Evaluation of Process Capability in Gas Carburizing Process to Achieve Quality through Limit Design

TRANSACTIONS OF MATERIALS AND HEAT TREATMENTVol.25 No.5PROCEEDINGS OF THE 147H IFHTSE CONGRESSOctober 2004Evaluation of Process Capability in Gas Carburizing Process to Achieve Qualitythrough Limit Design ConceptK. Palaniradja, N. Alagumurthi , V. SoundararajanDepartment of Mechanical Engineering, Pondicherry Engineering College, Indiaproperties under different conditions. In one condition it can be very mild, soft and suitable for any forming operation. Inanother condition the same steel can be very hard and strong. This versatility is made possible by the dfferent heattreatments that the steel can be subject to. One such treatment is Gas carburizing. This is the most widely used process forsurface hardening of low carbon steels. In this method the surface composition of the steel changes by diffusion of catbonand or nitrogen and result in hard outer surface with good wear resistance properties. A striking feature of Gas Carburizingprocess is that in this process the original toughness and ductility remains unaffected even afer heat treatment. 3% nickelchromium case hardened low carbon stels are widely used for critical automotive and machine aplications such as rackand pinion, gears, camshaft, valve rocker shafts and axles which requires high fatigue resistance.Fatigue behaviour of casecarburized parts depends to a great extent on the corret combination of Hardness Penetration Depth (HPD) and themagnitude of hardness at the surface and beneath the surface with low sire and shape distortion. In order to reduce themanufacturing costs in terms of material consumption and elimination of the number of processing steps, the effect of Gascarburizing parameters on the fatigue behaviour should already be considered in the parameter design stage. Therefore it isof importance to optimize the gas carburizing process variables to atain quality products with respect to hardness and casedepth. In the present paper, the evaluation of process capability was arried out through a Limit Design Concept calledorthogonal aray design of experiment. To optimize the process variables the infuence of several parameters (Holding time,Carbon potential, Furmace temperature and Quench time) of the gas carburizing process on the micro hardness, total casedepth, efctive case depth and level of distortion of AISI 8620 steel were discussed.Key words: Gas carburizing, optimization, carbon potential, holding time, quenching tine, fumace temperatureNUMEROUS INDUSTRIAL APPLICATIONS requirethe range of 870 to 940 °C. Then the fumace is floodeda hard wear-resistant surface called the case, and awith a suitable gas such as Propane, Butane, andrelatively soft, tough inside called the core. There areKerosene etc. Finally the components are held at thisfive principal methods of case hardening:temperature to allow diffusion of carbon into the case.●CarburizingAfter the carburizing treatment is over the components●Nitidingare quenched to get the required hardness, wear●Cyaniding or carbonitridingresistance and fatigue resistance on the surface,●. Flame hardeningsupported by a tougher core. A striking feature of gas●Induction hardeningcarburizing process is that in this process the originalThe first three methods change the chemicaltoughness and the ductility remains unaffected evencomposition, carburizing by the addition of carbon,after the heat treatment. [1]nitriding by the addition of nitrogen and cyaniding byGas carburizing is a complex process in itself as athe addition of both carbon and nitrogen.The last twonumber of variables affect the success of the processmethods do not change the chemical composition of theand quality of the components. An industrial surveysteel and are essentially shallow-hardening methods. Inindicates that there is a rejection of 10-12% of caseflame and induction hardening the steel must behardened components due to various defects. Qualitycapable of being hardened, therefore, the carboncontrol is the one way to minimize the percentagecontent must be about 0.30 percent or higher. Howeverrejection. Quality control is possible through hardeningthe aim of all the method is the same.the components under optimal conditions. [2]1. Gas Carburizing2. Review of LiteratureThis is the oldest and one of the cheapest methods ofIt is well known that the surface -hardened steelscase hardening in which the surface of the componentsserved the engineering requirements of good wear; saturated with carbon in a gaseous atmosphereresistance and better fatigue life. Surface hardeningcontaining carbon.To accomplish this, first theresults in a hard surface layer above a much softer andcomponents are heated in a gas tight furnace in a中国煤化工。Carburization is aneutral atmosphere to a predetermined temperature inhardening whichTYHCNMHG396PROCEEDINGS OF THE 14T IFHTSE CONGRESSOctober 2004produces higher carbon content and hardness whichThe 9 experiments were conducted by varying all theproduces higher carbon content and hardness of theparameters(conditions adopted in experimentation iscase than that of the inner core. However, due to thegiven in Table 1) identified to study the influence chigher carbon content at the case, the material becomesthese parameters (between low, medium & high) oninhomogeneous and may result in the overall change ofsurface hardness / case depth and every case hardenedits mechanical properties and fracture behaviors.component was taken to measure the surface hardnessThe distortion or fracture of heat treated componentsand case depth. The result as per the L9 array wasis major industrial problem, which may considerablygiven in the Table 2.increase the costs of operations and decrease theTable 1 showing the operating range ofqualities of core parts.During recent decadesparameters of GCFconsiderable interest has been devoted particularly bygear manufacturers