EDM OF TOOL STEEL

SS-EN ISO 9001SS-EN ISO 14001

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 3, 08.2007The latest revised edition of this brochure is the English version,which is always published on our web site www.uddeholm.com

EDM OF TOOL STEEL

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ContentsIntroduction ............................................ 3

The basic principles of EDM ................ 4

The effects of the EDM processon tool steels .......................................... 4

Measuring the effects ............................ 6

Achieving best tool performance ....... 9

Polishing by EDM .................................. 11

Summary ................................................. 11

EDM OF TOOL STEEL

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Fig.1. A “rough-machined”EDM surface with a crosssection through chips andcraters. Material: UddeholmOrvar 2 Microdized.

The effects ofthe EDM processon tool steelsThe influence of spark erosion onthe machined material is completelydifferent to that of conventionalmachining methods.

As noted, the surface of the steelis subjected to very high tempera-tures, causing the steel to melt orvaporize. The effect upon the steelsurface has been studied by Udde-holm Tooling to ensure that thetool maker may enjoy the manybenefits of the EDM process, whileproducing a tool that will have asatisfactory production life.

In the majority of cases, it hasbeen impossible to trace any influ-ence at all on the working functionof the spark-eroded tool. However,it has been observed that a trim-ming tool, for example, has becomemore wear resistant, while in somecases tool failure has occurred pre-maturely on changing from conven-tional machining to EDM. In othercases, phenomena have occurredduring the actual electrical dischargemachining that have caused un-expected defects on the surface ofthe tool. This due to the fact thatthe machining has been carried outin an unsuitable manner.

The basicprinciples of EDMElectrical discharge machining (sparkerosion) is a method involving elec-trical discharges between an anode(graphite or copper) and a cathode(tool steel or other tooling mate-rial) in a dielectric medium. Thedischarges are controlled in such away that erosion of the tool orwork piece takes place. During theoperation, the anode (electrode)works itself down into the work-piece, which thus acquires the samecontours as the former.

The dielectric, or flushing liquidas it is also called, is ionized duringthe course of the discharges. Thepositively charged ions strike thecathode, whereupon the tempera-ture in the outermost layer ofthe steel rises so high (10–50,000°C/18–90,000°F) as to cause the steelthere to melt or vaporize, formingtiny drops of molten metal whichare flushed out as “chippings” intothe dielectric. The craters (andoccasionally also “chips” which havenot separated completely) are easilyrecognized in a cross section of amachined surface. See figure 1.

Four main factors need to be takeninto account when considering theoperating parameters during anEDM operation on tool steel:

• the stock-removal rate

• the resultant surface finish

• electrode wear• the effects on the tool steel.

The influence of the EDM operationon the surface properties of themachined material, can in unfavour-

IntroductionThe use of Electrical DischargeMachining (EDM) in the productionof forming tools to produce plasticsmouldings, die castings, forging diesetc., has been firmly established inrecent years. Development of theprocess has produced significantrefinements in operating technique,productivity and accuracy, whilewidening the versatility of the pro-cess.

Wire EDM has emerged as anefficient and economic alternativeto conventional machining of aper-tures in many types of tooling, e.g.blanking dies, extrusion dies and forcutting external shapes, such aspunches.

able circumstances jeopardize theworking performance of the tool.In such cases it may be necessary tosubordinate the first three factors,when choosing machining para-meters, in order to optimize thefourth.

Special forms of EDM can now beused to polish tool cavities, produceundercuts and make conical holesusing cylindrical electrodes.

EDM continues to grow, there-fore, as a major production tool inmost tool making companies,machining with equal ease hardenedor annealed steel.

Uddeholm Tooling supplies a fullrange of tool steels noted for con-sistency in structure. This factor,coupled with very low sulphur lev-els ensures consistent EDM per-formance.

This brochure gives information on:

• The basic principles of EDM

• The effects of the EDM pro-cess on tool steels

• Achieving best tool performance

EDM OF TOOL STEEL

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Fig. 2. Section from a spark-machined surface showing changes in structure.Material: Uddeholm Rigor, hardened to 57 HRC.

Fig. 3. Pillar crystals formed duringsolidification.

“Surface strength”—an important factorAll the changes that can be ob-served are due to the enormoustemperature rise which occurs inthe surface layer.

In the surface layer, it has been ob-served that the four (main) factorsassociated with the all-important“surface strength” of the steel areaffected by this temperature in-crease:• the microstructure• the hardness• the stress condition• carbon content.

