The application of a pack cementation of Y-doped chromium and silicon coatings on low alloy steel type-T22 yields a significant improvement in the oxidation



resistance. Steel-T22 was coated with Ydopedchromizing-siliconizing. Diffusion coating was carried out at 1050 oC for 6 h under an Ar atmosphere by simultaneous Y-dopded chromizing-siliconizing process. Cyclic oxidation tests were conducted on the coated steel-T22 alloy in the temperature range 300-500 oC in H2O for 100 h at 5 h cycle. . The oxidation kinetics for coated system in H2O was found to be parabolic .Oxide phases that formed on coated system are SiO2 and Cr2O3. Finite element results show that there is a good agreement between FEM and experiments results

Abbas Kh. Husssein

Kadhim K. Resin

Sabah Kh. Hussein

Al-Nahrain Journal for Engineering Sciences

Directory of Open Access Journals

The 1stRegional Conference of Eng. Sci. NUCEJ Spatial ISSUE vol.11, No.3, 2008 pp 450-458  NUCEJ vol.11, No.3,2008                                Using MATLAB of Oxidation                                  450  Finite Element Analysis Using MATLAB of Oxidation Properties in H2O Vapor of Steel-T22 Alloy Coated by Simultaneous Y-Doped Chromizing-Siliconizing Process Dr.Sabah Kh.Hussein   Dr.Abbas Kh.Husssein   Dr.Kadhim K.Resin  Abstract The application of a pack cementation of Y-doped chromium and silicon coatings on low alloy steel type-T22 yields a significant improvement in the oxidation resistance. Steel-T22 was coated with Y-doped chromizing-siliconizing. Diffusion coating was carried out at 1050 oC for 6 h under an Ar atmosphere by simultaneous Y-dopded chromizing-siliconizing process. Cyclic oxidation tests were conducted on the coated steel-T22 alloy in the temperature range  300-500 oC in H2O for 100 h at 5 h cycle. . The oxidation kinetics for coated system in H2O was found to be parabolic .Oxide phases that formed on coated system are SiO2 and Cr2O3. Finite element results show that there is a good agreement between FEM and experiments results.    Keywords:SteelT22;Oxidation;Chromizing-Siliconizing;Pack cementation  1.Introduction        Low alloy steels are generally considered to comprise plain carbon steels and steels with a total alloying content of up to 12%. As such, they are much cheaper than more highly alloyed alloyed materials and are often used in large quantities in heavy engineering industries. Whilst these materials are not generally selected for resistance to high temperature corrosion (the material choice is largely dictated by cost, ease of fabrication and mechanical   properties) they are often required to operate in high temperature aggressive environments. For instance, the power generation, refuse incineration and chemical process industries use many miles of low alloy steel heat exchanger tubes. Hence, the high temperature oxidation properties of low alloy steels are often important in determining component life [1].  The development of a surface oxide scale limits the degradation of a pure metal or alloy in a hot oxidizing environment. The addition of reactive elements which have a high affinity for oxygen (such as Y, Ce, Hf) may further improve the oxidation resistance through various effects [2]:   Promotion of the selective oxidation of an element which forms a stable oxide of low diffusivity (such as Cr2O3).  Reduction of the growth rate of oxide scale. Inhibition of scale failure (i.e. through thickness cracking and scale/substrate interfacial decohesion) [2].      Reactive-Element (RE) additions may be provide either as metallic or oxide dispersoide components in bulk alloy, or as surface produced by coating. They are used predominantly with Cr2O3- and Al2O3-forming alloys to resist aggressive environment [3]. V.Provenzano and coworkers [4] proposed a model of mechanical keying due to the formation of oxide pegs rich in active elements, the role of these peg being to anchor the oxide scale to the coating alloy. It was found that the addition of yttrium  prevents the sulfur segregation to the alloy/scale interface, either by reacting with sulfur to form a stable sulfides or by tying up  the sulfur by segregation to internal oxides surfaces [5]. Thus, the addition of small amount of reactive element [ Y,Ce, La, Hf, Zr, Th] to an alloy resulted in substational improvements in the adherence of their oxide scales during thermal cycling [6].  