Gordana Bakić

Hydrogen Embrittlement of Industrial Components: Prediction, Prevention, and Models

Hydrogen embrittlement is a common, dangerous, and poorly understood cause of failure in many metal alloys. In practice, it is observed that different types of damage to industrial components have been tied to the presence and localization of hydrogen in metals. Many efforts have been made at understanding the effects of hydrogen on materials, resulting in an abundance of theoretical models and papers. However, a fully developed and practically-applicable predictive physical model still does not exist industrially for predicting and preventing hydrogen embrittlement. The connection of microstructure-based behaviors of materials and effects on the macroscopic measurable characteristics (stress levels, hardness, strength, and impact toughness) is of the utmost importance to achieve a unified model for hydrogen embrittlement. This paper gives an overview of the application of a model for structural integrity analysis of boiler tubes made of plain carbon steel exposed during operation to a local corrosion pro...

Determination of Stress Intensity Factors in Low Pressure Turbine Rotor Discs

An attention in this paper is focused on the stress analysis and the determination of fracture mechanics parameters in low pressure (LP) turbine rotor discs and on developing analytic expressions for stress intensity factors at the critical location of LP steam turbine disc. Critical locations such as keyway and dovetail area experienced stress concentration leading to crack initiation. Major concerns for the power industry are determining the critical locations with one side and fracture mechanics parameters with the other side. For determination of the critical locations in LP turbine rotor disc conventional finite elements are used here. For this initial crack length and during crack growth it is necessary to determine SIFs. In fatigue crack growth process it is necessary to have analytic formulas for the stress intensity factor. To determine analytic formula for stress intensity factor (SIF) of cracked turbine rotor disc special singular finite elements are used. Using discrete values of SIFs which correspond to various crack lengths analytic formula of SIF in polynomial forms is derived here. For determination of SIF in this paper, combined -integral approach and singular finite elements are used. The interaction of mechanical and thermal effects was correlated in terms of the fracture toughness.

Characterization of Tube Repair Weld in Thermal Power Plant Made of a 12%Cr Tempered Martensite Ferritic Steel

The heat resistant tempered martensite ferritic steel X20CrMoV121 (DIN) has been extensively used within the last few decades as a material for boiler tubing systems and pipelines in thermal power plants (TPP). Long-term behavior of this steel is vastly researched and very well known, but main disadvantage is its poor weldability. In situ welding of martensitic steels is always challenging task and is usually quite difficult to implement properly in a short time, during forced outages of TPP. In this paper, characterization and mechanical properties of undermatch welded joint made during partial replacement of boiler outlet superheater (SH) in TPP by austenitic filler material, after 10 years of service are presented. Due to “cold” technique of welding, which does not required post weld heat treatment, this procedure were regular and widely used repair welding technique in two TPP (620 MW) units. In the purpose of comparison, two other type of matching welding joints of the same SH were also characterized: shop welded joint made by electrical resistance flash butt welding, as well as field welded joint made by gas tungsten arc welding during assembling of SH, which were both in service approximately 150,000 h.

Structure Integrity of Pressure Vesels Repair Welding Joints

Vertical cylindrical vessel-chambers as a part of coal — drying plants, whose purpose is to collect wastewater, are supported at 3 points in upper dish head area and are made of fine-grained Mn steel plates, joined by welding. Significant thinning and leaking in upper dish head area of the vessel occurred due to original design provoking an intensive abrasion, cracking and rupture. After reconstruction, in the upper zone of cylindrical shell, two new joints were made in sity by manual arc welding, with subsequent, local post-weld heat treatment. However, cracks appeared firstly in new welded zones in radial and axial joints, and then in zones of openings. Later, the similar failure features began to appear in the area of original welded joints. All of these cracks were repaired by properly specified technology. Unfortunately, after some period of exploitation the initiation of new cracks was observed, at first by the visual inspection. This problem was detected in the repaired areas in all (16) vertical cylindrical vessels. Cracks have propagated in different directions with various penetration depths, up to 3 mm. It is interesting to note that crack appeared in the HAZ of vertical joints, while in the area of radial welded joints the cracks were randomly distributed in a larger zone. For cracks up to 3 mm, deep grinding was applied whereas for greater crack depths repair welding with local post-weld heat treatment was used. Since welding with subsequent heat treatment could only be used twice to repair the welded joints [1, 2], it is clear that in the areas repaired several times (up to 7) proper mechanical characteristics of the material is rather difficult to maintain.

Case Study of Supporting Tubes Failure

This paper deals with determination of the failures causes of supporting tubes in a 350 MW fossil fuel power plant. Due to frequent fractures of supporting pipes occurring always at the same location and the ensuing reduction of the plant availability as well as substation financial losses, there was an urgent need to resolve the problem [K. Takahashi, M. Yokouchi, S.K. Lee, K. Ando, J. Am. Ceram. Soc., 86-12, 2003]-[3]. This work describes the test results and the conclusions drawn from the results.