CourseWiki/Materials and Manufacturing Processes/Metals

Heat Treating Metals

Choose a conditioning process by its goal: soften, refine structure, harden, improve toughness or reduce residual stress.

Heat treatment is conditioning: change structure to change properties

Heat treatment uses controlled heating, holding and cooling to change a metal's internal structure. It belongs to Conditioning in Manufacturing Process Categories. It can change properties without intentionally changing the overall shape, although distortion can occur.

The result depends on alloy, starting condition, section thickness and the complete cycle. Heating more or cooling faster is not automatically better. A treatment that strengthens one alloy can soften another.

Steel microstructures are not the four steel families

Ferrite and austenite are different iron-based crystal phases. Cementite is iron carbide, Fe₃C, a hard, brittle phase. Pearlite is a layered arrangement of ferrite and cementite. Martensite can form from austenite during sufficiently rapid cooling in suitable steel. These terms describe structure, not the buying categories carbon, alloy, stainless and tool steel.

Carbon can diffuse during slower transformations. Quenching can suppress those transformations and produce martensite. The outcome is not determined by cooling speed alone: chemistry and section size affect what forms throughout the part.

Anneal to soften; normalize suitable steels to refine structure

Annealing is a family of treatments used to soften material, restore ductility or adjust its structure. Full annealing of suitable steels includes slow cooling; annealing cold-worked metal can form new grains and restore formability. Not every anneal uses the same temperature or cycle.

Normalizing heats suitable steel into its austenitic range and then cools it in air. It can refine and make the structure more uniform after earlier processing. Air cooling and slow furnace cooling produce different results.

Choose the process for the desired condition. Do not apply one universal temperature offset to all steels.

Quench hardening usually needs tempering afterward; Practice

Quench hardening usually needs tempering afterward

Quench hardening heats suitable steel to form austenite, holds as required, then cools fast enough to obtain the intended hardened structure. It commonly raises hardness and strength but can reduce ductility and toughness. Cracking and distortion are possible.

Tempering reheats hardened steel below the temperature at which austenite begins to form. It adjusts the balance of hardness and toughness and reduces some quenching stresses. It is not simply heating “just below” one critical temperature; the required cycle depends on grade and target properties.

Qualitative temperature-time sketches: annealing has slow cooling; hardening has rapid quenching followed by a separate lower-temperature tempering cycle. No numerical times or temperatures are implied. — Separate illustrative cycles, not a furnace program. Peak temperatures, holds and cooling rates are grade-specific.

These concepts explain specifications; they are not operating instructions. Use the approved process for the identified material, equipment and quench medium.

Practice the purpose of tempering

Surface hardening puts wear resistance near the outside

A part may need a hard wear surface and a tougher interior. Carburizing adds carbon near the surface of suitable steel before hardening. Nitriding introduces nitrogen to develop a hard surface layer. These change surface chemistry.

Induction hardening heats a surface region electrically and then quenches it. It uses the steel's existing composition rather than adding carbon. It therefore needs a suitable starting steel. Case depth, surface hardness and core properties must be specified together.

Cold work strengthens; stress relief addresses locked-in stresses; Practice

Cold work strengthens; stress relief addresses locked-in stresses

Work hardening, also called strain hardening, occurs as metal is permanently deformed during cold working. It commonly raises strength and hardness while reducing remaining ductility. “Cold” here means below the relevant recrystallization range, not necessarily cold to the touch.

Stress relief reduces residual stresses left by processing. These are stresses that remain without an external load. The goal is often dimensional stability during later machining or service. Stress relief is not automatically a full softening anneal and does not guarantee zero distortion.

Practice restoring formability

Aluminum strengthening does not use steel's martensite route

Some aluminum alloys can be strengthened by solution treatment, quenching and aging. Solution treatment puts selected elements into solid solution; subsequent aging develops small particles that obstruct deformation. Aging can occur naturally or through controlled heating, depending on the alloy and required condition.

Solid-solution strengthening and precipitation strengthening are different mechanisms. Dissolved atoms provide the first; particles of another phase provide the second. Other aluminum alloys obtain much of their strength through cold work. Annealing can soften cold-worked material.

An aluminum temper describes its processing condition. It is not synonymous with the steel process called tempering. Preserve suffixes such as -O and -T6 when reading or writing a material description; consult the relevant alloy-temper specification for their exact meanings.

Further reading on conditioning processes

Bodycote's heat-treatment overview provides entry points for annealing, hardening, tempering, stress relief and solution-and-age treatments. AEC's alloy and temper guide is a starting point for aluminum terminology. Neither an overview nor a qualitative sketch replaces a material-specific process specification.

Appears in

DET 130 DET 330 DET Metals and Heat Treatment Workpiece Materials for Machining