Spray-formed steel is steel manufactured using a unique new process technology called the Osprey process. The steel is melted in a crucible using two induction furnaces and is then atomized in a spray chamber under a protected atmosphere.

A specially designed spray head is used to deposit the semi-liquid steel onto a short cylindrical former (see Figure 1), which is simultaneously rotated and slowly moved in a direction parallel to the axis of the former. By carefully controlling the movements of the former and the spray parameters, it is possible today to manufacture cylindrical billets up to 550 mm in diameter and 2500 mm in length. The billets can afterwards be forged in order to obtain the dimensions needed.

The advantage of spray forming is the high solidification speed of the liquid steel. This allows for a production of highly alloyed materials, which until today only have been possible with the powder metallurgy (PM) process. When using the spray forming process, the size of the wear-resistant carbides can be controlled in order to increase the wear resistance compared to a PM steel.

The photos shown in Figure 2 clearly illustrate the difference in the microstructure when traditionally cast steel (Figure 2a), spray-formed steel (Figure 2b) and PM materials (Figure 2c) are investigated under a microscope.

The carbides in the traditionally cast steel are clustered in large strings and the sizes can be up to 100 µm in length. This can be a main reason for brittleness and this steel is often hardened to lower hardness in order not to lose too much ductility. The carbides in the traditionally cast steel are mainly formed by chromium, which has a hardness of approximately 1500 HV.

When spray forming steel or producing PM steel, the fast solidification of the steel enables the process to form very hard MC carbides, mainly based on vanadium, with hardness up to 2,950 HV. Furthermore, the spherical form of the carbides and the homogeneous distribution results in a more ductile material—giving the heat treatment shop the possibility of hardening the steel even more without losing the ductility. Carbide sizes are typically around 6 µm in spray-formed tool steels and below 3 µm in PM steel.

The main difference between spray-formed steel and PM steel is found in the number of process steps needed before the steel can be used. Using spray forming, the billets are produced from liquid metal in one step, whereas the PM billet has been through five to six expensive steps before hot forging can take place (see Figure 3). This results in spray-formed steel being less expensive than PM steel.

All this has been used in the development of two new steel alloys, which are to be used in high wear applications. One contains 8.9 percent Vanadium and is one of the most wear-resistant steel grades ever made. The other contains a little less Vanadium, 7.2 percent, but has a toughness of three to four times higher than the first.