1、06FTM13Economic Aspects of Vacuum Carburizingby: J. Kowalewski, SECO/WARWICK CorporationTECHNICAL PAPERAmerican Gear Manufacturers AssociationEconomic Aspects of Vacuum CarburizingJanusz Kowalewski, SECO/WARWICK CorporationThe statements and opinions contained herein are those of the author and shou
2、ld not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractThereisanincreasedinterestinfurnacesforvacuumcarburizingduetothedemandforproductswiththebestoverall metallurgical quality and lowest unit cost. Vacuum carburizing technology produces work withm
3、inimum distortion, the direct result of being cooled down with gas. The surface metallurgy is superiorbecause the carburization process is carried out in a vacuum environment. Vacuum furnaces systemsprovide”coldtocold”(coldworkgoingin,coldworkcomingout)andfullyautomaticoperationthatreducestheamounto
4、foperatorinvolvement,thusminimizinglaborcosts.Consideringupstreamanddownstreamcosts,vacuumcarburizingprovidesatotalreductionofprocessingcostsandisanaturalfitinaleanmanufacturingcell.An additional advantage is that vacuum furnace technology is a ”green” manufacturing process with nonegative impact on
5、 the environment.This technology differs considerably from traditional gas carburizing both in the equipment used and in theprocesseconomy.Thispaperpresentstheaspectsofvacuumcarburizingtechnologythathaveanimpactonprocess costs and quality improvements in the final product.Copyright 2006American Gear
6、 Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2006ISBN: 1-55589-895-51Economic Aspects of Vacuum CarburizingJanusz Kowalewski, SECO/WARWICK CorporationThere is an increased interest in furnaces for vacu-um carburizingduetothedemandfor productswiththebe
7、st overallmetallurgicalqualityandlowestunitcost. Vacuum carburizing technology producesworkwithminimumdistortion,thedirectresultofbe-ingcooleddownwithgas. Thesurfacemetallurgyissuperior because the carburization process is car-ried out in a vacuum environment. Vacuum fur-nacessystemsprovide“coldtoco
8、ld”(coldworkgo-ing in, cold work coming out) and fully automaticoperation that reduces the amount of operator in-volvement, thus minimizing labor costs. Consider-ing upstream and downstream costs, vacuum car-burizing provides a total reduction of processingcostsandisanaturalfitinaleanmanufacturingce
9、ll.An additional advantage is that vacuum furnacetechnology is a“green” manufacturingprocess withno negative impact on the environment.Thistechnologydiffersconsiderablyfromtraditionalgas carburizing both in the equipment used and inthe process economy. This paper presents the as-pects of vacuum carb
10、urizing technology that havean impact on process costs and quality improve-ments in the final product.Vacuum Carburizing is considerablyfaster than gas carburizingVacuum carburizing is characterized by anextraor-dinarily high coefficient of carbon transfer at thephase interface, which results in a h
11、igh carbontransfer.Intheinitialphaseofcarburizing,forexam-ple,atatemperatureof1740F(950C), thecarbonstream directed at the charge surface reaches therate of 250 g/m2h. This means that, in the case ofthin carburization layers, the process is consider-ably faster than the gas carburizing process. Thea
12、dvantage is smaller in the case of thick layers thatexceed for example, .00315 inches (0.8 mm),wherethecarbontransfer is muchmoredependanton the diffusion coefficient (DC).Thevacuumcarburizingprocessmayeasilybecar-ried out even at temperatures of up to 1900F(1050C), within thenatural temperaturerang
13、e of avacuum furnace. The process temperature in-creases to 1700-1800F (950-980C), comparedtotraditionalgascarburizingprocesses thattypical-ly operate within a temperature range of1600-1700F (880-930C). Operating at highertemperatures results in shorter carburizing cyclesduetotheconsiderableincrease
14、ofthediffusionco-efficient (DC). Boththeincreasedamount of carbonin the carburizing atmosphere, and faster diffusion(Dc)areresponsiblefortheincreaseinvacuumcar-burizingefficiencywhencomparedtothetraditionalgas carburizing.Figure 1. Single chamber vacuum carburizingfurnaceReduction of the processing
15、time andenergy-related factorsVacuum carburizing technology differs consider-ablyfromgascarburizinginthemethodofdeliveringthe carbon stream to the charge surface, processregulation, andinthecompletionoftheentirecycle.Moredifferences arefoundinthefurnaceconstruc-tion,theresultsofheatandchemicaltreatm
16、ent,andin the consumption of energy, and therefore, theprocess costs. Thenewtechnology consistentlyre-duces and/or eliminates deformations, eliminatesinternal oxidation, and reduces the exhaust gasemission into the atmosphere.It is commonly believed that shortening the cycleperiod accordingto this m
17、ethod willreduce thepro-cess cost. But, the reduction of the process dura-tion is higher for the same temperature, in the caseof thin carburized layers than thicker layers, where2the impact of the diffusion coefficient is dominant.For thin layers, especially those manufactured athigh temperatures in
