A Controlled New Process of Pack Aluminization

TRANSACTIONS OF MATERIALS AND HEAT TREATMENTVol.25 No.5PROCEEDINGS OF THE 14 IFHTSE CONGRESSOctober 2004A Controlled New Process of Pack AluminizationHUANG Zhi-rong, MA Liu-bao, LI Pei-ning, XU HongSchool of Mechanical Engineering, East China University of Science and Technology, Shanghai, ChinaAbstract: Aluminum diffusion coatings are often prepared by a pack aluminization technique, which is a specific variety ofchemical vapor deposition (CVD) method. The coating process takes place in a bed containing a mixed powder that servesas a source of the coatings forming element. The phase composition of the diffusion layer obtained depends on the activityof the Al during the pack aluminization processing. In this work, the proportion of Al to special additive powder in the packand the treatment temperature are adjusted to achieve the desired surface compostion of aluminized layer. The aluminized20 plain carbon steel and HK40 austenitic steel were investigated by optical microscopy (OM) , X-ray dffaction (XRD)and microsclerometer. The results showed that the desired FeAl, FezAl and NiAl were respectively formed on the 20 plaincarbon steel and HK40 austenitic steel, and the aluminides FeAl3, FezAls or NizAl3, NiAls could be inhibited.Key words: Pack aluminization, Contolled aluminization, Aluminized steelsIRON ALUMINIDES exhibit excellent corrosionphase inhomogeneities may occur, and therefore, theresistance in aggressive atmospheres at elevatedeffective activity of the bed may differ from whattemperaturel, which enables steels or alloys to beoriginally intended. As a result, a mixture of favorableprotected by Fe-Al intermetallic coatings. However, itand unfavorable phase may appear on the surface of theshould be borme in mind that aluminides with a highcoating. In the present work, the pack, with the desiredaluminum content- FeAlg, FezAls- -tend to embrittleAl activity, was prepared from a mixture of Al and athe coated layer and reduce its oxidation resistance. Byspecial additive. A controlled new process of packcontract, iron- rich iron aluminides possess excellentaluminization is developed for the purpose ofmechanical properties and oxidation resistancel5-olcontrolling the phases formed on the surface andAluminum diffusion coatings are often prepared by aobtaining the desirable aluminide phase.pack aluminization technique, which is a specificvariant of the chemical vapor deposition (CVD)1. Experimental Proceduresmethod. In the process, the coating is prepared by packThe alloys in the experiments are a commercial typecementation process involving heating the parts in20 plain carbon steel and a centrifugal casting HK40contact with a powder mixture of aluminum or anaustenitic steel furmace tube, the compositions are asaluminum alloy, a halide activator, usually a chloride orshowed in Table 1.fluoride salt, and an inert diluent such as alumina. TheAll specimens were in the form of rectangular sheetsassembly is then heated to promote diffusion, forof dimensions 10mmx10mmx3mm. The specimensexample at 800-1000C for 2-24 hours. The aluminumwere grounded with silicon carbide waterproof papercoating on the surface of the component penetrates intofrom No.200 to No. 800, degreased with acetone andhe alloy substrate to form a zone， the phasealcohol, ultrasonically cleaned in acetone and driedcomposition and properties of which are sensitive to thebefore pack cementation processing. The specimenprocess parameters---pack composition, temperaturesurrounded by uniformly blended powder mixtureand treatment time. The final microstructure of thecontaining Al powder, special additive(SA), NH4Clcoating is controlled by the substrate composition,activator and Al2O3 inert fllr was sealed in austenitictemperature and the pack aluminum activity. In moststainless retort with a paste of kaoline and