Stainless Steel Properties

Executive summary

Stainless steels are iron-based alloys defined by their ability to form and maintain a thin, self-healing chromium oxide (Cr₂O₃) passive film.

This passive film — established when chromium content reaches roughly ≥10.5 wt% — is the foundation of their corrosion resistance and makes stainless steel distinct from plain carbon steels.

By adjusting alloying (Cr, Ni, Mo, N, Ti, Nb, etc.) and microstructure (austenitic, ferritic, martensitic, duplex, precipitation-hardening), engineers obtain a broad palette of combinations of corrosion performance, strength, toughness, fabricability and appearance.

1. What is stainless steel?

Definition. Stainless steel is an iron-based alloy containing sufficient chromium (nominally ≥10.5 wt%) to form a continuous, protective chromium-oxide (Cr₂O₃) passive layer in oxygenated environments.

That passive film is thin (nm scale), self-repairing when oxygen is present, and is the fundamental basis for the material’s corrosion resistance.

Core Alloying Elements and Their Functions

• Chromium (Cr, 10.5%–30%): The most critical element. At sufficient concentrations, Cr reacts with oxygen to form a dense, adherent Cr₂O₃ passive film (2–5 nm thick) that blocks corrosive media from attacking the iron matrix.
  Higher Cr content enhances general corrosion resistance but may increase brittleness if not balanced with other elements.
• Nickel (Ni, 2%–22%): Stabilizes the austenitic phase (face-centered cubic, FCC) at room temperature, improving ductility, toughness, and weldability.
  Ni also enhances resistance to stress corrosion cracking (SCC) in chloride environments and low-temperature toughness (prevents brittle fracture below 0℃).
• Molybdenum (Mo, 0.5%–6%): Significantly improves resistance to pitting and crevice corrosion (especially in chloride-rich environments) by increasing the passive film’s stability.
  Mo forms molybdenum oxide (MoO₃) to repair local film damage, making it essential for marine and chemical applications.
• Titanium (Ti) and Niobium (Nb, 0.1%–0.8%): Carbide stabilizers. They preferentially combine with carbon (C) to form TiC or NbC,
  preventing the formation of Cr₂₃C₆ at grain boundaries during welding or high-temperature service—this avoids “chromium depletion” and subsequent intergranular corrosion (IGC).
• Manganese (Mn, 1%–15%): A cost-effective alternative to Ni for austenite stabilization (e.g., 200-series stainless steel).
  Mn improves strength but may reduce corrosion resistance and toughness compared to Ni-bearing grades.
• Carbon (C, 0.01%–1.2%): Influences hardness and strength. Low C content (≤0.03%, L-grade) minimizes carbide formation and IGC risk; high C content (≥0.1%, martensitic grades) enhances hardenability via heat treatment.

Microstructural Classification and Key Characteristics

Austenitic Stainless Steel (300-series, 200-series)

• Composition: High Cr (16%–26%), Ni (2%–22%) or Mn, low C (≤0.12%). Typical grades: 304 (18Cr-8Ni), 316 (18Cr-10Ni-2Mo), 201 (17Cr-5Ni-6Mn).
• Microstructure: Fully austenitic (FCC) at room temperature, non-magnetic (except after cold working).
• Core Trait: Excellent ductility, toughness (even at cryogenic temperatures down to -270℃), and weldability; balanced corrosion resistance.

Ferritic Stainless Steel (400-series)

• Composition: High Cr (10.5%–27%), low C (≤0.12%), no or minimal Ni. Typical grades: 430 (17Cr), 446 (26Cr).
• Microstructure: Ferritic (body-centered cubic, BCC) at all temperatures, magnetic.
• Core Trait: Cost-effective, good general corrosion resistance, and oxidation resistance at high temperatures (up to 800℃); limited ductility and weldability.

Martensitic Stainless Steel (400-series, 500-series)

• Composition: Medium Cr (11%–17%), high C (0.1%–1.2%), low Ni. Typical grades: 410 (12Cr-0.15C), 420 (13Cr-0.2C), 440C (17Cr-1.0C).
• Microstructure: Martensitic (body-centered tetragonal, BCT) after quenching and tempering; magnetic.
• Core Trait: High hardness and wear resistance (HRC 50–60 after heat treatment); moderate corrosion resistance.

