Heat treatment of tool steels in vacuum furnaces ... - Seco-Warwick

ARTICLE 2012Heat treatment of tool steels in vacuum furnaces with gas quenchPart 1. Process requirements and simulationsPhD. Emilia Wołowiec, Prof. Piotr Kula –Technical University of Lodz, Lodz, PolandPhD. Maciej Korecki, Eng. Józef Olejnik –SECO/WARWICK S.A., Swiebodzin, PolandIn a few wordsTool steels constitute the basic material forforming of all groups of materials and fortool manufacturing. A properly designedand controlled cooling process is of majorsignificance for the final effect of heattreatment and therefore it is underlying forthe durability and service suitability of thetreated element.Contemporary heat treating equipment andtechnologies facilitate massive and effectiveheat treatment of tool steels, whileprocess simulators such as G-Quench Pro ®enable designing of new treatment methodswith simultaneous reduction of the timeconsuming testing procedures.IntroductionTool steels are a widely used material formaking metal, polymer, ceramic and compositetools which may be formed by machiningor plastic forming. Since the crucialrequirement for a tool is its shape stability,the material it is made from is expectedto withhold loading without plasticstrain and to be highly resistant to abrasion.Tool steels are classified according to applications.Dies, punches, stamping dies,pressing rolls and other tools designed forcold processing are made of cold workingalloy steel such as NCLV, NC10,NC11LV, NZ3 (although in practice boththe material and the tools get slightly heatingwhen working, say as a result of frictionor deforming work). The large-sizeextrusion dies, pressure casting moulds,mandrels, punches and other elements forplastic working of other materials heatedup to higher temperatures (within the rangeof 250-700 o C) are made of hot workingalloy steels (such as WCL, WCLV,WNLV, etc.). This group of materials isexpected to show high degree of hardness,abrasion resistance, impact strength andgood hardening capacity. The third groupcomprises high speed steels (e.g. SW7M,SW18, SK5M) applied chiefly to machinemetals at high speeds (cutters, drills, millingcutters).Generally speaking, all properly quenchedtool steels are characterized by high hardness,abrasion resistance, small deformabilityand small susceptibility to overheating.Since the basic qualities of those steelgrades (abrasion resistance, strength, crackresistance, ductility) are greatly dependenton the hardness achieved, it is that parameterthat attracts a lot of attention. Properlyrun quench process is of major significancefor the target hardness of steel elementsand thus determines their functional suitability.Heat treatment of tool steelsHeat treatment of tool steels consists of ahardening process followed immediatelyby a tempering process, which ensures anappropriate structure of the material. Itmust be mentioned at this point that properlyselected chemical composition has amajor influence on the properties of thosematerials. However, this is beyond thescope of the present paper. Tool steels requireprecision treatment, usually individu-1/7

alized for each grade of steel. Detailedguidelines are to be found in branch literatureand manufacturers’ material specificationswhile new concepts and trends in heattreatment are discussed in specialist magazinesand at HT branch events [1-4] . Nevertheless,certain similarities can be foundwithin application groups of those steels.Quenching the cold working tool steelsneeds to ensure fine-grained steel resistantto abrasion and wear. Such properties areachieved by conducting the process in sucha way that some of the carbides remainundissolved in the austenite. Therefore, inpractice those steels are quenched down inoil from temperatures of up to 960 o C.The hot working steels are quenched in oilor gas from temperatures of up to 1120 o Cand then tempered at temperatures up to600 o C. The austenitizing temperature is acompromise between the necessity to limitthe growth of primary austenite grains andthe need to dissolve the alloy carbides.Depending on the tool size, quenching processis aimed at obtaining a martensitestructure (for smaller elements) or martensitewith bainite (for larger tools). This isfollowed by twice tempering at temperatureor above the point of secondary hardnessin order to reduce the retained austeniteand to increase ductility and resistanceto thermal fatigue. Sometimes supplementaryprocesses are introduced, such as applyingvarious coatings (CVD, PVD) ornitriding, which additionally increase thehardness of the working surfaces of thetreated tools as well as improve their resistanceto abrasion and corrosion.The high speed steels are quenched in oilor gas under pressure, down from temperaturesof up to 1250 o C and then temperedwithin 500-600 o C.A properly performed heat treatment isdecisive for the mechanical and functionalproperties of tools and the economy oftheir application. Allowing any irregularitiesleads to quicker wear, deformation ordefect to functional elements or, in extremecases, to their damage (cracking) as earlyas in the course of heat treatment, whichcauses notable financial losses. Needless tosay, proper quality and condition of theinitial material is also significant.Difficulties in ensuring quality to largesizetools (moulds and dies) have led to thecreation of processing standards. The mostfamiliar are the works published by NAD-CA (North American Die Casting Association)[5] and those published by the leadersin automotive industry, among others theconcerns of Ford [6] and General Motors [7] .These standards apply mainly to the steelgrade H13 (WCLV) and its modifications:they relate to quality control for initial material,guidelines for conducting and controllingHT processes and researching theresults.Guidelines for HT of hot working toolsteels acc. to NADCAAccording to NADCA guidelines, HT processshould be performed in a vacuum furnacewith high pressure gas quench whilethe surface and core temperatures of a processedworkpiece is monitored and controlled(precise locations for workloadthermocouples are predetermined).Heating up to austenitizing temperature iseffected gradually in order not to allow anysignificant temperature difference. Thefirst heating stop is at the temperature ofapprox. 650 o C and continues until the temperaturedifference between the core andsurface is below 110 o C (practically muchbelow that). The next stop is preset at850 o C and continues until the temperaturesequalize, with the difference not exceeding14 o C. Finally, the austenitizing temperatureof 1030 o C is reached at which soakingfollows for 30 minutes until the temperaturesequalize (with permissible temperaturedifference below 14 o C) or for maximum90 minutes from reaching 1030 o C atthe surface. These guidelines limit thermaldeformations and excessive growth of austenitegrain.2/7

