If you’re searching for a pre-hardened mold steel that balances mirror polishability, dimensional stability, and cost-effectiveness for mid-to-high-end molds—738 mold steel is the ideal solution. Widely used in automotive, consumer product, and medical device molds, this alloy solves common pain points like inconsistent surface finishes, mold warping, or slow production. In this guide, we’ll break down its key properties, real-world applications, manufacturing steps, and how it compares to other materials—so you can create reliable, high-quality molds that fit your budget.
1. Material Properties of 738 Mold Steel
738’s appeal lies in its well-rounded composition, which delivers premium performance without the ultra-high cost of top-tier mold steels. Let’s explore its properties in detail:
1.1 Chemical Composition
The elements in 738 work together to enhance polishability, toughness, and stability—tailored for versatile mold-making. Below is its typical composition (per industry standards):
| Element | Content Range (%) | Key Role |
|---|---|---|
| Carbon (C) | 0.28 – 0.35 | Provides moderate hardness (for pre-hardened use) while keeping the steel machinable. |
| 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) | 1.40 – 1.80 | Boosts corrosion resistance and wear resistance; supports fine grain structure for mirror polishability. |
| Nickel (Ni) | 2.80 – 3.40 | Enhances toughness and ductility—prevents mold cracking under stress. |
| Molybdenum (Mo) | 0.30 – 0.50 | Increases high-temperature stability (useful for plastic injection molds); improves dimensional stability. |
| Vanadium (V) | 0.05 – 0.15 | Refines grain structure further, enhancing fatigue strength and 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 738 behaves during mold manufacturing and use—such as heat transfer and shape retention. All values are measured at room temperature unless noted:
- Density: 7.85 g/cm³ (consistent with most mold steels, making it easy to calculate mold weight).
- Melting Point: 1450 – 1500 °C (high enough to withstand forging and pre-hardening heat treatment).
- Thermal Conductivity: 30 W/(m·K) (good heat transfer, ensuring plastic parts cool evenly in injection molds).
- Coefficient of Thermal Expansion: 11.8 × 10⁻⁶/°C (from 20 to 600 °C; low expansion reduces mold warping during production).
- Specific Heat Capacity: 470 J/(kg·K) (efficient at absorbing and releasing heat, reducing plastic injection cycle times).
1.3 Mechanical Properties
As a pre-hardened mold steel, 738 is supplied ready for machining—no post-machining heat treatment needed. Below are its typical mechanical properties (in pre-hardened state):
| Property | Typical Value | Test Standard | Why It Matters |
|---|---|---|---|
| Hardness (HRC) | 32 – 36 | ASTM E18 | Balanced hardness—hard enough for mold durability, soft enough for easy machining and polishing. |
| Tensile Strength | ≥ 1100 MPa | ASTM A370 | Handles the pressure of plastic injection without deformation. |
| Yield Strength | ≥ 950 MPa | ASTM A370 | Resists permanent damage, keeping molds dimensionally stable for 300,000+ cycles. |
| Elongation | ≥ 12% | ASTM A370 | Good ductility, reducing the risk of cracking during mold clamping or complex machining. |
| Impact Toughness (Charpy V-notch) | ≥ 45 J (at 20 °C) | ASTM A370 | Excellent toughness—prevents mold failure from sudden impacts (e.g., part jams). |
| Fatigue Strength | ~500 MPa (10⁷ cycles) | ASTM E466 | Resists wear from repeated use (key for high-cycle packaging molds or automotive molds). |
1.4 Other Properties
- Corrosion Resistance: Good. Chromium content protects against rust in workshop environments and mild chemicals (e.g., plastic additives or mold release agents).
- Wear Resistance: Moderate to Good. Suitable for most plastic and die casting applications; nitriding can boost wear resistance for high-wear molds.
- Machinability: Excellent. Its pre-hardened hardness (HRC 32–36) and fine grain structure make it easy to mill, drill, and turn—reducing machining time by 20–25% vs. unhardened steels.
