S7 Tool Steel
S7 tool steel is a versatile air-hardening shock-resistant tool steel known for its exceptional toughness, high impact resistance, and good wear resistance. It is designed to withstand heavy shock loads and repeated impacts, making it a preferred choice for tools and components that operate under high-stress, high-impact conditions.
Basic Information
Definition: S7 is a chromium-molybdenum tool steel classified as a “shock-resistant” grade. It achieves its properties through air-hardening heat treatment, which minimizes distortion and eliminates the need for rapid quenching media like water or oil.
Corresponding Standards: Conforms to ASTM A681 (USA) and SAE J437. Internationally, it is equivalent to DIN 1.2357 (Germany) and GB 5CrNiMo (China, with slight compositional differences).
Key Characteristics: Outstanding impact toughness (even at high hardness), good wear resistance, air-hardening capability (low distortion), and moderate machinability.
Chemical Composition
The composition of S7 tool steel is carefully balanced to optimize shock resistance and hardenability, with typical ranges as follows:
Element Content Range Role in the Steel
Carbon (C) 0.45–0.55% Provides a balance of hardness and toughness; forms carbides for wear resistance.
Manganese (Mn) 0.20–0.90% Enhances hardenability and contributes to toughness; improves hot workability.
Silicon (Si) 0.20–1.00% Acts as a deoxidizer; enhances strength and impact toughness.
Chromium (Cr) 3.00–3.50% A key element for hardenability and wear resistance; forms chromium carbides.
Molybdenum (Mo) 1.30–1.80% Improves high-temperature strength, toughness, and hardenability; reduces temper brittleness.
Vanadium (V) 0.10–0.30% Refines grain structure, enhancing toughness and wear resistance.
Phosphorus (P) ≤0.030% Harmful impurity; strictly controlled to prevent brittleness.
Sulfur (S) ≤0.030% Controlled to avoid reduced toughness and hot cracking.
Physical Properties
Density: Approximately 7.85 g/cm³ (similar to most alloy steels).
Melting Point: Around 1450–1500°C.
Thermal Conductivity: ~35 W/(m·K) at room temperature.
Coefficient of Thermal Expansion: ~11.0×10⁻⁶/°C (20–100°C).
Elastic Modulus: ~210 GPa.
Mechanical Properties
S7 tool steel’s mechanical properties are primarily optimized through heat treatment, with a focus on balancing hardness and toughness. Key properties include:
Annealed State (for Machining)
Hardness: ≤235 HB (machinable with conventional tools).
Tensile Strength: ~700 MPa.
Yield Strength: ~450 MPa.
Elongation: ~20% (moderate ductility for forming).
After Air Hardening + Tempering
Tempering Temperature Hardness Tensile Strength Impact Toughness (Charpy V-Notch) Key Property Focus
200°C (392°F) 58–60 HRC ~2000 MPa ~20 J Maximum hardness and wear resistance.
300°C (572°F) 55–57 HRC ~1800 MPa ~35 J Balanced hardness and toughness.
400°C (752°F) 50–52 HRC ~1600 MPa ~50 J High toughness for shock applications.
500°C (932°F) 45–47 HRC ~1400 MPa ~70 J Maximum toughness with moderate hardness.
Wear Resistance: Moderate to high, depending on hardness; suitable for applications with moderate abrasion.
Shock Resistance: Excellent—superior to many other tool steels (e.g., D2 or A2) due to its high toughness at operational hardness levels.
Heat Treatment Process
S7 tool steel’s air-hardening capability is a critical advantage, reducing distortion and simplifying processing:
Annealing
Purpose: Soften the steel for machining.
Process: Heat to 820–850°C, hold for 2–4 hours, then cool slowly (≤20°C/hour) to 600°C, followed by air cooling. Results in hardness ≤235 HB.
Austenitizing (Heating for Hardening)
Temperature: 925–950°C (1697–1742°F).
Hold Time: 30–60 minutes (depending on part thickness) to ensure uniform austenitization.
Air Hardening
Cooling: Cool in still air (no quenching oil/water needed). This slow cooling minimizes distortion and cracking.
Result: Forms martensite, achieving high hardness (60–62 HRC as-quenched).
Tempering
Temperature Range: 200–550°C (392–1022°F), with holding time of 1–2 hours per 25 mm of thickness (double tempering recommended to eliminate retained austenite).
Effect: Reduces internal stress, adjusts hardness, and maximizes toughness. Higher tempering temperatures increase toughness but reduce hardness.
Processing Performance
Machinability: Good in the annealed state (≤235 HB) using high-speed steel or carbide tools. After hardening, machining is difficult—grinding is preferred for finishing.
Weldability: Moderate; preheating to 200–300°C and post-weld tempering are required to prevent cracking. Welding may reduce toughness in the heat-affected zone.
Formability: Limited cold formability due to moderate strength in the annealed state. Hot forming is possible at 1000–1100°C, followed by annealing to restore machinability.
Grindability: Excellent—can be ground to tight tolerances and smooth surfaces, making it suitable for precision tools.
Application Fields
S7 tool steel’s standout feature—shock resistance—makes it ideal for tools and components subjected to heavy impacts or repeated loading:
Tooling & Dies:
Cold chisels, punches, and dies for cold heading, blanking, and piercing (resists chipping from impact).
Hammer dies, forging dies, and extrusion dies (handles repeated mechanical shock).
Shear blades and slitter knives for cutting thick or hard materials.
Industrial Components:
Rock drills, jackhammer bits, and mining tools (withstand extreme impact in harsh environments).
Machinery parts like gears, cams, and shafts in high-shock applications.
Aerospace & Defense:
Impact-resistant components in aircraft and military equipment, such as landing gear parts and armor components.
Automotive:
Stamping dies for car body panels and heavy-duty fasteners (resists wear and impact during high-volume production).
In summary, S7 tool steel is a go-to material for applications demanding a rare combination of high toughness, impact resistance, and moderate wear resistance. Its air-hardening property and minimal distortion further solidify its role as a reliable choice for precision and high-stress tooling.