EDM OF TOOL STEEL This information is based on our present state of knowledge and is intended to provide general notes on our products and their uses. It should not therefore be construed as a warranty of specific properties of the products described or a warranty for fitness for a particular purpose. Classified according to EU Directive 1999/45/EC For further information see our “Material Safety Data Sheets”.

Edition 3, 08.2007 The latest revised edition of this brochure is the English version, SS-EN ISO 9001 which is always published on our web site www.uddeholm.com SS-EN ISO 14001 EDM OF TOOL STEEL

Contents Introduction ...... 3 The basic principles of EDM ...... 4 The effects of the EDM process on tool steels ...... 4 Measuring the effects ...... 6 Achieving best tool performance ...... 9 Polishing by EDM ...... 11 Summary ...... 11

3 EDM OF TOOL STEEL

Introduction able circumstances jeopardize the The basic working performance of the tool. The use of Electrical Discharge principles of EDM In such cases it may be necessary to Machining (EDM) in the production Electrical discharge machining (spark subordinate the first three factors, of forming tools to produce plastics erosion) is a method involving elec- when choosing machining para- mouldings, die castings, forging dies trical discharges between an anode meters, in order to optimize the etc., has been firmly established in (graphite or copper) and a cathode fourth. recent years. Development of the (tool steel or other tooling mate- process has produced significant rial) in a dielectric medium. The refinements in operating technique, discharges are controlled in such a The effects of productivity and accuracy, while way that erosion of the tool or the EDM process widening the versatility of the pro- work piece takes place. During the cess. operation, the anode (electrode) on tool steels Wire EDM has emerged as an works itself down into the work- The influence of spark erosion on efficient and economic alternative piece, which thus acquires the same the machined material is completely to conventional machining of aper- contours as the former. different to that of conventional tures in many types of tooling, e.g. The dielectric, or flushing liquid machining methods. blanking dies, extrusion dies and for as it is also called, is ionized during As noted, the surface of the steel cutting external shapes, such as the course of the discharges. The is subjected to very high tempera- punches. positively charged ions strike the tures, causing the steel to melt or cathode, whereupon the tempera- vaporize. The effect upon the steel ture in the outermost layer of surface has been studied by Udde- the steel rises so high (10–50,000°C/ holm Tooling to ensure that the 18–90,000°F) as to cause the steel tool maker may enjoy the many there to melt or vaporize, forming benefits of the EDM process, while tiny drops of molten metal which producing a tool that will have a are flushed out as “chippings” into satisfactory production life. the dielectric. The craters (and In the majority of cases, it has occasionally also “chips” which have been impossible to trace any influ- not separated completely) are easily ence at all on the working function recognized in a cross section of a of the spark-eroded tool. However, Special forms of EDM can now be machined surface. See figure 1. it has been observed that a trim- used to polish tool cavities, produce ming tool, for example, has become Four main factors need to be taken undercuts and make conical holes more wear resistant, while in some into account when considering the using cylindrical electrodes. cases tool failure has occurred pre- operating parameters during an EDM continues to grow, there- maturely on changing from conven- EDM operation on tool steel: fore, as a major production tool in tional machining to EDM. In other • the stock-removal rate most tool making companies, cases, phenomena have occurred machining with equal ease hardened • the resultant surface finish during the actual electrical discharge or annealed steel. • electrode wear machining that have caused un- • the effects on the tool steel. expected defects on the surface of Uddeholm Tooling supplies a full the tool. This due to the fact that range of tool steels noted for con- The influence of the EDM operation the machining has been carried out sistency in structure. This factor, on the surface properties of the in an unsuitable manner. coupled with very low sulphur lev- machined material, can in unfavour- els ensures consistent EDM per- formance. This brochure gives information on: • The basic principles of EDM • The effects of the EDM pro- Fig.1. A “rough-machined” cess on tool steels EDM surface with a cross •Achieving best tool perform section through chips and ance craters. Material: Uddeholm Orvar 2 Microdized. 4 EDM OF TOOL STEEL

