values. It was found that during tightening, the washers are crushed and, therefore, their coefficient of friction decreases, which causes a considerable increase in the applied torque and the equivalent stress. The engineering failure analysis showed that a decrease in the friction coefficient of the washer from 0.16 to 0.07 causes the failure of boron-steel 27MnCrB5?2 bolts during tightening of railcar wheel-axle.