18-8 Stainless Steel: Composition, Performance & Uses

1. Executive summary

“18-8 stainless steel” is the common name for a family of austenitic stainless steels characterized by roughly 18% chromium and 8% nickel (hence “18-8”).

The best-known member is Type 304 (UNS S30400 / EN 1.4301). 18-8 alloys are the workhorses of stainless technology because they combine broad corrosion resistance, excellent formability, high toughness, and simple fabrication.

They are not, however, the best choice for aggressive chloride environments or high-temperature creep applications — in those cases alloys with added molybdenum, stabilized or duplex microstructures, or nickel-base alloys are preferred.

2. What “18-8” means — definition and scope

“18-8” is an informal, historical descriptor that designates stainless steels with approximately 18 wt.% chromium and 8 wt.% nickel—the classic austenitic stainless composition introduced in the early 20th century.

It typically refers to the 300-series austenitic family: principally Type 304 and its variants (304L, 304H), plus related stabilized grades (e.g., 321, 347) that share the 18–20% Cr / 8–10% Ni base but add titanium or niobium to control carbide precipitation.

Key points:

• “18-8” is a practical shorthand — specify exact grade (e.g., 304, 304L, 321) in procurement.
• The austenitic microstructure is stabilized by Ni; Cr provides passivity and oxidation resistance.

3. Typical grades and standards

Common commercially used 18-8 variants include:

• Type 304 (UNS S30400 / EN 1.4301) — standard 18-8 stainless; general purpose.
• Type 304L (S30403 / 1.4306) — low-carbon variant (≤0.03% C) to reduce sensitization during welding.
• Type 304H (S30409 / 1.4307) — higher carbon (≈0.04–0.10%) for improved strength at elevated temperatures.
• Type 321 (S32100 / 1.4541) — Ti-stabilized for better resistance to intergranular corrosion after exposure in 450–850 °C range.
• Type 347 (S34700 / 1.4550) — Nb-stabilized equivalent to 321.

Standards covering these grades include ASTM A240 / A240M (plate, sheet), ASTM A276 (bars), ASME/ASME II, and EN/ISO equivalents. Always reference the precise standard and UNS/EN number in specifications.

4. Chemical Composition of 18-8 stainless steel

5. Mechanical properties of 18-8 stainless steel

The table below gives representative mechanical properties for typical 18-8 austenitic stainless steels (e.g., Type 304 family) in the solution-annealed / annealed condition.

6. Physical & Thermal Properties

• Density: ≈ 7.9 g·cm⁻³.
• Modulus of elasticity (E): ≈ 193–200 GPa.
• Thermal conductivity: relatively low for a metal, ≈ 14–16 W·m⁻¹·K⁻¹ at 100 °C (falls with temperature).
• Coefficient of thermal expansion: ≈ 16–17×10⁻⁶ K⁻¹ (20–100 °C) — higher than carbon steel, important for thermal joint design.
• Melting range: solidus ~ 1375–1400 °C, liquidus ~ 1400–1450 °C (composition dependent).
• Magnetic behavior: essentially non-magnetic in annealed condition; cold work or formation of martensite imparts mild ferromagnetism.

Temperature service limits: continuous use up to ~400–800 °C is possible depending on alloy and environment; beware of sensitization zone (~425–850 °C) and carburization/oxidation at high temperatures.

For sustained high-T strength consider 304H, 309, 310 or other high-temperature alloys.

7. Corrosion behaviour — strengths and limitations

Strengths

• Good general corrosion resistance in oxidizing atmospheres and many chemicals (acids/bases) at ambient temperatures.
  The passive Cr₂O₃ film grants broad utility in food, architectural and many process environments.
• Good hygiene and cleanability, which is why 18-8 is widely used in food, beverage and medical equipment.

Limitations

• Pitting and crevice corrosion in chlorides: without Mo, 18-8 is susceptible to localized attack in chloride-bearing media (seawater, brines) especially at elevated temperatures or in crevices.
  If chlorides are present, Type 316 (with Mo) or duplex alloys are often chosen.
• Stress corrosion cracking (SCC): austenitic 18-8 steels are susceptible to chloride-induced SCC under tensile stress and elevated temperature; avoid combination of tensile stress + chlorides + temperature.
• Intergranular corrosion (sensitization): occurs after exposure to 425–850 °C unless low-C (304L) or stabilized grades (321/347) are used.
• Galvanic corrosion: when coupled to more noble alloys, 18-8 can act as an anode in certain electrolytes — design to avoid dissimilar metal contact or provide insulation.

Practical selection rule: For general service where chlorides or heavy reducing conditions occur, evaluate 316 (Mo), super-austenitics, duplex or nickel alloys.

