Page 24 - MetalForming February 2014

The Voice of the Customer
Form tools are coated by Oerlikon Balzers. “We use their Alcrona Pro, Lumena and X.Cell coatings,” Harant adds. Alcrona Pro, according to Oer- likon literature, is a “top-tier” AlCrN coating for punching and forming applications, and boasts a microhard- ness of 3200HV and a friction coeffi- cient (dry) of 035. For extra hardness requirements, the Lumena TiAlN coat- ing gets the call, rated to 3400 HV microhardness. X.Cell is at the lower end of the microhardness scale (2100 HV) with a friction coefficient of 0.5.
Keeler Weighs in on Forming the High-Strength Steels
I asked formability expert Stu Keeler (long-time contributor of MetalForm- ing’s Science of Forming column) to share some thoughts on forming 340XF and 550XF steels. He says:
“Compared to the lower strength 340XF steel, the higher strength 550XF steel has the following characteristics:
• “As yield stress increases, the
amount of elastic stresses in the atomic structure increases as the sheetmetal forms. This in turn increases the amount of springback, which is the release of the elastic stresses back to their natural position. The amount of springback is proportional to the yield strength (stress). Therefore, the 550XF steel will exhibit about 60-percent more springback in bend angle, side- wall curl, twist, global shape change, etc. Increased springback correction or compensation can be difficult to achieve.
• “Higher strength steel will require stronger tooling to resist deflection, wear, etc. Increased tooling downtime could occur.
• “Energy required to form the steel is the area under the true stress- strain curves. Higher-strength steel will require more energy to form the same amount of deformation. The press might be capable of withstand- ing the load but not the need for more energy.
• “The increased energy to form the stamping will raise the temperature of the material—not uniformly but a function of the amount of strain at each location. Temperature variation in the stamping can change the vis- cosity of the lubricant and coefficient of friction. Amounts of deformation (especially stretch) can change in dif- ferent areas of the stamping to change springback and even cause galling and scoring.
• “Workhardening capability (n- value) decreases with increase in yield strength. The n-value decrease with increasing yield stress can be shown on a curve. Higher strength means less allowable stretchability, as shown by the reduction in the height of the form- ing-limit curve.
• “As yield strength increases total elongation decreases; this lessens the ability to bend the steel. Numerically, the ratio of punch radius to sheet thick- ness must increase as total elongation decreases, to avoid splits.” MF
22 MetalForming/February 2014
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