Papers by Keyword: Thermal Shock

Abstract: In order to provide a theoretical basis for the study of thermal fatigue properties on surface composites, thermal shock cracks initiation and propagation of WCP reinforced high chromium steel substrate surface composites were studied by thermal shock test method at 500 °C. The results show that cracks initiation and propagation begin within a few thermal shock cycles, and after 15 thermal shock cycles, the composites remain intact, indicating a good thermal shock resistance. The thermal shock cracks consist mainly of longitudinal and transverse cracks. Within a few thermal shock cycles, the initiation and propagation of longitudinal cracks play a dominant role; however, with the increase in the number of thermal shock, the transverse cracks may play a key role as the length and number of both types of cracks increases. However, the increase is slow. The longitudinal cracks are mainly caused by the first class thermal stress and the transverse cracks result from the culminant effects of the first and second thermal stress, interacting with each other.

Abstract: In many machines and devices there are hydraulic drives. They should operate well in various weather conditions, also in low ambient temperatures. Some malfunctions may occur during operation of hydraulic system in a so called “thermal shock condition”, which happens when frozen hydraulic component (e.g. hydraulic pump, motor or directional spool valve) is suddenly supplied with hot oil. Transient thermal state emerges in these conditions. Particular elements of component warm up differently. This results in different thermal expansion of components during warm up, which is changing the size of clearance between cooperating elements. Experimental tests of hydraulic components in low ambient temperatures were conducted in the hydraulic laboratory of the Faculty of Mechanical Engineering of Gdansk University of Technology. They concerned: orbital motors, satellite motors, gear pumps, spool valves (also proportional), piston pumps, and hydraulic cylinders. It was proved, that in thermal shock conditions some malfunctions may occur, especially with large temperature differences between oil and component. Based on results of the tests of hydraulic components and systems start-up in low ambient temperatures, one can perform a change in components design, change the type of material of components elements, and even prepare a proper procedure to be followed during start-up of cold hydraulic components and systems. In the article discusses some designs of hydraulic components resistant to thermal shock, and how to prepare components to work in low temperatures, e.g. by providing system with an additional heater to ensure uniform heating of elements in components. Change in design of hydraulic components resistant to thermal shock conditions can be obtained through computer simulation method. Analytic and computer simulation methods can be used by engineers who design machines and devices that work in low ambient temperatures.

Abstract: This study aimed to understand the effect of TiO₂ content on the thermal shock performance of Al₂O₃/TiO₂ air plasma spray coatings. Three kinds of commercial feedstock powders were used, i.e. Al₂O₃, Al₂O₃-13wt%TiO₂ and Al₂O₃-40wt%TiO₂. The resulting coatings were studied in terms of micro Vickers hardness test, XRD, SEM and thermal shock test. The last was conducted using a water quenching method. The results showed that both the coating hardness and thermal shock performance decreased with the TiO₂ content.

Abstract: A sprayed MoB/CoCr cermet coating was formed on 310S stainless steel by low pressure plasma spraying (LPPS), and its durability in the molten Al-12.07wt.%Si has been investigated by SEM、EDS and XRD. The immersion test revealed that the MoB/CoCr coating has much higher durability without dissolution in the molten Al-12.07wt.%Si alloy. Little change of crystal structure, mainly composed of ternary borides of Co2MoB2 and CoMoB, suggesting that the ternary borides have much higher durability.

Abstract: When diamond abrasive layer becomes thinner, remaining expensive diamond grits will lose their cutting ability. The recovery of waste diamond tools can realize the recycling of material, is one kind of green manufacturing. Taken electroplated diamond tools for example, key technical problems for diamond tool recovery were discussed in this study. A thermal shock method was proposed for electroplated diamond tools recycling processes. Based on heat elastic-plastic theory, the stress state between abrasive layer and substrate of electroplated diamond tools was analyzed. Thermal shock processes of electroplated diamond tools were simulated with finite element method. Thermal shock experiments of electroplated diamond tools were carried out with experimental electric furnace. The results indicate that there exists stress variation between abrasive layer and tool substrate interface during thermal shock operation. The stress change will result in the interface cracking and the peeling of abrasive layers from substrate. There exists obvious stress gradient between abrasive layer and substrate interface. The stress extreme value occurs on the area of abrasive layer transition region, the corner or end of abrasive layer. Experimental results prove that the separating of abrasive layer and the tool substrate using thermal shock method is feasible.

