304 vs 202 – Composition, Heat Treatment, Properties, and Applications

Introduction

Stainless steels 304 and 202 are common austenitic grades used across fabrication, architectural, food processing, and consumer-product markets. Engineers, procurement managers, and manufacturing planners frequently decide between them when balancing corrosion resistance, mechanical performance, weldability, and unit cost. Typical decision contexts include specifying material for indoor food-contact equipment, architectural trim, or structural components where budget constraints push selection toward lower-nickel options.

The principal difference between 304 and 202 is their alloying strategy: 304 uses higher nickel and standard chromium to secure robust corrosion resistance and a stable austenitic microstructure, while 202 reduces nickel content (and increases manganese and nitrogen) to achieve a lower-cost alternative with somewhat reduced corrosion resistance but comparable strength. This trade-off—material cost versus corrosion performance and process behavior—drives most direct comparisons.

1. Standards and Designations

• 304
• Common designations: AISI 304, UNS S30400, EN 1.4301 (and 304L as 1.4306), JIS SUS304, GB 06Cr19Ni10.
• Classification: Austenitic stainless steel (stainless).

• Typical standards: ASTM A240/A480 (plates and sheets), ASME SA240, EN 10088 series.

• 202

• Common designations: AISI 202, UNS S20200, EN (not widely standardized in Europe, commonly specified by national standards), JIS SUS202 equivalents in some markets, GB 202.
• Classification: Austenitic stainless steel (stainless), often marketed as a low-nickel austenitic grade.
• Typical standards: Used in sheet/coil and formed products; proprietary spec ranges exist in different regions.

Both grades are stainless (corrosion-resistant) steels rather than carbon, alloy, tool, or HSLA steels.

2. Chemical Composition and Alloying Strategy

The table below shows commonly quoted nominal or range compositions (wt %) for wrought, commercially available grades. Values vary by standard and heat; use the relevant mill certificate for procurement.

How the alloying affects properties: - Chromium (Cr) provides the passive oxide film and general corrosion resistance; both grades have similar Cr content. - Nickel (Ni) stabilizes the austenitic phase and enhances formability and corrosion resistance; 304 has substantially more Ni than 202. - Manganese (Mn) and nitrogen (N) in 202 are used to stabilize austenite in place of some nickel and to increase strength by solid-solution strengthening. - Carbon influences strength and sensitization risk; 202 often has a slightly higher carbon limit. Lower-carbon variants (e.g., 304L) are used to reduce sensitization during welding. - Minor elements and impurities (P, S) affect machinability and localized corrosion.

3. Microstructure and Heat Treatment Response

• Typical microstructure (as-fabricated)
• Both 304 and 202 are principally austenitic (face-centered cubic) in the annealed condition. The austenite stability depends on Ni, Mn, and N content.
• 202 has higher Mn/N and lower Ni; its austenite may be slightly less stable at elevated temperatures and during heavy cold working, but it remains austenitic at room temperature for standard compositions.

• Delta ferrite is minimal in these compositions under normal cooling; some processing routes can introduce small ferrite fractions.

• Response to common thermal/mechanical treatments

• Annealing (solution treatment): Typical for both grades at ~1000–1150 °C followed by rapid cooling to dissolve carbides and reset microstructure. Both regain ductility and corrosion resistance after solution anneal.
• Cold work (rolling, drawing, bending): Both harden with cold work; 202, due to higher Mn and N, often achieves higher strength levels after cold work at the same deformation.
• Aging/precipitation: Austenitic stainless steels are not hardened by traditional quench-and-temper; prolonged exposure in the 400–850 °C range can cause embrittling phases (carbides, sigma phase) that reduce toughness and corrosion resistance—304 is more resistant to intermetallic precipitation due to higher Ni.
• Normalizing/quenching & tempering: Not applicable as strengthening routes for austenitic stainless grades; they do not respond to martensitic hardening like carbon steels.

4. Mechanical Properties

The following table provides representative ranges for wrought, annealed product forms (sheets, plates, or cold-rolled coils). Actual properties depend on cold work, thickness, and heat treatment.

Interpretation: - 202 tends to achieve higher strength—particularly after cold working—because of higher Mn and N solid-solution strengthening. Yield strength and tensile strength are often higher for 202 in comparable conditions. - 304 typically shows higher ductility and better retained toughness after thermal exposure or welding because of higher Ni content and greater austenite stability. - Selection should consider the as-supplied condition: a cold-worked 304 may surpass an annealed 202 in strength, but a cold-worked 202 will generally be stronger than a similarly worked 304.

5. Weldability

Weldability for austenitic stainless steels is generally good, but sensitivity depends on carbon level, alloying that affects hardenability, and susceptibility to intergranular corrosion.

