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.,Microalloyed Forging Steels for Automotive Applications,David K. MatlockAdvanced Steel Processing and Products Research CenterDepartment of Metallurgical and Materials EngineeringColorado School of MinesGolden, Colorado USA,.,Microalloy Steel Applications,http:/www.khulsey.com - Kevin Hulsey Illustration, Inc. (2005),Sheet steels for outer bodyand structural components,Engine Components:Crank shaftsConnecting rods,Suspensioncomponents,Transmissioncomponents,?Future Applications?,Gears, axles,shafts, hubs,.,Ultra Light Steel Auto Suspension, AISI (2001), www.autosteel.org,Etc. 7%,AutomotiveMaterials,.,Example: Trends in Engine Design,C. Lematre, AIST Bar Conf., Winter Park, Colorado (2006),New environmental legislation continued reductions in allowable emissionsDiesel: Increased injection pressuresGasoline: Increased use of turbo and/or compressors,European Data,.,Lighter weight powerplants for improved fuel economyRequire higher strength materialsMicroalloyed bar steels for crankshaftsIncreased stiffness (compared to cast iron)Better fatigue resistanceReduced component weightAllow smaller and lighter flywheels and clutch systems,AISI Report 2004 www.autosteel.org,Example: Trends in Engine Design,.,Historical: Use of Microalloyed Forging Steels for Automotive Components,Initial applications in Europe 1980sBased on direct-cooled ferrite-pearlite steelsGrowth in use in the US 1990sFerrite-pearlite steelsNon-traditional bainitic steels with retained austeniteWith improved processing control and understanding of microalloying fundamentals, use continues to expand.,.,Requirements for New Microalloyed Bar and Forging Steels Alloying, Processing and Product Developments,Higher Strength with Excellent Mechanical PropertiesFormability, Toughness, Fatigue Resistance, Machinability . Product BenefitsMore efficient vehicle designsHigher torque-capacity componentsHigher operating stressesSafer vehiclesImproved crashworthiness,.,Presentation Overview,Opportunities for use of Microalloyed Steels in AutomobilesReduce manufacturing costsBalance properties and performanceTailor materials for specific applicationElements of interest Ti, Nb, V, N, C, Al and their T-dependent solubilities Presentation based on Selected ExamplesAutomotive SpringsForgings for GearsThermomechanical ProcessingTransmission Components,.,Microstructural Classes of Microalloyed Steels,Ferrite-Pearlite Steels Direct CooledStrengthened by:Pearlite volume fractionFerrite grain size and substructurePrecipitation strengthening of ferrite(e.g. V and Nb)Heat Treated Steels Through hardened (martensitic or bainitic)Induction hardened surfacesSurface modified and heat treatedCarburized,.,Direct-Cooled Forging Steels,Kaspar et al., Microalloying Forging Steels, TMS, 1996,Benefit of Direct-Cooling: Reduced processing time and costRequirements: Equipment capable of controlled cooling,.,Fundamentals of Microstructure Control and Strengthening Mechanisms in Microalloyed Bar Steels,Solubility ConsiderationsMicrostructural Control and Grain Growth during ProcessingStrengthening and Toughening Mechanisms,.,Microalloy Precipitate Solubility,Microalloy elements in metals:As solid solution alloy additionsIn precipitatesAt grain boundariesWithin grainsPrecipitates form between solid solution elements and interstitials of C or NMicroalloy Interstitials Typical ElementsPrecipitatesNbCNbCNbNVNVCVNTiTiCTiNAlAlNSpecific precipitates that form depend on composition and temperature,.,Solubility Overview,Apply equilibrium thermodynamics to predict state of microalloy elements in alloy driving force for precipitate formation controlled by solubilityReactions of Interest:M + X = MXExample:Nb + C = NbCyM + zX = MyXz4V +3C = V4C3Reaction Rate: ks = Equilibrium constantks is referred to as the