Ever wondered why some die-casting molds last 500,000 cycles while others crack after 50,000? SKD61 tool steel—also called H13 or 1.2344—explains this performance gap. This special alloy handles hot-work tooling better than most. It resists thermal shock that destroys ordinary steels. Die casters, forgers, and extrusion shops rely on it for their production schedules. But here’s the catch: SKD61 only works well with proper heat treatment. You need good tempering control too.
1. Chemical Composition of SKD61
| Element | C | Cr | Mo | V | Si | Mn |
|---|---|---|---|---|---|---|
| Content (%) | 0.32–0.45 | 4.75–5.50 | 1.00–1.75 | 0.80–1.20 | 0.80–1.20 | 0.20–0.50 |
International Equivalents
| （Standard） | （Equivalent Grade） |
|---|---|
| U.S.A AISI/ASTM | H13 steel |
| Germany DIN | 1.2344（X40CrMoV5-1）steel |
| Japan JIS | J4404（SKD61）steel |
| China GB | 4Cr5MoSiV1 steel |
| Russia GOST | 4Х5МФ1С steel |
2. Mechanical Properties of SKD61
SKD61 delivers strong performance after proper heat treatment. You get tensile strength of 1200–1600 MPa and yield strength of 1000–1380 MPa. This puts the alloy in the high-performance category for hot work tooling.
Tempering temperature controls the mechanical properties. Higher tempering reduces hardness. But it boosts toughness for thermal cycling applications.
2.1 Hardness Across Processing Conditions
| Heat Treatment Stage | Hardness | Notes |
|---|---|---|
| Annealed | ~207 HB (≈ 90–95 HRB) | Soft; easy to machine before hardening |
| Quenched & Tempered | 40–52 HRC | Practical working hardness range |
| Typical Die Hardness | 42–48 HRC | Best balance of wear resistance & thermal shock protection |
| Low-Temperature Tempering (150–250°C, 2–3 hrs) | 58–62 HRC | For high-wear molds; lower toughness; limited thermal cycling |
2.2 Strength and Ductility Parameters
Standard quench-and-temper processing gives you minimum elongation of 9%. Area reduction reaches ≥ 45% at room temperature. The elastic modulus sits at 207–215 GPa. Poisson’s ratio ranges from 0.27–0.30. These elastic constants stay stable across typical working hardness ranges.
Impact resistance from Charpy V-notch testing shows clear tempering effects:
– 540°C temper (52 HRC) → 13.6 J
– 565°C temper (48 HRC) → 24.4 J
– 580°C temper (44 HRC) → 24.4 J
Look at the jump from 13.6 J to 24.4 J between 550°C and 565°C tempering. This shows the key temperature window for best toughness. Dies with severe thermal shock work better with tempering at 565–580°C. You lose a bit of hardness, but gain toughness.
2.3 Heat Treatment Response
Heat the material to 1020–1050°C (austenitizing). Then quench with air or oil. This preps the structure for tempering.
Standard practice uses triple tempering at 550–680°C. This locks in dimensions and removes residual stress. Each tempering cycle needs 2 hours minimum. Cool fully to room temperature between cycles.
3. Thermal Properties of SKD61
These stable heat properties give SKD61 strong resistance to thermal fatigue. This holds true during repeated temperature cycling in die casting, forging, and extrusion work.
| Temperature / Range | Thermal Conductivity | Notes |
|---|---|---|
| 100°C | 42.7 W/m·K | Higher conductivity at low temp |
| 215°C | 24.3 W/m·K | |
| 475°C | 24.3 W/m·K | Uniform heat spreading at high temp |
| Service Range Average | 16 W/m·K | Balanced conductivity reduces heat shock |
At lower temperatures, conductivity increases to 42.7 W/m-K at 100°C. But the practical service range average sits at 16 W/m-K. This balanced conductivity helps dies resist heat shock. Heat doesn’t move too quickly. This protects against cracks during fast cooling cycles.