Dies are hardened by being quenched atmaximum speed down to the temperatureof 150 o C in the core. The average surfacequench ratio down to 540 o C should be atleast 28 o C/min. In the event of large-sizedies (of cross-sections above 300 mm),isothermal cooling is applied at surfacetemperature of 400-450 o C when the coretemperature diverges by more than 100 o C.Isothermal stop is finished when one of thefollowing conditions occurs:⎯ core temperature differs from surfacetemperature by more than100 o C⎯ surface temperature drops below400 o C⎯ 30 minutes have elapsed from theonset of the isothermal stop.Quenching is continued until 50 o C is obtainedin the core, following which temperingstarts immediately. Workloads should notbe cooled down below the temperature of33 o C. The required cooling rate is essentialdue to the risk of emission of carbides atgrain limits, which results in an impairedimpact strength. Isothermal cooling restrictsthe temperature difference of thesurface and the core and thus decreasesstresses and deformations, protects thepiece against cracking and prevents creationof a pearlitic structure.The first tempering is performed at thetemperature of minimum 565 o C by holdingfor the time depending on the cross-sectionof the tool (1h / 25 mm), but not shorterthan for 2 hours. This is followed by coolingdown to ambient temperature and secondtempering at the temperature not lowerthan 550 o C. Third tempering is not necessaryand is applied only for final correctionof hardness. The tempering processes reducethe internal stresses, ensure dimensionalstability and proper structure as wellas the required hardness, usually within therange of 42-52 HRC.Fig. 1. Horizontal vacuum furnace model15.0 VPT (SECO/WARWICK)Vacuum furnace with gas quenchThe requirements set by NADCA concerningheat treatment of moulds and dies aremade feasible in a single chamber vacuumfurnace equipped with inert high pressuregas quench system (type HPGQ) [8-13] . Nationalcompany SECO/WARWICK hasdeveloped a type-series of 15.0 VPT furnacesdedicated specially to heat treatmentof tools. These furnaces meet the toughestrequirements set by the tool branch and aredelivered to customers all over the world(European countries, the USA, Mexico,Brazil, China, India, and even Australia).Furnaces of various volume of workingarea are available starting from400/400/600 through 600/600/900,900/800/1200, 1200/1200/1800 mm andlarger, featuring horizontal or vertical loadingsystem (Fig. 1).3/7

