What is Hot Stamping?
Hot stamping, known as press hardening in Europe, and hot press forming in Asia, is a thermal forming process for sheet metal where forming and metallurgical heat-treating take place during the stamping process. Press hardening was originally developed in the 1970s to manufacture hardened steel agricultural tools, but has since had a major commercial impact on the fabrication of lightweight and high strength white bodies in the automotive industry. Reduced fuel consumption and vehicle safety have driven the universal use of ultra-high strength steel components made possible by the advent of the hot stamping process.
Early development and progression of hot stamping was focused on low carbon manganese boron alloyed steel 22MnB5 due to incumbent use and availability in the white body sector. By heating 22MnB5 sheet above 900 °C the microstructure of the metal is converted from ferritic steel to austenitic steel, then with rapid cooling in the stamping die, the steel phase is transformed into martensite, with a strength of up to 1500 MPa [220 KSI]. Higher carbon steel grades with specialized coatings and advanced chemical compositions have since been developed with strength up to 2000 MPa [290 KSI] and many significant process and material property advantages.
Hot stamping requires a programmable servo hydraulic press with fast advance speeds and dwell capability to repeatably control the tonnage for hot forming and cooling processes. Ancillary equipment typically includes a roller hearth furnace or stack oven to heat the sheet metal, material transfer robotics or feeders, a die cooling system, and part trimming operations. Press frame style, guiding systems, speeds, tonnage, material feed direction, quick die change and off-center loading capability must all be carefully evaluated to ensure that a press is optimized for specific hot stamping applications.
Any heat treatable or difficult to form metal can benefit from hot stamping when the production of complex high strength parts is an advantage. Some example materials are presented in table 1 below.
|Boron Alloy Steel, 0.22% C||22MnB5||1500 MPa|
|Boron Alloy Steel, 0.30% C||30MnB5||1800 MPa|
|Boron Alloy Steel, 0.37% C||37MnB5||2000 MPa|
|Aluminum Alloy, 6000 series||6061-T6||300 MPa|
|Aluminum Alloy, 7000 series||7075-T76||500 MPa|
|Magnesium Alloy||AZ31B-H||300 MPa|
|Titanium Alloy||Ti-6Al-4V||900 MPa|
|Copper Nickel Alloy||C71500||500 MPa|
Table 1: Example sheet metal materials for hot stamping.
Not all metals will exhibit the high strength transformation found in boron steels when hot stamped. The addition of boron to carbon steels promotes the phase transformation to martensite when rapidly cooled. Steel sheets that are not alloyed for phase transformation cannot be hardened to ultra-high strength, however, hardness tailoring via zone cooling and heating can be used to control microstructure and therefore material properties. Most nonferrous alloys will exhibit limited improvements in hardness but hot press forming can improve formability of complex shapes, eliminate spring back, and reduce defects in many applications.
Corrosion, decarbonization and scaling of sheet materials at high furnace temperatures is an issue for hot stamping. Uncoated steels require inert gas atmospheres to minimize scaling. Corrosion resistant coatings, such as aluminum-silicon, are often applied to sheet steels to eliminate the need for scale removal. The addition of specific alloying elements can also reduce corrosion and in some cases reduce the cooling required to maintain hardness and permit multi step forming operations.
A significant amount of effort has been devoted by metallurgists and steel producers to improve materials used for hot stamping. Figure 1 below illustrates the relationship between elongation and tensile strength for boron steel 22MnB5 in annealed and hot stamped states compared to other steel compositions, such as mild steel and conventional high strength steels. The temperature time phase diagram overlay on the right illustrates the conditions required to produce martensitic steel and the time and temperature zones where other phases will occur in 22MnB5.
Advantages of Hot Stamping
Advantages of hot stamping structural components are the exceptional as-formed ultimate tensile strength and the complex geometries that can be formed. The elevated strength of the hot stamped parts allows the weight of the components to be reduced by using thinner gauge sheet metal while maintaining both structural integrity and crash performance. Further advantages are listed below:
- Fewer joining operations by welding or fastening.
