309 vs. 321 vs. 304 Stainless Steel Tool Wrap: Which Grade for Heat Treating?
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If you heat treat tool steel without a vacuum furnace, stainless steel tool wrap is the single most cost-effective way to keep your parts bright, in-tolerance, and free of scale. But walk into any shop and you'll hear the same question: which grade of foil wrap do I actually need — 309, 321, or 304? The three look nearly identical on the roll, yet they behave very differently once the furnace hits temperature.
This guide breaks down the real differences between Type 309, Type 321, and Type 304 stainless steel tool wrap so you can match the right foil to your steel, your furnace temperature, and your budget — and stop guessing at the counter.
Quick Answer: Match the Foil to Your Furnace Temperature
Every grade of heat treating foil has a ceiling. Push past it and the foil weakens, tears, and lets oxygen reach your part — exactly what you were trying to prevent. Here's the short version before we dig in:
- Type 304 — the economical choice for lower-temperature work up to roughly 1800°F.
- Type 321 — the everyday workhorse, titanium-stabilized, rated up to 2000°F. This is what most shops reach for.
- Type 309 — the high-temperature grade, good up to 2240°F for high-speed steels and demanding cycles.
Why the Grade Matters at All
All three foils do the same basic job. At about 0.002 inches thick, stainless steel tool wrap forms a sealed envelope around your workpiece. As the furnace heats, the small amount of oxygen trapped inside reacts with the foil instead of with your tool steel. That protects the part from two expensive problems: scaling (a flaky oxide layer that has to be ground or blasted off) and decarburization (carbon burning out of the surface, leaving a soft skin that won't reach full hardness).
The difference between grades comes down to how much heat the foil itself can survive before it stops protecting the part. Chemistry drives that ceiling. A foil that's perfect at 1800°F can fail well before 2200°F, and paying for a high-temperature grade you don't need just adds cost. Choosing correctly is what separates a bright, clean part from a scaled disaster.
Type 321 Stainless Steel Tool Wrap: The Standard Workhorse
Type 321 is the grade most heat treaters use by default, and for good reason. It's titanium-stabilized, which makes it stable and reliable at high heat. The titanium acts as a scavenger, reacting with stray oxygen inside the packet before that oxygen can pull carbon from your part's surface.
Its average operating temperature is around 1850°F, with a working ceiling of about 2000°F. Keep in mind that real-world performance depends on time at temperature as much as the temperature itself — at 1850°F a properly sealed packet holds up for well over four hours, and it still performs at 2000°F for shorter cycles.
Best for: air-hardening tool steels such as A2, D2, and CPM grades, and the large majority of general hardening work that lives at or below 2000°F.
Type 309 Stainless Steel Tool Wrap: The High-Temperature Grade
When your cycle runs hotter than 321 can handle, Type 309 is the answer. Its higher chromium and nickel content lets it withstand temperatures up to about 2240°F — a meaningful margin above the 321 ceiling.
That extra headroom matters for high-speed steels (like M2) and any austenitizing schedule that climbs into the 2100°F–2240°F range. Running 321 at those temperatures risks foil breakdown and oxygen infiltration; 309 gives you a safety margin so the packet stays intact through the soak. It costs more than 321, but for high-temp work that cost is cheap insurance against a ruined part.
Best for: high-speed steels, high-alloy grades, and any process that exceeds the 321 temperature rating.
Type 304 Stainless Steel Tool Wrap: The Economical Option
Type 304 is the general-purpose, budget-friendly grade. Offered in an annealed form for easy folding, it's well suited to lower-temperature processes up to roughly 1800°F. If your work stays in that range, 304 protects the surface without paying the premium for a higher-temp alloy.
Best for: lower-temperature hardening, tempering-adjacent work, and cost-sensitive jobs that never approach the 321 or 309 range.
