hardened steel grinding replacement by hard turning technology

1.JOZEF MAJERÍK, 2. NINA DANIŠOVÁHARDENED STEEL GRINDING REPLACEMENT BY HARDTURNING TECHNOLOGYAbstract:This article solves the problem of functional surfaces creation machined without and with coolantafter mixed cubical boron nitride application. Workpiece material is tool steel C120U. The hardnessvalue is within HRC = 49÷62. Various cutting speeds at constant feeds and depths of cut are chosenin depending on hardness in turning. The research article studies quality ratios according machinedsurface integrity is considered:• Microgeometry of machined surface,• The changes of physical-mechanic properties in surface layer,• Microstructure of the surface layers,• Physical-chemical condition of surface functionality,• Residual stresses beneath the machined surface,• Tribological characteristics of the surface functionality.These ratios creates the influence of fatigue strength assumption against wear, anti corrosion stability,fit quality etc.Keywords:Hard turning, cubical boron nitride, grinding, surface integrity, residual stresses beneath the surface,surface microgeometry, microstructure of surface layers, surface functionality, tool wearINTRODUCTIONThe assignment of surface integrity investigationis new theory creation in term of new trends intechnological praxis. Its very important toimprove surface functionality of machined partsin qualitative aspect. The hard turningreplacement is also possible to apply instead ofroug and finish grinding. Tard turningtechnology requires stabile and solid machinetool with minimal allowances in guiding and lastbut not least techological system Machine Tool –Cutting Tool – Workpiece – Fixture stiffness. Itwas applied the polycrystalline cubical boronnitride CBN, as cutting material which isadvisable to hard turning. Composition of CBNconsists of BN (50 ÷ 75%) and ceramics binder.Hard turning technology of hardened parts isvery useful, because maching time is reallyshorter, lower invested costs, lower amount ofoperating sections and integrated shape.Workpiece geometry is realized for oneclamping. Minimal charges for ecology – drymachining.THE EXPERIMENTAL PROBLEM PURPOSEThe main purpose of this experimental methodwas considered surface layers aspect ofspecimen samples made of C120U hardenedsteel. All samples were manufactured by dryhard turning, grinding and turning with coolantin term of surface integrity. C120U steel isalloyed tool steel with medium capacity of© copyright FACULTY of ENGINEERING – HUNEDOARA, ROMANIA 85

ACTA TECHNICA CORVINIENSIS – BULLETIN of ENGINEERINGCarbon. This type of steel was chosen asa compared material, by reason to reach highhardness after hardening (64 HRC) and which isthen tempered for 60±2 HRC. C120U achievesgood core toughness, worse heat texturization,good machinability and hardening disruptioninsensibility.In experimental process were evaluatedfollowing aspect of machined surface by hardturning and grinding on specimen samples:1. microgeometry of machined surface (Ra, Ry,Rz, Rq), machine SURFTEST SJ-301,2. The change of physical-mechanic surfacelayer properties (hardness HV0,1 a HV0,2) onLECO machine.3. Surface layer microstructure, at devicesOLYMPUS IX70 and NEOPHOT 32,4. Residual stress of I.st. type measurementbeneath the surface on HZG 4 apparatus.5. Machined surface wearing on tribologicalapparatus.0,2 mm, v c = 120 m.min -1Grinding parameters:n o = 155 min -1 , v o = 0,23 m.s -1 , v k = 30,6 m.s -1n k = 1670 min -1a p1 = 0,05 mm (roughing), a p2 = 0,01 mm(finishing)Grinding wheel: ø350 x 40/127 – A9960K9V,coolant E5% emulsion (EMULZIN-H)METALOGRAPHIC EVALUATIONThe microsturcture of surface layers evaluationwas executed on OLYMPUS IX7 microscope. Youcan see in all microstructures of dry turnedhardened steels thin white layer over the wholesurface with 0,002 ÷ 0,003 mm width. See onFig.2. Under the white surface layer can seeinfluenced area by cutting temperature. Its darklayer of temperable martensite with 0,002 ÷0,004 mm width.The next layer is situated bold martensite withresidual austenite and metal carbides. The samewhite laeyr can see also in hard turned samplewith coolant, but on the grinded sample cannotsee this, or is very thin and very unstable. It iscaused by grinding technology and very highcutting conditions during grinding.The microstructure of grinded surface shows usthe high tempered area. This argument is alsoconfirmed by HV0,2 microhardness measured onC120U steel after grinding.FIG. 1: Hard turning process with CBN insert atHARRISON ALPHA 1400 S machine toolHARD TURNING AND GRINDING PARAMETERSMachine tool: HARRISON ALPHA 1400S,Tool holder: DCLNR 2525 M12 – T – MAX – P(Sandvik COROMANT),Cutting tool: VRP – CNGA 120408 T01020 –cubical boron nitride 7020 (SandvikCOROMANT),Clamping: workholding device in universalchuck, upbear by centre, sample clamping forø20H8 an to face side by nut and washer,Coolant: emulsion E5% or without coolant(DASCOL 2500)Hard turning parameters: C120U without andwith coolant (G) HRC = 60±2n = 1345 min -1 , f = 0,15 mm, a p = 0,17 ažFIG. 2: Microstructure of C120U hardened steelafter hard dry turning process – see continuouswhite layer of residual austenite on samplesurface. White layer width is 0,002 ÷ 0,003 mm.Influenced area is situated under white layer (seedark cauterized layer) up until 0,004 mm.(tempering martensite)862010/Fascicule 3/July­September/Tome III