solubility product and is equal to the ratio of activities:,.,Solubility Overview,Temperature dependence of solubility product, ks, described by an Arrhenius equation; i.e. By convention ks given by:Example solubility diagram for NbC,.,Example: NbC Solubility Diagram,Assume:T = 1100 oCNb = 0.1 wt pctC = 0.2 wt pct,Solubility data from: Narita, Trans ISIJ (1975),.,Effects of Composition and Temperature,Example: NbC Solubility Diagram,Solubility data from: Narita, Trans ISIJ (1975),.,Carbide/NitrideSolubility,Lower solubility product=Increased precipitate stabilityExamples:At 1200 oC TiN is more stable than NbC, VNAt all temperatures VC has lowest stability, e.g. VC easily dissolves at higher temperatures,.,From: Matlock, Krauss, and Speer, Microalloying 05,Importance of Understanding Solubility,Temperature ranges in which carbides, nitrides, and carbonitrides form and dissolve determine suitability for a given microalloying design. For example:TiN, stable at temperatures in excess of 1200 oC, used for austenite grain size control at high forging temperatures and during high-temperature carburizingVN (and carbonitrides) dissolve at low austenitizing temperaturesV is available for fine-scale precipitation strengthening on cooling after forgingNbC (and TiC) dissolve and precipitate at temperatures intermediate to TiN and VPrecipitates prevent austenite recrystallization during finish hot rolling results in fine ferrite grain sizes.,.,Carbide/NitrideSolubility,CompareAustenite and Ferrite,.,Alloying and Process Control,Solubility controls driving force for precipitation, volume fraction, etc.Approach to use solubility productsSpecify alloy contentEvaluate solubilities of constituentsDetermine which precipitates form/dissolve at temperature/composition of interestPredict austenite composition and precipitate volume fractionsDesign process methodologyApproach will be illustrated with Nb-modified gear steels for high temperature carburizing.,.,Fundamentals of Microstructure Control and Strengthening Mechanisms in Microalloyed Bar Steels,Solubility ConsiderationsMicrostructural Control and Grain Growth during ProcessingStrengthening and Toughening Mechanisms,.,Consider grain growth controlDuring hot workingDuring heat treatingGrain growth limited by second phase particlesR = stable grain sizer = particle radiusf = volume fraction particlesFiner grain size = decrease r or increase fSize and volume fraction controlled by alloying and processingIf too small particles redissolveIf too large particles are ineffective Types of grain growthNormal grain growthAbnormal grain growth,Particle Effects on Grain Growth,.,Steel: Controlled Rolled with 0.02NbTime: 60 minutesHeating Rate: 145C/min,NGG = NormalAGG = AbnormalIAGG = Initial Abnormal,Grain Growth Terminology,K. AlOgab, PhD, ASPPRC, CSM (2004),.,Particle Effects on Grain Growth,1Moving Grain Boundary Approaches Particle,3 Particle RetardsBoundary Movement,.,Particle Effects on Grain Growth,From: Krauss, Steels, ASM, (2005),Plain Carbon SteelsNormal grain growth(without particles),Steel with ParticlesGrain growth slowed by particles,.,Particle Effects on Grain Growth,From: Krauss, Steels, ASM, (2005),Plain Carbon SteelsNormal grain growth(without particles),Steel with ParticlesGrain growth slowed by particles,Particle dissolution at higher temperatures results in abnormal grain growth,.,Particle Effects on Grain Growth,From: AlOgab, PhD, ASPPRC, Colorado School of Mines (2004),Example Light Optical Micrographs,Abnormal Grain Growth,Normal Grain Growth,.,Grain Growth Control Ti, V, Al, Nb,From: Krauss, Steels, ASM, (2005),.,From: Krauss, Steels, ASM, (2005),Grain Growth Control Importance of Nb,.,Grain Growth Control Importance of Nb,From: Krauss, Steels, ASM, (2005) andK. AlOgab, PhD, ASPPRC, CSM (2004),.,From: Krauss, Steels, ASM, (2005),Summary: