June 2005

Less carbide means fewer cracks in tools made from gas-atomised steel

With non-metallic impurities more or less eliminated from
modern gas-atomised powders, attention has focused on
the role of carbide clusters in crack initiation in tool steels...

The technology for production of high-performance PM tool steels and high-speed steels by gas atomisation, combined with encapsulation and hot isostatic pressing to full density has been subject to extensive development over the last 20 years.

Figure 1. Advanced process for PM tool and high-speed steels.

The driving force for this development has been the elimination of non metallic inclusions (NMI) in the powder to improve tool life by reducing the number of crack-initiating defects.
In particular the introduction of large tundish technology combined with electro slag heating (ESH) prior to gas atomisation resulted in a significant improvement in cleanliness. Today cleanliness levels of PM tool and high-speed steel products produced by such technologies is comparable to that of super alloys produced via double remelt technology (VIM/VAR or VIM/ESR) [1].

As-HIP structures of one PM high-speed steel with different powder particle sizes Figure 2. 45-63 µm Figure 3. 355-500 µm Figure 4. 800-1000 µm

Careful study of the fracture surfaces of samples of super-clean PM tool and high-speed steels subjected to fatigue testing threw up new suspects in crack initiation aside from NMI and surface defects. These were carbide clusters found in a study [2] on PM steels with different cleanliness levels. In previous studies carbide clusters had not been regarded as a problem since their effect was overshadowed by other more frequent sources for crack initiation. Carbide was a factor considered by technical staff evaluating options for a new atomiser at Böhler Uddeholm in Kapfenberg, Austria. The thinking behind their eventual decision was described in a paper published at the PM World Congress in Vienna last year entitled Fine tool steel powders using novel free-fall gas atomisation technology.
The reason for the formation of carbide clusters is believed to be the agglomeration of carbides to the centre of large powder particles during solidification. The following sequence of microstructures (Figures 2, 3, and 4) show the carbide structure in as-HIPped material with different particle size fractions of the powder. As clearly visible in Figure 4 the large particle size fraction (800µm - 1000µm) has considerably larger clusters than the other samples.
One possibility of further enhancing the toughness and fatigue resistance of PM tool steel and high-speed steel is to reduce the proportion of large powder particles. Normally this is done by sieving the atomised powder. This is costly and presents a certain risk of contamination by foreign particles if the sieving is done outside the atomiser.
Another effect of large powder particles is an increased tendency to particle segregation during capsule filling. This is illustrated in Figure 5 which shows how loose powders of two atomised particle size distributions (Dm = 150µm and 60µm) behave when filled into small transparent glass containers. The coarse powder sample always segregates into regions of small and large particles whereas this tendency is not observed in the finer powder sample.
An atomisation process generating a low fraction of large particles is consequently favourable for high performance PM tool and high-speed steel and the reason why Böhler Uddeholm invested in such technology in their new PM plant.

Turning to atomisation theory, the following relationship [3] has proved to be useful for estimation of particle size in a gas atomisation system:
For a certain material (i.e. where s and r can be regarded as constant) the average particle size is basically dependent on the relationship between gas and metal flow rate, the gas velocity and the system efficiency reflected in the system factor K. (The influence of melt viscosity is to our opinion unclear and relatively small in comparison to other factors and thus excluded here). The K-factor can be seen as characterising how efficiently the gas is interacting with and disintegrating the thick delivery stream into a fine powder. Such factors as nozzle design, location of nozzles, gas direction etc are decisive for the system's efficiency.
It is generally accepted that the disintegration of a tapping stream is done in separate steps by primary atomisation into liquid ligaments or clusters which are subsequently broken down in a secondary atomisation step into fine droplets which solidify as spherical particles [4], [5].

Figure. 5. Particle segregation of as-atomised powders

Figure 6. Close coupled system Figure 7. Free fall system.

