Root Cause Analysis of the Brittle Fracture of Pipes of Boiler Heating Surfaces after Long-Term Operation

Abstract

The study of the causes of destruction of power equipment is necessary for their elimination or limitation, as well as for the development and improvement of methods of technical diagnostics. The latter is especially important, since accidents or unplanned shutdowns of power equipment cause great economic damage. Standard methods of technical diagnostics in assessing the residual life of equipment include mechanical testing of samples cut out of equipment parts, determination of their chemical composition, structural studies, and fractography of the metal. Both fractures of samples specially cut for mechanical testing and surfaces of destruction that occurred during operation are studied. The danger of developing hydrogen embrittlement in metals is well known. The presence of brittle fracture areas or facets is usually associated with hydrogen-induced destruction or hydrogen embrittlement. Direct measurements of the concentration of hydrogen dissolved in metal samples are not regulated by standards; therefore, diagnostics of the development of hydrogen embrittlement is only qualitative. Our studies show that standard approaches are insufficient for technical diagnostics of the pipes of heating surfaces of boiler equipment. Hydrogen accumulation during operation may not have visible signs of corrosion or structural changes. It was found that hydrogen and external thermomechanical load induce anisotropy of mechanical and structural properties of steels of pipes of heating surfaces. As a result, the nature of destruction of samples cut from pipe walls depends on the orientation of test loads relative to the main axes of the stress tensor of operational (working) loads. The obtained experimental data allow us to conclude that, when establishing the causes of accidents and examining working heat exchangers in order to assess their residual life, it is necessary to additionally measure the distribution of the concentration of dissolved hydrogen in the metal and mechanically test ring samples.