202 vs 204 – Composition, Heat Treatment, Properties, and Applications

Introduction

Austenitic 200‑series stainless alloys such as "202" and "204" are commonly proposed as lower‑nickel alternatives to 300‑series grades where cost, formability, and corrosion resistance must be balanced. Engineers and procurement teams frequently face a selection dilemma: choose a higher manganese, lower‑nickel composition that minimizes material cost but can be harder to form (202), or select a newer, cost‑optimized alloy that reduces nickel without sacrificing formability by adding copper or adjusting other alloying elements (commonly referred to as 204 or 204Cu).

The primary directional difference between these two families is a cost‑performance tradeoff driven by alloy strategy: 202 reduces nickel primarily with higher manganese, whereas 204 variants attempt to retain or improve formability and corrosion performance using a different balance of Ni, Cr, Mn and controlled copper additions. Because both are austenitic stainless steels intended as economical substitutes for 304 in many applications, they are often compared during design, manufacturing and procurement.

1. Standards and Designations

• 202: Commonly referenced as AISI/UNS S20200 (sometimes abbreviated SS202). Found in commercial sheet and strip product ranges and referenced in flat‑rolled stainless specs (e.g., ASTM A240 for sheet/plate in some markets) and various national catalogs.
• 204: Frequently encountered as 204Cu (UNS S20430) or commercial designations that emphasize low‑nickel, copper‑bearing austenitic stainless. Not every national standard has a direct one‑to‑one EN/JIS/GB equivalent; manufacturers publish mill specs or UNS numbers.
• Standards to check for exact limits: ASTM/ASME (A240, A480 for plates/sheets), UNS listings (S20200, S20430), and supplier mill certificates. Regional EN (European) or GB/JIS equivalents may differ and should be verified for acceptance criteria.

Classification: Both 202 and 204 are austenitic stainless steels (non‑magnetic in the fully annealed condition), not carbon steels, tool steels, or HSLA.

2. Chemical Composition and Alloying Strategy

Table: typical composition ranges (wt%). Values are representative ranges from common commercial datasheets; always confirm with the mill certificate or specification for the batch you are buying.

Element	202 (typical, wt%)	204 / 204Cu (typical, wt%)
C	≤ 0.15 (low)	≤ 0.08–0.10 (low)
Mn	Relatively high (e.g., several wt% to substitute Ni)	Moderate (lower than 202)
Si	≤ ~1.0 (minor deoxidizer)	≤ ~0.8–1.0
P	≤ 0.03–0.05 (impurity limit)	≤ 0.03–0.045
S	≤ 0.03 (impurity limit)	≤ 0.03
Cr	~17–19	~18–20
Ni	Moderate (reduced vs 304, but higher than some 204 variants)	Lower than 304; similar or slightly lower than 202 depending on subgrade
Mo	Typically nil or trace	Typically nil or trace
Cu	Trace to ~0.5–1.0 (usually low)	Intentional addition (e.g., ~0.5–1.5) in 204Cu
N	Low to moderate (small amounts to stabilize austenite)	Low (controlled)
V, Nb, Ti, B	Not typical alloying additions	Not typical

How the alloying strategy affects properties: - Chromium (Cr) provides the passive film and baseline corrosion resistance common to austenitic stainlesses. - Nickel (Ni) stabilizes the austenite and improves toughness and formability; reducing Ni lowers material cost but can affect toughness and corrosion behavior unless compensated. - Manganese (Mn) is used to stabilize austenite when Ni is reduced — it is cheaper than Ni but increases work hardening and can affect ductility and machinability. - Copper (Cu) in 204 variants is used to improve strength and formability and partially compensate for reduced Ni in corrosion performance; Cu can also aid resistance to certain acids and improve surface finish. - Carbon and nitrogen are controlled to balance strength (via solid solution and interstitial strengthening) and avoid sensitization or excessive hardness.

