Steel Heat Treatment Processes: Hardening, Annealing, and Surface Modification

Thermochemical and Heat Treatments for Steel

I. Heat Treatments (Thermal Processes)

Heat treatments involve controlled heating and cooling cycles to modify the internal structure of steel, thereby improving its mechanical properties.

1. Quenching (Hardening)

• Function: To significantly increase the hardness and mechanical strength of the steel.
• Process: Rapid cooling is required to prevent unwanted transformations, ensuring the formation of a hard microstructure.

2. Tempering

• Function: To improve the material's characteristics by reducing brittleness and internal stresses caused by quenching.
• Process: Always performed immediately after quenching. It involves reheating the hardened piece to smooth the hardening effect and reduce internal tensions.

3. Annealing

• Function: To soften the material, making it easier to machine and work.
• Process: Very slow cooling is employed to fully facilitate the desired transformations. This slow process allows sufficient time for atoms to return to their stable, original lattice structure.

4. Normalizing

• Function: To refine the grain structure and eliminate internal stresses.
• Process: Medium-rapid cooling allows for the partial development of transformations. It is faster than annealing but results in slightly less ductility.

II. Thermochemical Treatments (Surface Modification)

Thermochemical treatments modify the chemical composition of the steel surface by adding elements (like carbon, nitrogen, or sulfur) to improve specific surface properties, primarily hardness or corrosion resistance.

Main Thermochemical Processes

• Cementation (Carburizing): Applied to parts requiring high resistance to wear and impact, achieving high surface hardness while maintaining a resilient core.
• Sulfidation (Sulfinization): Improves wear resistance, enhances lubrication properties, and helps prevent seizing (grabbing).
• Cyanidation: A process often followed by tempering to achieve a hard surface layer containing both carbon and nitrogen.
• Nitriding: A surface hardening treatment applied to specific steels and cast irons. It achieves extremely high surface hardness and excellent corrosion resistance.

III. Iron and Carbon Alloys

1. Steels

• Composition: Iron alloys with a carbon content typically between 0.1% and 1.76% (or up to 2.06%).
• Characteristics: Steels are generally considered wrought materials and can be alloyed with other elements to form specialized alloy steels.

2. Castings (Cast Irons)

• Composition: Iron alloys where the proportion of carbon is higher than 1.76% (up to 6.67%).
• Characteristics: Industrially, cast irons typically contain 2% to 5% carbon. They are generally not wrought.

IV. Iron Crystal Structure: Ferrite

Ferrite (Alpha Iron) is characterized by its body-centered cubic (BCC) structure.

• Carbon Solubility: Ferrite has a weak capacity to form solid solutions with carbon. The maximum amount of carbon it can dissolve is approximately 0.025%.
• Magnetic Properties: Ferrite is non-magnetic above the Curie temperature.

V. Objectives of Heat Treatment

Heat treatments are performed to achieve several specific metallurgical goals:

1. To obtain lower hardness (e.g., annealing).
2. To improve machinability.
3. To eliminate internal stresses caused by cold working (strain hardening).
4. To achieve a more homogeneous structure.
5. To obtain maximum strength and toughness.
6. To vary specific physical properties.