Craft
Heat treating carbon steel: anneal, harden, temper
A forged edge is soft until it is hardened and brittle until it is tempered. The order is not negotiable.
The same fire that moves a bar can change what the bar is. Shaping, as in drawing out and upsetting, uses heat as a way in; heat treating uses it on the material itself, to set how hard, how tough and how willing to bend the piece will be. The definition is narrow: only heating and cooling done deliberately to change a material’s properties count, not the warmth that happens as a by-product of hot forming or welding. Three processes carry the load in carbon steel: anneal, harden, temper.
What the heat is actually doing
Metal is a packing of small crystals called grains, whose size and composition do much to set hardness, strength, toughness, ductility and elasticity. Heat treating works by controlling two rates: diffusion, the movement of atoms through the crystal lattice, and cooling. Iron also reorders its crystal arrangement at particular temperatures, a behavior called allotropy, and one of those rearrangements is what lets carbon dissolve into the metal at all. Two mechanisms carry the changes: martensite formation, which deforms the crystals from within, and diffusion, which redistributes the alloying atoms.
Why does carbon content decide so much?
Carbon turns iron into steel, and its amount decides what heat treating can do. At 0.77 percent carbon, a eutectoid steel cooled slowly separates into alternating platelets of ferrite, which is iron, and cementite, a carbide: that layered structure is pearlite. Below 0.77 percent, hypoeutectoid steels grow islands of soft ferrite alongside the pearlite, gaining ductility and losing hardenability. Above it, hypereutectoid steels crystallize hard cementite first, buying hardenability at a cost in ductility. The austenitizing temperature, red to orange-hot, runs around 1,500 to 1,600 degrees Fahrenheit (820 to 870 degrees Celsius) depending on carbon content.
Annealing is deliberate slowness
Annealing heats the steel past its transforming point, then cools it slowly enough that the carbon has time to settle into soft pearlite. Full annealing cools slowly and aims at coarse pearlite, the softest condition, wanted before cold working or for machinability. Process annealing moves faster and settles for a uniform structure. Normalizing is the brisk relative: the steel is heated about 40 degrees Celsius (72 degrees Fahrenheit) above its upper critical temperature, held, then cooled in open air, leaving a uniform grain and a harder, stronger, less ductile piece than full annealing. The Wikipedia entry on heat treating reads the same ladder off its phase diagrams.
How fast is too fast when quenching?
Hardening is a race against the metal’s own diffusion. Cooled quickly enough, the carbon has no time to migrate and stays trapped in the lattice, which shears into martensite, a hard, brittle crystal that forms as a matter of temperature rather than time, at just under the speed of sound. The quench medium sets the pace, fastest to slowest: brine, polymer, fresh water, oil, forced air. Faster is not automatically better: quenching the wrong steel too fast cracks it. High-tensile AISI 4140 goes into oil, H13 hot work tool steel into forced air, medium-tensile AISI 1040 into brine. The eye for it passes from person to person: finding a class or a group is the practical route in.
Tempering, the second and gentler heat
Martensite as quenched is too brittle for most uses. Tempering holds steel below its lower critical temperature, commonly between 400 and 1,105 degrees Fahrenheit (205 to 595 degrees Celsius), trading a little hardness for toughness. Temperatures up to about 1,300 degrees Fahrenheit (700 degrees Celsius) buy further ductility at some cost in yield strength. Variants such as austempering and martempering hold the piece at a set point above the martensite start instead of cooling straight through.
Why does polished steel change color as it heats?
Freshly ground or polished steel grows an iron oxide layer as it warms, and at each temperature that layer reaches a thickness that interferes with light, so the surface takes on a specific color. These tempering colors have been used for centuries to gauge temperature, and the sequence holds steady enough to tabulate.
| Temperature | Color |
|---|---|
| 350 °F (176 °C) | light yellowish |
| 400 °F (204 °C) | light straw |
| 440 °F (226 °C) | dark straw |
| 500 °F (260 °C) | brown |
| 540 °F (282 °C) | purple |
| 590 °F (310 °C) | deep blue |
| 640 °F (337 °C) | light blue |
The color works as a clock as well as a thermometer. The oxide film thickens over time, so steel held at 400 degrees Fahrenheit long enough can drift brown or purple without ever getting hotter, and surface oil or the type of heat source shifts the reading. Very hard tools are often tempered in the light to dark straw range, springs often toward the blue, and higher-carbon tool steel still finishes harder than spring steel at the same temperature.
What the fire takes, the fire gives back
Heat can remove carbon as well as rearrange it. In an oxidizing atmosphere, austenitized steel decarburizes: carbon combines with oxygen and leaves the metal, and the scale keeps oxygen in contact with the surface even inside the oxygen-free coals of a forge. Blacksmithing practice answers by austenitizing for the shortest time that does the job, while a reducing fire lets carbon diffuse slowly deeper into the metal instead of out of it.
The next time a tool or blade is described as hardened and tempered, or a maker names a straw or a blue temper, the table above attaches numbers to the words: one heat past critical, one fast cooling, one moderate heat. Three steps, two fires, seven colors.
Where the numbers come from
The temperatures and colors above follow the English Wikipedia article on heat treating, a free encyclopedia entry. Past the three processes a smith meets, it covers the crystallography of iron, iron-carbon phase diagrams, time-temperature transformation curves, and furnace types from batch to fluidised bed, with references leaning on ASM International handbooks.