If you’re building molds that need better hardness and a smoother finish than basic grades like P20, 738H mold steel is a smart upgrade. It’s a pre-hardened steel designed for mid-to-high-end molds. It solves common problems like wear and inconsistent surfaces without the high cost of top-tier options. This guide covers its properties, uses, and how it compares to alternatives.
What makes 738H’s material properties stand out?
738H’s performance comes from a refined composition. It balances strength, polishability, and practicality for demanding mold applications.
Chemical composition
Each element in 738H is controlled to boost durability and precision.
| Element | Content Range (%) | Key Role |
|---|---|---|
| Carbon (C) | 0.30 – 0.38 | Provides higher hardness than standard 738 |
| Manganese (Mn) | 0.50 – 0.80 | Improves hardenability and reduces brittleness |
| Silicon (Si) | 0.20 – 0.40 | Enhances strength and oxidation resistance |
| Chromium (Cr) | 1.50 – 1.90 | Boosts corrosion resistance and wear resistance |
| Nickel (Ni) | 3.00 – 3.60 | Enhances toughness to prevent cracking under stress |
| Molybdenum (Mo) | 0.35 – 0.55 | Increases high-temperature stability |
| Vanadium (V) | 0.08 – 0.18 | Refines grain for better polishability |
Physical properties
These traits affect how 738H performs during manufacturing and use.
- Density: 7.85 g/cm³ – standard for mold steels, simplifying weight calculations
- Thermal conductivity: 29 W/(m·K) – ensures even cooling of plastic parts in injection molds
- Thermal expansion: 11.7 × 10⁻⁶/°C – lower than standard 738, reducing warping
- Specific heat capacity: 470 J/(kg·K) – helps cut plastic injection cycle times
Mechanical properties
As a pre-hardened steel, 738H is ready to machine without post-heat treatment.
| Property | Typical Value | Why It Matters |
|---|---|---|
| Hardness | 36–40 HRC | Better wear resistance for high-cycle molds |
| Tensile strength | ≥ 1,200 MPa | Handles high injection pressure without deformation |
| Yield strength | ≥ 1,000 MPa | Keeps molds stable for 400,000+ cycles |
| Elongation | ≥ 10% | Reduces cracking risk during clamping |
| Impact toughness | ≥ 40 J at 20°C | Avoids failure from sudden jams |
In one project, a manufacturer switched from standard 738 to 738H for water bottle molds. The higher hardness extended mold life by 35%. The pre-hardened state also cut lead time by skipping post-heat treatment.
Other key properties
- Corrosion resistance: Very good. Chromium protects against rust in workshops and mild chemicals.
- Wear resistance: Good to very good. Nitriding can boost it for high-wear applications.
- Machinability: Excellent. Even with higher hardness, machining time is only 10–15% longer than standard 738.
- Dimensional stability: Exceptional. Low thermal expansion prevents post-machining warping.
- Mirror polishability: Excellent. Fine grain and low sulfur allow finishes as smooth as Ra ≤ 0.015 μm.
Where is 738H mold steel used?
738H’s versatility makes it a top choice for mid-to-high-end molds across several industries.
Plastic injection molds (high-cycle)
738H works well for high-volume plastic parts like water bottles and electronic casings. A Chinese manufacturer used it for water bottle molds. Mold life increased by 35% compared to standard 738.
Automotive molds (precision)
Automotive exterior parts and under-hood components benefit from dimensional stability. A German auto supplier used 738H for sensor molds. Defect rates dropped by 40% after the switch.
Consumer product molds (high-gloss)
Luxury cosmetic containers and kitchenware need mirror finishes. A French cosmetic brand used 738H for lipstick tube molds. Customer satisfaction with surface quality rose by 90%.
Semi-optical and medical molds
Parts like LED light covers and pill dispensers need precision and basic hygiene. A U.S. medical supplier used 738H for pill dispenser molds. Tolerance accuracy improved by 20%.
How is 738H manufactured?
Turning 738H into quality molds follows a clear process. The pre-hardened state simplifies production.
Melting and casting
Raw materials melt in an electric arc furnace at 1550–1650°C. Strict controls keep sulfur and phosphorus low, which is critical for polishability. The molten steel is cast into ingots and cooled slowly to refine the grain structure.
Forging
Ingots heat to 1100–1200°C and are pressed into mold blanks. This step boosts toughness and eliminates internal voids.
