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

Introduction

Austenitic stainless steels 201 and 304 are among the most commonly considered grades when designers, procurement teams, and fabricators balance corrosion resistance, formability, mechanical performance, and cost. The typical decision contexts include: minimizing material cost for decorative or lightly corrosive environments versus ensuring long-term corrosion resistance in food, chemical, or outdoor applications; and choosing between easier cold forming or better long-term toughness and weld performance.

The primary practical distinction is that Type 201 intentionally uses reduced nickel content and higher manganese/nitrogen additions as a cost-driven substitution strategy, while Type 304 retains higher nickel and chromium levels to maximize austenite stability and corrosion resistance. Because of that substitution strategy, 201 and 304 are often compared wherever cost, corrosion resistance, and formability are traded off in component selection.

1. Standards and Designations

• 304: widely standardized as ASTM/ASME A240 (plate, sheet), A312 (pipe), and equivalents in other systems; European EN number commonly quoted as 1.4301 (often X5CrNi18-10); JIS designation SUS304; Chinese GB equivalents (commonly listed under Cr–Ni alloys). Classification: austenitic stainless steel.
• 201: commonly referenced by UNS S20100 and in some product specifications as ASTM/AISI Type 201 or SUS201 in JIS; regional standards and vendor designations vary. Classification: austenitic stainless steel (nickel-reduced, manganese- and nitrogen-stabilized).

Note: Both are stainless (austenitic) grades rather than carbon, alloy, tool, or HSLA steels.

2. Chemical Composition and Alloying Strategy

Table below gives typical composition ranges (wt%) for commercial Type 201 and Type 304 stainless steel. Values are given as representative industry ranges; specific standards or mill certificates should be consulted for tight tolerances.

Alloying effects (brief): - Chromium (Cr) provides the passive oxide film that gives stainless steels their corrosion resistance. - Nickel (Ni) stabilizes the face‑centered cubic (austenitic) structure, improving ductility, toughness, and resistance to sensitization; higher Ni also improves low‑temperature toughness. - Manganese (Mn) and nitrogen (N) are used in 201 to substitute for some nickel, stabilizing austenite but altering mechanical behavior and corrosion performance. - Carbon (C) affects strength and susceptibility to carbide precipitation (sensitization) during thermal exposure; lower C variants (e.g., 304L) mitigate intergranular corrosion after welding.

3. Microstructure and Heat Treatment Response

• Microstructure (as-produced, annealed): Both 201 and 304 are nominally fully austenitic at room temperature when annealed. Austenite stability in 201 is maintained by a higher Mn + N budget rather than Ni. As a result, 201 is more prone to deformation‑induced martensite formation during heavy cold working than 304 in some conditions, because its austenite can be less stable under strain.
• Heat treatment: Neither 201 nor 304 is hardenable by conventional quench-and-temper heat treatment (they are non‑heat‑treatable austenitic grades). Typical annealing practice is solution anneal at approximately 1010–1120 °C followed by rapid cooling (water quench or rapid air quench) to dissolve carbides and restore corrosion resistance and ductility.
• Cold work and thermomechanical processing: Strength in both grades is predominantly increased by cold work. Increased cold‑working raises yield and tensile strength and reduces elongation; 201 typically work‑hardens more rapidly.
• Sensitization: Both grades can suffer chromium carbide precipitation if exposed in the sensitization temperature range (approximately 500–800 °C) for prolonged times, leading to intergranular corrosion. Low‑carbon variants (e.g., 304L) or stabilized grades (with Ti or Nb additions) are used when welding or high‑temperature exposure is a concern.

4. Mechanical Properties

Table compares mechanical behavior qualitatively (typical, annealed product forms). Exact values depend on product form (sheet, plate, tube), temper, and supplier data sheet.

Interpretation: Type 201 often gives higher as‑received strength and hardness for the same processing route, but at the expense of ductility and sometimes toughness. Type 304 provides a better ductility–toughness combination, which benefits forming and service reliability in many corrosive and structural applications.

