Cause of bending fracture and control of inclusions in high-strength fasteners made of Al-bearing high carbon steel

Abstract

In this paper, the cause of bending fracture of high-strength fasteners after electroplating and the effect of T.Ca, T.O, and T.S contents in Al-bearing high carbon steel on the type, size, and shape of inclusions were studied. The fracture surface of fastener exhibited stepwise cracks and a lot of intergranular facets associated with ductile tearing, which were typical of hydrogen embrittlement. The agminated elongated MnS inclusions in the center of fasteners could aggravate the susceptibility of hydrogen embrittlement. Thermodynamic calculation showed that the ideal modification of MnS inclusions needed to meet the conditions that the inclusions in molten steel before solidification should be C₁₂A₇, and the mass ratio of CaS/MnS at the end of solidification was 23.7. Based on the above conditions, the quantitative relationships between the contents of T.Ca, T.O, and T.S were determined. Among them, the T.O content in steel increased with the increase of S content. Therefore, to suppress the formation of type B-stringer shaped inclusions and improve the cleanliness of steel, the sulfur content in steel should not be high.