Page 50 - MetalForming-Jun-2018-issue

48 MetalForming/June 2018
www.metalformingmagazine.com
The Science of Forming
grades, the ferritic grades become brit- tle at low temperatures.
SS430 is the most widely used fer- ritic stainless steel, with SS409 having greater corrosion risk owing to a lower chromium content. SS439 offers greater resistance to corrosion and improved high-temperature stability, making it suitable for exhaust systems. Using titanium and niobium to tie carbon and nitrogen into fine precipitates results in improved formability–the same mechanism employed in the pro- duction of interstitial-free extra-deep- drawing ultra-low-carbon steels.
Greater shear strength in annealed austenitic stainless steels as compared to carbon steels leads to more force being required to shear stainless alloys of equal thicknesses. Press and die sections should be built with greater rigidity to account for the increased shear strength. Austenitic grades workharden to a greater extent than do ferritic grades, giving higher strength to the rollover sec- tion of a cut edge. Flanging or otherwise expanding a poorly cut edge results in a
greater likelihood of edge cracks. Mini- mizing rollover, by using well-aligned cutting tools with tighter clearances, improves the cut edge. However, tight clearances accelerate the wear of shear knives, making it difficult to keep cutting tools sharp and sufficiently aligned.
Computer-simulation models used for low-carbon steels are insufficient to model the forming and structural per- formance of stainless steels. Ferritic grades have a relatively constant n-value, whereas austenitic grades have an n- value that changes with strain, test speed and temperature. Austenitic grades have a “TRIP-effect,” converting to martensite during forming, which must be incor- porated into any prediction involving austenitic stainless steels. M F
Want to learn more about different sheetmetals and their formability? Plan to attend PMA’s Sheetmetal Formability of Steel, Aluminum and Stainless Steels seminar in Cleveland, OH, June 26-27. Visit www.pma.org to register, or contact Marianne Sichi at msichi@pma.org for information.
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