718H Mold Steel: Properties, Uses & Manufacturing for High-Precision Molds

What Makes 718H Mold Steel a Premium Choice?

The performance of 718H comes from its carefully optimized chemical composition and its unique pre-hardened state. It is designed to be strong, stable, and ready for precision machining right from the start.

Chemical Composition

Every element in 718H is selected to enhance precision, polishability, and toughness. The combination of nickel and chromium is particularly important for achieving a mirror finish.

Mechanical Properties in the Pre-Hardened State

A key advantage of 718H is that it is supplied in a pre-hardened state. This means it is ready for machining with its final properties already achieved.

Where Is 718H Mold Steel Used?

The unique combination of mirror polishability, dimensional stability, and durability makes 718H the material of choice for the most demanding mold applications.

Optical Molds

This is one of the most critical applications for 718H. The surface finish of the mold directly impacts the clarity of the optical part.

• Lens Molds: A Japanese optical manufacturer used 718H for smartphone camera lens molds. They achieved a consistent mirror finish (Ra 0.008 μm). Compared to the stainless mold steel they used before, polishing time was reduced by 40%, and the scrap rate from blurry optics dropped from 15% to 0.5%.
• Other Uses: Molds for light guides in LED panels and optical fibers also rely on 718H for its ultra-smooth surface and precision.

Medical Device Molds

The medical industry demands precision, consistency, and corrosion resistance.

• Syringe and Component Molds: A U.S. medical supplier switched to 718H for plastic syringe molds. The material’s corrosion resistance stood up to harsh medical disinfectants used during cleaning cycles. Its precision ensured part consistency, dropping defect rates to just 0.05%, which is critical for regulatory compliance.

Automotive High-Precision Molds

Automotive parts, especially interior components, require excellent surface finish and tight tolerances.

• Sensor Housings and Interior Panels: A German automotive supplier used 718H for molds producing sensor housings with a tolerance of ±0.02 mm. The material’s high yield strength handled high injection pressure without warping. The result was a significant increase in mold life, from 200,000 parts to 500,000 parts.

Consumer Product Molds

For luxury goods and high-gloss plastic items, the surface finish is a key selling point.

• Cosmetic and Luxury Molds: A French cosmetic brand used 718H for lipstick tube molds. The mirror polishability of the steel delivered the high-gloss surface their customers expect. The durability of the mold also allowed for high-volume production, contributing to a 95% customer satisfaction rate with the product’s surface quality.

How Is 718H Mold Steel Manufactured?

Creating a high-precision mold from 718H is a streamlined process, thanks to its pre-hardened state, but it requires careful attention to detail.

Steelmaking and Pre-Hardening

The process begins with creating a clean, uniform material and then heat-treating it to its final hardness.

• Melting: Raw materials are melted in an Electric Arc Furnace (EAF) with strict controls to ensure ultra-low sulfur and phosphorus content, which is critical for polishability.
• Forging: The steel is forged at high temperatures to refine the grain structure and eliminate internal voids, creating a solid foundation for the mold blank.
• Pre-Hardening Heat Treatment: This is a critical step where the steel is quenched and then tempered to achieve its final hardness of HRC 36–40. This eliminates internal stress, which is the primary cause of post-machining warping.

Precision Machining and Polishing

Because the steel is pre-hardened, the final shape is machined directly without the risk of distortion from a later heat treatment.

• Precision Machining: Mold blanks are machined using 5-axis CNC machines to create complex cavities with tight tolerances (as precise as ±0.001 mm). Carbide tools are recommended to handle the pre-hardened steel.
• Polishing: This is the most critical step for achieving a mirror finish. Start with 400-grit sandpaper to remove machining marks. Progress to finer grits (1000, 3000, 8000) to gradually refine the surface. Finish with diamond paste (1–3 μm) to achieve a mirror polish of Ra ≤ 0.01 μm. The fine grain structure of 718H allows it to reach this level without surface pits.

718H vs. Other Mold Materials

Selecting the right mold steel involves balancing cost, machinability, and final part quality. This comparison shows where 718H excels.

718H mold steel is a premium material engineered to meet the most exacting demands of high-precision mold making. Its pre-hardened state, with a hardness of 36–40 HRC, eliminates the risk of post-machining warping, ensuring exceptional dimensional stability. Its ultra-low sulfur content and fine grain structure enable a mirror polishability down to Ra ≤ 0.01 μm, which is critical for optical lenses and high-gloss consumer products. Real-world case studies demonstrate its ability to reduce polishing time by over 50% and extend mold life by more than 150%. While it comes at a higher cost than standard pre-hardened steels like P20, its superior performance, durability, and the elimination of post-heat-treatment steps make it a highly cost-effective and reliable choice for applications where quality and precision are paramount.

FAQ About 718H Mold Steel

Can 718H be used for die casting molds for aluminum or zinc?
Yes, but with an additional treatment. While 718H has good toughness to handle die casting pressure, its wear resistance can be enhanced. We recommend nitriding the surface to boost hardness to HRC 60–65. With this treatment, 718H can achieve a mold life of up to 300,000 cycles for aluminum die casting.

Is 718H more difficult to machine than standard pre-hardened steel like P20?
It is slightly more challenging due to its higher hardness. Machining time for 718H is typically 10–15% longer than for lower-hardness pre-hardened steels. However, using sharp carbide tools and high-quality cutting fluid minimizes this difference, and the time saved by eliminating post-machining heat treatment makes it more efficient overall.

What is the maximum mold size that 718H can handle while maintaining uniform properties?
718H has excellent hardenability. It maintains uniform hardness and dimensional stability for mold sections up to 120 mm thick. For thicker molds, a slow tempering cycle of over 3 hours is recommended to ensure the core and surface have consistent properties.

How does the mirror polishability of 718H compare to stainless mold steel?
Both can achieve a mirror finish of Ra ≤ 0.01 μm. However, 718H is pre-hardened, meaning you can polish it directly without the risk of distortion that comes with post-machining heat treatment of stainless mold steels. This makes 718H a more efficient and predictable choice for achieving ultra-smooth surfaces.

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.

Our solution for 718H 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 718H Mold Steel: Properties, Uses

The short version

718H 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.