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1

Thermal Stresses in the Accessories of Heat Treatment Furnaces vs Cooling Kinetics

Bajwoluk A., Gutowski P.

Archives of Foundry Engineering

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2019

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iss. 3

88--93

EN

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.

2

Thermal micro- and macrostresses in the surface layer of cast steel accessories for carburizing furnaces

Bajwoluk A., Gutowski P.

Inżynieria Materiałowa

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2018

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Vol. 39, nr 5

178--183

EN

High heat resistance and creep resistance make the austenitic Fe–Ni–Cr alloys widely used as a material for accessories of the furnaces for heat treatment and thermochemical treatment. Due to the specific character of furnace operation, its tooling is exposed to the effect of numerous highly unfavourable factors, including high temperature and its rapid changes, strongly carburizing atmosphere as well as mechanical and thermal loads originating from the processed batch of items. As a result of the impact of the above-mentioned factors, in castings made from the Fe–Ni–Cr alloys, cracks and deformations occur, eliminating these castings from further use. The damages are mainly caused by stresses arising in the cast structure during subsequent work cycles. It is generally believed that stresses in furnace accessories are due to the temperature gradient generated during heating or cooling of components and differences in the thermal expansion of the structural constituents of cast steel from which these accessories are made. This article focuses on the results of numerical analysis of the stress distribution in the surface layer of a carburized austenitic alloy undergoing the cooling process. Cases are discussed when the examined sources of stress act as separate and independent factors and also when they operate simultaneously. This provides information about the stress formation and development of stress-initiated cracks on the surface of real castings.

PL

Dobra żaroodporność oraz żarowytrzymałość sprawiają, iż austenityczne stopy Fe–Ni–Cr są powszechnie stosowane jako materiał, z którego wykonywane są elementy osprzętu pieców do obróbki cieplnej i cieplno-chemicznej. Specyfika pracy oprzyrządowania pieców powoduje, że jest ono poddawane cyklicznemu działaniu wielu niekorzystnych czynników, w tym: wysokiej temperatury oraz jej gwałtownym zmianom, silnie nawęglającej atmosfery oraz obciążeniom mechanicznym i cieplnym pochodzącym od obrabianego wsadu, przyspieszającym procesy zmęczenia cieplnego. Pomimo dość dobrych właściwości wytrzymałościowych, którymi charakteryzują się austenityczne stopy niklowo-chromowe w podwyższonej temperaturze, w elementach z nich wykonanych dochodzi w wyniku wzajemnego oddziaływania wymienionych czynników do odkształceń oraz pęknięć eliminujących je z dalszego użytkowania. Za główne źródła naprężeń powodujących niszczenie odlewów (rys. 1, 2) użytkowanych w piecach uważa się różną rozszerzalność cieplną składników strukturalnych występujących w staliwie, z którego są one wykonane oraz gradient temperatury tworzący się na przekroju podczas chłodzenia lub nagrzewania. Celem pracy było przeprowadzenie analiz numerycznych rozkładów naprężeń powstających w warstwie wierzchniej gwałtownie chłodzonego, nawęglonego elementu wykonanego ze stopu Fe–Ni–Cr z uwzględnieniem równoczesnego występowania obydwóch tych źródeł.

3

Design Options to Decrease the Thermal Stresses in Cast Accessories for Heat and Chemical Treatment Furnaces

Bajwoluk A., Gutowski P.

Archives of Foundry Engineering

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2018

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Vol. 18, iss. 4

125--130

EN

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.

4

The Effect of Cooling Agent on Stress and Deformation of Charge-loaded Cast Pallets

Bajwoluk A., Gutowski P.

Archives of Foundry Engineering

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2017

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Vol. 17, iss. 4

13--18

EN

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.

5

Thermal Stresses in the Wall Connections of Cast Grate Structures

Bajwoluk A., Gutowski P.

Archives of Foundry Engineering

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2016

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Vol. 16, iss. 4

11--16

EN

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.

