Heat treating

Heat treating is a group of industrial processes used to change a material's properties.

These thermal processes alter physical and sometimes chemical characteristics. While most commonly used in metallurgy, heat treatment also applies to materials like glass. The goal is to achieve a specific result, such as softening or hardening a material. This is done by using extreme heating or chilling. While heating and cooling often happen incidentally during welding or hot forming, true heat treatment is done intentionally to reach a desired state.

To understand how this works, we must look at the microstructure of a metal. Metallic materials consist of tiny crystals called grains or crystallites. The size and composition of these grains determine how the metal behaves mechanically. Heat treatment manipulates these properties by controlling the rate of cooling and the rate of diffusion. Diffusion is the process where atoms spread out to create a more even, or homogenous, distribution within the crystals.

There are two primary mechanisms that change an alloy during treatment. The first is the diffusion mechanism, which changes the homogeneity of the alloy. In this process, atoms of a dissolved element move through the metal. If the alloy cools to an insoluble state, these atoms may migrate to grain-boundaries. This is called precipitation, which leads to nucleation, or the grouping of atoms. The second mechanism is the formation of martensite. This causes the crystals to deform intrinsically through a process called a diffusionless transformation.

In a diffusionless transformation, the metal is cooled so quickly that atoms cannot migrate to new spots in time. This often happens when using external media like oil, water, or polymers for quenching. Because the atoms are trapped within the crystal lattice, they prevent the matrix from reaching its low-temperature arrangement. This creates shearing stresses within the lattice. In steel, this transformation creates martensite, which hardens the metal. However, in other alloys like aluminum, this same process can actually make the metal softer.

Specific chemical compositions, or the amount of different elements in an alloy, dictate the results. A eutectoid alloy is a specific mixture that forms a single, continuous microstructure upon cooling. For example, a eutectoid steel contains exactly 0.77% carbon. When cooled slowly, it forms a layered structure of ferrite and cementite called pearlite.

If the carbon content varies, different structures form. A hypoeutectoid alloy has less than 0.77% carbon. These alloys often form islands of proeutectoid-ferrite, which increases ductility but lowers hardenability.

Conversely, a hypereutectoid alloy contains more than 0.77% carbon. When this type of steel is cooled slowly, cementite begins to crystallize first. This forms a proeutectoid phase before the rest of the metal becomes pearlite. Because cementite is much harder than pearlite, these alloys have greater hardenability, though they lose some ductility. A eutectic alloy is slightly different, as it has a single melting point lower than any of its individual parts. When a molten eutectic alloy cools, all constituents crystallize into their phases at the same temperature.

Successful heat treating requires precise control over temperature, time, and the cooling rate.

Most treatments begin by heating an alloy above a specific transformation temperature, known as an "arrest." At this temperature, the metal undergoes a period of hysteresis. This means the heat energy is used for the crystal change rather than raising the temperature. Consequently, the temperature may stop rising for a short time until the change is complete. For iron-based alloys, these critical temperatures are vital for determining whether the metal becomes austenite or other phases. Controlling these variables allows engineers to tailor metals for specific mechanical needs, such as strength, toughness, or elasticity.