304 vs 321 – Composition, Heat Treatment, Properties, and Applications

Introduction

Austenitic stainless steels 304 and 321 are among the most commonly specified grades in design, fabrication, and procurement. Engineers and purchasing professionals routinely weigh trade-offs between corrosion resistance, high-temperature stability, weldability, and cost when selecting between them. Typical decision contexts include pressure-vessel fabrication, food and beverage equipment, heat-exchanger components, and welded assemblies exposed to elevated temperatures.

The primary distinguishing feature is that one grade is stabilized against chromium carbide precipitation by intentional addition of a carbide former, improving resistance to intergranular corrosion in the 425–870 °C sensitization range. Because the two grades share similar chromium–nickel matrices, they are often compared for the incremental cost and performance benefits that stabilization offers in high-temperature or welded applications.

1. Standards and Designations

• Common international standards:
• ASTM/ASME: 304 (UNS S30400 / ASTM A240), 321 (UNS S32100 / ASTM A240)
• EN: 1.4301 (304), 1.4541 (321)
• JIS: SUS304, SUS321
• GB (China): 06Cr19Ni10 (approx. 304), 0Cr18Ni9Ti (approx. 321)
• Classification: Both are austenitic stainless steels (stainless), non-magnetic in the annealed condition, not carbon, alloy, tool, or HSLA steels.

2. Chemical Composition and Alloying Strategy

Notes: - The table lists common commercial ranges; exact limits depend on the specific standard and product form. - Grade 321 deliberately contains titanium in a controlled range to tie up carbon as titanium carbides/nitrides rather than allowing chromium carbide precipitation at grain boundaries. - Alloying strategy: Chromium provides corrosion resistance; nickel stabilizes the austenitic structure; titanium in 321 prevents sensitization, improving intergranular corrosion resistance after welding or high-temperature exposure.

3. Microstructure and Heat Treatment Response

• Typical microstructure (annealed): Both grades are fully austenitic with uniformly distributed austenite grains. Carbides are minimal in properly stabilized 321 and typically distributed fine in annealed 304.
• Sensitization and stabilization:
• 304 is susceptible to chromium carbide precipitation at grain boundaries when held in the 425–870 °C range (sensitization), which can promote intergranular corrosion.
• 321 forms titanium carbides/nitrides that preferentially consume carbon and nitrogen, minimizing chromium carbide formation and thus reducing susceptibility to intergranular attack.
• Heat treatment response:
• Austenitic stainless steels are not strengthened by quench-and-temper typical of ferritic/pearlitic steels. Solution annealing (e.g., 1010–1150 °C depending on spec) followed by rapid cooling restores corrosion resistance and ductility.
• Cold work increases strength via strain hardening and may alter corrosion performance; recovery/annealing is used to restore ductility.
• Normalizing is not applicable in the same sense as for carbon steels—solution anneal and quench differ from normalizing/quench & temper processes used to adjust microstructure in alloy steels.

4. Mechanical Properties

Explanation: - In the annealed condition both grades have very similar baseline mechanical properties because the matrix chemistry (Cr–Ni austenite) is comparable. - Cold working or strain-hardening markedly increases strength and hardness for both. - Stabilization with titanium has minimal influence on room-temperature strength but improves performance under thermal cycles by preventing grain-boundary carbide precipitation that can embrittle grain boundaries in sensitized 304.

5. Weldability

• Both 304 and 321 are considered readily weldable using common processes (GMAW/MIG, GTAW/TIG, SMAW). Low carbon content helps avoid weld metal hardening and hydrogen cracking issues typical in higher-carbon steels.
• Welding considerations:
• 304 can be susceptible to intergranular corrosion in the heat-affected zone (HAZ) if cooling is slow through the sensitization range or if service exposes welded assemblies to sensitizing temperatures.
• 321's titanium stabilizer ties up carbon, reducing chromium carbide precipitation in the HAZ; 321 is therefore preferred for welded components that will see prolonged high-temperature exposure.
• Common weldability indices (interpret qualitatively):
• Carbon equivalent (IIW):
  $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
  Lower $CE_{IIW}$ indicates easier welding; both grades have low carbon equivalents compared with high-alloy steels.
• Pitting resistance equivalent number (PREN) is not a weldability index but useful for corrosion ranking (see next section).
• Simplified combined parameter:
  $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
  Higher $P_{cm}$ can indicate higher cracking tendency; titanium increases $P_{cm}$ slightly but provides beneficial stabilization against sensitization.
• Practical guidance: For general fabrication where post-weld high-temperature exposure is unlikely, 304 is acceptable and cost-effective. For welded components that will operate in or be exposed to sensitization temperatures, 321 reduces the risk of intergranular corrosion without requiring post-weld solution annealing.

