ANNEALING

Annealing - The process of annealing makes metal less brittle by subjecting to
heat followed by cooling, changing  the composition and properties of the metal.
This process is also used on glass to relieve internal stresses present from
cooling from its molten state.  Thin pieces of glass can be annealed in less than
30 minutes, while a lens such as the one on the telescope at the Palomar
Observatory, which is 200 inches thick, took more than a year.

When a steel seam is welded, it leaves a brittle martensite.  It is important to
relieve the stress in that area and increase your ability to work with the metal.  
Depending on whether you are using a low  frequency welding operation which
produces a wide heat-affected zone or high frequency welding operation which
produces a narrow heat-affected zone, would determine the amount of heat
required for induction annealing.  Typically you would equate the wall thickness
to the reference depth of steel above the Curie temperature to determine your
frequency requirements for induction annealing.

For instance, if your item was over .38 inches thick, you would usually choose a
frequency of 1,000 Hz.  The localized heating through your heat-affected zone
would be provided by a split-return coil and a flux concentrator in the form of a
laminated core.  Charts can be found in books such as Elements of Induction
Heating: Design, Control and Applications by S. Zinn and S.L. Semiatin, to help
you determine your exact needs.

Definitions:

(martensite «MAHR tuhn zyt», noun. a hard, brittle, solid solution of up to 2 per
cent of carbon in alpha iron, a constituent of steel which is quenched rapidly.  
[< Adolf Martens, 1850-1914, a German metallurgist ]

Works Cited:

Bernstein, Melvin. "Annealing." World Book Online Reference Center. 2005.
World Book, Inc. 6 Jan. 2005.
<http://www.aolsvc.worldbook.aol.com/wb/Article?id=ar023040>.

Zinn, S. and Semiatin, S.L. "Elements of Induction Heating: Design, Control and
Applications",1988. Electric Power Research Institute.