“Surface strength”— invariably follows the direction of Tempered layer an important factor the crystals. In normal rough In the tempered layer, the steel machining, this layer has a thickness All the changes that can be ob- has not been heated up so much as of about 15–30 m. served are due to the enormous µ to reach hardening temperature and The carbon content in the surface temperature rise which occurs in the only thing that has occurred is layer can also be affected, for in- the surface layer. tempering-back. The effect naturally stance, by carburization from the decreases towards the core of the In the surface layer, it has been ob- flushing liquid or from the elec- material – see the hardness curve in served that the four (main) factors trode, but decarburization can also figure 2. associated with the all-important occur. In order to study the structural “surface strength” of the steel are changes incurred with different affected by this temperature in- Rehardened layer machining variables, different tool crease: In the rehardened layer, the tem- steels—see table 1—were “rough- • the microstructure perature has risen above the auste- machined” and “fine-machined” with • the hardness nitizing (hardening) temperature graphite electrodes. • the stress condition and martensite has been formed. • carbon content. This martensite is hard and brittle. Figure 2 shows a section from a normal rough-spark-machined surface with the typical, different structural changes.

Melted and resolidified layer The melted and resolidified layer produced during the EDM process is also referred to as the “white zone”, since generally no etching takes place in these areas during metallographic preparation. Figure 3, nevertheless, shows clearly that it is a rapidly solidified layer, where long pillar crystals have grown straight out from the surface 1000 x of the metal during solidification. Fig. 3. Pillar crystals formed during A fracture occurring in this layer solidification.

400 600 800 1000 H v Melted and resolidified layer

Rehardened layer

Tempered layer

Unaffected matrix

200 X Typical hardness distribution in the surface layer

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

5 EDM OF TOOL STEEL

Austenitizing, time 20 min Tempering, time 2 x 30 min Hardness Uddeholm Temperature Temperature Hardened Annealed steel grade AISI °C °F °C °FHRCHBNote: As Uddeholm Corrax is a precipitation hardening steel ARNE O1 810 1490 220 430 60 190 the EDM surface has different CALMAX – 960 1760 200 392 58 200 characteristics. The “white RIGOR A2 940 1725 220 430 60 – layer” consists of melted and SVERKER 21 D2 1020 1870 250 480 60 220 resolidified material with a IMPAX SUPREME P20 850 1560 580 1075 30 – hardness of approx. 34 HRC. ORVAR SUPREME H13 1025 1875 560 1040 50 180 There will be no other heat affected zone of importance. Table 1. The tool steels were tested in the hardened and tempered condition, and some of them also in the annealed condition.

Thickness µm Measuring 80 Graphite electrode the effects 60 Fig. 4a. Layer thicknesses and 40 Melted zone fissure frequency in the surface The thicknesses of the heat- layer in electrical discharge 20 machining of hardened (52 HRC) affected zones have been measured. Hardended zone The hardnesses in these zones 0 Uddeholm Orvar Supreme at Matrix different pulse durations. have also been measured, as have 100 200 500 1000 ti µ sec crack frequencies and crack 21 25 43(A) depths. Strength values have – – 3(B) – – –(C) been obtained through bending tests. No. of cracks per cm: (A) in melted zone The layer thicknesses appear to (B) in hardened zone (C) in matrix be largely independent of both steel grade and electrode material. On Thickness µm the other hand, there is a definite difference between the specimens Graphite electrode 60 which have been hardened and those which were in the softanneal- 40 Fig. 4b. As above, but for 20 Melted zone ed condition. Figure 4 shows, in the electrical discharge machining of form of graphs, the layer thicknesses 0 Hardended zone Uddeholm Orvar Supreme in and fissure frequency with different Matrix the annealed condition. 100 200 500 1000 t sec i µ pulse durations for Uddeholm 5 19 15(A) Orvar Supreme. – – –(B) In the annealed material, the zones – – –(C) are thinner and the fissures fewer. No. of cracks per cm: (A) in melted zone (B) in hardened zone The brittle, hardened zone is scar- (C) in matrix cely present at all (figure 4b). The layer thicknesses can vary considerably, from 0 µm to maxi- mum values slightly below the Rmax specified in the machining directions. In the rough-machining stages

(ti ≥100µ sec), the thicknesses of the layers vary far more substantially than in the fine-machining stages. Fig. 5. Fine-machined Uddeholm The thickness of both the melted Rigor, pulse duration 10µ sec. and the hardened zone increases 100 x with spark duration, which appears the beneficial effect of “fine-finish- to be the most important single ing”, i.e. to produce a very thin re- controlling variable. Figure 5 shows melted and heat-affected zone.

