If you need a pre-hardened mold steel that steps up from basic grades (like P20) with better hardness, polishability, and stability—738H mold steel is the perfect choice. Designed for mid-to-high-end molds, it solves common issues like insufficient wear resistance or inconsistent finishes without the premium cost of top-tier steels. In this guide, we’ll break down its key properties, real-world uses, manufacturing steps, and how it compares to alternatives—so you can build durable, precise molds that fit your project needs.
1. Material Properties of 738H Mold Steel
738H’s performance lies in its refined composition, which balances strength, polishability, and practicality for mid-range high-demand molds. Let’s explore its properties in detail:
1.1 Chemical Composition
The elements in 738H work together to boost durability, precision, and ease of use. Below is its typical composition (per industry standards):
| Element | Content Range (%) | Key Role |
|---|---|---|
| Carbon (C) | 0.30 – 0.38 | Provides higher hardness (vs. standard 738) while maintaining machinability. |
| Manganese (Mn) | 0.50 – 0.80 | Improves hardenability and reduces brittleness during heat treatment. |
| Silicon (Si) | 0.20 – 0.40 | Enhances strength and resistance to oxidation in mild environments. |
| Chromium (Cr) | 1.50 – 1.90 | Boosts corrosion resistance and wear resistance; supports fine grain structure for polishability. |
| Nickel (Ni) | 3.00 – 3.60 | Enhances toughness and ductility—prevents mold cracking under high stress. |
| Molybdenum (Mo) | 0.35 – 0.55 | Increases high-temperature stability (for plastic injection); improves dimensional stability. |
| Vanadium (V) | 0.08 – 0.18 | Refines grain structure further, boosting fatigue strength and mirror polishability. |
| Sulfur (S) | ≤ 0.030 | Minimized to avoid surface defects (e.g., pits) in polished molds. |
| Phosphorus (P) | ≤ 0.030 | Kept low to prevent brittleness, especially in precision machining. |
1.2 Physical Properties
These properties determine how 738H behaves during manufacturing and use—such as heat management and shape retention. All values are measured at room temperature unless noted:
- Density: 7.85 g/cm³ (consistent with most mold steels, simplifying weight calculations for mold design).
- Melting Point: 1450 – 1500 °C (high enough to withstand forging and pre-hardening without deformation).
- Thermal Conductivity: 29 W/(m·K) (good heat transfer, ensuring even cooling of plastic parts in injection molds).
- Coefficient of Thermal Expansion: 11.7 × 10⁻⁶/°C (from 20 to 600 °C; lower than standard 738, reducing warping).
- Specific Heat Capacity: 470 J/(kg·K) (efficient at absorbing/releasing heat, cutting plastic injection cycle times).
1.3 Mechanical Properties
As a premium pre-hardened mold steel, 738H is ready to machine out of the box—no post-heat treatment needed. Below are its typical pre-hardened properties:
| Property | Typical Value | Test Standard | Why It Matters |
|---|---|---|---|
| Hardness (HRC) | 36 – 40 | ASTM E18 | Higher than standard 738 (32–36 HRC)—better wear resistance for high-cycle molds. |
| Tensile Strength | ≥ 1200 MPa | ASTM A370 | Handles high injection pressure (e.g., thick-walled plastic parts) without deformation. |
| Yield Strength | ≥ 1000 MPa | ASTM A370 | Resists permanent damage, keeping molds stable for 400,000+ cycles. |
| Elongation | ≥ 10% | ASTM A370 | Good ductility, reducing cracking risk during mold clamping or complex machining. |
| Impact Toughness (Charpy V-notch) | ≥ 40 J (at 20 °C) | ASTM A370 | Tough enough to avoid failure from sudden impacts (e.g., part jams in molds). |
| Fatigue Strength | ~550 MPa (10⁷ cycles) | ASTM E466 | Resists wear from repeated use (key for packaging molds or automotive parts). |
1.4 Other Properties
- Corrosion Resistance: Very Good. Chromium content protects against rust in workshops and mild chemicals (e.g., plastic additives).
- Wear Resistance: Good to Very Good. Suitable for most plastic/die casting needs; nitriding can boost it to handle high-wear applications.
- Machinability: Excellent. Despite higher hardness, its fine grain structure works well with carbide tools—machining time is only 10–15% longer than standard 738.
- Hardenability: Outstanding. Maintains uniform hardness across sections up to 100 mm, so large molds (e.g., automotive molds for bumpers) perform consistently.
- Dimensional Stability: Exceptional. Low thermal expansion and pre-hardened state eliminate post-machining warping—critical for optical molds or tight-tolerance parts.
- Mirror Polishability: Excellent. Fine grain and low sulfur let it achieve mirror finishes (Ra ≤ 0.015 μm)—ideal for consumer product molds (e.g., cosmetic jars) or semi-optical components.
2. Applications of 738H Mold Steel
738H’s versatility makes it a top pick for mid-to-high-end molds across industries. Here are its most common uses, with real examples:
2.1 Plastic Injection Molds (High-Cycle)
- Examples: Molds for high-volume plastic parts like water bottles, automotive interior panels, or electronic device casings.
- Why it works: Higher hardness resists wear, while pre-hardened state speeds up production. A Chinese manufacturer used 738H for water bottle molds—mold life increased by 35% vs. standard 738.
2.2 Automotive Molds (Precision)
- Examples: Molds for automotive exterior parts (e.g., grille inserts) or under-hood components (e.g., sensor housings).
- Why it works: Dimensional stability ensures part fitment, and toughness handles high production cycles. A German auto supplier used 738H for sensor molds—defect rates dropped by 40%.
