Heat treatment for EDC parts: D2, SKD11, H13 and what it adds to your schedule

Most EDC hardware never needs heat treatment. The parts that do — lock faces, detent components, pivots, wear inserts — need it absolutely, and the sequence it forces changes how the whole job is planned.

The sequence

Hardened steel cannot be machined efficiently at final hardness. So the job runs:

1. Machine soft, leaving stock on the critical surfaces
2. Heat treat to the specified hardness
3. Grind, lap or wire EDM to final dimension
4. Finish

Step three exists because parts move during hardening. Not much, but enough: a few hundredths of a millimetre of distortion is normal, and on a lock face that is the whole tolerance.

What this does to lead time

Heat treatment is almost always an outsourced step, which means a handoff out and a handoff back. One to two weeks added to the schedule is normal for a small EDC batch, sometimes more if the treater batches by furnace load and yours is small.

That is the honest number to plan around. A four-week part becomes a six-week part, and no amount of pressure changes it, because the furnace cycle has a fixed duration.

The common grades

D2 — high carbon, high chromium, air hardening. Hardens to about 58 to 62 HRC. Excellent wear resistance, the standard choice for lock bar inserts and wear surfaces. Not stainless despite the chromium content, so it will rust if left bare.

SKD11 — the Japanese equivalent of D2, essentially interchangeable. Frequently what a Vietnamese shop will actually stock, so specifying “D2 or SKD11” rather than D2 alone can shorten material lead time at no functional cost.

H13 — hot work tool steel, tougher and less wear-resistant than D2. Around 48 to 52 HRC typically. Use it where the part takes impact rather than sliding wear.

440C — covered in stainless for EDC. Hardens to around 58 to 60 HRC and is the option when the part must be both hard and corrosion resistant.

Specifying hardness properly

Give a range, not a number. “58-62 HRC” is achievable; “60 HRC” is not a thing anyone can guarantee.

Say where it is measured if the part has thin and thick sections, because through-hardening a thin section is easy and a thick one less so.

If the part must be hard on one surface and tough underneath, that is case hardening or selective treatment, which is a different process and a different conversation. Say what the part does rather than prescribing a process you are not sure about.

Distortion and how to design around it

Sharp internal corners, abrupt section changes and asymmetric geometry all concentrate stress during quenching and increase movement. Where you can:

• Add generous radii at internal corners
• Keep section thickness reasonably uniform
• Avoid deep blind slots near a thin wall

None of this is always possible on a lock insert. Where it is not, leave more grind stock and expect a slightly higher cost.

Stress relief

If the part is machined heavily from a solid block, ask about stress relieving between roughing and finishing — even for parts that are not being hardened. Material removed unevenly from a rolled plate releases residual stress and the part bows. It is a common cause of scales and liners that arrive flat-ish rather than flat.

What to write in the RFQ

State the grade with an acceptable equivalent, the hardness range, which surfaces are critical after hardening, and that you understand grinding follows. Something like: “SKD11 or D2. Machine soft, harden 58-62 HRC, grind lock face and pivot bore to drawing. Other surfaces as-heat-treated.”

That paragraph tells a shop you know what you are asking for, and it gets quoted properly the first time. Send the part and we will route it to shops with the treater relationship already in place.

What actually happens in the furnace

Understanding the sequence helps you read a quote and a schedule.

Austenitising. The part is heated to a temperature where the steel’s structure changes — around 1010 to 1040 degrees Celsius for D2. It is held long enough for the change to complete through the section, which is why thick parts take longer than thin ones.

Quenching. Rapid cooling locks in a hard, brittle structure called martensite. D2 and SKD11 are air-hardening, which means still or forced air rather than an oil or water quench. Air hardening is gentler and produces less distortion, which is one reason these grades are popular for precision parts.

Tempering. The as-quenched part is too brittle to use. Reheating to a lower temperature — typically 200 to 550 degrees Celsius depending on target hardness — trades a little hardness for toughness. Usually done twice, sometimes three times.

Cryogenic treatment. Optional, increasingly common. Cooling to around minus 80 or minus 196 degrees Celsius between quench and temper converts retained austenite to martensite, improving hardness stability and wear resistance. For a lock face that must not move over years, it is worth asking about.

The whole cycle is typically one to three days of furnace time, plus queue, plus transport each way. That is where the one-to-two-week schedule addition comes from.

Vacuum versus atmosphere furnaces

Vacuum heat treatment leaves the part clean and bright with no scale or decarburisation. It costs more and it is what you want for a precision part that will not be ground on every surface.

Atmosphere or salt bath leaves surface scale and some decarburisation — a softened outer skin. Fine if every critical surface is being ground afterwards, a problem if not.

Ask which is being used. For small EDC parts with some as-heat-treated surfaces, vacuum is usually the right answer and the cost difference at these quantities is small.

Decarburisation

Worth its own note because it is a silent failure. In an uncontrolled atmosphere, carbon leaves the surface layer at temperature, leaving a skin that is softer than the core — sometimes 0.1 to 0.3 mm of it.

On a lock face that is then ground, the soft skin is removed and nothing is wrong. On a surface left as-heat-treated, you have a part that measures the right hardness at the core and wears like mild steel at the surface.

Specify vacuum, or specify grind stock on every surface that must be hard.

How much grind stock to leave

A practical guide for small EDC parts:

• Air-hardened D2/SKD11, simple geometry: 0.1 to 0.2 mm per surface
• Complex or asymmetric geometry: 0.2 to 0.4 mm
• Long thin parts: more, and expect some to be unrecoverable

Leave it only on the surfaces that need to be accurate. Grind stock on every face means grinding every face, which is where the cost goes.

Wire EDM as an alternative to grinding

For internal features, slots and profiles that a grinding wheel cannot reach, wire EDM cuts hardened steel directly and holds excellent tolerance.

It is slower per millimetre than grinding but it needs no fixturing gymnastics and it produces no heat-affected distortion of consequence. For a lock bar insert with a precise profile, EDM after hardening is frequently the correct route and it is how many production knife parts are made.

Ask for it by name if your part has a profile feature that must be accurate after hardening.

Verifying hardness

Ask for a hardness test report with the first article. Rockwell C is the standard scale for these grades.

Note that the test leaves a small indentation, so specify where it may be taken — on a non-cosmetic face, or on a sacrificial coupon run with the batch. A coupon is the usual answer for a small batch and it costs almost nothing.

For how to test and report the result, see Rockwell hardness testing of blades and clips.

FAQ

Can I heat treat after anodizing or coating? No. Heat treatment temperatures destroy anodize and most coatings. The sequence is always machine, harden, grind, then finish.

Will the part change size? Yes, slightly, and unpredictably in direction. D2 typically grows a small amount. That is exactly why grind stock exists.

Is SKD11 genuinely the same as D2? Close enough that they are used interchangeably for this work. Compositions differ marginally. Writing “D2 or SKD11” in the RFQ lets the shop use whichever it stocks, which can save two weeks of material lead time at no functional cost.

Do I need heat treatment for a titanium part? No. Titanium Grade 5 comes in the condition you want and is not hardened further for EDC use. See Grade 2 vs Grade 5.

What hardness should a lock face be? Depends on the lock geometry and what it bears against. Typical wear inserts sit around 58 to 60 HRC. Harder is not automatically better — a lock face harder than its mating surface will wear the mating surface instead.

Can small quantities be heat treated economically? Yes, because treaters batch multiple customers’ work in one furnace load. The cost per part at 50 pieces is reasonable; the lead time is the same as at 5,000. Send the part and the treater queue gets built into the schedule you are quoted.