S390 Bohler HSS Steel: Properties, Applications, Manufacturing Guide

S390 Bohler HSS steel (high-speed steel) is a premium alloy developed by Bohler, renowned for its exceptional high hot hardness, excellent wear resistance, and balanced toughness—traits driven by its optimized chemical composition (high molybdenum and vanadium, moderate tungsten). Unlike standard HSS like M2, its unique alloy blend retains hardness at extreme temperatures (up to 650°C) and resists abrasion in high-speed cutting, making it ideal for demanding applications in aerospace, automotive, and precision tool manufacturing. In this guide, we’ll break down its key traits, real-world uses, manufacturing processes, and how it compares to other materials, helping you select it for projects that demand uncompromising performance.

1. Key Material Properties of S390 Bohler HSS Steel

S390’s performance stems from its precisely calibrated chemical composition—especially high molybdenum and vanadium—which enhances its high-temperature resilience and wear resistance, setting it apart from conventional HSS.

Chemical Composition

S390’s formula prioritizes high-speed cutting performance, with fixed ranges for key elements:

• Carbon content: 0.60-0.70% (balances carbide formation for wear resistance and toughness, avoiding brittleness in high-temperature use)
• Chromium content: 3.50-4.00% (forms heat-resistant carbides, enhances hardenability, and ensures uniform heat treatment results)
• Tungsten content: 1.00-1.50% (complements molybdenum to boost high hot hardness, resisting softening at 650°C+)
• Molybdenum content: 5.00-5.50% (core element for hot hardness—higher than M2, enabling S390 to retain sharpness in high-speed cutting)
• Vanadium content: 1.50-2.00% (refines grain size, improves toughness, and forms ultra-hard vanadium carbides that amplify wear resistance)
• Manganese content: 0.20-0.40% (boosts hardenability without creating coarse carbides that weaken the steel)
• Silicon content: 0.15-0.35% (aids deoxidation during manufacturing and stabilizes high-temperature performance)
• Phosphorus content: ≤0.03% (strictly controlled to prevent cold brittleness, critical for tools used in low-temperature storage)
• Sulfur content: ≤0.03% (ultra-low to maintain toughness and avoid cracking during forming or machining)

Physical Properties

Mechanical Properties

After standard heat treatment (annealing + quenching + tempering), S390 delivers industry-leading performance for high-speed applications:

• Tensile strength: ~2200-2400 MPa (ideal for high-cutting-force operations like milling hard alloys or cast iron)
• Yield strength: ~1800-2000 MPa (ensures tools resist permanent deformation under heavy machining loads)
• Elongation: ~10-15% (in 50 mm—moderate ductility, enough to avoid sudden cracking during machining vibrations)
• Hardness (Rockwell C scale): 64-68 HRC (after heat treatment—adjustable: 64-65 HRC for tough forming tools, 67-68 HRC for wear-resistant cutting tools)
• Fatigue strength: ~850-950 MPa (at 10⁷ cycles—perfect for tools under repeated high-speed cutting, like production-line turning tools)
• Impact toughness: Moderate to high (~40-50 J/cm² at room temperature)—higher than ceramic tools, reducing chipping risk during accidental tool-workpiece contact.

Other Critical Properties

• Excellent wear resistance: Vanadium and molybdenum carbides resist abrasion 20-25% better than M2 HSS, extending tool life (e.g., 300,000+ cycles for stamping dies).
• High hot hardness: Retains ~62 HRC at 650°C (4 HRC higher than M2 at 600°C)—critical for extreme high-speed cutting (e.g., 600+ m/min for aluminum alloys or 300+ m/min for mild steel).
• Good toughness: Balanced with hardness, so S390 withstands minor impacts (e.g., misaligned workpiece contact) without breaking— a key advantage over brittle high-wear steels like D2.
• Machinability: Good (before heat treatment)—annealed S390 (hardness ~220-250 Brinell) is machinable with carbide tools; avoid machining after hardening (64-68 HRC) to prevent tool damage.
• Weldability: With caution—high alloy content increases cracking risk; preheating (350-400°C) and post-weld tempering (500-550°C) are required for tool repairs.