3.1 Thermal Expansion Behavior
The coefficient of thermal expansion (CTE) shows expected growth across working temperatures.
| Temperature Range | CTE Value | Notes |
|---|---|---|
| 25–95°C | 11.0 µm/m·°C | Low-temperature expansion |
| 25–205°C | 11.5 µm/m·°C | Moderate growth |
| 25–540°C | 12.4 µm/m·°C | High-temperature expansion behavior |
| Service Range Average | 16–17 × 10⁻⁶/K | Expected in tooling applications |
| Heat Treatment Size Change | 0.07% | After 1010°C hardening + 540°C tempering |
3.2 Heat Capacity and Working Range
Specific heat capacity measures 0.460 J/g-°C from 0–100°C. Between 50–100°C, it reaches 477 J/kg-K. The service range average holds at 500 J/kg-K. Dies absorb heat energy without sharp temperature jumps because of this capacity.
The practical working temperature range spans 0–500°C. The melting point sits between 1370–1400°C. This large gap between service and melting temperatures gives you safety margin for high-heat uses.
3.3 Physical Characteristics
Material density ranges from 7.7–8.03 × 10³ kg/m³. Service applications use 8000 kg/m³ for calculations. Electrical resistivity measures 0.7 Ohm·mm²/m. These stable heat properties give SKD61 strong resistance to thermal fatigue. This holds true during repeated temperature cycling in die casting, forging, and extrusion work.
4. Tempering Response of SKD61
Triple tempering changes SKD61 from brittle to reliable. You start with a hard but fragile piece after quenching. Three tempering cycles turn it into a tough hot work tool. The process controls final hardness. It also balances wear resistance with thermal shock protection. This relationship matters for picking the right tempering temperature for your job.
4.1 Hardness Development Across Tempering Temperatures
Quench SKD61 from 1000–1030°C using air or oil. The steel shows clear hardness patterns during temperin
| Temperature (°C) | Hardness (HRC) | Microstructural Changes / Notes |
|---|---|---|
| 400 | 54 | Initial high hardness; fine carbides form in martensite structure, causing the boost. |
| 500 | 56 | Peak hardness observed; continued formation of fine carbides. |
| 550 | 54 | Hardness begins to drop after peak. |
| 600 | 49 | Further decrease in hardness. |
| 650 | 47 | Hardness keeps dropping beyond this point. |
Most factories stick with the 540–580°C window. This range balances all the properties you need.
Impact resistance from Charpy V-notch testing shows clear tempering effects:
For precision control, follow these benchmarks with 4-hour tempering cycles:
– 540°C (1000°F) → 52 HRC – max hardness with decent toughness
– 550°C (1050°F) → 50 HRC – general-purpose hot work dies
– 565°C (1100°F) → 48 HRC – better impact resistance
– 580°C (1150°F) → 44 HRC – severe thermal cycling jobs
4.2 Application-Specific Tempering Guidelines
- Plastic injection molds need different treatment than wear parts. Heat to 1020–1030°C first. Then temper at 250°C to hit 50–52 HRC. This low-temperature approach keeps sharp details intact. It also stops sink marks from forming.
- Wear-resisting parts and shrink rings need tempering at 575–600°C. Core hardness lands at 45–50 HRC. Surface treatments can push surface hardness to ~1000 HV1. Deep support meets hard working surfaces with this setup.
The 538–649°C tempering range handles most hot work jobs. Hardness runs from 53–38 HRC. Exact temperature and part size determine where you land.
4.3 Dimensional Stability During Tempering
Size changes stay small through tempering.
| Process | Condition/Parameter | Effect/Outcome | Application Notes |
|---|---|---|---|
| Hardening & Tempering | Harden at 1010°C
Temper at 540°C | +0.07% longitudinal growth
52 HRC hardness | Plan for these changes during die design |
| Alternative Tempering | Temper at 595°C | Growth increases to +0.08%
Hardness drops to 47 HRC | Adjust design for altered dimensions and hardness |
| Triple Tempering | Temper at 550–680°C
Hold each cycle for 2–4 hours | Boosts mold life by more than 50% compared to single tempering | Recommended for longer-lasting molds |
Heavy sections need the full 4 hours. Cool to room temperature between cycles. This completes stress relief and carbide formation.
