Knowledge base
Solution annealing and precipitation hardening
The heat treatment for stainless steels and nickel-based alloys
Term explained
The heat treatment for stainless steels and nickel-based alloys
Solution annealing is the heat treatment for materials that do not gain strength through hardening and tempering: austenitic stainless steels such as 1.4571, duplex steels, titanium and nickel-based alloys. The material is heated to a high temperature, around 1,050 °C for austenitic steels, until carbides and precipitates in the microstructure dissolve, and is then cooled rapidly so that they stay in solution. The result is a uniform, soft microstructure with the best corrosion resistance; on certificates, this delivery condition is designated +AT.
Why solution annealing follows forging
When austenitic steel is hot formed, chromium carbides precipitate at the grain boundaries and deplete the material of chromium there; it becomes susceptible to intergranular corrosion. Solution annealing after forging reverses this. That is why solution annealing is part of the production sequence for forged stainless steel bolts with corrosion resistance requirements, whereas for cold-worked property classes such as A4-80 it is weighed against the required strength.
Solution annealing and precipitation hardening
Precipitation-hardening materials such as the creep-resistant austenitic steel 1.4980, the martensitic steel 1.4534 or Alloy 718 are precipitation hardened after solution annealing, a step also known as ageing: held at an intermediate temperature for several hours, they form extremely fine precipitates that raise the strength to the level of a quenched and tempered steel without impairing corrosion resistance. The solution annealed and precipitation hardened condition is then stated on the mill certificate of the raw material or is produced after forging.
How RS Randack goes about it
Solution annealing and precipitation hardening are carried out by qualified partners in accordance with the material standard or customer specification; strength is verified in our own laboratory on the test bar, and corrosion resistance, on request, through microstructure and corrosion tests at the partner laboratory. Threads are rolled after heat treatment.
What you should specify
Material and required delivery condition, standard or specification for the heat treatment, minimum values and tests, certificate type.
Enquire about bolts in austenitic steel or nickel-based alloys
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