Figure 2 shows a section from anormal rough-spark-machinedsurface with the typical, differentstructural changes.

Melted and resolidified layerThe melted and resolidifiedlayer produced during the EDMprocess is also referred to as the“white zone”, since generally noetching takes place in these areasduring metallographic preparation.Figure 3, nevertheless, shows clearlythat it is a rapidly solidified layer,where long pillar crystals havegrown straight out from the surfaceof the metal during solidification.A fracture occurring in this layer

Rehardened layerIn the rehardened layer, the tem-perature has risen above the auste-nitizing (hardening) temperatureand martensite has been formed.This martensite is hard and brittle.

1000 x

Tempered layerIn the tempered layer, the steelhas not been heated up so much asto reach hardening temperature andthe only thing that has occurred istempering-back. The effect naturallydecreases towards the core of thematerial – see the hardness curve infigure 2.

In order to study the structuralchanges incurred with differentmachining variables, different toolsteels—see table 1—were “rough-machined” and “fine-machined” withgraphite electrodes.

Typical hardness distributionin the surface layer

200 X

400 600 800 1000

Hv

Melted andresolidified layer

Tempered layer

Unaffected matrix

Rehardened layer

invariably follows the direction ofthe crystals. In normal roughmachining, this layer has a thicknessof about 15–30 µm.

The carbon content in the surfacelayer can also be affected, for in-stance, by carburization from theflushing liquid or from the elec-trode, but decarburization can alsooccur.

EDM OF TOOL STEEL

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Austenitizing, time 20 min Tempering, time 2 x 30 min Hardness

Uddeholm Temperature Temperature Hardened Annealedsteel grade AISI °C °F °C °F HRC HB

ARNE O1 810 1490 220 430 60 190CALMAX – 960 1760 200 392 58 200 RIGOR A2 940 1725 220 430 60 –SVERKER 21 D2 1020 1870 250 480 60 220IMPAX SUPREME P20 850 1560 580 1075 30 –ORVAR SUPREME H13 1025 1875 560 1040 50 180

Measuringthe effectsThe thicknesses of the heat-affected zones have been measured.The hardnesses in these zoneshave also been measured, as havecrack frequencies and crackdepths. Strength values havebeen obtained through bendingtests.

The layer thicknesses appear tobe largely independent of both steelgrade and electrode material. Onthe other hand, there is a definitedifference between the specimenswhich have been hardened andthose which were in the softanneal-ed condition. Figure 4 shows, in theform of graphs, the layer thicknessesand fissure frequency with differentpulse durations for UddeholmOrvar Supreme.

In the annealed material, the zonesare thinner and the fissures fewer.The brittle, hardened zone is scar-cely present at all (figure 4b).

The layer thicknesses can varyconsiderably, from 0 µm to maxi-mum values slightly below the Rmax

specified in the machining directions.In the rough-machining stages(t

i ≥100µ sec), the thicknesses of the

layers vary far more substantiallythan in the fine-machining stages.

The thickness of both the meltedand the hardened zone increaseswith spark duration, which appearsto be the most important singlecontrolling variable. Figure 5 shows

Fig. 5. Fine-machined UddeholmRigor, pulse duration 10µ sec.

Table 1. The tool steels were tested in the hardened and tempered condition, and some of themalso in the annealed condition.

Note: As Uddeholm Corrax isa precipitation hardening steelthe EDM surface has differentcharacteristics. The “whitelayer” consists of melted andresolidified material with ahardness of approx. 34 HRC.There will be no other heataffected zone of importance.

100 200 500 1000 ti µ sec

5 19 15(A) – – –(B) – – –(C)

Melted zone

Hardended zone

Matrix

No. of cracks per cm: (A) in melted zone (B) in hardened zone (C) in matrix

Graphite electrode

Fig. 4b. As above, but forelectrical discharge machining ofUddeholm Orvar Supreme inthe annealed condition.

60

40

20

0

Thickness µm

Melted zone

Hardended zone

Matrix

100 200 500 1000 ti µ sec

21 25 43(A) – – 3(B) – – –(C)

No. of cracks per cm: (A) in melted zone(B) in hardened zone(C) in matrix

80

60

40

20

0

Graphite electrode

Thickness µm

the beneficial effect of “fine-finish-ing”, i.e. to produce a very thin re-melted and heat-affected zone.

100 x

Fig. 4a. Layer thicknesses andfissure frequency in the surfacelayer in electrical dischargemachining of hardened (52 HRC)Uddeholm Orvar Supreme atdifferent pulse durations.

