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Influence of the base metal grain size and heat input on decarburization in single pass SAW and multi pass SMAW welded joints of dissimilar ferritic steels

Nimko M. O., Skulskyi V. Yu.

Journal of Achievements in Materials and Manufacturing Engineering

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2023

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Vol. 119, nr 1

14--26

EN

Purpose This paper aims to assess a separate influence of heat input and base metal grain size on microstructural evolution in the weld adjacent zone of bainitic steels with 1.5…2.0% Cr, welded or overlayed with consumables with 9% Cr after post-weld heat treatment. Design/methodology/approach Analysis of the width of decarburised layer on microphotographs of welded or overlayed specimens after tempering at 750°C. Specimens were made by using different welding approaches: single-pass welding, multi-pass welding and overlaying. Findings It is shown that with an increase of the heat input energy, the width of the resulting decarbonised layer decreases linearly; the increase of the base metal grain size leads to a decrease in the layer width after tempering at 750°C. The microhardness testing showed the average hardness in the decarburised layer of 15Kh2M2FBS steel was 161 HV0.1 (minimum – 154 HV0.1), while the average hardness in the rest of the heat-affected zone was 192 HV0.1. Research limitations/implications Future research may include comparing the creep rupture strength of the weldments made with different welding parameters or base metal grain size to assess the influence of these factors on creep rupture strength. Practical implications Results permit to achieve minimisation of the rate of carbon diffusion in the weld-adjacent area of the heat-affected zone by means of variation of welded parameters and base metal grain size. Originality/value An influence of high-diffusivity paths (grain boundaries) on carbon diffusion in the heat-affected zone of dissimilar weldments was confirmed experimentally; the correlation between base metal grain size/welding parameters and the rate of the diffusion during high-temperature exposure was found.

2

Influence of welding parameters on decarburization in heat affected zone of dissimilar weldments after post weld heat treatment

Nimko M.O.

Archives of Materials Science and Engineering

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2021

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Vol. 112, nr 1

23--31

EN

Purpose: This paper aims to assess an influence of thermal welding parameters on microstructural evolution in the weld adjacent zone of P91 steel, overlayed by austenitic consumables, after post weld heat treatment. Design/methodology/approach: Analysis of the width of decarburized layer on microphotographs of overlayed specimens after tempering 750°C, 7 and 18 hours. Specimens were made by using different heat input and preheating temperature parameters. Findings: It is shown that with increase of the heat input energy, the width of the resulting decarbonized layer decreases linearly; the effect of heating temperature on the layer width is parabolic with a minimum at a temperature of ~195°C. Research limitations/implications: Future research may include comparison of the creep rupture strength of the weldments, made with different welding parameters, to assess the influence of kinetics of decarburization and variation of the parameters on creep rupture strength. Practical implications: Results permit to achieve minimization of rate of carbon diffusion in the weld adjacent zone of the HAZ by means of variation of welded parameters. Originality/value: Experimentally was confirmed a role of high-diffusivity paths (grain boundaries) on carbon diffusion in the HAZ of dissimilar weldments; found correlation between welding parameters and the rate of the diffusion during high temperature exposure.

3

Modelowanie metodą automatów komórkowych cyklu przemian fazowych w stalach

Opara J., Jabłoński G., Rudzki D., Pietrzyk M

Hutnik, Wiadomości Hutnicze

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2012

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Vol. 79, nr 6

447--451

PL

Praca dotyczy zastosowania automatów komórkowych do modelowania przemian fazowych w stalach. Model obejmuje przemiany ferrytyczną, bainityczną i martenzytyczną. Model ma zastosowanie do większych szybkości chłodzenia stali o niższej zawartości węgla, czyli dla warunków, kiedy nie pojawia się przemiana perlityczna. W pracy przedstawiono przyjęty schemat automatu komórkowego oraz opisano reguły przejścia dla każdej z trzech przemian. Uwzględniono dyfuzję węgla oraz tworzenie się węglików w bainicie, kiedy mały współczynnik dyfuzji węgla w austenicie nie pozwala na jego transport na duże odległości. Przedstawiono przykładowe mikrostruktury składające się z ferrytu, bainitu i martenzytu.

EN

Cellular Automata model for phase transformations in steels is presented. The model covers ferritic, bainitic and martensitic transformations and is applicable to larger cooling rates and steels with lower carbon content. Therefore, pearlitic transformation is not considered. The scheme of the CA space is presented and transition rules for transformations are formulated. The model accounts for the diffusion of carbon and precipitation of carbides in bainite, when low diffusion coefficient prevent transport of carbon on larger distances. Examples of microstructures containing ferrite, bainite and martensite are presented.

