Peculiarities of degradation of metal in welded joints of steam pipelines

Abstract

Extension of life of steam pipelines, operated under creep conditions, determines the necessity of evaluation of their damageability and in many respects depends on carbide transformations in metal. The work considers peculiarities of diffusion and structure of carbide phases in metal of welded joints of pearlite steels after long-term operation. It is determined that concentration of coagulating carbide phases (M23C6, M7C3 etc.) Metal of welded joints of steam pipelines is characterized by structural, chemical and mechanical inhomogeneity, formation of which is promoted by welding heating at their manufacture. Namely, inhomogeneity under conditions of long-term operation provides for more intensive physical-chemical processes in welded joint metal than in the base metal, that, respectively, provokes its degradation and limits the life.Evaluation of damageability level in welded joints of steam pipelines of long-term operation, which allows specifying their residual life, is of current importance. Investigation of peculiarities of physical-chemical processes, taking place in welded joint metal, is relevant for evaluation.It is a well-known fact that welded joints of elements of steam conduit channel from 15Kh1M1F and 12Kh1MF steels, being operated under 545-585 °C temperature and 20-25 MPa pressure [1][2][3][4][5][6][7], mostly suffer from damages at long-term running. Transfer of power generating units in cycling operation mode provides for acceleration of their damageability. Damageability of metal of welded joints mainly takes place by creep and fatigue mechanisms, which are forced by corrosion constituent (Figure 1). Damageability by creep mechanism is characterized by formation of nucleation (critical) pores of around 0.03-0.07 μm size as well as their coarsening up to 3-7 μm, further branching and transformation of pores in creep cracks.The most intensive formation of creep pores is observed in areas of incomplete recrystallization and fusion of HAZ of welded joints in their external, most elongated area. The fatigue cracks

Features of pore formation in welded joints of steam lines in long-term operation

“…Their formation under creep conditions is provided by development of inner slipping and sliding along the boundaries of α-phase grains. This is the most intensive in the locations of coagulating second phase precipitates [2].…”

“…The objective of the work was studying the features of the mechanism of pore formation in the metal of welded joints in steam lines from 15Kh1M1F and 12Kh1MF steels in longterm operation. Investigation results allowed lowering the intensity of pore formation and refining the diagnosis of residual life of steam line welded joints [1][2][3][4][5][6][7], which are damaged more intensively than the steam line proper.…”

“…Diffusion-induced movement of alloying elements (chromium and molybdenum) from central zones of α-phase grains into their near-boundary zones, as well as movement of reduced elements along grain boundaries leads to formation of segregations, promoting running of carbide reactions of I gr [1]. Movement of alloying elements leads to decrease of strength characteristics of α-phase grains, that is confirmed by fragmentation (polygonization) of grains and higher level of their deformation, respectively [2]. It is found that the level of deformation of HAZ regions is much higher than mass deformation of the steam lines equal (at welded joint operating time of more than 250,000 h) to approximately 0.5-0.7 % ( Figure 1).…”

Structural changes in metal of welded joints of steam pipelines

“…такі зміни викликають зменшення сил зчеплення між атомами як у кристалах α-фази, так і на межі розділу фаз (в більшій мірі). наприклад, на границі розділу зерен α-фази та коагулюючих карбідів м 23 с 6 [1], що призводить до зародження та подальшого розвитку пор повзучості.…”

“…У процесі тривалої експлуатації паропроводів в умовах повзучості (більше 270 тис. год) масова деформація паропроводів складає приблизно 0,5…0,7 %, а деформація ділянок Зтв -0,7…8 % [1,6]. механізм пластичної деформації металу зварних з'єднань доцільно розглядати шляхом використання теорії дислокацій [7,8].…”