Features of damage to welded joints of steam pipes made of steel 12X1M1F and 15X1MF

Abstract

A significant number of heat and power equipment, including welded connections of steam pipelines made of 15x1m1f and 12x1mf Steels, have now reached the design service life. Their practical experience is about 200,000 hours at an estimated temperature of 545°C, and the park resource according to regulatory documentation is 250,000 hours. Note that aging welded joints are operated in conditions of start-ups and stops, which have become more frequent, which contributes to an increase in the intensity of their damage. In this connection, the question arose about the possibility of their operation longer than the estimated period in the new conditions. Goal. The goal is study of the features of damage to welded elements of steam pipelines operating in conditions of creep and low cycle fatigue. Methodology. The study of samples from existing damaged steam pipelines was carried out by the method of structural analysis, based on the consideration of the processes of structural and phase changes occurring in the areas of welded joints and conclusions were drawn regarding their resistance to destruction during operation. Results. The mechanisms of crack formation, their causes and the main provisions of metallographic analysis on the study of damaged welded joints operating under creep conditions and representing a certain number of types of cracks are presented. Such cracks are cold-breaking cracks, which are brittle cracks at moderate temperatures (up to 100–150 0C) during hydraulic tests and start-ups; long-term ductility failure cracks, which are brittle cracks caused by the failure of long-term ductility creep temperatures and rated stresses below permissible; cracks on exhausted areas, which are cracks when operating in the range of creep temperatures and rated stresses above permissible and fatigue cracks, which are cracks caused by the action of variable stresses with an amplitude greater than permissible values. Scientific novelty and practical significance. It is established that the mechanism of aging of long-term operated welded joints of steam pipelines is associated with structural transformations that cause the appearance of cracks in the welded joints.