Heat treatment

Why it matters

A spring, a file, a chisel and a car body panel can all be made of steel, yet one is springy, one is hard enough to cut other steel, and one is soft enough to press into shape. Much of that difference comes from heat treatment: heating and cooling a metal in a controlled way to change its microstructure without changing its shape or composition. This lesson explains the main treatments for steel, and briefly for aluminium, at the level an engineer needs to read a drawing note such as "quench and temper to 30–35 HRC" and understand what it means.

Heat treatment is industrial work with furnaces at hundreds of degrees, hot metal and quench tanks of oil or water, which can cause burns, fires and steam explosions. This lesson is for understanding only. Do not try it at home; it is done in properly equipped shops or labs, with training and supervision.

The starting point: austenite

From the iron-carbon diagram, heating a steel above roughly 727–912 °C (depending on carbon content) turns it fully or partly into austenite, FCC iron that can dissolve carbon. Every steel heat treatment starts here (austenitising) and differs in how the steel is cooled.

Slow cooling: annealing and normalizing

• Full annealing: heat into the austenite region, then cool very slowly, usually in the furnace. The result is coarse pearlite and ferrite: soft, ductile and easy to machine or form. Annealing also removes the effects of cold work.
• Normalizing: heat a little higher, then cool in still air. Faster cooling gives finer pearlite, so the steel is somewhat stronger and harder than annealed, with a more uniform grain structure. It is often used to refine the structure after forging or welding.
• Stress relieving: heating well below the transformation temperature to relax residual stresses from welding or machining, without changing the microstructure much.

Fast cooling: quenching to martensite

If austenite is cooled very rapidly, by plunging it into water, brine or oil, the carbon atoms do not have time to diffuse out to form ferrite and cementite. Instead the FCC lattice shears into a distorted body-centred tetragonal structure supersaturated with carbon: martensite. Martensite is very hard and strong, but also brittle and full of internal stress. Its hardness depends mainly on carbon content: low-carbon steels (below about 0.25 % C) cannot be hardened much by quenching, which is one reason they weld easily.

Quenching has risks. Water quenches are severe and can distort or crack parts, especially with sharp corners or sudden changes in section. Oil and polymer quenchants are gentler. Hardenability is a steel's ability to form martensite deep below the surface rather than only at the skin. Alloying elements such as chromium, molybdenum and nickel slow the transformation to pearlite, increasing hardenability so thicker sections harden with a gentler quench. Hardenability is measured by the Jominy end-quench test.

Time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) diagrams show how fast a given steel must cool to avoid pearlite and bainite (another, tougher microstructure) and form martensite.

Tempering

Freshly quenched martensite is usually too brittle to use. Tempering reheats it to a temperature below 727 °C, often somewhere between about 200 and 650 °C, for a set time. Carbon comes out as tiny carbide particles, internal stresses relax, and the result, tempered martensite, trades some hardness for much better toughness.

The tempering temperature sets the balance:

• low tempering (around 200 °C): high hardness and wear resistance, for cutting tools and bearings;
• higher tempering (around 500–650 °C): lower hardness but much greater toughness, for shafts, bolts, axles and springs.

The combination is called quench and temper (Q&T). A drawing might specify a steel grade, "Q&T to 28–34 HRC", where HRC is the Rockwell C hardness scale. Standards such as SAE/AISI grade data give the tempering temperatures that achieve a given hardness.

Surface hardening

Many parts need a hard, wear-resistant surface over a tough core: gear teeth, camshafts, bearing races.

• Carburizing: diffuse carbon into the surface of a low-carbon steel at around 900–950 °C (the diffusion lesson computed how deep it goes), then quench and temper. The surface becomes high-carbon martensite; the core stays tough.
• Nitriding: diffuse nitrogen at lower temperatures to form hard nitrides, with little distortion.
• Induction or flame hardening: heat only the surface of a medium-carbon steel quickly, then quench it.

Aluminium: precipitation hardening

Heat-treatable aluminium alloys such as 6061 and 7075 are strengthened differently, because aluminium does not form martensite:

1. Solution treatment: heat to dissolve the alloying elements into a single solid solution.
2. Quench: cool rapidly to trap them in a supersaturated solution.
3. Ageing: hold at a moderate temperature (artificial ageing) or at room temperature (natural ageing) so that tiny precipitates form and block dislocations.

The temper designation "T6" means solution treated and artificially aged. Overageing (too long or too hot) makes the precipitates coarsen and the alloy weaker. Welding can locally undo the treatment, so welded heat-treated aluminium is weaker near the weld.

Common mistakes

• Expecting every steel to harden. Low-carbon steel barely responds to quenching.
• Skipping tempering. As-quenched martensite can crack in service, or even on the bench.
• Assuming harder is better. Higher hardness usually means lower toughness and ductility.

Recap

• Annealing and normalizing cool slowly: soft, ductile pearlite and ferrite.
• Quenching forms martensite: hard, brittle, stressed. Tempering restores toughness.
• Hardenability (alloying) controls how deep martensite forms.
• Aluminium alloys are hardened by solution treatment, quench and ageing (T6).