Fig.2. Austenitizing and quenching withisothermal stop and with monitoring offurnace temperature and temperature ofdie surface and coreThe furnaces feature a compact design andas they do not emit pollutants or other noxiousagents, they may be installed and operatedin clean working spaces. They areequipped with a graphite heating chamberwhich permits heating the workload up tothe temperature of 1300 o C with uniformityof +/- 5 o C or better. This is achieved due tothe circumferentially placed heating elementsproviding radiation heating in vacuumand inert gas (convection, ConFlapsystem), which ensures effective and uniformheating also at low temperatures. Thefurnace quenches in inert gas at high pressure(15 atm) forced in a close circuit by ablower. Cooling gas is forced through thecircumferentially placed nozzles directlyonto the workload wherefrom the heat iscollected and passed to the internal heatexchanger. The cooling system providesfor isothermal quenching by controllingcooling intensity with the blower’s capacityand gas pressure (Fig. 2). Effectivenessof gas cooling in 15.0 VPT furnaces wasvalidated through tests on a reference steelblock 400/400/400 mm (Fig.3.) whenspeeds from 40 to 80 o C/min were achieved(cf. required by NADCA – 28 o C/min andGM – 39 o C/min).Fig.3. Test of quench speed acc. to NAD-CA on a reference steel block of400/400/400 mmA vacuum furnace enables carrying out theentire treatment in a single unit, withoutworkload transfer, and in a single workingcycle which contains the following subsequentstages: heating for austenitizing,isothermal quenching, repeated temperingand also nitriding. The process may bemonitored through workload thermocoupleslocated in an optional place of the die.Ideally clean surface of the workpieces isthe effect of the treatment run in vacuumand in inert gases (Fig. 4).Fig.4. A die in the vacuum furnace chamberfollowing complex heat treatmentTool steel quench simulatorDetermination of the interdependenciesbetween the structure, technological processand functional properties is of key importancefor correct and optimal tool4/7

manufacturing. It is so because selection ofan appropriate material combined with anappropriate technology ensures the bestproduct durability at the lowest cost.Accelerated advancement of civilizationhas initiated increased consumer expectationstowards a number of industries, concerningproduct quality and durability withsimultaneously expected reduction in hazardousemissions and minimized energyconsumption. This in turn has broughtabout a number of changes in the approachto manufacturing.Nowadays, the traditional method of reachingoptimum product properties and technologicalparameters by trial and error iscommonly replaced with simulation andprediction methods which allow the productand its technology to be computerdesigned.In numerous cases computer hastaken over the control of manufacturingwhile the growing popularity of this solutionis visible in the expanding market ofdigital equipment. Heat treatment has alsowitnessed an interest in applications forprocess modeling and simulation. Thisconcerns the technological process itself aswell as the final properties of heat treatedelements [14-19] .The general use simulation software consideronly the standard parameters of agiven phenomenon, which leads to the finalresults being burdened with an error. Amethod to increase the precision of calculationsis to take into account the parametersof the environment in which a processis effected, in this instance the individualcharacteristics of a quenching furnace as itis the case with the G-Quench Pro simulator.It may seem that the individual propertiesof a machine would tie the applicationto a given piece of equipment and thusrender it useless for others. However, appropriateparametrization of the settingspermits to retain the versatility of the software,thereby it may be used on variousunits of equipment. Despite the above, thissolution is rarely found in industrial practice.Fig. 5. Overall view of a software for simulationand control of tool steel quenchingprocessThe G-Quench Pro software (Fig.5) is appliedto simulate and control tool steelquenching in gas, thus reducing the needfor test runs. The mathematical principlesof the quench process and the dependenceof material hardness from quench timehave been developed on the basis of theresearch carried out at the University ofTechnology in Łódź, Poland andSECO/WARWICK company, with dueconsideration of available literature.A direct result of such simulation is determinationof a cooling curve for a givenmaterial under given conditions. The coolingcurve is determined basing on the parametersof the material, the process andthe workpiece such as quench temperature,pressure and type of cooling gas, dimensionsand curvature of the workpiece anddensity of workpieces in the quench chamber.Combined with an individual phasediagram for the material, the curve providesinformation on the phases throughwhich the steel goes during treatment. Theultimate effect of simulation is determinationof quench speed and predicted finalhardness of the material.As it was mentioned earlier, individualparameters of a quench unit largely determinethe actual process and, as a result, thesame parameters preset on two differentunits may render differently hardened5/7

