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.
Complete Annealing
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.
Isothermal Annealing
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.
Spherical annealing
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.
Recrystalization Annealing
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.
Diffusion Annealing
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.
Copious quantities of nickel and chromium bolster a base ferrous structure. That’s what gives Austenitic Stainless Steel its face-centred microcrystalline identity. Infused with a little carbon, the alloy family gains its mainstream mechanical properties. So, what is that austenitic edge? The nickel-based flavouring is clear enough, but we want to know more about its practical features and applications, plus the reason such a popular alloy group can’t undergo heat treatment.
Questioning the Face-Centred Differences
Austenitic steels are highly formable, weld-amenable and non-magnetic. They’re popular, there’s no denying that point. In fact, 300 series stainless steel is part of this family, and that’s the most popular type of steel on the market today. Members of that series are polished and tough, and they’re found in high and low-temperature applications. In the kitchen or on a high-end automobile, 3xx alloys function equally well. Their crystal matrices, formed as face-centred cubes, cram toughness and ductility into a compact package. Having said all that, there’s that heat treatment problem to solve.
Austenitics Refuse Heat Treatment
Tough and malleable, there’s just one problem that prevents the alloy from being an all-around winner: austenitic stainless steels cannot experience conventional heat treatment procedures. The high quantities of nickel and chromium do gift the alloy with that signature look. The metal is polished and corrosion-resistant. Without sufficient amounts of carbon in the amalgamated blend, however, it’s a hard, almost impossible form of stainless steel to heat treat. While martensite forms without too much energy expenditure in other types of steel, that metallic state and its quench-facilitated austenitic structure cannot develop without a requisite quantity of alloying carbon.
Cold Worked and Annealed
Because of the above issues, austenitic steels are work hardened. They’re exposed to bending operations and drawing work, which causes the metal’s microcrystalline structure to deform. The result is material stress. Now, with that deformation energy locked inside the alloy, heat treatment technology can finally make its contribution. Instead of a hardening process, however, it’s an annealing service that heats the stainless steel workpiece until the work hardened stress is neutralized. Again, heat treatment hardening is not applicable here, but an annealing routine will help when cold work stress threatens the structural integrity of an austenitic stainless steel product run.
Carbon-rich steels are a heat treatment engineers bread-and-butter. Opting to break away from that microcrystalline paradigm, austenitic stainless steels pack their cubic ferrous matrices with nickel and chromium, although there is a small amount of carbon inside that crystal framework. It’s this nickel high base that gives the alloy its corrosion resistant character, but that feature comes at a cost. The cost is exacted in the form of a heat treatment-less fabrication process.