Tool steel is the default choice for CNC‑machined tooling, dies, and wear components that must survive high loads, abrasion, and long production runs. This page explains what tool steel is, how it behaves in CNC machining, and how to choose between grades such as A2, A3, D2, H13, O1, and S7 for your specific application. Use it as an engineering reference for material selection, design details, and manufacturing capabilities at Clarwe.
Overview of Tool Steel in CNC Machining
What is tool steel?
Tool steel is a family of alloy steels engineered for high hardness, wear resistance, and toughness, used for tools, dies, molds, and heavily loaded components. In CNC machining, it is specified when parts must maintain tight tolerances and sharp edges under impact, abrasive wear, or high contact pressures.
How is it used in CNC machining?
CNC-machined tool steel parts include punches and dies, mold inserts, wear plates, fixtures, and precision tooling components used in high-volume or harsh production environments. These applications demand dimensional stability and edge retention across long production runs - where standard steels would degrade rapidly.
Why choose tool steel over standard steels?
Tool steels can be hardened far beyond mild or carbon steels while retaining enough toughness for cyclic and impact loads. For production tooling, this translates to fewer changeovers, better dimensional control, and lower total cost of ownership over the life of the line.
Key Properties and Benefits
Hardness and wear resistance
After heat treatment, tool steels can achieve hardness levels standard steels cannot, enabling edges and surfaces to withstand sustained abrasive contact in high‑cycle tools, cutting dies, and wear components.
Toughness and dimensional stability
Tool steels are not simply hard - they are engineered to absorb impact and cyclic loading without cracking. Air-hardening and shock-resisting grades maintain tight dimensional stability through heat treatment, which is critical for CNC-machined parts that must hold tolerances after hardening and tempering.
Machinability in the annealed condition
Most tool steels are efficiently CNC milled and turned in their annealed or pre-hardened state before final heat treatment and grinding. This process window gives shops the opportunity to achieve precise geometry before hardening, reducing scrap and rework on finished tooling.
Share your tool steel CNC requirements with Clarwe and get fast, engineering-backed manufacturability feedback and pricing.
Common Tool Steel Grades for CNC Machining (A2, A3, D2, H13, O1, S7)
The six grades below cover the majority of cold-work, hot-work, and impact-loaded CNC machining applications. Each has a distinct performance profile - selecting the right one starts with understanding your load type, operating temperature, and wear conditions.
A2 - Air-Hardening Cold-Work Tool Steel
A2 is the benchmark general-purpose cold-work grade. It air-hardens with minimal distortion, making it well suited to precision punches, dies, and tooling inserts where dimensional stability after heat treatment is non-negotiable. Balanced hardness and toughness make it easier to machine than D2 while still offering good wear resistance.
A3 - High-Carbon Air-Hardening Cold-Work Steel
A3 is a higher-carbon variant of the A-series with slightly greater wear resistance than A2, achieved through increased carbon and vanadium content. It is less commonly specified than A2 but finds use in tooling where improved abrasion resistance is needed without moving to the more brittle D2. Like A2, it air‑hardens with good dimensional stability and is used for cold‑work dies and punches where incremental wear resistance is needed.
D2 - High-Carbon, High-Chromium Cold-Work Steel
D2 delivers the highest wear resistance in this group, driven by its elevated carbon and chromium content. It is the preferred choice for cutting blades, shear tools, and forming dies that see prolonged abrasive contact. Trade-off: lower toughness than A2 or S7, so it is less suited to shock-loaded applications. Machinability is lower than A2 due to higher carbide content, so expect slower cutting speeds and more tool wear in CNC machining.
H13 - Chromium-Molybdenum Hot-Work Tool Steel
H13 is engineered for elevated-temperature service. It retains strength and hardness under thermal cycling, making it the standard material for die-casting dies, extrusion tooling, and high-temperature mold cores. Thermal fatigue resistance is what sets it apart from cold-work grades - H13 is designed to survive rapid heating and cooling cycles that would crack or soften other tool steels. In the annealed state, H13 machines reasonably well, but tool selection and coolant control are important because of its alloy content and work hardening tendencies.
O1 - Oil-Hardening Cold-Work Tool Steel
O1 is the most machinable grade in this list and is oil-hardened for reliable performance in lower-demand cutting and forming tools. It is a cost-effective starting point for general-purpose tooling where extreme wear resistance is not required.
