Browsing by Author "Sonderegger, Bernhard"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    Open Access
    Development of the small punch test platform to evaluate the embrittlement of power plant materials
    (2017) Tshamano, Lavhelesani Oliet; Sonderegger, Bernhard; Knutsen, Robert D; Becker, Thorsten
    Life assessment of critical components and piping is performed in the electrical power plants in order to prevent structural/component failure and prolong safe operation of the equipment. In the event that these components fail, the consequences can be very costly since this may result in power supply disruptions, component replacements, environmental damages and the loss of human life. Regulations, standards and codes are designed to ensure the safe operation of the power plants. However, on their own, they are not adequate to account for aging power plants that have been in service for more than half of their originally designed lifespans, since failures have been experienced due to in-service aging mechanisms (i.e. temper embrittlement, creep, etc.) and poor engineering and maintenance practises. Mechanical, metallurgical and non-destructive techniques are used to evaluate the condition of the in-service materials in order to aid in these life assessments. The structural integrity assessments utilise material toughness properties as determined through fracture toughness testing, which requires a significant quantity of material, and is therefore cumbersome and expensive. Consequently, several other material property testing techniques are used to aid in structural integrity assessments, such as impact energy, tensile and hardness testing. Through empirical correlations, these test results are used to estimate fracture toughness properties and, consequently, the error bands are expected to be as high as 50%. Due to its small size, the small punch test (SPT) technique can be regarded as a quasi-non-destructive test, and is therefore a preferred method for determining the fracture toughness in aid of structural assessment. The SPT technique involves a compression load from the punch to a sample (ϕ8mm x 0.5mm thick) clamped between clamping and receiving dies. This study aims to develop a test rig that will be used to perform the SPT in order to quantify the level of embrittlement on the ex-service, low-pressure steam turbine material (NiCrMoV steel). The data results acquired from the SPT technique are the reaction load of the punch and the deformed displacement of the sample performed at a constant displacement rate according to CWA 15627:2007. Two SPT rigs were designed, manufactured and commissioned. These two were commissioned using FEM and tensile test results for validations. The steel was subjected to three different conditions: as received (AR), de-embrittled (DE) and hardened (HD). The three types of steel illustrated that the SPT can quantify embrittlement levels through the correlation of tensile, Charpy impact energy and fracture toughness testing.
  • Thumbnail Image
    Item
    Open Access
    Influence of heat treatment condition on the stress corrosion cracking properties of low pressure turbine blade steel FV520B
    (2017) Naicker, Leebashen; Knutsen, Robert D; Sonderegger, Bernhard
    Stress corrosion cracking (SCC) is a corrosion phenomenon which continues to plague the power generating industry especially in low pressure (LP) steam turbine blades operating in the phase transition zone. An investigation has therefore been conducted to examine the effect of heat treatment condition on the microstructure, mechanical properties and SCC properties of one such LP turbine blade material, FV520B, used in the steam turbines of coal-fired power stations in South Africa. The three stage heat treatment cycle of the FV520B turbine blades consists of homogenisation at 1020°C for 30 minutes, solution treatment at 790°C for two hours and precipitation hardening at 545°C for six hours. In this study, the precipitation hardening temperature was varied in the range 430-600°C to investigate how this variation would affect the material and SCC properties. Hardness and tensile testing were performed to obtain mechanical properties while the investigative techniques used to characterise the microstructures were light microscopy, dilatometry, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Stress corrosion susceptibility for the different heat treatment conditions was quantified using U-bend specimens while crack growth rates and threshold stress intensities for SCC (KISCC) were measured using fatigue precracked wedge open loaded (WOL) specimens. Both SCC tests were conducted in a 3.5% NaCl environment maintained at 90°C. XRD results revealed the presence of reverted austenite in the higher tempered specimens due to the precipitation hardening temperature being close to the Ac1 temperature for the material. The presence of reverted austenite was shown to adversely affect mechanical strength and hardness which decreased with increasing precipitation hardening temperature. Light and electron microscopy (SEM and TEM) revealed the presence of Cr-rich precipitates along the prior austenite grain boundaries in all tested heat treatment conditions. The propensity, quantity and size of the Cr-rich precipitates increased as the specimen temper temperature increased. SCC susceptibility was shown to be dependent upon yield strength and decreased as precipitation hardening temperature increased with specimens in the overaged condition showing no cracking after more than 5000 hours in the test environment. WOL testing only produced cracking in the three highest strength specimens after 2000 hours. Crack growth rates and threshold stress intensities were found to be dependent on yield strength and decreased with increasing precipitation hardening temperature. Analysis of fracture surfaces revealed crack propagation along prior austenite grain boundaries in all test heat treatment conditions indicating intergranular stress corrosion cracking (IGSCC) as the dominant cracking mechanism.
