SKD61 Mold Steel: A Comprehensive Overview
SKD61 is a high-performance hot-work mold steel celebrated for its exceptional heat resistance, toughness, and durability in high-temperature applications. As a staple in industries like die casting, hot forging, and high-temperature plastic molding, it excels in withstanding repeated thermal cycles, mechanical stress, and abrasive wear—making it a cornerstone of modern manufacturing.
Basic Information
Definition: SKD61 is a chromium-molybdenum-vanadium alloy steel engineered for hot-work environments. It retains strength and toughness at temperatures up to 600–700°C, resisting thermal fatigue, heat checking, and deformation during prolonged exposure to high heat and pressure.
Standards & Equivalents: Originating from Japan (JIS G 4404), SKD61 is globally recognized and equivalent to:
AISI H13 (USA)
DIN 1.2344 (Germany)
GB 4Cr5MoSiV1 (China)
These grades share similar compositions and performance, ensuring interchangeability in international manufacturing.
Key Advantages: High-temperature strength, excellent thermal fatigue resistance, good wear resistance, and consistent hardenability across thick sections.
Chemical Composition
SKD61’s alloying elements are precisely balanced to optimize hot-work performance:
Element Content Range Role in Performance
Carbon (C) 0.32–0.42% Provides a foundation for hardness and carbide formation, balancing strength and toughness.
Chromium (Cr) 4.50–5.50% Enhances hardenability, oxidation resistance, and thermal stability; forms protective oxides at high temperatures.
Molybdenum (Mo) 1.00–1.50% Boosts high-temperature strength, creep resistance, and thermal conductivity; reduces temper brittleness.
Vanadium (V) 0.80–1.20% Refines grain structure, improves wear resistance via hard vanadium carbides, and resists heat-induced cracking.
Silicon (Si) 0.80–1.20% Enhances oxidation resistance and high-temperature strength; acts as a deoxidizer for purity.
Manganese (Mn) 0.20–0.50% Improves hardenability and machinability without compromising toughness.
Phosphorus (P) ≤0.030% Strictly controlled to prevent brittleness, especially under thermal stress.
Sulfur (S) ≤0.030% Minimized to avoid reducing toughness and causing hot cracking during processing.
Physical Properties
Density: ~7.85 g/cm³ (consistent with most alloy steels).
Melting Point: ~1450–1500°C.
Thermal Conductivity: ~35–40 W/(m·K) at room temperature; remains stable at high temperatures, aiding heat dissipation.
Thermal Expansion Coefficient: ~11.0×10⁻⁶/°C (20–500°C), minimizing distortion during rapid heating/cooling cycles.
Elastic Modulus: ~210 GPa at room temperature; retains ~80% of this value at 600°C, ensuring structural stability.
Mechanical Properties
SKD61’s performance is tailored for high-temperature durability, with properties varying by heat treatment:
Annealed State (For Machining)
Hardness: ≤235 HB (easily machinable with conventional tools).
Tensile Strength: ~700–800 MPa.
Elongation: ~18–22% (sufficient ductility for complex mold shaping).
After Hardening + Tempering (For Hot-Work Use)
Tempering Temperature Hardness Tensile Strength (20°C) High-Temp Strength (600°C) Impact Toughness (Charpy V-Notch)
500°C (932°F) 48–52 HRC ~1600–1800 MPa ~1000–1100 MPa ~30–40 J
550°C (1022°F) 45–48 HRC ~1400–1600 MPa ~900–1000 MPa ~40–50 J
600°C (1112°F) 40–45 HRC ~1200–1400 MPa ~800–900 MPa ~50–60 J
Thermal Fatigue Resistance: Outstanding—resists microcracking from repeated heating/cooling, critical for die casting dies.
Wear Resistance: Good at high temperatures, thanks to hard chromium and vanadium carbides, ensuring long service life.
Heat Treatment Process
Proper heat treatment unlocks SKD61’s full potential for hot-work applications:
Annealing
Purpose: Soften the steel for machining and ensure a uniform microstructure.
Process: Heat to 850–880°C, hold 2–4 hours, then cool slowly (≤30°C/hour) to 500°C, followed by air cooling. Results in spheroidal carbides and hardness ≤235 HB.
Austenitizing
Temperature: 1020–1050°C (1868–1922°F).
Hold Time: 30–60 minutes (based on thickness) to dissolve carbides and form uniform austenite.
Quenching
Cooling: Oil quenching (most common) or air cooling for thin parts. Ensures full hardening with minimal distortion.
Result: As-quenched hardness of 50–55 HRC (martensitic structure).
Tempering
Temperature: 500–600°C (932–1112°F), with 2–4 hours holding time (double tempering recommended to eliminate retained austenite).
Effect: Optimizes high-temperature strength, toughness, and resistance to thermal fatigue. Higher temperatures prioritize toughness over hardness.
Processing Performance
Machinability: Good in the annealed state (≤235 HB) with carbide or high-speed steel tools. Carbides may cause mild tool wear, so sharp tools and proper feeds/speeds are advised.
Weldability: Moderate—requires preheating (300–400°C) and post-weld tempering to prevent cracking. Typically used for mold repair rather than primary fabrication.
Formability: Limited cold formability; hot forming is possible at 1000–1100°C, followed by annealing to restore machinability.
EDM Performance: Excellent—produces a smooth recast layer, ideal for intricate mold details like cooling channels or textured surfaces.
Application Fields
SKD61 is indispensable in high-temperature manufacturing processes where heat, wear, and fatigue resistance are critical:
Die Casting:
Molds for aluminum, zinc, and magnesium die casting (withstands molten metal contact and rapid cooling cycles).
Core pins, ejector pins, and die inserts in die casting systems.
Hot Forging & Extrusion:
Forging dies, extrusion dies, and punches for hot forming of metals (resists deformation under high pressure and temperature).
Plastic Injection Molding:
Molds for high-temperature resins (e.g., PEEK, LCP) or thick-walled parts requiring elevated mold temperatures.
Other Hot-Work Tools:
Hot shear blades, rolling mill rolls, and glass molding tools (endures continuous high-heat exposure).
In summary, SKD61’s unique blend of heat resistance, toughness, and wear resistance makes it the gold standard for hot-work mold steel. Its global equivalence ensures reliability across industries, solidifying its role in high-performance manufacturing worldwide.