Duplex Stainless Steel (2205, 2507)

• Composition: Balanced austenitic-ferritic phases (50%±10% each), high Cr (21%–27%), Ni (4%–7%), Mo (2%–4%), N (0.1%–0.3%). Typical grades: 2205 (22Cr-5Ni-3Mo), 2507 (25Cr-7Ni-4Mo).
• Microstructure: Dual-phase (FCC + BCC), magnetic.
• Core Trait: Superior strength (twice that of austenitic grades) and resistance to SCC, pitting, and crevice corrosion; suitable for harsh marine and chemical environments.

Precipitation-Hardening (PH) Stainless Steel (17-4PH, 17-7PH)

• Composition: Cr (15%–17%), Ni (4%–7%), Cu (2%–5%), Nb (0.2%–0.4%). Typical grade: 17-4PH (17Cr-4Ni-4Cu-Nb).
• Microstructure: Martensitic or austenitic base with precipitates (Cu-rich phases, NbC) after aging treatment.
• Core Trait: Ultra-high strength (tensile strength >1000 MPa) and good corrosion resistance; used in high-load aerospace and medical applications.

2. Core Performance: Corrosion Resistance

Corrosion resistance is the defining property of stainless steel, rooted in the passive film’s stability and alloying element synergies. Different grades exhibit distinct resistance to specific corrosion mechanisms.

Passive Film Mechanism and General Corrosion Resistance

The Cr₂O₃ passive film forms spontaneously in oxygen-containing environments (air, water) and is self-healing—if damaged (e.g., scratches), Cr in the matrix rapidly reoxidizes to repair the film.
General corrosion (uniform oxidation) occurs only when the film is destroyed, such as in strong reducing acids (hydrochloric acid) or high-temperature reducing atmospheres.

• Austenitic grades (304, 316): Resist general corrosion in atmospheric, fresh-water, and mild chemical environments. 316 outperforms 304 in chloride-rich media due to Mo addition.
• Ferritic grades (430): Good general corrosion resistance in air and neutral solutions but susceptible to pitting in high-chloride environments.
• Duplex grades (2205): Exceptional general corrosion resistance, combining Cr’s film-forming ability with Mo’s pitting resistance.

Specific Corrosion Types and Grade Adaptability

Pitting and Crevice Corrosion

Pitting corrosion occurs when chloride ions (Cl⁻) penetrate local defects in the passive film, forming small, deep corrosion pits.
Crevice corrosion is similar but localized in narrow gaps (e.g., weld seams, fastener interfaces) where oxygen depletion accelerates corrosion.

• Key Influencing Elements: Mo and N significantly improve resistance—each 1% Mo addition reduces the critical pitting temperature (CPT) by ~10℃.
  316 (CPT ≈ 40℃) outperforms 304 (CPT ≈ 10℃); 2507 duplex steel (CPT ≈ 60℃) is ideal for seawater applications.
• Preventive Measures: Use Mo-bearing grades, avoid crevice designs, and perform passivation treatments (nitric acid immersion) to enhance film integrity.

Intergranular Corrosion (IGC)

IGC arises from chromium depletion at grain boundaries: during welding or high-temperature service (450–850℃), carbon combines with Cr to form Cr₂₃C₆, leaving a Cr-depleted zone (Cr < 10.5%) that loses passivity.

• Resistant Grades: L-grades (304L, 316L, C ≤ 0.03%), stabilized grades (321 with Ti, 347 with Nb), and duplex grades (low C + N stabilization).
• Mitigation: Post-weld heat treatment (solution annealing at 1050–1150℃) to dissolve Cr₂₃C₆ and redistribute Cr.

Stress Corrosion Cracking (SCC)

SCC occurs under the combined action of tensile stress and corrosive media (e.g., chloride, caustic solutions), leading to sudden brittle fracture.
Austenitic grades (304, 316) are susceptible to SCC in hot chloride environments (>60℃), while ferritic and duplex grades exhibit higher resistance.

• Resistant Grades: 2205 duplex steel, 430 ferritic steel, and PH grades (17-4PH).
• Mitigation: Reduce tensile stress (stress relief annealing), use low-Cl⁻ environments, or select duplex grades.

High-temperature and oxidation resistance

Oxidation resistance improves with Cr and Si; high-Cr ferritics (e.g., 446 with ≈25–26% Cr) resist oxidation to ~800 °C. Austenitics like 310S (≈25% Cr, 20% Ni) are used for oxidation resistance up to ~1 000 °C.
For continuous high-temperature strength or carburizing atmospheres, select purpose-designed heat-resistant alloys or Ni-base superalloys.

