High Temperature Oxidation Behavior of High Speed Steel for Roll in Water Vapor

TRANSACTIONS OF MATERIALS AND HEAT TREATMENTVol.25 No.5PROCEEDINGS OF THE 14TH IFHTSE cCONGRESSOctober 2004High Temperature Oxidation Behavior of High Speed Steelfor Roll in Water VaporZHOU Li', LIU Fang', LiU Changsheng', SUN Da-le2; Cllege of Material & Mtallurgy, Norheastern Universis, Shenyang 100,China2. BAOSHAN lron & Seel co, LiD. Technology Center Shanghai 201900, ChinaAbstracti. The oxidation behavior of high speed seel (HSS) was rsearched by high temperature thermo balace at 500to 800C in water vapor. The morphology was observed byningelectronmicroscope, the microstructure of oxidescale was analyzed by eenergy diprive spectrometer and x ray dffaction specrum. The results indicate that the massgain of HSS iceases with oxidain tmpraure rsig,. the efe of aidain temperaure on the mopholoy isobvious, water vapor temperature only afete mas gain and afets hardly morphology> scale at the sameoxidation temperature. The relevant oxidation mechanisms ar;o discussed.Key words: high seed seel, high temperature oxidation, iothermal oxidation, mass gain, carbideHSS has been sussully used for hot rolls in order to .paper, cleaned in acetone by ultrasonic for 10 minutes,make rolled plates having homogeneous thickness andand then kept sealed in an inert (Ar) atmosphere beforeuniform surface during hot rlling, thereby leading tothe test. The kinetics of the degradation process due toenhance surface quality of rolled plates and extend rolloxidation was followed after putting the sample in highlife because of its excellent hardness, wear resistance,temperature thermo balance, the experiments wereand high temperature properties [1]. The use of HSScarried out at 500C、575C、650C、725"C、800C forrolls has resulted in an improvement of the strip surfaceup to 2 hours, the water vapor temperature was at 40quality up to 20%. The working temperature of hotC、60C、80C respectively, the air fux density withrolls often exceeds 500"C, the thermal cyclic workingwater vapor was 100ml/min.of rolls in hot metal forming causes a superficial oxidescale, which plays a major role because a hard oxidescale may give abrasive behavior whereas a ductile- Armoxide scale may decrease the friction coefficient as wellas the wear, the oxidation resistance must be high一-outpuryursasenough to avoid the deterioration of the workingsurface, the oxidation behavior of HSS materials hasX的Xththerefore a great importance for the technologicalproperties of hot rolls [2-5].quthe/The purpose of this paper is to investigate hightemperature oxidation behavior of HSS used as hotrolls at different temperatures in water vapor and findoxidation rule in order to control the oxide scale atFig.1 Schematic diagram of high temperaturebest conditions, which is significant to improve stripthermo balancesurface quality and extend roll life.2. Results and Discussions1. Experimental proceduresThe microstructure of HSS comprises a martensiticThe oxidation behavior of HSS was researched bymatrix and a noticeable volume fraction of carbides, thehigh temperature thermo balance (Sartorius 4410) atcarbides are mainly the MgC7-type V carbides, and a500 to 800°C in water vapor. The schematic diagram ofpart M6C-type Mo carbides ( Fig.2).the apparatus is ilustrated in Fig.1. The morphologyThe mass gain responses of HSS samples arewas observed bscanning electron microscopeillustrated in the oxidation time curves shown in Fig. 3.(SSX-550)，the microstructure of oxide scale wasThis figure indicates that the mass gain increases withanalyzed by energy dispersive spectrometer (EDS) andtemperature rising as expected. The oxidation rate ofX-ray diffraction spectrum (D/max-rb) using Cu KaHSS is very slow at 500"C (Fig3(a))， as well as atradiation.575"C(Fig.3(b)), because the oxidation temperature isThe oxidation sample size was 20mm X 10mmXvery low for the HSS containing a lot of oxidation1.5mm, mtalurgrapbically polished by 800 grit siC中国煤化工，Mo, Cr, and theYHCNMHGVol.25 No.sTRANSACTIONS OF MATERIALS AND HEAT TREATMENT135oxide scale formed on the surface prevents theoxidation process. The oxidation rate of HSS samplesincreases gradually at 650Cand 725"C(Fig.3(c-d)). At(a800 C (Fig3(e))， the oxidation rate increases:3dramatically because of the oxidation temperature is:too high. Each mass gain curve almost follows anoxidation kinetics with a parabolic trend, hightemperature parabolic oxidation indicates that thermaldiffusion of ions is controlling the oxidation rate, sucha process may be therefore connected with a uniformdiffusion of the reactants through a growing oxideTimeminutescale. At 500"C, the effect