Plunge hot steel into water and its crystals rearrange near sound speed
Heat steel until it glows red to orange, around 820 to 870 degrees Celsius, then dunk it in water. Carbon atoms get trapped before they can escape the shifting crystal lattice, straining it into a hard phase called martensite. That change, once triggered, races through the metal at just under the speed of sound.
Heat treating covers a family of techniques, from annealing and tempering to quenching and case hardening, that deliberately heat or chill a material to change its properties. Metals are made of tiny crystals called grains, and their size and makeup largely decide how hard, strong, tough or bendy the metal will be. By controlling how fast atoms diffuse and how fast the piece cools, a smith or factory can dial those qualities up or down. Glass gets similar treatment.
Many metals rearrange their atomic lattice at particular temperatures, a behaviour called allotropy. In an alloy, such a shift can suddenly let another element dissolve, or force it back out. Cool slowly, and dissolved atoms have time to wander out and cluster at the grain boundaries. Steel cooled gently from its hot austenite state separates into alternating thin layers of ferrite and cementite, a soft layered structure called pearlite. Cool fast, and the result is martensite instead. Oddly, rapid cooling hardens steel but softens aluminium.
Carbon content steers the outcome. Steel with exactly 0.77 percent carbon turns entirely into pearlite on slow cooling. Less carbon than that, and softer ferrite forms first, making the metal more ductile but less hardenable. More carbon, and hard cementite crystallises first, raising hardenability at the cost of ductility. Alloys that lack the ferrite change, like many non-iron ones, instead strengthen slowly over time by precipitation, known as age hardening.
Watching a thermometer reveals the transformations. As metal passes a critical point, the temperature briefly stalls, because all the incoming heat goes into reshaping crystals; metallurgists call these pauses arrests. Pure iron has four such points. Heating only just past the upper one keeps grains small, which matters because big grains leave large boundaries that act as weak spots where a part may crack.
Source: Heat treating
