Carbon Steel: Definition, Fe-C Diagram, TTT Curves and Heat Treatments

Definition of Carbon Steel: Carbon steel is an alloy of iron and carbon (Fe-C) containing between 0.008% and 2.1% carbon, along with residual elements from the manufacturing process, such as phosphorus, sulfur, silicon and manganese.

Considerations:

• Used in most metal-part constructions. It accounts for up to 90% of metal materials produced.
• Low price relative to its mechanical strength.
• For practical use, steels with 0.05% to 1.5% C provide resistance to tension ranging from 32 to 60 kgf/mm².
• The yield strength of carbon steels can range from 17 to 27 kgf/mm².

Fe-C Diagram Definitions

• Hypoeutectoid steels: Steels with a carbon content below 0.8%.
• Eutectoid steels: Steels with a carbon content of 0.8%.
• Hypereutectoid steels: Steels with a carbon content between 0.8% and 2.1%.

Phases of the Fe-C System

• Ferrite (α-iron): Body-centered cubic (BCC) crystal structure, magnetic, and practically does not dissolve carbon in solid solution. It is stable below 910 °C.
• Austenite (γ-iron): Face-centered cubic (FCC) crystal structure, non-magnetic, dissolves up to 2% carbon in solid solution. It is stable between 910 °C and 1390 °C.
• Ferrite: Solid solution of carbon in α-iron.
• Austenite: Solid solution of carbon in γ-iron.
• Cementite: Chemical compound (iron carbide, Fe₃C) containing 93.33% Fe and 6.67% C, with high hardness and brittleness.
• Pearlite: Mixture of ferrite and cementite.
• Martensite: Supersaturated solid solution of carbon in α-iron. It results from rapid cooling of austenite.
• Bainite: Ferrite plus very fine carbides (cementite). Its formation depends on the cooling rate and the alloy composition.

TTT Curves (Time-Temperature-Transformation)

TTT curves are generated through experimental studies. Several small specimens of eutectoid steel are heated above the critical zone and then rapidly cooled to different temperatures and held there. At each temperature, the waiting time until transformation differs for each specimen. Micrographic evaluation is used to determine the start and end of the transformations. The curves are then plotted from the recorded data.

Factors Influencing the Curves

1. Chemical composition: In general, alloying elements and the % C delay the transformation of austenite, shifting the curve to the right and favoring the formation of martensite.
2. Grain size: The larger the grain, the longer the transformation time; a larger grain size shifts the curve to the right.
3. Homogeneity of austenite: Greater homogeneity shifts the curve to the right.

Heat Treatments for Carbon Steel

1. A) Annealing: Consists of austenitizing the steel (about 50 °C above A₃ or A₁), followed by slow cooling, which produces the normal transformation products.

   • Objectives: Remove residual stresses from mechanical treatment; reduce hardness and increase ductility; refine the coarse grain structure; eliminate the effects of previous heat treatments.

2. B) Normalizing: Consists of austenitizing the steel (about 50 °C above A₃ or A_cm), followed by faster cooling than annealing, normally in air. It also produces normal transformation products, but with a more refined grain.

   • Objectives: Same as annealing; a finer grain improves mechanical properties compared with annealing.

3. C) Quenching: Consists of austenitizing the steel (about 50 °C above A₃ or A₁), followed by cooling fast enough to avoid the normal transformation products of austenite.

   • Objectives: Obtain martensite, which provides high tensile strength and high hardness, with low toughness and marked residual stresses.

4. D) Tempering: Consists of reheating the quenched steel to a specific temperature below the critical zone, followed by cooling. It is performed right after quenching to correct its difficulties.

   • Objectives: Obtain tempered martensite (slightly lower hardness, greater toughness, and reduced residual stresses).

5. E) Martempering: Similar to quenching and tempering. However, before martensite begins to form, cooling is interrupted for some seconds so that the temperature of the center and surface of the part becomes equal.

   • Objectives: Obtain martensite with lower residual stresses and less distortion.

6. F) Austempering: Isothermal (non-continuous) cooling treatment consisting of austenitizing the steel (about 50 °C above A₃ or A₁), followed by rapid cooling to a certain temperature to avoid the normal transformation products of austenite. It is then held at that isothermal temperature, followed by continuous cooling in air.

   • Objectives: Obtain bainite (for steels with more than 0.5% C or Mn), with good mechanical properties and absence of residual stresses.

Thermochemical Treatments for Fe-C Alloys

These are treatments that harden the surface or near-surface region through partial chemical modification of the material.

• A) Carburizing (Cementation): Consists of introducing carbon into the surface of low-carbon steels. The part is then heated and quenched, aiming for the formation of martensite on the surface.
• B) Nitriding: Treatment preceded by quenching and tempering, carried out at 600 °C to 700 °C. It consists of heating the steel in a nitrogen-rich atmosphere, forming complex nitrides on the surface to achieve high hardness.
• C) Cyaniding: Consists of heating the steel at a suitable temperature in a molten cyanide salt bath, enriching the surface with carbon and nitrogen. It is then quenched in water.
• D) Carbonitriding: Consists of heating the steel in a carbon- and nitrogen-containing atmosphere.
• E) Boriding: Consists of introducing boron into the surface, forming iron boride.

Applications of Carbon Steel

Applications depend on the influence of composition, weldability, heat treatments, temperature effects (high and low), carbon content, corrosion, and types of carbon steel (low-carbon, medium-carbon, high-carbon for low-temperature use, structural quality, and high-strength grades).