Grain Growth Control,Microalloying elements produce particles that suppress grain growthParticle dissolution leads to abnormal grain growth and steels with large average grain sizesParticle size, volume fraction, and distribution controlled by microalloy additions and temperatureTi and Nb effective in creating particles that suppress grain growth,.,Fundamentals of Microstructure Control and Strengthening Mechanisms in Microalloyed Bar Steels,Solubility ConsiderationsMicrostructural Control and Grain Growth during ProcessingStrengthening and Toughening Mechanisms,.,Strengthening Mechanisms,Grain Size RefinementIncrease strengthIncrease fatigue resistanceIncrease toughnessRefines transformed microstructuresPrecipitation StrengtheningTransformation Strengthening,.,Hall- Petch EquationGrain boundary blocks slip bandStress concentrated at head of blocked slip band,Strengthening: Grain Size Effects,Barrett et al. (1973) and Gladman (1997),.,Grain Size Refinement,From: Krauss, Steels, ASM, (2005),.,Grain Size Refinement,Austenite refinement also modifies martensite packet size in quenched and tempered steels.,From: Krauss, Steels, ASM, (2005),Martensite packet size v. austenite grain size,Yield strength depends on packet size,.,Fatigue Endurance Limits: Carburized Gear Steels,Cornelissen et al., ASM (2000),Modified 4320 and 8620 Gear Steels,Finer Austenite Grain Size,.,Fatigue Endurance Limits: Carburized Gear Steels,Cornelissen et al., ASM (2000),Modified 4320 and 8620 Gear Steels,Finer Austenite Grain Size,Refined Prior Austenite Grain Size Leads Directly to Improved Fatigue Resistance,.,Grain Size Effectson Toughness,Charpy V-Notch DataFine grain size lowers transition temperatureFine grains resistcleavage fracture,From: Hertzberg (1989),.,Precipitation Strengthening,Strength increases with smaller particle sizes or greater volume fractions,Pinned Dislocation0.1 C - 0.04 Nb (wt pct) Steel(From Gladman, 1985),.,D. Ponge, www.materialsknowledge.org (2005),Combined Effects:Grain Refinement and Precipitation,.,Direct-Cooled Microalloyed Bar Steels,Sawada et al., ISS-AIME, (1994),Direct cool after forging ferrite-pearlite or “non-traditional bainite” microstructuresHigh carbon critical for strengthMicroalloying precipitation hardeningStrength Increase toughness decrease,.,Direct-Cooled Microalloyed Bar Steels,S. Thompson, ASPPRC, (2006),Pearlite + Interphase Precipitation,.,Importance of Interlamellar Spacing,Pearlite strength and toughness depend on interlamellar spacing,From: Gladman,in Microalloying Forging Steels, TMS, 1996,S = true interlamellar spacingt = pearlitic carbide thickness,.,Summary: Strengthening Mechanisms,Contributions of different strengthening mechanisms are additiveMany equations available in the literature to summarize properties (e.g. see Gladmans text)Opportunities exist to design materials with specific mechanical properties,From: Gladman,in Microalloying Forging Steels, TMS, 1996,.,Thermomechanical Processing of Microalloyed Bar Steels,.,TMP Control of Microstructures,Example Reference:Boyd and Zhao, in New Developments in Long and Forged Products, AIST, (2006)Purpose of Study: Compare forging schedules designed to control austenite recrystallization with alloy content in modified 1541 alloysVariables: Forging history (temperature), cooling rate, microalloy additions,Boyd and Zhao, AIST, (2006),.,Boyd and Zhao, AIST, (2006),Warm forging: Designed to finish in ferrite-pearlite region,Forging Schedules Applied to Nb/V/Ti Steels,Goal: Produce refined ferrite and pearlite colonies + precipitation strengthening,Deformation in Austenite: Designed to produce non-recrystallized austenite,.,Boyd and Zhao, AIST, (2006),Warm Forged 1541 with Nb,SEM Micrographs,Result: successfully produced refined microstructuresStrengthened also by dislocation substructure in