By the very nature of their work, castings used in furnaces for heat treatment and thermo-chemical treatment are exposed to the effect of many unfavorable factors causing their deformation and cracking, significantly shortening the lifetime. The main source of damage are the micro- and macro-thermal stresses appearing in each cycle. As the cost of furnace instrumentation forms a significant part of the total furnace cost, in designing this type of tooling it is important to develop solutions that delay the damage formation process and thus extend the casting operation time. In this article, two structural modifications introduced to pallets castings to reduce thermal stresses arising at various stages of the cooling process are proposed. The essence of the first modification consists in making technological recesses in the wall connections, while the aim of the second one is to reduce the stiffness of the pallet by placing expanders in the external walls. Using the results of simulation analyses carried out by the finite element method, the impact of both proposed solutions on the level of thermal stresses was evaluated.
The results of research on the effect of the type of cooling agent used during heat treatment and thermal-chemical treatment on the formation of temperature gradient and stress-deformation distribution in cast pallets, which are part of furnace accessories used in this treatment, are disclosed. During operation, pallets are exposed to the effect of the same conditions as the charge they are carrying. Cyclic thermal loads are the main cause of excessive deformations or cracks, which after some time of the cast pallet operation result in its withdrawal due to damage. One of the major causes of this damage are stresses formed under the effect of temperature gradient in the unevenly cooled pallet construction. Studies focused on the analysis of heat flow in a charge-loaded pallet, cooled by various cooling agents characterized by different heat transfer coefficients and temperature. Based on the obtained temperature distribution, the stress distribution and the resulting deformation were examined. The results enabled drawing relevant conclusions about the effect of cooling conditions on stresses formed in the direction of the largest temperature gradient.
The paper discusses the impact of the geometry of foundry pallet components on the value of temperature gradient on the wall crosssection
during heat treatment. The gradient is one of the most important factors determining the distribution of thermal stresses in these
items. Analysis of quantitative simulation was carried out to detect possible effect of the type of connection between pallet walls and
thickness of these walls (ribs) on the interior temperature distribution during rapid cooling. The analysis was performed for five basic
designs of wall connections used in pallets. Basing on the results obtained, the conclusions were drawn on the best connection between the
ribs in foundry pallets.
Depending on the course of the processes of heat treatment and thermo-chemical treatment, the technological equipment of heat treatment furnaces is exposed to different operating conditions, as the said processes differ among themselves in the temperature of annealing and atmosphere prevailing in the furnace chamber, in the duration of a single work cycle and in the type and temperature of the coolant. These differences affect the magnitude of stresses occurring in each cycle of the operation of furnace accessories, and thus play an important role in fatigue processes leading to the destruction of these accessories. The kinetics of temperature changes during each cooling process plays an important role in the formation of thermal stresses on the cross-section of the cooled parts. It depends on many factors, including the initial cooling temperature, the type and temperature of the cooling medium, or the dimensions and shape of the object. This article presents a numerical analysis of the effect of the initial temperature on the distribution of stresses on the cross-section of the grate ribs, generated in the first few seconds of the cooling process carried out in two cooling media, i.e. hardening oil and water. The analysis was carried out by the finite element method, based on the results of experimental testes of temperature changes in the rib during its cooling.
The purpose of this study was to establish a relationship between the type of wall connection used in the cast grates, which are part of the
equipment operating in furnaces for heat treatment and thermal-chemical treatment, and stresses generated in these grates during the
process of rapid cooling. The places where the grate walls are connected to each other are usually characterized by the thickness larger
than the remaining parts of walls. Temperature variations in those places are responsible for the formation of hot spots, and in the hot spots
temperature changes much more slowly. The type of wall connection shapes the temperature gradient in the joint cross-section, and hence
also the value of thermal stresses generated during cooling. In this study, five different designs of the grates were compared; the difference
in them was the type of the designed wall connection. The following design variants were adopted in the studies: X connections with and
without holes, T connections with and without technological recesses, and R (ring) connection. Numerical analysis was performed to
examine how the distribution of temperature changes in the initial phases of the cooling process. The obtained results served next as a tool
in studies of the stress distribution in individual structures. The analysis were carried out by FEM in Midas NFX 2014 software. Based on
the results obtained, the conclusions were drawn about the impact of different types of wall connections on the formation of thermal
stresses in cast grates.
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