Pre-hardening heat treatment
This is the key step that makes 738H ready for use.
| Process | Temperature | Result |
|---|---|---|
| Annealing | 800–850°C, slow cool | Softens to HRC 22–25 for machining |
| Quenching | 880–920°C, oil cool | Hardens to HRC 45–48 |
| Tempering | 560–600°C, air cool | Sets final hardness at HRC 36–40, relieves stress |
Precision machining
5-axis CNC machines with carbide tools create complex cavities. Tight tolerances of ±0.005 mm are achievable for precision parts.
Polishing
Polishing steps progress from 400-grit to 8000-grit sandpaper. A final diamond paste finish (1–3 μm) achieves mirror polish at Ra ≤ 0.015 μm.
Surface treatment (optional)
- Nitriding: Creates a HRC 60–65 surface layer for high-wear molds
- Electroplating: Chrome or nickel coating adds corrosion resistance for medical or food-contact parts
How does 738H compare to other mold materials?
Choosing the right mold steel means balancing hardness, polishability, stability, and cost.
| Material | Hardness (HRC) | Mirror Polish (Ra μm) | Dimensional Stability | Cost vs. 738H | Best For |
|---|---|---|---|---|---|
| 738H | 36–40 | ≤ 0.015 | Exceptional | 100% | Mid-high-end plastic and automotive molds |
| 738 | 32–36 | ≤ 0.02 | Outstanding | 85% | Basic mid-range molds |
| P20 | 28–32 | ≤ 0.05 | Good | 70% | Low-mid precision molds |
| 718H | 36–40 | ≤ 0.01 | Exceptional | 130% | Ultra-precision optical or medical molds |
| S136 Stainless | 30–32 | ≤ 0.01 | Very good | 180% | Corrosion-prone molds for PVC |
Key takeaways:
- 738H offers better value than 718H or stainless steel for most mid-range high-demand molds
- It outperforms standard 738 and P20 in hardness and polishability
- For projects needing a step up from basic grades, 738H hits the sweet spot
What does a real project look like?
A Korean automotive parts manufacturer had a problem. Their standard 738 molds for grille inserts wore out after 200,000 cycles. Surface finish also faded over time.
They switched to 738H. Here are the results:
- Process: Mold blanks at HRC 38 were CNC-machined, polished to Ra 0.02 μm, and nitrided
- Mold life: Increased to 350,000 cycles – a 75% improvement
- Surface finish: Remained consistent even after 300,000 cycles
- Downtime: Dropped by 30% due to fewer mold changes
Why did it work? Molybdenum and vanadium in 738H boosted wear resistance. The fine grain structure maintained polishability over repeated cycles. The switch solved both durability and finish issues.
Conclusion
738H mold steel delivers a strong balance of hardness, polishability, and dimensional stability. It’s a practical upgrade from standard 738 and P20 for mid-to-high-end molds. With its pre-hardened state, it simplifies manufacturing while providing better wear resistance and surface quality. For automotive, consumer, and precision molds, it offers reliable performance without the premium cost of top-tier steels.
FAQ
Can 738H be used for die casting molds?
Yes, with nitriding. 738H’s toughness handles die casting pressure. Adding nitriding boosts surface wear resistance. We’ve seen it used for aluminum die casting molds with life up to 300,000 cycles.
Is 738H harder to machine than standard 738?
Slightly. Its higher hardness (36–40 HRC vs. 32–36 HRC) adds about 10–15% to machining time. Using sharp carbide tools and quality cutting fluid minimizes the difference. The longer mold life usually offsets the extra time.
Can 738H achieve a mirror finish like 718H?
Close. 738H reaches Ra ≤ 0.015 μm, while 718H hits Ra ≤ 0.01 μm. For most consumer and automotive parts, 738H’s finish is sufficient. Only ultra-precision parts like high-end optics need 718H’s finer polish.
What hardness range is best for 738H molds?
The standard pre-hardened range is 36–40 HRC. This balance provides good wear resistance while maintaining machinability. For higher wear applications, nitriding can add a surface hardness of 60–65 HRC.
How long do 738H molds typically last?
In plastic injection applications, mold life often reaches 350,000 to 500,000 cycles. With proper maintenance and surface treatments like nitriding, some molds exceed 1 million cycles.
Discuss Your Projects with Yigu Rapid Prototyping
At Yigu Rapid Prototyping, we help manufacturers select and process the right mold steels for their applications. From 738H for automotive molds to custom machining and finishing, our team brings practical experience to your project. Contact us to discuss your next mold build.