GRINDING OF TOOL STEEL

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GRINDING OF TOOL STEEL

This information is based on our present state of knowledge and is intended to provide generalnotes on our products and their uses. It should not therefore be construed as a warranty ofspecific properties of the products described or a warranty for fitness for a particular purpose.Classified according to EU Directive 1999/45/ECFor further information see our “Material Safety Data Sheets”.Edition 7, Revised 12.2012The latest revised edition of this brochure is the English version,which is always published on our web site www.uddeholm.comSS-EN ISO 9001SS-EN ISO 14001

GRINDING OF TOOL STEELCONTENTSIntroduction4Grinding wheel design4How the grinding wheel works6The grinding machine9Grinding fluid9The tool steel10Recommendations for grinding ofUddeholm tool steel13Cutting speed and feed14Grinding wheel dressing15Examples of suitable grinding wheels15–173

GRINDING OF TOOL STEELGrindingwheel designIn principle, a grinding wheel consistsof the following components: Abrasive Binder Air poresBinderExample:AbrasiveGrain sizeA 46 H VGradeABRASIVEIt is important that the abrasive fulfilsrequirements in respect of: hardness sharpness thermal resistance chemical stabilityToday, the following four main groupsof abrasives (all synthetic) are used,fulfilling the above requirements togreater or lesser extents.1. Aluminium oxide designation A (SG)2. Silicon carbide designation C3. Cubic boron nitride designation B4. Diamond designation SDAbrasives have different applicationareas, depending on their particularcharacteristics, as shown partially inthe table below.AbrasiveABRASIVECertain special grinding wheels, suchas metallically bonded diamondwheels, contain no air pores.It is the composition and variationof the above components that determines the characteristic of a grindingwheel. An identification system, whichhas now been ratified as an interna-4ABRASIVENormalcorundumMixedcorundumRed aluminaWhite aluminaCOLOURPROPERTIESBrown, greyYellowbrownRedWhiteBinderAirporesFigure 1. The arrangement and proportionsof abrasives grains, air pores and bondbridges (made up of binder) determinegrinding wheel characteristics.The table below shows how thecharacteristics of aluminium oxideabrasive can be varied by alloying it.TougherThe high alloy content of tool steelmeans that such steel are often moredifficult to grind than conventionalstructural steel.In order to achieve successfulresults when grinding tool steel, it isnecessary to choose the grindingwheel with care. In turn, choosing theright grinding wheel and grinding datarequires an understanding of how agrinding wheel works.This brochure provides a quitedetailed description of the make-upof the wheel, of how it works whengrinding and of the parameters thatdetermine the final result. It alsoincludes recommendations forgrinding wheels for use with Uddeholm tool steel.tional standard by ISO, indicates thecomposition of grinding wheels. Theidentification consists of numeralsand letters in a particular sequence,defining the abrasive, grain size, gradeand ideCBNDiamondHARDNESSKNOOPTHERMALDURABILITYIN AIR C21002000250047007000120014006501. Aluminium oxide, is the abrasivemost commonly used for grindingsteel, and is available in several variants. It can be alloyed with otheroxides, of which the most common istitanium oxide.Unfortunately, the colour of a grinding wheel does not always necessarilyindicate the type of abrasive used init, due to the fact that some grindingwheel manufacturers colour theirabra-sives and binders.There is also another type of aluminium oxide named ceramic orsintered aluminium oxide. This abrasive has a fine crystalline structure,which means that the grains retaintheir sharpness better. However, itsuse requires higher grinding pressure.A typical application for it is grindingtool steel in rigid grinding machines.Examples of this type of abrasive areSG (Seeded Gel) from Norton andCubitron from 3M.2. Silicon carbide is an abrasive that isused primarily for grinding cast ironand austenitic stainless steel, althoughit can also be used for hardened toolsteel. It occurs in two main variants:the black silicon carbide and a somewhat harder green variant, which ismore brittle than the black material.3. Cubic Boron Nitride (CBN) is produced in approximately the same wayas synthetic diamond, and is an abrasive that is used primarily for grindinghardened high-carbide tool steel andhigh-speed steel. A drawback of CBNis its high price—almost twice that ofsynthetic diamond.4. Diamond is seldom used, despiteits high hardness, for grinding toolsteel as a result of its low thermalresistance. Diamond is used primarilyfor grinding cemented carbide andceramic materials.