  • Thumbnail Image
    Item
    Open Access
    The influence of prior creep damage on the fracture localisation in X20 CrMoV12-1 cross-weld creep tests
    (2017) Rasiawan, Trisha; Sonderegger, Bernhard; Knutsen, Robert D; Shuro, I
    Many of Eskom's coal fired power plants have an average age of 170 000 hours and a few operating close to 300 000 hours. Main steam temperatures experienced in a power plant vary between 535-555°C. These operating conditions place main steam pipe components to operate within the creep regime. It is of utmost importance for safety and plant health that these critical components are managed to determine the remaining life and risks associated with high temperature exposure for prolonged periods of time. Non-destructive testing (NDT) methods are utilised extensively on Eskom power plants to determine the remaining life and replacement strategies for critical components. Surface replication is used as a life assessment tool for creep damage quantification of main steam pipe work. A large part of maintaining plant is repair welding on creep aged and sometimes creep aged material as entire system replacements are impractical and time consuming. By repair welding new material onto creep aged material, mechanical and microstructural properties of the creep aged material deteriorates. The study of this work is focused on characterising the as-received materials from Eskom power plants and using these creep aged materials to create cross-weld samples with virgin material. The cross-weld samples were creep-rupture tested at high temperature and low stress conditions to determine the fracture location of repair welded cross-weld samples. Once ruptured, the zone of rupture, was identified and created in a larger volume by simulation using Gleeble® thermo-mechanical equipment. The as-received base materials were subjected to different operating conditions hence contain different degrees of creep damage. The microstructural evaluation of the creep damaged material was conducted using optical microscopy, scanning electron microscopy (SEM), coupled with more advanced electron backscattered diffraction (EBSD). Microhardness and hot tensile testing were included to characterise the mechanical degradation of the as-received material. The fracture location of the creep-ruptured cross-weld samples were investigated using optical microscopy, SEM and EBSD and occurred on the outer region of the heat affected zone (HAZ) of the creep aged material. The fine grained microstructure with coarse precipitation of this region is characteristic of the fine grain heat affected zone (FGHAZ). The occurrences of voids predominantly occur in this narrow region with very few voids in the adjacent base/weld material. As this zone is of particular interest due to it being the weakest region in repair welded joints, the need to investigate it further is important. A larger testing volume of the FGHAZ was created by applying a weld thermal cycle simulation to the as-received base materials. The impact of this simulation was determined microstructurally by optical microscopy and mechanically by hardness and tensile testing. The FGHAZ has low creep resistance and is most susceptible to failure due to the small grained microstructure. Due to the numerous small grains, there is a high effective diffusion coefficient (HEDC). The multi axial stresses induced during in service/ creep testing conditions together with the HEDC causes voids to form at an accelerated rate. Significant void coalescence promotes the formation of micro cracks which in turn lead to macro crack formation and eventually failure.
