| Abstract [eng] |
The dissertation analyses the impact of heat treatment parameters on the mechanical properties of chrome-molybdenum (5% Cr–0,5% Mo) steel welded joints. The key objects of research are heat-treated chrome-molybdenum steel welded joints and piping elements operated at high temperature for an extensive period of time, where degradation of mechanical properties has been observed. Detailed analysis of these objects allow selecting the most appropriate heat treatment parameters for the investigated steel welded joints. The main objective of the thesis is to investigate the causes of degradation of alloy steel mechanical properties during operation and fabrication of the equipment, and to develop a methodology for identification of optimum heat treatment parameters for chrome-molybdenum steel welded joints. A few key issues are addressed in the thesis: identification of dependence of chrome-molybdenum (5% Cr–0,5% Mo) steel welded joint mechanical properties on heat treatment parameters, identification of the optimum value of temperature/time parameter, and identification of causes of the investigated steel mechanical property changes and degradation. The first objective was formulated considering the fact that the equipment made of those steels are very expensive and hazardous, whereas welding and heat treatment parameters reducing the service life may be chosen during their fabrication. The performed reference literature analysis revealed that mechanical property changes of these steels, using very intensive local heat treatment of welded joints, which is frequently used while fabricating the equipment, has not been yet investigated. The second objective is related to steel microstructure testing. Upon the performance of this testing it is possible to identify the causes of steel, operated for an extensive period of time, degradation as well as causes of new welded joint mechanical property changes. The dissertation consists of the introduction, three chapters and general conclusion. The introduction analyses the topicality of the issue, formulates the objective and tasks of the thesis, describes the scientific novelty of the thesis, and introduces author’s presentations and publications, and structure of the dissertation. The first chapter covers the literature reference review. The chapter provides the review of works analyzing the peculiarities of industrial appliance of chrome-molybdenum steels, the issues of heat treatment and long-term service of welded joints of those steels. The second chapter, the methodology section, describes the research methodology. The essence of testing methods, their application, testing performance and the methods of result processing are described in this chapter. The third chapter, the experimental section, describes the experiment performance sequence, the results obtained and their review. Arguments for the defended statements are presented. 6 presentations were made at the conferences in Lithuania and other countries and 7 articles published on the topic of the dissertation. |
