Understanding Stainless Steel Classification

Stainless steel fasteners are essential for applications requiring corrosion resistance, but not all stainless is created equal. This guide covers every major grade you'll encounter, helping you choose the right one for your application.

Understanding Stainless Steel Classification

Stainless steels are categorized by their crystalline structure, which determines their properties. The three main families used in fasteners are:

• Austenitic (300 series) — Non-magnetic, excellent corrosion resistance, cannot be hardened by heat treatment
• Martensitic (400 series) — Magnetic, moderate corrosion resistance, can be hardened
• Precipitation Hardening — High strength with good corrosion resistance

300 Series (Austenitic) — The Workhorse Family

304 Stainless Steel (18-8)

The most common stainless steel fastener material worldwide.

304 gets its nickname "18-8" from its nominal composition: 18% chromium and 8% nickel. This combination creates the passive chromium oxide layer that gives stainless steel its corrosion resistance.

Chemical Composition:

Mechanical Properties (A2-70 Rating):

• Tensile Strength: 700 MPa (101,500 psi) minimum
• Yield Strength: 450 MPa (65,300 psi) minimum
• Proof Load: 500 MPa (72,500 psi)
• Elongation: 40% minimum
• Hardness: Rockwell B70-90

The "A2-70" designation means: A2 = austenitic stainless, 70 = tensile strength class (700 MPa ÷ 10).

Corrosion Resistance: ★★★★☆ (4/5)

Excellent resistance to atmospheric corrosion, mild chemicals, and most food products. Will corrode in chloride-rich environments (seawater, road salt) and strong acids.

Magnetic Properties:

Non-magnetic when annealed. Cold working (threading, heading) can induce slight magnetism, but this doesn't affect corrosion resistance.

Applications:

• General industrial equipment
• Food processing machinery
• Architectural applications
• Kitchen equipment
• HVAC systems
• Water treatment (freshwater)

Cost Comparison: Baseline (1.0x) — Most economical stainless option

When NOT to Use 304:

• Marine/saltwater environments (use 316)
• Swimming pool equipment (chlorine attacks it)
• High-strength structural applications (use 410 or 17-4 PH)
• Chemical processing with chlorides or acids

316 Stainless Steel (Marine Grade)

The premium choice for harsh environments.

316 adds 2-3% molybdenum to the 304 formula, dramatically improving resistance to chlorides and acids. This is the standard for marine, chemical, and medical applications.

Chemical Composition:

Mechanical Properties (A4-70 and A4-80 Ratings):

A4-70 (standard):

• Tensile Strength: 700 MPa (101,500 psi) minimum
• Yield Strength: 450 MPa (65,300 psi) minimum
• Proof Load: 500 MPa (72,500 psi)

A4-80 (strain hardened):

• Tensile Strength: 800 MPa (116,000 psi) minimum
• Yield Strength: 600 MPa (87,000 psi) minimum
• Proof Load: 640 MPa (92,800 psi)

Corrosion Resistance: ★★★★★ (5/5)

Superior resistance to chlorides, acids, and pitting corrosion. The molybdenum content provides excellent protection against crevice corrosion—critical for fastener applications where threads create natural crevices.

Magnetic Properties:

Non-magnetic. Slightly less prone to work-hardening magnetism than 304.

Applications:

• Marine hardware and boat fittings
• Swimming pool and spa equipment
• Medical devices and surgical instruments
• Chemical processing equipment
• Pharmaceutical manufacturing
• Coastal architectural applications
• Food processing (acidic foods)

Cost Comparison: 1.3-1.5x the cost of 304

When NOT to Use 316:

• Budget-conscious indoor applications (304 is sufficient)
• High-strength requirements without proper design (still limited to ~116 ksi)
• Concentrated sulfuric acid (requires special alloys)
• High-temperature applications above 800°F continuous

303 Stainless Steel (Free Machining)

Optimized for manufacturing, not end-use performance.

303 adds sulfur to improve machinability, making it easier to produce on automatic screw machines. However, this comes at the cost of corrosion resistance and weldability.

Chemical Composition:

Mechanical Properties:

• Tensile Strength: 620 MPa (90,000 psi) typical
• Yield Strength: 275 MPa (40,000 psi) typical
• Hardness: Rockwell B84-92

Corrosion Resistance: ★★★☆☆ (3/5)

Lower than 304 due to sulfur inclusions creating initiation points for corrosion. Acceptable for mild environments but avoid outdoor or wet applications.

Magnetic Properties:

Non-magnetic, similar to 304.

