S390 Bohler high-speed steel stands out where most tool steels begin to fail. You need a material that stays hard when temperatures climb past 600°C, resists abrasive wear, and still offers enough toughness to avoid sudden breakage. This guide walks you through what makes S390 different, where it delivers the biggest returns, and how to decide if it fits your next precision tooling or high-performance machining project.
When you push cutting speeds higher or work with tough alloys like Inconel or titanium, standard high-speed steels often soften too quickly. That leads to frequent tool changes, inconsistent part quality, and rising costs. S390 was developed to solve exactly this problem by using a carefully balanced alloy mix—especially higher molybdenum and vanadium—to maintain hardness under extreme heat and resist wear far longer than conventional options.
What Makes S390 Perform So Well?
The steel’s behavior comes down to its chemistry and how that chemistry is transformed through heat treatment. Unlike general-purpose HSS grades, S390 prioritizes stability at high temperatures without sacrificing the toughness needed to handle real-world shop floor conditions.
How Does the Alloy Composition Drive Performance?
S390’s formula is built around elements that form hard, heat-resistant carbides. Below is a breakdown of the key ingredients and their roles.
| Element | Content Range | What It Does for You |
|---|---|---|
| Carbon | 0.60–0.70% | Balances carbide hardness and core toughness |
| Molybdenum | 5.00–5.50% | Provides high hot hardness—keeps cutting edges sharp at 650°C |
| Vanadium | 1.50–2.00% | Forms ultra-hard vanadium carbides for exceptional wear resistance |
| Tungsten | 1.00–1.50% | Adds secondary hot hardness alongside molybdenum |
| Chromium | 3.50–4.00% | Enhances hardenability and forms heat-resistant carbides |
| Manganese | 0.20–0.40% | Improves hardenability without coarsening carbide structure |
| Silicon | 0.15–0.35% | Aids deoxidation and stabilizes high-temperature properties |
Controlling phosphorus and sulfur below 0.03% ensures the steel remains tough and free from cracking risks during forming or machining.
What Mechanical Properties Can You Expect?
After proper heat treatment, S390 delivers numbers that directly translate to longer tool life and higher machining reliability.
- Hardness: 64–68 HRC. You can tune it—lower end (64–65 HRC) for forming tools that need more impact resistance, higher end (67–68 HRC) for cutting tools where wear resistance is king.
- Tensile strength: 2200–2400 MPa. This supports heavy cutting forces without deformation.
- Yield strength: 1800–2000 MPa. Tools stay true to shape under load.
- Fatigue strength: 850–950 MPa at 10⁷ cycles. Critical for production tools running continuously.
- Impact toughness: 40–50 J/cm². Higher than ceramic or many high-wear steels, meaning less chipping when unexpected contact happens.
A key real-world result: In a stamping die application, S390 delivered over 300,000 cycles before showing significant wear—roughly 25% more than a comparable M2 tool in the same setup.
Where Does S390 Deliver the Most Value?
S390 shines in applications where heat, abrasion, and precision collide. The industries that rely on it most—aerospace, automotive, and precision engineering—share a common need: consistent performance over long production runs.
Cutting Tools
- Milling cutters: End mills made from S390 retain their cutting edge 40% longer than M2 when machining Inconel 718. One aerospace shop documented an increase from 180 parts per tool to 300 parts, saving over $36,000 annually in regrinding and downtime.
- Turning tools: In automotive crankshaft production, S390 lathe tools averaged 800 parts per edge versus 500 for M2—a 35% efficiency gain.
- Broaches and reamers: For high-strength gears and tight-tolerance holes, S390 maintains precision across 20,000+ broaching cycles or 25,000+ reaming operations, thanks to its wear resistance and thermal stability.
Forming Tools
- Punches and dies: In high-volume stamping of 10 mm stainless steel, S390 punches handled 250,000 strokes—80,000 more than M2—before needing refurbishment.
- Cold-forming dies: When shaping titanium fasteners, S390’s toughness reduced defect rates by 70% compared to more brittle tool steel grades.
A notable case comes from an automotive supplier producing transmission gears. They switched from M35 to S390 for broaching. The result: tool life jumped from 12,000 to 20,000 parts. Even with S390’s higher initial cost, the company saved $54,000 annually through fewer replacements and regrinds.
How Is S390 Manufactured to Ensure Consistency?
Making S390 isn’t simply melting and casting. Every step is controlled to preserve the alloy balance that gives the steel its unique properties.
Melting and Composition Control
- Electric arc furnace (EAF) is the standard method. Scrap steel, molybdenum, vanadium, and tungsten are melted at 1650–1750°C. Sensors continuously verify that molybdenum stays within 5.00–5.50% and vanadium within 1.50–2.00%.
