Heat treatment methods 

Two main methods are used

• In-line or separate annealing with High Frequency Induction Current coils.
• Separate annealing in electric or gas fired furnaces with continuos or batch process.

Either an open air or an inert gas atmosphere can be applied for all methods.
The holding time is 5-10 seconds at in-line annealing, and 2-5 minutes at separate furnace annealing.

Reasons for heat treatment

The main reason for heat treating standard austenitic SS welded pipes with low C-contents is to stress-relieve the tubes by reducing stresses and hardness caused by the forming of the strip/plate to a pipe.

This is only necessary for tubes with a small OD, and then in-line HF-annealing will be sufficient.

The soaking time is not prescribed in any tube standard.

Welded pipes in standard austenitic steels with medium to low Carbon content can in many applications at ambient temperatures be used in an un-annealed and pickled condition.

Other reasons for heat treatment are:

• Reduce segregations of Cr and Mo in the weld metal.
• Reduce the ferrite content in the weld metal
• Dissolve carbides and other precipitates in the HAZ

Duplex grades

For the ferritic-austenitic grades the amounts of Cr, Mo and N must be balanced in the ferrite and austenite, in order to optimise the corrosion resistance. A proper water quenching is the necessary tool to achieve this. Both HF- and separate furnace annealing can be used. With bright annealing it is difficult to achieve the necessary fast cooling times.

Austenitic grades with high Mo

Pipes made of austenitic SS with Mo contents above 3% will, due to the segregations of Cr and Mo in the weld metal, have a reduced corrosion resistance in the as-welded condition. 

The short soaking time at an in-line induction heat treatment does not reduce the negative effect of the segregations. Separate furnace annealing with much longer soaking times is necessary. 

IGC-tests do not detect inadequate corrosion resistance when Mo-segregation is the problem.

Pickling

The standard method of pickling austenitic stainless tubes is to use an acid mixture containing about 15% nitric acid and 3% hydrofluoric acid. Pickling is carried out at a bath temperature of 40-50 °C. Pickling times for austenitic steels are around 30 minutes.