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Advancements in furnace design improve hot stamping

Temperature control, oxygen, dew point regulation facilitate nonuniform tempering, hardening

Advances in continuous roller furnaces, the standard for hot stamping lines, have improved temperature control, roller drives, input and output, and overall performance. Photo courtesy of Benteler Mechanical Engineering.

Editor’s Note: This article was adapted from the authors’ presentation, “Advanced design of continuous furnace for hot stamping line,” at the Great Designs in Steel conference, May 2016, Livonia, Mich., by the WorldAutoSteel and The Steel Market Development Institute.

Generally, the type of furnace used in hot stamping, or press hardening, is a continuous roller furnace (see lead image). Based on the manufacturing experiences and development of the machinery for the automotive and glassmaking industries, new roller furnaces have been designed using finite element method (FEM) analysis for numerical simulations of heating processes and heating power distribution.

Advanced designs focus on optimal heating, modern roller drives, a new approach to the furnace’s input and output without movable parts, and other items with respect to the optimal technological and technical aspects. Carrying out the numerical analyses allowed the simulation of a range of external and internal conditions and different technological processes—the actual testing of which would be extremely expensive, dangerous, or absolutely impossible.

During the R&D period, most advanced technologies for heating and process control units were tested and later used for design and construction. Designers and technologists, in collaboration with computer-aided engineering (CAE) specialists, have simulated and optimized various technological processes. Specialized research institutes, including the Technical University of Liberec, Czech Republic, were involved in the research and testing.

Design Characteristics

An important parameter of the furnace design is its modularity, which allows an OEM to build the furnace according to user requirements (see Figure 1). The design allows for comfortable access to the furnace for maintenance, safety features that meet updated international safety standards, and technological requirements such as the dew point regulation system and oxygen rate control.

The newly developed furnace is equipped with a number of temperature sensors for precise temperature regulation, oxygen, and dew point sensors for process regulation. In addition, the whole concept is characterized by temperature stability and the option of uniform or nonuniform (Taylor) tempering and hardening. (Read “How 2016 Car of the Year Honda Civic maneuvered around AHSS obstructions,” Sept./Oct. 2016 STAMPING Journal, p. 26.)

In nonuniform and uniform tempering, the technical aspects and conditions have different technical parameters for controlling the furnace temperature. Nevertheless, the system is engineered to control the tempering process inside the furnace for both technical options. The nonuniform tempering requires, in principle, the same design as the furnace for homogenous tempering, but with mechanical and technical adaptations.

Heating Concepts. The advanced furnace designs include recuperative gas burners with radiant tubes or electric resistance heaters. The most advanced system is based on highly efficient recuperative burners using the flameless oxidation (FLOX) technology equipped with the silicon-reinforced silicon carbide radiant tubes produced (see Figure 2).

The main advantages of these burners are the higher efficiency achieved (see Formula 1) and reduction of nitrogen oxides (NOx) emissions.1 They are up to 10 percent more efficient than common burners.

The design also offers other benefits, such as homogenous temperature distribution, reduction of the thermal stress in the burner, noise reduction, and lower restrictions on fuels.

Figure 1
Modularity allows for customization of the furnace to accommodate user requirements. Image courtesy of Benteler Mechanical Engineering.

Numerical Simulation of Heat Transfer. WS GmbH Co., Germany, performed an analysis of heat transfer as a reference for the heating system design. This calculation takes into account all mechanisms of heat transfer, such as conduction, convection, and radiation (see Figure 3).

It is possible to calculate radiation energy flux from the so-called “black” body eb (Wm-2) using the Stefan-Boltzmann law as ( see Formula 2) where σ = 5.6707e-8 (Wm2.K-4) is Stefan-Boltzmann constant and T (K) is thermodynamic temperature. It is known that nonblack bodies absorb and emit less energy than black bodies, which are ideal emitters. It is possible to characterize the emissive power on a non-black body (sometimes called a gray body) using a surface property called emittance. So the equation for non-black bodies should be written in the form2: (see Formula 3)

The emittance is a property of a surface, which depends on temperature and is considered wavelength. For simplification, the concept of nonblack body is established and supposes that emittance depends only on temperature.

When radiation is exchanged between bodies that are nonblack, the total net heat flux is calculated as: (see Formula 4)

where the transfer factor F1-2 depends on the emittances of both bodies as well as the geometric view F1-2, which is called the view factor. Transfer factor or view factor must be determined to calculate the radiative heat transfer. Generally, view factor can be calculated according to the equation: (see Formula 5)

where β1 and β2 are angles between surface dA1 and dA2 normal lines and a ray between dA1 and dA2, and s is distance between dA1 and dA2. Nevertheless this calculation is impossible for complicated geometries. By FEM numerical simulations, the computational domain division into individual finite elements is advantageously used.

The view factor is then numerically determined between each two elements in the system. Even here, the direct integration would be very difficult, so it is necessary to use some appropriate statistical methods.3

Numerical Simulation of Radiation Heat Transfer. Presented numerical simulations use a pixel-based modified hemicube method4. The foundation of radiation is based upon its being received from multiple directions and being emitted in multiple directions.

This leads to the concept of a hemisphere, where one considers an element being projected onto the hemisphere.

This geometric projection can be easily calculated for 2-D or axisymmetric, but is problematic in 3-D. To overcome this problem of exact projections, consider dividing the hemisphere into pixels, and then simply count the pixels.

Figure 2
Recuperative burners achieve higher efficiency and reduce nitrogen oxide emissions. Image on left courtesy of WS GmbH Co., Germany; image on right courtesy of J. Wünning.

