Red-hot steel meets high-precision cooling

Text: Thomas Masuch; Photos: Zikomm / Th. Masuch, SMS Group - 31/08/2018

At temperatures of 1,250 degrees Celsius, the red-hot steel in forges places extreme demands on employees and materials alike. That’s why cooling the dies is a key factor in the production process. At the forge company Hammerwerk Fridingen a plastic spray head, manufactured using additive techniques, has been tested for the first time. And even after several months, it is still standing up to the infernal conditions.

Entering Workshop 7 at the Hammerwerk Fridingen is like stepping into a bygone industrial age. The air reeks of steel, and the presses, each weighing several thousand tons, rock the surroundings with each deafening stroke. Rugged, powerful men grip the red-hot steel parts with long iron tongs hanging on chains, heaving them skillfully into the machines or from one die into the next. The steel hammers and presses tirelessly forge the glowing metal into universal joints and pistons that weigh several kilograms and will later be used in trucks.

The earplugs and steel toe-capped boots that visitors like us have to wear are certainly needed here. There could hardly be a greater contrast than between these surroundings and a sterile, clean car plant or even an additive manufacturing facility, where the environment is spotless, and conversations can be held in a whisper.

»The 3D printed spray head looked great, but we wondered if it would last.«

survived the hellish conditions

And yet additive manufacturing is also venturing into this tough, power-packed, and unforgiving world: or to be more precise, into the belly of a 5.5-meter-high Manzoni SR 2000 hot forging press, which effortlessly crushes glowing steel sections with its regular, 2,000- ton strokes into cross-shaped automotive components. The press contains a spray head created using additive manufacturing techniques, which repeatedly sprays a mist of saline solution onto the dies between strokes. This not only cools the dies, but also ensures that the red-hot blocks, which have a temperature of 1,250 degrees Celsius, are easily released from the mold.

The 3D printed spray head has already withstood 200,000 strokes. »We really didn‘t expect this at the outset,« explains Axel Roßbach. The engineer from the SMS Group developed the spray head in cooperation with Hammerwerk Fridingen and tailored it to local requirements. Both project partners already rate the fact that the laser-sintered polyamide spray head has survived these hellish conditions for months as a great success. According to Roßbach, installing and connecting the head also went off smoothly and without significant additional effort. In the 3D printed version the cooling

In the 3D printed version the cooling medium (a mixture of various salts and water) is fed to the desired points via curved pathways. At the outlet, the coolant is converted into a fine aerosol by compressed air and sprayed onto the hot dies. An additional control air line for switching the coolant on and off, directly at each nozzle, was rapidly installed. »We can optimally adapt the droplet size and distribution to the conditions,« says Roßbach. Quite apart from this, the spray head was also nominated for the German Design Award 2018, no doubt thanks to its eye-catching contours.

Usually, the SMS Group supplies die presses for forging companies such as the hammer mill or builds entire production plants. Individual additive components are visible only on closer inspection, but the group nevertheless sees the technology as playing an important role. SMS has set up its own internal additive manufacturing group, which is looking for ways to achieve optimization through additive manufacturing across the entire organization. »We want to be an innovative project partner for our customers and, of course, also demonstrate this through the use of state-of-the-art technologies,« says Roßbach, who worked in the drop forging industry for several years and thus has close ties to the sector.

»a foam gun«

However, deployment of the additive spray head in Hammerwerk Fridingen got off to anything but a flying start. »The 3D printed spray head looked great, but we wondered if it would last,« says Dr. Marco Laufer, Technical Plant Manager, Metal Forming Technology. The first model did not hold up, confirming Laufer’s initial skepticism. The wall thicknesses were too small, causing the cooling medium to penetrate them, was Roßbach’s subsequent diagnosis. And Laufer, a man who doesn’t mince his words, remembers »a foam gun«.

Roßbach and his team optimized the spray head, and it has now been in service reliably for four months – in close conjunction with a conventional stainless-steel spray head. The 3D printed variant weighs considerably less, lightening the load on the underlying mechanical components. This is a benefit that cannot be quantified directly but, according to plant manager Laufer, it will certainly become apparent during the next maintenance.

THE FINE ART OF SPRAYING

The hammer mill was founded in 1953 in Fridingen, where the upper Danube is still a small river that loops gently through the fircovered valleys. With 458 employees and sales of 83.5 million euros, it is one of the larger drop forges in Germany. 38,000 tons of steel are processed here annually into around six million parts, which are subsequently used in applications in areas including the automotive industry, mechanical engineering, or agricultural technology.

The plant also has its own tool shop and CNC production facility, where forged parts are processed further. But it is the eight spindle and eccentric presses that are the heart of the company. And their production output depends to a great extent on cooling and spraying the dies correctly. »It’s important that the die is cooled down quickly and effectively,« explains Dr. Laufer.

At the same time, the spray mist should cover the entire die without any liquid remaining in the mold. »Otherwise the red-hot component will literally shoot upwards through explosively evaporating water.« If cooling is inadequate, on the other hand, the strain on the dies increases. »Thermal cracks form. This can reduce service life by up to a quarter,« says Kai Allweil, who was closely involved in the development process as a foreman at the forging shop.

no one-size-fits-all solution

The importance of spraying the dies correctly is also illustrated by the fact that »Industrieverband Massivumformung«, the German industry association for massive forming, is pursuing its own research project with Technische Universität Darmstadt. (Mentor group »Spray Cooling« IMU 58). The initiative investigates the effects of spraying on hot surfaces and how the temperature of the dies can best be reduced. »There’s no one-size-fits-all solution for good spraying. Often the right spraying process also depends on the die and the component,« explains Laufer.

To further improve the procedure, the spray heads would have to be individually adapted to the specific molds. This is something that Dr. Laufer, Allweil, and Roßbach, in their respective roles as plant manager, foreman, and engineer, agree on. »Based on the current additive-manufactured spray heads, we can now go a step further and create additional added value for the forging process,« says Roßbach. Allweil also has specific added value in mind: »Additive manufacturing is so flexible that we may also be able to develop three-dimensional spray heads with outlets in different directions.«

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