15.2 Heat Treatment of Metals

Sub-task A-3.07 (Performs heat treatment of metal) covers what a millwright does at the bench and in the field: annealing a work-hardened part before forming, normalizing after welding, hardening and tempering a punch or wear part, stress-relieving a fabrication, and specifying preheat for welding.

Critical Temperatures

Steel changes crystal structure as it is heated. Two temperatures matter:

• Lower critical temperature (A1) = 727 C — the point at which pearlite begins to transform to austenite. All hardening treatments must exceed it; all tempering and stress relieving must stay below it.
• Upper critical temperature (A3) — the point at which the steel is fully austenitic. It varies with carbon content, from about 900 C at 0.20% carbon down to 727 C at the 0.83% eutectoid composition.

For hardening and annealing, steel is heated roughly 30–50 C above the upper critical temperature, soaked long enough for the section to reach temperature throughout (a common rule is one hour per 25 mm of thickness), and then cooled at a rate that determines the result.

The Four Core Processes

Stress relieving is a fifth, separate treatment: heat to roughly 550–650 C (well below A1), soak, then cool slowly. It relaxes residual stress from welding, machining or cold work without changing the microstructure or hardness. It is what a millwright specifies for a welded frame that must hold alignment.

The hardening rule: only steels with more than about 0.30% carbon harden appreciably by quenching. Mild steel heated cherry red and dropped in water will not get hard, because there is not enough carbon to form significant martensite. A part that must be hard on the outside and tough in the core is case hardened instead.

Quench Media

Quench severity determines whether the part hardens fully — and whether it cracks or distorts.

Parts are quenched vertically, with the longest axis down and agitated, so cooling is even. Quenching a long shaft horizontally cools one side first and bows it permanently. A vapour blanket clinging to a surface causes soft spots, which is why agitation matters.

Tempering and Temper Colours

After quenching, a part is at maximum hardness and maximum brittleness, with high residual stress. It must be tempered promptly — a hardened part left overnight can crack on the shelf.

Tempering temperature sets the balance. On a clean polished surface, the oxide film colour gives a usable field indication:

Higher tempering temperatures (400–650 C) are used on alloy steels such as 4140 to reach a specified hardness band like 28–32 HRC for a shaft. Note that some alloy steels suffer temper embrittlement if held in the 375–575 C range, so the manufacturer's tempering specification must be followed rather than a colour chart.

Case Hardening

Case hardening produces a hard, wear-resistant surface over a tough, shock-absorbing low-carbon core — exactly what a gear tooth, a pin or a cam follower needs.

Nitriding is the low-distortion choice because it runs below the lower critical temperature, so no phase change and no quench occur. That is exactly why a precision component already ground to size can be nitrided but not carburized.

Heat Treatment of Non-Ferrous Metals

Copper, brass and aluminum behave differently, and confusing them with steel is a common error.

• Copper and brass are annealed by heating to a dull red and then either quenching in water or air cooling. Unlike steel, quenching does not harden them — copper alloys have no hardening phase transformation. They harden only by work hardening (cold forming), and heating removes it.
• Aluminum alloys in the heat-treatable series (2xxx, 6xxx, 7xxx) are solution treated, quenched, and then aged to a -T4 or -T6 temper. Overheating a T6 part with a torch annealsit locally and permanently destroys the temper — a fact that matters when a millwright straightens or welds aluminum guarding or structure.

Welding, Preheat and Post-Weld Heat Treatment

Heat treatment and welding meet in the heat-affected zone (HAZ). Welding is a localized quench: molten metal against a cold mass cools fast enough to form brittle martensite in medium- and high-carbon steel, and hydrogen from moisture then drives hydrogen-induced (cold) cracking, which can appear up to 48 hours after welding.

Carbon equivalent (CE) predicts the risk:

$CE = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15}$

Preheat slows the cooling rate, lets hydrogen diffuse out, and reduces shrinkage stress. It is applied to the whole joint region, not just the weld line, and interpass temperature is maintained through the weld. Post-weld heat treatment (PWHT) — typically a 595–675 C stress relief — is applied to thick, restrained or code-governed weldments.

Welding cast iron is a special case: preheat to about 260–320 C, weld with nickel-based electrodes (ENi-CI or ENiFe-CI) in short stringer beads, peen each bead while hot to relieve shrinkage, and cool very slowly under insulating blankets. Skipping the slow cool cracks the casting alongside the weld every time.

Field Safety

• Heat-treating creates severe burn and fire hazards. Use dry tongs, face shield, leather gloves and apron, and keep quench tanks away from combustibles.
• Never quench in a container of oil that has water in the bottom — the water flashes to steam and can throw burning oil.
• Galvanized coating vapourizes near 900 C, producing zinc oxide fume and metal fume fever. Remove the coating and ventilate.
• Hardened parts can shatter. Wear eye protection when striking, cutting or grinding a hardened component.