Bending and roll bending
Bending can be an open die/closed die bending or a roll bending operation. The major issue, especially for high strength steel, is springback. The amount of springback varies between different groups of steel, and a change of grade and/or sheet thickness in roll bending often requires machine adjustments in order to achieve the right shape.
In open die/closed die bending the strategy is to over-bend the part. The formability in this type of bending is represented by the minimum allowed inner radius in the bend divided by the sheet thickness ratio. For austenitic and ferritic stainless steel this ratio is 1, while for duplex grades it is 2.
Spinning is used for cylindrical and symmetrical components. Particularly suited to the production of components with relatively thin walls, this method allows advanced shapes and sharp corners to be made. Thinning of the sheet is possible, and spinning can be used on all stainless steel grades, but it is important to keep in mind that higher forces are needed for duplex grades. Machine power can be an issue for high strength material.
With roll forming, the sheet passes through a series of roll stands that gradually shape it into a profile or profile-shaped component, with no thickness reduction. This method is suitable for all stainless steel grades, especially for high strength grades like duplex or temper rolled austenitic steels. However, higher forces are required and the springback tendency is more significant for duplex and high strength grades.
Press forming is a generic term covering processes such as deep drawing, stretch forming, ironing, and hydroforming. The focus should be on choosing a robust forming process with safety margins for cracking and a shape accuracy within given tolerances. The tool surfaces and correct lubrication are very important in deep drawing and stretch forming operations in order to obtain high surface quality of the formed parts.
The standard austenitic grades 4307 and 4404 have excellent formability compared to most other materials and they are well suited for applications where there is extensive thinning during forming, as is the case in stretch-forming operations. The higher alloyed austenitic grades such as 904L, 254 SMO®, 4565, and 654 SMO® also have excellent formability, despite their high alloying content, but 654 SMO® and 4565 have higher strength and require higher press forces.
Typically, ferritic steels have better drawability properties in comparison to austenitic grades where the material is not subjected to thinning as it is in stretch forming. A phenomenon called ridging, or roping, may occur when deep drawing low alloyed ferritic grades, but this can be avoided by using a Ti/Nb stabilized grade such as 4509 or 4521. Ridging/roping primarily leads to aesthetic rather than functional effects. Austenitic grades are also well suited to deep drawing if thinning is allowed.
The high strength of duplex grades can be a challenge and sets higher demands on machines, tools, and lubrication performance. The higher springback may also be an issue for shape accuracy. The remedy is to utilize the material’s strength and use thinner gauges
Hole flanging is typically done to support tubular or rod connections, and the flanges are sometimes threaded. The cut edges of the hole must be of good quality in order to avoid cracked edges. This sets high demands on tool performance, especially when punching holes. The burr should be positioned toward the tool during flanging to reduce the risk of edge cracking.
Hole flanging should be avoided in areas where the component has been severely cold worked in a previous forming operation, since formability in these areas may be more limited than the areas subjected to less cold working.