Intergranular brittle cracking of truck diesel fuel filter-cases

Several truck diesel fuel filter-cases split successively only after 500 km–1300 km of running mileage on highway. All the cracked filter-cases are made from the hot-dip galvanized cold-working extra low carbon steel plate using deep-drawing shaping process. Visual inspection indicates that longitudinal cracking occurred on the filter-cases and crack initial locations are all situated at the contact points of inner surface of the filter-case with the three convexes of the cover on the filter-core. Fractographic analysis indicates that intergranular fracture is much more dominant. The fracture surfaces of the tensile specimens taken from the steel plate of the cracked filter-cases reveal ductile fracture with dimples formation. Fracture mechanism considerable difference between the tensile specimens and the failed filter-cases suggests that intergranular brittle cracking of the filter-cases depends heavily on the state of applied stress. Occurrence of the great contact compressive load between the inner surface of the filter-case and the convexes of the cover on the filter-core makes the circumferential tensile stress on the outer surface of filter-case at the contact locations increase intensely, which lower the threshold value of hydrogen embrittlement (HE). Hydrogen atoms were introduced into the steel plate mostly probably during acid pickling operating before hot-dip galvanizing or fluxing operating in hot-dip galvanizing. The cold working microstructure greatly enhances the hydrogen diffusion capacity and a large number of defects produced in cold-worked steel may increase the density of hydrogen traps.