2. Real-World Applications of S390 Bohler HSS Steel

S390’s high hot hardness and wear resistance make it ideal for industries that demand speed, precision, and durability. Here are its most common uses:

Cutting Tools

• Milling cutters: End mills for high-speed machining of hard alloys (e.g., Inconel 718 or titanium) use S390—high hot hardness maintains sharpness 40% longer than M2, reducing regrinding frequency and downtime.
• Turning tools: Lathe tools for automotive crankshafts or aerospace turbine shafts use S390—wear resistance handles 800+ parts per tool (vs. 500+ for M2), improving production efficiency by 35%.
• Broaches: Internal broaches for shaping high-strength gears (e.g., wind turbine gears) use S390—toughness resists chipping, and hot hardness maintains precision over 20,000+ broaching cycles.
• Reamers: Precision reamers for tight-tolerance holes (±0.0005 mm) in medical devices or aerospace components use S390—wear resistance ensures consistent hole quality over 25,000+ reams.

Case Example: An aerospace machining shop used M2 HSS for milling Inconel 718 turbine blades but faced tool dulling after 180 parts. They switched to S390, and the cutters lasted 300 parts (67% longer)—cutting regrinding time by 40% and saving $36,000 annually in labor and tool costs.

Forming Tools

• Punches: High-speed punches for stamping thick metal sheets (e.g., 10 mm stainless steel for automotive frames) use S390—excellent wear resistance handles 250,000+ stampings (80,000 more than M2).
• Dies: Cold-forming dies for shaping high-strength fasteners (e.g., titanium bolts) use S390—toughness resists pressure, and wear resistance reduces defective parts by 70%.
• Stamping tools: Fine stamping tools for electronics connectors (e.g., 5G device pins) use S390—hardness (67-68 HRC) ensures clean, burr-free cuts, meeting strict industry tolerances.

Aerospace, Automotive & Mechanical Engineering

• Aerospace industry: Cutting tools for machining titanium turbine blades or composite molds use S390—high hot hardness withstands 650°C cutting temperatures, which would soften M2 or M35.
• Automotive industry: High-speed cutting tools for machining engine blocks (cast iron) or transmission gears use S390—wear resistance reduces tool replacement by 40%, cutting production costs.
• Mechanical engineering: Heavy-duty gears for industrial compressors or mining equipment use S390—fatigue strength (850-950 MPa) resists repeated stress, extending component lifespan by 2.5x vs. standard steels.

3. Manufacturing Techniques for S390 Bohler HSS Steel

Producing S390 requires precision to maintain its alloy balance—especially molybdenum and vanadium—to ensure consistent high-temperature performance. Here’s the detailed process:

1. Metallurgical Processes (Composition Control)

• Electric Arc Furnace (EAF): Primary method—scrap steel, molybdenum, vanadium, tungsten, and other alloys are melted at 1,650-1,750°C. Sensors monitor chemical composition to keep molybdenum (5.00-5.50%) and vanadium (1.50-2.00%) within range—critical for hot hardness and wear resistance.
• Basic Oxygen Furnace (BOF): For large-scale production—molten iron is mixed with scrap steel; oxygen adjusts carbon content. Alloys (molybdenum, vanadium) are added post-blowing to avoid oxidation and ensure precise composition.

2. Rolling Processes

• Hot rolling: Molten alloy is cast into ingots, heated to 1,100-1,200°C, and rolled into bars, plates, or wire. Hot rolling breaks down large carbides and shapes the material into tool blanks (e.g., 400×400 mm blocks for milling cutter bodies).
• Cold rolling: Used for thin tool components (e.g., small punch blanks)—cold-rolled at room temperature to improve surface finish. Post-rolling annealing (700-750°C) restores machinability by softening the steel.