Previous work [7,8,9,10,11], has shown that such reactive element additions are effective in improving the high temperature corrosion of iron-base alloys by improving the resistance of protective scales to spallation.It was found that the scale formed on yttrium-free alloy is typically convoluted or wrinkled and poorly adherent, the yttrium-containing alloy produces a flat and adherent oxide. In  NUCEJ vol.11, No.3, 2008                                                                            Hussein, et.al               451  recent years yttrium has become the most commonly used of these reactive elements. The amount of the reactive element needed to produce the beneficial effect is small, (typically 1wt.% or less). Heat-resisting alloys depend on the formation of a protective oxide on the metal surface to limit section loss by oxidation. Generally, this protective oxide is chromia (Cr2O3), or silica (SiO2). In practice, the most common way that a protective oxide fails is by exfoliation or spalling from the metal surface. This spallation may be induced by stresses arising from the oxide growth process itself, strains resulting mechanical flexing of the component in the service, or from stresses arising from thermal cycling because of the difference in the coefficients of thermal expansion of the oxide and the metal. Spallation of oxide may involve fracture in the oxide adjacent to the metal surface, fracture in the metal immediately below the interface, or by separation at the interface itself. In the last case, failure involves not only the magnitude of the stresses, but also a consideration of the interfacial adhesion.          In this study, the applicability to deposit yttrium-doped chromium and silicon on the surface of steel-T22 alloy using single step pack cementation to enhance the oxidation resistance in steel-T22 was examined. The cyclic oxidation behavior of un-coated  and coated steel-T22 alloy will be studied in the temperature range 300-500oC in H2O  2. Experimental procedure  The substrate alloy used in this study was low alloy steel (Type T22-ASTM). The nominal composition and the spectrochemical  analysis of low alloy steel (Type T22-ASTM) arte shown in Table(1) and Table(2) respectively. The low alloy steel samples were cut into squares shapes with dimensions (20mm× 20×mm×5mm) with small hole of 2mm diameter was drilled in each sample for holding. All surfaces , including the edges were wet ground using 120, 220, 320, 600, 800, and 1200 grit silicon carbide papers. These samples were then cleaned with water, degreased with acetone, and then ultrasonically cleaned for 30 minutes using ethanol as a medium. After drying, the samples were stored in polyethylene zip-lock bags. The dimensions of all samples were measured. The pack mixture used for chromium-silicon diffusion coating consisting of 16 Wt.%Cr powder (50-60 m in particule size) as an chromium source, 6 Wt.%Si powder (50-60 m in particule size) as a silicon source, 2Wt.% NaF and 2Wt.%NaCl as activator and the balance was silica-powder (70-120 m in particule size). All pack powders was sized by sieving method and 1Wt.