Forming techniques for stainless steel (2) drawing and spinning
Introduction
Stainless steel can be formed in the same way and using the same type of equipment as for most other steels. Some differences in technique apply and these are explained below. The information in this article is specifically directed towards austenitic stainless steels, the most frequently used family of stainless steels.
Austenitic stainless steels exhibit strong work hardening characteristics which affect many of the forming techniques. Machinery capability levels need to be increased, (or existing capability levels down-rated), compared with carbon steels. A higher rigidity of tools and machinery is necessary for working stainless steel.
All fabrication processes should be carried out in a clean, and if possible, a dedicated environment.
Drawing
This is the forming operation whereby flat sheet is pressed into cylindrical and rectangular shapes by means of a punch and die set up. The work is best carried out in a double action press using a bolster or pressure plate to hold back the material whilst forming is taking place, thus avoiding wrinkling of the component flange.
Within the austenitic family, similar materials will give marked differences in their deep drawing characteristics, depending on their precise chemical composition.
Mechanical and hydraulic presses in general use can be used for stainless steel drawing operations, but again the capability is significantly reduced. For deep drawing work, the hydraulic press is preferred because speed and pressures can be accurately controlled.
Punch and die radii of nominally 5 to 10 times the material thickness will allow a smooth draw in of the flat sheet. Too large a radius will cause wrinkling, too small a radius will limit the amount of reduction achieved.
For deep drawing work, several operations are necessary with inter-stage annealing to achieve a high depth to diameter ratio. Where lubricants are used to aid drawing, these should be removed prior to annealing. Descaling after annealing and prior to further working is essential.
Certain plastic protective coatings applied to sheet products can remain in position whilst drawing takes place and will act as an aid to pressing.
For information, annealing of the austenitic stainless steels is carried out at a temperature of 1050/1120ºC with rapid cooling thereafter.
Spinning
Spinning can form cylindrical and bowl shaped components. This is often a one step operation using a round nose tool working over a rotating mandrel. Manual and power operated tools are used.
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