- Hardenability: Very Good. It maintains uniform hardness across thick sections (up to 100 mm), so large molds (e.g., automotive molds for door panels) have consistent performance.
- Dimensional Stability: Outstanding. Low thermal expansion and pre-hardened state prevent mold warping—critical for optical molds or precision consumer product molds.
- Mirror Polishability: Very Good. Fine grain structure and low sulfur content let it achieve mirror finishes (Ra ≤ 0.02 μm)—ideal for consumer product molds (e.g., cosmetic jars) or semi-optical parts.
2. Applications of 738 Mold Steel
738’s versatility makes it suitable for a wide range of mid-to-high-end mold applications. Here are its most common uses, with real examples:
2.1 Plastic Injection Molds
- Examples: Molds for making plastic parts like automotive interior components (e.g., center consoles), consumer electronics casings (e.g., tablet shells), or medical device housings.
- Why it works: Pre-hardened state speeds up production, while dimensional stability ensures part consistency. A Chinese plastic manufacturer used 738 for tablet casing molds—production time dropped by 30% vs. using unhardened steel.
2.2 Automotive Molds
- Examples: Molds for automotive exterior parts (e.g., mirror housings) or under-hood components (e.g., sensor brackets).
- Why it works: Toughness handles high-volume production stress, and corrosion resistance stands up to workshop chemicals. A German automotive supplier used 738 for sensor bracket molds—mold life increased from 180,000 to 350,000 parts.
2.3 Consumer Product Molds
- Examples: Molds for cosmetic containers (e.g., lotion bottles), kitchenware (e.g., high-gloss plastic spatulas), or toys (e.g., detailed action figures).
- Why it works: Mirror polishability delivers premium aesthetics, while machinability lets you create complex shapes. A French cosmetic brand used 738 for lotion bottle molds—customer complaints about surface flaws fell by 85%.
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 bottle caps).
- Why it works: Dimensional stability ensures part accuracy, and corrosion resistance meets basic medical hygiene standards. A U.S. medical supplier used 738 for pill bottle cap molds—part tolerance accuracy improved by 15%.
3. Manufacturing Techniques for 738 Mold Steel
Turning 738 into high-quality molds is streamlined thanks to its pre-hardened state. Here’s a step-by-step breakdown:
- Melting: Raw materials (iron, nickel, chromium, etc.) are melted in an electric arc furnace (EAF) at 1550–1650 °C. Strict control ensures uniform element distribution (critical for consistent hardness).
- Casting: Molten steel is poured into ingot molds or continuous casters to form slabs. Slow cooling (50–100 °C/hour) prevents internal defects and refines grain structure.
- Forging: Slabs are heated to 1100–1200 °C and pressed/hammered into mold blanks (e.g., 500x500x200 mm for injection molds). Forging improves toughness and eliminates voids.
- Pre-hardening Heat Treatment:
- Annealing: Heat to 800–850 °C, hold 2–4 hours, cool slowly. Softens steel to HRC 22–25 for initial machining.
- Quenching: Heat to 880–920 °C, hold 1–2 hours, quench in oil. Hardens steel to HRC 45–48.
- Tempering: Reheat to 580–620 °C, hold 2–3 hours, cool. Sets final pre-hardened hardness (HRC 32–36) and relieves internal stress.
- Machining: Mold blanks are milled, drilled, or turned into mold cavities using CNC machines. Carbide tools are recommended for tight tolerances (±0.005 mm).
- Polishing: Molds are polished to the desired finish—from matte (for functional parts) to mirror (for consumer products). 738’s fine grain structure lets it achieve Ra ≤ 0.02 μm with diamond paste.
- Surface Treatment (Optional):
- Nitriding: Creates a hard surface layer (HRC 60–65) to boost wear resistance for high-cycle molds.
