The heat-treated steels are subject to more or less complex cycles of heating, maintaining and cooling to defined temperatures. After speaking in previous posts by steels hardening, today we want to talk about another heat treatment: the annealing.
What is the annealing heat-treatment for stainless steels?
The process of annealing (intended for austenitic steels such as those stainless steel and duplex) is an heat-treatment for precipitation-hardening. This treatment is performed at high temperatures, usually > 1000°C. Indeed, it provides heat at a temperature between 1000 and 1100°. This ensures the annealing of the alloying elements inside of austenitic grains. Maintaining the temperature of heating should last the time required to ensure that this temperature reaches the steel piece heart. Only thanks to this stasis you can eliminate structural alterations that were caused by previous machining. Next, we expected a rapid cooling of the piece in air or water, just to prevent the precipitation of chromium that is usually between 450° and 850°.
What is the annealing treatment for?
Generally, this type of heat-treatments allow you to directly improve the characteristics of steels as:
- mechanical resistance
- to room temperature
- to high temperature
- corrosion resistance.
In particular, one of the main purposes of annealing heat-treatment of austenitic steels is to maximize resistance to oxidation. In addition, this treatment is done
- to completely eliminate hardening caused by plastics processing (cold) of material
- to relieve residual stresses.
The annealing of Duplex steel.
The only heat-treatment done on Duplex steel is to annealing (made at a temperature between 920 – 1100°C). After this type of treatment, the biphasic structure of these steels results in a state of equilibrium (50% austenite – 50% ferrite).
This makes the annealing of Duplex steel extremely critical. In fact:
a subsequent heating of steel would involve the risk of formation of unwanted crystalline phases
a prolonged stay between 600-900°C would involve embrittlement at room temperature
a prolonged stay between 350 – 550°C would involve in the deterioration of resilience and resistance to corrosion.
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I found it interesting when you explained that annealing is the process is maximizing the resistance to oxidization. I would imagine that this process would be very useful for making metals that are used for construction. It seems like it would make the materials much more durable for use in structures.