As promised, in this blog we’ll review the Direct and Indirect Hot Forming processes. The goal of hot-forming is to achieve complex geometries and part consolidation at strength levels previously unimaginable, while minimizing springback and distortion. While several steels are applicable, the most common hot forming steels are Boron-based (between 0.001% and 0.005% boron); the industrial workhorse is 22MnB5. Hot Forming with these steels has been in use since the 1990s for various body structure components and two types of press-hardening or hot forming applications, Direct Hot Forming and Indirect Hot Forming, are currently available.
Direct Hot Forming Process
Figure 1 shows the process flow for the Direct Hot Forming process. During Direct Hot Forming, all deformation of the blank is done in the high temperature austenitic range (minimum temperature of 850 °C), followed by quenching with a cooling rate greater than 50 °C/s. The initial HF steel microstructure is composed of ferrite and pearlite, with the following room temperature properties: yield strength of 350-400 MPa, tensile strength of 550-600 MPa, and total elongation around 25%.
Figure 1: Graphic showing steps in the Direct Hot Forming process.1
In Step 1 (Marker 1 in Figure 2), the steel coil is cut into blanks that will be loaded into a high temperature oven with multiple zones to achieve uniform temperatures across the material.
Figure 2: Steel properties during the forming process.
In Step 2, the blank is fed into the furnace. To enable the material to harden, it must first be heated above 900 ºC to change the microstructure to austenite. Generally, this is accomplished in continuous furnaces to ensure a continuous heating process. Typical furnace time is 5-8 minutes. The exposure of the tool steel to the high temperatures necessary for hot-forming can result in large variations in friction because of changes in the surface topography, removal of oxide layers, and excessive wear of the tool. One approach to overcome the issues of friction is to apply suitable coatings or various surface treatments to the tool steel.
Currently, an aluminium-silicon (AS) coating is the most common coating applied to blanks to prevent the formation of this surface oxide. Other coatings include hot-dipped galvanized (GI), galvannealed (GA), zinc-nickel (Zn-Ni), and organic substances. Inert gasses can be used for special applications. The coatings also help prevent in-service corrosion in part areas difficult to shot blast or otherwise remove the surface oxide prior to application of additional corrosion protection treatments.
In Step 3, the “hot” blank is transferred into a forming die. Robots or linear transfer systems (feeders) can transfer the blank to the water-cooled die in about three seconds. To protect the transfer system from overheating and minimize the heat loss of the blank, insulation should be used – an example is the placement of heat shields between the blanks and the transfer system. Once transferred, positioning aids ensure that the blank is located precisely in the die.
In Step 4, the hot blank is formed into the part geometry. Forming temperature typically starts at 850 ºC and ends at 650 ºC. While in the austenitic range, the true yield stress is relatively constant at 40 MPa with high elongations greater than 50%. This enables stampings with complex geometries and part consolidation to form successfully with limited springback issues.
Step 5 reflects In-Die Quenching (Marker 3 in Figure 2), where the die is liquid-chilled. When forming is completed, the stamping now contacts both the punch and die for both side quenching. The minimum quench rate is 50 ºC/sec. Some actual cooling rates are two or three times the minimum rate. Quenching the formed part leads to a significant increase in the strength of the material and a greater precision in its final dimensions. The quench process transforms the austenite to martensite throughout the entire stamping, which accounts for the increase in strength. The room temperature properties of the final stamping are 1000-1250 MPa yield strength, 1400 -1700 tensile strength, and 4-8% elongations (See the true stress-strain curve in Figure 3, lower graph). Total time for robot transfer, forming, and quenching is about 20-30 seconds and depends heavily on the quench rate and quenching system. With smaller stampings, forming and quenching of multiple stampings in the die reduces per stamping processing time.
Figure 3: True stress-strain curves for different sheet thickness of as-received boron-based HF steel tested at room temperature (left curve) and tested after heat treatment and quenching (right curve).2
Step 6 is the post-forming operations. The very high strength and low elongations of the final stamping restrict these final operations. The room temperature stamping should not undergo additional forming. Any special cutting, trimming, and piercing equipment must utilize appropriate materials and equipment capacities to withstand the high loads generated during these operations. Production speeds range between 2 to 4 parts per minute, slowed to allow sufficient quenching.
Indirect Hot-Forming Process
The Indirect Hot Forming process (Figure 4) accomplishes initial forming and trimming cold (prior to hot forming), shown as preform Step 1A. Here, 90-95% of the stamping geometry is pre-formed in conventional dies at room temperature, based on incoming steel properties. The stamping is trimmed (2A) and then subjected to the usual heating cycle in Step 3A. Additional hot-forming (4A) is now possible for areas of the stamping too severe to form at room temperature. However, the in-direct forming process has a cost increase over the Direct Hot Forming process since two forming dies are required instead of one.
Figure 4: Graphic showing steps in the In-Direct Hot-Forming process.1
Indirect Hot Forming was developed to reduce wear on the tool when dealing with uncoated steel. The added cold forming stage reduced movement between the steel and the tool, thus leading to less wear on the tool. On the contrary, with Indirect Hot Forming rapid cooling of the finished stamping takes place via the surfaces of the tool. The Indirect Hot Forming process is introduced to develop stampings with more complex form features. Since the stamping cavity depth is formed during cold stamping and the detail features are formed thereafter in the Hot Forming press, more complex geometry can be achieved and distortion is minimized.
Figure 5: Post forming heat treating. Lower strength, more formable steel is formed to final shape (Marker 1), heated, and quenched to achieve the final high strength (Marker 2).
Another process similar to the Indirect Hot Forming process is Post-Forming Heat-Treating (PFHT). Very high strength steels generally have greatly reduced stretchability. The PFHT goal is to create the stamping from lower strength, but more formable steels (Marker 1 in Figure 5) by traditional Cold Forming processes. The final processing heat and quench sequence creates a very high-strength stamping (Marker 2 in Figure 5). The major issue restricting widespread implementation of PFHT typically has been maintaining stamping geometry during and after the heat treatment process. Fixturing the stamping and then heating (furnace or induction) and immediate quenching appear to be the solution for production applications. Current quenching processes are water, air hardening, or water-cooled dies.