Alloy Steel Machining

Why Choose Alloy Steel

High Strength with Heat Treatability

Alloy steel achieves higher strength and hardness than carbon steel after heat treatment — 4340 can reach 1,900+ MPa UTS in the fully hardened condition. This makes alloy steel the correct choice when loads exceed what carbon steel can handle and stainless or titanium would be over-specified or too costly.
Wear Resistance

Tool steels (H13, D2, M2) are designed for maximum wear resistance at operating temperature. H13 retains hardness up to 600°C — making it the standard for hot-work tooling, die casting dies, and injection mold inserts.
Fatigue Strength

4340 and 300M have excellent fatigue resistance — they are specified for aerospace landing gear, crankshafts, and high-cycle structural components where fatigue failure is the critical design mode.
Toughness at High Hardness

The alloy additions in 4140, 4340, and H13 allow these steels to achieve hardness levels not obtainable in carbon steel while retaining adequate toughness to resist brittle fracture. This balance is why alloy steel dominates tooling and high-load structural applications.

Common Alloy Steel Grades

4140

Chromium-molybdenum alloy — through-hardenable to ~54 HRC, 655–1,020 MPa UTS depending on condition. Standard for shafts, gears, couplings, bolts, and structural parts requiring more strength than carbon steel can provide.

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4340

Higher nickel content adds toughness at high hardness — achieves 1,900+ MPa UTS after heat treatment. Used for aerospace landing gear, crankshafts, and extreme-load structural components. Excellent fatigue resistance.

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D2

High-carbon, high-chromium — excellent wear resistance at room temperature. 60–62 HRC after heat treatment. Standard for punches, dies, and cutting tools where abrasion resistance is paramount.

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M2

Tungsten-molybdenum high-speed steel — retains hardness at cutting temperatures up to 600°C. Standard for drill bits, milling cutters, and cutting tools used in production machining.

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Our Machining Capability

We machine alloy steels across the full hardness range — annealed, pre-hardened (P20, NAK80), and through-hardened conditions (H13, D2 at 60+ HRC). Hard machining of tool steels at 55–62 HRC is performed using CBN tooling and rigid setups to hold ±0.005 mm. Heat treatment (hardening, tempering, stress relief) is coordinated through qualified partners with hardness verification. EDM (wire and sinker) is available for features that cannot be hard-machined at the required tolerance.
Limitations & Cost Considerations

Alloy steels in the hardened condition are significantly more expensive to machine than annealed material — slower speeds, higher tooling cost, and more frequent tool changes. Design for machineability where possible: rough-machine features before heat treatment, then finish-machine critical dimensions after. High-alloy tool steels (H13, D2) are susceptible to cracking if thermal shock is applied during machining or heat treatment — controlled process parameters are required.

FAQ

What is the difference between 4140 and 4340?

4340 has higher nickel content (1.65–2.0% vs. nil in 4140), which improves toughness at high hardness and enhances hardenability for large cross-sections. 4140 is the standard choice for shafts and gears up to ~75mm diameter. For larger sections, aerospace parts, or extreme-load applications, 4340 provides better through-hardening and fatigue performance.
Can you machine H13 or D2 in the hardened condition?

Yes. We hard-machine H13 (48–52 HRC) and D2 (60–62 HRC) using CBN tooling, high-rigidity setups, and light finishing cuts. This is the correct approach for mold inserts and die components where post-machining heat treatment would cause distortion. Wire EDM is used for features where hard machining cannot achieve the required geometry.
What tolerances can you hold on hardened alloy steel?

±0.005 mm on hard-machined features. Wire EDM achieves ±0.003 mm on profile cuts in hardened material. For very tight tolerances (±0.001–0.002 mm) on bore diameters, cylindrical grinding after heat treatment is the correct process — available on request.