Key formulas used to assess weldability risk (interpret qualitatively):

• Carbon equivalent (IIW method): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ Higher $CE_{IIW}$ indicates increased hardenability and potential cracking risk in some steels; for austenitic stainless steels, these elements are used differently, but the formula gives a comparative concept.

• Pitting resistance equivalent number (for corrosion rather than weld cracking): $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$

• Pcm (weldability index used more for carbon steels, but useful conceptually): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation for 304 vs 202: - 304 typically exhibits very good weldability with standard filler metals (e.g., 308L for 304) and low risk of hot cracking and post-weld embrittlement when proper procedures are used. Carbon control (304L) is used where sensitization is a concern. - 202 welds acceptably with common methods, but reduced Ni content increases risk of certain weld-related problems, such as slightly reduced ductility in the heat-affected zone and potentially higher susceptibility to certain forms of localized corrosion in aggressive environments. - Pre- and post-weld cleaning, proper filler selection, and avoiding prolonged exposure to sensitization temperatures are standard controls. For critical applications, perform weld procedure qualification and corrosion testing on welded assemblies.

6. Corrosion and Surface Protection

• Stainless behavior
• Both 304 and 202 rely on a chromium-rich passive oxide film for corrosion resistance. 304, with higher nickel, generally provides superior general corrosion resistance, better performance in chloride-containing environments, and better formability without surface cracking that can compromise passivity.

• 202 has acceptable corrosion resistance for many indoor and mildly corrosive environments (decorative trim, indoor appliances, HVAC components), but it is not recommended for chloride-rich environments (coastal, marine, chlorinated process streams) where 304 or higher-alloy grades are preferred.

• Use of PREN (where applicable)

• For pitting-corrosion assessment use: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$

• PREN is more commonly applied to duplex and austenitic grades containing Mo and elevated N; it will illustrate why 304 (low Mo, low N) and 202 (low Mo, moderate N) are both limited in severe chloride environments.

• Surface protection for non-stainless steels (not applicable here)

• If a project specification requires galvanizing or painting, note that these are separate strategies used for non-stainless steels; for stainless grades, surface finish and passivation treatments are the primary protection methods.

7. Fabrication, Machinability, and Formability

• Formability and bending:
• 304 has excellent formability, deep drawability, and springback characteristics due to higher Ni and stable austenite—preferred where complex forming is required.

• 202 can be formed but has higher work-hardening rate; dies and tooling must accommodate increased forming forces and more frequent anneal cycles may be needed for tight-radius forming.

• Machinability:

• Austenitic stainless steels are generally less machinable than carbon steels. 202's higher strength and work-hardening tendency make machining marginally more challenging than 304; however, because 202 often contains higher sulfur limits in some commercial variants (improving machinability), real-world results depend on product spec.

• Use appropriate tooling (carbide inserts, high positive rake), coolant, and conservative feeds/ speeds for consistent results.

• Surface finishing:

• Both polish, pickling, and passivation treatments are standard. 304 accepts common finishes (2B, BA, No.4) with predictable results; 202 may show slightly different etching behavior and requires attention to maintain uniform appearance in decorative applications.

8. Typical Applications

Selection rationale: - Choose 304 when corrosion resistance, long-term durability in humid or mildly aggressive conditions, or extensive forming/welded assemblies are required. - Choose 202 when initial material cost is a dominant constraint and the environment is non-aggressive (indoors, controlled atmosphere) and higher strength at lower cost is acceptable.

9. Cost and Availability

• Cost
• 202 is typically less expensive per kilogram than 304 because it substitutes manganese and nitrogen for part of the nickel content. Nickel is a high-cost alloying element, so lower-Ni grades tend to be marketed as economical alternatives.

• Market prices fluctuate with commodity nickel and stainless scrap values; the relative premium for 304 can widen significantly in periods of high nickel price.

• Availability

• 304 is globally ubiquitous in sheet, coil, plate, bar, tubing, and fasteners. Lead times are typically short for standard product forms.
• 202 is widely available in many markets for sheet and coil but may be less common in specialty product forms or specific international standards. Confirm availability for large-volume orders or specialty dimensions.

10. Summary and Recommendation

Concluding recommendations: - Choose 304 if corrosion resistance, superior formability, established global availability, and long-term durability in mildly aggressive environments are critical. It is the safer choice for food-contact, hygienic, and coastal/marine-exposed applications. - Choose 202 if initial material cost is a primary driver, the service environment is benign (indoor, non-chloride), and higher as-built strength or lower nickel content is acceptable. Validate through application-specific corrosion testing and confirm supplier specifications for N, Mn, and S to ensure required machinability and surface finish.

Final note: Always specify the required product form, surface finish, mechanical property condition (annealed vs. cold-worked), and applicable standard on purchase orders. For weldments and critical components, carry out welding procedure qualification and, where corrosion resistance is a safety or life-cycle concern, perform application-specific corrosion testing rather than relying solely on general grade comparisons.