GRINDING OF TOOL STEELABRASIVE GRAIN SIZEGRINDING WHEEL GRADEThe grain size of the abrasive is animportant factor in selecting thecorrect grinding wheel.Grain sizes are classified in accordance with an international mesh sizein mesh/inch, ranging from 8 (coarse)to 1200 (superfine). Grain sizes forgrinding tool steel are generally inthe range 24–100 mesh. Coarse grainsizes are used for rapid rate of removal, when grinding large workpieces,grinding softer materials or when thecontact surface of the grinding wheelis large. Fine grain sizes are used toproduce high surface finish, whengrinding hard materials or when thecontact surface of the grinding wheelis small.The surface smoothness of theground part depends not only on thegrain size of the grinding wheel. Thesharpness of the wheel, the bondingmaterial used and the hardness of thewheel also play a considerable part indetermining the surface finish produced.In the case of diamond and CBNgrinding wheels, European grindingwheel manufacturers indicate grainsize by the diameter of the abrasivegrains in microns, while American andJapanese manufacturers indicate it inmesh size.The grade of a grinding wheel refersto its hardness, i.e. how securely theabrasive grains are held by the binder.It does not, therefore, depend on thehardness of the abrasive used in thewheel.The grade of a grinding wheel isdetermined primarily by the quantityof binder used in the wheel. A higherproportion of binder reduces theamount of air pores and produces aharder wheel.The grade of a wheel is indicatedby a letter, indicating the hardness inalphabetical order:E very soft compositionZ very hard composition.Vitrified grinding wheels are thosemost commonly used for grindingtool steel.Resinoid is used as a binder ingrinding wheels intended for highperipheral speeds, such as certainCBN wheels.Rubber-bonded wheels are used forhigh specific grinding pressures, suchas for control wheels in centrelessgrinding.Metallic binders are used fordiamond and certain CBN wheels.Such wheels can withstand very highperipheral speeds.For tool steel, the most commonlyencountered compositions are withinthe hardness range G–K. Indication ofthe grade is sometimes followed by anumeral, which indicates the spreadof the abrasive particles in the wheel.GRINDING WHEEL BINDERSThe following binders are used tobind the grains in a grinding wheel: Vitrifieddesignation: V Resinoid,,B Rubber,,R Metal,,MThe photo shows the difference between a CBN wheel and a conventional grinding wheel.As a result of the high price of CBN, wheels made from it consist of a thin layer of abrasiveapplied to a central hub, usually of aluminium.5

GRINDING OF TOOL STEELHow the grindingwheel worksGrinding is a cutting process in whichthe cutting edges are formed by thegrains of abrasive. The same principles apply for grinding as for otherchip-cutting methods, although various factors mean that it is necessaryto consider the theory of grindingsomewhat differently.Conditions that are special forgrinding. The cutting tool has an irregularcutting geometry and the abrasivegrains are irregularly placed, whichmeans that cutting, ploughing andsliding will occur, see figure 2. The cutting geometry can change.The method of working of an abrasive tool includes a certain degreeof “self-sharpening”, which meansthat grains of abrasive break or arereplaced as they wear. Negative cutting angles. The irregular “blunt” shapes of the grainsmean that the rake angles are oftennegative.CuttingChipGrindingdirectionAbrasive grainWorkpieceGrindingdirectionPloughing A very large number of cuttingedges. Very high cutting speed. The mostcommon cutting speed for precision grinding, 35 m/s 2100 m/min.,is far above what is normal forother cutting processes. Very small chips, i.e. very smallcutting depth for each cutting edge.GRINDING FORCESThe grinding forces that act on eachindividual grain of abrasive are referred to as specific forces. A meanvalue of the specific forces can beobtained by dividing the total forceby the number of cutting edges,which depends on the size of thecontact area and the number of cutting edges in the grinding path. Thespecific forces determine variouseffects, including the degree of selfsharpening of the grinding wheel, i.e.its “working hardness”. The total forceis the force arising between thegrinding wheel and the workpiece.GRINDING WHEEL WEARThe grains of abrasive are initiallysharp, but in the same way as with allother cutting edges they wear downin use and become blunt. Finally, thegrains will have become so blunt thatthey have difficulty in penetrating intothe material of the workpiece. TheySmall chipcease to remove material and generate only heat. The grinding wheel isthen said to be burning the material,which can cause cracks in it.For a grinding wheel to workcorrectly, the stresses in the binderand the strength of the binder mustbe so balanced that, as the grainsbecome as blunt as can be accepted,they are pulled out of the binder andare replaced by new, sharp grains.The grinding wheel, in other words,sharpens itself. Self-sharpening alsooccurs through grain breakage, whichcreates new cutting edges.The degree of self-sharpening, i.e.whether the grinding wheel is hardor soft, is affected by the composition of the wheel (its design hardness) and by the conditions underwhich it is working.AVERAGE CHIP THICKNESSAlthough the chips removed bygrinding are small and irregular, themean value of their thickness at anytime is relatively constant. This valuevaries, depending on the type ofgrinding operation and in response tothe changes in grinding data.If a grinding wheel is cutting largerchips, this means two things:1. Higher loading on each cuttingedge, i.e. higher specific forces. Thisincreases the self-sharpeningcharacteristic of the wheel andLarge chipAbrasive grainWorkpieceGrindingdirectionSlidingLow forces onthe abrasive grainHigh forces onthe abrasive grainAbrasive grainWorkpieceFriction heatFine surfaceRough surfaceFigure 2. Different conditions during grinding (highly schematic). Cutting angles aregenerally negative.6Figure 3. A large chip size results in a rougher surface finish onthe workpiece.