% of the pack silica filler was replaced by yttrium reactive element.. Low alloy steel (type T22-ASTM) was placed in a sealed stainless steel cylindrical retort of 50mm in a diameter and of 80mm in a height in contact with the pack mixture. The retort was then put in another stainless steel cylindrical retort of 80mm in a diameter and 140mm in a height. The outer retort has a side tube through which argon gas passes and second in the top cover for argon gas outlet. Type-k calibrated thermocouple was inserted through the cover of the outer retort for recording real temperature near inner retort.Pack cementation process was carried out at 1050 oC for 6 h under an Ar atmosphere. After coating, the samples were ultrasonically cleaned , and weighed. It was found that the diffusion coating time of 6 h at 1050 oC give a coating thickness of 79-82 m.  Alloy with   yttrium -doped chromium-silicon diffusion sample were accurately weighed and then placed into ceramic coating crucible. Cyclic oxidation tests were carried out in the temperature range 300-500oC in H2O. Each heating cycle includes heating in the furnace for 5 hours and cooling heating in still air.   3. Results and Discussion 3.1 cyclic oxidation of coated system in H2O Acomputer program (Matlab Language) is used to calculate the best fit to the equations ( W/A=Ktn and W/A=Kt1/2 ). A plot of the specific weight change vs. time and square root time, for coated systems (Y-doped chromizing-siliconizing ) diffusion coatings coated low alloy steel type-T22 cyclic oxidized in H2O at temperatures 300,400 and500oC  conditions for 100 hr at 5 hr cycle, is shown in Figures (1), this figure indicates that the minor elements (Y) additions, seems to play an important role on scale adherence of  low alloy steel type-T22 .            The improved oxidation resistance of  (Y)-doped chromizing-siliconizing diffusion coatings, also attributed to promotion of the selective oxidation of an elements (Cr,Si) which forms a stable oxide diffusivity, (Cr2O3  and SiO2), on substrate as well as reduction of the growth rate for (Cr2O3  and SiO2).   Table (3) show that the KP values obtained at temperature range (300-500oC) for all coated systems were higher than that for oxidation under air and this result is in good agreement with reference [12]. It was observed that the reaction rates under water vapor cyclic  NUCEJ vol.11, No.3,2008                                Using MATLAB of Oxidation                                  452  oxidation are induced because of hydrogen to permeate from water vapor environment. A considerable amount of (Cr2O3  and SiO2) is also being evident as an oxidation product of all systems coated low alloy steel type-T22 cyclic oxidized in H2O at temperature 500oC, for 100 hr at 5 hr cycle Figures (2 a-f) ,show the scale formation on coated low alloy steel type-T22 cyclic oxidized in H2O at temperatures 300,400 and 500oC.  It is apparently, that the three coated systems  ( Y-doped chromizing-siliconizing ) diffusion coatings coated low alloy steel type-T22 cyclic oxidized in H2O at temperatures 300,400 and500oC, do not show severe scale spalling, and appeared uniform and smooth after thermal cycling as shown in above figures. A detailed examination of the alloy/scale interface on the coated system reveals that the scale remain in contact with the metal during oxidation over almost the whole surface.  3.2 Finite Element Analysis   3.2.1 Method of weighted residual  Methods of weighted residual is useful to obtain aproxmate solution to different gravity equation . in order to explain the methods , we consider the following sample  problem :    at first step , we assumed a trial function as : which satisfy boundary condition.The residual (R) become : xxaxaxudxudR )1(2~~22   We next step is to determine the unknown constant (a) such that the chosen test function best approximates the exact solution . a weighted function (w) is selected and the weighted average of the residual over the problem domain is set to zero . That is :  1022100)~~( dxxudxudwwRdxI                              0))1(2(10dxxaxaxawI                          In this work ; the weak formulation is used to solve the differential equation :   Where  x:Specific weight change (cm2/sec). t :Oxidation time (hr). k: oxidation rate constants (mg/cm4)/hr. There are two method to describe the model of solving the weak formula in this method as : First method :                   B.C The weak formulation of the equation is :   The second weak formulation [13]    3.2.1 Galerkins finite element formulation  This section shows how to compute weighted residual in a symmetric manmer using finite element and piece wise continuous functions. Consider a sub domain or finite element shown in Figure(3) the element has two nodes , one at each end. The corresponding coordinate value (Xi or Xi+1 ) and the nodal variable (Ui or Ui+1) are assignment .  