April 1, 2009
Hot stamping, also called hot forming or press hardening, is the process of forming metal while it is very hot (in excess of 900 C degrees) and then quenching it (cooling it quickly) in the die. The process converts low-tensile strength metal to a very high-strength steel (150 to 200 kilopounds per square inch (KSI).
In its simplest definition, hot stamping, also called hot forming or press hardening, is the process of forming metal while it is very hot (in excess of 900 degrees C) and then quenching it (cooling it quickly) in the die. The process converts low-tensile-strength metal to a very high-strength steel (150 to 200 kilopounds per square inch (KSI).
The press-hardenable material—boron steel or aluminized steel—is heated to more than 900 degrees to an austenite temperature in an oven in the first stage of the press line. The material is transferred quickly to a press, and the part is formed while the material is very hot. Then the part is quenched—by being held in a water-cooled die cavity for a few seconds at the bottom of the stroke, which is when the material's grain structure is converted from a austenitic state to a martensitic state.
"On the surface, the hot- stamping process sounds rather simple—heat a steel blank until it is red-hot, press it into a die cavity, and keep it there for a few seconds while it cools. The whole process conjures up visions of a blacksmith working a hot blank over an anvil with a sledge hammer and a quench bucket by his side," said Rich Marando, president of Graebener Group Technologies, Napoleon, Ohio.
High Tensile Strength."In the case of modern hot stamping, relatively complex parts can be formed in a single-step die," Marando continued. "The result is a complex, near-net-shape part with yield strengths many times stronger than a mild steel part."
Boron steel, in its original state, has tensile strength of around 50 KSI but is about 200 KSI after it is hot-formed.
Complex Components. Because hot stamping allows the forming of complex parts in one stroke, multi-component assemblies can be redesigned and formed as one component, eliminating some downstream joining processes such as welding.
No Springback. Perhaps hot stamping's most significant advantage, after its strengthening capability, is its stress-relieving capability that resolves problems with springback and warping, a common problem when forming high-strength steel (HSS) and advanced high-strength steel (AHSS).
Cosma International Inc., Troy, Mich., a division of Magna International, designs, develops, and manufactures automotive systems, assemblies, modules, and components. The Tier 1 supplier relayed its experiences with hot stamping in "Hot forming: Strong meets light; Caught between NHTSA and CAFE," published in the May 2006 issue of The FABRICATOR®.
One of the challenges inherent in stamping AHSS is that cold-stamped AHSS parts have a tendency to spring back. This interferes with fit-up and causes difficulties during welding and assembly, said Swamy Kotagiri, director of R&D.
"It becomes really difficult to cold-stamp complex, angular parts or parts with a 3-D twist when they are made with very high-strength materials—150 to 200 KSI.
Simply put, hot stamping is best-suited to form components that must be both lightweight and very strong. Hot stamping's most prevalent application has been for structural automotive components such as body pillars, rockers, roof rails, bumpers, and door intrusion beams that must be strong enough to withstand a large load with minimal intrusion into the passenger compartment during a rollover and impact (see Figure 1).
Cosma sought ways to meet the National Highway Traffic Safety Administration's (NHTSA) structural requirements for increased vehicle safety. The requirement to reduce the roof intrusion into the compartment area in a rollover has become even more demanding.
"The roof crush standards are based on gross vehicle weight [GVW]," Kotagiri said. "Basically, the standard is saying that if a vehicle's GVW is 4,000 pounds, it has to withstand a force of up to 1.5 times that 4,000 pounds, or 6,000 pounds, in a roof crush test intended to simulate a rollover incident. New rules require the vehicle to be 2.25 to 2.5 times the GVW."
To meet those requirements, the components' material must be stronger—normally achieved by increasing in thickness and weight.
In an apparent contradiction, the components also have to be engineered to be as light as possible to meet increasingly stringent corporate average fuel economy (CAFE) standards for lower emissions, he said. "So the objective is to get the required structural performance, but add as little weight as possible," said Kotagiri.
Heating System. An oven or inductive heating system must be installed before the forming station in the press line to heat the blank to about 960 degrees C.
