Page 33 - MetalForming May 2009

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0 50 100 150 Yield Strength (ksi)
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Schematic showing properties of hot-forming steel from 1) as-received to 2) hot forming to 3) final quenched part.
formed and quenched at the same time, reducing the process time allocated to each part. However, the real comparison must be against forming the part com- pletely at room temperature with 150- ksi steel having only 5 to 6 percent total elongation. If making the part requires four subparts, four forming dies, a num- ber of line dies to correct for springback, welding costs and uncontrollable dimensional variations, this usual form- ing mode could well exceed the cost of hot forming. If the part absolutely can- not be made at room temperature, is the cost now infinite?
Hot forming has been around for a number of years. The recent automotive weight-reduction programs (increased strength and reduced sheetmetal thick- ness) have allowed hot forming to prove itself as a competitive process. Howev- er, if you are set in your ways and always have processed parts the same way for decades, hot forming will never even come up on your radar. MF
220 ksi min. tensile strength and a typ- ical total elongation of 5 percent. This is the strength specified for the final part.
• The very high strength and low elongations of the final part restrict final operations to special cutting, trim- ming and piercing equipment designed to withstand the high loads generated by these operations. No additional forming should be attempted.
The above steps describe the “direct” hot-forming process during which all forming is done on the hot blank in the water-cooled die. A second process, called an “indirect” process, also is used.
Here the part is partially formed (to 90 percent complete) in traditional dies at room temperature. Instead of the blank being heated, the partially formed part is heated to 1650 F. The hot part is transferred to the die for forming the severe design features that tear during room-temperature forming. Quenching to the martensitic-steel microstructure completes the process.
One can argue that the process is too slow to be profitable. The total time for transfer to the die, forming and quenching is about 20-30 sec. If the part is small, multiple parts can be
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Total Elongation (%)