liquidcases, the pack composition is varied to obtain acoating with the desired properties. Thesodium silicate, and subsequently heated in a furmace. Itcoating-forming elements may be introduced inwas at first heated to 720~820C, held for 4h and thenelemental form， or as an alloy of appropriateheated to 920~1000C, held for 2h, and at last the retortwas fumace-cooled to below 350 C before beingcomposition, usually prepared by melting andcooled in the air.solidification followed by homogenization and grindingThe structure and properties of the coating layerinto a powder. In that process, some chemical anTable 1 Chemical compositions of tested samples (mass %)中国煤化工-MaterialsCrNi_SFeEFYHC NMH G-20~0.230.3| 0.U.U55| u.035balHK40-0.424.821.10.75| 1.0.0270.0100.053bal.Vol.25 No.5TRANSACTIONS OF MATERIALS AND HEAT TREATMENT693were investigated using optical microscope (OM),as AlCI, AlCl2 and AlCl3. Lower aluminum chlorides,scanningelectron microscope (SEM)， X-raysuch as AICI and AlCl2， diffuse through andiffractometer (XRD) and micro-hardness tester.aluminum-depleted zone in the pack adjacent to thespecimen surface. These react at the surface to release2. Results and Discussionaluminum into the coating with the formation of ahigher aluminum chloride, such as AlCl3. The higher2.1 Structures and PhasesThe specimen of 20 plain carbon steel, which ischloride then diffuses back to the undepleted packsurrounded by blended powder mixture of 10%-Al,where it reacts with more aluminum to reform the5%-SA, 2%-NH4Cl with the balance Al2O3, was heatedlower chlorides. Activator thus circulates continuouslyto 720C at first, held for 4h and then heated to 930C,between the undepleted pack and the specimen surfaceheld for 2h. The optical microscope, shown in Fig.1,transporting aluminum to deposit aluminum on thesurface to be coated. When 20 plain carbon steel heatedindicates that the coating layer treated by the newat 720C, the matrix is ferrite (body centered cubic, bcc,process has the same toothed feature as that of thecrystal structure) phase in which the solubility ofcoating treated by conventional process. The X-rayaluminum is very high, maximum is about 36 %",difraction spectra, ilustrated in Fig.2, indicate that thewhich is prone to formation of aluminum-richcoating layer was primarily FeAl intermetallicaluminides in diffusion aluminide coating. The specialaluminide and a small amount of FezAl, with no FeAl3additive in the pack reacts with AlCl, AlCl2 to formand FezAls phases appeared.SA2Cls and SAAl3, the AlCl and AlCl2 are partiallyconsumed and reduced to get to the surface ofspecimen, which inhibits formation of aluminum-richaluminides and corrosion of surface of specimen.Therefore, the new aluminizing process has advantagesof controlled the coating structure, smoother hardnessdistribution, shorter treating time and finer surface ofas-treated workpiece.100The SA2Cls and SAAl3 decompose and releasealuminum into the coating, which increases the growthrate of coating.The specimen of HK40 austenitic steel surroundedby blended powder mixture of 10%-Al， 5%-SA,Fig. 1 Optical micrograph of the 20 steel specimen1.5%-NH4Cl with the balance Al2O3 was heated to 780after aluminizationC at first, held for 4h and then heated to 1000C, heldfor 2h. The results of the photomicrograph and X-rayFeAldiffraction study, given in Fig.3- 4, indicated that thecoating was made up of nickel aluminides layer,primarily NiAl phase, and transition layer, a +NizAl,without iron and nickel-rich aluminides.Fe,AlFPeAl FeAI20，.Fig. 2 XRD spectra from the 20 steel specimen afteraluminizationThe pack was prepared as a mixture of 5%-Al,100日12%-SA，2.5%-NH4Cl with the balance Al2O3, thespecimen was first heated to 720C, held for 4h andFig. 