Resistance to fracture of weldable carbon structural steel with ferrite ? Pearlite and Widmanstatten structures

Abstract

1. The resistance to fracture of steel St3 determined from the ductile-brittle transition temperature depends on structural factors — the shape and quantity of ferrite precipitates, grain size, and distribution of structural components; the static characteristics (σb, σT, δ, ψ) are practically the same for samples with different microstructures. 2. The susceptibility of steel St3 to cold brittleness is affected by the evenness of the microstructure and the mutual distributions of the structural components. The transition temperature rises with increasing grain size, the extent of the Widmanstatten structure, and the unevenness in the distribution of the structural components. The loop-shaped microstructure with fine grains is more susceptible to brittleness than the even coarse-grained structure. 3. The susceptibility to brittleness is highest for samples with a coarse-grained loop-shaped structure due to the effect of both heterogeneity and grain size. 4. The work of crack propagation depends on the quantity of ferrite in the structure: The larger the amount of ferrite and, consequently, the higher the concentration of carbon in the quasieutectoid, the lower the value ofap. The work of crack initiationai varies little with changes in the structure within the limits investigated. 5. For the preparation of structures with stress concentrators such as welded structures it is best to use steels with a network and an even fine-grained structure and also Widmanstatten structure containing no more than 45–50% ferrite.