Cold Forming for Stainless Steel
Do you know cold forming for stainless steel? All Columbus Stainless Steels supplied in the annealed condition (No.1, HRA, 2D, 2B or BA) can be cold formed by any of the conventional processes. These processes include blanking, bending, piercing, roll forming, coining, embossing, pressing, spinning, flow forming and deep drawing. In general the equipment such as presses, press brakes, guillotines, etc. used for carbon steels can be used for stainless steels.
The cold forming process is similar to the cold heading process. However, the process uses vertical presses instead of horizontal cold heading machines. The cold forming process is also volume specific and the process uses dies and punches to convert a specific "slug" or blank of a given volume into a finished intricately shaped part of the exact same volume. As more power is required to work stainless steels, the capacity of the equipment is effectively reduced, eg. Guillotines and press brakes that are rated up to 6mm thickness for carbon steel are restricted to 4mm for stainless steels. In operations where austenitic grades are cold worked, eg. Pressing and deep drawing the capacity of the equipment can be effectively reduced by up to 60% due to rapid work hardening characteristics of these materials.
In addition to the extra power requirements to from austenitic and marstenitic stainless steel, greater demands are made on the form tooling. Stainless steels not only have greater strength than carbon steels but work harden more, cause more wear, are susceptible to pressure welding, exhibit more springback and have lower heat conductivity. As a result of this, tool materials must be harder, smoother and better designed than tooling for carbon steel.
Higher grade tool steels are required and frequently tungsten carbide or aluminium bronze are utilized. However, in the case of the latter these materials can be used as "inserts" in critical areas rather than the complete tool. In many cases tools designed for carbon steels are used for Stainless steels, in such cases one must expect to make changes in "setting", use higher quality lubricants, have more frequent maintenance and reduced life.
As a result of the above, lubrication requirements are critical and high quality products are essential. Heavy duty pastes and emulsions, sulphurised or sulphurchlorinated oils, chlorinated oils or waxes are used depending on the forming operation. Mineral oils, soap solutions and general purpose soluble oils are not usually used. Ease of removal after forming is also a consideration since all traces of lubricant must be removed prior to subsequent heat treatment or before putting into service. Surface contamination can increase finishing coasts and adversely affect corrosion resistance. In some cases, especially pressing or deep drawing of ferritic grades, material is PVC or PE coated prior to forming. This not only acts as a very effective lubricant, but increases the formability, prevents galling and/or pressure welding and minimises damage to the surface finish.