Research into defectiveness of welded joints of steam pipes operated over a long time

Abstract

The welded joints of steam pipelines of the power units of thermal power plants, which are operated for long periods under conditions of creep and low-cycle fatigue, are the most damaged elements of power units. By the attributes of cohesion, we presented the classification of the types in the defectiveness of the welded joints of steam pipelines, which is appropriate to use for refining the special features of physical-chemical processes and structural changes that occur in metal of the welded joints of steam pipelines. The represented types of damages are characteristic for the welded joints, which exhausted their base resource, including the prolonged one. We revealed the most characteristic places of defectiveness in metal of the welded joints and orienting of cracks towards the weld metal, as well as the metallographic attribute of defectiveness. The physical-chemical processes are examined, which specify structural changes in the long-operated welded joints of steam pipelines. We examined the process of slowing down the dislocations by the carbide precipitates, which coagulate lengthwise and, in the form of intermittent chains, are concentrated along the boundaries of grains of the α-phase. We established that the creep pores to the largest degree are formed in the places of contact between coagulating carbides and grains of the α-phase. It was revealed that the creep cracks are formed at the sections of incomplete recrystallization of the heat-affected zone of welded joints (about 75 %), the fusion (approximately 15 %), while the rest is at other sections of HAZ, as well as in the weld metal and in the base metal. It is demonstrated that the creep cracks developed by the brittle mechanism, which is contributed by the presence of segregations in the near-boundary zones of grains of the α-phase. We established that the fatigue cracks predominantly develop at the section of fusion, overheating and incomplete recrystallization of HAZ, as well as near the backing rings in the places of contact of the pipe elements with different thickness. Dependence is detected of the reduction in metal resistance of the long operated welded joints of steam pipelines to the formation of fatigue cracks on the level of degradation of their structure. Data of present study are necessary for obtaining welded joints with decreased defectiveness in the process of prolonged operation under conditions of creep and low-cycle fatigue.