AUSTENITIC AND DUPLEX STEELS
Chromium added steels are known as ferritic and find a little use in the industrial field. Austenitic steels are obtained adding nickel as well, essential in the chemical and petrochemical sectors where corrosion and temperature resistance are required.
Austenitic-ferritic stainelss steels (duplex) combine the corrosion resistance and temperature resistance of the austenitic steels with the mechanical resistance of ferritic steels.
SOLUTION ANNEALING OF AUSTENITIC AND DUPLEX STEELS
Solution annealing is the only treatment that can be performed on austenitic steels with higher carbon content than 0,03% and on duplex steels.
It is during the process at the temperature of about 1000 ÷ 1100 °C, that the carbon is made soluble at the solid state. The carbon is contained in the steel under the form of carbides (mainly Chromium carbides), precipitated after welding processes or, generally, heating processes in the temperature range included between 450° and 850°C.
The material, after a suitable soaking time at the indicated temperature, completely regains its corrosion resistance characteristics, which are reduced in case the content of chromium in the alloy matrix has fallen below the 12% threshold (sensitization).
The final cooling must be quick, about two minutes, to prevent a new precipitation of carbides, in the temperature range of 450 ÷ 850 °C . The heating mechanism + soaking + rapid cooling is similar to hardening treatment of martensitic steels, but in austenitic steels it gives the maximum softening state instead of increasing the resistance values.
The solution annealing treatment is also performed in order to completely eliminate the state of hardening due to plastic cold work machining of the material and to relieve the residual stresses, even though the possible manifestation of new states of tension that easily occur during the stage of drastic cooling following the inevitable differences in temperature and consequent different expansions on the item must not be underestimated.
The structures at 1000 °C can collapse under their own weight and are therefore to be reinforced adequately through materials having the same thermal expansion.
The deformations due to the thermal shock, which the items undergo at contact with water, cannot be limited.
In case it is planned to remove work hardening, solution annealing can be performed on not yet assembled components in order to allow recalibration.
The heat treatment of solution annealing of the duplex steel (NON CHIARO) is particularly critical, since if the soaking temperature is not respected, it leads to a lack of balance between the austenitic and ferritic stages with lower corrosion resistance and/or very low mechanical characteristics.
The treatment of superduplex steels is even more critical and it requires a particular cooling curve in order for the component to have the proper final technological properties.
STRESS RELIEVING OF AUSTENITIC STEELS
Stress relieving is essential when the items operate in an aggressive environment (risk of cracks for stress-corrosion cracking), to achieve their geometric stability in mechanical processing and when they are operating.
As previously mentioned, austenitic stainless steel structures of the type AISI 301, 302, 303, 304, 305, 308, 309, 310, 316 and 317, when heated even for a few minutes at a temperature included between 450 °C and 850 °C, are subject to sensitization and, therefore, exposed to corrosion.
For this reasons, stress relieving treatments cannot be performed at the classical temperature of 620 °C, as for carbon steels, but it will be unavoidable to perform heating at a lower temperature than 450 °C for a high number of hours, without sensitizing the material, in relation to the characteristics of the operating environment (more or less aggressive).
Stress relieving, directly connected with the flowing and annihilation of dislocations induced by temperature, will be in this case at maximum equal to the material yield strength at 450 ° C.
"Low Carbon" austenitic steels or "L" such as the 304L and 316L, with carbon content lower than 0.03%, not subject to sensitization, can undergo the stress relieving also in the critical temperature range obtaining geometrical stability and minor sensitivity to stress corrosion, after a careful analysis of all the operating conditions of the item.
STABILIZATION OF AISI 321, 347 AND 348 STEELS
For stainless steels such as AISI 321 and AISI 347, characterized by the presence in the alloy of Ti and Nb, the precipitation of chromium carbides is avoidable through the stabilization treatment, which consists in heating the manufactured item in a temperature range between 845°C and 900°C, maintaining it at this temperature for two or four hours and subsequently cooling it with the appropriate cooling rates, in relation to the geometry of the item.
Due to the bigger affinity of carbon with Titanium and Niobium, preferential precipitation of the relative carbides will occur without having sensitization, leaving chromium unchanged.
Therefore, for these steels, the classical stress relieving becomes possible as well or, in the case of plated or overlayed items, the PWHT that allows excellent reduction of the residual stresses and reduces the possible phenomena of corrosion stress cracking.
In particular, for the stabilization treatment, specific care should be given to the geometry of complex items with concentrated welds, due to the risk of reheating cracking.
Also for components manufactured with these steels, in which temperature, uniformity, heating and cooling rates and soaking time are crucial, always subject to testing with mechanical test on test coupons or corrosion tests, it is important that our customers can count on a structured and trustworthy supplier as TRATER, which is provided with personnel and means to execute the thermal cycles at best and deliver items always compliant to specifications.