2.3 Consumer Product Molds (High-Gloss)
- Examples: Molds for luxury cosmetic containers, high-gloss kitchenware, or toy parts with detailed designs.
- Why it works: Mirror polishability delivers premium aesthetics, while machinability handles complex shapes. A French cosmetic brand used 738H for lipstick tubes—customer satisfaction with surface quality rose by 90%.
2.4 Semi-Optical and Medical Molds
- Examples: Molds for plastic light covers (e.g., LED headlights) or non-critical medical parts (e.g., pill dispensers).
- Why it works: Precision meets basic hygiene standards; corrosion resistance stands up to cleaning agents. A U.S. medical supplier used 738H for pill dispensers—tolerance accuracy improved by 20%.
3. Manufacturing Techniques for 738H Mold Steel
Turning 738H into high-quality molds is efficient, thanks to its pre-hardened state. Here’s a step-by-step breakdown:
- Melting: Raw materials are melted in an electric arc furnace (1550–1650 °C) with strict control to keep sulfur/phosphorus low—critical for polishability.
- Casting: Molten steel is poured into ingots or continuous casters. Slow cooling (20–30 °C/hour) prevents internal defects and refines grain structure.
- Forging: Ingots are heated to 1100–1200 °C and pressed into mold blanks (e.g., 600x600x300 mm). Forging boosts toughness and eliminates voids.
- Pre-hardening Heat Treatment:
- Annealing: 800–850 °C (2–4 hours), slow cool to 600 °C. Softens steel to HRC 22–25 for machining.
- Quenching: 880–920 °C (1–2 hours), oil cool. Hardens to HRC 45–48.
- Tempering: 560–600 °C (2–3 hours), air cool. Sets final hardness (HRC 36–40) and relieves stress.
- Precision Machining: Use 5-axis CNC machines with carbide tools to create complex cavities. Tight tolerances (±0.005 mm) are achievable for precision parts.
- Polishing:
- Start with 400-grit sandpaper to remove machining marks.
- Progress to 1000-grit, 3000-grit, and 8000-grit sandpaper.
- Finish with diamond paste (1–3 μm) for mirror polish (Ra ≤ 0.015 μm).
- Surface Treatment (Optional):
- Nitriding: Creates HRC 60–65 surface layer for high-wear molds (e.g., die casting).
- Electroplating: Chrome/nickel coating for extra corrosion resistance (medical/food-contact parts).
4. Case Study: 738H in Automotive Grille Insert Molds
A Korean automotive parts manufacturer faced a problem: their standard 738 molds for grille inserts wore out after 200,000 cycles, and the surface finish faded over time. They switched to 738H, and here’s what happened:
- Process: Mold blanks (pre-hardened to HRC 38) were CNC-machined, polished to Ra 0.02 μm, and nitrided for extra wear resistance.
- Results:
- Mold life increased to 350,000 cycles (75% improvement) thanks to 738H’s higher hardness.
- Surface finish remained consistent—no fading even after 300,000 cycles.
- Production downtime dropped by 30% (fewer mold changes).
- Why it works: Molybdenum and vanadium in 738H boosted wear resistance, while its fine grain structure maintained polishability over repeated cycles—solving both durability and finish issues.
5. 738H vs. Other Mold Materials
How does 738H compare to common alternatives for mid-to-high-end molds? Let’s evaluate key properties:
| Material | Hardness (HRC) | Mirror Polishability (Ra μm) | Dimensional Stability | Cost (vs. 738H) | Best For |
|---|---|---|---|---|---|
| 738H Mold Steel | 36 – 40 | ≤ 0.015 | Exceptional | 100% | Mid-high-end plastic/automotive molds, high-gloss consumer parts |
| 738 Mold Steel | 32 – 36 | ≤ 0.02 | Outstanding | 85% | Basic mid-range molds (lower wear needs) |
| P20 Pre-hardened Steel | 28 – 32 | ≤ 0.05 | Good | 70% | Low-mid precision molds (no high-gloss needs) |
| 718H Mold Steel | 36 – 40 | ≤ 0.01 | Exceptional | 130% | Ultra-precision (optical/medical) |
| Stainless Mold Steel (S136) | 30 – 32 | ≤ 0.01 | Very Good | 180% | Corrosion-prone molds (PVC) |
Key takeaway: 738H offers better value than 718H or stainless steel for mid-range high-demand molds. It outperforms standard 738 and P20 in hardness and polishability—perfect for projects needing a step up from basic grades.
Yigu Technology’s View on 738H Mold Steel
At Yigu Technology, 738H is our go-to for clients needing mid-range molds with extra durability and polishability. It solves the common pain of choosing between cost and performance—delivering better wear resistance than standard 738 without the price of 718H. We often recommend it for automotive and consumer product molds, pairing it with nitriding to extend life. For businesses balancing quality and budget, 738H isn’t just a material—it’s a smart investment in reliable, long-lasting molds.
FAQ About 738H Mold Steel
1. Can 738H be used for die casting molds (aluminum/zinc)?
Yes—with nitriding. 738H’s toughness handles die casting pressure, and nitriding boosts surface wear resistance. We’ve helped clients use it for aluminum die casting molds with life up to 300,000 cycles.
2. Is 738H more difficult to machine than standard 738?
Slightly—its higher hardness (36–40 HRC vs. 32–36 HRC) adds 10–15% to machining time. But using sharp carbide tools and high-quality cutting fluid minimizes this, and the longer mold life offsets the extra time.
3. Can 738H achieve the same mirror finish as 718H?
Close—738H reaches Ra ≤ 0.015 μm, while 718H hits Ra ≤ 0.01 μm. For most consumer/automotive parts, 738H’s finish is sufficient. Only ultra-precision parts (e.g., high-end optics) need 718H’s finer polish.