ACTA TECHNICA CORVINIENSIS – BULLETIN of ENGINEERINGTRIBOLOGICAL CHARACTERISTICS MEASUREMENTThe experimental samples made of heattreatment materials hard dry turned, hardturned with coolant and grinded were realizedon tribological apparatus. Time duration of onetribological testing was designated to 300 min.Testing tribological arrangement continuallyevaluated reaction force and temperature infrictional event. We have emanated fromfriction ratio and driving force from frictionalevent construction during the calculations.Experiment approved that whereby the partsurface is harder, therethrough wear is reallyslower.profile or surface influencing during the cuttingprocess. Its stabilization let you say smalltempering after hard turning, or big tempering ofsurface layers after grinding. It is also the surfacehardness.FIG. 4: Measured values of radial wear resultsCONCLUSIONSFIG.3: Tribological arrangement for radial weartestingFrom individual tribological characteristics ofindividual hardened steels measurement andfrom graphs comparison comes the followingknowledges:6. radial wearing ΔD of measured samples isalways the biggest after grinding, because itsdone by little bigger surface roughness ofgrinded surfaces and heat influence whereoccurs to martensitic structure tempering.That is different at hard turned surfaces,7. there is not difference between dry hardturned and hard turned with coolantsamples in radial wear,8. There are the biggest friction force andfriction ratio at grinded surfaces.From experimental measurement results thatsurface wearing in tribosystems influencesmicrogeometry and material surface averageIt was appreciated surface layer aspect ofexperimental samples made of hardened C120Usteel hard turned with CBN and grinded.Experimental measurement were accomplishedby microgeometry, microhardness,metalographic evaluation of surface layers andalso tribological characteristics. These aspects ofsurface integrity are very important for surfacefunctionality and operation durability ofmachined parts. Machined surface, itsinvestigation, evaluation and technologicalprocess improvement creates important way tomake better quality of production process.REFERENCES[1.] MAJERÍK, J.: Náhrada brúsenia kalenýchocelí tvrdým sústružením. In: Strojárstvo/Strojírenství – Mechanical EngineeringJournal, ISSN 1335-2938. - Roč. 12, č.5(2008), s. 170-171,180.[2.] MAJERÍK, J., PAVLÍK, P.: Overenie možnostináhrady brúsenia tvrdým sústružením sPKNB z pohľadu integrity povrchu, In:Funkčné povrchy 2006 [zborník] : Zborníkprednášok z medzinárodnej vedeckejkonferencie. - ISSN 1336-9199. - Trenčín. -277 s. - ISBN 80-8075-137-4. - s. 121-128.[3.] ZGODAVOVÁ, K., MAJERÍK, M.: MechatronicProduct Proporcionality and Inter-2010/Fascicule 3/July­September/Tome III 87

ACTA TECHNICA CORVINIENSIS – BULLETIN of ENGINEERINGChangeability Management: MechanicalComponents, November 25-28,2009, In:Annal of DAAAM for 2009 & Proceedings ofthe 20th International DAAAM SymposiumIntelligent Manufacturing & Automation:Focus on Theory, Practice and Education :World Symposium. - ISSN 1726-9679. -Vienna: Vienna University of Technology,2009. - ISBN 978-3-901509-70-4. - p.1933-1934.AUTHORS & AFFILIATION1.JOZEF MAJERÍK,2.NINA DANIŠOVÁ1.DEPARTMENT OF ENGINEERING TECHNOLOGY ANDMATERIALS, FACULTY OF SPECIAL TECHNOLOGY,ALEXANDER DUBCEK UNIVERSITY IN TRENČÍN,SLOVAKIA2.INSTITUTE OF MANUFACTURING SYSTEMS ANDAPPLIED AND MECHANICS, FACULTY OF MATERIALSSCIENCE AND TECHNOLOGY,STU TRNAVA,SLOVAKIAACTA TECHNICA CORVINIENSIS– BULLETIN of ENGINEERINGISSN: 2067-3809 [CD-Rom, online]copyright ©University Politehnica Timisoara,Faculty of Engineering Hunedoara,5, Revolutiei,331128, Hunedoara,ROMANIAhttp://acta.fih.upt.ro882010/Fascicule 3/July­September/Tome III