1、07FTM05Vacuum Carburizing System for Powder MetalParts and Componentsby: J. Kowalewski and K. KucharskiSECO/Warwick CorporationTECHNICAL PAPERAmerican Gear Manufacturers AssociationVacuum Carburizing System for Powder Metal Partsand ComponentsJanusz Kowalewski and Karol Kucharski, SECO/Warwick Corpo
2、rationThe statements and opinions contained herein are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractCarburizingisoneoftheleadingsurface-hardeningprocessesappliedtothesintered,low-alloyedsteelpartsintheautomotiv
3、eindustry. Whilediffusionofcarbon inwroughtsteeliswelldocumented,thisisnotthecasefor PM steel subject to carburizing in vacuum furnaces. In this paper we present results that show that thedensityofthepowdermetalisthemainfactorforthefinalcarboncontentanddistribution. Alsoimportantisthestateofthe surf
4、aceofthepart;eithersintered withopen porosityor machinedwith closedporosity. Thewaythe carburizing gas moves through the furnace might be of some influence as well.Copyright 2007American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2007ISBN: 978-1
5、-55589-909-71Vacuum Carburizing System for Powder Metal Parts and ComponentsJanusz Kowalewski, Karol Kucharski, SECO/Warwick CorporationIntroductionVacuum carburizing is a non-equilibrium process1. Unlike atmospherecarburizing itis notpossibleto set the carbon potential of the atmosphere andcontrol
6、its composition in order to obtain a desiredcarburized case.Currentlytheboost-diffusiontechniqueisappliedtocontrol the surface carbon content and carbon dis-tribution in this case. In the first boost step the flowofthecarburizinggashastobesufficienttosaturatethe austenite while avoiding soot deposit
7、ion andformation of the massive carbides. To accomplishthis goal, the calculation of the proper gas flow ratehastobemade. Howeverinthecase ofP.M. parts,theamountofcarbonabsorbedbythepartssurfacecan be a few times higher thanks to additionalinter-nal surface created by pores present in the carbu-rize
8、d case 2,3. This amount will depend on thedensity of the part, the densification grade of thesurface layer and the stage ofthe surface “asma-chined”or“assintered”. Itisbelievedthatenhancedgas diffusion after initial evacuation of the P.M.parts leads to faster carburization from within thepores,espec
9、iallywhenporesareopensurface“assintered” and interconnected low density.A serious problem with vacuum carburizing isdeliv-ering the carbon in the uniform manner to the workpieces. This led to the development of the differentmethods of carburizing gas circulation i.e., pulse/pump method developed in
10、1960s or pulse/pausetechnique applied in most of todays vacuum fur-naces. 4,5. In both cases each pressure changemay deliver fresh carburizing atmosphere into thepores and leads to faster carburization from withinthe pores.Since todays control of vacuum carburizing isbasedlargelyonempiricalresults,p
11、resentedexper-iments may lead to better understanding andimproved control of the process 6, 7.Materials and experimental procedures.A standard TRS bars compacted at the pressurefrom 480 MPa to 1080 MPa, from the blend equiva-lent to 8620 steel to green densities 7.0, 7.1, 7.2,7.3,7.4and7.5g/ccmwereu
12、sedforthecarburizingtest.ThebasicironpowderwasaQMPAtomet1001HP.Alloyingelementswhereadmixedasferro-alloysorelementalpowderstogetherwithgraphiteand0.2%lubricant. Sintering was at 1280 C in the nitrogen+10% hydrogen atmosphere.As a reference coupons of 8620 steel were used.Chemical composition of the
13、samples andreference coupons is given in table 1.To get a machined surface, half of the sampleswhereground0.1mmwithtwopasses. Carboncon-tent was measured at glow with a dischargespectrometer Leco GDS400. Usually five (5) runswhere made for each depth level. Carbon profilingwas achieved by subsequent
14、 grinding and spectralanalysis.Vacuum carburizing was carry furnace with pulse/pause method and in furnace with pulse/pumpmethod of gas circulation.In both furnaces process temperature was main-tained at 900 C and the same boost and diffusiontime 16 min and 21 min respectively.Table 1. Chemical comp
15、osition of the samples.Material C Si Mn Ni Cr Mo Cu8620 P.M. 0.19 0.21 0.84 0.52 0.51 0.21 0.218620 steel 0.21 0.323 0.81 0.57 0.54 0.20 0.202In pulse/pause furnace the boost period consists of3 pulses 3 min + 1.5 min + 1.5 min separated by 5min pauses. During each pulse the pressure wasfluctuating
