QUENCHING AND CARBURISING Proceedings ofthe Third International Seminar ofthe International Federation for Heat Treatment and Surface Engineering Held in conjunction with the Annual Conference of The Institute ofMetals and Materials Australasia (The Materials Society ofIEAust) THE INSTITUTE OF MATERIALS Book 566 Published 1993by The Instituteof Materials ICarltonHouseTerrace London SWIY 5DB ©The InstituteofMaterials 1993 Producedunderthe auspices of The InternationalFederationfor HeatTreatment and Surface Engineering BritishLibraryCataloguinginPublicationData Quenching and Carburising: Proceedings of the 3rd International Seminar ofthe InternationalFederationfor Heat Treatment(Melbourne, 1991) 620.1121 ISBN 0-901716-51-0 Typeset inGreat Britain by AldenMultimediaLtd, Northampton PrintedinGreat Britain by The Alden Press, OsneyMead, Oxford Introduction Quenching and Carburising are two ofthe most basic and widelypractised steel heat treatmentprocesses. Each allowsthe base propertiesand performanceofthe steel to be significantly enhanced, such that a relatively inexpensive and simple starting material can be used for a wide range of demanding applications. Nevertheless, thetechnologicaldevelopmentswithinthose twoprocessesare often ignored in favour of'high tech' surface treatments. The aim of QuenchingandCarburisingwas to reviewthe recent advancements thathave beenmade in thesefields.Theconferencewasthe thirdbiennial seminar of the International Federation for Heat Treatment and Surface Engineering (IFHT) and aptly formed the centrepiece ofthe InstituteofMetals and Materials Australasia's(IMMA)annualconference, MaterialsProcessingandPerformance. The sessions devoted to quenching covered a range of topics from the measurement of quench intensity, through to applications within the heat treatment industry and the steel processing industry. The current status ofthe application and scienceofquenching was reviewed by Professor Liscicfrom the University of Zagreb. Contributed papers covered developments in the use of fluidisedbedfurnaces, helium quenchinginvacuumfurnaces and theenvironmen tal issues associated with the disposal of quenching media. Aswithmost industrialprocessesthereisnowastrongneedforaccuratemodels ofthequenchingprocess, and anumberofexamplesweregiven:quenchingofhigh carbonsteels,wherethe roleofresidual stressisimportant,airmistcooling during continuous casting, spray cooling in heat treatment and after hot rolling and gas quenchingwithhelium. Oneimportantfeature ofsuchmodellingisthe generation ofaccuratedatain the laboratory, and then incorporatingdifferenceswhich exist between controlled laboratory conditions and full-scale industrial plants. It is also apparent there there is now a degree ofcommonality between conventional heat treatment operationsutilising controlledcooling and modern inline thermo mechanical processing routes where the properties are generated during the shaping ofthe final product. Carburising currently receives less exposure in the recent technical literature than quenching; largely because it is now an established and widely practiced technique for surface engineering. However, there are still a number of issues which need to be addressed, and these werecovered in the two reviewpapers by Professors G. Krauss (Colorado School of Mines) and J. Grosch (Technical University of Berlin), with particular emphasis on the microstructures and toughness of the carburised steels. The contributed papers cover modelling, mechanical properties and industrial carburising processes. viii Introduction Overall the conference demonstrated that there is a great deal ofknowledge regarding many aspectsofQuenchingandCarburising, and thatthe challengeisto apply this knowledge to control the process, and reduce the product variability. Steel is now a highly engineered material, which is still developing improved properties at lower cost to the user. To this end it is hoped that the material contained within this volume will enhance the application of these engineering processes throughout the heat treatment and thermomechanical processing industries. The success of the conference was largely due to a small group of highly enthusiastichardworkingpeople, most notably BruceHinton(DSTO-ARL), Bill Sinclair(BHP Research) and the staffofthe IMMAoffice,particularlyMargaret Kirk and Angela Krepcik. Great support was also provided by members of the IFHT,and particularthanksisexpressedto ProfessorT. Bell,ProfessorB. Liscic and DrJ. Naylor for their assistance during the conference organisation. Finally, Iwouldliketothanktheauthors,many ofwhomventureddown-under for the first time, who provided us all with the opportunity to learn, discuss and benefit from their experiences in heat treatment research and industrial appli cation. They have helped create a volume which, it ishoped, willservefor many years as an important reference for those either working in the heat treatment industry, or teaching these subjects. Peter D. Hodgson BHP Research - Melbourne Laboratories Mulgrave, Victoria, Australia. Conference Chairman Contents Introduction P.D. HODGSON vii State ofthe art in quenching 1 B. LISCIC 2 Measurement andevaluation ofthe quenching power ofquenching media for hardening 33 J. BODIN AND S. SEGERBERG 3 Quench severity effectson the properties of selected steel alloys 55 CHARLES E. BATES,GEORGE E. TOTTEN AND KIMBERLEYB. ORSZAK 4 Use and disposal ofquenching media. Recent developments with respect to environmental regulations 71 E.H. BURGDORF 5 Use offluidised beds for quenching in the heat treatmentfield 85 RAY W. REYNOLDSON 6 Gas quenching with helium in vacuum furnaces 107 BENOIT LHOTE AND OLIVIER DELCOURT 7 Residual stress in quenched spheres 119 D.W. BORLAND AND B.