Abstract: MoB/CoCr, a novel material for thermal spraying, with high durability is used to resist erosion by molten Al-12.07wt.%Si alloy. The durability of the MoB/CoCr coating prepared by low pressure plasma spraying (LPPS) has been investigated using a molten-metal immersion tester. The test revealed that the MoB/CoCr coating has much higher durability without dissolution in the molten Al-12.07wt.%Si alloy. Little change of crystal structure, mainly composed of ternary borides of Co₂MoB₂ and CoMoB, is observed after the immersion test, suggesting that the ternary borides have much higher durability.

Abstract: The temperature field in the adhesively bonded single lap steel joint after a thermal shock (100 °C, 10s) and the influence of the restraint on the thermal stress in the joint was investigated using elasto-plastic finite element method (FEM). The results showed that the temperature in the overlap region is symmetrically distributed to the bondline after the thermal shock. Five kinds of constraints were applied in the study and it is found that the peak value of the thermal stresses were occurred at the points near the free ends of the adherend lap zone along the mid-bondline under the action of constraining the adherends in the direction of both transversal and longitudinal. The symbol of the longitudinal stress Sx and peel stress Sy is negative.

Abstract: The effect of five constraint conditions in cooling process on the residual stress distributed in the adhesively bonded single lap steel joint under a 100°C thermal shock was investigated using the elasto-plastic finite element method (FEM). The results from the numerical simulation show that the effect of constraint model is significantly to the residual stress distributed in both mid-bondline and adherend near the interface of the adhesively bonded single lap steel joint. The constraint to the longitudinal of the adherend is much significantly to the residual stress than that of the transversal one. Comparing the action of the constraint model C and model E, the effect of the model E on the residual stress along the mid-bondline is higher than that of the model C.

Abstract: This work addresses damage evaluation of porous mullite refractory subjected to thermal shock. Incommunicating circular pores were distributed randomly at a volume percentage up to 40% in a cylinder of 20 cm diameter. The analysis was performed by means of the software ANSYS® combined with a pre-program that generates randomly distributed pores of given size. The analysis procedure was divided into two stages. In the first, transient thermal analysis considering temperature-dependent material property was dealt with different thermal shock temperatures under natural cooling condition. The following structure analysis ran based on the obtained temperature distribution. The material damage was defined by that the local tensile stress reached to or was over the strength of the refractory. The extent of damage was determined as the ratio of the area of the damaged regions to the section area of the cylinder. The results show that the porosity, thermal shock temperature and cooling time have a high effect on the material damage. The lower the porosity is, the larger the extent of damage. The thermal damage increases with the raise of thermal shock temperature and the cooling time. The damage develops rapidly within 10 minutes but slows down after one hour cooling. The damage difference at high shock temperature stage (≥ 1000°C) is less than at low shock temperature stage. The pore size effect gets into practice only at high shock temperature stage: the damage increases with the raise of the pore size. The present research confirms that high porosity and small pore size could decrease greatly thermal shock damage and should be considered in the micro structural design of refractory.

Abstract: The experiments of thermal shock damage on piston were conducted by shaped high power laser. Damage mechanism of thermal shock specimen was characterized by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The mechanical properties of thermal shock specimen were evaluated by microhardness. The corresponding mechanisms were discussed in detail. The results show that cracks originated from the interface of Al-matrix and intermetallic phase due to the thermal and mechanical misfit between these brittle components of the microstructure and the surrounding ductile matrix. Oxides of thermal shock crack can accelerate the damage of piston. There is a drop in the hardness of piston at all locations after thermal shock tests.