Metal Materials
738H Mold Steel: Properties, Uses & Manufacturing for Mid-Range Molds
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.
Key takeaways
- 738H’s performance comes from a refined composition. It balances strength, polishability, and practicality for demanding mold applications.
- 738H’s versatility makes it a top choice for mid-to-high-end molds across several industries.
- Turning 738H into quality molds follows a clear process. The pre-hardened state simplifies production.
- Choosing the right mold steel means balancing hardness, polishability, stability, and cost.
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
How to write a spec that gets priced correctly
A drawing is a contract. If it leaves a number open, the shop fills that number in with its own default. The default is rarely the one you had in mind. Put these five lines on every RFQ and most of the back-and-forth disappears before it starts.
- Alloy designation and the standard behind it
- Temper or heat treatment condition
- Mechanical properties you actually need
- Mill certificate and traceability requirement
- Finish and the faces it applies to
The point is not paperwork. Each line removes a decision the shop would otherwise make for you. A complete RFQ comes back priced. An incomplete one comes back guessed.
What actually drives the price
Buyers usually push on material because it is the line they can see. In practice the material is rarely the biggest number on a quote. These five lines decide most of it:
- alloy grade and mill availability
- stock size versus finished size
- heat treatment and testing
- machinability
- certification
Ask for the quote split by line. A supplier who cannot break it down is guessing, and you will pay for the guess. The line you can usually renegotiate is the tolerance band, not the raw material.
Problems you will meet, and the fix
Every process has a failure mode it prefers. The same four show up again and again on this kind of work, and none of them are mysteries once you know what to look for.
- Substituting a similar grade without telling anyone
- Hardness that varies across a single bar
- Corrosion in a weld heat-affected zone
- Distortion after heat treatment on a thin part
Catch the fault on the first part, not the hundredth. A first-article check costs an hour. A bad batch costs the whole order. Write the check into the purchase order so nobody has to remember it.
What to check on the first article
The first article is the cheapest inspection you will ever buy. Use it properly and the rest of the run becomes boring, which is exactly what you want.
- Every dimension on the drawing, with the measured value written beside it.
- The material certificate, matched to the heat or lot number on the parts.
- The finish against the sample you approved, under the same light.
- Fit against the mating part, assembled rather than measured.
- Packaging, because parts that arrive damaged were never in tolerance.
Check in this order: datums, then mating features, then cosmetic faces. If the datums are wrong, every number after them is noise. Keep the report on file. When a problem shows up in month six, that document is the only thing that tells you what changed.
How to compare two quotes fairly
Two numbers are not comparable unless the scope matches. Line them up before you decide, because a low number with three gaps is not a low number.
Same drawing, same material, same finish, same inspection. Anything else is a different job wearing the same part number. Then look at what each quote leaves out. Freight, packaging, certificates and tooling are the usual gaps.
Ask each supplier to state their assumptions in writing. The one who does is usually the one who has run the process before.
Scaling from one part to a batch
The first part teaches you the process. The hundredth teaches you the process cost.
Watch three things as volume grows: setup amortisation, tool wear, and how much hand work the finish still needs. If the unit price barely moves between ten and a hundred, the cost sits in material or in hand work, not in setup. Ask which, because only one of them is negotiable.
Lead time behaves the same way. A one-off is scheduled around other jobs. A repeat order is scheduled around your forecast. Give the shop a forecast and the lead time gets shorter, not longer.
How to read a supplier's quote
A quote is a short document with a long shadow. Four things in it decide whether the job runs smoothly, and only one of them is the price.
- The scope line. It should name the process, the material and the finish, not just the part.
- The tolerance basis. General tolerance or the drawing, and which standard it follows.
- The exclusions. Tooling, freight, certificates and inspection are the usual omissions.
- The validity date. Material prices move, and an open-ended quote is a promise nobody can keep.
Read the exclusions before you read the total. A quote that leaves out tooling is not cheaper. It is a quote for a different job.
What to send with the first order
The first order sets the pattern for every order after it. Send a complete package once and the shop files it, and the second order takes a fraction of the time.
- A STEP or native model plus a PDF drawing carrying the same revision letter.
- The material callout by standard number, not by trade name.
- The finish and the exact faces it applies to.
