Root cause analysis of the brittle fracture of pipes of boiler heating surfaces after long-term operation

Research into the causes of failure of power equipment is necessary to eliminate or restrict their impact, as well as to develop and improve technical diagnostic methods. The last reason is especially important, since accidents or unscheduled shutdowns of power equipment result in serious damage to the economy. Standard methods of technical diagnostics used to assess the residual life of the equipment include mechanical tests of the samples cut from parts of the equipment, determination of their chemical composition, structural studies and fractography of metal fractures. The danger of hydrogen embrittlement in metals is well known. Presence of the areas of brittle fracture or facets is usually associated with hydrogen-induced fracture or hydrogen embrittlement. Direct measurements of the concentration of hydrogen dissolved in metal samples are beyond the scope of regulatory requirements, thus making quantification of the development of hydrogen embrittlement rather difficult. We have shown that technical diagnostics of the pipes of heating surfaces of boiler equipment necessitates additional approaches to the evaluation of hydrogen embrittlement. Hydrogen accumulation during operation may not show visible signs of corrosion or structural changes. It is shown that hydrogen and external thermomechanical load induce the anisotropy in the mechanical and structural properties of the pipe steels. As a result, the nature of the destruction of samples cut from pipe walls depends on the orientation of the test loads relative to the main axes of the stress tensor of operational (working) loads. Experimental data obtained indicate that when determining the causes of accidents and examining heat exchangers to assess their residual life, it is necessary to measure the distribution of the concentration of dissolved hydrogen in the metal and to carry out mechanical tests of ring samples.

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