Daniel Schaeffler Daniel Schaeffler
President

Work Hardening is Your Enemy

July 12, 2021
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Engineered stampings result from the plastic deformation of sheet metal caused by forming forces exceeding the material’s yield strength. Work hardening or strain hardening strengthens the material as deformation continues throughout the press stroke. Grades with higher levels of n-value, a parameter which characterizes work hardening, will exhibit greater formability. Many benefits come from using metal alloys with greater levels of work hardening, including the delay of strain localization, which provides the ability to form more complex shapes.  However, the strengthening associated with work hardening leads to several challenges.

Despite having the same flat-sheet strength as shipped from the mill, newer steel grades work-harden to a greater extent than do conventional high-strength grades. High work hardening also characterizes the 3XX-series of austenitic stainless steels. Greater work hardenability may lead to issues in the press shop, especially when attempting to restrike or flange previously formed sheet metal.  

Creating a draw panel or a panel from the first forming station of a progressive die involves straining the sheet metal, leading to work hardening in the formed areas. Moving those work-hardened areas in subsequent operations requires greater force than required to initially form the flat sheet.  Those formed areas now exhibit an even higher strength, so press shops must adjust their practices to working with an even higher-strength steel.  

The amount of strengthening from work hardening relates to the amount of strain applied to each area. Higher levels of work hardening accentuate the strength differences in a panel between areas of lower and higher forming strains. This in turn may lead to panel twisting and other dimensional issues.
Where a conventional high-strength low-alloy (HSLA) steel might see a strength increase of 20 percent, rough calculations show that even a few-percent strain with dual-phase steels can increase the strength by 50 percent as compared to the flat-sheet properties.  

As more parts transition to higher strength to take advantage of potential thickness reduction and associated weight savings, work hardening puts additional stresses on edges, tools and presses.

Cut Edges

Cut-edge quality is of utmost importance if the part design calls for stretching when flanging a punched hole or sheared edge. It is important to optimize the size of the burnish and fracture zones, and to minimize the burr. Also critical: having these zones uniform across the cut.  

The microstructure of advanced high-strength steels (AHSS) contains phases of differing hardness. Reductions of sheared-edge ductility occur when the microstructural components are not of uniform hardness, such as in some multiphase steels.

Due to work hardening, expanding cut edges becomes even trickier. The shearing or punching operation that creates the edge is itself a forming operation because it moves the sheet metal, and, as such, the material at the edge increases in strength. Strengthening the edge further limits edge ductility—precisely why edge quality takes on a heightened importance in these grades.

Lubrication and Tool Wear

Even during room-temperature stamping, line operators can feel the energy of forming when a part leaves the press hot to the touch. Higher-strength parts require more energy to form, especially when production requirements push for increased cycle time. The highly strained work-hardened regions of those parts require still more energy. When localized in the higher-strained regions, some of this energy is lost in the form of heat dissipation. Friction with the tooling increases in these areas, leading to more heat buildup. Lubricant burnoff may occur unless operators apply an appropriate lube containing proper additives active at the real forming temperature, in contrast to the perceived room-temperature forming.

Without sufficient lubrication, the risk of accelerated tool wear exists.  Best practices include upgraded tool materials with engineered coatings, especially at the high-contact-pressure regions occurring in the areas of greatest work hardening. Use coated inserts for longer life in these high-contact areas to achieve better results from lower friction and reduced heat buildup.

Press Requirements—Force and Energy

Press selection for parts made from conventional steels centers around bed size and maximum tonnage requirements. Typically, the selection process also includes multiple safety factors. Press allocators may elect to place a part estimated as needing 490 tons into a 600-ton press rather than using a 500-ton press. Even when using the smaller press, a rating of 500 tons as the peak load that the press safely can deliver without causing damage to the frame and press components may also include a built-in safety factor.

These safety factors come into play when forming HSLA parts, where strengthening of 20 percent occurs from work hardening. On the other hand, the 50-percent strengthening from work hardening when forming AHSS grades may overwhelm the safety factors, resulting in the press lacking sufficient forming tonnage.

The energy that a press can supply takes on increased importance when working with advanced grades. Higher-strength parts require more energy to form, and work hardening increases these requirements. Forming higher-strength parts in a press incapable of supplying the necessary energy risks stalling the press. MF

Industry-Related Terms: Alloys, Bed, Burnish, Burr, Die, Draw, Ductility, Edge, Flange, Form, Forming, Hardenability, Plastic Deformation, Shearing, Stroke, Thickness, Work Hardening
View Glossary of Metalforming Terms

 

See also: Engineering Quality Solutions, Inc., 4M Partners, LLC

Technologies: Materials

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