304 vs 316L vs 321 Stainless Steel Plate: Which Grade to Buy?

For most general fabrication, 304 plate is the right choice; specify 316L when chlorides or chemicals are present; and choose 321 when the plate will be welded and run between 400–800 °C. Those three sentences cover 80% of real-world decisions — the rest of this article explains why, with the numbers to back them up.

On this page: 1. Side-by-Side Chemistry (ASTM A240) 2. Mechanical Properties (ASTM A240 Minimums) 3. Corrosion Resistance: The PREN Ranking 4. Temperature & Welding Behavior 5. Cost Reality Check 6. Frequently Asked Questions 7. Talk to a Stockist

1. Side-by-Side Chemistry (ASTM A240)

The only chemistry differences are molybdenum (316L) and titanium stabilization (321) — but they change performance dramatically.

2. Mechanical Properties (ASTM A240 Minimums)

3. Corrosion Resistance: The PREN Ranking

Pitting Resistance Equivalent Number (PREN = %Cr + 3.3×%Mo + 16×%N) is the quickest way to rank chloride performance:

Key insight buyers miss: 321 is a temperature grade, not a corrosion grade. Its titanium carbides prevent chromium-carbide precipitation during welding and long service in the 400–800 °C sensitization range — which is why it dominates in exhaust systems, furnace parts and welded pressure equipment, not because it resists salt better.

4. Temperature & Welding Behavior

All three grades weld readily with standard austenitic filler (308L for 304, 316L for 316L, 347 or 321 filler for 321) — no preheat required, and post-weld annealing is generally unnecessary thanks to their low-carbon or stabilized chemistries.

5. Cost Reality Check

As a rough procurement index (304 = 1.00):

• 304 plate — 1.00 (the benchmark)
• 321 plate — 1.05–1.15 (small titanium addition)
• 316L plate — 1.2–1.4 (molybdenum is expensive)

Paying the 316L premium when you don't have chlorides is wasted budget; not paying it when you do is a corrosion failure waiting to happen. Match the grade to the medium, not to the brochure.

6. Frequently Asked Questions

Can I substitute 321 plate for 304 plate?

Yes in most cases — 321 meets or exceeds 304's mechanical minimums and adds weld-temperature stability, at roughly 5–15% higher cost. The reverse substitution (304 for 321) is risky above 400 °C in welded structures.

Is 316L magnetic?

In the fully annealed state, essentially no. Cold working (cutting, forming) can induce slight magnetism in all three grades — this is a structure effect, not a quality defect.

Which grade for a welded hot-water tank?

304 for municipal water; 316L if the water is chlorinated, brackish, or the tank is within a few kilometers of the coast.

Do you supply these grades in plate and sheet?

Yes — Shanghai Hangbo Alloy Group stocks 304/304L, 316L and 321 plate from thin sheet to heavy plate, with ASTM A240 / GB/T 24511 certification and CNAS/SGS bilingual test reports on every heat.

7. Talk to a Stockist, Not a Middleman

Need 304, 316L or 321 plate cut to size? Shanghai Hangbo Alloy Group supplies full-size and custom-cut plates with plasma/flame cutting, heat treatment and third-party inspection under one roof.