Introduction:
Choosing between SKD61、SKD6 and SKT4 might seem simple, but the wrong steel can shorten die life, raise defect rates, and push your production costs through the roof. Although these steels are all used for hot work, their heat resistance, thermal-fatigue behavior, and machining performance are very different—and those differences matter once temperatures climb above 500°C.
This guide cuts through the confusing specs and explains what actually affects your tooling: alloy composition, heat-treatment response, high-temperature hardness, thermal cycling durability, and real performance data from die-casting, extrusion, and forging shops.
By the end, you’ll know exactly which steel matches your operating temperature, cycle frequency, and production needs—so your dies last longer and your processes run more smoothly.
Chemical Composition
| Alloy Element | SKD61 | SKD6 | SKT4 |
|---|---|---|---|
| Carbon (C) | 0.35–0.42% | ~0.32–0.42% | 0.50–0.60% (highest) |
| Chromium (Cr) | 4.8–5.5% | ~4.0% | 0.8–1.2% (lowest) |
| Vanadium (V) | 0.80–1.15% | 0.3–0.5% | 0.05–0.15% (very low) |
| Molybdenum (Mo) | 1.0–1.5% | ~1.0% | 0.35–0.55% |
| Silicon (Si) | 0.80–1.20% | 0.80–1.20% | 0.10–0.40% |
| Manganese (Mn) | 0.25–0.50% | ~0.5% | Higher variation |
Key Takeaways：
SKD61 = High-alloy, best for oxidation/heat stability
SKD6 = Lower Cr but optimized for thermal fatigue
SKT4 = C-rich, low-alloy, cheaper but weaker above 450°C
Mechanical Properties
| Property | SKD61 (H13 / JIS SKD61) | SKD6 (H11 / JIS SKD6) | SKT4 (L6 / JIS SKT4) |
|---|---|---|---|
| Hardness (HRC) | ~44–52 HRC (typical working); up to 55 HRC after Q&T | ~45–56 HRC typical after hardening/tempering | ~45–60 HRC (depends on heat treatment) |
| Tensile Strength (Ultimate) | ~1200–1600 MPa (hardened) | ~1450–1680 MPa typical | ~1950 MPa (hardened then tempered to ~400°C) |
| Yield Strength | ~1000–1380 MPa (hardened) | ~1240–1410 MPa typical | ~1910 MPa (tempered low) |
| Elongation | ~10–14% typical | ~12–13% typical | ~2% at high-strength temper |
| Thermal Performance | Maintains hardness & toughness up to ~500–600°C | Good hot strength and thermal fatigue resistance to ~500–540°C | Sharp drop in hardness above ~300–400°C |
| Crack / Thermal Fatigue Resistance | Good | Excellent (best for rapid heating/cooling) | Low |
| Best Applications | Die casting, extrusion, high-heat tooling | Forging punches, thermal-cycling tools | Moderate-temperature tooling, cost-sensitive projects |
Insights:
SKD61 = Best for holding hardness at heat
SKD6 = Longest life under repeated thermal cycling
SKT4 = High RT strength but weak at high temps
Heat Treatment
| Steel | Key Process | Temper / Hardness | Advantage |
|---|---|---|---|
| SKD61 | Preheat 790°C → Austenitize 1000–1050°C → Air quench → Triple temper | 550–680°C → 50–52 HRC | High-temp stability, toughness, dimensional control |
| SKD6 | Same as SKD61 | 550–600°C → 45–50 HRC | Thermal-fatigue resistance, long tool life under rapid cycles |
| SKT4 | Preheat 780–820°C → Austenitize 1000–1030°C → Optional anneal | Above 550°C → softens quickly | Easy machining, cost-efficient for moderate heat |
Interpretation:
SKD61 = Most stable dimensions after heat treatment
SKD6 = Same schedule but lower temper for fatigue resistance
SKT4 = Needs tighter control; softens fast if overheated
Key Differences
Compare these three steels side by side under real pressing conditions. The performance gap becomes clear.
Hardness & Structural Stability
- SKD61: Maintains 50–52 HRC at high temperatures, keeping die edges and cavity dimensions accurate through 100,000+ aluminum die-casting shots.
- SKD6: Softer (45–50 HRC) to absorb thermal stress; excels under rapid heating/cooling, preventing surface cracks in hot forging.