of water vapor on the massgain isn't obvious because the oxidation temperature isso low that the oxidation reaction is slow. At 575C、g 0x0-650C and 725"C, the effect of water vapor on themass gain is obvious, at each oxidation temperature,mass gain increases with water vapor temperaturerising. But at 800C, the trend is opposite, mass gainincreases with water vapor temperature dropping,which is because the oxide scale in higher temperatureTmeiminutewater vapor has better protection than that in lowerone.00Tieieioure: hadoimocTieelmieuteFig.2 Microstructure and XRD pattermThe Morphologies of isothermal oxidation samplesat different oxidation temperatures in 40C watervapor are shown in Fig.4, it is well evident that theMorphologies are obviously different at differentoxidation temperatures. At 500C, the oxide scaleFig.3 Mass gain curves of samplesgrows mainly on the metal matrix (area A in Fig.4(a)),(a) at 500"C; (b) at 575"C; (C) at 650"C;and the MgC-type V carbides (area B in Fig.4(a)),which are analyzed by XRD and EDS, aren't oxidized,中国煤化工800CMHCNMHG136PROCEEDINGS OF THE 14TH IFHTSE CONGRESSOctober 2004the oxide scale is even and compact.The reason of the selective oxidation is that theHSS contains a high amount of oxidation resistanceelements such as V, W, Mo, Cr, which are strongcarbide formers, so they are mainly inside the carbides,which leads to the difference of oxidation resistancebetween matrix and carbides. With the oxidationtemperature rising, the carbides are also oxidized altte, the oxide scale is still even and compact on theBmatrix (Fig.4(b). At 650"C, the selective oxidationcan't be seen, the oxide scale is uneven, the oxidationproduct are rich Fe by EDS, there are a lot of strumaeson the sample surface (Fig. 4()), which indicates thatthe devastating oxidation happens at some locationsbecause of some defects under the oxide scale. At 725"C, the oxide scale is even than that at 650C, this isbecause the oxidation temperature is so high that theoxide scale can't protect the HSS from oxidation verywell, the oxidation rate on the whole sample surface isvery high and almost the same, so the oxidationsurface is even (Fig.4()). At 800C, the oxide scale isuneven, there are a lot of strumaes on the samplesurface (Fig.4(e)), but which is different from those inFig.4(c), there are a lot of cracks on the strumaes, thisis because the growth stress and thermal stress makethe oxide scale crack, which makes the fresh metalexpose to the high temperature water vapor, in theselocations, the oxidation rate is higher than others.Fig.5 shows the XRD patterms of samples, it can beseen that at 500°C, 575°C and 650°C, Fe( Ferrite)、Fe2O3 (hematite) and Fe3O4 (magnetite) are revealed, at725°C and 800°C , Fe disappears, Fe2O3 and FegO4 arethe main phases. Under the experimental conditions,the depth analyzed by XRD amounts to only a fewmicrometers, so the results are only representative ofthe constutions of the outer part of the oxide scale.4000.=Fe,O,0010 RFe0123465∞0708201P1Fig.5 XRD patterns of oxidation samplesFig.4 Morphologies of samplesThe morphologies of oxidation samples at 575C inin 40°C water vapordifferent temperature water vapor are shown in Fig.6.(a) at 500C; (b) at 575C; (c) at 650C;中国煤化; almost the same, the(d) at 725"C; (e) at 800°CIlation product evenly,iYHCNMHGVol.25_ No.5TRANSACTIONS OF MATERIALS AND HEAT TREATMENT137and the carbides are hardly oxidized or justly begin tooxidation disappears, the oxide scale covers the wholebe oxidized a lttle in some locations. This indicatessurface，but at some locations， the devastatingthough the mass gain increases with the water vaporoxidation happens because of some defects, there are atemperature rising (Fig.3(b)), the effect of water vaporlot of strumaes on the surface (Fig 4()). The surfacetemperature on the morphology is negligible (Fig.6becomes even because the temperature is so high that(a-C)) compared with the oxidation temperature (Fig.4the oxidation rate of each place rises dramatically,(a-e)).which leads to the even growth of oxide scale(Fig.4()). At certain temperature because the growthstress and thermal stress make the oxide scale crack,which makes the fresh metal expose to the highatemperature water vapor, on these locations, thoxidation rate is faster than others, there are a lot ofstrumaes on the sample surface (Fig.4 (e))fe,O、0S75C n6 t water Wepe0★Fe,05t inot银vapr100500-510152025303455050507500520"1Fig.7 XRD patterms of oxidation samplesat 575C in different temperature vaporFig.8 is the mechanism diagram of selectiveoxidation. Oxidation nucleates at the carbide- -matrixinterface (Fig.8(a-b)), due to the high free energycontent