GRINDING OF TOOL STEELgives it the characteristics of asofter wheel.2. The surface of the part beingground is coarser, see Figure 3.A reduction in the average chip thickness represents the opposite. It istherefore important to understandhow changes in grinding data andother conditions affect the averagechip thickness.STOCK REMOVAL RATEWhen grinding, the amount of chipsremoved per unit of time can mosteasily be expressed as mm3/s. This isoften referred to as the stock removal rate, and depends on the machine feed, the composition of thegrinding wheel, its cutting speed(peripheral speed) and (in certaincases) on the dimensions of theworkpiece.It is often more meaningful to talkabout stock removal rate rather thanabout table feed speed, feed depthetc., and it is also quite easy to calculate. Cost considerations often dictate that the stock removal rateshould be as high as possible. If thestock removal rate is increased without increasing the number of grainsof abrasive performing the work, e.g.by greater infeed depth, the chip sizewill also naturally be increased.CUTTING SPEEDThe peripheral speed of a grindingwheel has a direct effect on thenumber of cutting edges that actuallyperform the machining work. If, forexample, the cutting speed is doubled, twice as many grains of abrasivewill pass the workpiece per unit oftime. If the workpiece speed is notincreased, the mean chip thicknesswill decrease, thus also reducing thecutting forces on each grain. Selfsharpening will be less effective, i.e.the grinding wheel will be effectivelyharder, producing a finer surfacefinish, but with greater risk of burningthe surface.Conversely, reducing the speed ofthe wheel will increase the chipthickness, with the result that thegrinding wheel behaves as a softerwheel.Generally, both peripheral velocityand workpiece speed are increased inorder to increase the total rate ofremoval.THE G-RATIO OFA GRINDING WHEELThe G-ratio of a grinding wheel refersto the relationship between theamount of material removed and theamount of grinding wheel consumed.The G-ratio is a measure of howeffectively a grinding wheel workswith the particular workpiece material.If the wheel is to sharpen itself properly, it must be of a softer composition than one intended for externalcylindrical grinding of a similar part. Inthis latter case, the contact length isshorter, which means that there arehigher cutting forces on each grain.The contact width may be equal to thewidth of the grinding wheel as, forexample, in plunge grinding. Howeverin operations such as surface grindingwith a moving table, only part of theCylindrical grindingGRINDING WHEELCONTACT SURFACEIt is at the contact surface betweenthe grinding wheel and the workpiecethat the actual cutting operationoccurs. A large contact surface meansthat a greater number of cuttingedges participate in the process, thusreducing the chip size and specificforces. Similarly, a reduced contactsurface area results in greater chipsize and higher specific forces.In principle, the contact surface isin the shape of a rectangle. Its extentin the cutting direction is referred toas the contact length or contact arc,while its extent perpendicular to thecutting direction is referred to as thecontact width.The contact length depends primarilyon the type of grinding operation. Inaddition, it depends on the diameterof the grinding wheel, the cuttingdepth and in all cases—except forsurface grinding—the dimensions ofthe workpiece. Differences in thecontact length are the main reasonfor having to select different grindingwheel compositions for differentgrinding operations.If, when performing internal grinding, a grinding wheel is used that hasa diameter only a little less than thatof the ground hole, the contactlength will be very large, resulting inlow cutting force per grain.Surface grindingInternal grindingSegmental surfacegrindingFigure 4. Differences in contact length fordifferent grinding operations.7

