Heat Treatment

Heat Treatment

Controlled thermal processing used to modify a material so that it develops desired properties or a more desirable material condition.

Four Primary Heat-Treatment Routes in the Source

Stress relief; annealing; austenitize, quench, and temper; and solution anneal, quench, and age.

Stress Relief

A heat treatment used for most alloys that undergo a fusion process to eliminate residual stresses that can cause distortion, decreased fatigue life, or reduced corrosion resistance.

Residual Stress

Internal stress remaining in a material that can contribute to distortion, decreased fatigue life, or reduced corrosion resistance.

Why is stress relief important after fusion-based additive manufacturing?

Fusion processing can leave residual stresses in a part, so stress relief is used to reduce or eliminate those stresses and their undesirable effects.

When is stress relief often performed on fusion-AM parts?

It is often performed before the part is removed from the build plate.

Typical Stress-Relief Temperature Range

Approximately 30–40% of the material's absolute melting point in kelvin.

Annealing

A heat treatment whose primary goal is to provide a more stable microstructure for a better balance of mechanical properties and corrosion resistance.

Primary Goal of Annealing

To provide a more stable microstructure that gives a better balance of mechanical properties and corrosion resistance.

Typical Annealing Temperature Range

Approximately 50–60% of the material's absolute melting point in kelvin.

Examples of Alloys Commonly Annealed

Austenitic stainless steels, copper alloys, and various aluminum, cobalt, nickel, iron, and refractory alloys.

Austenitize–Quench–Temper

A heat-treatment sequence in which a material is heated to a high temperature, rapidly cooled, and then reheated to reduce brittleness.

Austenitizing

In the austenitize–quench–temper sequence, heating the material to approximately 60% of its absolute melting point.

Quenching

Rapidly cooling a heated material, usually using water or another liquid in the treatment described in the source.

Tempering

Reheating a quenched material, around 300°C in the treatment described in the source, to remove brittleness.

Why is tempering performed after quenching in the austenitize–quench–temper process?

Tempering is used to remove the brittleness associated with the quenched condition.

Examples of Alloys Commonly Austenitized, Quenched, and Tempered

Alloy steels, tool steels, martensitic stainless steels, and some titanium alloys.

Solution Anneal–Quench–Age

A heat-treatment sequence consisting of solution annealing at high temperature, rapid cooling, and subsequent aging to precipitate strengthening phases.

Solution Annealing

In the solution anneal–quench–age process, heating an alloy to approximately 90% of its absolute melting point before quenching.

Aging

In precipitation-hardening treatments, holding the alloy at an elevated temperature after solution annealing and quenching so that strengthening phases precipitate.

Typical Aging Temperature in the Source

Approximately 25% of the material's absolute melting point.

Precipitation Hardening

Strengthening an alloy by using solution treatment, quenching, and aging so that phases precipitate and strengthen the material.

Why does aging strengthen a precipitation-hardenable alloy?

Aging causes phases to precipitate from the alloy, and these precipitated phases strengthen the material.

Examples of Alloys Commonly Solution Annealed, Quenched, and Aged

Aluminum alloys, precipitation-hardening stainless steels, nickel alloys, some copper alloys, magnesium alloys, and some titanium alloys.

Why must heat-treatment conditions be carefully controlled?

The actual temperatures, times, heating rates, and cooling rates can strongly affect the resulting treatment and therefore often need close control for a particular alloy.

Why might conventional heat-treatment schedules not be optimal for AM alloys?

Most alloys used in AM were originally developed for casting, wrought, or welding applications, so the optimum heat treatment for the same alloy produced by AM may be different.

Possible Problems Caused by Rapid Cooling

Rapid cooling during some heat treatments can induce residual stresses, distortion, and even cracking.

Heat Treatment and Microstructure

Heat treatment can modify the material's microstructural condition, which can change its resulting properties.

Thermal History

The sequence of heating and cooling experienced by a material; different thermal histories can leave the same alloy in different material or microstructural conditions.

Heat Treatment in the Processing–Structure–Property Relationship

Heat treatment is a processing step that can alter microstructure, which in turn influences material properties.