Using natural gas to make safer automotive parts At the Volkswagen factory in Kassel, high-strength chassis parts are being manufactured – thanks to natural gas.
Natural gas and vehicles – it's a good combination for more than just motors: Natural gas also plays an important role in automobile production. Volkswagen uses the energy source in the hot working of steel plates that are later used to form the car's passenger compartment. Gas-powered furnaces heat the plates before they are formed in the press and then cooled. Once cooled, the material is twice as hard as before being heated. Thanks to the extreme rigidity of the ultra high-strength steel, the total weight of the chassis of the Golf VII, for instance, could be reduced by some 23 kilograms. The result: Reduced material costs and use of resources, as well as lower fuel consumption and CO2 emissions. All of this with the same protection for the car's passengers. Every day, 60,000 hot worked parts leave the second-largest German VW plant near Kassel.
Each unit has its own furnace
It's hot and loud in the huge hall. Massive machines carry out their work behind protective screens. The eleven hot working systems cover a 250 meter long and 60 meter wide area. Each of the units has its own furnace with several dozen gas burners and one press.
The system follows a specified program, forming either B-pillars, sills or other components of the passenger compartment out of the steel sheets. The entire process is largely automated and requires only a few personnel. Among other things, their tasks include transporting parts with the forklift or performing quality checks.
1000 tons of sheet steel each day
The steel is rolled onto coils weighing 30 tons, with each strand between just 1 to 2.5 millimeter thick. "We process some 1000 tons of sheet steel every day, more than 30 of these coils", says Carl Steffen Maurer, who is responsible for hot working. The machine cuts blanks, or plates, from these sheets. A robot takes a blank from the rack and puts it on transport rolls that lead into the furnace chamber.
The glow of the fire at the furnace inlet is deceptive: Inside, there are no open flames, only gas flames burning in ceramic pipes. "That is enough to set the entire ceramic-clad chamber aglow with the heat," explains Maurer.
From room temperature to 950 degrees
While the blank is being transported slowly but continuously through the 25 meter long chamber, it is heated to 950 degrees Celsius. By the time it gets to the press, it has cooled by 250 to 300 degrees. But the heat it radiates can still be felt several meters away. A robot uses clamps to pick up the hot metal and lays it on the open press. The sheet is set on pins to keep it in position. The hydraulic press then lowers and the floor of the hall literally starts to shake.
The forming tools that shape the metal are water-cooled and reduce the temperature of the plate to approx. 150 degrees. Simultaneous pressing and cooling takes exactly 5.5 seconds and changes the microstructure of the steel: The material is now twice as hard as before. It has also changed on the outside – its surface is no longer shiny but dark and matt. The metal is now transported on a rack by another robot, the hot working is complete. The edges of the piece are later refinished and holes cut out for assembly by a special laser cutting machine. The steel is now too hard for regular mechanical metal cutting instruments.
1. The steel sheet is 1 millimeter to 2.5 millimeters thick and is rolled in the beginning.
2. Cut and stacked: blanks before being processed in the hot forming line.
3. A robot removes blanks from a stand and places them onto the unit's rollers.
4. The blanks begin to glow as soon as they enter the 25-meter-long furnace chamber.
5. The red-hot sheets leave the furnace at a temperature of more than 600 degrees Celsius.
6. The sheets travel from the furnace on ceramic-coated rollers, ready for the press.
7. In the fully automated laser-cutting machine, the contours of the pressed parts are finished and the holes for the assembly line work cut out.
VW has used the technology since 2004, and was once a pioneer among automobile manufacturers. The plant's capacity has expanded to include eleven hot working lines since then. By way of comparison: The VW head office in Wolfsburg has two. Today, no automobile leaves the Volkswagen production line without hot worked components.
They form the cage-like passenger compartment. Stability is everything: In order to give passengers the greatest possible protection in the event of an accident, the material may not break or bend. "Ultra high-strength steel has proven to be an ideal material", states Carl Steffen Maurer.
Facts / Figures:
Founded in 1957, the VW plant in Kassel now has a workforce of more than 16,000 employees.
The company uses 41 million kilowatt hours of natural gas each year for hot working processes. Approx. 3.75 million kilowatt hours are needed per system.
The location as a whole uses 285 million kilowatt hours of natural gas each year.
"Natural gas is more economical and climate-friendly" Interview with Carl Steffen Maurer, technical specialist production/hot working, and Sebastian Barden, energy officer from the forming technology division
Hot working is very energy-intensive. Why the effort?
Carl Steffen Maurer: The thermal input is indeed immense. However, since the properties of steel are enhanced through this process – that is to say, it becomes harder – we can reduce material usage by 30%. That lowers our production costs; in addition, we can build the passenger compartment to be substantially lighter. In vehicle manufacturing, every kilogram matters. The lighter a vehicle is, the less fuel it will consume. Over time, that pays off – in terms of the environment and climate too.
Why did Volkswagen opt for natural-gas powered furnaces?
Sebastian Barden: As a primary energy source, natural gas is more economical and climate-friendly. If we used electricity to generate heat, we would need additional conversion processes, and that always leads to losses. At the same time, we would emit three times the amount of CO2-equivalents. And there are practical reasons for not using oil – the technology would be difficult to integrate in the production area.
What happens to the waste heat from the systems?
Sebastian Barden: Some of the heat is used to preheat the furnace through heat exchangers. The rest contributes to the heating unit and is used to produce electricity using a reciprocating engine.