The proposed unknown trial function to be (to solving eq.(1)).   We want to express eq.(7) interms of nodal variables .In other words , (C1 and C2) need to be replaced by Ui and Ui+1 . To this end , we evalute (u) at X=Xi and X=Xi+1            then :  Solve eq(8)  xudxud220<x<1      u(0)=0  ; u(1)=1)1(~ xaxuxkdtdxxkdtdxL Ldxwdxdtdxwkx0 01)(Ldxwdxdxdtwxk01)(21 cxcuiii ucxcxu 21)(12111)( iii ucxcxuiiiiXXUUC111iiiiiiXXXUXUC1112 NUCEJ vol.11, No.3, 2008                                                                            Hussein, et.al               453            and eq(9) for          obtain (C1)                and   (C2) :  Sub eq(10 and 11) into eq.(6) :   Where:   : element length H1, H2 : linear shape functions (Figure 4) Or in another form [14].  Figure (5) illustrates the comparison between theory, finite element, and experimental results, it is clear that there is a good agreement between theory, finite element, and experimental results. 4. Conclusions         From the cyclic oxidation tests of uncoated and coated low alloy steel type-T22 in water-vapour temperatures range between 300-500 oC for 100 hr at 5 cycle the following results can be concluded: 1. Coated system (Y-doped chromizing-siliconizing diffusion coating) revealed good cyclic oxidation resistance for low alloy steel type-T22,and oxidation kinetics was following the parabolic oxidation rates. 2.Phases present on the surface of all coated systems subjected cyclic oxidation and hot corrosion are SiO2 and Cr2O3.  3.There is  a good agreement between theory, finite element, and experimental results.   5. References 1.L.LShier, Corrosion , Vol.1, Metal/Environment Reactions, Planta Tree, 2000, p.16-30. 2. P.Moon,  Role of Reactive Elements in Alloy Proptection , Materials Science and Technology, Vol.5, August 1989,pp.754-762. 3. D.P.Moon,  The Reactive Element Effect on the Growth Rate of Nickel Oxides Scales at High Temperature , Oxidation of Metals, Vol.32, 1989,pp.47-66. 4. V.Provenzano et. al,  Void Formation and Suppressing during High Temperature Oxidation of MCrAl-Type Coatings  , Surface and Coatings Technology,A356(2003), pp.136-144. 5. Ahmed A.Moosa,Jalil K.Ahmed and Ali H.Hoobi , Corrosion Properties of Inconel 600 Coated by Simultaneous Aluminizing-Chromizing Process , Materials Science Forum Vols.546-549(2007) , pp. 1763-1768. 6. Ahmed A.Moosa,Jalil K.Ahmed and Ali H.Hoobi ,  Oxidation Properties in CO2 of Inconel Alloy 600 Coated by Simultaneous Aluminizing-Chromizing Process , Chinese Journal of Aeronautics 20(2007) , pp.134-139. 7. V.K.Tolpygo and H.J.Grabk,   The Effect of Impurities on the Alumina Scale Growth: An Alternative View , Acta Metallurgica, Vol.38, No.1, September, 1998, pp.123-129. 8.John Stringer ,  The Reactive Element Effect in High Temperature Corrosion ,Materials Science and Engineering, A120,May, 1989,pp.129-137 9..B.A.Pint , P.F.Tortorelli and I.G.Weight,  Long-Trem Oxidation Performance of Ingot-Produced Fe3Al Alloys , Materials at High Temperatures , 16(1), May,1999, pp.1-13. 10.F.H.Stott,   Method of Improving Adherence , Materials Science and Technology, Vol.4, May, 1988,pp.431-438. 11.A.Paual, R.Sanchz, O.M.Montes and J.A.Odriozola,  The Role of Silicon in the Reactive Elements on the Oxidation of Conventional Austenitic Stainless Steel , Oxidation of Metals, Vol.67, Nos.1/2,February, 2007, pp.87-105. 