6 EDM OF TOOL STEEL

Structures of The cause of “arcing” must have time to become de- spark-machined layers Short off-times, or pause times, give ionized. Too short an off-time can result in double sparking “ignitions” With longer pulse duration, the more sparks per unit of time and which lead to constantly burning heat is conducted more deeply into thus more stock removal. During arcs between the electrode and the the material. Higher current inten- the off-time, the dielectric fluid work piece, resulting in serious sity and density (and thus spark surface defects. The risk of arcing is energy) do, indeed, give a higher increased if flushing conditions for “amount of heat” in the surface, but the dielectric fluid are difficult. the time taken for the heat to dif- As a result of “arcing”, i.e. a con- fuse, nevertheless, appears to have dition in which arcs are formed the greatest significance. The pic- between local parts of the elec- tures below show how the surface trode and the workpiece, large cra- zones are changed in Uddeholm ters or “burns” are formed in the Sverker 21 (in hardened and tem- surface. These have frequently been pered condition) with different confused with slag inclusions or pulse durations and electrode mate- t = 200 µs. Magnification 500 x porosity in the material. Figures 7 rials. i Figur 6d. Copper electrode and 8 show the surface of a tool with a section through one of the suspected “pores”. One of the primary causes of this type of defect is inadequate flushing, or machining of narrow slots, etc., resulting in chips and other loose particles forming a bridge between the electrode and the workpiece. The same effect can be obtained with a graphite electrode which t = 10 s. Magnification 500 x bears traces of foreign material. i µ ti = 500 µs. Magnification 500 x Figur 6a. Copper electrode Figur 6e. Graphite electrode On modern machines featuring so- called adaptive current control, the risk of “arcing” has been eliminated.

1:1 ti = 10 µs. Magnification 500 x Figur 7. The suspected “pores” can Figur 6b. Graphite electrode be seen on the surface of the tool

t = 100 µs. Magnification 500 x 65 x i Figur 8. A section through one of the Figur 6c. Graphite electrode suspected “pores” 7 EDM OF TOOL STEEL

Fissure frequency Melted Hardened also increases with pulse zone zone Matrix duration High-alloy cold-work steel UDDEHOLM SVERKER type 20–50 2–10 0–5 With times in excess of 100µ sec, Hot-work steel all steels reveal several cracks in UDDEHOLM ORVAR type 10–40 2–5 0–2 the melted layer. High-carbon and/ Cold-work steels UDDEHOLM RIGOR and or air-hardening steels show the UDDEHOLM ARNE types 10–30 0–5 0–2 highest frequency of fissures. The Plastic-moulding steel annealed specimens contain no UDDEHOLM IMPAX SUPREME type 0–5 0–2 0 cracks at all in the matrix. The number of cracks which con- Table 2. The table shows the occurrence rate of fissures. tinue down into the hardened zone is roughly 20%, while only a very few cracks penetrate into the ma- trix. In the matrix, the fissure depth The difference in hardness and vol- be eliminated, some different is seldom more than about some ume between the layers gives rise related operations can be used: tens of a µm. Here too, it applies to stresses which, upon measure- • Stress-relief tempering at a that cracks in the matrix are mainly ment, have been found to have the tempering temperature approx. encountered in the highly-alloyed same depth as the affected surface 15°C (30°F) lower than that cold-working steels. Table 2 shows layers. These stresses can be sub- previously used tempering tem- the occurrence rate of fissures in a stantially reduced by extra heat- perature, lowers the surface number of tested tool steels. treatment operations. hardness without influencing the Renewed tempering (235°C/ hardness of the matrix. The difference in stock-removal 455°F 30 min) of the specimen in • Grinding or polishing will re- rate amounts to a maximum of figure 9 resulted in lowering of the move both the surface structure approx. 15% between the different hardness level to the curve drawn and cracks, depending of course grades of tool steel with the same with a broken line. on how deeply it is done (approx. machine setting data. If electrical discharge machining is 5–10 µm in fine-machining). The hardnesses in the different properly performed with a final layers can also vary considerably, fine-machined stage, surface defects but in principle the same pattern are largely eliminated. If this is not applies to all grades. Figure 9 shows possible for one reason or another, a typical hardness distribution. or if it is necessary for all effects to