  • Thumbnail Image
    Item
    Open Access
    Influence of the heat treatment procedure on the stress corrosion cracking behaviour of low pressure turbine blade material FV566
    (2017) Seumangal, Nicole; Sonderegger, Bernhard; Knutsen, Robert D
    Stress corrosion cracking is one of the leading damage mechanisms in low-pressure turbines in the power generation industry; in LP turbine blades it primarily occurs in the last stage blades. The research investigated the influence of tempering temperature on the microstructure, mechanical properties, and stress corrosion cracking properties of 12% chromium FV566 stainless steel, which is used to manufacture LP turbine blades. The standard heat treatment of the steel comprises of austenitising, quenching and double tempering. Austenitising is carried out at 1050°C for one hour - which is sufficiently long to generate a fully austenitic matrix and to dissolve carbon completely. Subsequently, the material is quenched in air. The high level of alloying elements ensures the complete martensitic transformation, with carbon atoms trapped in the matrix and distributed homogeneously. Thereafter, tempering of the material at 580-600°C enhances the ductility and toughness. Tempering replaces the solid solution strengthening of the dissolved carbon with precipitation strengthening by carbides. The final microstructure of the FV566 steel blades is referred to as tempered martensite. van Rooyen showed that for 12% chromium steel tempering at and above 600°C induces passivity of the material against SCC, while tempering of 12% chromium steels at 450-550°C causes sensitisation of the material and the material exhibits intergranular SCC. From such studies, the motivation arises to investigate the impact of heat-treatment parameters - specifically the impact of tempering temperature on the stress corrosion behaviour of the material. The testing methodology comprises heat treatment of FV566 samples at 1050°C for 1 hour, at 350°C for 1 hour, and thereafter tempering for 1 hour at various tempering temperatures. Each stage of heat treatment is followed by air cooling - followed by analysis of the microstructure, mechanical testing and stress corrosion cracking testing of the specimens at the different temper conditions. Stress corrosion testing was divided into two categories. The first set of tests was carried out with U-bend specimens to determine the susceptibility of materials at different heat treatments to SCC, the time taken for SCC to initiate, and the mode of cracking. The second set of tests was conducted to determine the threshold stress intensity, as a function of crack growth rate, for each heat treatment. The SCC failure mechanism observed was intergranular SCC (IGSCC) by anodic dissolution for the 550°C, 560°C, 570°C, 580°C, 590°C, 600°C and 620°C specimens. The material's resistance to SCC improved with increasing tempering temperature. Specimens tempered at 480°C and 550°C were most susceptible to SCC, while specimens tempered at 600°C The material's resistance to SCC improved with increasing tempering temperature. Specimens tempered at 480°C and 550°C were most susceptible to SCC, while specimens tempered at 600°C were immune to SCC in a 4000-hour period. A change in tempering temperature results in a change in the quantity and type of precipitates formed which results in changes in SCC properties of FV566.
  • Thumbnail Image
    Item
    Open Access
    The modelling of damage due to diffusional creep in high chromium steels
    (2016) Weyer, Royden; Knutsen, Robert D; Sonderegger, Bernhard
    Understanding the creep deformation of high chromium steels in use in modern power plants has become important in predicting the behaviour and stability of these materials over their operational lifetime. At the deformation rates and conditions recorded in modern power plants, diffusional creep by vacancy migration is seen to be the dominant creep mechanism. However, the understanding of diffusional creep in particle stabilized materials is heavily incomplete. The aim of this project was to model the damage caused by diffusional creep, while considering the microstructure of high chromium steels and the evolution of this microstructure. This problem is addressed by expanding the existing Nabarro-Herring theory on lattice diffusion into a spatially resolved FEM model using MATLAB. This model focussed on adapting the Nabarro-Herring creep model to handle vacancy concentration changes over time. This allowed the model to produce the primary, secondary and tertiary creep stages present in experimental creep tests. As for microstructure, the focus was on adding precipitates (one of the strongest creep strengthening mechanisms) and voids (the largest cause of material damage). During creep exposure, precipitates were subject to coarsening while voids were subject to growth. The primary creep stage was formed by the initial rapid flux of vacancies into the body of the grain, due to large chemical potential gradients. A dynamic equilibrium of vacancy concentration would form within the grain, leading to the secondary creep stage. The creep rate produced was similar to that of the existing theory and it was found to decrease with the introduction of precipitates. This was evaluated by analysing the stress gradients caused by hard particles in a softer matrix. These stress fields lowered the stress in the grain boundaries and thus resulted in fewer vacancies being generated. Coarsening led to a reduction in the stress field distribution and thus resulted in creep strength loss in the material. The inclusion of voids was shown to decrease the initial creep rate, with void growth lessening this effect and leading to the tertiary creep stage. The initial strengthening was due to the void surface replacing the grain boundary as a source of vacancies. As the void surface is a very inefficient source, fewer vacancies were generated, resulting in lower diffusion rates. A slight steady increase in the creep rate over time was shown with the inclusion of void growth. The increase in vacancy generation was caused by the higher stress fields around voids. Initially the stress increase due to a loss in area was accounted for as a stress concentration around the void. Once this void grew too large in relation to the grain size, the stress concentration no longer accounted for all of the stress increase due to load bearing area loss. This resulted in the damage equation coming into play, causing a rapid increase in the stress throughout the grain and leading to the rapid tertiary creep stage.