Engineering handbooks describe annealing as a stress reduction process. A workpiece goes into a furnace, is heated to its transformative temperature, then it slowly cools to room temperature. Hard and stressed when going into a heat treatment furnace, the part exits with a softer, plastic-deformable structure. Back with the different grades of steel, there’s more than one way to anneal those alloy workpieces.
With this method, steel parts are heated until they’re roughly 30°C hotter than their critical transformative temperature. Held like this for a prescribed period of time, the metal is allowed to cool naturally. After the cooling phase is over, and the internal stresses fade, machine workability and a fine-grained microcrystalline structure develop.
On taking the temperature of a steel workpiece to its critical transformative temperature, similar to the full annealing process, the alloy is forcibly cooled. At its lower critical temperature, the hold period begins. The steel grain changes into its pearlite phase. A final stage sees the steel cool slowly. Isothermal annealing uses complex thermal curves to achieve granularly controlled results.
A slight variation in the cyclical thermal curve takes the furnace temperature 20-30°C higher. Maintained like this for 2-to-4 hours, thermal soakage produces globular pearlite, which distributes uniformly throughout a steel part’s length and breadth. The ball-shaped carbides form the backbone of carbon steel tools, which must retain a fatigue and impact resistant form.
Also referred to as intermediate annealing, this variation on a theme soaks steel in furnace heat until the deformation grain becomes uniform and homogenous. Intermediate annealing neutralizes process hardening and steel-locked residual stress so that a multi-stage cold working operation can proceed without incident.
On raising the furnace temperature until the steel reaches its solid phase line state, the thermal energy level is maintained. After more than 10-hours, but not more than 15-hours, the steel workpieces is allowed to cool. Having reached room temperature, the chemical composition of the steel is more consistent than it was before the process. Diffusion work usually requires a second pass, which is effected as a complete annealing operation or an alloy normalizing pass.
The full annealing procedure generally suits most applications and steels, but then there are the high and low carbon variants to consider. Think of the contrasting procedures as material and application-specific options. For example, tool or bearing steel parts should be spheroid annealed so that they gain carbide hardened-strength. For residual stress control, the temperatures change, fine-tuned annealing curves are initiated, and grain structures homogenize or develop application-specific properties.