Graphite electrode t = 200 sec i µ HV

1000

800

600

400 Hardness immediately after EDM Fig. 9. Typical hardness distribu- 200 Hardness after re- tion in hardened Uddeholm tempering Sverker 21 immediately after EDM and then after re-tempering. 0 0 50 100 150 µm

8 EDM OF TOOL STEEL

Bending test Background to Achieving best To evaluate the likely effect of the the bending test results tool performance remelted layer, surface irregularities The hard, re-solidified rehardened and cracks produced in the EDM layers cause, in the first instance, EDM using solid electrodes process on the strength of a tool, a those cracks which are formed (copper/graphite) bending test was carried out. Vari- upon application of the load and in As noted, in most cases where the ous combinations of EDM surface the second instance those which EDM process has been carefully finish and post treatments, e.g. were already present to act as initia- carried out no adverse effect is stress-relieving/polishing, were tors of failure in the matrix. At experienced on tool performance. tested on 5 mm square test pieces 57 HRC, the matrix is not tough As a precautionary measure, of Rigor at 57 HRC. The test pieces enough to stop the cracks from however, the following steps are were spark-machined on one face growing and consequently the fail- recommended: to different EDM stages and bent ure occurs already on the elastic severely, with the EDM surface on part of the load curve. Normally, EDM OF HARDENED AND the outside of the bend. there should have been a certain TEMPERED MATERIAL Figure 10 shows that the sample amount of plastic bending of a test with a fine-spark machined finish bar in this material. A Conventional machining which had been polished afterwards B Hardening and tempering gave the best result. The rough C Initial EDM, avoiding “arcing” and spark-machined sample, without any excessive stock removal rates. Finish with “fine-sparking”, i.e. low post treatment, had the lowest current, high frequency. bending strength. D (i) Grind or polish EDM surface or D (ii) Temper the tool at 15°C (30°F) lower than the original Bending strength tempering temperature. N/mm2 1200 or D (iii) Choose a lower starting hardness of the tool to 1100 improve overall toughness. 1000 900 800 EDM OF ANNEALED MATERIAL 700 600 A Conventional machining 500 B Initial EDM, as C above. 400 C Grind or polish EDM surface. This reduces the risk of crack 300 formation during heating and 200 quenching. Slow pre-heating, in stages, to the hardening tempera- 100 ture is recommended.

ough spark-machined

ough spark-machined

Fine spark-machined Stress-relieved

R Stress-relieved

Fine spark-machined, Polished Fine spark-machined,

Fine spark-machined 0 R Fig. 10. Bending strength at different EDM stages and with different subsequent operation. Material Uddeholm Rigor Note: When EDM’d in solution 57 HRC. The shaded areas show the spread of the results measured. annealed condition the toughness of Uddeholm Corrax is not affected. It is recommended that all EDM’ing of Uddeholm Corrax is done after aging since an aging after EDM’ing will reduce the toughness. It is recommended that the “white layer” is removed by grind- ing, stoning or polishing.

9 EDM OF TOOL STEEL

Wire EDM These stresses take the form of In certain cases the risk can be The observation made about the tensile stresses in the surface area reduced through different pre- EDM surface in earlier pages are and compressive stresses in the cautions. also mostly applicable to the wire centre and are in opposition to each 1: To lower the overall stress level EDM-process. other. During the wire erosion pro- in the part by tempering at a high The affected surface layer, how- cess a greater or lesser amount of temperature. This assumes the use ever, is relatively thin (<10 µm) and steel is removed from the heat- of a steel grade with high resistance can be compared more to “fine- treated part. Where a large volume to tempering. of steel is removed, this can some- sparking” EDM. Normally there are 2: By drilling several holes in the times lead to distortion or even no observable cracks in the eroded area to be removed and to connect cracking of the part. The reason is surface after wire erosion. But in them by saw-cutting, before harden- that the stress balance in the part is certain cases another problem has ing and tempering. Any stresses disturbed and tries to reach an been experienced. released during heat treatment are equilibrium again. The problem of After heat treating a through then taken up in the pre-drilled and crack formation is usually only hardening steel the part contains sawn areas, reducing or eliminating encountered in relatively thick cross high stresses (the higher the tem- the risk of distortion or cracking section, e.g. over 50 mm (2") thick. pering temperature, the lower the during wire-erosion. Fig. 13 illu- With such heavier sections, correct stresses). strates how such pre-cutting may hardening and double tempering is be done. important.

