March 2004

The Hagen Symposium

Spray forming bypasses PM for tool steels

High quality tool-steel billets are the result when steel powders are compacted by spraying and then forged to eliminate porosity…

The benefits of tool steels produced by the PM route compared with conventional ingot casting followed by forging are well known. PM produces finer grain size and more homogeneous microstructures with less segregation of the carbide, and has the ability to increase carbide content in order to improve wear resistance.

However, whilst the PM route requires some 10 process steps and careful handling of the tool steel powder to avoid introducing oxidation or contaminants, the spray forming route bypasses powder handling and powder consolidation in order to achieve essentially the same good properties as PM technology, but with only half the process steps. This led Dan Spray A/S to install an Osprey Metals spray forming plant in Taastrup, Denmark, in 1999 for the production of new commercial grades of tool steels.

Dr Ralf Piotrowiak of Edelstahl Witten-Krefeld GmbH, Germany, a partner to Dan Spray, reported on some of the benefits of the spray formed products at the Hagen PM Symposium. He said that tool steel feedstock is melted in a four-tonne induction furnace with the melt poured into a casting furnace which bottom feeds into the atomising unit. Here a nitrogen gas stream atomises the melt into droplets of 50µm to 500µm diameter, which are then accelerated from the two oscillating atomisation nozzles onto a rotating target allowing uniform compaction of the droplets and homogeneous growth of the billet. A properly adjusted downward movement of the growing billet allows for a permanently constant distance between the atomizing unit and the billet during spray forming. The final billet dimensions are a maximum of 500 mm diameter and 2.5 metres in length with a weight of approximately four tonnes.

The billets are shipped to Edelstahl Witten-Krefeld where they are forged to eliminate any residual porosity using the world's largest forging machine, heat treated, machined and inspected. He outlined some of the properties and applications of the new grades of spray formed tool steel billets such as ESP23 and ESP32. ESP23, he said, is a cold-work spray-formed tool steel with 23-26 per cent carbide content giving hardness of 64HRC. ESP32, which is based on a high-speed steel composition, has a carbide content of 28 per cent and hardness approaching 70HRC.

The microstructure of the spray-formed cold-work steel ESP 23 shows an average carbide size of 8 - 14µm near the surface and 25 - 30µm in the centre of the billet indicating a rather homogeneous distribution of the carbides. Oxygen content in the cold work tool steel was approximately 30 ppm. The cold-work tool steel showed outstanding performance in applications such as fine blanking where tool life was improved by a factor of seven.

Fig.1 Spray forming plant used for producing tool steels at Dan Spray A/S in Denmark. 1. Induction furnace 2. Casting furnace 3. Spray chamber 4. Heat treatment furnace 5. Spray formed billet