This procedure is easier to implement in 3-D, though it is still expensive. As an alternative, approximate the hemisphere with a hemicube as used in presented simulations.

FEM Analysis. Figure 4 shows the model for FEM heating simulation. This model is designed to monitor the temperature development of the blank during its way through the furnace. Processes inside the radiant tubes are not simulated.

Uniform heat flux at the inner surface of the radiant tubes is set to simulate the heat flux from the burner. For different tubes, various heat flux distribution along the length of the tube can be considered. Regardless of the method used to calculate the view factors, there are two consequences:

  1. The number of view factors becomes large, growing quadratically with the number of radiating surfaces.
  2. The subsequent calculation in the analysis program is dependent upon the number of view factors calculated.

It was necessary to develop a special method for radiation simulation considering the number of elements.

Because of the large temperature ranges, it was necessary to consider all the material properties (specific heat, thermal conductivity, density, and emittance) as temperature-dependent. It is similar to thermoelastic analysis. Relevant material properties were measured on the devices at TU Liberec.

Results Show Steady Temperature

Blank Heating Simulation Results. The results shown in Figure 4 show a steady temperature field during operation of the furnace and heating of the blanks. The surface temperature of the product at the beginning of the furnace is also visible. Figure 5 shows the temperature inside the furnace and temperature on products (center and the sides) during the transport of the blanks through the furnace. The deviation in temperature across the furnace is negligible.

Design of Furnace Openings—Input and Output. During the development phase, the most attention was devoted to the design of the input and output opening. The opening concept was designed to eliminate the necessity of placing movable curtains or barriers to reduce maintenance problems, eliminate heat loss, and ensure safety. The tunnel solution was chosen as the best one.

Results of the numerical simulations and tests show that the heat losses through the opening are negligible and reach approximately 650Wm-2. Bodies placed in the vicinity of the openings are not heated to more than 45 degrees C. Figure 6 shows the calculated temperature field in the opening and the distribution of temperature inside the furnace input. The decrease of the temperature inside the input is evident.

Dew Point Regulation System. The dew point is the temperature at which the water vapor in air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates. Basically, the dew point temperature is a function of the content of water vapor inside the furnace atmosphere.

Hydrogen embrittlement occurs when various metals, especially high-strength steel, become brittle and fracture following exposure to hydrogen. Hydrogen embrittlement increases material cracking and often is the result of the unintentional introduction of hydrogen into susceptible metals during heating, forming, and finishing operations. The source of hydrogen is water inside the furnace.

Figure 3
An overview of various approaches to heat transfer for different temperatures between the blank and the furnace shows that radiation is the dominant heat transfer mechanism. Chart courtesy of Benteler Mechanical Engineering.

The chemical reaction during the embrittlement process is described by a basic equation: (see Formula 6)

This equation shows that it is quite important to control the mass of H2O inside the furnace to prevent the risk of hydrogen embrittlement. The risk is high especially for coated blanks. When the hydrogen diffuses along the grain boundaries and combines with the carbon, which is alloyed with the iron inside the steel, it forms methane gas. The methane is collected inside the small voids along the grain boundaries where it builds up enormous pressures that initiate cracks (see Figure 7). This is quite important during the processing of aluminum silicon (AlSi)-coated blanks, because the hydrogen can be easily collected and locked below the coating.

Possible sources of water (hydrogen) inside the furnace causing hydrogen embrittlement are:

  • Endo-Exo gas protective atmosphere (H2, CO, N2, CO2, H2O)
  • Nitrogen + natural gas mixture (CH4, CO, N2, CO2, H2O)
  • Open flame (N2, CO2, H2O, CO)
  • Missing controlled atmosphere (N2, CO2, H2O)

Nowadays most OEMs accept only the parts produced under the controlled atmosphere with a reduced dew point temperature of -10 degrees C and lower.

The influence of hydrogen embrittlement can be decreased by using pure nitrogen gas (N2) for uncoated blanks. This approach may reduce the dew point temperature and protect the surface against oxidation. Alternatively, for blanks with AlSi coating, using dried air (with dew point temperature -40 degrees C) atmosphere (CO2, N2, O2) reduces the dew point temperature and supports the Al-Fe diffusion.

Cognizant of the problems that hydrogen embrittlement causes, researchers developed a system for the dew point regulation that saves energy and is based on the use of dried air or nitrogen (patented in 2011)5. The dew point level is real-time-analyzed

and-regulated to reach the required value permanently.

Notes

1. J.A. Wünning and J.G. Wünning, “Flameless Oxidation to Reduce Thermal NO-Formation,” Progress in Energy Combustion Science, Vol. 23 (1997), pp. 81-94.

2. J.G. Wünning, “Energy Saving Potentials for Gas Fired Industrial Furnaces,” Thermoprocess Symposium 2007, June 13 -15, 2007, Düsseldorf.

Figure 4
Results of FEM analysis and temperature field in the furnace. The temperature distribution inside the blanks at different positions and heating times during the heating process are also simulated. Image courtesy of Benteler Mechanical Engineering and TU Liberec.

3. H.C. Hottel and A.F. Saroffim, Radiative Transfer, McGraw Hill, 1967.

4. M.F. Cohen and D.P. Greenberg, “The hemi-cube: a radiosity solution for complex environments,” ACM SIGGRAPH Computer Graphics, v.19 n.3, p.31-40, July 1985.

5. Patent 10 2011 053 634.5, Benteler Automobiltechnik GmbH, 33102, Paderborn, DE, 15.9.2011.

About the Authors

Jad Tawk

Global Sales Manager, Hot Forming Technology, division automotive, Business Unit Mechanical Engineering