Peculiarities of degradation of metal of welded joints of steam pipelines of heat power plants

Abstract

The peculiarities of degradation of structure and damageability of metal of welded joints of the steam pipelines of heat power plants of long-term operation period (more than 250,000 h) under the conditions of creep and low-cyclic fatigue are given. It is shown that welded joints of steels 15Kh1M1F and 12Kh1MF are damaged mostly along the areas of fusion, overheating and partial recrystallization of metal in the near-weld zone and also in the places of joining pipe elements of different thicknesses. 13 Ref., 1 Table.

Tracking Supervision of Service Performance and Life Assessment of Defective 15Cr1Mo1V Steel Pipeline

“…The main failure causes of high temperature and high pressure steam pipeline are roughly divided into the following categories: excessive deformation caused by overpressure; brittle fracture caused by original defects; fatigue failure caused by alternating load; creep damage caused by high temperature and high pressure environment [1,2,3,4]. The theory and method of life prediction of the main steam pipeline have been studied for nearly 80 years [5,6,7,8,9]. In the early stage, there were time fraction method of creep life loss, creep deformation fraction method, and later resistance method.…”

Peculiarities of degradation of metal in welded joints of steam pipelines

“…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.…”

“…M 23 C 6 carbides form intermittent chains along the boundaries of α-phase grains [12,18]. Creep pores are nucleated close to such carbides, transforming in crack in process of their development (Figure 8) [4,11,18].…”

Structural changes in metal of welded joints of steam pipelines in operation

“…Structures of the welded joints in process of long-lasting operation under creep conditions (temperature of operation T op = 545-585 °C, operating pressure P op = 20-25 MPa) are transformed with various intensity into different on content ferrite-carbide mixtures. Intensity of structural changes, significantly depending on their initial condition, to the most extent after more than 250,000 h of running of welded joint, is obvious in areas of fusion, overheating and incomplete re-crystallization of HAZ metal [8,9]. Structural changes are caused by diffusion displacement of chromium and molybdenum from central zones of α-phase crystals in their nearboundary zones (volumetric diffusion) and diffusion displacement of given elements over the grain boundaries (grain-boundary diffusion) as well as formation of segregations in the nearboundary zones of crystallites and displacement of dislocations by means of glide and climb, that resulted in appearance of fragmentation (polygonization) of α-phase grains; by transfer of chromium and molybdenum from α-phase into carbides as well as formation of new carbides VC and Mo 2 C; passing of carbide reactions M 3 C → → M 7 C 3 → M 23 C 6 ; coagulation of carbide phases of the 1st group; nucleation and propagation of creep micropores as well as their transformation in macropores and macrocracks.…”

“…In process of aging, self-diffusion of chromium and molybdenum from central zones of α-phase grains in their near-boundary zones provokes change of bonding forces between the atoms. Diffusion displacement of atoms of chromium and molybdenum, caused by gradients of their chemical potentials, results in formation of near-boundary segregation zones in α-phase crystals [8,9]. Presence of such zones develops the conditions for carbide reactions M 3 C → M 7 C 3 → M 23 C 6 as well as coagulation of M 7 C 3 carbides and, mainly, M 23 C 6 .…”