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.
Dear Danny Schaeffler,
We heard many times that indirect hot stamping is cost more than direct hot stamping.
What is cost difference between direct and indirect hot forming in high-level numbers?
How much higher will be investments to indirect line versus direct line?
Thanks you in advance.
Dear Alexey Shubin,
Thank you for your question. We are very happy that this post continues to generate views and interest.
The indirect method needs another forming station, so there’s the cost of another form die or another prog die station.
Along with this comes additional costs associated with maintenance, logistics, and processing associated with the extra tooling and extra step.
Depending on the coating, there could be process control differences, and an associated cost increment.
Remember that properties after hot forming are a function of the cooling rate. In the indirect process, there is the room temperature forming step. Any dimensional issues on this panel needs to be accounted for in the hot forming step, otherwise there will be gaps between the panel and the tooling, changing the local cooling rate, resulting in property variability.
On the positive side, there might be reduced costs in tool coatings, and potentially energy savings, in the indirect method.
– Danny Schaeffler
Engineering Quality Solutions, Inc. and 4M Partners, LLC
EQSgroup.com and Learning4M.com
Danny Schaeffler- exploring cost opportunity, proposing Boron Steel without coating.
I would like to get experts opnion, can we use Boron steel with out any coating? what are pro’s and cos’s
Thank you for your question.
This topic will be explored in greater detail in the next version of the Guidelines, so we encourage you to sign up for updates. [upper right of this page]
It may be expected that the purchase price of an uncoated product will be lower than a coated steel alternative, but there will be additional costs you should consider.
Many of the potential issues with using an uncoated steel are associated with the development of the scale barrier layer if the heated blanks are exposed to the atmosphere.
Only some of the issues you may encounter include:
– Uncoated blanks will develop a scale barrier layer.
– Scale is a rough oxide, which affects metal flow – even when forming at these high temperatures.
– The scale coating will change heat transfer in the dies, possibly preventing the critical cooling rate from being achieved. Without this, the targeted microstructure/properties will not be produced. Or possibly worse – it will not be uniform throughout the part, and you will not know where. The different strength levels can lead to dimensional issues.
– Even if you have a “good” part coming out of stamping, the scale coating needs to be removed before the next operation. Scale removal is a separate operation.
– Shot blasting may be used to remove scale, but this induces surface stresses, which may affect dimensional accuracy.
– The part you now have has no corrosion protection.
There are certainly applications where the use of uncoated PHS is appropriate, but the extra operations and constraints should be considered as well.
Remember, there is sometimes a high price to cost savings.
– Danny Schaeffler
Engineering Quality Solutions, Inc. and 4M Partners, LLC
https://www.EQSgroup.com and https://www.Learning4M.com
What are the alternatives of Al-Si coating for hot stamping of steel sheets?
Is there any hot forming grades of steel having final strength > 2000 MPa?
What is the suitable welding method for 22MnB5 or higher strength hot forming grade of steel?
What are the direction of developments in the grades of hot forming steels?
Zinc based coatings are being used by some companies – especially with the indirect method. Each approach has their strengths and challenges, as well as optimal processing conditions. You should be prepared to adjust your thermal cycle to work best with your chosen sheet grade + coating combination, and your steel supplier may be able to help you with optimizing the settings.
Going higher in strength is possible with certain chemistries, and several companies are working on volume production of those grades.
You’ll be able to learn more about the options as WorldAutoSteel develops V7 of the AHSS Guidelines.
– Danny Schaeffler
Engineering Quality Solutions, Inc. and 4M Partners, LLC
https://www.EQSgroup.com and https://www.Learning4M.com
Thank you.
We are developing steel to achieve >2GPa strength after hot forming.
It is said that certain companies have patents for the most common hot forming process, and if these patents are applied in practice, many stampers are expected to suffer.
It is worrisome how to deal with the hot-forming process patent of aluminum coating material.
What do you think?
Exactly! You just highlighted one of the many challenges of using GI/GA zinc coatings. This is addressed by keeping the blanks in the furnace at a time/temperature appropriate to alloy the coating to produce galvanneal, and then ramping up to the 900C for press hardening. (The galvanneal alloy has a higher melting temperature than pure zinc). But leaving the blanks in the furnace too long leads to over-alloying and a degradation of properties.
The furnace type, thermal controls, time/temperature profiles, and so on are highly controlled balance of conditions and productivity and quality.
Similar challenges exist on a smaller scale in Al-Si coatings. Processing conditions must be controlled, but this type of coating has a wider window of conditions that will produce an acceptable coated product.
– Danny Schaeffler
Engineering Quality Solutions, Inc. and 4M Partners, LLC
https://www.EQSgroup.com and https://www.Learning4M.com
Dear Sir,
How long the Galvanneal blanks will be kept in the Furnace even very small increase in soaking time will increase the diffusion between Iron and Zinc.
And ramping unto 900 will also leads to heavy powdering during press forming ???
Like any engineer, I must respond with “it depends.”
Your comments are correct as it pertains to the thermal cycle seen on a galvanizing line.
A rotary hearth oven vs a multi-chamber vertical box “pizza” oven vs a roller hearth oven each have different control zones and constraints. They can be heated in different ways.
You might be limited in the thickness of your coating and the time to ramp up to 900C – you need to avoid microcracks which increases the risk of liquid metal embrittlement (LME).
The questions you raise speak directly to the tighter process control window found on zinc coated PHS grades compared with Al-Si coated.
There is no one right answer. Your WorldAutoSteel supplier and your furnace supplier can provide valuable insight to guide you based on your specific needs and constraints.
– Danny Schaeffler
Engineering Quality Solutions, Inc. and 4M Partners, LLC
https://www.EQSgroup.com and https://www.Learning4M.com
When we expose the coated materials like galvanized and Galvannealed product to Such high temperature around 800°c the Zn coating will melt and it will create surface defect .How these issue will be rectified ??
Danny Schaeffler addressed your question, but it posted as a new comment on the blog. Please be sure and go to the blog and see his comment to you. Best regards!