- SKT4: Hard at room temperature but loses 30% hardness above 400°C; best for moderate-temperature jobs (<450°C) where cost matters more than durability.
Heat Resistance
- SKD61: High chromium (4.8–5.5%) forms a self-healing oxide layer, resisting scaling in long production runs.
- SKD6: Similar heat resistance but superior thermal-fatigue performance; prevents heat-checking cracks under localized hot spots.
- SKT4: Low chromium limits high-temperature use; oxidation accelerates above 500°C, reducing die life by 40–60%, but easier to machine and cost-effective for prototypes or short runs.
Machinability & Distortion
- SKD61: Minimal distortion during heat treatment; complex dies maintain ±0.05 mm accuracy.
- SKD6: Good machinability like SKD61; slightly softer matrix reduces quenching stress, helping preserve die shape during thermal cycling.
- SKT4: More sensitive to temperature changes; requires careful heat treatment but costs 20–30% less per kg than SKD61.
Uses for High-Temperature Pressing
Each steel works well in certain high-temperature pressing settings. The steel properties match what the job needs.
SKD61 in Die-Casting and Extrusion
- Aluminum Die-Casting: Used for cavity blocks, cores, and ejector pins. Prevents metal fusion under molten aluminum (~700°C) and 80–150 MPa pressure. Chromium-vanadium carbides stop soldering. Common applications include automotive engine blocks, transmission housings, and heat sinks. Dies last over 100,000 shots.
- Zinc & Magnesium Die-Casting: Handles lower metal temperatures (Zinc: 420°C, Magnesium: 650°C) and fast thermal cycling stress. Applications include electronics housings, automotive trim parts, and medical devices.
- Hot Extrusion Dies (Aluminum Profiles): Standard choice for extrusion dies. Billet temperatures reach 480–520°C. SKD61 dissipates heat rapidly, preventing die overheating. Used for window frame extrusions, automotive structural profiles, and heat sink channels.
SKD6 – Thermal Cycling Work
- Hot Forging Punches: Handles rapid heating from 200°C to 600°C, avoiding cracks that occur in SKD61 under fast cycles. Used in connecting rod forging, crankshaft rough forming, and gear blank making. Longer tool life.
- Hot Punching and Trimming Dies: Works with 400–500°C workpieces. Survives thousands of heating-cooling cycles without surface cracking.
SKT4 – Moderate-Temperature Work
- Hot Rolling Guide Equipment: Operates below 450°C. Cost-effective choice. Used in guide rolls, side guards, and structural supports. Strength at room temperature sufficient; lower high-temperature resistance acceptable.
- Prototype Forging Dies & Short-Run Tooling: Suitable for low-volume production. Machining is faster, heat treatment is simpler, and replacement cost is low.
Picking Steel for High-Temperature Pressing
- SKD61 – High Temp
For pressing >1000°C, SKD61 resists softening with 4.8–5.5% Cr and 1–1.5% Mo. Dies stay strong; factory data shows 50% longer life vs SKT4, and 2.5× longer life in automotive casting at 700°C. - SKD6 – Thermal Cycling
Ideal for rapid heating/cooling (200–600°C). Softer 45–50 HRC matrix absorbs stress, preventing cracks. Case: 60% fewer thermal failures; punches lasted 10,000+ cycles vs 6,000 for SKD61. - SKT4 – Moderate Temp
Works <450°C, low-volume runs. Strength at room temp, easy machining, 20–30% cheaper than SKD61. Example: guide rolls ran 5 years with 35% total cost savings. - Insight
Select steel based on temperature, cycle frequency, and budget: SKD61 for extreme heat, SKD6 for thermal cycling, SKT4 for moderate-temp, cost-sensitive work.
Conclusion
Choosing SKD61 or SKD6/SKT4 depends on your application and operating conditions. SKD61 excels in high-temperature work (>500°C), resisting thermal fatigue and maintaining toughness, making it ideal for heavy die-casting and hot forging. SKD6/SKT4 performs well at moderate temperatures, offering excellent wear resistance and dimensional stability for precision work and cost-sensitive projects. The right steel choice directly affects tool life, product quality, and operating costs—using the optimal grade prevents early failures, reduces downtime, and ensures consistent production results.