GRINDING OF TOOL STEELgrinding wheel is actually cutting andthis part changes as the wheel wearsdown. It is sometimes possible toreduce the contact width, if this isrequired, by truing of the grindingwheel. This reduces contact surfacearea, resulting (as already described)in a greater chip thickness, higherloading on the abrasive grains and aneffectively softer grinding wheel.THE NUMBER OF CUTTINGEDGES IN THE CONTACT AREAThe number of cutting edges in thecontact area is a factor that has aconsiderable effect on the chipthickness and thus on the grindingprocess.A large number of cutting edgesper unit area mean that the work ofremoving material is spread over alarger number of grains, reducing thechip thickness and the specific forces.The grain size of the abrasive alsoaffects the number of cutting edges,which is the reason for the commonobservation that fine-grained cuttingwheels seem to be harder.DRESSING AND TRUINGGRINDING WHEELSDressing and truing of a grindingwheel are often considered to be thesame thing because they are oftenperformed as one operation.Truing is made to produce any profilewhich may be required on the faceof the wheel and to ensure concentricity.8Dressing is a conditioning of thewheel surface to give the desiredcutting action. Dressing the wheelexposes sharp cutting edges. One andthe same grinding wheel can be givencompletely different grinding characteristics through application of different dressing tools or different dressing methods. Dressing is therefore aparticularly important parameter inachieving good grinding performance.Dressing resulting in a smoothsurface on the wheel results in thecutting edges of the grains of abrasivebeing close together, while dressingresulting in a rough surface of thewheel gives the wheel a more openstructure. Dressing provides a meansof making the same grinding wheelgive completely different grindingresults.The degree of self-sharpeningaffects the structure of the grindingwheel surface, i.e. the number ofcutting edges per unit of area.A grinding wheel that has a high selfsharpening performance has a different, more open structure than onehaving poorer self-sharpening performance.There are many different tools available for dressing and truing grindingwheels, e.g. crushing rolls and diamond tools. CBN wheels are bestdressed using a diamond coatedroller.Certain types of grinding wheels, e.g.resinoid bonded CBN wheels, needto be “opened” after dressing. Thisreveals the abrasive particles andcreates space for chip formation. Inpractice this can be done by pushinga wet aluminium oxide stone into thewheel for a few seconds.

GRINDING OF TOOL STEELThe grindingmachineThe type of grinding operation andthe machine available has a considerable effect on the choice of appropriate grinding wheel composition.A grinding machine should be as rigidas possible, in order to allow it towork at high grinding pressures. Thisis because it is the rigidity of thegrinder and the method of clampingthe workpiece that determine thepermissible grinding pressure andtherefore restrict the choice ofwheels. If the machine is not sufficiently rigid, a softer grinding wheelcomposition or a smaller contactarea between the grinding wheel andthe workpiece should be chosen, inorder to achieve the required degreeof self-sharpening performance.The speed of the grinder alsoaffects the choice of grinding wheel.CBN wheels often require peripheralspeeds of 45 m/s in order to providegood cutting performance. Emulsions. These consist of waterwith an ad-mixture of 2–5% of oilin an extremely finely distributedform. Sulphur or chlorine additivesmay also be used as EP additives. Cutting oils. These are composed ofa mineral oil base with EP-typeadditives. Cutting oils provideeffective lubrication but poorercooling.Water solutions are most suitablewhen grinding with diamond wheels.Emulsions are used nowadays forthe majority of grinding operationsbecause they are ecologically beneficial and perform adequately.Cutting oils give the best resultsfor profile and plunge grinding withfine grained wheels, e.g. when grinding threads. Cutting oil also providesthe longest life for resinoid bondedCBN wheels, although high-oilemulsions are often chosen in theinterests of pollution reduction.Grinding fluidWhen grinding, as with all othercutting operations, a cutting fluid isused primarily to: cool the workpiece act as a lubricant and reducefriction between the chips, workpiece and grinding wheel remove chips from the contactareaFine gridning of detailsin hardened UdddeholmMirrax ESRThere are three main types of cuttingfluids that can be used when grinding. Water solutions. These are liquidsthat consist of water with syntheticadditives in order to increase itswetting performance and preventcorrosion. Such fluids contain no oiland provide good cooling performance but poorer lubrication performance.9