-A. HUGAAS 8 Computer simulation ofresidual stresses during quenching 127 A.K. HELLIER, M.B. MCGIRR, S.H. ALGER AND M. STEFULJI 9 A mathematical model to simulatethe thermomechanical processing ofsteel 139 P.D. HODGSON, K.M. BROWNE, D.C. COLLINSON, T.T. PHAM AND R.K. GIBBS 10 Measurement and characterisation of air-mist nozzles for spray quenching heat transfer 161 M.S. JENKINS, S.R. STORY AND R.H. DAVIES 11 Investigation ofquenchingconditions and heat transfer in the laboratory and in industry 177 s. SEGERBERG AND J. BODIN VI Contents 12 The design and performance ofa laboratory spray cooling unit to simulate in-line heat treatment ofsteel 189 R.E. GLOSS, R.K. GIBBS AND P.D. HODGSON 13 Microstructure, residual stresses and fatigue ofcarburised steels 205 G. KRAUSS 14 Fundamentals ofcarburising and toughness of carburised components 227 J. GROSCH 15 Martempering and austempering ofsteel and cast iron 251 GEORG WAHL 16 Property prediction of quenched and case hardened steelsusing a PC 267 T. RET!, M. GERGELY AND C.C. SZILVASSY 17 Effectofcarburising on mechanical properties ofsteels 281 NORIO KANETAKE 18 Heat treatment with industrial gases- Linde Carbocat and Carboquick processes 293 ABDUL KAWSER Index 305 1 State of the Art in Quenching B. LISCH: Facultyof MechanicalEngineeringandNaval Architecture, UniversityofZagreb ABSTRACT A survey ofactual methods (based on direct temperature measurement) for quenching intensityevaluationinlaboratoryconditionsaswellasinworkshoppracticeisgiven,and commented,especiallyinrelationtopossiblepredictionofmetallurgicaltransformations and hardnessdistribution on the cross-sectionofreal workpieces. Apart from the widely used immersion quenching technique, the fundamental and latest developments of other quenching techniques such as spray quenching, gas quenching in vacuum furnaces and hot salt bath quenching are briefly described. Informationabout 'intensivequenching' isgiven, and the prospects offurther develop ments in the fieldofquenchingoutlined. 1 INTRODUCTION Quenching has a historical background centuries old, and has always played an importantrolein the manufactureofmetallic products. Certainly,the technology ofquenchinghas changedaswellas the kindofworkpiecesbeingquenched:from axes and swords to gears and automotive components and, as the latest development, to a gas turbine blade that has been quenched in a hot isostatic pressing (HIP) quenching unitin argon under 2000barsofpressure.\Even more important is the fact that simultaneously quenching has changed from an empirical skill to a scientifically founded and controlled process which now belongsto thearea of'intelligentprocessing'of'materials.lThisisthegoaltowhich wearestriving, andhopefullybythebeginningofanewmilleniumwewillbemuch closertoitthanwearetoday,because inpresentworkshoppracticewestillhaveto rely on practicalexperience. Fora long periodin the recent historyofquenching, besideswater, oilwas the main quenching medium. Correspondingly, manufacturers and suppliers of quenching oils have played the main role in the development of quenching technology and related testing methods and specifications. Nowadays the assortment of quenchants in use ismuch broader. There are at least seven groups of quenching media having different chemical and physical 2 Quenching and Carburising propertiesand, ofcourse, different quenchingintensities. They are (indescending order ofquenching intensities): • Water solutions ofinorganic substances. • Plainwater. • Polymer solutions in water. • Quenching oils of different grades. • Hot salt baths. • Fluidised beds. • Pressurised and circulated gases (N2, He, Ar). In spray quenching techniques water, water-oil emulsions, polymer solutions in water or water-air mixtures are used. One has to take into account that with each of above mentioned groups of quenchants verymanydifferent quenchingintensitiescan be realised. On the one hand this is due to different substances and their concentration, and to different quenching parameters (bath temperature, agitation rate, pressure) on the other. This situationcallsfor a universallyapplicablemethodfor testing andevaluation of quenching intensity and makes the selection of optimum quenchant and quenching conditions both from the technological and economical point ofview an important consideration. Followingmodernproductionconceptssuchas'justintimemanufacturing'and 'total quality control', when reproducibility of results becomes necessary, heat treatment processes and relevant equipment have become fully automated and CNC controlled. Ofall heat treatment operations quenching (especially immer sion quenching) is still the least controlled. Even in a fully automated and CNC controlled sealed batch furnace, where temperature-time cycles as well as the atmosphere can be automatically changed according to the requirements at the time, quenching intensity depends on the