PREDICTION OF BRITTLE FRACTURE OF OIL AND GAS PIPELINES BASED ON INSTRUMENTAL INDENTATION DATA

The paper proposes a new approach for determining the critical brittle fracture temperature of steels used in oil and gas pipelines, based on the results of tests using the instrumental indentation method. This method relies on the analysis of static indentation results of an indenter into the surface of the wall or metal sample. The established correlation between true fracture stress and the Zener-Holloman parameter opens up new opportunities for analyzing the effects of temperature and plastic deformation rate on the steel embrittlement process. The developed methodology allows not only for the assessment of the critical brittle fracture temperature after prolonged service but also for the prediction of its dynamics based on operational conditions. This approach contributes to more accurate forecasting of material durability and its behavior under extreme conditions, which is crucial for enhancing structure reliability. The practical applicability of the method is demonstrated using X70 pipeline steel. Experimental data on the indenter penetration depth were obtained using tests on standard samples with various cut orientations. Deformation characteristics were determined by using static tensile tests of flat samples. The thermal activation parameters of plastic deformation were established based on reference data concerning the temperature dependence of yield strength. Successful application of the method requires strict control of deformation characteristics and indentation rate. This approach may be useful for assessing the residual life and safe operation of pipelines and pressure vessels.