ferrite,.,D. Ponge, www.materialsknowledge.org (2005),Thermomechancial Processing AlsoEnhances Precipitation,.,Boyd and Zhao, AIST, (2006),Effect of TMP on Mechanical Properties,Warm Forged: 1541+ Nb,Forged to produce non-recrystallized austenite,Conventional Forging,1541+ Ti/V,1541+ Nb,.,Boyd and Zhao, AIST, (2006),Summary Thermomechanical Processing,Microstructures with refined ferrite and pearlite can be successfully produced to increase both strength and toughnessPrecipitates resulting from Nb additions successfully suppressed grain growth to yield the superior properties.Thermomechanical processing of microalloyed bar steels offers opportunities for property development in new steels.,.,ExampleAutomotive Springs,.,Example: Automotive Springs,Design Requirements:Higher strengthImproved fatigue resistanceImproved toughnessLighter weightHistorically, spring steels based on SAE 5160 and more recently, SAE 9259Recently, microalloyed steels, based on Nb + V have led to improved materials,.,M. Head, et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,.,Spring Steel Compositions,New Microalloyed Grade: Lower C, with Si, Nb, and VUse Thermomechanical Processing to control grain size,With (max values) 0.02 P; 0.021 S; 0.012 Ni (in wt pct),M. Head, et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,.,Spring SteelProcessing,Hot RolledFormedHeat TreatedAustenitize 940 oCTEM Showed Nb Precipitates for Control of Austenite Grain Size,M. Head, et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,.,Front Suspension Coil Spring in North American Minivan: Improved with Microalloyed Steel,M. Head, et al., Stelco Inc., SAE (2006)and GDIS, www.autosteel.org,.,Summary Spring Steel Development,Controlled thermomechanical processing is critical in order to be able to utilize benefits of microalloying to control and refine final microstructureFinal properties benefit from contributions of both Nb and V in alloys.Development of improved microalloy spring steels will continue.,M. Head, et al., Stelco Inc., SAE (2006) and GDIS, www.autosteel.org,.,Example:Vacuum Carburized Gears,.,Bending Fatigue In Gears,.,Trends for Automotive Gear Steels,Carburize at higher T: 930 oC 1050 oCShorter heat treat cycle = $Use alternate technologiesVacuum or low-pressure plasma carburizingHigher T gas carburizingDesign alloys to respond to modified thermal histories use microalloy additionsImprove material propertiesFatigue resistanceBending fatigue Contact fatigue,.,Example Time Saving with “Vacuum” Carburizing,Klinkenberg and Jansto, AIST, (2006),.,Grain Size and “Vacuum” Carburizing,Advanced Carburizing: Plasma and Vacuum may operate at higher temperatures = shorter cycle times, but.,From Davidson et al. 2001,.,Austenite Grain Size Importance to Fatigue of Carburized Steels,Fine Grain Size = Increased Endurance Limit,Cornelissen et al. (2000),.,Trends for Automotive Gear Steels,Methods to refine austenite grain sizes in carburized gear steelReheat after carburize and quench - Refine by transformation cyclingRequires extra heat treat cycle = $Utilize microalloy precipitates to suppress grain growthAlloy designs based on solubility and process temperature considerations,.,Laboratory BendingFatigue Sample,Production Gear Set:Ring and Pinon,Bending Fatigue of Carburized Steels,ASPPRC, Golden, CO (2006),.,Bending Fatigue Failure Mechanisms in Carburized Steels,Gas Carburized 4820 Steel,Matlock et al. ABM, So Paulo, Brazil (2004),.,Grain Refinement: Ti Additions,Titanium + Nitrogen,Titanium NitridePrecipitates,From Davidson et al. 2001,.,200 mm,Simulated Carburizing8620 927oC (no Ti),Plasma Carburizing8620 1093oC,Plasma Carburizing 1093oC8620 Modified: Ti + N,200 mm,50 mm,d = 15 mm,d = 123 mm,d = 57 mm,From Davidson et al. (2001),.,Ti additions only produce grain refinementTo optimize alloy to resist grain