5. Key Performance Characteristics of SKD61
SKD61 stands out in hot work tooling. It combines multiple critical properties in one alloy. The steel delivers 40–52 HRC core hardness after proper quench and temper treatment. Most production facilities target 42–48 HRC for their applications. This hardness range stays stable during service. The material keeps effective hot hardness up to 600–650°C without major softening.
5.1 Strength and Toughness Balance
Heat-treated SKD61 delivers strong mechanical performance. Tensile strength reaches 1200–1600 MPa depending on tempering temperature. Yield strength spans 1000–1380 MPa across the working hardness range. The Charpy impact toughness rates as “good compared to other hot-work steels” in industry tests. This toughness comes from fine Cr-Mo-V carbides spread evenly throughout the microstructure.
JIS standards set minimum properties for annealed base material: tensile strength ≥960 MPa, yield strength ≥770 MPa, elongation ≥9%, and reduction of area ≥45%. Impact value must reach ≥47 J/cm². These benchmarks ensure consistent quality from all suppliers.
5.2. Physical and Thermal Performance
The steel’s physical properties support reliable die behavior. Density measures 7.8 g/cm³. Elastic modulus sits at 210 GPa. Room-temperature thermal conductivity runs 25–30 W/m·K. The coefficient of thermal expansion equals 10.4×10⁻⁶ /°C between 20–100°C. These stable numbers help engineers design dies with accurate thermal calculations.
5.3. Superior Heat Resistance Properties
Three features make SKD61 excel in thermal cycling environments. First, high-temperature strength retention keeps the material hard up to 650°C. Second, resistance to softening at high temperatures prevents hardness loss during long production runs. Third, thermal fatigue resistance stops heat-checking cracks from forming. Electro-slag-remelted (ESR) SKD61 versions show better fatigue life. They have fewer inclusions and a cleaner microstructure.
5.4. Real-World Performance Gains
Field data proves SKD61’s advantages. Forging operations switching from H11 to SKD61 report ~30% increase in mold life. Dies show less thermal fatigue cracking. They also hold dimensional tolerances over 500,000 cycles. Aluminum die-casting customers see 30–40% longer mold life versus standard hot-work steels. This applies to complex, thin-walled automotive parts.
6. Industrial Applications of SKD61
| Application Category | Typical SKD61 Parts |
|---|---|
| Die Casting | Core inserts, cavities, sliders, ejector pins, runners, sprue bushings |
| Extrusion | Dies, liners, mandrels, pressure pads, followers, die cases, die holders |
| Hot Forging | Press forge dies, hot heading dies, swaging dies, gripper dies |
| Hot Cutting | Shear blades for hot-rolled steel cutting |
| Plastic Molds | High-temp molds for glass-filled polymers |
6.1 Surface Treatment for Extended Die Life
Facilities gas or plasma nitride SKD61 tooling to fight erosion in metal flow zones. Standard nitriding at 525°C creates controlled case depths:
– 10 hours → 0.125 mm case
– 20 hours → 0.180 mm case
– 40 hours → 0.250 mm case
– 60 hours → 0.300 mm case
Die casting cores and extrusion die orifices gain most from nitriding. The hard surface layer stops aluminum soldering. It also cuts erosive wear from high-velocity metal flow.
6.2 Modern Machining Approaches
Most shops now hard-mill SKD61 at 40–50 HRC instead of grinding finished dies. This saves 30–50% manufacturing time for complex 3D cavity shapes. TiAlN-coated carbide end mills at 0.01–0.03 mm/tooth feed rates produce mirror finishes on hardened material. Research-based cutting force data now helps CAM software predict tool deflection. It optimizes tool paths for 50 HRC SKD61 workpieces.
Conclusion:
I’ve walked you through SKD61’s complete technical profile—from chemical specs to real-world die performance. Now you know why proper heat treatment matters and how tempering temperature controls your tool’s lifespan. The data shows clear patterns: temper at 565°C for thermal shock resistance, aim for 42–48 HRC in most applications, and don’t skip triple tempering if you want maximum mold life. Armed with this knowledge, you can specify SKD61 confidently and squeeze every cycle out of your tooling investment.