Fig. 13. Pre-drilled holes connected by a saw-cut, before hardening and tempering, will help to prevent distortion or cracking when wire eroding thick sections.

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

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

10 EDM OF TOOL STEEL

Wire erosion layer produced is very thin and in connection with the working of cutting punches equal in the these grades. The thick- performance of spark-machined ness is about 2–4 m. Since there is tools should arise, however, there When producing a cutting punch µ no sign of any heat-affected layer, are some relatively simple extra by wire erosion, it is recommended the influence of the EDM on me- operations that can be employed, as (as with conventional machining) to chanical properties is negligible. indicated above. cut it with the grain direction of the A slightly striped appearance has tool steel stock in the direction of been re-ported in materials rich in the cutting action. This is not so carbides, such as high-carbon cold- important when using PM steels work steels and high-speed steels, due to their non-directional grain Summary where there is always a certain structure. In summing up it can be said that properly executed electrical dis- amount of carbide segregation or in charge machining, using a rough and material with high sulphur content. a fine machining stage in accordance The difference in bending strength Polishing by EDM with the manufacturer’s instruction, between rough-spark-machined and To day some manufacturers of EDM- eliminates the surface defects ob- fine-spark-machined test pieces is equipment offer, by a special tech- tained in rough machining. Naturally, largely due to the difference in the nique, possibilities to erode very certain structural effects will always distribution of the cracks and to the fine and smooth surfaces. It is pos- remain, but in the vast majority of presence of the in spots distributed sible to reach the surface finish of cases these are insignificant, pro- white layer on the fine-spark- machined specimens. The rougher about 0,2–0,3 µm. Such surfaces are vided that the machining process sufficient for most applications. The has otherwise been normal. Struc- surface finish of the rough-machined greatest advantages are when com- tural effects, more-over, need not specimen has not really been signi- plicated cavities are involved. Such necessarily be regarded as entirely ficant. Regardless of circumstances, cavities are difficult, time consuming negative. In certain cases the surface such surface irregularities are rela- and therefore expensive to polish structure, i.e. the rehardened layer, tively harmless as crack initiators manually, but can be conveniently has—on account of its high hard- compared with the solidification done by the EDM- machine during a ness—improved the resistance of cracks. During the polishing of the night-shift, for example. the tool to abrasive wear. In other fine-machined test piece which was Investigations made on our grades cases it has been found that the carried out, the depth of the white Uddeholm Impax Supreme, Udde- cratered topography of the surface and rehardened layer was merely holm Orvar Supreme, Uddeholm is better able to retain lubricant reduced and not completely elimi- Stavax ESR and Uddeholm Rigor than conventional surfaces, resulting nated. Further polishing would show that the hard re-melted white in a longer service life. If difficulties probably result in complete restora- tion of the bending strength. Highly stressed tools and parts thereof, e.g. very thin sections that are far more liable to bending, can justify an extra finishing operation. The lower the hardness in the matrix, the less sensitive the mate- rial will be to adverse effects on the strength as a result of electrical discharge machining. Lowering of the hardness level of the entire tool can, therefore, be another alterna- tive.

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

11 Network of excellence

Uddeholm is present on every continent. This ensures you 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.

www.assab.com www.uddeholm.com HAGFORS KLARTEXT U0708XX

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 partner and tool steel provider.

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 or Assab representative close at hand to give local advice and support. For us it is all a matter of trust – in long-term partnerships as well as in developing new products. Trust is something you earn, every day.

For more information, please visit www.uddeholm.com or www.assab.com