S7 - Shock-Resisting Tool Steel
S7 is the grade of choice when impact and shock dominate the application. It combines high toughness with sufficient hardness to handle chisels, punches, and hammer tooling subject to repeated high-energy impacts - conditions that would crack less tough grades.
Mechanical Properties
Mechanical properties for tool steels can vary significantly with grade, supplier, and heat treatment condition, so it is important to treat published values as indicative rather than absolute. The table below summarizes typical ranges for key properties such as density, strength, elongation, hardness, and fatigue performance to support early‑stage material selection for CNC‑machined tool steel parts.
| Material | Density (g/cm³) | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation at Break (%) | Hardness (HBW) | Fatigue Strength (MPa) |
|---|---|---|---|---|---|---|
| A2 Tool steel | 7.8 | 350-450 | 700 - 900 | 8 - 15 | 200 - 230 | 250 - 350 |
| A3 Tool steel | 7.8-7.85 | 350-600 | 700 - 900 | 10 - 20 | 210 - 240 | 250 - 400 |
| D2 Tool steel | 7.7-7.8 | 450-600 | 800 - 1000 | 5 - 15 | 200 - 230 | 350 - 500 |
| H13 Tool steel | 7.75-7.8 | 800-1100 | 1200 - 1500 | 8 - 15 | 190 - 240 | 450 - 600 |
| O1 Tool steel | 7.8-7.85 | 600-900 | 900 - 1200 | 8 - 18 | 180 - 220 | 350 - 500 |
| S7 Tool steel | 7.7-7.8 | 900-1100 | 1300 - 1600 | 8 - 20 | 180 - 210 | 450 - 650 |
Design Considerations, Machinability, and Surface Finish
Designing for tool steel
Tool steel's elevated strength and hardness demand careful attention to geometry. Generous internal radii, adequate wall sections in loaded areas, and avoidance of sharp internal corners all reduce stress concentrations and extend tooling life. Where hardening follows machining, leave grinding or finishing stock on critical surfaces — final tolerances are far easier to achieve after heat treatment through precision grinding than by trying to hold them through the hardening cycle itself.
Clarwe’s DFM review will flag sharp internal corners, insufficient radii, or under‑supported sections that could shorten tooling life or complicate heat treatment.
CNC machinability
Most tool steels are roughed and semi‑finished in the annealed or pre‑hardened state using carbide tools, appropriate coolant, and rigid setups. After hardening and tempering, final dimensions are achieved via grinding, EDM, or controlled hard‑milling rather than conventional roughing strategies.
Achievable surface finishes
Standard CNC milling on tool steel delivers Ra 1.6–3.2 μm. Optimised toolpaths and dedicated finishing passes bring this to Ra 0.8–1.6 μm. For sealing faces, sliding surfaces, or polished mold cores, secondary grinding and lapping can achieve Ra 0.4 μm or better where the application demands it.
Applications and Grade Selection Guide
Tool steel is specified across industries wherever tooling must outlast the parts it produces. Typical CNC-machined tool steel applications include:
Stamping and blanking: Punches, dies, shear blades - typically D2 or A2
Die casting and extrusion: H13 dies, cores, and extrusion mandrels
Injection molding: Mold inserts, slides, and lifters - A2 or D2 for abrasive-filled polymers
Impact tooling: Chisels, hammer tools, and striker blocks - S7
General-purpose cold-work tooling: O1 for moderate-duty cutting and forming applications
Wear components and fixtures: Wear plates, gibs, and precision inserts - D2 or A2
High-abrasion cold-work tooling: A3 where improved wear resistance is required over A2 without moving to the higher brittleness of D2.
Grade selection quick reference
| Application Type | Recommended Grade | Key Reason |
|---|---|---|
| High-wear cold-work (dies, blades) | D2 | Maximum wear resistance |
| Precision punches and dies | A2 | Dimensional stability, balanced toughness |
| High‑abrasion cold‑work with moderate shock | A3 | More wear resistance than A2 without full D2 brittleness |
| Hot-work (die casting, extrusion) | H13 | Thermal fatigue resistance |
| General cold-work tooling | O1 | Machinability, cost efficiency |
| Shock and impact tooling | S7 | Toughness, crack resistance |
Tool Steel vs Aluminum, Stainless, and Mild Steel
Material selection for production tooling often comes down to a trade-off between performance, machinability, and cost. Here is how tool steel compares against the three most common alternatives considered at the design stage.
