The dissertation solves the scientific and practical problem of increasing efficiency of correction technology of casting defects using advanced welding technology.
The object of the research is the technology of welding defects of constructions made of pearlitic steel, using the method of transverse slide electrodes CROMOCORD Kb and OVERCORD without preliminary, concomitant heating and heat treatment after welding.
The subject of the research is the connection of the structural state of weld metal in thermal influence with a set of properties, which characterize reliability of welded joints obtained during welding of casting defects in relation to chromium-molybdenum-vanadium steels.
In the introduction the relevance of research objectives is justified, the relation of study with scientific programs, plans, topics is shown, scientific novelty is provided and practical value of results is formulated.
The first chapter provides an analytical review of steels, which are used for hull turbines, their properties, general characteristics and peculiarity of their welding and thermal processing. Modern welding technologies in manufacturing and repair of oversized constructions of pearlitic steels, their difficulties and drawbacks are considered. Factors, which influence crack formation and susceptibility to brittle fractures of heat-resistant steels are estimated.
The main tasks of the dissertation are set, the directions of research are chosen.
The second chapter provides experimental and computational methods, which were used in the process of study.
The third chapter provides studies’ results of the quality of welded joints from 15CrMoV5-10 steel, performed with method of transverse slide without further heat treatment by macro control.
According to the data of research in this chapter, the following results were obtained:
- microhardness measurements in the zone of thermal influence of welded joint confirm credibility of hardness HV measurements, which leads to higher hardness in the middle of the zone of thermal influence, compared to near the fusion boundary;
- welding using the method of transverse slide does not reduce resistance to brittle fracture in the zone of thermal influence and the seam, in comparison to the base metal in the initial state, and after high tempering performed after welding, the toughness of the zone of thermal influence is even higher than the original base metal;
- while using the method of transverse slide, due to the increase in the frequency of thermal cycling, auto heating acts more effectively, which leads to deceleration of cooling speed.
The fourth chapter is dedicated to the research on the ability of weld metal and zone of thermal influence to resist brittle fracture after welding with method of transverse slide, performed by standard technology and without additional heat treatment, by determining the critical temperature of brittleness and crack resistance at normal and sub-zero temperatures.
It is established, that a significant difference in the welded joints stress state made in reverse step method with heating up to 350 – 400ºС in comparison to the method of transverse slide without heating is not detected, therefore such heating is not appropriate for defects correction.
The fine structure of samples after using the method of transverse slide without additional heat treatment was spectroscopically investigated, the chemical composition of carbides was identified.
The fifth chapter is dedicated to studying the impact of welding using the method of transverse slide on hardness distribution, level of mechanical properties and structure of welded joints from steel 25L. The research was performed on samples in volume: with preliminary heating, with high tempering after welding and without preliminary and concomitant heating.
The dependence of the change in hardness on the method of welding and on the measurement is presented in graphs. It is established, that regardless of the welding technology there is a hardness increase in the zone of thermal influence near the fusion boundary.
Hence, after welding with the method of transverse slide the high-temperature area of the zone of thermal influence, regardless increased hardness, has the same ability to resistance to formation of cold cracks as the base metal. This may be due to the grinding of the grain structure of the zone of thermal influence as a result of thermal and deformation action during welding.
According to the results of metallographic studies, it is established that in multilayer welding using the method of transverse slide, the microstructure of the zone of thermal influence and weld metal recrystallizes and becomes fine-grained due to high temperature heating during surfacing of layers and is bainite with coaxial ferrite grains.