3. Mechanical Properties

Stainless steel’s mechanical properties vary widely by microstructure and heat treatment, enabling customization for load-bearing, wear-resistant, or cryogenic applications.

Mechanical snapshot (typical, ranges):

Ductility and Toughness

• Austenitic grades: Excellent ductility (elongation at break 40%–60%) and toughness (notch impact toughness Akv > 100 J at room temperature).
  They retain toughness at cryogenic temperatures (e.g., 304L Akv > 50 J at -200℃), suitable for LNG storage and cryogenic vessels.
• Ferritic grades: Moderate ductility (elongation 20%–30%) but poor low-temperature toughness (brittle transition temperature ~0℃), limiting use in cold environments.
• Martensitic grades: Low ductility (elongation 10%–15%) and toughness in the quenched state; tempering improves toughness (Akv 30–50 J) but reduces hardness.
• Duplex grades: Balanced ductility (elongation 25%–35%) and toughness (Akv > 80 J at room temperature), with good low-temperature performance (brittle transition temperature < -40℃).

Fatigue Resistance

Fatigue resistance is critical for components under cyclic loads (e.g., shafts, springs).
Austenitic grades (304, 316) have moderate fatigue strength (200–250 MPa, 40% of tensile strength) in the annealed state; cold working increases fatigue strength to 300–350 MPa but raises sensitivity to surface defects.
Duplex grades (2205) exhibit higher fatigue strength (300–380 MPa) due to their dual-phase structure, while PH grades (17-4PH) reach 400–500 MPa after aging.
Surface treatments (shot peening, passivation) further enhance fatigue life by reducing stress concentrations and improving film stability.

4. Thermal and Electrical Properties

Thermal properties

• Thermal conductivity (20 °C): 304 ≈ 16 W·m⁻¹·K⁻¹; 316 ≈ 15 W·m⁻¹·K⁻¹; 430 ≈ 25–28 W·m⁻¹·K⁻¹. Stainless steels conduct heat much less effectively than carbon steel or aluminium.
• Coefficient of thermal expansion (20–100 °C): Austenitics ≈ 16–17 ×10⁻⁶ K⁻¹; ferritics ≈ 10–12 ×10⁻⁶ K⁻¹; duplex ≈ 13–14 ×10⁻⁶ K⁻¹.
  Austenitics’ higher CTE leads to larger thermal movements and greater welding distortion risks.
• High-temperature strength: Austenitics retain strength at moderate temperatures; specialized grades (310S, heat-resistant ferritics) extend maximum use temperature. For continuous creep applications, choose creep-resistant steels or Ni-based alloys.

Electrical Properties

Stainless steel is a moderate electrical conductor, with resistivity higher than copper and aluminum but lower than non-metallic materials.
Austenitic grades (304: 72 × 10⁻⁸ Ω·m) have higher resistivity than ferritic grades (430: 60 × 10⁻⁸ Ω·m) due to alloying element additions.
Its electrical conductivity is not suitable for high-efficiency conductors (dominated by copper/aluminum) but suffices for grounding rods, electrical enclosures, and low-current components where mechanical strength and corrosion resistance are prioritized.

5. Processing Performance

Stainless steel’s processability (welding, forming, machining) is critical for industrial manufacturing, with significant differences across grades.

Welding Performance

Weldability depends on microstructure, carbon content, and alloying elements:

• Austenitic grades (304, 316): Excellent weldability via arc welding, gas welding, and laser welding.
  Low C grades (304L, 316L) and stabilized grades (321, 347) avoid IGC; post-weld passivation enhances corrosion resistance.
• Ferritic grades (430): Poor weldability due to grain coarsening and brittleness in the heat-affected zone (HAZ). Welding requires low heat input and preheating (100–200℃) to reduce HAZ cracking.
• Martensitic grades (410): Moderate weldability. High C content causes HAZ hardening and cracking; preheating (200–300℃) and post-weld tempering (600–700℃) are mandatory.
• Duplex grades (2205): Good weldability but requires strict heat control (interpass temperature < 250℃) to maintain phase balance (50% austenite/ferrite). Post-weld solution annealing (1050–1100℃) restores corrosion resistance.

Forming Performance

Formability is linked to ductility and work hardening rate:

• Austenitic grades: Excellent formability due to high ductility and low work hardening rate.
  They can be deep-drawn, stamped, bent, and rolled into complex shapes (e.g., 304 for food cans, architectural panels).
• Ferritic grades: Moderate formability but prone to cracking during cold forming due to low ductility; warm forming (200–300℃) improves workability.
• Martensitic grades: Poor cold formability (low ductility); forming is typically performed in the annealed state, followed by quenching and tempering.
• Duplex grades: Good formability (similar to 304) but requires higher forming force due to higher strength.