- Less part spring back and part warping.
- Fewer part defects such as cracks and splits.
- Reduced press tonnage compared to cold forming.
- Ability to tailor material properties by part zone.
- Ability to improve as received microstructures.
- Fewer operational steps to a finished product.
When to Use Hot Stamping
The number of body parts hot formed and the degree of weigh reduction continues to increase in this highly competitive and strictly regulated market.
Uses in aviation, aerospace, defence, and other emerging markets are beginning to experience the benefits higher strength and lighter weight made possible by hot stamping of difficult to form advanced alloys.
Processes capable of achieving similar results include;
Metal Stamping [Cold]: If the material being formed is ductile enough at room temperature then similar high strength parts can be made without the need to heat and anneal the sheet metal. More traditional part and die design methods for managing spring back are typically required to produce accurate parts.
Warm Stamping: Typically used for non-ferrous alloys that are difficult to form at room temperature. A controlled cooling and ageing process is often required after forming to restore solution hardened microstructures present before the sheet is annealed by heating.
Hydroforming: An advanced sheet and tube forming process that uses hydraulic pressure instead of a fixed punch to produce geometries not suitable for stamping, including undercut or bulged shapes.
Fabrication: For prototyping or small production runs, parts can be produced from multiple sheets via welding or other joining processes. In most cases, fabrication is only appropriate when production volume is not high enough justify the cost of dedicated tooling.
Hot stamping involves the rapid quenching of ultra-high strength steels that have been heated and formed to shape. The process begins with the de-stacking of a blank that is loaded into an oven or furnace to be heated. The heated blank is transferred by a press feeding system to a hydraulic hot stamping press, which closes to generate tonnage very quickly and then holds the part until it is cooled. When this process is complete, the hardened component is automatically unloaded from the hydraulic press by a press unloading system for finishing. Figure 3 below illustrates the basic elements of hot stamping.
Fig. 3: Direct and indirect hot stamping process elements
Direct hot stamping is more prevalent in industry. Indirect hot stamping adds a cold forming step before heating.
A comprehensive examination of all parameters that affect hot stamping is beyond the scope of this paper. Instead, we will look at a few key factors that translate well for acceptable process economics: material properties, quench tooling, production rates, and defect reduction.
High material hardenability translates into higher strength, which directly reduces the weight of the part. In the automotive sector this means better fuel economy and increased passenger safety. High tensile strength is not the only requirement. Crashworthiness also relies on energy absorbing ductile steel properties to be present in crumple zones. Advanced high strength steels must meet both of these objectives. Key methods to achieve these results are tailored blanks and zone-based heat treating in the quench tooling.
Form and Quench Tooling:
A well-designed form and quench tool is critical to the success of the part being produced. High strength zones must be quenched quickly to produce the desired material properties. Isolated component zones requiring more ductile properties are cooled more slowly and in some instances the tooling is heated to achieve the proper microstructure. Specialized hot work tool steels that can survive the rigors of repeated thermal cycling are required to minimize die wear and reduce tooling failure. The working surfaces of the tool are often hardened by nitriding and other material deposition techniques to resist abrasion and increase tool life.
Press hardening is a significantly slower process than traditional stamping primarily due to the time required to quench the part. Typical press cycle rates are 10 to 30 seconds with thicker materials taking longer because of the extra time needed to quench. Additional delay can also be attributed to the need for laser trimming when the as quenched part is too hard for post process press trimming and punching. One technique that has been employed to reduce cycle times is the addition of silicon to the alloy composition that allows for higher quench temperature exits to facilitate faster multi-step forming with hot press punching and trimming.
Hot stamping gets a hall pass on spring back as a defect due to the annealing process that makes forming of deep and complex shapes possible, but high temperatures mean sticky steel. The high temperature also means that no lubrication can be applied to the sheet and friction is a significant problem. Additionally, material coatings used to prevent decarburization and scale formation during blank heating can be very abrasive to tool surfaces. As a result, managing friction is critical to avoid excessive thinning, splitting, and cracking in the part, as well as excessive die wear from the abrasion.
