title = "Annealing: Full, Process, and Spheroidize Annealing"

content = """
Annealing is a heat treatment in which a metal is heated to a specified temperature, held for a sufficient time, and then cooled slowly—in the furnace or in still air—to produce a soft, ductile microstructure with reduced hardness and internal stress. At Ridgecrest Metalworks, annealing is used to soften work-hardened material before further cold forming operations, to improve machinability, and to relieve internal stresses from welding or cold working.

Full annealing of steel involves heating above the upper critical temperature (Ac3 for hypoeutectoid steels, approximately 1475–1650°F / 800–900°C depending on carbon content) to fully austenitize the steel, then cooling slowly in the furnace at a rate of 25–50°F (14–28°C) per hour through the critical range (1400–1000°F / 760–540°C). This produces a coarse pearlitic microstructure with maximum softness and ductility. Full annealing produces the lowest hardness levels but requires long furnace cycles and careful atmosphere control to prevent oxidation and decarburization.

Process annealing (subcritical annealing, also called stress relief annealing for steel) heats the material to below the lower critical temperature (Ac1, approximately 1300–1380°F / 705–750°C for most carbon steels), avoiding any austenite transformation. It is used to relieve cold-working stresses and partially restore ductility in heavily cold-worked steel, at lower cost than full annealing. Sheet metal undergoing multiple passes of cold rolling is process-annealed between passes.

Spheroidize annealing produces a microstructure of spheroidal (globular) carbides in a ferritic matrix—the softest and most machinable condition for high-carbon steels (0.60% C and above) and alloy tool steels. It is accomplished by prolonged holding just below Ac1 (1300–1380°F), or by cycling above and below Ac1 repeatedly. The resulting structure machines easily, allowing cutting tools, die steels, and bearing steels to be rough-machined before final hardening. Spheroidize annealing can require 12–24 hours or more for high-carbon and high-alloy steels.

Atmosphere control is critical during all annealing operations to prevent decarburization (loss of surface carbon, which would reduce hardenability and surface hardness after final heat treatment) and scaling. Endothermic, exothermic, or nitrogen-based controlled atmospheres are used.
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version = "0.0.1"