here. Because of the high thermal stability andthe low oxidation rate of carbides, at the beginning theoxidation product grows mainly on the metal matrix,giving rise to an uneven oxide scale (Fig.8()). Bygrowing, the oxide scale tends to cover the wholesurface (Fig.8 ()).The main oxidation reactions of HSS in highFig.6 Morphologies of samples at 575"Ctemperature water vapor are as follows:(a) in 40"C water vapor; (b) in 60"C water vapor;4Fe+3O2= =2Fe2O3(I(c) in 80C water vapor3Fe+4H2O=FegO4+4H2(2)Fig.7 shows XRD patterns of oxidation samples atIn fact, the amount of H2O taking part in the575 C in different temperature vapor. It is verychemical reaction is very ltte (Fig.1), it is difficult toobvious that with the water vapor temperature rising,simulate the roll working conditions, so the equation 1the amount of Fe3O4 increases, which manifests that theis the dominant, this is the reason that the Fe2O3 is themain oxidation product by XRD (Fig.5), but the FezO4amount of H2O affects the proportion of oxide scale.From above experiment results, the high temperatureis only a lttle, the amount of FezO4 increases with theoxidation behavior of HSS for roll can be analyzed. Atwater vapor temperature rising (Fig.7), it can belower temperature, selective oxidation happens becauseforecasted that the FezO4 will become the main phaseoxidation resistance of alloy elements are different, theif the amount of water vapor is enough. The rolls workoxide scale grows mainly on the metal matrix, theunder cycle water cooling, the outer layer of the oxidecarbides are hardly oxidized or a lttle oxidized (Fig.4scale is FezO4, which just proves it.(a-b)). With the oxidation temperature rising, selective中国煤化工god coion of he_ethod to reduce theYHCNMHG138PROCEEDINGS OF THE 14 IFHTSE CONGRESSOctober 2004excessive growth of oxide scale， avoiding thetemperature on the morphology is negligible at eachexposition of the roll surface to high temperature, so asoxidation temperature.to maintain a high resistance to detachment and to(3) In 40C water vapor, selective oxidationavoid the consequent formation of peeling and banding.happens at 500C, the oxide scale grows mainly on theThe working temperature of HSS rolls should bemetal matrix, and the carbides are not oxidized; thecontrolled as possible as low, it is at least under 575C,carbides begin to be oxidized a lttle at 575 C; selectivehere the oxide scale is compact, even and adhesive,oxidation disappears above 575"C, and devastatingwhich can obviously improve the strip surface qualityoxidation appears at 800C.and extend roll life. So it is critical to optimize roll(4) At 500°C, 575°C and 650°C, Fe、Fe2O3 andcooling water under satisfying finishing rollingFe;O4 are revealed, at 725°C and 800°C, Fe disappears,temperature.Fe2O3 and FezO4 are the main phases by XRD. At575°C, the amount of the FezO4 increases. with theO2+water vaporwater vapor temperature rising.(5) The working temperature of HSS rolls should becontrolled as possible as low, so it is critical to optimize(aroll cooling water under satisfying finishing rollingMatrix-^CarbideAcknowledgementThe authors are grateful to BAOSHAN lron & Steel Co,TD. Technology Center for the support during the period of(6)this research.References1 Huang K C, Lee s, Lee H C. Effects of elements onmicrostructure and fracture properties of cast high speedsteel rolls, Materials Science and Engineering A, 1998, 254(1-2): 282-285.O:+water vapor2 Munther P A, Lenard J G The efect of scaling ont↑↑↑ Growhinterfacial friction in hot rlling of steels, Joumal of(d)Materials Processing Technology, 1999, 88(1): 105-113.IONFe Ro2; Vergne C, Boher C, Levaillant C, et al. Analysis of thefriction and wear behavior of hot work tool scale:application to the hot rlling process, Wear, 2001, 250():Fig.8 Mechanism diagram of selective oxidation32-333.Molinari A, Staffelini G Tomasi A, et al. Oxidationbehavior of ledeburitic steels for hot rolls, Materials3. ConclusionsScience and Engincering A, 2000, 280 (2): 255-262.(1) The oxidation rate of HSS is very slow at 500C5 Colas R, Ramyrez J, Sandoval 1, et al. Damage in hotand 575"C, increasing gradually at 650C and 725"C,rolling work rlls, Wear, 1999, 230(1): 56-60.and increasing drastically at 800 C. Mass gainincreases with water vapor temperature rising when theCorresponding author. Dr. Zhou Li,oxidation temperature is below 725C, but the massE-mail: zhouli .neu@sohu com,gain trend is opposite at 800C.Mail adres: College of Material & Metallugy, Northeastem(2) The effect of oxidation temperature on theUniversity, Shenyang 10004,China,morphology is obvious, but the effect of water vaporTel: +86-24-83681680中国煤化工MHCNMHG