EDM OF TOOL STEEL

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Figur 8. A section through one of thesuspected “pores”

Figur 6d. Copper electrode

ti = 200 µs. Magnification 500 x

Figur 6e. Graphite electrode

Figur 6b. Graphite electrode

Figur 6c. Graphite electrode

ti = 100 µs. Magnification 500 x

Figur 6a. Copper electrode

ti = 10 µs. Magnification 500 x

ti = 10 µs. Magnification 500 x

ti = 500 µs. Magnification 500 x

Figur 7. The suspected “pores” canbe seen on the surface of the tool

Structures ofspark-machined layers

65 x

1:1

With longer pulse duration, theheat is conducted more deeply intothe material. Higher current inten-sity and density (and thus sparkenergy) do, indeed, give a higher“amount of heat” in the surface, butthe time taken for the heat to dif-fuse, nevertheless, appears to havethe greatest significance. The pic-tures below show how the surfacezones are changed in UddeholmSverker 21 (in hardened and tem-pered condition) with differentpulse durations and electrode mate-rials.

The cause of “arcing”Short off-times, or pause times, givemore sparks per unit of time andthus more stock removal. Duringthe off-time, the dielectric fluid

must have time to become de-ionized. Too short an off-time canresult in double sparking “ignitions”which lead to constantly burningarcs between the electrode and thework piece, resulting in serioussurface defects. The risk of arcing isincreased if flushing conditions forthe dielectric fluid are difficult.

As a result of “arcing”, i.e. a con-dition in which arcs are formedbetween local parts of the elec-trode and the workpiece, large cra-ters or “burns” are formed in thesurface. These have frequently beenconfused with slag inclusions orporosity in the material. Figures 7and 8 show the surface of a toolwith a section through one of thesuspected “pores”.

One of the primary causes of thistype of defect is inadequate flushing,or machining of narrow slots, etc.,resulting in chips and other looseparticles forming a bridge betweenthe electrode and the workpiece.The same effect can be obtainedwith a graphite electrode whichbears traces of foreign material.On modern machines featuring so-called adaptive current control, therisk of “arcing” has been eliminated.

EDM OF TOOL STEEL

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HV

1000

800

600

400

200

0

Graphite electrodeti = 200 µ sec

Fig. 9. Typical hardness distribu-tion in hardened UddeholmSverker 21 immediately after EDMand then after re-tempering.

The difference in stock-removalrate amounts to a maximum ofapprox. 15% between the differentgrades of tool steel with the samemachine setting data.

The hardnesses in the differentlayers can also vary considerably,but in principle the same patternapplies to all grades. Figure 9 showsa typical hardness distribution.

Fissure frequencyalso increases with pulsedurationWith times in excess of 100µ sec,all steels reveal several cracks inthe melted layer. High-carbon and/or air-hardening steels show thehighest frequency of fissures. Theannealed specimens contain nocracks at all in the matrix.

The number of cracks which con-tinue down into the hardened zoneis roughly 20%, while only a veryfew cracks penetrate into the ma-trix. In the matrix, the fissure depthis seldom more than about sometens of a µm. Here too, it appliesthat cracks in the matrix are mainlyencountered in the highly-alloyedcold-working steels. Table 2 showsthe occurrence rate of fissures in anumber of tested tool steels.

0 50 100 150 µm

Table 2. The table shows the occurrence rate of fissures.

The difference in hardness and vol-ume between the layers gives riseto stresses which, upon measure-ment, have been found to have thesame depth as the affected surfacelayers. These stresses can be sub-stantially reduced by extra heat-treatment operations.

Renewed tempering (235°C/455°F 30 min) of the specimen infigure 9 resulted in lowering of thehardness level to the curve drawnwith a broken line.

If electrical discharge machining isproperly performed with a finalfine-machined stage, surface defectsare largely eliminated. If this is notpossible for one reason or another,or if it is necessary for all effects to

be eliminated, some differentrelated operations can be used:

• Stress-relief tempering at atempering temperature approx.15°C (30°F) lower than thatpreviously used tempering tem-perature, lowers the surfacehardness without influencing thehardness of the matrix.

• Grinding or polishing will re-move both the surface structureand cracks, depending of courseon how deeply it is done (approx.5–10 µm in fine-machining).