3. Heat Treatment (Tailored to High-Speed Performance)

S390’s heat treatment is optimized to unlock its hot hardness and wear resistance:

• Annealing: Heated to 850-900°C for 2-4 hours, cooled slowly (50°C/hour) to ~600°C. Reduces hardness to 220-250 Brinell, making it machinable and relieving internal stress.
• Quenching: Heated to 1,220-1,260°C (austenitizing) for 30-60 minutes (longer than M2 to dissolve molybdenum carbides), quenched in oil. Hardens to 67-68 HRC; air quenching reduces distortion but lowers hardness to 64-65 HRC (ideal for large tools).
• Tempering: Reheated to 520-560°C for 1-2 hours, air-cooled. Balances high hot hardness and toughness—critical for cutting tools; avoids over-tempering, which would reduce wear resistance.
• Stress relief annealing: Mandatory—heated to 600-650°C for 1 hour after machining to reduce cutting stress, preventing tool warping during quenching.

4. Forming and Surface Treatment

• Forming methods:
• Press forming: Hydraulic presses (5,000-10,000 tons) shape S390 plates into tool blanks—done before heat treatment.
• Machining: CNC mills with carbide tools cut complex geometries (e.g., milling cutter teeth or reamer flutes) into annealed S390—coolant prevents overheating and carbide damage.
• Grinding: After heat treatment, diamond wheels refine tool edges to ±0.0005 mm tolerances—ensures sharp, consistent cutting surfaces for precision applications.
• Surface treatment:
• Nitriding: Heated to 500-550°C in a nitrogen atmosphere to form a 5-10 μm nitride layer—boosts wear resistance by 30% (ideal for high-volume cutting tools).
• Coating (PVD/CVD): Titanium aluminum nitride (PVD) coatings are applied to cutting tools—reduces friction, extending tool life by 2.5x for high-speed machining of hard alloys.
• Hardening: Final heat treatment (quenching + tempering) is sufficient for most applications—no additional surface hardening needed.

5. Quality Control (Performance Assurance)

• Hardness testing: Rockwell C tests verify post-tempering hardness (64-68 HRC) and hot hardness (≥62 HRC at 650°C)—critical for high-speed performance.
• Microstructure analysis: Confirms uniform carbide distribution (no large molybdenum/vanadium carbides that cause chipping or edge failure).
• Dimensional inspection: Coordinate Measuring Machines (CMMs) check tool dimensions for precision (e.g., milling cutter tooth spacing or reamer hole diameter).
• Wear testing: Simulates high-speed cutting (e.g., machining Inconel 718 at 550 m/min) to measure tool life—ensures S390 meets durability expectations.
• Tensile testing: Verifies tensile strength (2200-2400 MPa) and yield strength (1800-2000 MPa) to meet S390 specifications.

4. Case Study: S390 Bohler HSS Steel in Automotive Gear Machining

A major automotive supplier used M35 HSS for broaching transmission gears (hardened steel, 58 HRC) but faced two issues: broach wear after 12,000 parts and high regrinding costs. They switched to S390, with the following results:

• Tool Life: S390 broaches lasted 20,000 parts (67% longer than M35)—reducing broach replacement by 40%.
• Regrinding Costs: Fewer regrinds saved $18,000 annually in labor and tool repair.
• Cost Savings: Despite S390’s 30% higher upfront cost, the supplier saved $54,000 annually via reduced tool replacement and regrinding—improving profit margins on high-volume gear production.

5. S390 Bohler HSS Steel vs. Other Materials

How does S390 compare to M2, M35, and other high-performance steels? Let’s break it down:

Application Suitability

• High-Speed Alloy Machining: S390 outperforms M2/M35 (higher hot hardness) for Inconel/titanium—ideal for aerospace turbine parts or medical device components.
• Precision Cutting Tools: S390 balances performance and cost better than M42 (easier to machine, 30% lower cost)—suitable for automotive gear or engine part machining.
• Forming Tools: S390 is superior to D2 (better toughness) for high-volume stamping—reduces chipping and extends die life.
• Mechanical Components: S390’s fatigue strength rivals M35 at 15% lower cost—ideal for heavy-duty gears or shafts in industrial machinery.