Steel Heat Treatment and Corrosion Types

Posted on Mar 13, 2025 in Electromechanical Engineering

Steel Heat Treatment

Temple-heat treatment consists of heating some Fe alloys followed by continued rapid cooling in a suitable medium (water, oil, or air). It is used to obtain harder martensitic steels. The ability of steel to become martensitic is its hardenability.

Martensitic Tempering

In martensitic tempering, the steel is heated to the austenitizing temperature and continues until all the austenite transforms. The bath is cooled in a carbon-supersaturated salt at a constant temperature. During this period, the austenite must not transform. The BCC structure of austenite becomes BCT (martensite). This temper holds up to a maximum of 0.6% C. The hardness and mechanical strength increase.

Precipitation Tempering

Precipitation tempering is used for Mg alloys, Al, and Cu. The tightening comes after cooling. During cooling, fine particles of a chemical precipitate oppose dislocation crystals of the alloy, but the cooling is too fast for the compound to mix with the alloy. The final product is the same as that obtained during heating.

Austenitizing Continuous Tempering – Full

This process is used for hypoeutectoid steels. The material is heated 50°C above the upper critical temperature and cooled in the most appropriate medium. Its main structural component is martensite.

Austenitizing Continuous Tempering – Incomplete

This process is used for hypereutectoid steels. The process is similar, but the austenite becomes pearlite, and cementite remains. It cools very quickly, and you get a structure of martensite and cementite.

Influential Factors in Heat Treatment

  1. Steel composition (heating)
  2. Percentage of C and alloy
  3. Heating time (as mass)
  4. Cooling rate (most important factor)
  5. Characteristics of the cooling medium (conditions its velocity): oil, water, air.
  6. Size and geometry of the sample (thickness)

Surface Hardening Treatments

Cementing

Cementing involves adding C to the steel surface at a given temperature (Feγ + 2CO → (Feγ + C) + CO2). It is used for steels with low carbon content to increase surface hardness while maintaining core toughness.

Carbonitriding

In carbonitriding, sodium cyanide is used as a binder liquid: 2CNNa + O2 → 4NCONa → 2NCONa → CO3Na2 + NaCN + CO + 2N. Nitrogen forms iron nitrides, which are very hard.

Nitriding

Nitriding is applied to certain steels and castings with Al and Cr to obtain very high hardness. It is performed in special ovens (500º – 525ºC) with ammonia flows: 2NH3 → 2N + 3H2. Nitrogen forms iron nitrides (Fe4N).

Types of Corrosion

  • Uniform: Cathodic and anodic areas are exchanged in a metal close to an electrolyte.
  • Galvanic: Two different metals are exposed to an electrolyte; the more electronegative one corrodes.
  • Differential Aeration: Occurs in cracks and crevices where dirt and moisture penetrate; the inland areas become impoverished or experience corrosion.
  • Pitting: Corrosion starts from the inside surface. A small crack initiates corrosion, and the surface beneath it runs out of oxygen.
  • Intergranular: Occurs when a second phase precipitates at the grain boundaries, creating a galvanic cell. Ferrite is anodic with respect to cementite, producing a circulation of electrons.
  • Selective: One metal corrodes in a single-phase alloy (e.g., Zn in brass).
  • Erosion: The protective layer of metal oxide is destroyed by mechanical action (wear); occurs in fluid-conducting elements.
  • Stress Corrosion Cracking: Strain or internal tension creates small cracks. Areas with less tension act as the cathode, and corrosion starts (often after cold deformation).