Abstract
This paper presents a numerical method for prediction of fracture toughness, local cleavage strength and characteristic distance in structural steels at low temperatures. This method, established on a statistical analysis of microstructure and conditions of brittle fracture initiation, enables to calculate how the integral probability of fracture varies with the stress intensity factor, temperature and deformation, and microstructural parameters of steel. The application of the proposed numerical method on Ni-Cr steel demonstrates very good agreement of predicted temperature dependence of scatter in fracture toughness with experimental results. Mechanism of initiation and propagation of brittle main crack and its stages are identified from the results of fractography analysis. In this paper also a new numerical method calculating the characteristic distance as a function of temperature is suggested. The proposed procedure represents a base for systematic control of microstructure so as to ensure the optimum relation between strength characteristics and fracture toughness of steels.

Key words
Ni-Cr steel, carbides, cleavage fracture, fracture toughness, cleavage strength, characteristic distance

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