UNI C40 Alloy Steel Bar UNE C35K Round Bars GOST 40 Rods

Product Description

Medium-Carbon Structural Steels (UNI / UNE / GOST)

These designations represent unalloyed medium-carbon steels across different European and Russian standards. While they are often referred to as "alloy steels" in a general industrial sense, they are technically carbon steels (non-alloy) designed for structural components that require higher strength than mild steel but better ductility than high-carbon tool steels.

These grades are "workhorse" materials for machinery, providing a good balance of strength, toughness, and machinability.

International Designations & Equivalents

These steels are largely interchangeable for general engineering purposes, though specific requirements for "K" (killed) or "S" (sulfur-controlled) variants may vary.

Comparison of Key Properties

1. UNI C40 / GOST 40

These are functionally identical medium-carbon steels. They are frequently used in the Normalized or Hardened & Tempered condition.

• Tensile Strength ($R_m$): 550–700 MPa (Normalized)

• Yield Strength ($R_e$): $\ge$ 300 MPa

• Hardness: ~170–210 HBW (Normalized)

• Characteristics: Responds well to induction or flame hardening to achieve surface hardness for wear resistance.

2. UNE C35K

The "K" suffix in the Spanish UNE standard typically denotes Killed Steel (fully deoxidized during melting), ensuring a uniform chemical composition and better internal soundness.

• Tensile Strength ($R_m$): 520–650 MPa

• Yield Strength ($R_e$): $\ge$ 270 MPa

• Characteristics: Slightly lower carbon than C40 makes it more ductile and easier to cold-form or weld, though it has lower ultimate strength.

Product Forms & Applications

Round Bars & Rods

Standard diameters are used in the production of components that must withstand moderate mechanical stresses:

• Shafts & Axles: Due to their reliable fatigue strength.

• Gears & Pinions: Often induction hardened on the teeth.

• Bolts & Studs: High-strength fasteners (Grade 8.8 equivalent when heat treated).

• Connecting Rods: For small engines and mechanical linkages.

Key Technical Attributes

• Heat Treatment: These grades can be through-hardened (quenched in oil or water) and tempered to achieve a specific balance of hardness and ductility.

• Machinability: Excellent. These steels are known for their consistent chip formation and ability to achieve a high-quality surface finish using standard carbide tooling.

• Weldability: Moderate. Due to the carbon content (0.35%–0.45%), there is a risk of crack formation in the heat-affected zone (HAZ). Pre-heating to 150–250°C is generally recommended for thicker sections.

• Forgeability: Very good. These steels are easily forged into complex shapes for automotive and industrial machinery.

Condition of Supply

Depending on the application, these bars are typically supplied in one of the following states:

1. As Rolled (+AR): Direct from the mill, no specific heat treatment.

2. Normalized (+N): Heated and air-cooled to refine grain structure and improve uniformity.

3. Quenched & Tempered (+QT): Heat treated to a specific tensile strength range.

4. Cold Drawn (+C): Provides tighter dimensional tolerances and a bright surface finish, at the cost of some ductility.