to copying with the deformationS.No. Parameters| NotationT Unit| Value withwhich occur during case hardening. One of the reasonsRangefor this work is that it is desirable to avoid the)1CarbonCPMv 1110 topotential1120expensive post-hardening grinding operations. [302HoldingTHMin| 180 to 220There is a unique optimization technique namedtimeutesTaguchi's Robust Design Method.Parmeters design by03Fumace| FIC890 to 930Taguchi's Method is offline quality control method.Tempffline quality method or quality and cost control04Quenching| QMin 20 to 30activities conducted on the product and process designstages to improve product manufacturability andTable 2 L9 Orthogonal array of Gas Carburizingreliability and to reduce product development and lifeprocess parameters and test resultstime costs. Parameter design can be used to make aprocess robust against sources of variation and henceExpt.HT| F| QTHRA| TCD_No.improve field performance.In view of Taguchi's01802079Concept, the product must be produced at optimal02 1115 180 9100.7levels at which minimal variation in its functionalcharacteristics. Controls factors and noise factorsL031120.180 930308(affect the products quality. Control factors can be04 111 200 910810.9easily controlled and on the other hand noise factors are05 1115200 930nuisance variables that are expensive to control. [4]06| 1120 200 89025.00The present research was carried out to evaluate the07 1110 220 9300.8various parameters and to optimize the gas carburizing220890_8process by subjecting AISI 8620 steel specimen for9 11202209101.00case hardening technique.3.5 Case Depth/Hardness Measurement3. Experimental ProcedureThe surface hardness (HRA) was measured withRockwell Hardness tester and Total case depth (TCD)3.1 Materialin mm was found through Rockwell Hardness testerThe steel under investigation is of the typeand visual metallurgical examination.designated as AISI 8620 (cylindrical samples of3.6 Metallographic Examinationdiameter 20mm and length 300mm).For some of the Specimens microstructure was also3.2 Heat Treatmentanalyzed to find out the Total Case Depth and EffectiveThe investigation was performed on cylindricalcase depth.samples of AISI 8620 in a 3 / m depth, 130kW, and3.7 Distortion Check upmethanol-acetone Unitherm Gas carburizing fumaceDistortion was also checked with mechanicalwith oil as Quenchant.comparator.3.3 Process VariablesThe four process variables namely holding time,4. Results and Discussioncarbon potential, Furmnace temperature and quenchingANOVA analysis was carried out to determine thetime which affect the hardness and case depth wereinfluence of main factors and to determine theselected for experimentation.percentage contribution of each factor. The average3.4 Design of Experimenteffect of main factors on surface hardness and TotalA L9 (3) orthogonal array design of experimentcase denth were oiven in Tahle 3 and Table 4.were adopted for experimentation.中国煤化工TYHCNMHGVol.25 No.5 .TRANSACTIONS OF MATERIALS AND HEAT TREATMENT397Table 3 Average effect of main factors ona) To achieve High surface hardness thesurface hardness (HRA)optimum conditions were:ParameterLow _MediumHighHolding time 220 minutes, quenching time 30Holding time79.6680.66minutes, Carbon potential 1110 一1115mv,Carbon potential79.3380.3381.23Furmace temperature 890 - 930°CFurnace Temp.80.00b) To achieve more case depth the optimumQuenching time79.0081.00conditions wereHolding time 200 -220 minutes, quenching timeTable 4 Average effect of main factors on30 minutes, Carbon potential 1110 -1115mv,Total Case depth (TCD)Low Medium High(3) The confimmation of experiment shows that the0.730.860.90experimental observations are within 95% confident0.830.85level. The error in the experimental analysis is very low.Furmace Temperature0.80The results indicate that the carbon potential is havingthe major role in the hardness and case depth effect.(4) For selected specimens microstructure and microTable 3 indicates the Fumace temperature is nothaving any effect on the surface hardness. Thehardness study was done and it was found that therepercentage contribution of holding time (22%),were no defects in the carburized specimen.Quenching time (21%), Fumace Temperature (0%),Acknowledgementand Carbon potential (57%) on the surface hardnessThe authors are thankful to Rane (Madras) Pvt. Ltd,indicates that the Carbon potential having moreThirubhuvanai, Pondicherry for providing the experimentalinfluence on the surface hardness.facilities.Table 4 indicates the influence of Gas carburizingReferencesprocess parameters on case depth. Carbon potential in1. Avner. H SidneyIntroduction to Physical Metallurgy",the fumace is having more influence (49%) on the caseMcGraw Hill, Newyork, 1982.depth whereas the percentage contribution of holding2. Shewmon, G. Paul, "Diffusion in solids, series in materialtimeis 26%， Furmace temperature is 15% andscience and engineering”, McGraw Hill, Tokyo, 1963.Quenching time is 8% and % of error is 2%.Optimum process condition were obtained by3. SHEN-CHIH LEE and WECO-YOUE HO,“The effect ofsurface hardening on fracture toughness of carburized steel",employing the higher is better (High surface hardnessIntermational Joumnal of Metallurgical Transactions A 1989,and high case depth) as strategy.Vol 20A, pp 519-524.4. Alagumurthi N etal, “An insight into optimum grinding5. Conclusionsconditions from DOE point of view-An experimental study(1) Time parameters have some significant influenceon quality and productivity improvement", Proceedings ofon the hardness and case depth of the case hardenedthe National conference on Challenges in achieving globalcomponents, however, carbon potential has moreQuality, TCE-Madurai, INDIA, pp 277-282, May 2004.influence on the surface hardness and case depth of theCorresponding author. K.Palaniradjacase hardened components.Email: palaniradja72 @ rediffmail.com(2) The present analysis shows that中国煤化工MYHCNMHG