A comparison of toughness of C-Mn steel with different grain sizes

The low-temperature toughness of C-Mn weld steel with different grain sizes was investigated with notched and precracked specimens. The results indicated that the fine grain steel, evaluated by notched specimens (Charpy V-notch and 4 point bending specimens), is tougher than that of the coarse grain steel over a temperature range from -196 °C to -30 °C. On the other hand, the coarse grain steel, evaluated with precracked specimens, has a remarkably greater plane strain fracture toughness compared to the fine grain steel. The microstructural analysis revealed that the fracture toughness of both the fine grain and the coarse grain steel is not directly related to the distance of the fracture initiation site from the precrack tip or the size of the ferrite grain. The behavioral discrepancy can be explained in terms of the ratio of local fracture stress to yield stress,i.e., σ f f/σ y. The fine grain steel had a higher σ f f/σ y in the notched specimens but a lower value in the precracked specimens compared to the coarse grain steel. The scatter of toughness data can be mainly attributed to the probabilistic distribution of the weakest particle. We suggested that σ f f/σ y may be a useful parameter for the engineering evaluation of toughness