Effects of the alloying elements ratio in the Ni–Al–Cr–W–Mo–Ta system on the thermal fatigue resistance of the deposited metal

The article presents an analysis of the influence of the alloying elements ratio in the Ni-Al-Cr-WMo-Ta system on the resistance of the deposited metal against thermal fatigue. The combined effect of alloying elements on the crack resistance of weld metal under cyclic temperature changes in the range of 20–1150°C is established. It is shown that in the alloying system, the sensitivity of a metal to the formation of thermal fatigue cracks mainly depends on the number of refractory elements that cause the formation of topologically closepacked (TCP) phases. The content in the deposited metal of 3.5 wt.% tungsten, 3.0 wt.% molybdenum, 2.5 wt.% tantalum does not cause the appearance of fatigue cracks. The developed deposited metal provides high level of thermal and oxidation wear resistance compared to highly doped nickel and cobalt industrial alloys.

Methods for Determining the Thermal State of Medium-Speed Diesel Engines Taking into Account Boiling of the Coolant

Forcing medium-speed diesel engines by increasing the mean effective pressure leads to an increase in the thermal factor of the engine parts. High temperatures of the cylinder head fire deck and the cylinder liner working surface cause thermal fatigue cracks in these parts as well as piston scuffing. Therefore, the development of new methods of intensifying heat transfer in the cooling gallery and refined methods for determining the engine parts thermal state is currently relevant. In both areas of research, a significant role is played by the process of coolant boiling, which significantly intensifies heat transfer in the cooling system. A review of the literature showed that the existing methods of mathematical modeling of conjugate heat transfer in the cooling cavities taking into account the boiling process have a number of significant drawbacks. This paper presents the developed mathematical model and methods for determining the thermal state of medium-speed diesel engine parts taking into account boiling of the coolant, thus making it possible to combine the advantages of both the engineering approach and numerical simulation based on computational fluid dynamics. The thermal state of a new generation medium-speed diesel engine D500 was calculated and the thermal factor of the main engine parts was estimated.

Assessment of thermal behavior of a cooling system to reduce thermal fatigue cracks in aluminum injection molds

The present article aimed to test changes in cooling water temperatures of males, present in aluminum injection molds, to reduce failures due to thermal fatigue. In order to carry out this work, cooling systems were studied, including their geometries, thermal gradients and the expected theoretical durability in relation to fatigue failure. The cooling system tests were developed with the aid of simulations in the ANSYS software and with fatigue calculations, using the method of Goodman. The study of the cooling system included its geometries, flow and temperature of this fluid. The results pointed to a significant increase in fatigue life of the mold component for the thermal conditions that were proposed, with a significant increase in the number of cycles, to happen failures due to thermal fatigue.

Comparative study of protective coatings for heat resistance

Modern gas turbine engines operate under changing temperature loads. Therefore, one of the important characteristics of the protective coatings used on the turbine blades is their high resistance to the occurrence and development of cracks under mechanical and thermal loads. The applied effective systems of internal heat removal of the cooled turbine blades lead to an increase in their heat stress. At present, cracks arising from thermal fatigue are one of the common defects of the protective coatings used on turbine blades. The heat resistance of coatings at high temperatures is determined by three factors: the shape of the part on which the coating is applied, the thickness of the coating and the phase composition of the surface layers or the maximum aluminum content in the coating. Therefore, when choosing a protective coating for these operating conditions, it is important to know the impact of these factors on the thermal stability of the coating. The paper presents a comparative study of various coatings on their resistance to crack formation under cyclic temperature change. The dependence of the heat resistance of the considered coatings on the method of their application and phase-structural state is established. Especially valuable is the established mechanism of formation and propagation of thermal fatigue cracks depending on the phase composition of the initial coating. It is shown that the durability of protective coatings with cyclic temperature change depends on the chemical composition of the coating and the method of its formation. The dependence of the formation of thermal fatigue cracks on the samples with the coatings under study on the number of cycles of temperature change is established.