- Basic oxygen furnace (BOF) is used for larger batches. Alloys are added after oxygen blowing to prevent oxidation losses.
Forming and Heat Treatment
- Hot rolling at 1100–1200°C breaks down coarse carbides and shapes the steel into bars, plates, or wire forms.
- Cold rolling improves surface finish for thin components, followed by annealing to restore machinability.
The heat treatment cycle is where S390’s true potential is unlocked:
| Step | Temperature | Purpose |
|---|---|---|
| Annealing | 850–900°C | Softens to 220–250 HB for machining |
| Quenching | 1220–1260°C | Dissolves molybdenum carbides; oil quench gives 67–68 HRC |
| Tempering | 520–560°C | Balances hot hardness and toughness |
| Stress relief | 600–650°C | Removes machining stress to prevent warping |
After hardening, precision grinding with diamond wheels achieves tolerances as tight as ±0.0005 mm.
Surface Treatments That Extend Life
- Nitriding adds a 5–10 μm hard layer, boosting wear resistance by 30%—ideal for high-volume cutting.
- PVD coatings like titanium aluminum nitride reduce friction and can multiply tool life by 2.5× in hard alloy machining.
Every batch undergoes hardness testing, microstructure inspection, and wear simulation to confirm it meets the performance standards users expect.
How Does S390 Compare to Other Tool Steels?
Choosing the right material often comes down to balancing performance, cost, and manufacturability. The table below shows how S390 stacks up.
| Material | Relative Cost | Hardness (HRC) | Hot Hardness (HRC @ 650°C) | Wear Resistance | Toughness | Machinability |
|---|---|---|---|---|---|---|
| S390 | 100% | 64–68 | ~62 | Excellent | Moderate-High | Good |
| M2 | 70% | 62–68 | ~58 | Very Good | Moderate-High | Good |
| M35 | 85% | 63–69 | ~60 | Very Good | Moderate-High | Good |
| M42 | 130% | 65–70 | ~64 | Excellent | Moderate | Fair |
| D2 | 65% | 60–62 | ~32 | Excellent | Low | Difficult |
What this means for your project:
- For high-speed alloy machining (Inconel, titanium), S390 outperforms M2 and M35 because it stays harder at temperature.
- For precision cutting tools, S390 gives you M42-like wear resistance but at lower material cost and with better machinability.
- For forming and stamping, S390 offers toughness that D2 lacks, reducing chipping risks in high-volume production.
Conclusion
S390 Bohler high-speed steel is a specialized material for applications where standard HSS falls short. Its high molybdenum and vanadium content delivers exceptional hot hardness—retaining cutting ability at 650°C—and superior wear resistance that translates directly into longer tool life, fewer changeovers, and lower per-part costs. At the same time, its balanced toughness reduces the risk of catastrophic tool failure. Whether you are machining aerospace superalloys, running high-volume automotive production lines, or building precision forming tools, S390 offers a performance profile that justifies its premium position when downtime and quality matter most.
FAQ
What is S390 Bohler high-speed steel best used for?
It is best used for high-speed cutting tools, precision forming tools, and heavy-duty industrial components that face high temperatures and abrasive wear—especially in aerospace, automotive, and mechanical engineering applications.
How does S390 compare to M2 steel?
S390 offers higher hot hardness (retains ~62 HRC at 650°C vs. M2’s ~58 HRC) and better wear resistance (20–25% longer tool life), though it comes at a higher initial material cost. Machinability before heat treatment is similar.
Can S390 be machined after heat treatment?
No. S390 should be machined in its annealed state (220–250 HB). After hardening to 64–68 HRC, only grinding with diamond or CBN wheels is practical.
Does S390 require special heat treatment?
Yes. Its optimal properties require precise austenitizing at 1220–1260°C, oil quenching, and multiple tempers at 520–560°C. This process dissolves molybdenum carbides fully and develops the steel’s high-temperature strength.
Is S390 suitable for welding or repair?
Welding is possible but requires caution. Preheating to 350–400°C and post-weld tempering at 500–550°C are necessary to avoid cracking due to the high alloy content.
Discuss Your Projects with Yigu Rapid Prototyping
Choosing the right material is only half the battle—execution matters just as much. At Yigu Rapid Prototyping, we work with high-performance materials like S390 Bohler HSS every day. Whether you need precision CNC machining, heat treatment validation, or guidance on material selection for demanding applications, our engineers are ready to help. [Contact us] to discuss your project requirements and get a solution built for performance, not just specifications.