5. Weldability

Weldability of both grades is generally good compared with carbon steels because austenitic stainless steels do not harden by martensitic transformation upon cooling. Considerations:

• Carbon equivalents and weld cracking risk can be estimated with accepted empirical formulas. Two commonly used indices are:
• $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
• $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
• Interpretation (qualitative): Lower nickel in 201 reduces austenite stability compared with 304, which can influence solidification mode, hot cracking susceptibility, and the degree of strain-induced martensite in the heat‑affected zone (HAZ) under some conditions. Higher Mn and N in 201 can also change filler metal selection and weld metal composition.
• Practical guidance:
• Use appropriate filler metals. For 304 base metal welded to 304, 308/308L fillers are common. For 201, many fabricators choose fillers that restore a higher Ni content in the weld metal to improve corrosion resistance and ductility.
• Preheating is generally not required; post‑weld annealing is not typically used for austenitics in normal applications.
• For critical corrosion‑resistant joints, select filler chemistry to ensure the weld metal and HAZ meet the corrosion resistance needs.

6. Corrosion and Surface Protection

• Stainless behavior: Both grades form a chromium‑rich passive film; however, overall corrosion resistance differs.
• PREN (useful mainly for assessing pitting resistance in chloride environments when Mo and N are present) is calculated as:
• $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Note: For Type 304 (Mo ≈ 0) PREN is dominated by Cr and N; for Type 201 lower Cr and different N result in a lower PREN than 304, so pitting resistance in chlorides is generally inferior.
• Practical aspects:
• Type 304 has superior general corrosion resistance in many aqueous and atmospheric environments and is the preferred minimum for food contact, medical equipment, and many chemical exposures.
• Type 201 performs adequately in indoor, mildly corrosive environments (decorative panels, kitchen equipment in low‑chloride conditions, appliances) but is not recommended for applications with significant chloride exposure (coastal environments, deicing salts) or where long-term passive stability is required.
• Non‑stainless protection: If a non‑stainless steel is being compared, common surface protections include galvanizing, painting, or plating—but these do not replace stainless behavior. For both 201 and 304, surface finishes (electrochemical polishing, passivation treatments) can significantly affect corrosion performance.

7. Fabrication, Machinability, and Formability

• Forming and deep drawing: Type 304 generally offers superior formability and stretchability in the annealed condition because of higher nickel content and greater ductility. Type 201 can be formed, but springback is greater and the metal work‑hardens more rapidly; tooling and process parameters must account for that.
• Bending and welding distortion: 304’s better ductility reduces cracking risk during severe forming; 201 may require more force and tighter process control.
• Machinability: Austenitic stainless steels are generally more difficult to machine than carbon steels. Type 201 tends to work‑harden rapidly, which can reduce machinability; tooling with higher rake angles, rigid setups, and slower feed rates are commonly used. 304 is also gummy and requires proper tooling, but many machinists find 304 marginally easier to machine in comparable tempers.
• Surface finishing: Both grades can be polished to high finishes. Because of its higher susceptibility to localized corrosion in aggressive environments, 201 may show staining earlier if surface finish and passivation are not adequate.

8. Typical Applications

9. Cost and Availability

• Cost: Type 201 is typically less expensive than Type 304 because of significantly lower nickel content. Nickel is the main cost driver in stainless steels; substituting Ni with Mn and N reduces material cost sensitivity to nickel markets.
• Availability: Type 304 is ubiquitous worldwide in sheet, plate, coil, tube, and bar forms and is generally easier to source in certified material conditions for critical applications. Type 201 is regionally common and widely available for commodity product forms, but certified mill data and certain product forms may be less readily stocked than 304 in some markets.

10. Summary and Recommendation

Summary table (qualitative):

Recommendations: - Choose Type 201 if: - Project is cost‑sensitive and exposure is limited to mild, indoor, or low‑chloride environments. - Higher as‑received strength and cost savings outweigh the need for maximum corrosion resistance. - Surface appearance and low cost in consumer products are prioritized. - Choose Type 304 if: - Long‑term corrosion resistance, hygienic service, or exposure to chlorides is expected. - Formability, weldability, and established material certification are important. - The application must meet common industry standards for food contact, pharmaceuticals, or outdoor exposure.

Final note: Material selection should always be validated against the specific environmental conditions, mechanical loads, welding and forming sequences, and procurement/supply constraints of the project. For critical applications, consult mill certificates and perform corrosion testing or engineering evaluations to confirm the suitability of 201 versus 304 for the intended service.