Heat treatment of Austenitic steels, Duplex and nickel alloys

Heat treatments of solution annealing and stabilization of 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 is the only treatment that can be performed on austenitic steels with higher carbon content than 0,03% and on duplex steels. It is the process that, at the temperature of about 1000 ÷ 1100 °C, makes soluble at the solid state the Carbon 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 + soaking + rapid cooling mechanism is similar to hardening treatment of martensitic steels, but in austenitic steels it gives the maximum softening state instead of increasing the resistance values. For this reason solution annealing is also called “negative hardening” or “inverse hardening”. The solution annealing treatment is also performed in order to completely eliminate the state of work hardening due to plastic cold work machining of the material and to relieve the residual stresses, even though the possible manifestation of new stress states 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, instead, cannot be limited. In case it is finalized to remove work hardening, solution annealing can be performed on not yet assembled components to allow their, possible, recalibration. The heat treatment of solution annealing of the duplex steel 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 so that the component has the correct final technological properties.
Stress relieving is essential when the items must be machined or they operate in an aggressive environment (risk of cracks due to stress-corrosion cracking). 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 necessary to perform heatings at a temperature lower 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). The relieving, directly connected with the flowing and annihilation of dislocations induced by temperature, will be, in this case and equal, at maximum 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.
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 that temperature for two or four hours and subsequently cooling it down 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 leaving chromium unchanged, without having sensitization. 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, particular 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.

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Heat treatments of nickel alloy components

The alloys with high percentage of nickel are used for the exposure to high temperature or highly corrosive agents, or in presence of both conditions. Nickel alloys are divided in function of the main element and of the secondary ones:
  • Nickel – Carbon: Nickel 200 – Nickel 201
  • Nickel – Copper: Monel 400
  • Nickel – Chromium: Nimonic 80A, Nimonic 90
  • Nickel – Chromium – Iron: Inconel 600
  • Nickel – Chromium – Iron – Molybdenum – Copper: Hastelloy G
  • Nickel – Chromium – Iron – Molybdenum – Cobalt: Hastelloy X
  • Nickel – Chromium – Molybdenum – Niobium: Inconel 625
  • Nickel – Molybdenum: Hastelloy B
  • Nickel – Molybdenum – Chromium: Hastelloy C276
  • Nickel – Iron: Invar
  • Nickel – Iron – Chromium: Incoloy 800, Incoloy 825
The heat treatments that are generally executed in the metallurgy of Nickel are “annealing”, “solution annealing, and the aging” and stress-relieving.

The annealing heat treatment generates particularly sweet final structures, ideal for the execution of mechanical work machining, forming or welding. Many alloys are initially bought in this state for construction.
Solution annealing

Solution annealing consists in heating the alloy at an adequate temperature, maintaining that temperature for a sufficient period of time to cause the transformation of one or more constituents into a solid solution and, then cooling it in a sufficiently rapid way in order to maintain those components in the solution. The annealing is generally performed at temperatures included between 1800 and 2450°F, followed by a rapid cooling at ambient temperature. Many materials have specific cooling speeds that must be reached in order to obtain the correct metallurgical microstructure in the final product.
Aging heat treatments

Aging is generally performed at temperature between 1000 and 2080°F in inert atmosphere or air, for soaking times that span from 2 hours to more than 40 hours according to the exact material and the specified metallurgical microstructure. To obtain the desired final results, various stages could be necessary (with reduction of temperature in each phase).

Executed at a lower temperature than annealing or solution annealing, it reduces the residual stresses due to welding and mechanical working processes, reducing the stress-corrosion cracking phenomenon. On the components with the highest value, which undergo strict inspections and with treatments monitored by third institutions, it is important that our customers can count on a structured and reliable supplier as Trater, which is provided with personnel and means to execute the thermal cycles at best and deliver items always correctly treated.
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