Heat treatment

Heat Treatment: The Core Process for Material Property Control

Heat treatment is a key process in materials engineering. Through controlled heating, holding, and cooling of materials (primarily metals, but also including ceramics and polymers), it modifies the material's internal microstructure (such as grain size, phase composition, and distribution of precipitated phases), thereby precisely controlling its mechanical properties (hardness, strength, toughness, and plasticity), physical properties (conductivity and magnetism), or chemical properties (corrosion resistance).

The effectiveness of heat treatment is entirely determined by the parameters of the three phases: heating, holding, and cooling. These three phases are known as the "three elements of heat treatment" and are the core of process design:

The purpose of heat treatment varies in different scenarios, but its core is to "adjust material properties as needed." It primarily falls into the following four categories:

Based on the process purpose, treatment scope, and principle, metal heat treatment can be divided into three categories, with "integrated heat treatment" being the most widely used type:

Overall heat treatment is the most basic type, suitable for workpieces requiring overall performance control. It primarily includes the following four core processes (using steel as an example):

--Annealing: Heating to above/below the phase transition temperature → prolonged holding → slow furnace cooling. This eliminates internal stresses, softens the material, and refines the grain size, resulting in lower hardness and improved ductility. It is suitable for pre-processing (e.g., before cutting).

--Normalizing: Heating to the austenitizing temperature → holding → cooling in air (faster cooling than annealing). This refines the grain size, homogenizes the structure, and improves mechanical properties. Strength is higher than in the annealed state. It is suitable for the final heat treatment of structural parts (e.g., shafts).

--Quenching: Heating to the austenitizing temperature → holding → rapid cooling (water/oil cooling) produces a martensitic structure, significantly improving hardness and wear resistance. This results in extremely high hardness but also increased brittleness, requiring combined tempering (e.g., tool hardening).

Tempering: Reheating the quenched workpiece to a temperature below the phase transition temperature, then holding the temperature, and then cooling (in air or oil) to reduce quench brittleness and adjust the hardness-toughness balance. Quenching + tempering ("quenching and tempering") is the most commonly used strengthening process and is suitable for parts subject to impact (such as crankshafts).

The core of surface heat treatment is "surface strengthening." It is suitable for workpieces that require high hardness and wear resistance on the surface and high toughness in the core (such as gears and shafts) to avoid core embrittlement caused by full quenching. There are two main types:

When only a specific area of ​​the workpiece needs to meet performance requirements (such as the weld seam of a welded part or the raceway of a bearing), a localized heating and cooling method is used to avoid unnecessary deformation or performance loss caused by overall treatment. Examples include localized quenching of large machine tool guideways and localized stress relief annealing of welded joints.

Heat treatment is a fundamental supporting technology in nearly all industrial sectors. Without heat treatment, it is impossible to achieve the material properties that meet engineering requirements: