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

Introduction

Stainless steels 304 and 321 are two of the most widely specified austenitic grades in industry. Engineers, procurement managers, and manufacturing planners commonly weigh corrosion resistance, high‑temperature stability, weldability, and cost when choosing between them. The selection dilemma typically centers on whether to prioritize general corrosion resistance and cost-efficiency (304) or to resist carbide precipitation and intergranular attack at elevated temperatures (321).

The principal metallurgical distinction is that 321 is stabilized by titanium additions that tie up carbon as carbonitrides, markedly improving resistance to intergranular corrosion after exposure to sensitizing temperature ranges. Because both are austenitic, they are frequently compared for piping, vessels, heat exchangers, and fabricated components used in service that may experience elevated temperatures and welding.

1. Standards and Designations

• 304
• Common designations: AISI 304, UNS S30400, EN 1.4301, JIS SUS304, GB 06Cr19Ni10
• Type: Austenitic stainless steel (stainless)
• Relevant standards: ASTM A240 (plates), ASTM A276 (bars), ASTM A312 (pipes), ASME SA-240, EN 10088
• 321
• Common designations: AISI 321, UNS S32100, EN 1.4541 (or 1.4541/1.4878 variants), JIS SUS321, GB 06Cr19Ni10Ti
• Type: Austenitic stainless steel (titanium-stabilized stainless)
• Relevant standards: ASTM A240, ASTM A312, ASME SA-240, EN 10088

Both are classified as stainless austenitic steels; they are not carbon, tool, or HSLA steels.

2. Chemical Composition and Alloying Strategy

The table below shows typical composition ranges (weight %) per widely used standards. Exact limits depend on specific standards and product forms; listed values are representative.

Alloying strategy summary: - Chromium and nickel produce a stable austenitic matrix and provide general corrosion resistance. Nickel also improves toughness and formability. - Titanium in 321 preferentially forms titanium carbides/nitrides (TiC, TiN) that prevent chromium carbide precipitation at grain boundaries when steel is exposed to sensitizing temperatures (approximately 450–850 °C). This stabilization reduces susceptibility to intergranular corrosion after welding or prolonged high‑temperature service. - Low carbon limits reduce the driving force for carbide precipitation; in 304L (low‑carbon 304) the “L” variant offers another route to reduce sensitization without stabilization.

3. Microstructure and Heat Treatment Response

• Typical microstructure: Both 304 and 321 are fully austenitic (face-centered cubic) in the annealed condition. They contain occasional delta ferrite depending on melting and processing, but predominantly austenite.
• Response to thermal cycles:
• Annealing: Solution annealing (e.g., 1010–1150 °C depending on product) dissolves carbides and homogenizes. Rapid cooling is used to avoid sensitization, but 321 is less sensitive because Ti forms stable carbides.
• Welding: Localized heating in the 450–850 °C range can allow chromium carbide precipitation at grain boundaries in unstabilized grades. 321’s titanium ties up carbon and nitrogen, limiting chromium carbide formation and preserving intergranular corrosion resistance.
• Thermo‑mechanical processing: Cold working increases dislocation density and can lead to strain‑induced martensite in some austenitic stainlesses (less common in fully stabilized grades). Both grades can be work-hardened; recovery occurs on heating.
• Heat treatment routes like normalizing, quenching & tempering are not typically applied to austenitic stainless steels for strengthening — they are strengthened by work hardening and solid solution; precipitation hardening is not applicable to 304/321.

4. Mechanical Properties

The properties below are typical for annealed condition; product form (sheet, plate, bar), thickness, and standard influence values.

Interpretation: - In the annealed condition, 304 and 321 have very similar tensile, yield, and ductility characteristics because their matrix chemistry is broadly comparable. Differences in mechanical performance are typically minor and overshadowed by processing history (cold work) or product form. - Toughness is high for both at ambient temperatures; both maintain reasonable impact resistance at moderately elevated temperatures. Strength increases with cold work for both grades.

5. Weldability

Both 304 and 321 are considered readily weldable with standard austenitic stainless welding practice. Welding considerations: - Carbon content and stabilization: Higher carbon content increases the risk of chromium carbide precipitation in the heat‑affected zone (HAZ). 321’s titanium reduces this risk by forming TiC/TiN, which is especially valuable when post‑weld heat exposure or long service at sensitizing temperatures is expected. - Hardenability is low; austenitic stainless steels are non‑hardenable by quench; hydrogen and solidification cracking concerns must be managed via appropriate filler metal and technique.

Use of weldability indices (qualitative guidance): - Example carbon equivalent for welding: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ This formula helps predict susceptibility to cold cracking and need for pre/post‑weld heat treatment in steels. For austenitic stainless steels, the absolute CE is less directly applicable, but the approach emphasizes that alloying elements influence weld behavior. - An extended parameter used in stainless steels: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$ $P_{cm}$ is used to estimate the propensity to form intermetallics or delta ferrite in welds; qualitative interpretation suggests that small Ti additions (as in 321) alter the balance of phases in the HAZ and weld metal.

Practical implications: - Use matching or low‑carbon filler metals for 304 applications to avoid sensitization (e.g., ER308L for welding 304). - When joining 304 that will see service in the sensitizing range, consider 321 base material, low‑carbon 304L, or stabilized filler metal, depending on application and cost.

6. Corrosion and Surface Protection

• For stainless grades only (304, 321):
• General corrosion resistance is governed largely by chromium content and the continuity of the passive Cr2O3 film. Neither grade contains molybdenum, so their pitting resistance in chloride environments is limited compared with Mo-bearing grades.
• PREN (Pitting Resistance Equivalent Number) is not particularly discriminating here because Mo and N contributions are small or absent; for reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ For 304/321 (Mo = 0), PREN is essentially the Cr content plus any small N effect — both grades have similar PREN.
• Intergranular corrosion: 304 is susceptible to intergranular attack after exposure in the sensitization range if carbon is present and carbide precipitation occurs at grain boundaries. 321’s titanium scavenges carbon and reduces this corrosion mode, making 321 preferable for welded components or components exposed to sustained elevated temperatures.
• For non‑stainless steels (not applicable here): common protections include galvanizing, painting, and coatings — irrelevant to 304/321 selection except where mixed material systems are used.

7. Fabrication, Machinability, and Formability

• Cutting and Machinability:
• Austenitic stainless steels are generally more difficult to machine than carbon steels due to work hardening and low thermal conductivity. 304 is slightly easier to machine than 321 in practice, but differences are modest.
• Use rigid setups, sharp tooling, higher feed rates, and appropriate coolant to minimize work hardening.
• Formability and bending:
• Both grades have excellent formability in the annealed condition and can be deep‑drawn and formed. 321’s titanium stabilization can slightly reduce susceptibility to strain aging effects in some cases but does not materially change formability for most operations.
• Surface finishing:
• Both respond well to polishing and electropolishing; note that grinding or welding will need post‑process passivation to restore corrosion resistance in critical applications.

8. Typical Applications

Selection rationale: - Choose 304 for cost‑sensitive, general‑purpose corrosion resistance and where operating temperatures remain below the sensitization range or where low‑carbon 304L is specified for welded structures. - Choose 321 where welded assemblies or components are exposed to repeated or sustained temperatures in the sensitizing range and intergranular corrosion risk must be minimized, or where oxidation resistance at moderate elevated temperatures is required.

9. Cost and Availability

• Cost: 321 is typically priced modestly higher than 304 due to titanium additions and lower demand volumes. The difference varies with market conditions and product form.
• Availability: Both grades are widely available in sheet, plate, coil, tubing, pipe, and bar. 304 is more ubiquitous globally, so lead times and sourcing flexibility are generally better for 304 than for 321, particularly in specialty product forms.
• Procurement tip: For large projects, specify finish, product form, and any certification requirements early; consider 304L as an alternative to 321 when the principal concern is weld sensitization and cost control.

10. Summary and Recommendation

Conclusion and practical guidance: - Choose 304 if you need a cost‑effective, general‑purpose austenitic stainless steel for ambient‑temperature service, food and beverage environments, or applications where welding can be controlled by low‑carbon filler metals or where post‑weld solution annealing is feasible. - Choose 321 if the component will be welded and then exposed to temperatures in the sensitization range (e.g., 450–850 °C) or if the part must withstand repeated thermal cycling or prolonged elevated‑temperature service where carbide precipitation would otherwise compromise corrosion resistance and mechanical stability.

Final note: material selection must consider the exact service conditions (temperature profile, chemicals present, stress state, and fabrication route). Where uncertain, consult corrosion testing data or a metallurgical engineer and, when necessary, specify testing (e.g., intergranular corrosion tests) or choose low‑carbon or stabilized alloys to mitigate sensitization risk.