Tool steel is built to resist wear. That is exactly what makes machining tool steel so demanding, especially after hardening.
As a precision engineering and CNC manufacturer in Werl, Vulcanus machines cold and hot work steels, soft and hardened.
This guide covers steel groups, process chains, tooling and cutting data. It helps you plan parts that are precise and cost-effective.
Key Takeaways
- Tool steel is usually rough machined soft, then hardened, then finish machined hard.
- Annealed tool steel machines well, while hardened steel requires carbide or CBN.
- Chromium, vanadium and molybdenum carbides cause high abrasive tool wear.
- Sufficient stock allowance for heat treatment distortion is the most important planning factor.
- Rigid machines and stable clamping are essential for hard machining.
What Is Tool Steel?
Tool steels are alloyed or unalloyed steels for tools and highly stressed components. They offer high hardness, wear resistance and toughness. Typical uses include cutting, punching, forming and moulding tools.
The Four Tool Steel Groups
- Cold work steel: for service temperatures below about 200 °C, e.g. 1.2379 (D2) or 1.2842 (O2)
- Hot work steel: for die casting, forging and extrusion, e.g. 1.2343 (H11) or 1.2344 (H13)
- High-speed steel (HSS): for cutting tools, e.g. 1.3343 (M2)
- Plastic mould steel: often supplied pre-hardened, e.g. 1.2312 (P20+S) or 1.2738
Which steel suits your part is covered in our guide to CNC material selection.
Why Tool Steel Is Hard to Machine
The reason lies in the microstructure. Hard carbides of chromium, vanadium and molybdenum act like abrasive grains on the cutting edge. High strength and high cutting forces add to the challenge.
The more carbides a tool steel contains, the more wear-resistant the part and the faster the cutter wears.
Typical Tool Steel Components
- Cutting and punching tools, dies and punches
- Mould inserts for injection moulding and die casting
- Wear strips, knives and crushing tools in recycling technology
- Guides, gauges and fixture elements
In machines with heavy wear, such parts are frequent spare parts. Their service life depends directly on material and machining quality.
Machining Tool Steel: The Typical Process Chain
With tool steel, the sequence of operations matters as much as the operations themselves. The proven chain has three stages.
Stage 1: Soft Machining in the Annealed State
In the soft-annealed state, hardness is usually around 200 to 250 HB. Most of the geometry is produced cost-effectively here. CNC roughing removes the bulk of the material efficiently.
Stage 2: Heat Treatment
Next, the part is hardened and tempered. Cold work steels such as 1.2379 reach around 58 to 62 HRC. Our article on heat treatment of metals explains the processes.
Hardening distorts the part. Dimensions and shape change depending on geometry and material.
Stage 3: Hard Machining
After hardening, the functional surfaces are finished. This includes hard milling, hard turning, grinding or EDM. The result is tight tolerances despite heat treatment distortion.
Planning the Stock Allowance
Every functional surface needs a defined allowance for distortion. Too little causes scrap, too much prolongs expensive hard machining. Learn more in our article on machining allowance.
Alternative: Pre-Hardened Steel
Pre-hardened steels are supplied at working hardness, usually around 30 to 40 HRC. Hardening is skipped, and with it the distortion. For moulds and fixtures under moderate load, this is often the more economical route.
Common Tool Steels Compared
The overview below shows typical reference values. Exact figures depend on batch, delivery condition and heat treatment.
| Grade | Group | Delivery condition (approx.) | Hardened (approx.) | Soft machinability |
| 1.2379 (D2) | Cold work steel | max. 255 HB | 58 to 62 HRC | medium to difficult |
| 1.2842 (O2) | Cold work steel | max. 230 HB | 58 to 62 HRC | good |
| 1.2343 (H11) | Hot work steel | max. 230 HB | 48 to 54 HRC | good |
| 1.2312 (P20+S) | Mould steel | pre-hardened, approx. 30 HRC | not usual | good, sulphurised |
| 1.3343 (M2) | High-speed steel | max. 270 HB | 62 to 65 HRC | difficult |
Sulphurised 1.2312 machines well in the pre-hardened state. However, it can only be polished to a limited extent.
Tooling for Machining Tool Steel
Cutting Materials by Hardness
- Coated carbide: standard for soft machining and hardness up to about 55 HRC
- Micro-grain carbide with AlCrN or TiAlSiN coating: for hard milling up to about 65 HRC
- CBN: for hard turning and finishing very hard materials
- Ceramics: for special cases at high cutting speeds
Our overview of essential CNC machining tools helps with selection.
Geometry and Cutting Edge
Hardened steel needs strong cutting edges with edge honing. Fewer but sturdier flutes create chip space. Variable pitch and high helix angles dampen vibration.
Cutting Parameters for Machining Tool Steel
Reference Values for Milling
The values below are rough guidelines for carbide end mills. The tool manufacturer’s data always takes priority.
- Soft annealed: cutting speed around 100 to 200 m/min
- Hardened above 55 HRC: around 80 to 120 m/min with small depths of cut
- Feed per tooth: typically 0.05 to 0.2 mm, depending on diameter and hardness
Our guide explains how spindle speed and cutting speed relate.
