How Does Heat Treatment Change Metal Properties?

11 November 2019

Once upon a time, blacksmiths heat-treated swords and armour. The job came to an end as the object sizzled in a barrel of water. That action toughened the metal. Just like that, there was no brittleness to worry about during the heat of battle. Modern heat treatment processes have similar goals, that much hasn’t changed, but the advanced methods used to accomplish those goals have changed a great deal.

Fine Tuning the Alloy Percentages

For one thing, heat treatment shops rarely deal with iron. Ferrous-only metals rust and lack workability. To enhance those older metals, forging and smelting factories imbue iron with carbon and nickel. The carbon strengthens the metal while nickel makes it corrosion resistant. But there’s a problem, one that even those country blacksmiths knew about. By work hardening iron, it does toughen, but the rigid microcrystalline structure has now become overly brittle. One good smack against an unyielding rock, then the metal will shatter. The job of a heat treatment professional, then, is to harden a metal workpiece while keeping it malleable and workable. The final product can never be brittle. From here, other properties are incorporated. Structural strength is paired with a modulus of elasticity. Corrosion resistance is also important, but so is a need for fatigue resistance.

Effecting Changes in Alloy Properties

For the blacksmith, a few mechanical alterations were enough. Today, heat treatment professionals are expected to imbue target alloy workpieces with a whole range of physical and mechanical changes. To accomplish these feats of strength, they put atomic structural changes in their equipment crosshairs. Phase changes are effected when great quantities of furnace heat are applied. Basically, the metal becomes almost hot enough to melt. Upper and lower transformative temperatures are held until body-centred cubic structures transform into face-centred cubic lattices. Then, with the lattice of the grain temporarily transformed, carbon diffuses into the crystals.

The heat and alloy composition cause most of the changes. To accomplish full phase changes, we also need to know the carbon content of the alloy. That’s why heat treatment operations vary so much between mild (low carbon) steel and medium or high carbon alloys. With the critical transformative temperatures identified, both high and low, plus the carbon content locked known, then all sorts of remarkable metal properties can be introduced. Finally, after the heat curves and hold periods are sequenced, then comes the moment to temper and quench, to stress relieve and anneal. Cooled to room temperature, the workpiece exhibits ductility, tensile strength, fatigue, and corrosion resistance, plus the kind of surface finish that would’ve been the envy of any old-world blacksmith’s shop.

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