AHSS Tooling Requires Greater Protection
Forming fluids for extreme frictional forces
Tools are subject to extreme wear during AHSS forming. Using the right lubricant can help increase tool life.
Just as the coating on M&M's® breaks under pressure, tool coatings have a tough time standing up to the extreme pressure of advanced high-strength steel (AHSS) forming.
Because AHSS requires twice the energy force to form (Figure 1) and the tooling has less wear resistance but higher toughness than other steels, tool coatings can be removed prematurely or chipped off if a mechanism to reduce compressive and tensile tearing isn't in place. Much automotive tooling coated by chemical vapor deposition (CVD) or physical vapor deposition (PVD) must be recoated prematurely, because the lubricant selected wasn't matched to the AHSS forming conditions.
Trading Wear Resistance for Tool Toughness
Frictional forces that act on a tool as the work material is forced across the working surface cause adhesion between the tool and the work material. This adhesion is the primary tool wear. A tool steel's resistance to adhesive wear depends on many variables, such as the steel's carbide content and volume, hardness, and surface finish.
According to Ed Severson, technical manager, Bohler-Uddeholm, "High-strength work materials put a high stress on the tools used to form them. Tooling materials with a high toughness level are required in order to keep the tools from cracking. The downside of having high toughness is that the tooling materials with the highest wear resistance are not in that category. (See Figure 2.)
"The toughest tool steel types (for example, AISI S7) have the lowest wear resistance. Traditionally, high carbide content is used to reduce tool wear. Unfortunately, because carbon reduces tool toughness, tool steels used for AHSS must contain lower levels. Since toughness takes priority over wear in these demanding applications, finding ways to increase the tooling's lubricity is necessary."
The article "Advanced lubricants improve high-strength steel forming" discussed the differences between extreme-pressure (EP) and extreme-temperature (ET) fluids. EP oils decrease in viscosity or break down in high temperature/high-pressure applications (Figure 3).
Because of the increased frictional forces of AHSS forming, both coated and uncoated tool surfaces require a protective film barrier to guard against premature wear. Most conventional stamping fluids are designed to protect a cold or warm tool surface. Because oils break down during the high heat of AHSS forming, their viscosity is reduced over 200 percent. As a result, premature galling occurs, and in some cases, the tool coating is removed completely.
ET technology films can provide the needed protective film between the tool coating and formed part and reduce the potential for compressive and tensile tearing force and resulting tool surface damage.
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