GRINDING OF TOOL STEELThe quantity and the size of carbidesin a steel has a very considerableeffect on the ease of grinding of thematerial. The greater the number of,and the larger the carbides, the moredifficult the material is to grind.This is the reason why tool steelproduced by powder metallurgyprocesses, having smaller carbides, iseasier to grind than a conventionallyproduced steel having a similar composition.10Hardness 2000Figure 5. Thehardness of grindingabrasives, basicphases found in a toolsteel and carbidesfound in tool steel.15001000DiamondCubic boron nitrideSilicon carbideAluminium oxideTitanium carbideTungsten carbideVanadium carbideNiobium carbideChromium carbideCementiteMolybdenum carbide0Austenite500MartensiteThe alloying constituents of a toolsteel have a considerable effect on itsease of grinding.The Uddeholm range of tool steelextends from low-alloy steel, such asUddeholm UHB 11, to high-alloysteel, such as Uddeholm Vanadis 10.There is seldom any problem ingrinding low-alloy tool steel. At theother end of the scale, however, thehigh-alloy carbide-rich steel can causeproblems when being ground, andrequire a careful choice of grindingwheel and operating parameters.The higher the wear resistance of asteel, the more difficult it is to grind.The wear resistance of a steel, andthus also its ease of grinding, aredetermined by its basic hardness andby the size, hardness and quantity ofthe carbides in it.In order to enhance the wearresistance of a tool steel, the steel isalloyed with carbide-forming alloyingelements, of which the most important are chromium and vanadium. Thesteel must also have a high carboncontent if carbides are to be formed.The diagram, Figure 5, shows thehardness of the basic phases found ina tool steel, the hardness of the mostcommon carbides found in tool steeland the hardness of commonly usedgrinding abrasives.As can be seen in the figure, it isonly diamond and CBN that areharder than all the carbides that arefound in a tool steel. However, asmentioned earlier, diamond is unsuitable for grinding steel.In practice, powder metallurgy isemployed to increase the quantity ofcarbide in a tool steel, i.e. such steelare more highly alloyed than conventional steel, which generally meansthat they are more difficult to grind.The effect of hardness on ease ofgrinding is also dependent on thequantity of carbide-forming alloyingelements in the steel.FerriteThe tool steelAs can be seen in Figure 6, hardnesshas a greater effect on grindability forhigh-carbide steel.Grindability index100AB10C10,1Figure 6. The effect of hardness ongrindability for:A – a low-alloy tool steel of Arne typeB – a material of Sverker typeC – material of Vanadis 10 type.

GRINDING OF TOOL STEELIn order to obtain good grindingperformance with high-alloy carbiderich tool steel, it is important toselect the correct grinding wheel.Materials in the Uddeholm Vanadisrange, for example, contain a largequantity of vanadium carbides. To cutthrough a vanadium carbide requiresan abrasive that is harder than aluminium oxide or silicon carbide.CBN wheels are therefore recommended as first choice for grindingthis material. The fact that, despitethis, material can be removed fromUddeholm Vanadis steel by grindingwith aluminium oxide or silicon carbide is due to the fact that it is thematerial enclosing the carbides that isground away, so that the carbides aretorn out of the basic material of thesteel. However, this occurs at theprice of high wear of the grindingwheel and a risk of poor grindingperformance.The formation of grinding cracks,which tend to occur perpendicular tothe direction of grinding, usuallymeans the tool has to be scrapped.Hardened steel are more sensitive togrinding cracks than non-hardenedsteel. A material that has been onlyhardened, and not tempered, mustnever be ground: hardened materialsshould always be tempered beforegrinding.Formation of grinding cracks can beexplained as follows:Almost all the energy used ingrinding is converted into heat, partlythrough pure friction and partly as aresult of deformation of the material.If the correct grinding wheel hasbeen chosen, most of the heat will beremoved in the chips, with only asmaller part heating up the workpiece.The diagram below shows thehardness profile through the surfaceof a tool steel, incorrectly ground insuch a way as to produce re-hardening.Hardness, HRC64605652480,100,200,300,400,50Depth below ground surface, mmFigure 7. Hardness profile through thesurface layer of an incorrectly ground tool.GRINDING CRACKS ANDGRINDING STRESSESThe wrong choice of grinding wheelsand grinding parameters results in aconsiderable risk of causing cracks inthe workpiece.Generally, grinding cracks are notas easy to see as in Photo 2. It isusually necessary to examine the partunder a microscope, or with magnetic powder inspection, in order tosee the cracks.Grinding cracks.Re-hardened layer in anincorrectly ground tool.Incorrect grinding of a hardened toolsteel can result in such a high temperature at the ground surface thatthe tempering temperature of thematerial is exceeded. This results in areduction in the hardness of the surface. If the temperature is allowed torise further, the hardening temperature of the material can be reached,resulting in rehardening. This produces a mixture of non-temperedand tempered martensite in the surface layer, together with retainedaustenite, as shown in Photo 3. Veryhigh stresses arise in the material,often resulting in the formation ofcracks.11

