It's all in the dwell
October 28, 2008
Hot stamping of automotive structural safety components developed in response to mandates levied by the U.S. National Highway Traffic Safety Administration (NHTSA) 1 to improve vehicle crash integrity while also reducing vehicle weight to help meet fuel efficiency standards. Heating a high-strength steel (HSS) component of a boron-steel alloy to the austenitic range—a super-heated solid solution state, ~ 1,700 degrees F (950 degrees C)—improved drawability, and then quickly cooling the part in a water-cooled die, or quenching, transformed the crystalline structure, increasing the formed component's strength-to-weight ratio.
Hot stamping was developed to meet mandates for stamped automotive components to be both light and strong. The component is heated to an austenitic, solid-solution state for forming and then quenched in-die and cooled to a martensitic state, making it three times harder than before.
Click image to view larger Good candidates for the hot forming process are B-pillars, bumpers, and transmission tunnels.
Press hardening, or hot stamping, of automotive structural safety components is a relatively new process that the metal forming industry has developed in response to mandates issued by the U.S. National Highway Traffic Safety Administration (NHTSA)1 to improve vehicle crash integrity while also reducing vehicle weight to help meet fuel efficiency standards (seeFigure 1).
Applying standard metallurgical principles, the metal forming industry found that heating a high-strength steel (HSS) component made of a boron-steel alloy to the austenitic range (see Figure 2)—a superheated solid solution state, approximately 1,700 degrees F (950 degrees C)—improved drawability, and then quickly cooling the part in a water-cooled die, or quenching, transformed the crystalline structure, increasing the formed component's strength-to-weight ratio (seeFigure 3).
The European press industry (responding to similar requirements in Europe) has named this process press hardening, while U.S. manufacturers tend to use the term hot stamping.
The cooled component can have a tensile strength of up to 1,500 megapascals (MPa)—approximately three times stronger than steel that has not been press hardened.
Click image to view larger Forming the part in its austenitic state improves formability and eliminates springback, warping, stress fractures, and other defects.
Why is a hydraulic press especially suitable for hot stamping? The short response—it's all in the dwell time for quenching.
Currently the best way to transform HSS from the austenite range to the higher-strength martensite range is to quench the formed part quickly while the press dwells, maintaining constant pressure with the die closed to prevent stresses from warping the part during the cool-down (transformation) phase.
The typical dwell time needed to form and quench a B-pillar is five to eight seconds, depending on the number of parts being formed, material thickness, and die temperature. The die temperature is directly proportional to the volume of water flow, inlet temperature, and the configuration of die water ports within the die.
Click image to view larger Image courtesy of Encyclopaedia Britannica, 1994.
The dynamics of this process play right into the flexibility and capabilities of a hydraulic press. Its free programmability of force, speed, and dwell throughout the stroke range makes it especially suitable for press-hardening applications.
Because a hydraulic press's dwell capability is the most critical aspect of this process, the pumps have to exhibit maximum holding pressure during the dwell. This places a heavy load on them. Use of a fixed auxiliary pump for hold-down during the dwell phase, or a programmable pump configuration in multiple-pump systems, allows the operator to achieve adequate holding pressure.
Full control over the process with monitoring of all functions—blank feeding, heating, forming, and unloading—is particularly important in a production run in which cycle time is critical to the metal's composition and, therefore, to product quality.
Click image to view larger The 1,700-degree-F, red-hot--literally--steel blank must be transferred quickly from the furnace to the press and accurately positioned in the die with minimal heat loss.
Cold forming structural safety components in HSS and ultrahigh-strength steels (UHSS) with tensile strengths between 600 and 750 MPa has its challenges—mainly poor material drawability, draw fractures, and springback.
A standard mechanical press in the 2,000- to 3,500-ton range can be used to cold form these high-strength materials, but at a substantial investment because of the press's size and required multiple hits and cushion dampening equipment. Breakthrough forces—a sudden release of stored energy—can be greater than 40 percent with HSS.
A servo-driven mechanical press can be used to cold form or hot form HSS. Its servo drive with advanced controllers that achieve constant energy and torque can reduce the forces at the breakthrough stage, thus reducing the need to purchase additional cushion dampening equipment. This type of press also has the ability to dwell at any point in the stroke, but currently it carries a higher initial price tag than a hydraulic press.
Ultimately, using a crank or servo-driven mechanical press to cold form HSS is likely to increase the price per part compared to hot stamping the part with a hydraulic press. This is mainly because a mechanical press's typical comparable advantage—its higher strokes per minute—is negated by the required dwell time.
Click image to view larger
1. Blank Feeding Into the Furnace. Blanks can be carbon steel, but to obtain additional strength, boron is added to the steel in amounts ranging from 3 to 15 parts per million. Blanks are separated, picked, and centered for entry into the furnace. An infeed conveyor moves the blank into the furnace. Processes such as marking and punching can be performed at this stage.
2. Heating. The blank is heated in an electric or gas-fired flowthrough furnace to austenitic temperature, 1,700 degrees F (950 degrees C), before it is ejected from the furnace. Normally, the blank is transported through the furnace directly on ceramic rollers. In some cases, the furnace is equipped with an inert gas, such as nitrogen, to prevent scaling from developing on the blank's surface.
3. Press Feeding. A conveyor with a centering station locates the part for the press loading robot, which quickly transfers the hot blank into the die. Speed and stability are critical. The gripper system and the furnace ejection system must quickly and precisely position the blank to enter the press (seeFigure 4).
4. Forming/Quenching. The hydraulic press then cycles, forming the part, and then dwells (typically five to eight seconds), maintaining programmed pressure during the quenching process that water-cools the component in the die. The finished part exits the press at approximately 300 degrees F (martensite).
The press tool must be able to form very hot blanks and withstand extreme temperature differences. Therefore, materials and tool designs must meet very stringent requirements. The dies have features such as an integrated water-to-air cooling system.
5. Unloading Finished Part. An unloading robot removes the finished part to an output conveyor for postprocessing.
1. National Highway Traffic Safety Administration, Title 49 of the United States Code, Chapter 301, Motor Vehicle Safety Federal Motor Vehicle Safety Standards (FMVSS) and Regulations.