Introduction

S40C is a medium carbon steel grade according to the Japanese JIS G4051 standard. It is comparable to AISI 1040 and C40 grades in other standards. S40C carbon steel is known for its excellent balance of strength, toughness, and wear resistance. It can be heat treated to achieve higher hardness and strength levels, making it highly suitable for mechanical components subjected to wear and stress.

Properties of S40C Grade:

  • Moderate to high tensile strength
  • Good toughness and impact resistance
  • Good wear resistance after heat treatment
  • Excellent machinability in annealed condition
  • Capable of hardening by heat treatment
  • Moderate weldability with proper procedures
  • High surface hardness achievable through surface treatments like flame hardening

Uses/Applications of S40C Grade:

  • Automotive components (shafts, axles, gears)
  • Machine parts (spindles, bolts, keys)
  • Structural parts subjected to medium stress
  • Engine parts and crankshafts
  • Piston rods
  • Forged parts and heavy-duty fasteners
  • Agricultural and industrial machinery parts

Chemical Composition (Typical):

Element Content (%)
Carbon (C) 0.37 – 0.44
Manganese (Mn) 0.60 – 0.90
Phosphorus (P) ≤ 0.030
Sulfur (S) ≤ 0.035
Silicon (Si) 0.15 – 0.35
Iron (Fe) Balance

Mechanical Properties (Typical in Normalized Condition):

Property Value
Tensile Strength 570 – 700 MPa
Yield Strength 330 MPa min
Elongation (in 50 mm) 16% min
Reduction of Area 35% min
Hardness (Annealed) 160 – 210 HB
Hardness (After Hardening & Tempering) Up to 250 – 300 HB

Physical Properties:

Property Value
Density 7.85 g/cm³
Modulus of Elasticity ~205 GPa
Thermal Conductivity ~49 W/m·K
Specific Heat Capacity ~486 J/kg·K
Electrical Resistivity ~0.16 µΩ·m

Forging:

  • Forging temperature range: 1100°C – 850°C.
  • Start forging at approximately 1200°C and stop forging no lower than 850°C.
  • After forging, slow cooling in furnace or dry ashes is recommended to avoid cracking.
  • Normalizing after forging enhances mechanical properties.

Heat Treatment:

  • Annealing: Heat to 820°C – 850°C, hold, then slow furnace cool to improve machinability and soften material.
  • Normalizing: Heat to 870°C – 920°C, hold, then air cool to refine grain size and enhance toughness.
  • Hardening: Heat to 820°C – 870°C, then quench in oil or water.
  • Tempering: After hardening, temper between 550°C – 680°C to adjust hardness and relieve stresses.

Dimensional Tolerances:

  • Hot rolled bars: Tolerances according to JIS G4051 and general standards for carbon steels.
  • Cold finished bars: Tighter dimensional tolerances achievable as per JIS G3191 or customer specifications.
  • Custom sizing available upon request.

Machinability:

  • S40C offers good machinability in the annealed condition.
  • Machinability is slightly reduced after heat treatment due to higher hardness.
  • Recommended to use high-speed steel or carbide tools for machining, with appropriate coolant usage.

Corrosion Resistance:

  • S40C is not corrosion resistant under atmospheric or humid conditions.
  • Surface treatments such as painting, oiling, black oxide coating, or galvanization are recommended for protection.
  • Suitable for indoor or controlled environments if untreated.

Weldability:

  • Moderate weldability; preheating is strongly recommended for thick sections to avoid cracking.
  • Use low hydrogen welding electrodes.
  • Post-weld heat treatment may be necessary to restore mechanical properties.
  • Welding without proper procedure can lead to weld cracking or brittleness due to the carbon content.

Available Form:

  • Hot Rolled Round Bars
  • Hot Rolled Flats, Squares
  • Forged Bars (Rounds, Flats)
  • Cold Drawn and Peeled Bars
  • Precision Ground Bars
  • Custom cut lengths available
  • Supplied in annealed, normalized, or quenched & tempered condition

Conclusion:

S40C carbon steel is a versatile medium carbon grade offering a balance between strength, hardness, and machinability. With its ability to be heat treated for improved mechanical properties, it is ideal for manufacturing durable and wear-resistant components. Proper care with welding and corrosion protection will maximize its performance in demanding industrial applications.

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