Wire EDM cutting in tool steels is an indispensable method that enables the high-precision machining of complex geometries. However, errors made during the EDM process can reduce part performance, leading to cracks and premature failure. In this article, we address critical points to consider during wire erosion cutting in tool steels, both from a technical and practical perspective.
What is Wire EDM?
Wire erosion is a machining method that enables the processing of electrically conductive materials without contact, using a thin wire electrode and a dielectric fluid. In particular, cold work tool steels, hot work tool steels, and plastic mold steels are frequently processed using the EDM method.
Advantages of Wire Erosion Cutting in Tool Steels
- Cutting capability with very tight tolerances
- High precision in complex and sharp geometries
- Absence of mechanical cutting force
- Hardened steels can be easily machined
However, these advantages can turn into serious disadvantages if the correct parameters are not used.
Precautions to Take During Wire Erosion Cutting
1. Formation of the White Layer
During the EDM process, a recrystallized, brittle layer with high hardness forms on the surface. This layer:
- It may cause microcracks
- Reduces fatigue resistance
- Significantly shortens mold life
Suggestion:
- Multiple passes (rough + skim cut) should be applied
- Final passes should be made with low energy.
- Grinding or polishing should be applied to critical parts after EDM.
2. Micro Crack Risk
High-carbon and high-alloy tool steels (e.g., 1.2379, 1.3343, and powder metallurgical steel series) are sensitive to thermal shocks during EDM.
Important points to note:
- High cutting speeds should be avoided.
- Low energy parameters should be preferred, especially for hardened parts.
- Preheated (stress-relieved) material should be used.
3. Heat Treatment Status
The number of tempering cycles applied to tool steels has a direct effect on microcracks that may occur during wire erosion (EDM), white layer behavior, and part life.
Steels Subjected to Single Tempering
- Internal stresses cannot be completely eliminated.
- It is more sensitive to thermal shocks during EDM
- There is a high risk of microcracks forming under the white layer.
- The risk increases particularly in D2, high-carbon and high-alloy steels.
Steels Subjected to Double Tempering (Recommended Minimum)
- Residual stresses are significantly reduced
- A more stable cut is achieved during EDM
- The likelihood of microcrack formation is significantly reduced.
- Dimensional stability increases
Triple Tempering (Critical Mold and PM Steels)
- It is particularly preferred for PM tool steels and complex molds.
- Surface integrity is maximized after EDM
- Mold service life is significantly extended
Why is Tempering So Important for EDM?
The EDM process creates localized and sudden heat input. In insufficiently tempered tool steels:
- Martensitic structure contains high internal stress
- The white layer formed during EDM triggers these stresses and initiates cracks.
- Cracks are often not visible on the surface but progress during service.
Technical Advice:
- After hardening, at least 2 tempering cycles, preferably 3 tempering cycles
- EDM after the last tempering
- Low-temperature stress relief after EDM on critical parts
- Especially in wire erosion blocks of 100 mm and above, a minimum of 3 tempering (sometimes 4) should be applied.
4. Selecting EDM Parameters Suitable for the Material
Not all steel grades can be processed with the same EDM parameters.
- Low-toughness-steels → Lower energy
- Powder metallurgy (PM) steels → More stable cutting, but still controlled parameters
5. The Importance of Dielectric Fluid and Filtration
Dirty or inadequately filtered dielectric fluid:
- It reduces surface quality
- Causes wire breaks
- Causes dimensional problems
Suggestion:
- Filter maintenance should be performed regularly.
- The dielectric temperature should be monitored in long cuts.
6. Wire Selection and Wire Tension
Incorrect wire selection:
- Conicity errors
- Increased surface roughness
- Causes cutting stability to deteriorate
General advice:
- Plated brass wires for precision cutting
- Wires with higher tensile strength in thick cross-sections
Recommended Post-EDM Procedures
- Grinding or honing
- Manual or chemical polishing (wire erosion acid can be used)
- Stress relief at low temperatures if necessary
These processes minimize surface damage caused by EDM.
Summary
Wire erosion (EDM) cutting in tool steels offers excellent precision when applied correctly; when applied incorrectly, however, it significantly shortens part life. EDM parameters suitable for the material, the correct heat treatment sequence, and an approach focused on surface quality are the key to successful results.
If long life, high performance, and repeatable quality are targeted in mold and tool steels, the wire erosion process must be considered as a whole.