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 issues like inconsistent finishes, warping, and slow production. This guide covers its properties, applications, and manufacturing methods.
What are the key properties of 738 mold steel?
738’s appeal lies in its well-rounded composition. It delivers premium performance without the ultra-high cost of top-tier mold steels. Its properties are tailored for versatile mold-making.
Chemical composition
The elements in 738 work together to enhance polishability, toughness, and stability.
| Element | Content Range (%) | Key Role |
|---|---|---|
| Carbon (C) | 0.28 – 0.35 | Provides moderate hardness while keeping steel machinable |
| Manganese (Mn) | 0.50 – 0.80 | Improves hardenability, reduces brittleness |
| Silicon (Si) | 0.20 – 0.40 | Enhances strength and oxidation resistance |
| Chromium (Cr) | 1.40 – 1.80 | Boosts corrosion resistance, supports fine grain structure |
| Nickel (Ni) | 2.80 – 3.40 | Enhances toughness, prevents cracking under stress |
| Molybdenum (Mo) | 0.30 – 0.50 | Increases high-temperature stability, improves dimensional stability |
| Vanadium (V) | 0.05 – 0.15 | Refines grain structure, enhances polishability |
Physical properties
These traits determine how 738 behaves during mold manufacturing and use.
- Density: 7.85 g/cm³ – standard for mold steels, simplifying weight calculations
- Melting point: 1,450–1,500°C – withstands forging and heat treatment
- Thermal conductivity: 30 W/(m·K) – ensures even cooling in injection molds
- Thermal expansion: 11.8 × 10⁻⁶/°C – low expansion reduces warping
- Specific heat capacity: 470 J/(kg·K) – reduces injection cycle times
Mechanical properties
As a pre-hardened mold steel, 738 is supplied ready for machining. No post-machining heat treatment is needed.
| Property | Typical Value |
|---|---|
| Hardness (HRC) | 32 – 36 |
| Tensile strength | ≥ 1,100 MPa |
| Yield strength | ≥ 950 MPa |
| Elongation | ≥ 12% |
| Impact toughness | ≥ 45 J at 20°C |
| Fatigue strength | ~500 MPa (10⁷ cycles) |
A Korean automotive parts manufacturer faced problems with P20 mold steel for center consoles. Inconsistent finishes and post-heat treatment warping caused issues. Switching to 738 solved these problems. Surface finish consistency rose from 82% to 99% meeting automotive standards. Production time dropped from 3 weeks to 2 weeks—a 33% improvement. Console fitment issues reduced by 70%.
Other key properties
- Corrosion resistance: Good. Chromium protects against rust in workshop environments.
- Wear resistance: Moderate to good. Nitriding can boost it for high-wear molds.
- Machinability: Excellent. The pre-hardened state reduces machining time by 20–25% compared to unhardened steels.
- Dimensional stability: Outstanding. Low thermal expansion prevents warping.
- Mirror polishability: Very good. Achieves finishes as smooth as Ra ≤ 0.02 μm.
Where is 738 mold steel used?
738’s versatility makes it suitable for a wide range of mid-to-high-end mold applications.
Plastic injection molds
Used for automotive interior components like center consoles, consumer electronics casings like tablet shells, and medical device housings. A Chinese plastic manufacturer used 738 for tablet casing molds. Production time dropped by 30% compared to unhardened steel.
Automotive molds
Used for exterior parts like mirror housings and under-hood components like sensor brackets. A German automotive supplier used 738 for sensor bracket molds. Mold life increased from 180,000 to 350,000 parts.
Consumer product molds
Used for cosmetic containers like lotion bottles, kitchenware like high-gloss spatulas, and detailed toys. A French cosmetic brand used 738 for lotion bottle molds. Customer complaints about surface flaws fell by 85%.
Semi-optical and medical molds
Used for plastic light covers like LED headlights and non-critical medical parts like pill bottle caps. A U.S. medical supplier used 738 for pill bottle cap molds. Part tolerance accuracy improved by 15%.
How is 738 manufactured?
Turning 738 into high-quality molds is streamlined thanks to its pre-hardened state.
Melting and casting
Raw materials melt in an electric arc furnace at 1,550–1,650°C. Strict control ensures uniform element distribution. Molten steel is poured into ingot molds or continuous casters. Slow cooling prevents internal defects and refines grain structure.