- Any test the part must pass: fit, seal, drop, salt spray, dielectric.
- A contact who can answer a question inside one working day.
Then write down what was agreed. An email that confirms the material, the finish and the date is worth more than a longer purchase order that leaves them open.
When this is the wrong choice
A good supplier tells you when the process does not fit. These are the cases where we say so.
- The volume is so low that setup costs more than the parts are worth.
- The tolerance is tighter than the process can hold without a second operation.
- The geometry cannot be made without a tool feature that will not pay back.
- The material you need is not available in the form the process requires.
- The schedule is shorter than the tooling or setup time, and nobody has said so yet.
None of these are refusals. They are reasons to look at a different route, and the earlier that conversation happens the cheaper it is. The most expensive mistake in manufacturing is a process chosen before the question was understood.
Planning the ramp from prototype to production
The gap between one good part and a thousand identical ones is a plan, not a machine. Three decisions close it.
First, freeze the design. Every change after tooling starts is a re-cut, and re-cuts are where schedules go. Second, decide the inspection level. A prototype is checked by eye; a production part is checked against a report. Third, agree the change process in writing, including who pays for it.
Write the ramp down as dates and owners, not as intentions. A ramp plan with three names and three dates is worth more than a page of good intentions.
What documentation to expect
Paperwork is not bureaucracy when a part fails at the customer. It is the only record of what was agreed. A normal shipment should carry four documents.
- A dimensional report covering every callout on the drawing.
- A material certificate traceable to the heat or lot number.
- A finish or coating certificate where the specification requires one.
- A packing list that matches the carton labels.
If a supplier cannot produce these on request, they are not running a controlled process. Ask for one sample set before you place the first order and you will know exactly what you are buying.
How to handle a change after the order
Changes happen. What matters is whether the change is priced, scheduled and recorded, or absorbed quietly and discovered later.
Send the change as a new revision, not as a note in an email. Ask for three answers in writing: what it costs, what it does to the date, and what happens to material or tooling already committed. Then decide.
A change without a revision number is a rumour. The shop will build what it has on file, and both sides will be right and unhappy at the same time.
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.
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.
Our solution for 738H Mold Steel: Properties, Uses
YiguProto runs metal sourcing and machining in house, from a single prototype to a production batch. We quote the process, the material and the finish as separate lines, so you can see where the money goes instead of arguing with one number.
Send a STEP file or a drawing and you get a DFM report in one to two business days. It flags the features that add cost and the features that will not hold tolerance on the process you picked.
Every order ships with an inspection report on the features you call out. If a part misses, we make it again. Ready to price a part? Start with a free quote and DFM review, or read more about our metal sourcing and machining. You can also talk to an engineer before you commit to a drawing.
FAQ about 738H Mold Steel: Properties, Uses
How long does a first article take?
Usually a few business days once the drawing is frozen, and longer if tooling is involved. The date we give you is the date we confirm in writing, not the date we hope for.
Is there a minimum order quantity?
No hard minimum. One part is fine. What changes with volume is which costs dominate, so a single unit is priced mostly on setup and a thousand units are priced mostly on cycle time and material.
What should I check before approving the parts?
Datums first, then the features that mate, then the cosmetic faces. Ask for a dimensional report, a material certificate and a finish sample. A part can be inside every tolerance and still not fit, so assemble one before you sign.
What tolerance can you hold on 738H Mold Steel: Properties, Uses?
The working default is ±0.05 mm on machined features, ±0.1 mm on formed ones. Tighter than that the process changes and so does the price, because every step tighter adds inspection time. Call out only the faces that mate with something else and leave the rest on the general tolerance.
What material should I specify?
Specify by standard, not by trade name. A trade name can change under you between two orders; a standard number cannot. Then add the one property that actually decides the part, whether that is strength, heat resistance or chemical exposure.
What files do you need in order to quote?
A STEP file plus a PDF drawing that carries the same revision letter. The model carries the geometry and the drawing carries the intent. When the two disagree, the shop picks one, and it is never the one you meant.
The short version
738H Mold Steel: Properties, Uses rewards the buyer who reads the data sheet and the drawing together. Pick the option for the property that matters, not for the price per kilo.
Tighten only what mates. Test a sample before the run. Then let the process do the rest.