quenchant and quenching conditions selectedand nothingmuch can bedone duringquenchingtochangethe quenching intensity in order to obtain optimum structures, hardness and residual stresses. This is unfortunately so, because with immersion quenching the only quenching parameterwhichcan bechangedinthecourseofquenchingisthe rate ofagitation ofthe quenchant. This possibility is also limited to workpieces ofbigger cross sections (where the quenchingitselflastslonger), andcallsfor an adequatesensor which will automatically change the speed ofthe agitation pump exactly at the momentwhen acertaintemperaturehas been reached at the relevant pointin the cross-section of the workpieces treated. The best possible conditions for a fully automated CNC control of the quenching intensity exist with water-air spray quenching.' In order to predict the microstructure, the distribution of hardness (strength) andresidualstressesafterquenchingaswellas to optimisethe hardeningprocess, computer modelling is performed. The benefits of such optimised production process are as follows:" - Reducing scrap or rework rate. State ofthe Art in Quenching 3 - Reducing trial and errorin developing process parameters for new parts. - Allowing tightertolerancesto beachievedviabettercontroloftheheat-treating process. The mostcritical informationrelatingto the hardeningprocess isthe rate ofheat transferfrom the workpiece to the quenching medium. 2 IMMERSION QUENCHING Although quenching techniques, especially in recent decades, have become more and more diversified, immersion quenching stillconstitutes the largestpart ofall quenching processes. Immersionquenchingismostly performedinevaporablequenchantslikeoilsor water solutions, exhibitingthe three knownstages ofcooling, i.e.vapourblanket stage, boiling stage and convection stage. It is a non-stationary heat transfer process thatcan be described using the following equation, the left sideofwhich being Fourier's differential equation givingthe heatflowfrom the interiorto the surface oftheworkpiece, whiletheright sideisNewton'sequationforheattransfer from the workpiece surface to the ambience: aT q = - A.-a= oc (TN - Tamb) (1) XN where: q= heat fluxdensity, W m-2 A.=thermalconductivity within the workpiece, W/mK, aT a = temperature gradient normal to the surface of the workpiece, Kjm, XN oc=interfacial heat transfer coefficient,W m-2 K, TN= surface temperature ofthe workpiece, K, Tamb= ambienttemperature,i.e.temperatureofthequenchant,K. Inthe caseofquenchingrealpartsthere aremanyfactorswhichinfluencethisheat transfer/metallurgical transformation process: (a) Factors depending on the workpieceitself: alloy grade (transformation characteristics); mass ofthe workpiece (sizeof the critical cross-section); geometry (volume/surface ratio); surface roughness andcondition; load arrangement and density.* (b) Quenchant characteristics: density (molecular weight); viscosity; specificheat; thermal conductivity ofthe fluid; boiling temperature; 4 Quenching and Carburising Leidenfrost-temperature (transition temperature between the vapour blanket and the boiling stage); wetting property ofthe liquid. (c) Factors depending on quenching facility: Bath temperature;* Agitation rate;* Flow direction; Concentration of the solution (ifapplicable).* Of all these influencing factors listed, only a few can be changed in the heat treatment shop. They are marked with a * and are usually called technological parameters of quenching. Takinginto account the simultaneous influence ofso many factors during the quenching process, one logically wants to know what 'cooling power' or 'quenching intensity' will result in each specific case. There are actually three different reasonswhywewantto have aspreciseandcompleteinformationofthe quenchingintensity, as possible, namely: I. Checking the specific characteristics of the quenchant (e.g. for mutual comparison, monitoring ofdeterioration or development ofnew grades of quenchants). II. Selectionofoptimumquenchantand technologicalquenchingparametersfor the specifiedalloy and workpieces. III. Computer modelling ofresulting properties. On the other hand many methods exist for measuring and evaluatingquenching intensityandonehas to know whichmethodshouldbeappliedinthecasesofI,II or III. The IFHT Committee on Scientificand Technological Aspects of Quenching, established in 1978, has, from its very beginning, distinguished between a laboratory methodfor testingofthe coolingpower ofa quenchingmedium, anda practical method for the measurement ofthe quenching intensity in the quench tank, assuming that laboratory testing is carried out in a small quantity of quenchingmediumwithoutagitation, whilepracticaltestinginvolves all relevant technological parameters. 2.1 EVALUATION OF THE QUENCHING INTENSITY BY COOLING CURVES Ofallmethodsfor testingquenchingintensity,thosemeasuringthetemperatureas afunction of time (cooling curve) at a specifiedpointwithin the test specimen are mostly used. It seems also to be generally accepted to use test specimens of cylindrical shape, but unfortunately there is no general agreement as yet on specimen size, material and position of the thermocouple tip. In order to standardizeworld-widethe laboratory methodfor testing ofindustrial quenching oilsthe IFHT Committee on Scientificand Technological Aspects ofQuenching