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1 Nis 2014

Steels and Welded Joints of Thermal Power Engineering Facilities

Proje Adı: Investigation of In-Service Degradation of Steels and Welded Joints of Thermal Power Engineering Facilities

Başvuru Yapılan Program: TÜBİTAK 2512 Uluslararası İkili İşbirliği Programı

Proje Başvuru Tarihi: Haziran 2013

Açıklanma Tarihi: Nisan 2014

Proje Ortakları: Gedik Üniversitesi (Prof.Dr. Sunullah ÖZBEK), Ternopil Ivan Pul’uj National Technical University, Ukraine

Toplam Proje Bütçesi: 22.000 TL

Proje Özeti: Despite the intensive development of nuclear power engineering and alternative sources of energy the thermal power plants today are the major source of electricity and heat. They occupy a dominant position on the market and ensure the effective functioning of the equipment of various industries in the EU and in Eastern Europe.

In particular, the thermal power plants superheater collectors operate at the temperature of 400– 500 оС in the medium of steam under pressure of 155 MPa. The combined effect of both factors (corrosive environment and slow deformation during the time operation) reduces the ductility of collector steel and leads to the degradation of its other mechanical properties.

As a result of prolonged exploitation of 12Cr1MoV steel in the superheater collector its structure-phase state is being changed. Especially, the pearlitic colonies are disintegrating, its number is increasing and carbides become spheres, the pores emerge along the grain boundaries. The stress-corrosion leads to the emergence and propagation of cracks on the inner surface of the collector along the grain boundaries, weakened by pores and carbides precipitation. The internal decarbonization affects significantly on the degradation of 12Cr1MoV steel under intensive hydrogen embrittlement and loss of intergranular strength.

The main TPP pipelines, which operate at temperatures up to 570 оC and steam pressure up to 24 MPa, also belong to the large responsible elements, which damage is accompanied by significant loss of coolant, electricity shortfalls, the threat to environment and staff and large financial costs for repairs. However, during the pipelines installation the welds are unavoidable. They are considered as the weakest link of any structure because they are heterogeneous in chemical composition, structure and mechanical properties and serve as the stress concentrators. This is especially dangerous under the effect of hydrogenation environment, which is a steam with high parameters. As a result of prolonged (more than 105 h) operation of pipelines the integrated hydrogen content in the metal increases by 1 … 1.5 orders of magnitude, and in the vicinity of stress concentrators its contents may grow even 3 orders of magnitude. This explains that there is a constant interest in evaluation of metal operability of welded joints different zones and their susceptibility to degradation during operation. Also, structural, mechanical and corrosive aspects of the problem are important. Yet the degradation starts on microstructural level by diffusion redistribution of carbon and alloying elements with the precipitation of carbides along the grain boundaries. Pore formation of around them, its coalescence due to creep with the emergence of microcracks at first, and after that are the stages of the damage accumulation in the metal. The stresses in the pipe wall and the hydrogenation intensify both the redistribution of elements, and the initiation of cracking.

The degradation of boilers metal and TPP pipelines welds is intensified in the regions of stress localization and is caused by the development of deformation ageing, the influence of hydrogen absorbed by the metal during the high temperature operation on the diffusion redistribution of the carbon and alloying elements. Diffusing in the metal to areas with the triaxial stress state (in the vicinity of stress concentrators, crack defects, etc.), the hydrogen intensifies the redistribution of elements and accelerates the structural changes in the metal. The degradation is the change in the structure, the accumulation of defects such as pores that form the micro- and later the macrocrack by merging , the growth of which ends with the failure of the structural elements.

To ensure the efficient and safe operation of such critical elements of thermal power equipment as collectors and main pipelines the life and frequency of inspections of their technical condition are strictly regulated. The residual life is adjusted based on the actual mechanical properties of the metal.

To determine the life of the thermal power plants equipment which are being operated under the severe temperature-force conditions that creates the prerequisites for the creep of metal the deterministic approaches are used. The serviceability of other equipment elements that wear out quickly (due to corrosion, erosion and other forms of wear) is based on experimental studies during periodic testing.