3 Optical micrograph of the HK40 austenitic steelthen heated to 930C, held for 2h. The XRD analysissnecimen after aluminizationindicated that the coating was primarily FezAl phase中国煤化工and aluminum-rich aluminides were similarly absent.very strong affinityThe chlorine, which comes from decomposition offor aliYHC N M H Guminide compounds.NH4Cl at aluminizing temperature(720C), reacts withTherefore, with the diffusion of Al into the substrate, italuminum in the pack forming aluminum chloride, suchcan combine with Ni to precipitate locally the nickel694PROCEEDINGS OF THE 14TH IFHTSE CONGRESSOctober 20043. ConclusionsNIAI(1) Desired surface structure of the aluminidecoating can be obtained by controlling the proportion ofAl to special additive powder in the pack and theparameters of pack aluminization process. The phasesof FeAl, FezAl, NiAl, NizAl, which have excellentmechanical properties and corrosion resistance, can beNiAlachieved respectively for 20 plain carbon steel andHK40 austenitic steel.(2) The new aluminizing process has advantages ofcontrolling the coating structure, smoother hardness20，。distribution, shorter treating time and finer surface ofFig. 4 X-ray diffaction spectra from the HK40as-treated workpiece.austenitic steel specimen after aluminizationaluminide, thereby absence of iron aluminides in XRDAcknowledgmentspectra. The major constituents in HK40 austenitic steelThis work was supported by Nano-meter Research Programsubstrate besides Fe are Cr and Ni. The solubility of Crof Shanghai Commission of Science and Technology under thein aluminides is extremely limited. Hence， Cr iscontract No. 021 1nm093.repelled to Y phase in the front of growing aluminides,which triggers the austenite to ferrite phaseReferencestransformation locally. The Al diffusion rate in theferrite is about two orders of magnitude higher than in1 Streif R. Protection of materials by advancedthe austenitels， which increases nucleation rate ofhigh-temperature coatings. J Phys, 1993, 4: 17.aluminides. Thus, aluminide particles precipitate in theTortorlli P F, Devan J H. Behavior of iron aluminides inferrite phase, forming transition layer ( a +NigAI).oxidizing/sulfiding environments. Mater Sci Eng, 1992,2.2 Micro-hardnessA153: 573.Microhardness measurements were made with a3 Sun Chao, Guo Jianting, Wang Shuhe, et al. OxidationVicker indentor driven by 0.98N load. Figure 5 showsbehavior of FegAl and FeAl alloys. Corrosion Science andthe microhardness profiles of the aluminide coatings ofProtection Technique, 1993,2 (5) :109-113.4 Soliman H M, EI-Azim M E A. Performance of the900 raluminide coating of some low alloy steels. J Mater Sci十The new processTechnol, 1997, 13(5): 462-466.三- + Conventional process5 Soliman H M, Mohamed K E, El-Azim M E A, and600Hammad F H. Oxidation Resistance of the AluminideCoating Formed on Carbon Steels. J Mater Sci Technol,1997, 13(5): 383-388.300 t6 Bahadur A, Sharma T L, Parida N, Mukherjee A N,Mohanty 0 N. Structure-property correlation in Al-diffusioncoated steels. J Mater Sci, 1993, 28: 5375-5381.Hu Gangxiang, Qian Miaogeng. Metallurgy, Shanghai:0040Shanghai Scientfic and Technical Publishers, 1980.34.Distance from surface, μ m8 Krutenat, C Richard, Bangaru, et al. Aluminide dispersedferrite diffusion coating on austenitic stainless steelFig. 5 Microhardness distibution of the cross -sectionsubstrates. US Patent 4835010, 1989.of the aluminized 20 plain carbon steel specimens bydifferent processesCorresponding author: Huang Zhirong,20 plain carbon steel by the controllable newEmail: huangzr@ecust.edu.cn,Mail address: P.O. Box 402, East China University of Sciencealuminizing process compared withthat ofconventional aluminizing process, which indicated thatand Technology, Shanghai 200237 China,the hardness distribution of the treated layer by the newTel & Fax: +86-21-64253810aluminizing process is smoother and lower than that of中国煤化工y conventional aluminizing process. This furthersuggests that there is no the aluminum-rich aluminideMYHCNMHGphases in the coating.