Primary challenges for hot stamping are friction-induced wrinkling of the material and maintaining temperatures to avoid hardening and cracking before the part is complete.
To reduce friction, forming may be done without pressure pads when part geometry allows for the free flow of material into the die cavity. Surface contact between the blank and die should be minimized in the part design. Prevention of lateral shifting can be achieved with edge or pin gauges to locate the blank. Stick pads may be used to grip or form the material in the center of the part. These techniques are contrary to cold stamping where draw beads control material movement and ensure sufficient material stretch to eliminate free metal.
Compression of flanged shapes on inside radii can cause wrinkling, distortion, and double metal after forming. Gapped pads can be used to prevent excessive wrinkle height from forming. Gap cylinders hold the pad apart to allow free metal flow into the cavity then the gap cylinders are compressed at the bottom of stroke to iron out the wrinkles before quenching.
Thermal expansion of the blank followed by contraction of the formed part when it cools makes it more difficult to control part geometry. Gauging needs to accommodate shrinkage without losing effectiveness and also allow the part to move freely when necessary.
Contact pressure deterioration can result from surface abrasion of tool surfaces, which can result in slower quenching. Excessive tool wear will increase cycle time and part cost. Tool wear can also cause part distortion due to unbalanced quenching, leaving residual stresses from uneven part shrinkage.
Configuring a press to produce hot stamped parts starts with the requirements for the part manufacturing process. The configuration requires information regarding material type, part specifications, production volume, production speed, and target pricing. These factors then have a direct influence on the handling, forming and quenching requirements, which in turn influence the specifications of the installation and tooling. A diagram of temperature versus process time below illustrates the steps in the process that will dictate the process requirements and the achievable production rate.
There are several key variables to consider when purchasing a hot stamping press or fully automated hot stamping press line.
- Hydraulic and control systems for a hot stamping press must be fully programmable and offer repeatable tonnage control, for this reason many hot stamping applications use servo hydraulic presses. With these systems oil must be extremely clean and it may be worth considering online or offline filtering systems in additional to the standard cartridge filters.
- Servo hydraulics are generally much more complicated in design, and most customers enjoy having a remote connection from the OEM into the press to help with troubleshooting.
- While there are many different circuit designs, typically the press would want to have a pre-pressing circuit which can quickly be moved over to pressing at full tonnage when required.
- As Hot Stamping presses are expensive and typically need to run multiple products with multiple dies, die carts along with automatic die clamping systems are popular to reduce die change time and maximize annual production.
The bed of a press must be able to accommodate the footprint of the largest expected toolset. For rectangular or complex blank shapes, orientation of the part within the bed will determine overall bed dimensions. A rough estimation of bed size can be calculated based on blank size. Wide access to the bed from all four sides of a hot stamping press is advantageous for automated material transfer and quick die change tooling.
The control system for a hydraulic hot-stamping press should be capable of fully programmable and repeatable tonnage control to optimizes the process and reduce energy consumption. The press should be able to produce enough tonnage to form the part and hold/harden it, but excessive tonnage should be avoided. Tonnage that is applied beyond what is required may cause excess energy consumption and tooling wear. A typical range for hot stamping tonnage is between 500 and 1500 tons.
The heated blank begins to cool rapidly immediately after being removed from the furnace, so it is critical that the press close and generate tonnage to form the part very quickly. Automated part loading typically requires that the press be open a considerable amount to allow ample clearance. This large clearance makes it even more critical that the press be able to open and close very quickly. Typically, closing speeds of 500 to 1,000 mm per second are required. Rapid return speeds are somewhat slower but are optimized to reduce cycle times.
Some advanced hot stamping capabilities include:
- Multistep forming prior to quenching by utilizing advanced steels that inhibit the loss of martensite at higher temperatures.
- Hot punching and hot trimming before quenching and part hardening.
- Induction furnaces that can preheat blanks to different temperatures by part zone to produce parts with multiple microstructures.