GRINDING OF TOOL STEELThe surface exhibits a high hardnessdue to the untempered martensite.An overtempered zone occurs justbelow the surface, where the hardness is lower than the basic hardnessof the workpiece.Incorrect grinding, resulting in amodified surface layer, often revealsitself through burn marks—discoloration of the ground surface. In orderto avoid burning and grinding cracks,it is necessary to keep down thetemperature of the ground part, e.g.by means of good cooling, and toemploy properly dressed grindingwheels that cut the material withsharp cutting edges instead of simplygenerating heat through friction.A simple example of how incorrectgrinding can cause cracks is shown inFigure 8. A hardened punch with ahead is to be cylindrical-ground, withthe head (b) being ground flat in thesame operation.Alternative A shows the use of agrinding wheel trued with a 90 edge.The grinding wheel, which is suitablefor cylindrical grinding of the surface(a), produces a good result on surface (a). Here the contact surface issmall so the self sharpening performance is good. The head, on the otherhand, which is to be ground flat,presents a larger contact surface tothe grinding wheel. The specificforces on the abrasive grains are lowso that the wheel does not selfsharpen. Instead, surface (b) is subjected mainly to rubbing and the heatgenerated can cause grinding cracks.Alternative B shows a better way togrind the punch. In this case, the sideof the grinding wheel has been truedas shown so that the contact surfaceat (b) is smaller. This results in improved self-sharpening and “cooler”grinding.Case C shows the preferred way togrind this part. The grinding wheel isset at an angle, so that the two contact surfaces are of approximately thesame size.The retained austenite content of ahardened material can also affect thegrinding result. High retained austen12ite levels increase the risk of crackformation when grinding.The majority of grinding operationsleave residual stresses in the groundsurface. These stresses are usually ata maximum close to the surface, andcan cause permanent deformation ofthe ground part when grinding thinmaterials.Of the three examples shown inFigure 9, Example 1 is most at risk inrespect of crack formation. It exhibitstensile stresses in the surface whichcan, if they exceed the material’sultimate tensile strength, result in thematerial cracking.Examples 2 and 3 are not as dangerous—the surface stresses are compressive stresses, which result inimproved fatigue strength of theground parts.It is, unfortunately, very difficult toproduce a simple check to determinethe stress pattern set up in theground part unless the stresses areso high that grinding cracks are visible.Grinding stresses can be reduced bystress-relief tempering after grinding.The tempering temperature shouldbe about 25 C below the previoustempering temperature in order toavoid any risk of reducing the hardness of the workpiece.Another way of reducing grindingstresses is to tumble or blast theground parts.Aba Example 1BBetterb–Depth below the surface aExample 2TensionDepth below the surfaceCompression–CExample 3Bestb Depth below the surfacea–Figure 8. Incorrect grinding can often resultin grinding cracks.Figure 9. Three typical examples of stressdistribution in a ground surface.

GRINDING OF TOOL STEELRecommendationsfor grinding ofUddeholm tool steelGRINDING OFHIGH-CARBIDE TOOL STEELThe high carbide content of highcarbide tool steel gives them excellent wear resistance, and requirespecial recommendations in respectof grinding operations and selectionof grinding wheels. For the majorityof grinding operations, CBN wheelsare the best choice for such steel.There are two different types ofcarbide rich tool steel, conventionallymade steel and powder steel. Themain differences that affect the grinding properties are the hardness, sizeand distribution of carbides, seeFigure 10 below. Powder steel, such as UddeholmElmax, Uddeholm Vanadis andUddeholm Vancron, have in spite ofthe high alloying level relativelygood grinding properties due tothe small carbide/nitro carbide size.The small carbides will give thegrinding wheel good self-sharpeningproperties. Conventionally made steel, such asUddeholm Rigor, UddeholmSleipner and Uddeholm Sverker,have not so good self-sharpeningproperties as powder steel due tothe bigger carbide size. However,the lower carbide hardness andcarbide content will compensatefor the grinding properties.Figure 11 shows the results of surface grinding trials on UddeholmVanadis 10 with aluminium oxide, finecrystalline aluminium o

difficult to grind than conventional structural steel. In order to achieve successful results when grinding tool steel, it is necessary to choose the grinding wheel with care. In turn, choosing the right grinding wheel and grinding data requires an understanding of how a grinding wheel works

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