Hardness of SUS 321 vs 304 Stainless Steel: What Really Differs

Quick Answer: In the Annealed Condition There Is No Real Hardness Gap

SUS 304 and SUS 321 are both austenitic stainless steels supplied in the solution-annealed condition, and in that condition their hardness is essentially the same, typically about 150 to 190 HBW with a specified maximum of roughly 201 HBW for both. Hardness is not the property that separates the two grades. The real distinction is the titanium addition in 321, which stabilises the steel against sensitisation and preserves strength at elevated temperature. Where a hardness difference does appear between a piece of 304 and a piece of 321, it is almost always caused by cold work, grain size or an incomplete anneal rather than by the grade designation.

What Actually Controls Hardness in Austenitic Stainless Steel

Austenitic stainless steels are not hardened by heat treatment in the way martensitic or precipitation-hardening grades are. They are hardened by cold deformation and softened by annealing. Four factors dominate:

Cold work: rolling, drawing, bending or machining raises the dislocation density and can push hardness far above the annealed level.

Annealing practice: temperature, holding time and cooling rate decide whether the structure is fully recrystallised.

Grain size: a finer grain gives a slightly higher hardness at the same composition.

Composition: carbon, nitrogen and molybdenum give modest solid-solution strengthening, while chromium and nickel mainly decide phase stability and corrosion resistance rather than hardness.

Because 304 and 321 are chemically very close, the composition contribution is small enough that two heats of the same grade can differ more than the two grades differ from each other. This is also why the belief that 304 is the harder grade persists: it usually comes from comparing samples that were not in the same condition, for example a cold-drawn 304 bar against an annealed 321 bar, a 304 part that work-hardened during machining against a 321 part machined with lighter passes, or a cold-worked part whose slight magnetic response is mistaken for higher hardness. When both grades are fully annealed and tested with the same method, their hardness results overlap, and a few points of difference on the Rockwell B scale is normal scatter between heats.

Chemical and Mechanical Comparison

Item	SUS 304	SUS 321
UNS number	S30400	S32100
Carbon, max.	0.08%	0.08%
Chromium	18.0 - 20.0%	17.0 - 19.0%
Nickel	8.0 - 10.5%	9.0 - 12.0%
Titanium	not specified	5 x carbon, min.
Tensile strength, min.	515 MPa	515 MPa
Yield strength, min.	205 MPa	205 MPa
Typical annealed hardness	about 150 - 190 HBW	about 150 - 190 HBW

Two entries in this table correct a common misconception. First, the two grades share the same specified minimum tensile and yield strength, so neither is intrinsically the harder material. Second, 321 in fact carries the slightly higher nickel range and the slightly lower chromium range, which is the opposite of the usual claim that 304 is richer in both elements.

Where 321 Genuinely Outperforms 304

321 was developed for high-temperature service. Titanium is added at a minimum of five times the carbon content so that it combines with carbon to form stable titanium carbides. That has two consequences: resistance to intergranular corrosion after welding is retained without resorting to a low-carbon grade, and the steel keeps its strength at temperatures where 304 begins to lose it. Typical uses of 321 include superheater and heat exchanger tubing, expansion joints, exhaust and furnace components, and equipment operating continuously above roughly 500 degrees Celsius. 304 remains the workhorse for food, dairy, architectural and general piping applications where service temperature is moderate and corrosion demand is normal.

How to Specify Hardness Correctly

If hardness is a functional requirement, specify it together with the condition of supply. A useful purchase specification states the grade, the anneal condition, the hardness scale and the acceptance limit, for example a maximum of 201 HBW in the solution-annealed condition. Where cold-worked material is needed for strength, state the temper, such as quarter hard, half hard or full hard, because the temper and not the grade sets the hardness. Finally, ask for hardness results on the mill certificate for each heat so the delivered material can be verified against the order.

Frequently Asked Questions

Q: Is 304 harder than 321?
No, not in the annealed condition. Both grades are supplied solution annealed and typically fall in the same hardness range of about 150 to 190 HBW. Any difference measured between them is normally caused by cold work or by the heat treatment history of the individual piece.

Q: Which grade has higher nickel, 304 or 321?
321 has the higher nickel range, about 9.0 to 12.0%, compared with about 8.0 to 10.5% for 304. 304 has the higher chromium range, about 18.0 to 20.0% against 17.0 to 19.0% for 321.

Q: Can heat treatment be used to harden 304 or 321?
No. Annealing softens both grades. They can only be hardened by cold deformation or by a specialised surface treatment. Hardening by quenching, as used for martensitic stainless steel, does not apply to these austenitic grades.

Q: Why does my finished 304 part show a higher hardness than the mill certificate?
Because the part has probably been cold worked by machining, bending or drawing after the mill anneal. Hardness measured on a finished part reflects the fabrication history as well as the grade.

Q: Which grade should I choose for high-temperature service?
321 is the better choice for continuous high-temperature duty, because the titanium addition stabilises the carbon and preserves both strength and corrosion resistance. 304 is generally limited to moderate service temperatures.

Q: Do 304 and 321 have the same corrosion resistance at room temperature?
They are very similar in most ambient environments. 321 has the advantage after welding or after exposure to the sensitising temperature range, because the titanium prevents chromium depletion at the grain boundaries.