Machining Performance

Machinability is influenced by hardness, toughness, and chip formation:

• Austenitic grades: Poor machinability due to high toughness, work hardening, and chip adhesion to cutting tools. Machining requires sharp tools, low feed rates, and cutting fluids to reduce wear.
• Ferritic grades: Moderate machinability, better than austenitic grades but worse than carbon steel.
• Martensitic grades: Good machinability in the annealed state (HB 180–220); hardening increases difficulty, requiring cemented carbide tools.
• PH grades: Moderate machinability in the solution-annealed state; aging hardens the material, making post-aging machining impractical.

6. Functional Properties and Special Applications

Beyond core performance, stainless steel’s functional properties (biocompatibility, surface finish, magnetic properties) expand its application scope.

Biocompatibility

Austenitic grades (316L, 316LVM) and PH grades (17-4PH) are biocompatible—they are non-toxic, non-irritating, and resistant to bodily fluids (blood, tissue).

316LVM (low carbon, vacuum melted) is used for surgical implants (bone plates, screws, stents) due to its high purity and corrosion resistance in physiological environments.

Surface modifications (polishing, electrochemical etching) further enhance biocompatibility by reducing bacterial adhesion.

Surface Properties and Aesthetics

Stainless steel’s surface can be tailored for aesthetics and functionality:

• Mechanical finishes: 2B, No.4 (brushed), BA (bright annealed), mirror. Choose finish for intended aesthetic and cleanability.
• Electropolishing: improves surface smoothness and corrosion resistance; commonly used in medical/food equipment.
• Chemical passivation: nitric or citric acid treatments remove free iron and augment the passive layer, improving corrosion resistance for food and medical applications.
• Coloration & coatings: PVD or organic coatings can add color or additional protection; adhesion requires proper surface prep.

Magnetic Properties

Magnetism is determined by microstructure:

• Austenitic grades: Non-magnetic in the annealed state; cold working induces weak magnetism (due to martensitic transformation) but does not affect corrosion resistance.
• Ferritic, martensitic, and duplex grades: Magnetic, suitable for applications requiring magnetic responsiveness (e.g., magnetic separators, sensor components).

7. Typical applications by family

• Austenitic (304/316): food processing, architectural cladding, chemical plant, cryogenics.
• Ferritic (430/446): decorative trim, automotive exhausts (446 high-temp), appliances.
• Martensitic (410/420/440C): cutlery, valves, shafts, wear parts.
• Duplex (2205/2507): oil & gas (sour service), seawater systems, chemical process equipment.
• PH (17-4PH): aerospace actuators, high-strength fasteners, applications demanding high strength with moderate corrosion resistance.

8. Comparison with Competing Materials

Material selection requires balancing mechanical performance, corrosion resistance, weight, thermal behavior, fabrication characteristics, and life-cycle cost.

The comparison below focuses on stainless steel versus the most commonly considered metallic alternatives in engineering practice.

9. Conclusion

Stainless steels are a versatile materials family that combines corrosion resistance, mechanical performance and aesthetic flexibility.

Successful use depends on aligning grade, microstructure and finish with the service environment and manufacturing process.

Use PREN and validated corrosion tests as screening tools for chloride environments; control fabrication heat history and surface condition; require MTRs and first-article corrosion/ mechanical qualification for critical systems.

When properly specified and processed, stainless steels deliver long service life and competitive life-cycle economics.

 

FAQs

Is 316 always better than 304?

Not always. 316’s Mo content provides materially better pitting resistance in chloride environments; but for non-chloride indoor applications 304 is usually adequate and more economical.

What PREN value should I target for seawater service?

Target PREN ≥ 35 for moderate seawater exposure; for splash or warm seawater consider PREN ≥ 40+ (duplex or superaustenitics). Always validate with site-specific testing.

How do I avoid intergranular corrosion after welding?

Use low-carbon (L) or stabilized grades, minimize time in the sensitization range, or perform solution annealing and pickling when practical.

When to choose duplex instead of austenitic stainless?

Choose duplex when you need greater strength and improved chloride/pitting and SCC resistance at a lower life-cycle cost than superaustenitics—common in oil & gas, desalination and heat-exchanger applications.