Steel Production and Heat Treatment Processes Explained

The Electric Arc Furnace

The oven consists of a steel vessel cooled externally by a water circuit and internally lined with refractory ceramic. The process begins by introducing cast iron, oxidized scrap, and lime. As the cast iron and scrap are processed, they receive electric energy in the form of voltaic arcs from electrodes, which increases the temperature. Once the steel composition reaches the desired margins, it is poured into appropriate containers or molds. This oven allows for very precise control of both temperature and steel composition.

Commercial Steel Forms

When leaving converters or furnaces, steel is in a liquid phase. To be marketed, it must be solidified and shaped. There are two primary methods:

• Ingot Casting: Pouring the liquid into a mold and allowing it to cool.
• Continuous Casting: Pouring the liquid into channels to form rectangular section bars.

Heat Treatment Processes

Heat treatment involves subjecting steel to controlled temperature changes to alter its constituent properties. Commonly used treatments include tempering, tempering (revingut), annealing, and normalizing.

Tempering

Tempering is applied when high hardness and mechanical strength are required. The goal is to obtain a steel structure composed of a large proportion of martensite, which is achieved through the rapid cooling of austenite. The process consists of:

1. Heating: Raising the steel temperature until the structure transforms into austenite. Depending on the carbon percentage, the temperature must be adjusted accordingly.
2. Rapid Cooling: Ensuring the austenite transforms into martensite. Each steel type requires a specific minimum cooling rate.

Normalizing

Normalizing involves heating the steel to the operating temperature followed by air cooling. This process is applied to low-carbon steels after cold or hot deformation to remove internal stresses and restore original properties.

Tempering (Revingut)

This procedure increases the toughness of steels that have already been subjected to the tempering process. It consists of heating the steel to a temperature below 723°C followed by air cooling to reduce internal stresses and improve durability.

Annealing

Annealing is used to decrease hardness and increase the plasticity of steel, making it easier to deform and work. It involves high-temperature heating followed by slow cooling. Types include:

• Regeneration Annealing: For steels with a carbon content greater than 0.6%.
• Globular Supercritical Annealing: For alloy and tool steels.
• Softening Annealing: Used for high-strength alloy steels.
• Stress-Relief Annealing: To remove brittleness produced during cold forming processes.