In scientific processing and process optimization of metal materials, annealing, normalizing, quenching, and tempering are known as the "four cornerstones" of heat treatment. These four technologies profoundly affect the internal microstructure and macroscopic properties of metals by precisely controlling the heating, insulation, and cooling processes, thereby meeting the diverse engineering application needs. The following is a detailed analysis of these four heat treatment technologies, as well as their practical applications and prospects in metal processing.
1. Annealing: Restoration and improvement of mental flexibility
Annealing, as one of the most basic and widely used processes in heat treatment, is centered on slowly cooling after heating to a specific temperature so that the internal stress of the metal can be released, the organization tends to be uniform, the hardness is reduced, and the plasticity and toughness are enhanced. This process is usually divided into three stages:
Heating: Heating the metal to a range above its recrystallization temperature but below the melting point to ensure that the internal atoms obtain sufficient activity energy.
Insulation: Maintaining the heating temperature for a while allows sufficient atomic rearrangement and recrystallization to occur inside the metal, thereby eliminating internal stress and refining the grains.
Cooling: Cool to room temperature at a slower rate to avoid stress regeneration and structural deterioration caused by rapid cooling.
Annealing processes are diverse, such as full annealing, spheroidizing annealing, etc., which are suitable for a variety of metal products from castings, and forgings to welded parts, aiming to improve processing performance, eliminate defects, and stabilize dimensions.
2. Normalizing: Standardized adjustment of metal properties
Normalizing, a heat treatment method between annealing and quenching, aims to obtain a finer grain structure than annealing through rapid cooling in the air, thereby improving the hardness and strength of the metal while maintaining good toughness. Its steps also include heating, insulation, and cooling, but the cooling rate is faster than annealing:
Heating: Heat the metal above the critical point Ac3 or Acm to ensure complete austenitization.
Insulation: Maintain temperature to promote austenite homogenization.
Cooling: Natural cooling in the air, using a faster cooling rate to promote grain refinement and performance improvement.
Normalizing is widely used in low-carbon steel and medium-carbon steel as a preliminary heat treatment for subsequent quenching or as a final heat treatment to improve the comprehensive mechanical properties of the material.
3. Quenching: The ultimate pursuit of metal hardness
Quenching is the most critical and technically difficult link in heat treatment, aiming to obtain high-hardness martensitic structure through rapid cooling. This process also includes three steps: heating, insulation, and rapid cooling:
Heating: Heat the metal above the critical point to achieve austenitization.
Insulation: Ensure the homogenization of austenite.
Rapid cooling: Use water, oil, or other efficient cooling media to quickly cool the metal to below room temperature to induce martensitic transformation.
After quenching, the hardness of the metal is significantly improved, but the brittleness also increases, so it is often combined with tempering to balance hardness and toughness. Quenching technology is widely used in the manufacture of high-hardness parts such as tools, molds, and bearings.
4. Tempering: Restoration and improvement of metal toughness
Tempering, as a necessary supplement after quenching, aims to eliminate quenching stress, restore and improve toughness, and maintain a certain hardness by heating to a lower temperature and then cooling. The tempering process also includes annealing, insulation, and cooling, but the selection of temperature and cooling rate is more sophisticated:
Heating: Select an appropriate temperature below the critical point to avoid re-austenitization.
Insulation: Promote stress release and organizational stability.