- Electroplating: Adds a chrome or nickel coating to improve corrosion resistance (ideal for medical or food-contact molds).
4. Case Study: 738 in Automotive Center Console Molds
A Korean automotive parts manufacturer faced a problem: their P20 mold steel center consoles had inconsistent surface finishes and required post-heat treatment, which caused warping. They switched to 738, and here’s what happened:
- Process: Mold blanks (pre-hardened to HRC 34) were CNC-machined to console geometry, polished to Ra 0.4 μm (matte-gloss finish), and treated with a mold release coating.
- Results:
- Surface finish consistency improved—99% of consoles met automotive quality standards (up from 82% with P20).
- Production time per mold dropped from 3 weeks to 2 weeks (33% improvement) — no post-heat treatment needed.
- Warping eliminated—console fitment issues reduced by 70%.
- Why it works: 738’s pre-hardened state skipped heat treatment, while its nickel and molybdenum content prevented warping. Its fine grain structure also ensured uniform polishing across the console’s complex curves.
5. 738 vs. Other Mold Materials
How does 738 compare to common alternatives for mid-to-high-end molds? Let’s evaluate key properties:
| Material | Hardness (HRC) | Mirror Polishability (Ra μm) | Machinability | Dimensional Stability | Cost (vs. 738) | Best For |
|---|---|---|---|---|---|---|
| 738 Mold Steel | 32 – 36 | ≤ 0.02 | Excellent | Outstanding | 100% | Mid-to-high-end plastic molds, automotive parts, consumer products |
| P20 Pre-hardened Steel | 28 – 32 | ≤ 0.05 | Excellent | Good | 75% | Low-to-mid precision molds (no high-gloss needs) |
| 718H Mold Steel | 36 – 40 | ≤ 0.01 | Very Good | Exceptional | 130% | Ultra-precision molds (optical, medical) |
| Stainless Mold Steel (S136) | 30 – 32 | ≤ 0.01 | Fair | Very Good | 180% | Corrosion-prone molds (e.g., PVC) |
| Aluminum Mold Materials (7075) | 15 – 18 | ≤ 0.02 | Excellent | Poor | 80% | Prototype molds (low-volume) |
Key takeaway: 738 offers the best cost-performance ratio for mid-to-high-end molds. It’s more polishable and stable than P20, cheaper than 718H or stainless steel, and far more reliable than aluminum for production molds.
Yigu Technology’s View on 738 Mold Steel
At Yigu Technology, 738 is our top recommendation for clients needing versatile, cost-effective mid-to-high-end molds—like automotive parts or consumer products. Its pre-hardened state cuts production time, while its polishability and stability ensure consistent quality. We often use it for complex molds, as its machinability lets us create intricate designs without warping. For clients balancing quality and budget, 738 isn’t just a material—it’s a practical solution that delivers premium results without the premium price tag.
FAQ About 738 Mold Steel
1. Can 738 be used for die casting molds (e.g., aluminum or zinc)?
Yes, but we recommend nitriding to boost wear resistance. 738’s toughness handles die casting pressure, and its dimensional stability ensures part consistency—we’ve helped clients use nitrided 738 for aluminum die casting molds with life up to 250,000 cycles.
2. Is 738 suitable for molds that need ultra-mirror finishes (Ra ≤ 0.01 μm)?
738 can achieve Ra ≤ 0.02 μm, which is suitable for most consumer products. For ultra-mirror finishes (Ra ≤ 0.01 μm) (e.g., high-end optical parts), we recommend upgrading to 718H or stainless steel (S136), as their finer grain structure supports even smoother polishing.
3. How does 738’s cost compare to P20, and is it worth the extra expense?
738 costs about 25% more than P20, but it’s worth it for mid-to-high-end molds. It eliminates heat treatment costs, reduces scrap rates (better stability), and lasts 40–60% longer—saving money in the long run, especially for high-volume production.