4

Modelling of carbon diffusion and ferritic phase transformations in an unalloyed hypoeutectoid steel

Wolff M., Acht C., Bochm M., Meier S.

Archives of Mechanics

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2007

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Vol. 59, nr 4-5

435-466

EN

During the important industrial process of case hardening, phase transformations in a steel workpiece are essentially influenced by the (non-homogeneous) carbon distribution (near the surface). Moreover, the temperature course is of great importance. We develop a mathematical model of case hardening which takes diffusion of carbon in austenite, heat conduction and possible phase transformations into account. In this work, mechanical behaviour (thermo-elasticity, classical plasticity and transformation-induced plasticity) is not included in the model. As a result we obtain an initial-boundary-value problem for a coupled system of two parabolic partial and several ordinary differential equations. Finally, we present some numerical simulations.

5

Analysis of carbon diffusion during bainite transformation in ADI

Ławrynowicz Z., Dymski S.

Archives of Foundry Engineering

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2007

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Vol. 7, iss. 3

87-92

EN

The paper presents an investigation of the time required for the diffusion of carbon out of supersaturated sub-units of ferrite into the retained austenite. The analytical model estimates the decarburisation time of the sub-units of supersaturated bainitic ferrite. The purpose of the present paper is to demonstrate how a thermodynamic method can be used for solving a problem of the decarburisation of bainite subunits and carbon diffusion distances in the matrix of ADI. This should in principle enable to examine the partitioning of carbon from supersaturated ferrite plates into adjacent austenite and the carbon content in retained austenite using analytical method. The diffusion coefficient of carbon in austenite is very sensitive to the carbon concentration and this has to be taken into account in treating the large concentration gradients that develop in the austenite. The results are discussed in the context of displacive mechanism of bainite transformation. Experimental measurements of volume fraction of bainitic ferrite and volume of the untransformed austenite indicate that there is a necessity of carbides precipitation from austenite. The necessary carbon diffusion distance in austenite also illustrates that the estimated time is not capable of decarburising the ferrite subunits during the period of austempering. A consequence of the precipitation of cementite from austenite during austempering is that the growth of bainitic ferrite can continue to larger extent and that the resulting microstructure is not an ausferrite but is a mixture of bainitic ferrite, retained austenite and carbides.

6

Praktyczne efekty zastosowania komputerowego systemu sterowania procesami nawęglania z obliczaniem przebiegu dyfuzji węgla w trybie on-line

Maliński P., Nowacki Z.

Przegląd Mechaniczny

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2000

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nr 13

23-27

EN

Practical application og programming heat treatment processes based on the use of a mathematical model for calculation of the run of carbon diffusion in on-line mode. Discusion of the main effects of application of the computer control system on the bais of its 12-month operation.

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Przemiana bainityczna : ocena doboru współczynnika dyfuzji węgla w austenicie w oparciu o zmierzoną grubość warstw austenitu szczątkowego

Ławrynowicz Z.

Zeszyty Naukowe. Mechanika / Akademia Techniczno-Rolnicza w Bydgoszczy

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1998

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Z. 43 (216)

289-297

PL

Metoda szacowania współczynnika dyfuzji węgla w austenicie, wykorzystana w artykule, oparta jest na hipotezie, że przemiana bainityczna zachodzi mechanizmem ścinania. W artykule porównano grubość warstw austenitu szczątkowego w strukturze bainitycznej zmierzoną na cienkich foliach z grubością wyliczoną w oparciu o termodynamikę przemiany bainitycznej. Do obliczeń profilu stężenia węgla w austenicie przyjęto trzy różne, najczęściej stosowane w obliczeniach wartości współczynnika dyfuzji węgla w austenicie. Największą zgodność między grubością austenitu zmierzoną na cienkich foliach oraz obliczoną analitycznie uzyskano, gdy do obliczeń wykorzystywano średnią ważoną wartość współczynnika dyfuzji węgla w austenicie.

EN

The method used to calculate the thicknee of the austenite films is based on the hypothesis that transformation to bainite is controlled by the T'[o] condition. The thickness of fine austenite films was measured on thin foils and additionally it was estimated ther modynamically in bainitic microstructures. The carbon concentration profiles in austenite were evaluated as a function of particular diffusivity of carbon i the austenite. Measurements of the thickness of austenite films were coupled with a theromdynamic model of bainitic transformation for estimation of proper value of carbon diffusivity. The best results were found when weighted average diffuivity D was substituted into theoretical calculations.