C3‑80 Fasteners: Understanding Martensitic Stainless Steel Bolts, ISO 3506, and the C3 Grade

In the fastener industry, the designation C3‑80 appears regularly on engineering drawings and procurement specifications, yet its meaning is often misunderstood. This article clarifies exactly what a C3‑80 bolt is—its material group, chemical composition, mechanical properties, and how it compares with familiar grades like A2‑70 and A4‑80. RAYCHIN LIMITED, a specialist global manufacturer and supplier of stainless steel and special alloy fasteners with decades of experience, provides this authoritative guide to help you specify C3‑80 fasteners with complete confidence.

1. What Is a C3‑80 Bolt? The ISO 3506 Classification System

What is C3‑80 bolt? The designation comes from ISO 3506, the international standard for mechanical properties of corrosion‑resistant stainless steel fasteners. It uses a simple code:

• The letter identifies the stainless steel group: A for austenitic, C for martensitic, F for ferritic.
• The number after the letter (1–5 for austenitic, 1–4 for martensitic) indicates the specific alloy composition within the group.
• The two‑digit number after the hyphen represents 1/10th of the minimum tensile strength in MPa. So, 80 means the fastener must achieve a minimum tensile strength of 800 MPa.

Therefore, a C3‑80 fastener is a martensitic stainless steel fastener from the C3 composition group, with a minimum tensile strength of 800 MPa. The C3 group corresponds to a martensitic chromium steel with approximately 12–14% chromium and a carbon content typically in the range of 0.17–0.25%—most commonly the grade X20Cr13 (EN 1.4021) or similar. RAYCHIN manufactures C3‑80 bolts to this exact specification, with full mechanical property certification.

2. C3 Stainless Steel Group and Chemical Composition

The C3 stainless steel group encompasses hardenable martensitic chromium steels. Unlike austenitic grades (A2, A4) which cannot be hardened by heat treatment, C3 materials are quenched and tempered to achieve the required high strength. The typical C3‑80 chemical composition based on ISO 3506 and the common steel 1.4021 (X20Cr13) is:

In practice, C3‑80 material is typically the steel designation 1.4021 fasteners or X20Cr13 bolts as defined in EN 10088. RAYCHIN sources raw material from certified mills and verifies chemistry on every heat via optical emission spectrometry (OES).

3. C3‑80 Mechanical Properties

The defining feature of C3 fasteners properties is their high strength, achieved through quenching and tempering. Key mechanical properties per ISO 3506 for property class C3‑80 are:

• Tensile Strength (Rm): ≥ 800 MPa (116 ksi)
• Yield Strength (Rp0.2): ≥ 640 MPa (93 ksi) – approximate; ISO 3506 does not specify minimum yield for martensitic classes, but typical quenched and tempered product achieves this level.
• Hardness: 23–35 HRC (typical after tempering)
• Elongation: ≥ 12% – indicating reasonable ductility despite the high strength

It is important to note that C3‑80 fasteners are magnetic (due to their martensitic structure) and have only moderate corrosion resistance compared to austenitic grades. They are not suitable for highly corrosive environments such as seawater or acidic conditions, but they perform well in mildly corrosive atmospheres, steam, fresh water, and many industrial applications where the high strength is the primary design driver.

4. C3‑80 vs A2‑70 and A4‑80 — Choosing the Right Grade

C3‑80 vs A2‑70: A2‑70 (typically 304 stainless steel) is austenitic, non‑magnetic, and has a tensile strength of 700 MPa. It offers much better corrosion resistance than C3‑80, particularly in chloride and chemical environments. However, A2‑70 cannot be hardened by heat treatment, and its strength is lower. Choose C3‑80 when high tensile strength is required in mildly corrosive environments; choose A2‑70 when corrosion resistance is paramount and strength requirements are moderate.

C3‑80 vs A4‑80: A4‑80 (typically 316 stainless steel) provides a tensile strength of 800 MPa—the same as C3‑80—but with significantly better corrosion resistance, especially against chlorides (due to 2–3% molybdenum). A4‑80 is non‑magnetic and retains its corrosion resistance in marine and chemical applications. C3‑80 is a more economical alternative where high strength is needed and the corrosion exposure is mild. RAYCHIN manufactures all three grades and can advise on the optimal selection for your application.

5. RAYCHIN's C3‑80 Fastener Manufacturing Capabilities

RAYCHIN LIMITED is a trusted manufacturer of C3‑80 fasteners for global industries. Our production includes:

• Hex bolts – ASME B18.2.1, DIN 931/933, ISO 4014, sizes M6–M48, 1/4″–2″
• Socket head cap screws – ASME B18.3
• Stud bolts and threaded rod – ASME B16.5, DIN 976
• Heavy hex nuts – ASME B18.2.2, DIN 934, proof‑loaded to match
• Flat and spring lock washers
• Custom parts – per your drawings; flanged bolts, stepped studs

Our quench‑and‑temper heat treatment is digitally controlled, with 100% hardness testing on every production lot. All shipments come with EN 10204 Type 3.1 certification and Positive Material Identification (PMI) reports. Type 3.2 with third‑party witness is available on request.

6. Applications for C3‑80 Fasteners

Thanks to their high strength and moderate corrosion resistance, C3‑80 bolts are widely used in:

• Steam turbine and valve bolting
• Pump and compressor assemblies
• Automotive and heavy‑duty equipment
• Industrial machinery and tools
• Structural connections in mildly corrosive atmospheres

When a design requires a high‑strength, cost‑effective stainless steel fastener that is not exposed to aggressive chemicals or chlorides, C3‑80 is the ideal choice.