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Eren Billur Eren Billur
Technical Manager

Heat-Assisted Adiabatic Cutting of High-Strength Steel

October 23, 2020


In the last two Cutting Edge columns, I explained how heating can improve the formability of boron-alloyed steels and some aluminum alloys. In both cases, we heat the sheet metal in a furnace to their optimum forming temperature. Here we’ll discuss a heat-assisted cutting technology where the sheet metal blank itself generates the heat required for cutting, without the need for a furnace or other heating device.

How Does a Blank Heat Itself?

thermal camera images of tensile specimensMost metal formers should have experienced that a part exiting the press may be quite hot. This occurs because the sheet metal generates heat as it deforms. The temperature of the part may depend on several factors, including the amount of work done (press force and working stroke length), and how quickly the part is produced (directly proportional to stroke).

Fig. 1 illustrates tensile-test results, recorded with a thermal camera, for two different steel grades—DC04 (a mild steel similar to a deep drawing grade), and HCT980X (similar to DP 1000). The high-speed thermal camera captured the temperature increase in the fracture zone. 

When pulling the mild steel at 50 mm/sec., the temperature at the fracture zone peaks at 69.5 C. When quadrupling the test speed, heat cannot dissipate as quickly as during the first condition. Also, due to strain-rate effects, about 5 percent more work is performed, and the temperature rises to 107.2 C. Note: After only 0.5 sec., the maximum temperature drops to 94.0 C. The heating and cooling cycle occurs in very short time intervals and is localized in a very confined area of deformation. 

When we pull the higher-strength HCT980X alloy, the peak temperatures are 123.9 C and 135.4 C, respectively.

How is This an Advantage?

comparison of the heating zonesHeating the blank typically is a disadvantage for metal formers. An automotive-OEM engineer once told me that if the draw-tool temperature of a door-inner panel increases by 7 C, the binder tonnage must be reduced to avoid cracks. The heating effect also limits the maximum parts produced per minute, since press speed must be reduced in order to control the tool temperature. 

During blanking, the plastic deformation—and so the heating—is very localized to just the periphery of the punch (Fig. 2a).

A typical blanking press would operate at a maximum speed of 80 strokes/min., depending on the feed length. If the press has a 300-mm stroke length and is cutting a 1.0-mm-thick blank, punch speed at contact would be approximately 25 mm/s. With this punch speed, the sheet first deforms and then is sheared (Fig. 2a). Heat would be generated in the deformation area (to around 150 C) but would quickly dissipate. 

shear zonesIf punch speed exceeds 3 m/sec., then the time required to cut the sheet decreases to a few milliseconds (Fig. 2b). There would be no time to dissipate the heat in the shear zone, thus the name adiabatic. This would generate a very narrow adiabatic shear band (10 to 50 μmm) and reach 700 C (Fig. 3).

When to Use Adiabatic Cutting

We can gauge the edge quality of a blank by its surface roughness (burr vs. shear), perpendicularity and the presence of microcracks. To improve surface roughness, stampers use smaller blanking clearances, which will increase tool wear. Blanking higher-strength materials using relatively tight clearances also increases the risk of microcracking. Microcracks can shorten the life of a fatigue-limited component or reduce the impact/crash resistance of a formed part. 

We recommend adiabatic cutting when blanking high-strength steels in excess of 1300-MPa tensile strength, including hot-stamped steels. I know of at least one Tier One supplier planning to replace laser trimming of hot-stamped steel with adiabatic cutting. MF

Industry-Related Terms: Alloys, Blank, Blanking, Drawing, Edge, Forming, Gauge, LASER, Periphery, Perpendicularity, Stroke, Surface, Tensile Strength, Binder
View Glossary of Metalforming Terms

 

See also: Billur Metal Form

Technologies: Materials

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