3. Microstructure and Heat Treatment Response

Both 202 and 204 are fully austenitic when produced and annealed. Key points:

• Typical microstructure: stable face‑centered cubic (FCC) austenite with evenly distributed austenitic grains; absence of ferrite or martensite in correctly processed, fully annealed material.
• Cold work response: Both grades strain‑harden easily (austenitic stainless steels harden by deformation), but 202 — with higher Mn — tends to work‑harden more rapidly, which can reduce formability limits on deep draws and complicate stamping unless intermediate anneals are used.
• Heat treatment: Austenitic stainless steels cannot be hardened by conventional quench/tempering. Annealing (typically in the 1000–1150 °C range followed by water quench) restores ductility and toughness. 204Cu may show modest age‑hardening behavior under certain temperatur/time conditions due to copper precipitation, but this is not comparable to precipitation‑hardening stainless grades and is typically not exploited for high‑strength design.
• Thermo‑mechanical processing: Hot rolling followed by controlled annealing establishes grain size and final properties; controlling residual strain and avoiding cold‑work induced martensite (if present) improves toughness.

4. Mechanical Properties

Table — qualitative comparison (annealed condition unless otherwise stated). Exact values depend on product form, thickness, temper, and specific supplier datasheet.

Property	202	204 / 204Cu
Tensile Strength (approx, annealed)	Moderate — comparable to other 200‑series; higher than some low‑Ni variants due to Mn	Comparable to 202; may exhibit slightly improved strength/ductility balance due to Cu
Yield Strength	Moderate	Moderate; similar or slightly lower than 202 in annealed state
Elongation (ductility)	Good but reduced relative to 204 after cold work (work hardens faster)	Generally slightly better formability and elongation in many supplier plats
Impact Toughness	Good at ambient; can be lower at cryogenic temperatures if Ni is very low	Good at ambient; copper and controlled Ni help maintain toughness
Hardness (annealed)	Low to moderate (work hardened quickly)	Low to moderate (tends to maintain better softness during forming)

Why: 202’s higher Mn and lower Ni increase austenite stability but also intensify work hardening; this produces higher instantaneous strength during forming but reduces total achievable elongation before failure. 204Cu’s alloy balance aims to preserve ductility and toughness while still reducing nickel content and cost.

5. Weldability

Weldability is influenced by carbon equivalent, hardenability, and microalloying. Useful predictive equations:

• Carbon Equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• Pcm (European weldability index): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative): - Both 202 and 204 are readily welded using standard austenitic stainless steel procedures (GMAW, TIG, etc.) with low risk of cold cracking because austenitic stainless steels maintain ductility in the weld metal. - Higher Mn (202) increases $CE_{IIW}$ and $P_{cm}$ moderately, meaning slightly greater attention to heat input and interpass temperature may be needed to avoid local hardening or undesirable microstructures in the heat‑affected zone. - 204Cu, with copper additions, generally welds comparably to other austenitics; copper may slightly influence filler selection — matching composition with appropriate austenitic fillers (e.g., 308/309 family equivalents or dedicated low‑Ni fillers specified by supplier) avoids microsegregation issues. - Post‑weld annealing is rarely required for typical service, but stress relief and control of intergranular corrosion risk should be considered for sensitization‑prone geometries or elevated service temperatures.

6. Corrosion and Surface Protection

• Both grades are austenitic stainless steel with chromium‑based passivity. Neither contains significant molybdenum, so chloride pitting resistance is lower than Mo‑bearing 300‑series (e.g., 316).
• PREN is not typically useful for these grades (it is meaningful for Mo‑containing stainlesses). For reference, PREN formula: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ Because Mo ≈ 0 for 202/204, PREN will not indicate high pitting resistance.
• Practical guidance:
• 202: acceptable in mildly corrosive environments (indoor, food contact, decorative), but less resistant to chlorides and acidic environments than 304/316. Susceptible to surface staining and pitting in aggressive chloride environments.
• 204/204Cu: intended as a more robust low‑Ni alternative to 304; copper can marginally improve resistance to certain acids and suppress crevice corrosion initiation in some service conditions, but it is not a substitute for Mo in heavy chloride exposure.
• For non‑stainless steels (not applicable here): use galvanizing, painting or coatings. For these stainless grades, consider electro‑polishing, passivation, or protective coatings for aggressive environments.

