Papers by Keyword: Thermal Fatigue

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Authors: Woo Sung Choi, Eric Fleury, Gee Wook Song, Jung Seob Hyun
Abstract: An important characteristic of a steam power plant is its ability to maintain reliability and safety of the plant against frequent start-ups and load changes. Transient regimes arising during start-ups, shut-downs and load changes give rise to unsteady temperature distribution with time in steam turbine rotor(HP/IP), which results in non-uniform strain and stress distribution. The rapid increase of temperature and rotational speed during starts-ups, especially, makes conditions more severe and causes main components’ damage and reduction of life span for steam turbine. Thus accurate knowledge of thermal and centrifugal stresses are required for the integrity and lifetime assessment for the turbine rotor. So far, only elastic calculations are currently performed for simplicity. However, it is well known that the materials of steam turbine rotor deform inelastically at high temperature. Existing models proposed to describe the viscoplastic(rate-dependent) behavior are rather elaborate and difficult to incorporate with computer simulations in the case of complex structures. In this paper, the life assessment for steam turbine rotor was established by combining the inelastic behavior and the finite element method. The inelastic analysis was particularly focused on viscoplastic behavior that is simple enough to be used effectively in computer simulation and matches the essential features of the time-dependent inelastic behavior of materials reasonably well for cyclic loading under non-isothermal conditions. Using this study, life consumption of steam turbine rotor can be obtained.
Authors: Shuai Lu, Hong Wei Li, Dun Bo Yu, Ming Pang, Bo Wang
Abstract: In single roll rapid quenching molding, especially for the preparation of amorphous ribbons and rare earth permanent magnetic materials, the chilling roll is an extremely important component of the preparation equipment. However, frequent repair of the roll is not conducive to continuous large-scale production because of its poor thermal fatigue resistance. Molybdenum is gradually being used as chilling roll material in some applications, and in the present paper a new approach to study the thermal fatigue resistance of molybdenum is based on laser pulse irradiation on the molybdenum surface. A new designed device effectively prevents the molybdenum to be oxidized during the laser heating test. The experiments of thermal fatigue damage on molybdenum were conducted by the high power laser. The results shows that the fatigue cracks had been propagated in the irradiated region after the specimen was loaded by 200 times pulsed laser heating. There is a greater crack density in the laser heating brim region than in the center of the irradiated region. According to the transgranular mode of crack propagation in the laser heating brim region and intergranular mode in the center of the irradiated region, the maximum stress took place at the intersection region due to the great temperature gradient in the laser heating brim region. Continued basis-oriented experiments are planned, regarding the mechanism of thermal fatigue crack initiation and propagation for chilling roll materials.
Authors: Michael Katcher, Dwaine L. Klarstrom
Abstract: HAYNES® 230® and 617 alloys are competing for use on Generation IV, high temperature gas cooled reactor components because of good high temperature creep strength in the temperature range between 760°C and 982°C and resistance to oxidation in the gas cooled reactor environment. A review of the metallurgy affecting the properties in each alloy will be discussed. Grain size and carbide precipitation developed during fabrication effect short term and long term ductility, fatigue, and creep. For example, 230 alloy has a finer grained structure which promotes fatigue strength with a slight sacrifice in creep strength. The 617 alloy has a coarser grain structure which provides slightly higher creep resistance while sacrificing some fatigue strength. Thermal aging also introduces gamma prime precipitation to the 617 alloy as well as grain boundary carbides, and this, in addition to grain boundary oxidation, reduces the low cycle fatigue strength of 617 alloy compared to 230 alloy. Independent studies have shown that 230 alloy possesses higher resistance to thermal fatigue than 617 alloy. However, welds of both base metals with similar weld composition have about the same thermal fatigue life. Cooling rates from solution annealing temperatures during processing effect the ductility and creep strength of these alloys with the highest cooling rates preferred for retention of ductility and creep strength. The reason; slow cooling rates promote carbide precipitation in the grain boundaries which reduces ductility and creep strength.
Authors: Xiu Hai Zhang, Can Wei Lai, Man Feng Xian, Guang Cai Su
Abstract: The thermal fatigue behaviors of traditional Al-Si-Mg casting alloy and optimized Al-Si-Mg casting alloys at different thermal fatigue temperatures were investigated. Fatigue cracking appeared on the surface of traditional Al-Si-Mg alloy after 450 thermal cycles at 300 °C thermal fatigue temperature. However, the fatigue cracking was not found on the surface of optimized Al-Si-Mg alloy at the limited thermal fatigue cycles (less than 4450 times). Moreover, the optimized Al-Si-Mg alloy only occurred to elastic deformation and could not emerge in irreversible deformation. The grain refinement in the optimized Al-Si-Mg alloy could make the thermal fatigue cracking appeared much later and propagated much slowly. These results showed that the thermal fatigue resistance of optimized Al-Si-Mg alloy was superior to that of traditional Al-Si-Mg alloy.
Authors: Yong Seok Kim, Dong Keun Lee, Jeong Min Lee, Hyun Woo Song, Sung Hyuk Kim, Jae Mean Koo, Chang Sung Seok, Myoung Rae Cho
Abstract: Thermal barrier coating. Thermal fatigue. Exposure time. Thermal fatigue test is one of the most widely used method to evaluate the durability of thermal barrier coating (TBC). However, thermal fatigue test can be concluded in totally different results according to the test variations. Especially, Exposure time of thermal fatigue test can affect the delamination life cycle of TBC. In this study, using the same test equipment which Kim et al. used, thermal fatigue tests were performed with different holding time at high temperature, and the test results by Kim et al. and those by this study were compared. In addition, delamination map was come to perfection from the test results to define more accurate thermal fatigue life.