8. Fabrication: forming, machining, welding and joining

Forming

• Excellent formability in annealed condition due to high ductility. Use proper tooling to account for springback (higher than mild steel) and the strong work-hardening behavior.
• Deep drawing & spinning are common for cookware and thin-wall vessels.

Machining

• Notoriously “gummy” compared with carbon steel; austenitic stainless steels work-harden in the cut, which increases tool wear. Best practice:

• • Use rigid tooling, positive rake carbide tools.
  • Employ moderate cutting speeds, high feed for roughing, and abundant coolant to avoid built-up edge and heat.
  • Use sharp edges and chip breakers.

Welding & joining

• Excellent weldability by common methods (GTAW, GMAW, SMAW, FCAW). Key points:

• • Use low-carbon (304L) for welded assemblies where post-weld sensitization is a concern.
  • Use appropriate filler metals (e.g., 308L/308 stainless filler for 304 base metal) to match chemistry and avoid hot cracking.
  • Control heat input & interpass temperature; excessive heat widens the sensitized zone.
  • Post-weld solution anneal (1050–1100 °C) followed by rapid quench can restore corrosion resistance where practical; often not feasible for assembled structures.
    Alternatively, use low-C or stabilized grades to avoid the need for PWHT.
  • Beware of solidification cracking in some weld configurations — follow qualified WPS and prequalified procedures.

Other joining

• Brazing, soldering, adhesive bonding are used with appropriate fluxes and surface preps. Adhesive bonding frequently requires surface activation (flame, plasma, chemical etch).

9. Heat treatment & thermal processing

• Not hardenable by quench & temper (austenitic 18-8 does not form martensite through heat treatment like carbon steels).
• Solution anneal: typical at 1010–1120 °C followed by rapid quench (water) to dissolve carbides and restore corrosion resistance and ductility. Used after welding/heavy cold work when feasible.
• Stress relief anneal: limited benefit; if performed, avoid temperatures in the sensitization range unless followed by solution anneal.
• Aging: prolonged exposure to 475 °C (475 °C embrittlement) in some iron-nickel-chromium alloys can embrittle the material — not typical for 304, but be cautious in long-time exposures.

10. Surface finishing, passivation and cleaning

• Mechanical finishes: 2B, BA, No.1, No.4 (brushed) etc. Select finish for application: polished for sanitary, matte for architectural.
• Pickling & passivation: chemical pickling removes heat tint and embedded iron; passivation (nitric or citric acid treatments) restores and strengthens the passive film—critical after welding or fabrication.
  Citric acid passivation is increasingly preferred for safety and environmental reasons.
• Electropolishing: reduces surface roughness and improves corrosion resistance (useful in pharmaceutical/food industries).
• Cleaning: avoid chlorinated cleaners; prefer mild alkaline cleaners or detergents followed by potable water rinse. For critical sanitary use, validate cleaning regimen.

11. Typical Applications of 18-8 stainless steel

• Food service and processing equipment: sinks, conveyors, tanks — hygienic, easily cleaned.
• Architectural surfaces and trim: durable, corrosion-resistant finishes.
• Household goods: cutlery, cookware, appliance panels.
• Chemical process equipment (mild services): piping, valves for non-chloride environments.
• Fasteners, springs (when cold-worked), instrumentation: using work-hardening for mechanical function.
• Medical devices and implants (select grades, controlled manufacturing): because of biocompatibility and sterilizability (but not all 18-8 variants are medical-grade).

12. Comparison to Related Alloys

13. Conclusion

18-8 stainless steel represents one of the most balanced and widely adopted material systems in modern engineering.

By combining approximately 18% chromium and 8% nickel, it achieves a stable austenitic microstructure that delivers an exceptional blend of corrosion resistance, mechanical reliability, formability, and weldability.

These characteristics explain its long-standing dominance across food processing, chemical equipment, architectural structures, pressure vessels, and general industrial applications.

 

FAQs

What does “18-8” mean in stainless steel?

“18-8” refers to the nominal chemical composition of approximately 18% chromium and 8% nickel.

This composition stabilizes an austenitic structure, providing corrosion resistance, ductility, and non-magnetic behavior in the annealed condition.

Is 18-8 stainless steel the same as Type 304?

Type 304 is the most common standardized grade within the 18-8 family.

While “18-8” is a general industry term, Type 304 (and its variants such as 304L and 304H) represents a precisely defined specification under international standards.

Is 18-8 stainless steel magnetic?

In the solution-annealed condition, 18-8 stainless steel is essentially non-magnetic. However, cold working can induce partial martensitic transformation, resulting in slight magnetic response.

What are the main advantages of 18-8 stainless steel over duplex stainless steels?

18-8 stainless steel offers superior formability, easier welding, better low-temperature toughness, and lower material and fabrication costs.

Duplex stainless steels provide higher strength and improved chloride resistance but are more demanding to process.