steel. Therefore, at the installation stagethe software is configured for a particularphysical piece of equipment assignedthereto. In this way the individual characteristicsof a furnace is also taken into considerationwhen defining final properties ofthe product.Quench process controlMonitoring of quench process is an essentialelement of the manufacturing cycle.Although process control is a cyclic procedureand occurs at every stage of production(design, process, testing final product),here it is exceptionally important as this isthe last stage which offers a possibility ofintervention and change to quench parameters.Testing the final product merely givesa possibility to acknowledge a faultytreatment afterwards. On the other hand,monitoring of the quench process in realtime provides for immediate interventionof the operator should any irregularities inthe progress of the treatment be found.Fig. 6. Monitoring of proper progress ofquench process in real time with the G-Quench Pro softwareIn the monitor mode the software requiresa remote or direct connection to the quenchunit. Co-operating with the furnace computerit downloads feedback on the currenttemperature in the chamber or in thequenched workpiece and then plots thereadouts onto the phase diagram. Real timeprocess curve is plotted upon the coolingcurve determined by the simulator (Fig. 6).This way the quench process undergoescurrent verification.SummaryTool steels are basic material for formingall types of materials and for tool manufacturing.A properly designed and controlledcooling process is of major significance forthe final effect of heat treatment and thereforeit is underlying for the durability andservice suitability of the treated element.Contemporary heat treating equipment andtechnologies facilitate massive and effectiveheat treatment of tool steels, whilequench process simulators facilitate highlyaccurate prediction of process results withsimultaneous reduction of the time consumingtesting procedures.Bibliography[1] M. Korecki, J. Olejnik, R. Gorockiewicz: Rozwójpieców HPQ na przykładzie aplikacji nawęglaniapróżniowego FineCarb, obróbki cieplnej staliHSLA oraz nowoczesnej obróbki cieplnej narzędzi.XI Seminarium Nowoczesne trendy w obróbcecieplnej. Bukowy Dworek 2007.[2] M. Korecki, J Olejnik, Z. Szczerba, M. Bazel:Jednokomorowy piec próżniowy HPGQ z efektywnościąhartowania porównywalną z olejem. IVKonferencja Naukowa Nowoczesne Technologie wInżynierii Powierzchni. Spała 2010.[3] E. Wołowiec, L. Małdziński, M. Korecki: Komputerowenarzędzia wspierające obróbkę cieplną icieplno-chemiczną. Piece Przemysłowe & KotłyXI/XII 2011, 8-14.[4] E. Wołowiec, P. Kula, M. Korecki, J. Olejnik:Simulation and Control of Tool Steel QuenchingProcess. 25th European Conference on Modelingand Simulation. Kraków 2011, 357-361.[5] Nord American Die Casting Association: SpecialQuality die steel & heat treatment acceptancecriteria for die casting dies. Vacuum heat treatment,2008.[6] FORD Motor Company, Advanced ManufacturingDevelopment - DC2010: Die insert material andheat treatment performance requirements, 2005.[7] GM Powertrain Group DC-9999: Die insertmaterial and heat treating specification, 2005.[8] J. Olejnik: Vacuum furnaces with high pressurecharge cooling. Metallurgy 3/2002.[9] M. Korecki: Technical and Technological Propertiesof Gas Cooling in High Pressure Chamber.6/7

IX Seminarium Nowoczesne trendy w obróbcecieplnej. Bukowy Dworek 2005.[10] J. Kowalewski, M. Korecki, J. Olejnik: NextGeneration HPQ Vacuum Furnace. Heat TreatingProgress 8 2008.[11] M. Korecki, J. Olejnik, Z. Szczerba, M. Bazel:Single-Chamber 25 bar HPGQ Vacuum Furnacewith Quenching Efficiency Comparable to Oil.Industrial Heating 9 2009, 73-77.[12] M. Korecki, J. Olejnik, Z. Szczerba, M. Bazel,R. Atraszkiewicz: Piec próżniowy Seco/Warwicktyp 25VPT z hartowaniem w azocie i helu podciśnieniem 25 bar i jego nowe możliwości technologiczne.XIII Seminarium Nowoczesne trendy wobróbce cieplnej. Bukowy Dworek 2010.[13] M. Korecki, P. Kula, J. Olejnik: New Capabilitiesin HPGQ Vacuum Furnaces. Industrial Heating3 2011.[14] L.A. Dobrzański, J. Madejski, W. Malina, W.Sitek: The prototype of an expert system for theselection of high-speed steels for cutting tools.Journal of Materials Processing Technology 56/1-41996, 873-881.[15] L.A. Dobrzański, J. Trzaska: Application ofneural network for the prediction of continous coolingtransformation diagrams. Computational MaterialsScience 30/3-4 2004, 251-259.[16] P. Kula, R. Atraszkiewicz, E. Wołowiec: Moderngas quenching chambers supported by SimVacPlus hardness application. AMT Heat Treatment,Detroit 2007.[17] P. Kula, M. Korecki, R. Pietrasik et al.: Fine-Carb - the Flexible System for Low-pressure Carburizing.New Options and Performance. The JapanSociety for Heat Treatment 49 2009, 133-136.[18] W. Sitek: Methodology of high-speed steelsdesign using the artificial intelligence tools. Journalof Achievements in Materials and ManufacturingEngineering 39/2 2010, 115-160.[19] E. Wołowiec, L. Małdziński, M. Korecki:Nowe inteligentne programy wspierające produktySeco/Warwick. XIV Seminarium Nowoczesnetrendy w obróbce cieplnej. Bukowy Dworek 2011,71-80.7/7