Die materials and wear in stamping AHSS
Part I: Die wear and die coatings
Advanced high-strength steels (AHSS) are used extensively in the automotive industry to help improve crash safety and reduce weight. With the increased strength of AHSS, however, come dramatically increased springback and forming load (and, therefore, contact pressure) compared to milder steel grades.
In many cases, forming pressures are increased on purpose to reduce springback, but this can contribute to increased tool wear and galling. Therefore, it is necessary to explore the use of new tool materials and surface modifications.1
Tool Failure Types and Causes
Figure 1 summarizes the principal modes of tool failure related to die wear and the steps required to avoid these failures,2 defined as follows:
- Wear is damage to a solid surface involving loss or displacement of material. Wear is caused by sliding contact between the workpiece and tool. Two main types of wear are abrasive, caused by hard particles forced against and moving along a solid surface, and adhesive, caused by localized bonding between contacting solid surfaces and leading to material transfer between these surfaces.
- Plastic deformation is caused by contact pressure exceeding the compression yield stress of the tool material.
- Chipping is a result of stresses exceeding the fatigue strength of the tool material.
- Cracking is caused by stresses exceeding the fracture toughness of the tool steel.
- Galling is a form of damage caused by sliding of two solids. It often includes plastic flow, material transfer, or both.
Four main factors have an effect on these failures:
- Contact pressure—Local contact pressure between the sheet and tool affects all types of tool failure. As stamping of AHSS requires increased contact pressure, the probability to observe tool failures increases significantly compared to stamping milder steel grades. For a given sheet material, contact pressure can be reduced by die design, such as using larger radii or reducing the sheet thickness.
- Surface quality—Although the surface of the tool is much smoother than the surface of the sheet, the tool's surface quality affects galling. Polishing the tool surfaces before and after coating helps to reduce galling. The sheet's roughness has little influence on tool failure.
- Tool coating—The proper coating with a low coefficient of friction is crucial to reduce galling and tool wear.
- Lubrication—Forming AHSS requires better-performing lubricants, possibly with extreme-pressure (EP) additives, because of the high contact pressure and temperature that occur during the process.
Surface Treatments and Coatings
Surface treatments and coatings help increase tool life and reduce friction. Heat treatments, nitriding, and hard chrome plating are among the most common techniques used. However, they can fail under the high contact pressure that is present when stamping AHSS.
Three coatings typically are used for dies that stamp AHSS: physical vapor deposition (PVD), chemical vapor deposition (CVD), and thermal diffusion (TD).
CVD requires higher temperatures than PVD (see Figure 2) and similar temperatures as TD. The high temperature can soften the die, which then necessitates a rehardening process. It also can cause dimensional distortion.3,4
The most common coating materials for PVD and CVD are nitrides (such as TiN and CrN), carbides (such as TiC and WC/C), solid lubricant films (such as DLC and MoS2), or a combination of these. Since many PVD and CVD coatings are brittle, they are applied to stiffen substrate materials, so the tool steel typically is surface-hardened before coating. Duplex coating, which consists of nitriding and PVD coating, is a common method.5,6
Considering the high cost of coated tool steels, large forming tools are made of relatively inexpensive and soft materials, such as cast iron or low-grade tool steel. Locations subject to severe wear are replaced with inserts of high-grade tool steels with coating (see Figure 3).
1. J. Bergstrm, P. Krakhmalev, A. Grd, and F. Lindvall, "Galling in Sheet Metal Forming," in proceedings of the IDDRG 2008.
2. Uddeholm & SSAB, "Tooling Solutions for Advanced High Strength Steels, Selection Guidelines," 2008.
3. S. Hogmark, S. Jacobson, and M. Larsson, "Design and Evaluation of Tribological Coatings," Wear, 246, pp. 20-33, 2000
4. B. Janoss, "Selecting, Using Tool Coatings to Stamp AHSS," The FABRICATOR®, July 2008.
5. Hogmark et al.
7. M. Liljengren, K. Kjellsson, T. Johansson, and N. Asnafi, "Die Materials, Hardening Methods and Surface Coatings for Forming of High, Extra High & Ultra High Strength Steel Sheets (HSS/EHSS/UHSS), in proceedings of the IDDRG 2008.
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