6

Inicjacja i propagacja mikropęknięć w stabilnym staliwie austenitycznym

Bajwoluk A., Gutowski P.

Inżynieria Materiałowa

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2015

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Vol. 36, nr 6

401--404

PL

Osprzęt pieców do nawęglania jest wykonywany ze stabilnego staliwa austenitycznego. W czasie jego użytkowania dochodzi do zniszczeń, które dyskwalifikują go z dalszej eksploatacji. Praca skupia się na zagadnieniu powstawania mikropęknięć w osprzęcie podczas jego chłodzenia. Literatura przedmiotu wskazuje, że kluczowe znaczenie dla powstawania pęknięć ma duża różnica rozszerzalności cieplnej składników strukturalnych staliwa. Dostępny w literaturze model wykazuje, że obecność węglików przy powierzchni stopu może powodować powstawanie silnych naprężeń rozciągających, mogących inicjować powstawanie pęknięć. Nie uwzględnia on jednak warstwy tlenków, która powstaje na powierzchni stopu. Nie wyjaśnia on również dlaczego pęknięcia powstają w praktyce jedynie na granicach ziaren, a nie w innych miejscach wydzieleń węglików. W artykule przedstawiono model obliczeniowy MES dla przypadku granicy ziaren znajdujących się przy powierzchni stopu pokrytego warstwą tlenków. Zaproponowany model posłużył do analizy numerycznej wpływu geometrii warstwy węglików (głębokość warstwy) oraz warstwy tlenków na rozkład naprężeń powstających w warstwie przypowierzchniowej podczas procesu chłodzenia. Otrzymane wyniki wykazują wyraźny związek pomiędzy warstwą węglików a naprężeniami pojawiającymi się przy powierzchni. Pokazują one również, że pomimo obecności warstwy tlenków przy powierzchni mogą powstawać naprężenia rozciągające. Niezbędne obliczenia wykonane na potrzeby pracy zostały zrealizowane za pomocą programu Midas NFX 2014.

EN

Carburizing furnaces tooling is made of stable austenitic cast steel. During the service life of the tooling damages occur, which disqualify it from further usage. This paper focuses on the issue of mini-cracks formation in the tooling, during the cooling process. The literature shows, that a great difference in thermal expansion between cast structural components has a pivotal meaning for a problem of the cracks creating. A model which is presented in the literature shows, that the presence of carbides near the alloy surface can cause strong tensile stresses, which may initiate cracks formation. However the model does not include an oxide layer which forms on the alloy surface. It also does not explain why the cracks arise in practice, only at grains boundaries and not in other places where carbides are located. In the article computational model for finite element analysis of grains boundary located near the alloy surface which is covered with oxide layer was presented. The proposed model was used for the numerical analysis of the influence of the carbide layer (the depth of the layer) and oxide layer geometry on the stress distribution in the surface layer, during the cooling process. The results show a clear relationship between the carbide layer and the stresses occurring at the surface. They also show that, despite the presence of the oxide layer, tensile stresses may occur at the alloy surface. The necessary calculations were made in the Midas NFX 2014 software.

7

The Effect of Pallet Component Geometry on Temperature Gradient During Cooling

Bajwoluk A., Gutowski P.

Archives of Foundry Engineering

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2015

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Vol. 15, iss. 1

5--8

EN

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.

8

Analysis of Thermal Stresses in Components of Pallets Operating in Furnaces for the Carburising Treatment

Bajwoluk A., Gutowski P.

Archives of Foundry Engineering

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2014

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Vol. 14, iss. 1 spec.

175--180

EN

The study is devoted to an analysis of thermal stresses arising in the basic elements of pallets used in furnaces for the carburising treatment. The construction of a pallet was presented and an example of its operating cycle was described. Using finite element method, a simulation analysis of the kinetics of cooling and heating of selected pallet components was performed. The results obtained were verified experimentally and used as a basis for the analysis of thermal stresses generated in these components during their cooling.