growth, use both Nb and Ti to further suppress austenite grain growth,Summary: Ti Additions,.,Precipitate Control of Austenite Grain Size During Carburizing,Microalloy Design:Ti TiN then Nb NbC to further suppress austenite grain growthAlloys:Base: 8620 with 0.03Ti0.02Nb, 0.06Nb, 0.11NbLab melted Heats (Timken) hot or controlled rolled,AlOgab et al. ISIJ (2007),.,Add Ti to precipitate all N as TiN,Review Basis for Alloy Design,AlOgab et al. ISIJ (2007),.,Design of Nb Alloys,With N as TiN, Evaluate NbC solubility,AlOgab et al. ISIJ (2007),.,Effect of carbon increase during carburizing for 0.1 and 0.3 Nb alloy additions,Review Basis for Alloy Design,AlOgab et al. ISIJ (2007),.,SAE 8620 steel with 0.03 Ti,Review Basis for Alloy Design,AlOgab et al. ISIJ (2007),.,Predictions of dissolved Nb content and NbC contents as a function of carburizing temperature,Review Basis for Alloy Design,Ti-modified SAE 8620 Steel with 0.06 Nb wt pct,AlOgab et al. ISIJ (2007),.,Solubilities at finishing rolling finishing temperaturesHot Rolled: Finish at 1100 oCControlled Rolled: Finish at 850 oCSolubility Considerations predicts higher precipitate volume fraction in controlled rolled material,Importance of Thermomechanical Processing,Ti-modified SAE 8620 Steel with 0.06 Nb wt pct,AlOgab et al. ISIJ (2007),.,500 nm,HR 0.06Nb Steel,CR 0.06Nb Steel,Importance of Thermomechanical Processing,Ti-modifiedSAE 8620 Steel0.06 Nb wt pct,AlOgab et al. ISIJ (2007),.,100 mm,Nb-Free 60 min.,Hot Rolled 0.02Nb 240 min.,Influence of Nb Additions: At 950 oC,K. AlOgab, PhD, ASPPRC, CSM (2004),.,Pseudo Carburizing Microstructures: 950oC,Base Alloy (T),HR 0.02Nb,60,90,240,360 minutes,100 mm,K. AlOgab, PhD, ASPPRC, CSM (2004),.,Pseudo Carburizing Results: Grain Growth,Hot Rolled,Control Rolled,Ti-only,Nb,Increased Nb,K. AlOgab, PhD, ASPPRC, CSM (2004),.,Fatigue Performance of Nb-Modified Carburized Gear Steels,Fatigue Test at Room TemperatureUtilize modified Brugger SampleVacuum Carburized at 1050 oCFrequency = 30 HzBase alloy = SAE 8620,Ti-modified SAE 8620 steelWith0.8 Mn, 0.6 Cr, 0.43 Ni, 0.2 Mo (wt pct),R. Thompson et al., ASPPRC, CSM SAE (2007),.,0.06 Nb,0.1 Nb,0.02 Nb,R. Thompson et al., ASPPRC, SAE (2007),Fatigue of Vacuum Carburized (1050 oC) Steel Modified with Nb,.,SummaryGrain Size Control in Carburized Steels,Nb + Ti additions effective in suppressing grain growth during carburizingOpportunities exist for new alloy development for high T carburizingMicrostructural control critical to fatigue performance,K. AlOgab, PhD, ASPPRC, CSM (2004),.,Example:Microalloyed Steel Tubing for Induction Hardened Transmission Components,.,http:/www.khulsey.com Kevin Hulsey Illustration, Inc. (2005),Automatic Transmission:One-Way Clutch Race,Automatic Transmission:One-Way Clutch Race,http:/www.innerauto.com (2005),.,http:/www.len-ind.com (2005),Automatic Transmission:One-Way Clutch RaceQ: Can part be manufactured form hot-rolled microalloyed tubing and processed by induction hardening?,.,Automatic Transmission:One-Way Clutch Race,Design requirementsCore: 22 to 30 HRCSurface: HRC 60 (aim after tempering), 58 (min)MachinableManufacturing methods (typical)Machined from medium carbon alloy steelCarburized and heat treated (twice) to achieve desired core and case properties,.,From: G. Krauss, ASM, 1980,Property Development: Minimum Carbon Content to Achieve Desired Surface Hardness by Induction Hardening + Tempering,Requires carbon content greater than 0.5 wt. % to insure properties after tempering,Aim:As-Quenched61 - 62,.,Property Development: Alloying to Produce Core Ferrite-Pearlite Core Hardness of 22 to 30 HRC (equivalent to UTS of 800 to 1000 MPa),Sawada et al., 1994,Plain carbon steels not strong enough!Use Microalloyed Steel,.,Automatic Transmission:One-Way Clutch Race,
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