C3‑80 Fasteners: Complete Technical Data – Tensile Strength, Hardness, Torque, Heat Treatment & More

For engineers specifying C3‑80 bolts, precise knowledge of mechanical properties, heat treatment response, and assembly behavior is non‑negotiable. While these martensitic stainless steel fasteners provide the highest strength among standard stainless groups, their reliable performance hinges on correct manufacturing processes and informed application. RAYCHIN LIMITED, a specialist global manufacturer and supplier of high‑strength stainless steel fasteners with decades of experience, provides this comprehensive technical reference. We detail everything from tensile strength and hardness to torque values, galling resistance, magnetic permeability, corrosion limitations, and hydrogen embrittlement risks, drawing directly from ISO 3506 and our own production data.

1. Mechanical Properties: Tensile Strength, Yield Strength & Hardness

The property class C3‑80 is defined by ISO 3506, where "80" signifies a minimum tensile strength of 800 MPa. This high strength is achieved through martensitic hardening, unlike austenitic grades (A2, A4) which rely solely on cold work. The typical room‑temperature properties for C3‑80 fasteners in the correctly quenched and tempered condition are:

C3‑80 yield strength is not explicitly mandated by ISO 3506 for martensitic classes, but RAYCHIN's tempered product consistently delivers 80–90% of tensile strength—typically above 640 MPa. C3‑80 hardness HRC is controlled during tempering; a minimum of 23 HRC ensures the required strength, while the 35 HRC maximum preserves ductility and resistance to brittle fracture. RAYCHIN performs 100% hardness testing on every fastener to guarantee this balance.

2. Torque Values and Tightening Recommendations

Achieving the correct preload without damaging threads requires accurate C3‑80 torque values. Martensitic stainless steels have a lower friction coefficient than austenitic grades, but surface finish and lubrication still significantly impact the torque‑tension relationship. The table below provides conservative C3‑80 bolt tightening torque guidelines based on 50% of yield strength (~320 MPa) and a lubricated nut factor K = 0.16.

These values assume clean, lightly oiled threads. For dry threads, increase torque by approximately 20–30%. Always use calibrated torque wrenches with a slow, steady application speed. RAYCHIN can provide a detailed torque chart specific to your fastener geometry and lubrication condition upon request.

3. Heat Treatment: Hardening and Tempering

The C3‑80 heat treatment process is essential to achieve the required 800 MPa tensile strength while maintaining sufficient ductility. RAYCHIN follows a strict two‑stage procedure:

1. Austenitizing and quenching (hardening): Heat to 950–1000°C (1740–1830°F), hold for sufficient time to dissolve carbides, then rapidly quench in oil. This transforms the microstructure to hard, brittle martensite.
2. Tempering: Reheat to 350–550°C (660–1020°F), hold, and air‑cool. This relieves internal stresses, precipitates fine carbides, and restores ductility. The exact tempering temperature is adjusted to achieve the target 23–35 HRC hardness range.

C3‑80 hardening and tempering must be precisely controlled. Over‑tempering above 600°C can reduce hardness below 23 HRC and compromise tensile strength, while under‑tempering leaves the fastener susceptible to brittle fracture. RAYCHIN's digitally controlled furnaces and 100% hardness verification ensure every fastener hits the correct property window.

4. Machining C3‑80 Fasteners

Machining C3‑80 fasteners in the annealed condition (prior to hardening) is relatively straightforward due to the alloy's lower carbon content compared to tool steels. RAYCHIN performs all turning, drilling, and thread rolling operations in the annealed state, followed by heat treatment. Key practices include:

• Thread rolling before hardening for optimal thread surface finish and fatigue life.
• Post‑heat treatment thread chasing when necessary to remove minor distortion from quenching.
• Rigid CNC setups with carbide tooling to maintain tight dimensional tolerances.

5. Galling Resistance and Assembly Best Practices

C3‑80 galling resistance is moderate—better than austenitic stainless steels like 304 or 316 due to the higher hardness and martensitic microstructure. However, under high contact stress and dry assembly, galling can still occur. RAYCHIN recommends:

• Always lubricate threads with a nickel‑based or MoS₂ anti‑seize compound.
• Use rolled threads rather than cut threads for a smoother surface finish.
• Avoid impact tools; use slow, controlled torque wrenches.
• Consider a nut of slightly different hardness to disrupt metal‑to‑metal adhesion.

6. Magnetic Permeability and Corrosion Resistance

C3‑80 magnetic permeability is high. Like all martensitic stainless steels, C3‑80 is ferromagnetic and will attract a magnet strongly. This is an important design consideration where non‑magnetic fasteners are required—in those cases, A2 or A4 austenitic grades are necessary.

C3‑80 corrosion resistance is moderate and suitable for mildly corrosive environments: fresh water, steam, atmospheric exposure, and many industrial fluids. However, it is not resistant to chlorides (seawater, de‑icing salts, bleach) and will pit if exposed to such media. It is also not suitable for acidic environments. For marine or chemical service, A4‑80 or duplex grades are required.

7. Hydrogen Embrittlement and Passivation

C3‑80 hydrogen embrittlement is a critical risk during manufacturing and in certain service conditions. Atomic hydrogen can be introduced during electrolytic plating or acid pickling, leading to delayed brittle fracture in hardened martensitic structures. RAYCHIN mitigates this risk through:

• Avoiding electrolytic zinc or cadmium plating entirely on C3‑80 fasteners.
• Using alkaline cleaning rather than acid pickling wherever possible.
• Baking at 190–220°C for 4–24 hours immediately after any wet chemical process that could introduce hydrogen.

C3‑80 passivation is an optional treatment that enhances the natural corrosion resistance of the martensitic stainless steel. Per ASTM A967 or AMS 2700, passivation removes free iron from the surface and enriches the chromium oxide layer. RAYCHIN can passivate C3‑80 fasteners upon request, providing improved rust resistance in mildly aggressive atmospheres. Note that passivation does not make C3‑80 suitable for chloride or acid service.