Hardness immediatelyafter EDM

Hardness after re-tempering

Melted Hardenedzone zone Matrix

High-alloy cold-work steelUDDEHOLM SVERKER type 20–50 2–10 0–5

Hot-work steelUDDEHOLM ORVAR type 10–40 2–5 0–2

Cold-work steelsUDDEHOLM RIGOR andUDDEHOLM ARNE types 10–30 0–5 0–2

Plastic-moulding steelUDDEHOLM IMPAX SUPREME type 0–5 0–2 0

EDM OF TOOL STEEL

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Achieving besttool performanceEDM using solid electrodes(copper/graphite)As noted, in most cases where theEDM process has been carefullycarried out no adverse effect isexperienced on tool performance.As a precautionary measure,however, the following steps arerecommended:

EDM OF HARDENED ANDTEMPERED MATERIAL

Bending testTo evaluate the likely effect of theremelted layer, surface irregularitiesand cracks produced in the EDMprocess on the strength of a tool, abending test was carried out. Vari-ous combinations of EDM surfacefinish and post treatments, e.g.stress-relieving/polishing, weretested on 5 mm square test piecesof Rigor at 57 HRC. The test pieceswere spark-machined on one faceto different EDM stages and bentseverely, with the EDM surface onthe outside of the bend.

Figure 10 shows that the samplewith a fine-spark machined finishwhich had been polished afterwardsgave the best result. The roughspark-machined sample, without anypost treatment, had the lowestbending strength.

Background tothe bending test resultsThe hard, re-solidified rehardenedlayers cause, in the first instance,those cracks which are formedupon application of the load and inthe second instance those whichwere already present to act as initia-tors of failure in the matrix. At57 HRC, the matrix is not toughenough to stop the cracks fromgrowing and consequently the fail-ure occurs already on the elasticpart of the load curve. Normally,there should have been a certainamount of plastic bending of a testbar in this material.

Bending strengthN/mm2

1200

1100

1000

900

800

700

600

500

400

300

200

100

0

A Conventional machining

B Hardening and tempering

C Initial EDM, avoiding “arcing” andexcessive stock removal rates.Finish with “fine-sparking”, i.e. lowcurrent, high frequency.

D (i) Grind or polish EDM surface

or D (ii) Temper the tool at 15°C(30°F) lower than the originaltempering temperature.

or D (iii) Choose a lower startinghardness of the tool toimprove overall toughness.

A Conventional machining

B Initial EDM, as C above.

C Grind or polish EDM surface.This reduces the risk of crackformation during heating andquenching. Slow pre-heating, instages, to the hardening tempera-ture is recommended.

Fig. 10. Bending strength at different EDM stages and withdifferent subsequent operation. Material Uddeholm Rigor57 HRC. The shaded areas show the spread of the resultsmeasured.

Note: When EDM’d in solutionannealed condition the toughness ofUddeholm Corrax is not affected.

It is recommended that allEDM’ing of Uddeholm Corrax isdone after aging since an aging afterEDM’ing will reduce the toughness.

It is recommended that the“white layer” is removed by grind-ing, stoning or polishing.

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EDM OF ANNEALED MATERIAL

EDM OF TOOL STEEL

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Wire EDMThe observation made about theEDM surface in earlier pages arealso mostly applicable to the wireEDM-process.

The affected surface layer, how-ever, is relatively thin (<10 µm) andcan be compared more to “fine-sparking” EDM. Normally there areno observable cracks in the erodedsurface after wire erosion. But incertain cases another problem hasbeen experienced.

After heat treating a throughhardening steel the part containshigh stresses (the higher the tem-pering temperature, the lower thestresses).

Fig. 11. Wire erosion of a hardened and temperedtool steel blanking die.

Fig. 13. Pre-drilled holes connectedby a saw-cut, before hardening andtempering, will help to preventdistortion or cracking when wireeroding thick sections.

Fig. 12. This block of D2 steel, approx.50 x 50 x 50 mm (2" x 2" x 2"), crackedduring the wire EDM operation.

These stresses take the form oftensile stresses in the surface areaand compressive stresses in thecentre and are in opposition to eachother. During the wire erosion pro-cess a greater or lesser amount ofsteel is removed from the heat-treated part. Where a large volumeof steel is removed, this can some-times lead to distortion or evencracking of the part. The reason isthat the stress balance in the part isdisturbed and tries to reach anequilibrium again. The problem ofcrack formation is usually onlyencountered in relatively thick crosssection, e.g. over 50 mm (2") thick.With such heavier sections, correcthardening and double tempering isimportant.

In certain cases the risk can bereduced through different pre-cautions.