12. Parm Valentin Rohr ,  Developpment De Revetemnts Pour Les Aciers D eechangeurs Thermiques et Ameliorhangeurs Thermiques et Amelioration de Leur Resistance Ala Corrosion En Environnment Simalant Les Fumees De Combustion de Charbon ,Soutenue Le, 21 Octobere 2005. 13. Darly L.Llogana  A first course in the finite element method .Wadsworth Group. 2002. 14. James Doyle Finite Element Methods John Wiley & Sons, Ltd  2004.  121 )()( ii UxHUxHUiihXXxH 11 )(iihXXxH )(2iiiXXh1 NUCEJ vol.11, No.3,2008                                Using MATLAB of Oxidation                                  454                          Fig.(3) Two nodes linear element Table(2) Spectrochemical analysis of low alloy steel (Type T22-ASTM)Element Fe C Mn P Sm Si Cr Mo V Ti Wt.% Rem. 0.06 0.35 0.012 0.011 0.46 1.96 0.91 0.002 0.01 Table(1) Nominal composition of low alloy steel (Type T22-ASTM)Element Fe C Mn Pmax Smax Si Cr Mo Wt.% Rem. 0.05-0.15 0.3-0.6 0.025 0.025 0.5max 1.9-2.6 0.87-1.13 Table (3) n values and parabolic oxidation rate constants KP for cyclic oxidation of coated system in H2O for 100 hr at 5 hr cycle. Temperature oC Y-doped Cr-Si n KP (mg/cm4)/hr300 0.4705 7.29×10-6400 0.5064 5.336×10-4500 0.6038 5.490×10-3 Xi Xi NUCEJ vol.11, No.3, 2008                                                                            Hussein, et.al               455    Fig.(4) Linear shape functions   Figure (2a-f)  LOM of low alloy steel type-T22 coated with Y-doped chromizing-siliconizing after cyclic oxidation under H2O at temperature range 300-500oC. (top & cross section views).  H (x) H (x)XXi NUCEJ vol.11, No.3,2008                                Using MATLAB of Oxidation                                  456     Figure (1a) Parabolic fitted results of specific weight change vs. time plot for Y-doped chromizing-siliconzing diffusion-coated low alloy steel type-T22 cyclic oxidized in H2O at temperatures 300,400 and 500oC .  Figure (1b) Linear fitted results of specific weight change vs. t1/2 plot for Y-doped chromizing-siliconzing diffusion-coated low alloy steel type-T22 cyclic oxidized in H2O at temperatures 300,400 and 500oC         NUCEJ vol.11, No.3, 2008                                                                            Hussein, et.al               457    Figure (5a) comparison between theory, finite element, and experimental results at 300oC.  Figure (5b) comparison between theory, finite element, and experimental results at 400oC.     10 20 30 40 50 60 70 80 90 1000.0080.010.0120.0140.0160.0180.020.0220.0240.026Time(hr)Specific Weight Change (mg/cm2 )KINETICS OF OXIDE GROWTH (POWER) OF H2O FOR COATING SYSTEM at 300oC [Y-DOPED-Cr-Si]  Theory ResultsFinite Element ResultsExperimental Results10 20 30 40 50 60 70 80 90 1000.060.080.10.120.140.160.180.20.22Time(hr)Specific Weight Change (mg/cm2 )KINETICS OF OXIDE GROWTH (POWER) OF H2O FOR COATING SYSTEM at 400oC [Y-DOPED-Cr-Si]  Theory ResultsFinite Element ResultsExperimental Results NUCEJ vol.11, No.3,2008                                Using MATLAB of Oxidation                                  458    Figure (5c) comparison between theory, finite element, and experimental results at 500oC. T22T22T221050oC300-500SiO2Cr2O310 20 30 40 50 60 70 80 90 1000.150.20.250.30.350.40.450.50.550.60.65Time(hr)Specific Weight Change (mg/cm2 )KINETICS OF OXIDE GROWTH (POWER) OF H2O FOR COATING SYSTEM at 500oC [Y-DOPED-Cr-Si]  Theory ResultsFinite Element ResultsExperimental ResultsThis document was created with Win2PDF available at http://www.daneprairie.com.The unregistered version of Win2PDF is for evaluation or non-commercial use only.

Finite Element Analysis Using MATLAB of Oxidation Properties in H2O Vapor of Steel-T22 Alloy Coated by Simultaneous Y-Doped Chromizing-Siliconizing Process

https://nahje.com/index.php/main/article/download/541/413

Finite Element Analysis Using MATLAB of Oxidation Properties in H2O Vapor of Steel-T22 Alloy Coated by Simultaneous Y-Doped Chromizing-Siliconizing Process

Abstract

Similar works

Full text

Available Versions

Directory of Open Access Journals