16、between 4.5 Torr to 8 Torr with the fre-quency1fluctuationper30sec. Thecarburizingat-mosphere was a mixture of 30% acetylene + 30%ethylene + 40% hydrogen.Boost in the pulse/pump furnace consist of threepulses separated by two pumping periods. Eachpulse last 5 min and include 8 injections, causingpre
17、ssure increase up to 100 Torr. Pumping peri-odsbetweenpulseslast1minandpressurewasre-duced till 2 Torr before beginning of the next pulse.Carburizing atmosphere consist of acetylenediluted with nitrogen in the ratio 1:7. Diffusion wascarry on under vacuum 0.2 Torr.ResultsFigures 1 and 2 show experim
18、entally obtained car-bondistributionsfordifferentdensitiesandsamplessurface “assintered” or“machined” forpulse/pausecarburizing process carry on in Seco/Warwickfurnace.Based on this results the othergraphs showingsur-facecarboncontentandcasedepthat0.5%Cinde-pendenceofthedensity canbe made,see figure
19、s3and 4.Figure 1. Carbon distribution in dependence of the density.Seco/Warwick carburizing process. Surface “as sintered”.Figure 2. Carbon distribution in dependence of the density.Seco/Warwick carburizing. Surface machined.3Figure 3. % C on the carburized surface in dependence of the density.Seco/
20、Warwick process.Figure 4. Case depth at 0.5% C in dependence of green density for machined surfaceand “as sintered”. Seco/Warwick puls-pause process.Figures 5 through 8 show similar results obtainedfor the pulse/pump carburizing process carry on inthe furnace.In both cases the highest surface carbon
21、 contentaround 2.8% C is observed in “as sintered” sam-ples, compacted to 7.0 g/ccm. This amount de-crease lineally to around 1.2 % C for samples com-pacted to 7.5 g/ccm. The same samples areshowing the higher case depth increase from 0.2mmin8620steelreferencesampleupto0.6mmforpulse/pump process and
22、 0.47 mm in pulse/pauseprocess in a vacuum furnace.Similar tendency are showing samples machined.The surface carbon content from 0.69%C - 0.7%Cin 8620 steel reference sample increase up to 1.0 1.1 %C at density 7.0 g/ccm and 0.8%C for density7.5g/ccm. Thecasedepthincreasefrom0.2mmupto 0.32mm 0.35mm.
23、4Figure 5. Carbon distribution in dependence of density.Hayes puls-pump process. Surface “as sintered”.Figure 6. Carbon distribution in dependence of density.Hayes puls-pump process. Ground surface.Figure 7. % C on the carburized surface in dependence of the green density.Hayes puls-pump process.5Fi
24、gure 8. Case depth at 0.5% C in dependence of density for machinedand “as sintered” surface. Hayes puls-pump process.ConclusionVacuum carburizing of PM materials is much fasterthan of solid steel. The most important factors areporosity and type of surface. In presented experi-ments P.M. samples with
25、 lowest density and openporosity showed the dramatic increase of thesurface carbon content up to 2.5%C and 3 timesdeeper case.Inthemicrostructureformationof massivecarbidesand increased amount of retained austenite is ob-vious. Thedifferencescausedbydifferentmethodsof gas circulation are not conclus
26、ive.References1 Kula, P., Olejnik, J., and Kowalewski, J., Fine-Carbt-the Smart System for Vacuum Carbu-rizing(IndustrialHeating,September11,2002),Meadville, PA.2 Weber,R.G.,Carburizingand CarbonitridingofPowderMetallurgyFerrousAlloys(PowderMe-tall. Int. 15 (1983) 2 94).3 Chen, Y.T. and Kiefer, Jr.,
27、 R.W., CarburizingOfP/M Materials (Carburizing Processing AndPerformance, Proceedings on an InternationalConference, p. 199, July 12-14, 1989,Lakewood, Colorado).4 St.Pierre,J.,RecentDevelopmentsInVacuumCarburizing (Carburizing Processing And Per-formance, Proceedings on an InternationalConference,J
28、uly 12-14,1989, Lakewood,Col-orado).5 Grafen, W. and Edenhofer, B., AcetyleneLow-Preassure Caburizing a Novel and SuperviorCarburizing Technology (Heat Treatment ofMetals, Vol. 26, 1999.4, pp. 79-83).6 Antes, H.W., Calculating Gas Flow Rate ForVacuum Carburization (An ASM InternationalPublication: Heat Treating Process, August2005).7 Sugiyama, M., Ishikawa, K., and Iwata, H.,Using Acetylene For Superior PerformanceVacuum Carburizing (18thASM HTS Confer-ence, November 1998).