1: To lower the overall stress levelin the part by tempering at a hightemperature. This assumes the useof a steel grade with high resistanceto tempering.

2: By drilling several holes in thearea to be removed and to connectthem by saw-cutting, before harden-ing and tempering. Any stressesreleased during heat treatment arethen taken up in the pre-drilled andsawn areas, reducing or eliminatingthe risk of distortion or crackingduring wire-erosion. Fig. 13 illu-strates how such pre-cutting maybe done.

EDM OF TOOL STEEL

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Fig. 14. This Uddeholm Stavax ESR mould insert was finishedby EDM “polishing”.

Polishing by EDMToday some manufacturers of EDM-equipment offer, by a special tech-nique, possibilities to erode veryfine and smooth surfaces. It is pos-sible to reach the surface finish ofabout 0,2–0,3 µm. Such surfaces aresufficient for most applications. Thegreatest advantages are when com-plicated cavities are involved. Suchcavities are difficult, time consumingand therefore expensive to polishmanually, but can be convenientlydone by the EDM- machine during anight-shift, for example.

Investigations made on our gradesUddeholm Impax Supreme, Udde-holm Orvar Supreme, UddeholmStavax ESR and Uddeholm Rigorshow that the hard re-melted white

Wire erosionof cutting punchesWhen producing a cutting punchby wire erosion, it is recommended(as with conventional machining) tocut it with the grain direction of thetool steel stock in the direction ofthe cutting action. This is not soimportant when using PM steelsdue to their non-directional grainstructure.

SummaryIn summing up it can be said thatproperly executed electrical dis-charge machining, using a rough anda fine machining stage in accordancewith the manufacturer’s instruction,eliminates the surface defects ob-tained in rough machining. Naturally,certain structural effects will alwaysremain, but in the vast majority ofcases these are insignificant, pro-vided that the machining processhas otherwise been normal. Struc-tural effects, more-over, need notnecessarily be regarded as entirelynegative. In certain cases the surfacestructure, i.e. the rehardened layer,has—on account of its high hard-ness—improved the resistance ofthe tool to abrasive wear. In othercases it has been found that thecratered topography of the surfaceis better able to retain lubricantthan conventional surfaces, resultingin a longer service life. If difficulties

in connection with the workingperformance of spark-machinedtools should arise, however, thereare some relatively simple extraoperations that can be employed, asindicated above.

A slightly striped appearance hasbeen re-ported in materials rich incarbides, such as high-carbon cold-work steels and high-speed steels,where there is always a certainamount of carbide segregation or inmaterial with high sulphur content.

The difference in bending strengthbetween rough-spark-machined andfine-spark-machined test pieces islargely due to the difference in thedistribution of the cracks and to thepresence of the in spots distributedwhite layer on the fine-spark-machined specimens. The roughersurface finish of the rough-machinedspecimen has not really been signi-ficant. Regardless of circumstances,such surface irregularities are rela-tively harmless as crack initiatorscompared with the solidificationcracks. During the polishing of thefine-machined test piece which wascarried out, the depth of the whiteand rehardened layer was merelyreduced and not completely elimi-nated. Further polishing wouldprobably result in complete restora-tion of the bending strength.

Highly stressed tools and partsthereof, e.g. very thin sections thatare far more liable to bending, canjustify an extra finishing operation.

The lower the hardness in thematrix, the less sensitive the mate-rial will be to adverse effects on thestrength as a result of electricaldischarge machining. Lowering ofthe hardness level of the entire toolcan, therefore, be another alterna-tive.

layer produced is very thin andequal in the these grades. The thick-ness is about 2–4 µm. Since there isno sign of any heat-affected layer,the influence of the EDM on me-chanical properties is negligible.

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high-quality Swedish tool steel and local support wherever you

are. Assab is our wholly-owned subsidiary and exclusive sales

channel, representing Uddeholm in various parts of the world.

Together we secure our position as the world’s leading supplier

of tooling materials.

Uddeholm is the world’s leading supplier of tooling materials. This

is a position we have reached by improving our customers’ everyday

business. Long tradition combined with research and product develop-

ment equips Uddeholm to solve any tooling problem that may arise.

It is a challenging process, but the goal is clear – to be your number one

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Our presence on every continent guarantees you the same high quality

wherever you are. Assab is our wholly-owned subsidiary and exclusive

sales channel, representing Uddeholm in various parts of the world.

Together we secure our position as the world’s leading supplier of

tooling materials. We act worldwide, so there is always an Uddeholm

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