6. Corrosion and Surface Protection

• Stainless grades:
• Use PREN for evaluating pitting resistance when Mo and N vary:
  $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
  For 304 and 321, Mo and N are low, so PREN values are modest and both are not highly pitting-resistant compared with Mo-bearing duplex or superaustenitic grades.
• General corrosion: Both grades exhibit excellent resistance to general aqueous corrosion at ambient temperatures owing to their chromium content.
• Intergranular corrosion: 304 can be vulnerable after exposure to sensitization temperatures; 321 is stabilized to reduce this risk.
• Non-stainless steels:
• Not applicable here; common non-stainless surface protection methods (galvanizing, painting) are irrelevant for 304/321 intended to provide corrosion resistance by composition.
• When stainless is insufficient:
• For chloride-polluted environments or where pitting is a concern, consider Mo-bearing grades (e.g., 316) or duplex/superaustenitic alloys.

7. Fabrication, Machinability, and Formability

• Machinability:
• Austenitic stainless steels are generally more difficult to machine than carbon steels due to high work hardening and low thermal conductivity.
• 304 and 321 have comparable machinability; specialized tooling, slower speeds, and heavier feeds are typical.
• Formability:
• Excellent ductility and formability in the annealed condition allow deep drawing and complex forming for both grades.
• Springback and work hardening are greater than for mild steel; plan forming allowances and intermediate anneals for severe deformation.
• Surface finishing:
• Both take standard finishes (polish, passivation, electropolish) well. Passivation with nitric or citric acid is commonly used to restore surface chromium oxide after fabrication.
• Welding and post-processing:
• For assemblies that must avoid sensitization and cannot be solution-treated after welding, 321 can eliminate the need for expensive post-weld heat treatment.

8. Typical Applications

Selection rationale: - Choose 304 where general corrosion resistance, formability, and cost are primary criteria and where long-term exposure to sensitization temperatures is unlikely. - Choose 321 when welded assemblies or components will experience cyclic or sustained temperatures in the sensitization range, or where intergranular corrosion following thermal exposure is a critical failure mode.

9. Cost and Availability

• Relative cost:
• 304 is more common and generally less expensive than 321 due to higher production volumes and simpler chemistry.
• 321 incurs a modest premium for titanium addition and for being a specialty, high-temperature grade.
• Availability by product form:
• 304 is widely available in sheet, plate, coil, tube, bar, and fasteners globally.
• 321 is widely available but lead times and stock sizes may be smaller for some product forms (e.g., specialty tubes or large-diameter forgings) compared with 304.
• Procurement considerations:
• For large projects, price volatility in nickel markets affects both grades; 321 may show slightly higher sensitivity due to tighter production runs.

10. Summary and Recommendation

Recommendations: - Choose 304 if: - The application requires general-purpose corrosion resistance at ambient or moderate temperatures. - Cost, availability, and forming/machining convenience are primary concerns. - Welded assemblies will not be exposed to prolonged service in the 425–870 °C sensitization range or post-weld heat treatment is feasible. - Choose 321 if: - The part will be welded and will operate or be exposed repeatedly to temperatures that promote chromium carbide precipitation (sensitization), or where post-weld solution annealing is impractical. - High-temperature stability and resistance to intergranular corrosion are mission-critical (e.g., heat exchangers, exhaust systems). - Slightly higher material cost is acceptable for reduced maintenance and improved long-term reliability.

Final note: Both grades are durable, widely used austenitic stainless steels. The decision between 304 and 321 typically hinges on the exposure-temperature profile of the component and whether stabilization against intergranular corrosion (via titanium addition) is warranted to ensure long-term performance after welding or thermal cycling.