Evaluation of intergranular embrittlement of a low carbon steel in austenite temperature range

The hot ductility of a low carbon steel was investigated at various strain rates in austenite temperature range, The embrittlement with intergranular fracture occurs in the specimen deformed in lower temperature range of aust enite following solution-treatment at higher temperature range of austenite , especially at 1680 K just under the melting point. Sulphides are dissolve d and sulphur is segregated to grain boundary by volume diffusion during th e solution-treatment. Fine particles of FeS precipitate on grain boundaries during cooling to deformation temperatures. Voids are initiated at such fi ne precipitates because of incoherence with the matrix. The precipitation o f FeS is caused by decrease in sulphur solubility with lowering temperature . The embrittlement occurs when a difference in the solubility of sulphur b etween at solution-treatment temperature and at deformation temperature rea ches higher than 65 ppm. The critical reduction in area, at which brittle f racture was distinguished from ductile fracture, was 60%. The intergranular fracture feature depends on strain rate. At high strain rates, the specime n was fractured after plastic deformation and the dimple pattern is found i n the intergranular fracture. At low strain rates, the specimen was fractur ed with extremely small plastic deformation after a crack is initiated and intergranular decohesion fracture is found. The energy required for crack p ropagation is calculated to be 3.45 J/m(2) at low strain rates.