Hard Milling With an HSC Strategy
Hard milling follows a clear rule: small depths of cut, high spindle speeds, constant tool engagement. Trochoidal toolpaths keep the load on the edge even. This approach is closely related to high speed cutting.
Prefer Climb Milling
In climb milling, the edge enters at maximum chip thickness. This reduces rubbing and work hardening on the surface. Tool life improves noticeably.
Cooling: Dry, Air or Emulsion?
Hard milling is often done dry with compressed air. Emulsion can cause thermal shock and edge chipping there. For soft machining, coolant lubricants are usually beneficial.
Our article on dry machining vs wet machining compares both approaches.
Turning, Drilling and Threading in Tool Steel
Turning
In the soft state, tool steel turns well with coated carbide inserts. Strong corner geometry and adequate feed prevent built-up edges. Hardened surfaces are hard turned with CBN.
Drill Before Hardening
Holes should be drilled in the soft state wherever possible. Drilling hardened tool steel is costly and risky. Precision bores get an allowance and are ground or eroded after hardening.
Planning Threads Correctly
Threads are usually cut before hardening. In the hardened state, thread milling is the safer option. Our comparison of thread milling vs. thread drilling explains why.
Hard Milling, Hard Turning or Grinding?
Hard Milling
Hard milling suits freeform surfaces, pockets and complex contours. Modern machines achieve excellent surface finishes. This often eliminates polishing or EDM.
Hard Turning and Grinding
Rotationally symmetrical surfaces can be hard turned or ground. Hard turning is more flexible, grinding achieves the finest tolerances. Our article on hard turning and grinding compares both.
EDM
Sharp internal corners and deep, narrow slots are often only possible with EDM. The process works regardless of hardness. However, it is much slower than milling.
Common Mistakes When Machining Tool Steel
Planning Mistakes
- Too little allowance: functional surfaces lack material after hardening.
- Sharp internal corners: they increase the risk of cracking during hardening.
- Wrong delivery condition: hard milling pre-hardened stock can be cheaper than hardening.
Cost Drivers for Tool Steel Parts
- Material price, especially for high-alloy grades
- Tool wear during hard machining
- Number of setups before and after hardening
- Scope of grinding, EDM and measurement
Our guide on how CNC quotes are calculated shows how these factors affect price.
Machining Mistakes
- Unstable clamping: vibration causes edge chipping.
- Feed too low: the edge rubs instead of cutting and overheats.
- Worn tools: they cause grinding burn and poor surfaces.
Our article on CNC milling errors covers more causes and fixes. To stop chatter, see how to avoid vibrations during milling.
Securing Surface Finish and Accuracy
Surface Finish
Hard milling of tool steel often achieves roughness below Ra 0.4 µm. That requires sharp tools and a stable process. Our article on surface roughness explains the parameters.
Clamping Hardened Parts Safely
Hardened workpieces do not tolerate point loads from clamping. Hardened supports and full-surface clamping prevent marks and distortion. Zero-point systems also secure position between soft and hard machining.
Temperature and Measurement
Hardened parts should cool to room temperature before final inspection. Even a few degrees change dimensions in the micron range. Climate-controlled measurement provides certainty.
Machining Tool Steel at Vulcanus
Vulcanus-Stahl & Maschinenbau in Werl machines materials other shops avoid. These include hardened steels, manganese steel, nickel-based alloys and tungsten.
- CNC milling: 4-axis and 5-axis machining with travels up to 4,500 x 1,200 x 1,800 mm
- Large parts: workpieces up to 10 tonnes
- Hard sawing: cutting hardened materials up to 65 HRC
- Metrology: tactile FARO measuring technology and ISO 9001 quality management
We plan allowances for heat treatment distortion and machine before and after hardening. Learn more on our CNC milling services page. If you want to outsource complete part production, see our guide to CNC contract manufacturing.
Conclusion: Machining Tool Steel Starts With the Right Process Plan
Tool steel demands more than powerful machines. Material condition, process chain, allowances and tooling are what count.
Roughing soft and finishing hard saves time and tooling cost. To find the right partner, read how to choose a CNC machining supplier.
Need parts made from hardened tool steel? Contact Vulcanus with your drawing for a technical assessment and a quote.
FAQ: Machining Tool Steel
Can hardened tool steel be milled?
Yes. With micro-grain carbide and the right strategy, hardness up to about 65 HRC can be milled. Rigid machines and small depths of cut are essential.
Which tool steel is easiest to machine?
In the annealed state, 1.2842 (O2) and 1.2343 (H11) machine well. Sulphurised, pre-hardened 1.2312 (P20+S) is also easy to machine.
How much allowance does tool steel need before hardening?
It depends on geometry, size and material. Typical values are a few tenths of a millimetre per surface.
Should tool steel be milled wet or dry?
Soft usually with coolant, hardened often dry with compressed air. This avoids thermal shock at the cutting edge.
What cutting speed applies to D2 (1.2379)?
Around 100 to 200 m/min soft annealed, and 80 to 120 m/min hardened with carbide. The tool manufacturer’s data always takes priority.
When is EDM better than milling?
For sharp internal corners, deep narrow slots and very fine details. EDM works regardless of hardness but is slower.