18、 the steel grades with higherhardening capacity, the vacuum cycles will be verycompetitive compared to gas carburizing. The im-plementation examples below illustrate theefficiency of vacuum carburizing.The vacuum carburizing method allows a uniformcarburized layer to be easily produced in openingsof
19、 small diameter and considerable depth.Agoodexampleofthisisfoundinelementsofdieselinjectors made of such materials as EN32B,18CrNiMo7-6 (17HNM). The vacuum carburizingcycle, usually operating in a temperature range of1540-1690 (900-920oC), requires 11 minutes ofcarburizing for a .01969inches (0.5 mm
20、) layer, and120 minutes of diffusion. A similar cycle performedin an atmosphere furnace required the process tobe carried out at a temperature range of1540-1560F (840-850oC) took three times aslongtoobtaincomparablequality. Theconspicuousimpact of the process temperature difference ispossible for th
21、in layers (CCAT 550HV0,5mm) pro-duced in the steel types, where the Grossman co-efficient H has negligible impact on CCAT 550HV(figure 2).A comparison of gas carburizing and FineCarbvacuum carburizingwas conductedtodemonstratethe differences in the process cycle for typical car-burized materials.The
22、 tests were carried out for a net charge of 770pounds (350 kg), consisting of 16MnCr5 and15CrNi6 steels. The tests of 16MnCr5 steel werecarried out in a Casemaster integral quench fur-nacewitha24inchx24inchx 36inchloadcapacityand ina double-chamber NVPT 24inch x 24 inchx36 inch (600mm x 600mm x 900
23、mm) vacuum fur-nace, while the tests of 15CrNi6 steel were carriedout in the same Casemaster IQ furnace and in asingle-chamber VPT 4035/36 vacuum furnace.The comparison was performed for two layer thick-ness values: .02362 and .04724 inch (0.6 and1.2mm). The process of gas carburizing is usuallycarr
24、ied out at temperatures of up to 1690-1700F(920-930C), while the process of vacuum carbu-rizing is normally carried out at temperatures of upto 1760-1800F (960-980C). Therefore, thecom-parison was carried out for the temperatures of1690F (920C) and 1760F (960C), respectively.Moreover, the time of he
25、ating up to carburizationtemperaturefor agivenchargeis assumedtobe50minutes,andthetimeofburn-inaftercool-downforhardeningisassumedtobe30minutes.Theresultsare presented in the tables below.Figure 2. Injector elements material: EN39B,17HNMFigure 3. Approximate duration of vacuumcarburizing process for
26、 16MnCr5 steels,correlated to the temperature and the requiredthickness of the AHTlayer16MnCr5 (16HG) steel;EHTmm Total cycleminutes(N+D) cycleminutesGas carburizing0.6 315 1761.2 660 520FineCarb vacuum carburizing0.6 210 63 (carburizing: 13minutes)1.2 525 380 (carburizing: 27minutes)315CrNi6 (15HN)
27、 steel;EHTTotal cycle (N+D) cycleGas carburizing0.6mm 250 1091.2mm 495 352FineCarb vacuum carburizing0.6mm 220 50 (carburizing: 9minutes)1.2mm 450 280(carburizing: 19minutes)The above results confirm the claimed efficiency,especially in the case of thin layers. Figure 3 pres-entsthegeneral,demonstra
28、tivediagramoftimees-timates of theFineCarbprocessatahightemper-ature range, easy to obtain in a vacuum furnace for 16MnCr5 steel and the most common layerthickness values.The economic competitiveness of the process(installation cost excluded) is a separate question.The above cycle periods have consi
29、derable impacton the consumption of energy-related factors.While disregarding detailed list of components oftheveryprocess(i.e.thestoptime,thetimeofmain-taining the furnace during weekends, etc.), the en-ergyconsumptionfora15CrNi6chargeand0.6mmand 1.2 mm layers is presented below.Gas carburizing Fin
30、eCarb vacuumcarburizing0.6 mm: 200 kWh,which includescharge heating -65kWh0.6 mm: 180 kWh,which includescharge heating -65kWh1.2mm: 290 kWh 1.2 mm: 315kWhThetableshows the vacuum carburizing methodtobe competitive in the case of thin layers, while gascarburizing is slightly more profitable in the ca
31、se ofthicker layers, whichisduetolarger heatloss oftheinsulation of the heating chamber in a vacuum fur-nace. Vacuum carburizing is more competitivewhen compared to the consumption of the processatmosphere.Theatmosphereconsumptionforboth0.6mm and 1.2mm layers is presented below.Gas carburizing FineC
32、arb vacuumcarburizingFeeding time approx.4.5 h - Endo atmos-phere consumption35 Nm3per cycleFeeding time approx.9 minutes gas con-sumption (ethylene/acetylene/hydrogen)0.45Nm3per cycleFeeding time approx.8.5h - 65Nm3Feeding time: 19 min-utes 0.95Nm3As a consequence, the post-processing gas emis-sion
33、isconsiderablylowerinthevacuumcarburizingtechnology, which concerns toxic CO and CO2inparticular. The vacuum carburizing technology alsoinvolvesconsumptionofcoolinggasusedinthegashardening cycle (the cost of about 0.4 PLN/Nm3xthe volume of the cooling chamber x the processpressure). Inthe caseof 15C