Bionic Repair of Thermal Fatigue Cracks in Ductile Iron by Laser Melting with Different Laser Parameters

Nodular iron brake discs typically fail due to serious thermal fatigue cracking, and the presence of graphite complicates the repair of crack defects in ductile iron. This study presents a novel method for remanufacturing ductile iron brake discs based on coupled bionics to repair thermal fatigue cracks discontinuously using bio-inspired crack blocking units fabricated by laser remelting at various laser energy inputs. Then, the ultimate tensile force and thermal fatigue crack resistance of the obtained units were tested. The microhardness, microstructure, and phases of the units were characterized using a digital microhardness meter, optical microscopy, scanning electron microscopy, and X-ray diffraction. It was found that the units without defects positively impacted both the thermal fatigue resistance and tensile strength. The unit fabricated at a laser energy of 165.6 − 15 + 19 J/ mm 2 had sufficient depth to fully close the crack, and exhibited superior anti-cracking and tensile properties. When the unit distance is 3 mm, the sample has excellent thermal fatigue resistance. In addition, the anti-crack mechanism of the units was analysed.

In-Situ Monitoring via Synchrotron Radiation Laminography of Thermal Fatigue Cracks at Die-Attached Joints Under Cyclic Energization Loading

Recently, due to the increasing heat density of printed circuit boards (PCBs), thermal fatigue damage in the joints has exerted a more significant influence on the reliability of electronic components. Accordingly, the development of a new nondestructive inspection technology is strongly desired by related industries. The authors have applied a synchrotron radiation X-ray micro-tomography system to the nondestructive observation of micro-cracks. However, the reconstruction of CT images is difficult for planar objects such as PCB substrates, due to insufficient X-ray transmission in the direction parallel to the substrates. In order to solve this problem, a synchrotron radiation laminography system was developed to relax size restrictions on the observation samples, and was applied to the three-dimensional nondestructive evaluation of several kinds of solder joints, which were loaded under accelerated thermal cyclic conditions via thermal shock tests. Moreover, the thermal fatigue crack propagation process that occurs under actual PCB energization loading conditions will differ from that under the usual acceleration test conditions. In this work, the possibility of in-situ monitoring of the thermal fatigue crack propagation process using the laminography system was investigated at die-attached joints subjected to cyclic energization loading, which is close to the actual usage conditions of PCBs. The optical system developed for use in the laminography system was constructed to provide a rotation stage with a tilt from the horizontally incident X-ray beam, and to obtain X-ray projection images via a beam monitor. In this manner, the X-ray beam is sufficiently transmitted through the planar specimen in all projections. The observed specimens included several die-attached joints, in which 3 mm square ceramic dies had been mounted on a 40 mm square FR-4 substrate using Sn-3.0wt%Ag-0.5wt%Cu solder. Consequently, the laminography system was successfully applied to the in-situ monitoring of thermal fatigue cracks that appeared in the solder layer under cyclic energization. This was possible because the laminography images obtained in the energization state have a quality that is equivalent to those obtained in a non-energized state, provided that the temperature distribution of the specimen is stable. In addition, the fatigue crack propagation process can be quantitatively evaluated by measuring the crack surface area and calculating the average crack propagation rate. However, in some cases, the appearance of thermal fatigue cracks was not observed in a solder layer that had been loaded by the accelerated thermal cycle test. This result strongly suggests that delamination occurred at the interface, which indicates that the corresponding fracture mode was significantly influenced by the type of thermal loading.

Improving Ultrasonic Examination Procedures for Detection of Thermal Fatigue

This paper describes improvements to the ultrasonic procedures to be used for the detection of thermal fatigue in nuclear power plants in accordance with the requirements of the Electric Power Research Institute (EPRI) Material Reliability Program (MRP) inspection and evaluation guidelines. These examinations have been performed at nuclear plants in the USA since the 1980s with very few detections of degradation. However, since 2013 there have been ten instances of thermal fatigue cracks. The MRP formed a thermal fatigue focus group to analyze these leaks and flaws related to thermal fatigue inspection programs. Then the group developed recommendations to address these recent operational experiences. The MRP has been developing improvements to the ultrasonic examination process and this paper will share these. A computer based training program for the ultrasonic personnel has been developed that will be described. And finally, the MRP has fabricated a variety of thermal fatigue mockups that are loaned to member utilities prior to an outage so the ultrasonic personnel can practice detecting thermal fatigue just prior to the examinations. Implementation of these mockups will also be described.