Steel Heat Treatments and Corrosion Mechanisms

Steel Heat Treatments

Standardizing (Normalizing)

Steels often exhibit plastic deformation; for example, they might have a pearlitic structure with relatively irregular grain sizes. A standard thermal treatment, known as normalizing, is applied to refine the steel and achieve a uniform grain distribution. Normalizing involves heating the material to a temperature between 55°C and 85°C above the upper critical temperature, followed by relatively fast cooling in air.

Annealing

This procedure is applied to low and medium-carbon steels that have been machined or significantly deformed through cold forming. The alloy is heated to austenitize it, typically 15°C to 40°C above the critical temperature. It is then allowed to cool slowly inside the furnace, a process that usually lasts for several hours. The resulting microstructure from this process is coarse pearlite.

Spheroidizing (Globularization)

Medium and high-carbon steels often have a microstructure consisting of coarse pearlite, which can be too hard for plastic deformation or machining. These grades can be annealed to develop a spheroidite microstructure. This spheroidized steel exhibits maximum softness and ductility, making it easily deformable or machinable. The thermal treatment involves heating the alloy to a temperature just below the eutectoid point (around 700°C). Spheroidizing typically lasts between 15 and 25 hours.

Tempering

Tempering is a treatment that follows quenching (hardening) to reduce brittleness and relieve internal stresses caused during the hardening process. It consists of heating the hardened parts to a temperature below the A1 point (lower critical temperature) to transform the martensite into a more stable structure. The process concludes with relatively fast cooling. The most influential factors on the results of tempering are the temperature and the heating time.

Understanding Steel Corrosion

Definition and Mechanism

Corrosion is defined as the transition of a metal from a free state to a combined state (like oxides or salts). This occurs due to the natural tendency of metals to return to their more stable ore state through the destructive action of oxygen in the air and electrochemical agents that promote the corrosion process. Essentially, the phenomenon consists of an oxidation of the metal, which can lead to the destruction of the component.

Types of Corrosion

Common types include:

• Uniform corrosion
• Galvanic corrosion
• Differential aeration corrosion
• Pitting corrosion
• Intergranular corrosion
• Selective leaching (dealloying)
• Erosion corrosion
• Stress corrosion cracking

Differential Aeration Corrosion

This type takes place in cracks and crevices within components made of the same metal. Dirt and moisture can enter these areas, meaning the outer surfaces are more exposed to air (more aerated), while the interior areas have less access to oxygen. This difference in oxygen concentration drives the corrosion process, as the areas with less oxygen become anodic (where oxidation occurs) and cannot be renewed easily.