The Mechanism of Failure of the Axisymmetrical Welded Joint under Thermomechanical Loading

Abstract:

The purpose of the paper is to present the mechanism of deformation and failure of welded joints during the process of retrofit. The considered joints are made of conventional steels used in the power industry, as well as new-generation steels. The problems of failure and the design of such joints are considered in the paper. The model of welded joint was built with properties of low alloyed chrome-molybdenum steel and a new-generation steel of P/T91 type. All metallurgical zones, i.e. the weld and the heat-affected zone were considered, modelled and discussed. A simulation of thermomechanical loading was conducted under various loading conditions to explain the causes of the failure of energy structures, which appears more frequently during their loadings or steam dropping (i.e. in conditions of changes of thermal loadings) than during their stable work. The results of calculations were confirmed by experimental metallurgical studies. It is observed that the brittle and soft zones of coarse-grained ferrite with minimal impact strength, resulting from internal and external operation parameters, appear in regions where oscillation of the highest von Mises and normal stresses takes place. The relevant literature does not describe the analysis of the phenomenon of acceleration of the failure of the welded structure leading to its failure, while taking into account the complex thermomechanical conditions of their operation. This mechanism is based on the accumulation of structural weakening of the weld as the result of the diffusion process within the axially symmetrical joint, the growth of the brittle zone and their additional degradation, and the influence of the oscillation of maximal normal and von Mises stresses. It was noted that in the process of thermomechanical loading, the state of failure initiation in circumferential and longitudinal directions appeared in the weld. A few illustrations presenting the mechanism of the deformation and failure of the welded joint are presented. The effects of the distortions of the weld are also discussed in the paper.

• Periodical: Materials Science Forum (Volumes 638-642)
• Pages: 3805-3810
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.638-642.3805
• Online since: January 2010
• Authors: Piotr Lacki, Kwiryn Wojsyk, Andrzej Służalec
• Keywords: Thermal Ratchet, Welded Joint Failure

References

[1] Holloway G., Lobinger F.: Boiler construction, materials and consumables, Central & Eastern European Seminar: Welding Practice in Power Engineering Industry Bielawa 18 - 21. 02. (2009).

[2] Hernas A., Dobrzański J.: Trwałość i niszczenie elementów kotłów i turbin parowych (The durability and failures of boiler and steam turbine components). A monograph, Part II. The Publishing House of the Gliwice University of Technology, Gliwice (2003).

[3] Wojsyk K.: Analiza mechanizmu degradacji osiowosymetrycznego złącza spawanego poddanego obciążeniom cieplnomechanicznym (Analysis of the mechanism of degradation of the axially symmetrical welded joint subjected to thermo-mechanical loading). A doctoral thesis, Czestochowa, 18 Dec., (2007).

[4] Hernas A.: Żarowytrzymałość stali i stopów, cz. I (The high-temperature creep resistance of steels and alloys, Part I). The Publishing House of the Gliwice University of Technology, Gliwice (2000).

[5] Lacki P., Wojsyk K.: Model rurowego hybrydowego złacza spawanego poddanego cyklicznym obciążeniom termomechanicznym. Problemy badawcze energetyki cieplnej (A model of the pipe hybrid welded joint subjected to thermo-mechanical loading. The research problems of thermal power engineering). Prace naukowe Mechanika no. 202. The Publishing House of the Warsaw University of Technology, (2003).

[6] Tasak E., Ziewiec A.: Problemy spawania stali różnorodnych stosowanych w energetyce. Transactions of the Conference on Welding in power engineering, 2002, pp.46-57.