34、rNi6steel hardenedinVPT4035/36furnaceatthepressureof10bars,thecost of used Nitrogen is about 20 PLN per cycle.When module furnaces are used and the demandfor the cooling gas is much higher, there exists thepossibility of constructing a recycling system withthe efficiency of 98%, which considerably i
35、mprovesthe calculation of cost.The examinations carried out with a speciallyequippedfurnaceexplainthecauses of so lowcon-sumption of the carburizing factors in the FineCarbtechnology. To achieve this, a vacuum carburizingfurnace was modernized to carry out a series ofmeasurements (table 1). Addition
36、al furnace equip-ment (presented in figure 2) allowed the chemicalcompositionofoutputgassestobeconstantlyregis-teredontherun.Astheproportions ofthefedatmo-sphere were known, it was possible to determinethe most probable directions of chemical reactionsoccurringduringtheprocess,andtodeterminetheirkin
37、etics.Acapillarytube(2)wasconnectedtothefurnace(1)to allow sampling of the carburizing atmosphere,sent over to be analyzed to the mass spectrometer(3).Particleswereionizedintheionizationchamber(6) inside the spectrometer, and later detected bythedetector (7) andanalyzed inthe quadrupleana-lyzer(5).T
38、heresultswereconvertedintoameasur-able signal registered by the computer (8).Thediagramspresentingthepeakintensitiesofpar-ticular masses were prepared after the series ofmeasurements for vacuum carburizing processes4at thetemperature of 1740F (950oC) and inaccor-dance with the diagram presented in t
39、able 1, andcomparedtotheinput levelof theprocessingatmo-sphere.Figures4and5showtheobtainedvaluesofethyleneandacetylenedecomposition, whilefigure4 shows the increase of the amount of hydrogen inthe processing atmosphere.Table 1. The percentage of components in theexamined processing atmospheres.Proce
40、ssA B C D E FMixture composition, %Ethylene 80 70 55 33 27 0Acetylene 0 5 13 23 27 40Hydrogen 20 25 32 44 46 60Figure 4. The value of ethylene decomposition as a function of mixture type.91,693,8602865,4MIX B MIX C MIX D MIX E MIX F%ACETYLENEDECOMPOSITION1009080706050403020100Figure 5. The value of
41、acetylene decomposition as a function of mixture type531,4107,319,833,793,430,4MIX A MIX B MIX C MIX D MIX E%INCREASEOFHYDROGENAMOUNT020406080100120MIX FFigure 6. The value of hydrogen amount increase as a function of mixture typeThe obtained results allow one to observe, that therelative decomposit
42、ion of ethylene proceeds fasterif its content in the mixture is lower (fig. 2). Thehighestintensityofacetylenedecompositionoccursfor DandE mixtures, yet it should benoted that theresults obtained for acetylene are burdened with acertain error, due to partial decomposition of ethy-lene into acetylene
43、, which is further on partly de-composed, while it is also partly responsible for theincreasedlevelofC2H2intheoutputgasses.There-fore, the decomposition percentage presented inFig. 3 can be slightly underrated.The decomposition of hydrocarbons results in theemission of hydrogen and its level in the
44、outputgasses increases, as compared to the expectedatmosphere. Fig. 4 illustrates the increase inpercents of the observed hydrogen amount. Themaximum values are observed for mixtures D andE, which correlate well with the data for the usedhydrocarbons. This behavior of D and E mixturescovered by the
45、patent can be explained by thesynergic operation of C2H4,C2H2and H2for thediscussed proportions of the fed processingatmosphere, whichensures optimum conditionsforthe process realization aimed at the quality ofmanufactured layers and short processing times.CONCLUSIONThis paper discusses operational
46、results that allowonetoobserveaclear,synergicimpactof themutu-al interaction of acetylene and ethylene in connec-tion with hydrogen, while the feeding proportionsare determined. The phenomenon results in a de-cidedly higher carbon supply, which leads to the in-tensification of vacuum carburizing pro
47、cess, andconsequently to shortening the treatment time, re-duction of gas consumption and negative environ-mental impact.Theeconomic efficiency ofvacuum carburizingcanbe considerably more competitive than traditionalgas carburizing. At present, the useof vacuum car-burizingconcentratesonmaterialswit
48、hbetterhard-ening capacity and (or) on details with a limitedcross-section, where the use of this technologybrings measurable economic benefits. The imple-mentation results of this technology will continue tobesupportedbyotherfactorsthatinfluencethefinalcost. The following factors are indicated: red
49、uctionof hardening distortion, reduced emission of post-processing gases, limitations of gas carburizing re-sultingfrom theuseof hardening oiland therelatedproblems,andthedevelopmentofprocessautoma-tion,etc. Whenviewingtheentireprocess,bothup-6stream and downstream, vacuum carburizing is aneconomical solution that produces optimum qualitywork with lower unit cost.REFEReNCES1. J.Olejnik “Nowoczesne konstrukcje piecwprzniowych w technologii obrbki cieplnej staliHSLA, stali do pracy na goraco WCLV oraz donaweglania przniowego”
