Metallurgy and Heat Treatment Processes for Steel
Fundamentals of Heat Treatment Processes
Heat treatments are processes where temperature is used as a variable to modify the microstructure and constitution of metals and alloys without changing their chemical composition. Thermochemical or surface treatments are also used to alter the chemical composition of a surface layer through chemical reactions that take place on the surface of the piece.
Quenching (Temple)
Quenching is a conventional heat treatment used to obtain martensitic steels. It is characterized by rapid and continuous cooling in a suitable medium such as water, oil, or air.
Factors Influencing Quenching
- Steel composition
- Heating temperature
- Warm-up time: Related to the mass of the piece to achieve austenitizing.
- Cooling speed: The most important factor.
- Characteristics of the quenching medium
- Size and geometry of the sample
Types of Quenching
- Continuous full austenitizing (hypoeutectoid steels)
- Continuous incomplete austenitizing (hypereutectoid steels)
- Martempering
- Austempering: The difference with the previous method is that the time spent in the cooling salts must be long enough to traverse the curves and transform austenite into bainite.
- Surface quenching
Annealing Processes
Annealing is a heat treatment where a material is heated to a specific temperature for a set duration and then cooled slowly. It is designed to remove defects caused by quenching.
Types of Annealing
- Regeneration (complete austenitizing)
- Globular (incomplete austenitizing)
- Subcritical globular softening
Tempering
Tempering is a treatment that follows quenching to eliminate brittleness and internal stresses. It involves heating the steel to a temperature below the critical point, followed by controlled cooling. Cooling can be fast to achieve high toughness or slow to minimize thermal stresses and potential deformations.
Types of Corrosion
- Uniform: Occurs when metal in an electrolyte has cathodic and anodic areas, resulting in uniform corrosion. This is the most common form and is prevented by varnishes.
- Pitting (Stings): Penetrates from the surface to the interior, starting at small cracks where oxidation begins.