Effects of microstructure and specimen size on the fracture toughness of EH47 shipbuilding steel at low temperature

•Grain size of EH47 steel increases from the plate surface to central location.•Surface specimen’s fracture toughness of EH47 steel is higher than central specimen.•Microstructure effect on fracture toughness is more significant than specimen size when the microstructure was strongly inhomogeneous distributed and tested at low temperature.•Fracture toughness of full-size sample can be evaluated with small-sized sample. This study investigated the effects of microstructure and specimen size on the fracture behavior of EH47 shipbuilding steel at low temperature. The fracture toughness (characterized by the Crack Tip Opening Displacement, i.e., CTOD) was evaluated using compact tensile specimens with various microstructures and sizes at − 60 °C. The results revealed that due to inhomogeneous microstructure along the steel plate thickness direction, fracture toughness decreased as the sampling location shifted from the surface to the center of the steel plate. Furthermore, the fracture toughness values of 19, 25, and 38 mm-thick specimens taken from the center of an 80 mm-thick steel plate remained almost constant. Based on the analysis of fracture surfaces, metallography and the characteristics of cleavage initiation site, it was found that grain size had significant effect on the cleavage fracture toughness. When the microstructure was strongly inhomogeneous distributed along the thickness direction and tested at the properly low temperature range, the influence of the microstructure on the fracture toughness was more significant than that of the specimen size. In this condition, small specimens taken from the central location of steel plate can be used to estimate the fracture toughness of specimens with full thickness.