The generalized diagram of fracture toughness for pipeline steels

Transit oil and gas pipelines usually exhaust their design service life after 30–40 years. Embrittlement is one of the most dangerous consequences of metal aging. Standard linear fracture toughness KIC testing of pipeline steels is possible only at a sufficiently low temperature (−196 °C) when the pipe wall thickness satisfies plane strain conditions. In this case there arises the problem of calculating KIC values at operational temperatures, by means of the results of low-temperature small specimen testing. The aim of the present work is building up the concept of a generalized fracture toughness diagram for different pipeline steels within the absolute temperature range 77 ≤ T ≤ 293 K. Nine different pipeline steels in “as-received” state were tested. A linear relationship between yield stress and Brinell hardness HB was revealed within the temperature range mentioned above. The generalized correlation between relations of KIC,T/KIC,243 and HBT/HB243 was found. Here subscripts “T” and “243” correspond respectively to any temperature and a phase transition temperature of 243 K. The correlation described makes it possible to propose a new simplified method for different pipeline steels fracture toughness estimations.

► Different pipeline steels exibit different KIC values despite having similar σ0.2 or HB.
► A linear relationship HB (σ0.2) keeps on within the temperature range of 77 ≤ T ≤ 243 K.
► The generalized relationship between KIC and HB, common to all the steels investigated, was established.
►  The simplified technique for KIC estimation within the temperature range 77 ≤ T ≤ 293 K was developed.