309 vs. 321 vs. 304 at a Glance
| Feature | Type 304 | Type 321 | Type 309 |
|---|---|---|---|
| Max temperature | ~1800°F | ~2000°F | ~2240°F |
| Key chemistry | Standard austenitic | Titanium-stabilized | High chromium & nickel |
| Relative cost | Lowest | Moderate | Highest |
| Typical use | Lower-temp, budget jobs | General tool steel (A2, D2, CPM) | High-speed & high-temp steels |
| Thickness | 0.002 in | 0.002 in | 0.002 in |
How to Choose the Right Tool Wrap in Three Steps
- Start with your austenitizing temperature. Pull the target hardening temperature for your specific steel grade from your data sheet or machinist's manual. That number, not price, drives the decision.
- Pick the grade with headroom above that number. Staying at or below 1800°F? Type 304 works. Living in the common 1800–2000°F band? Type 321. Running high-speed steels or anything above 2000°F? Step up to Type 309.
- Size the roll to your parts. Cut enough foil to cover the part twice with at least 3 inches of overlap on all sides for a clean, airtight seal. Not sure how much you'll go through? Our Tool Wrap Calculator does the math for you.
A Few Things That Apply to Every Grade
No matter which foil you choose, a few fundamentals decide whether you get vacuum-furnace-quality results:
- Clean the steel first. Degrease with acetone before wrapping. Oil, fingerprints, and grinding residue react badly at temperature and can weld the foil to the part.
- Seal it airtight. Fold every seam at least twice — three times for sharp-edged parts — and flatten with a seam roller. Tiny leaks cause the scaling and decarb you're trying to avoid.
- Don't reuse foil. After one cycle the foil becomes brittle and its protective chemistry is spent, so it can't form a tight seal again. Fresh foil every time.
- Vent oddly shaped parts. Large or awkward parts trap expanding air that can rupture the packet; leave a small vent during preheat, then close it with tongs.
For the full wrapping procedure, see our detailed guide on wrapping tool steel in foil wrap.
The Bottom Line
Choosing between 309, 321, and 304 stainless steel tool wrap isn't complicated once you anchor the decision to furnace temperature. Type 304 covers economical, lower-temperature work; Type 321 is the reliable everyday standard for most tool steels; and Type 309 buys you the extra margin high-speed steels and hot cycles demand. Match the grade to your process, seal it tight, and use fresh foil, and even a basic furnace setup can turn out clean, hard, in-tolerance parts.
Ready to stock up? Browse the full Tool Wrap catalog of 304, 321, and 309 stainless steel foil — with same business day shipping available.
Frequently Asked Questions
Is 309 or 321 tool wrap better?
Neither is universally "better" — it depends on temperature. Type 321 is the standard choice up to about 2000°F and costs less. Type 309 handles up to about 2240°F, making it the better pick for high-speed steels and hotter cycles where 321 would break down.
Can I use 304 stainless foil for high-temperature heat treating?
No. Type 304 is best kept to lower-temperature work up to roughly 1800°F. Above that range, step up to Type 321 or Type 309 so the foil survives the full soak without letting oxygen reach your part.
Why is Type 321 tool wrap so popular?
Type 321 is titanium-stabilized, which gives it excellent stability at high heat and lets the foil scavenge stray oxygen inside the packet. Combined with a 2000°F ceiling that covers most common tool steels, that makes it the default grade for the majority of shops.
What thickness of tool wrap should I use?
Standard heat treating foil is 0.002 inches thick across all three grades. That thickness balances flexibility for folding with enough durability to resist tears and transfer heat efficiently.
Can tool wrap be reused?
No. High temperatures make the foil brittle and consume the protective chemistry that blocks oxygen, so a used packet can't seal tightly or protect the part again. Always wrap with fresh foil.
How much tool wrap do I need for a part?
Cut a piece large enough to cover the part twice — top and bottom — with at least 3 inches of overlap on every side for sealing. For roll quantities across a batch of parts, use the Tool Wrap Calculator.