Cooling System. The press also must be equipped with a cooling system with a high number of variably controlled cooling circuits.
Automated Handling. Because the heated part is very hot to the touch, an automated part handling system, such as a shuttle or robot transfer system, must be integrated into the system.
Dwell Capability. A hydraulic or servo press with dwell capabilities is required to maintain tonnage at the bottom of stroke while the part is quenched.
Safety Hot Operation. The press system must conform to all safety requirements for a hot operation.
Tooling. The hot-stamping die is a single-step die with internal cooling channels, and it must be made of tooling materials that are resistant to thermal shock.
Because deep forming can be done in one hit, multipart assemblies can be formed as a single complex component.
"In the past, to beef up the safety performance of side-impact beams, the solution has been to add parts—to add more metal into the body side. In many cases, with hot stamping we're able to eliminate the need for additional parts," Kotagiri said in the article.
"Let's say you have a body side B pillar inner, B pillar reinforcement, and a B pillar outer. If I use hot-formed components in that assembly, now I'm able to eliminate the reinforcements. In roof crush case studies where conventional stampings and reinforcements have been substituted with hot-stamped components, the mass is reduced by roughly 30 to 40 percent," he said.
Scale. After forming, the part may need special treatment to remove scale and improve rust resistance. Uncoated boron steel requires a nitrogen-rich (or other inert gas) atmosphere in the furnace; otherwise, once it is exposed to the atmosphere, it develops a scale on it that must be removed by shot peening or sandblasting. Aluminum silicon-coated boron steel does not require scale removal.
Lower Tonnage Requirement. Kotagiri said the tonnage requirement to hot-form a high-strength steel part is substantially lower than is needed to cold-form it. Hot forming the draw portion of a B pillar requires only 30 percent to 40 percent of the tonnage needed to cold-form it using a similar-strength material, he said. So the part, which requires 1,000 tons to be cold-stamped, needs only 300 to 400 tons to be hot-formed, he said.
Complex Part Features. Hot forming can achieve very complex geometries at high strengths that cannot be cold-formed. "Honestly, there is not one other process today where you can form a part like an A pillar at the same strength, because of springback and because tonnage and strength requirements," Kotagiri said.
Laser Trim. The final trim must be done with a laser because once the part is press-hardened, it is too hard to be trimmed using traditional steel trimming dies.
Material Cost. Boron steel costs more than low-carbon materials.
Cannot Be Re-formed. Hot stamping is a one-step process—in fact, multiple processes, such as additional draws or flanging, cannot be performed—because the material is hardened.
Slower Forming. The hot-forming process is slower than cold stamping. From stroke to stroke, it takes about 15 to 20 seconds.
Limited Materials. This hot-forming process cannot be applied to galvanized or prepainted steel; only boron materials can be hot-stamped. Boron is the element that gives the material the capacity to change from a regular material to a martensite material when it cools.
Calculating the weight and cost differences can be a challenge because a direct comparison of a cold-formed part to a hot-formed part would not be accurate, Kotagiri said.
"For example, you cannot take a cold-formed B pillar and compare it to a hot-stamped B pillar and ask, 'What's the cost difference?' because you're getting more out of your part in the hot-forming process," Kotagiri said. "The question is, 'We need this performance; now give us your design with hot stamping and tell us how much you saved in mass.'"
Cosma's Kotagiri anticipates that the rising demands for high strength-to-weight ratios will heighten demand for hot-formed components. "We think it will increase until about 8 to 12 percent of body structures will be martensite material, and then it will plateau at that point,"
R&D efforts are under way to shorten quench cycle times and improve part integrity verification and lubrication and trimming methods. Research is also being done to improve control over thermal stress concentration so that strength and stress can be varied throughout the component.
Kotagiri said that one of the most important aspects of successfully applying hot-forming technology is being able to understand how the material is affected as it goes through the heating process in the furnace, and to understand the technology for quenching in the die.
"How can I make parts consistently over a period of time and know exactly how quickly they cooled, how much they have cooled, and control the part temperature?" What we have done is found the applications and ways to mass-produce components using this process. We have a very controlled, consistent cooling process," Kotagiri said.
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