Forging
Slabs are heated to 1,100–1,200°C and pressed into mold blanks. Forging improves toughness and eliminates voids.
Pre-hardening heat treatment
This is the key step that makes 738 ready for use.
| Process | Temperature | Result |
|---|---|---|
| Annealing | 800–850°C, slow cool | Softens to HRC 22–25 for initial machining |
| Quenching | 880–920°C, oil cool | Hardens to HRC 45–48 |
| Tempering | 580–620°C, 2–3 hours | Sets final hardness at HRC 32–36, relieves stress |
Machining and polishing
Mold blanks are milled, drilled, and turned into cavities using CNC machines. Carbide tools achieve tight tolerances of ±0.005 mm. Polishing reaches finishes from matte to mirror. 738’s fine grain structure allows Ra ≤ 0.02 μm with diamond paste.
Surface treatment (optional)
- Nitriding: Creates a hard surface layer of HRC 60–65 for high-cycle molds
- Electroplating: Chrome or nickel coating improves corrosion resistance for medical or food-contact molds
How does 738 compare to other mold materials?
Understanding how 738 stacks up helps with material selection for mid-to-high-end molds.
| Material | Hardness (HRC) | Mirror Polish (Ra μm) | Machinability | Dimensional Stability | Cost vs. 738 | Best For |
|---|---|---|---|---|---|---|
| 738 | 32–36 | ≤ 0.02 | Excellent | Outstanding | 100% | Mid-to-high-end plastic molds, automotive, consumer products |
| P20 | 28–32 | ≤ 0.05 | Excellent | Good | 75% | Low-to-mid precision molds |
| 718H | 36–40 | ≤ 0.01 | Very good | Exceptional | 130% | Ultra-precision optical and medical molds |
| S136 stainless | 30–32 | ≤ 0.01 | Fair | Very good | 180% | Corrosion-prone molds for PVC |
| 7075 aluminum | 15–18 | ≤ 0.02 | Excellent | Poor | 80% | Prototype and low-volume molds |
Key takeaways:
- 738 offers the best cost-performance ratio for mid-to-high-end molds
- It’s more polishable and stable than P20
- It’s cheaper than 718H or stainless steel
- It’s far more reliable than aluminum for production molds
Conclusion
738 mold steel delivers a practical balance of polishability, stability, and cost for mid-to-high-end molds. Its pre-hardened state eliminates post-machining heat treatment, speeding up production. Its fine grain structure enables high-quality surface finishes, while its dimensional stability prevents warping. For automotive, consumer product, and medical molds where quality matters but budgets aren’t unlimited, 738 offers reliable performance at a reasonable price.
FAQ
Can 738 be used for die casting molds like aluminum or zinc?
Yes, but nitriding is recommended to boost wear resistance. 738’s toughness handles die casting pressure, and its dimensional stability ensures part consistency. Nitrided 738 can achieve mold life up to 250,000 cycles for aluminum die casting.
Is 738 suitable for molds needing ultra-mirror finishes below Ra 0.01 μm?
738 achieves Ra ≤ 0.02 μm, which suits most consumer products. For ultra-mirror finishes below Ra 0.01 μm, such as high-end optical parts, upgrade to 718H or stainless steel S136. Their finer grain structure supports smoother polishing.
How does 738’s cost compare to P20, and is it worth the extra expense?
738 costs about 25% more than P20. For mid-to-high-end molds, it’s worth the investment. It eliminates heat treatment costs, reduces scrap rates from better stability, and lasts 40–60% longer. For high-volume production, the return on investment is clear.
What hardness range works best for 738 molds?
The pre-hardened range of 32–36 HRC balances wear resistance and machinability. For applications needing extra wear resistance, nitriding adds a surface hardness of 60–65 HRC while maintaining core toughness.
Does 738 require special machining tools?
Standard carbide tools work well. Its pre-hardened state at 32–36 HRC is soft enough for efficient machining but hard enough to maintain precision. Use sharp tools and appropriate cutting speeds for best results.
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 738 for automotive and consumer product molds to custom machining and finishing, our team brings practical experience to your project. Contact us to discuss your next mold build.