Authors: Shu Hung Yeh, Liu Ho Chiu, Shou Chi Lin, Yeong Tsuen Pan
Abstract: A hard-coating on hot work tool steel can be used to obtain higher corrosion resistance, as well as better wear resistance. This study investigates the thermal fatigue performance of AISI H13 hot work tool steel with and without hard chromium plating. Treated specimens were characterized using microstructural analysis, X-ray diffraction analysis and microhardness measurement. The thermal fatigue test is based on cyclic induction heating and water cooling. The specimen was heated to the maximum surface temperature of 670°C followed with water injection to bring the specimen down to a minimum temperature of 25°C. The thermal fatigue testing in this study was conducted using 500 cycles. A vacuum heat treated specimen with a hardness of 47 HRC was used as the reference material. The hard chromium plated layer with a thickness 35 μm had a hardness of 930 HV0.1. The damage factor, defined as crack depth × crack width, of quenched and tempered H13 specimens and hard chromium plated specimens were 800 and 1760, respectively. The damage factor evaluation verified the vacuum heat treated specimen thermal fatigue resistance is superior to that of the hard chromium plated specimen.
Authors: R.R. Keller, N. Barbosa, R.H. Geiss, D.T. Read
Abstract: A novel approach for measuring thermal fatigue lifetime and ultimate strength of patterned thin films on substrates is presented. The method is based on controlled application of cyclic joule heating by means of low-frequency, high-density alternating current. Such conditions preclude electromigration, but cause cyclic strains due to mismatch in coefficients of thermal expansion between film and substrate. Strain and stress are determined from measurement of temperature. Fatigue properties are a natural fit to testing by alternating current. Stress-lifetime (S-N) data were obtained from patterned aluminum lines, where stress amplitude was varied by changing current density, and lifetimes were defined by open circuit failure. Electron microscopy and electron backscatter diffraction observations of damage induced by a.c. testing suggested that deformation took place by dislocation mechanisms. We also observed rapid growth of grains – the mean diameter increased by more than 70 % after a cycling time of less than six minutes – which we attribute to strain-induced boundary migration. Ultimate strength was determined by extrapolating a modified Basquin relation for high cycle data to a single load reversal. A strength estimate of 250 ± 40 MPa was determined based on a.c. thermal fatigue data. In principle, an electrical approach allows for testing of patterned films of any dimension, provided electrical access is available. Furthermore, structures buried beneath other layers of materials can be tested.
Authors: Kuk Tae Youn, Young Mok Rhyim, Jong Hoon Lee, Chan Gyu Lee, Yun Chul Jung
Abstract: It is well known that the main failure mechanisms in die-casting mould are heat checking due to thermal fatigue and melt-out caused by chemical reaction between die and molten alloys. Thermal fatigue tests were carried out using the thermal cycle simulator to establish the proper method to estimate the thermal fatigue resistance of hot die steel. In this study, the thermal shock tester consisted of induction heating and water spray cooling unit was constructed to evaluate thermal crack propagation resistance and the sum of crack length per unit specimen length, Lm is proposed as the index representing the susceptibility to crack initiation and propagation. Also, new concept of measurement for the melt-out behavior was suggested. AISI H13 hot work tool steel was solution treated and tempered at various temperatures, to control the hardness and toughness that have effect on the behavior of thermal crack propagation. The result of thermal fatigue test showed that there is optimum value of hardness and impact energy to maximize the thermal crack propagation resistance. The influence of nitriding on melt-out resistance was also investigated. The dissolution rate due to melt-out phenomenon tended to be smaller for thicker compound layer. Furthermore, the resistance to melt-out was affected by the compound layer thickness rather than that of diffusion layer. The results of the both tests properly reflect the effect of materials properties on failure modes of die-casting mould and it means those test methods are suitable to evaluate the durability of hot work tool steel for die-casting.
Authors: Kuk Tae Youn, Young Mok Rhyim, Jong Hoon Lee, Young Sang Na, Wee Do Yoo, Chan Gyu Lee
Abstract: For hot die steel, failure is mostly caused by heat checking and melt-out on its surfaces which are in contact with molten metals. In the present research, resistances to melt-out and heat checking of surface modified H13 hot die steels, such as gas nitriding(GN) and micro-blasting(MB), were investigated. The evaluation of melt-out behavior was carried out by measuring the mean depth from the original surface after immersion. To examine the thermal fatigue resistance, a cyclic thermal shock system consisting of induction heating and water spray quenching was constructed. The value of Lm is proposed as the index representing the susceptibility to crack initiation and propagation. The melt-out depth was the lowest for the GN treated surface. MB-GN and MB-GN-MB treated specimens also showed good resistance to melt-out. In the case of GN treatment, while the white layer was completely melted out, the diffusion layer still existed even after immersion for 43 hours. This implies that nitriding significantly reduced the rate of melt-out. From the total crack number, it is seen that crack initiation was reduced to the extent of half after surface treatment. This result means that the residual compressive stress and nitrided layer were beneficial to crack initiation resistance.
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