Page 71 - MetalForming October 2011

Evident in this photo of a Graebner hot-stamping line is the plumbing system on the back wall, for the line’s chiller.
There is no comprehensive book on modern hot stamp- ing, and while academics gather yearly at an event in Europe to discuss the process, along with participation from the pri- vate stamping sector, there is no equivalent in North Amer- ica. And it’s nearly impossible for Tier Two or Three stampers to view a hot-stamping line in North America, as the manu- facturers running such lines cautiously guard their process- es from visitors’ eyes.
The hot-stamping process, stripped to its bare essen- tials, begins with manganese-boron steel blanks—flat or preformed, depending on the process variation—fed either by robot or pick-and-place mechanism into an oven, where they are carefully heated. The heating process can cause scaling on the surface of the blanks, resulting in excessive die wear downstream—stampers can minimize the amount of scaling by specifying that the steel be supplied with a special coating.
Most ovens move the blanks in a linear manner over rollers that expose the blanks to intense heat over a period of several minutes. As stampers evaluate technology options, they should take care to inquire about other heating options, as there are several new oven designs entering the market.
As displayed in the accompanying (and simplified) con- tinuous-cooling transformation (CCT) diagram, the blue line represents the heating and cooling process, with its tight timing window within which the hot-stamping process must occur. The trajectory of the blue line begins with the manganese-boron steel blank heated in an oven to about 950 deg. C, to allow the blank to reach the austenite phase.
Hot Potato
After exiting the oven, each heated blank transfers to a hydraulic press via robot or pick-and-place mechanism. At this stage the blank has a strange muted glow to it as it set- tles into the lower die section. The press closes tightly to clamp the die shut at a relatively high tonnage—not so much for the forming process but for the need to create an extreme- ly tight bond between the die surfaces and the part. The die
These photos from Schuler showcase a robot-tended hot- stamping line and the feeding of heated blanks into the die.
is permeated with
channels in its upper
and lower sections, designed to distribute cooling water throughout. The tightly closed die functions as a heat sink, absorbing the heat from the part. The blank cools quickly, within seconds, by several hundred degrees until it reaches the martensite phase. This results in a lightweight blank with phenomenal strength characteristics.
Cool the blank too slowly and it will enter the bainite, fer- rite or pearlite phases and exhibit out-of-spec properties. Conversely, by controlling the cooling process in the die, it is possible to create hot-stamped parts exhibiting various strength characteristics throughout their length. This proves critical in applications requiring a part to crumple in one area while maintaining stiffness in another, such as an automo- tive safety pillar.
After a few seconds, the die opens and the automation device removes the part from the die and places it on a rack. Blank trimming typically is performed by laser cutting; alter- native trimming techniques also are used.
The most engineering- and design-intense part of the entire process is the die. The die designer must be thor- oughly familiar not only with conventional design and sim- ulation software, but he also must be fluent with thermal- simulation software. Such software might include metallurgical-simulation software for the understanding of the metallurgical transformations of a particular part with- in the die. Other critical success factors include design of the cooling channels and the chiller, and the use of sensors to monitor performance.
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