Failure analysis of Grade-80 alloy steel towing chain links

An experimental investigation of the causes of failure of chain links that occurred during towing operation of heavy-weight army vehicles is reported in this study. All failures took place at the weld area of the links after a short service life. Tensile tests on both base metal and the weld samples indicated high tensile strength of 800 MPa for both materials. However, the weld exhibited brittle fracture at relatively small strains of ε ≈ 0.05, while the base metal failed at strains ε ≈ 0.20, indicating moderate ductility. Optical metallography and scanning electron microscopy (SEM) analysis revealed that fatigue was initiated due to inherited cracks at the outer circumference of the weld. Fatigue crack propagation was evident by progressive marks and intervening striations. Their distribution was not rotationally symmetric, indicating a possibility of combined cyclic loading on the links. The large area of final rupture indicated a ductile rupture in the weld center and brittle fracture in the outer region of the weld due to overloading. The results suggest that the major causes of chain failure are as follows: high cyclic loading, weld defects, improper post-weld heat treatment, and decrease in material hardness and corrosion resistance due to insufficiency of some alloying elements.