7. Fabrication, Machinability, and Formability

• Cold forming: 202 work‑hardens faster — designers must account for springback and may require intermediate anneals; deep draws can be more challenging. 204Cu tends to offer better formability and is often preferred where tight bends and complex stamping are required.
• Machinability: Neither grade machines as easily as free‑cutting carbon steels. Higher Mn in 202 can reduce machinability; 204Cu often machines similarly or slightly better, and controlled Cu can improve chip formation and surface finish.
• Surface finishing: Both take polishing and finishing well; 204Cu may produce slightly better as‑formed surface appearance in some operations.
• Welding and post‑fabrication: Proper fit‑up and filler selection are important; avoid rapid cooling that may trap stresses; pickling/passivation after welding recommended to restore corrosion resistance.

8. Typical Applications

202 — Typical Uses	204 / 204Cu — Typical Uses
Consumer goods, kitchenware, light architectural trim, decorative panels, indoor appliances where cost is a driver	Household appliances (washers, dryers), kitchen sinks, decorative architectural panels, sanitary fixtures where improved formability and surface finish are required
Automotive trim and small structural components with limited chloride exposure	Tubing and formed components that require good drawability and improved dimensional control
Light structural sheet where corrosion exposure is mild	Applications replacing 304 where nickel reduction is desired but formability or toughness must be retained

Selection rationale: choose 202 where upfront material cost is the primary constraint and corrosion exposure is moderate; choose 204/204Cu where slightly higher material or processing cost is justified by better formability, more consistent surface finish and retention of toughness with reduced Ni.

9. Cost and Availability

• Relative cost: Both are positioned as lower‑cost alternatives to 304. 202 often has the lowest nickel content strategy (compensated by higher Mn), which historically yields cost savings—but seasonal raw material prices (Mn vs Ni vs Cu) can change the relative economics. 204/204Cu is engineered to balance nickel reduction with preserved properties and can be competitive, especially when Ni is expensive and Cu is economical.
• Availability: 202 is widely available globally as standard commercial sheet/strip. 204/204Cu availability depends on region and manufacturer; it is increasingly common but may be less ubiquitous than 202 in some markets and product forms (e.g., specific coils, pre‑finished sheet temper options).

10. Summary and Recommendation

Table — quick comparison (qualitative):

Attribute	202	204 / 204Cu
Weldability	Good; monitor heat input (work hardening)	Good; copper has minimal impact on standard welding practice
Strength–Toughness balance	Moderate strength, increased work hardening; toughness adequate at ambient	Similar strength, slightly better ductility/formability and maintained toughness
Cost (material)	Typically lower in many markets (depends on Mn/Ni pricing)	Cost‑optimized for Ni reduction while retaining formability; may be similar or slightly higher than 202

Choose 202 if: - Your priority is minimizing material cost and the application involves mild environments, simple forming operations, and where suppliers can readily provide the required coil/sheet forms. - You can design fabrication steps to account for stronger work hardening (intermediate anneals, tooling allowances).

Choose 204 (204Cu) if: - You need tighter formability, better surface finish after forming, or a closer mechanical balance to 304 while still lowering nickel content. - The application demands consistent toughness and reduced risk of process issues from rapid work hardening (complex stampings, deep draws, high‑volume appliance parts).

Closing note: Both grades are useful cost‑optimized austenitic stainless options. The final selection should always be made against verified supplier mill certificates, product form (coil, sheet, tube), required temper/finish, and a lifecycle cost analysis that considers material cost volatility, fabrication cycle time (e.g., need for anneals), and long‑term corrosion exposure.