Tool steel vs. aluminum
Aluminum machines quickly, is lightweight, and works well for prototypes, low-pressure molds, and fixtures where wear is not a concern. Tool steel is the right choice when the part must survive abrasive contact, sustained impact, or high contact pressures - environments where aluminum would deform or wear out in short order. The cost and machining time premium for tool steel is justified by a significant difference in tool life under production conditions.
Tool steel vs. stainless steel
Stainless steel offers inherent corrosion resistance that tool steel cannot match without coatings or surface treatments. Where parts are exposed to moisture, cleaning agents, or process chemicals, stainless is often the more practical specification. Tool steel is preferred when hardness, edge retention, and dimensional stability after heat treatment are the governing requirements - performance characteristics where stainless grades generally fall short.
Tool steel vs. mild steel
Mild steels such as 1018 and 1045 are the easiest and least expensive to machine, making them suitable for structural brackets, frames, and non-wear components. They cannot be heat-treated to the hardness levels tool steel achieves, which makes them unsuitable for cutting edges, forming surfaces, or any application where wear resistance is a functional requirement. When a part needs to outlast the production run it supports, tool steel is the appropriate upgrade.
Clarwe’s Tool Steel CNC Machining Capabilities
Clarwe machines tool steel components across 3-axis, 4-axis, and 5-axis CNC milling centers, with precision CNC turning for rotational and cylindrical parts. We work across A-series, D-series, H-series, O-series, and S-series tool steels, supporting applications from single prototypes through to high-volume production runs of punches, dies, mold inserts, wear plates, and precision fixtures.
Our manufacturing network holds general tolerances to ±0.020 mm on CNC-machined features, with sub-±0.010 mm precision achievable on critical dimensions when design and fixturing support it. Post‑processing options include heat treatment, precision grinding, polishing, black oxide, PVD coatings, and EDM so tool steel parts arrive finished and ready for service.
All projects are executed under ISO 9001:2015, AS9100D, and ISO 13485:2016 certified process controls, with full material traceability and in-house inspection as standard. Our engineering team reviews every tool steel project for manufacturability before production begins, helping catch design issues before they become machining problems.
Frequently Asked Questions
What is tool steel and why is it used in CNC machining?
Tool steel is a category of alloy steel engineered specifically for high hardness, toughness, and wear resistance. It is used in CNC machining for components such as punches, dies, mold inserts, and wear plates that must hold tight tolerances and maintain edge sharpness under heavy loads or abrasive contact over long production runs.
Which tool steel grade should I choose for my CNC part?
Grade selection depends primarily on your application. D2 suits high-wear cold-work tooling, A2 is a balanced general-purpose cold-work grade, H13 is preferred for elevated-temperature applications like die casting and extrusion, O1 is a cost-effective option for moderate-duty tooling, and S7 is the right choice when impact toughness and shock resistance are the dominant requirements.
Can tool steel be CNC machined directly in the hardened condition?
Most tool steel parts are machined in the annealed or pre‑hardened condition, then hardened and finished via grinding or EDM to hit tight tolerances and surface finish. Fully hardened tool steel can be CNC hard‑milled for light finishing cuts, provided tooling and setups are appropriately rigid.
What tolerances and surface finishes are achievable on CNC-machined tool steel parts?
Standard CNC milling on tool steel typically achieves tolerances of ±0.020 mm and surface finishes around Ra 1.6–3.2 μm. Tighter tolerances down to ±0.010 mm and finer finishes below Ra 0.8 μm are achievable through precision setups, optimized toolpaths, and secondary operations such as grinding or polishing.
Does tool steel rust and how can it be protected?
Most tool steels do not have significant inherent corrosion resistance and will rust if exposed to moisture without protection. Common protective measures include controlled storage, application of rust-inhibiting coatings, black oxide treatment, or physical vapor deposition (PVD) coatings, depending on the service environment and application requirements.
Upload your CAD files and requirements through our online customer portal, and our engineering team will recommend suitable tool steel grades and provide a detailed CNC machining quote.