Applications:

• High-volume machined components
• Bushings and spacers
• Precision shafts
• Applications where manufacturing cost outweighs longevity

Cost Comparison: 0.9-1.0x (slightly lower due to easier machining)

When NOT to Use 303:

• Any corrosive environment
• Outdoor applications
• Food contact (sulfur inclusions)
• Welded assemblies (cracks easily)
• When long service life is required

400 Series (Martensitic) — Strength and Hardness

410 Stainless Steel

When you need hardness and some corrosion resistance.

410 is the basic martensitic stainless steel. It can be heat-treated to high hardness levels, making it suitable for applications requiring wear resistance or high strength.

Chemical Composition:

Mechanical Properties (varies by heat treatment):

Annealed:

• Tensile Strength: 450 MPa (65,000 psi)
• Yield Strength: 275 MPa (40,000 psi)
• Hardness: Rockwell B80-90

Hardened and Tempered:

• Tensile Strength: 1,100 MPa (160,000 psi)
• Yield Strength: 860 MPa (125,000 psi)
• Hardness: Rockwell C38-45

Corrosion Resistance: ★★☆☆☆ (2/5)

Lower chromium content means less corrosion resistance than 300 series. Adequate for mild atmospheric exposure but will rust in humid or wet conditions.

Magnetic Properties:

Strongly magnetic in all conditions.

Applications:

• Self-tapping screws
• Set screws requiring hardness
• Valve components
• Pump shafts
• Cutlery and blade applications
• Fasteners requiring high shear strength

Cost Comparison: 0.8-0.9x (less nickel = lower cost)

When NOT to Use 410:

• Marine or coastal environments
• Chemical exposure
• Food processing equipment
• When corrosion resistance is priority over strength
• Wet or humid conditions without additional protection

416 Stainless Steel (Free Machining Martensitic)

410's machinable cousin.

416 adds sulfur to 410 for improved machinability while retaining the ability to be hardened. It's the go-to for high-volume production of hardened stainless components.

Chemical Composition:

Mechanical Properties:

• Tensile Strength: 520-760 MPa (75,000-110,000 psi) depending on heat treatment
• Yield Strength: 275-585 MPa (40,000-85,000 psi)
• Hardness: Rockwell C30-40 (hardened)

Corrosion Resistance: ★★☆☆☆ (2/5)

Similar to 410, with slightly reduced resistance due to sulfur content.

Magnetic Properties:

Strongly magnetic.

Applications:

• High-volume screw machine products
• Studs and bolts requiring hardness
• Precision turned parts
• Valve stems
• Gear shafts

Cost Comparison: 0.85x (machining savings offset material cost)

When NOT to Use 416:

• Same limitations as 410
• Welded assemblies (sulfur causes hot cracking)

Precipitation Hardening — Aerospace Performance

17-4 PH (UNS S17400)

The strongest commonly available stainless steel.

17-4 PH combines high strength with good corrosion resistance through a unique precipitation hardening process. It's the standard for aerospace, nuclear, and high-performance applications.

Chemical Composition:

Mechanical Properties (Condition H1025):

• Tensile Strength: 1,070 MPa (155,000 psi)
• Yield Strength: 1,000 MPa (145,000 psi)
• Hardness: Rockwell C35-40
• Elongation: 12% minimum

Condition H900 (maximum strength):

• Tensile Strength: 1,310 MPa (190,000 psi)
• Yield Strength: 1,170 MPa (170,000 psi)
• Hardness: Rockwell C40-47

Corrosion Resistance: ★★★★☆ (4/5)

Comparable to 304 in most environments. Better than 400 series due to higher chromium content.

Magnetic Properties:

Magnetic after heat treatment.

Applications:

• Aerospace structural fasteners
• Nuclear reactor components
• High-performance motorsports
• Oil and gas equipment
• Medical implants
• Firearms components
• Valve and pump components

Cost Comparison: 2.0-3.0x the cost of 304

When NOT to Use 17-4 PH:

• Cost-sensitive applications where strength isn't critical
• Cryogenic temperatures (becomes brittle below -100°F)
• Marine environments (316 is more suitable)
• When non-magnetic properties are required

Quick Reference: Stainless Steel Grade Selection

Specification Cross-Reference

Key Takeaways

1. For most applications: 304 (18-8) provides the best balance of cost and performance

2. For marine/chemical: 316 is worth the premium—it prevents expensive failures

3. For high strength: Consider 17-4 PH or hardened 410, but understand the corrosion tradeoffs

4. For machined parts: 303 or 416 save manufacturing costs but sacrifice some corrosion resistance

5. Always specify the grade: "Stainless steel" alone is not a specification—insist on knowing the exact grade

When in doubt, 316 provides the best all-around protection and is rarely a wrong choice except where cost is the primary driver.