Stainless Steel
By increasing the chromium and other elements in steel, it is possible to make it resistant to rust and/or heat. Most stainless steels fall into one of three major classes, depending on their crystal structure and alloying elements:
1. Austenitic. 200 and 300 series stainless steels usually have 16 to 24 percent chromium, 3.5 to 37 percent nickel, 0.8 to 0.25 percent carbon, and other elements. Austenitic stainless steels are ductile, but work hardening limits forming. The most commonly used alloys of this group are the 304 and 316 types. Austenitic stainless steels are nonmagnetic under most conditions.
2. Ferritic. Some 400 series stainless steels have limited ductility and, thus, restricted formability. Some alloys work-harden quickly, and it is difficult to weld them. However, they are usually less expensive than the 300 series steels. Most ferritic stainless steels are magnetic.
3. Martensitic. This type of steel, with the exception of the 403, 410, and 414 types, cannot be cold formed. Most martensitic stainless steels are magnetic.
These properties are only typical. Stainless steels come in a large variety, and their properties fluctuate greatly, depending on alloying elements, cold work, and heat treatment.
Most stainless steel can be formed similarly to the carbon steels, but some of its characteristics must be considered: work hardening properties, high springback, more power required to form, better and different lubrication required (better wetting properties and higher pressure resistance), high-luster surface, and orange peel ridging and rapping.
An understanding of the effects of cold forming and the metallurgical and mechanical properties of stainless steel is essential for successful roll forming. Figure 1 provides a guideline to the cold-forming properties of stainless steels.
 |
| Figure 1 |
Aluminum
Roll forming of aluminum is not as difficult as forming steel, but more attention has to be paid to setting and adjusting roll gaps, applying the right lubricant to prevent pickup of aluminum on the rolls, and using well-designed and finished rolls and good lubricant to prevent surface marks.
To specify the material simply as aluminum is one of the most common mistakes made by designers who are unfamiliar with metals. There are as many different aluminum alloys as steels, with vastly different properties. Therefore, it is essential to use the proper designation for aluminum used in roll forming.