In order to achieve most efficient disintegration of the tapping stream it is favourable to control the gas-metal
interaction as long as possible in the atomisation zone.
There are a number of different atomisation methods for making fine gas atomised powder. The decising factors in the choice of method are mainly the scale of operation and economic factors that need to be considered to achieve a desired particle size distribution.
Perhaps the most common way to make fine powder is by close-coupled technology. Here the melt is interacting with high pressure (high velocity) gas directly at the exit of the ceramic delivery tube. The short distance between the gas nozzle exit and the melt means that the high kinetic energy in the gas can be effectively transferred to the melt. However the rapid expansion of the gas also means that a large portion of the gas is not directly participating in the disintegration but assists by building up pressure around the atomisation zone which is helpful for the secondary atomisation by drag force interaction at some distance from the primary atomisation region. The higher the gas flow rates, the more efficient is secondary atomisation since the pressure around the atomisation zone increases. One way of improving the efficiency of a close-coupled system could be to add one or more gas nozzles under the primary nozzle, thereby increasing the pressure and consequently the drag force around the secondary atomisation zone.

Fine-tuned balance

Although close-coupled systems are efficient in producing fine powders, the fine-tuned balance between melt and gas conditions at the delivery tube exit make them sensitive to disturbances when atomising large melt quantities. This lack of robustness makes close-coupled systems less attractive for large scale atomisation processes.
In the PM tool and high-speed steel business the heat sizes are generally large (5-8 tons) for commercial efficiency and consequently the atomisation process must run for many hours without interruption. This calls for atomisation processes with a high degree of stability against variation. For this reason so-called free fall atomisation is the preferred method. Here the metal falls freely some distance before being hit by high velocity gas. In order to maintain stability in free fall systems the gas jet distance to the melt stream has to be sufficient to avoid possible instabilities at the nozzle outlet which could lead to metal splats sticking to the jets and consequently stopping the process. Furthermore gas circulation around the jet may result in droplets sticking to the jets if the angles and distances are not properly balanced. This all leads to the necessity of having a relatively large distance between jet and gas-metal impingement point to be able to operate the atomizer repeatedly without interruption. This means that a lot of the energy in the gas is lost. As a consequence, powder size distribution in a common free fall tool steel atomizer is coarse compared to a close-coupled system and generally has a mean size around 130-150 µm with a relatively large portion above 500 µm (>10 per cent). In order to further enhance the efficiency of a free-fall system the distance from gas jet exit to atomisation zone should be reduced.

Figure 8. BUPT system [6]

Another important parameter in a free fall system is the angle between gas and metal stream - the larger angle the better for atomisation efficiency. At some point in traditional systems unfavourable recirculation around the gas jets will prevent stable conditions and restrict the angle.
The Böhler Uddeholm Powder Technology (BUPT) atomiser is operated with an 8 tonne melt size in a three-shift operation. The liquid metal delivery system is based on large tundish technology with an advanced heating system (ESH) which means that any disturbances in the atomiser will affect the whole operation drastically. This means the system must be very reliable. For that reason we chose to install a free fall atomising system which was developed in house.
The basic philosophy has been to arrange the gas nozzles for maximum energy utilisation, that is as close as possible to the melt stream and with as large angle of attack as possible.
This has been made possible by using a three-nozzle arrangement.
The first nozzle serves to divert the unbroken melt stream into a basically flat horizontally extended film of metal. The gas exit from the third nozzle is located downstream and as close as possible to the thinned and partly disintegrated melt. This apparently high-risk location for the gas nozzle is possible due to the introduction of an intermediate gas stream from the second nozzle which serves to protect the third nozzle from being struck by liquid particles emanating from the thinning of the stream in the first step. The gas jet from the third nozzle has high velocity due to the short distance to the melt, resulting in very efficient final disintegration of the thin liquid film after the first disintegration step. An added advantage is that the angle of attack between the first and the third nozzle can be made large in comparison to traditional systems due to the protective action of the second gas stream.

Fig. 9. Particle size distributions

Of course certain measures must be taken in the design of the nozzle arrangement to avoid unfavourable reflow of gas as well as balancing the interaction between the gas streams. A typical particle size distribution is shown in Figure 9. The average particle size is about half that of a traditional large-scale tool steel atomiser. A valuable result of the method is that the fraction of large particles (> 500 µm) in the as atomised powder is low. This fraction is removed by sieving in the atomisation process (no external handling) so the yield of useful powder below 500 µm is high.

Crack-initiating carbide clusters in tool steels can be tackled by using the right approach to atomisation, making for longer tool life and better economics

In summary, say the authors, patent applications covering this free-fall gas atomisation system developed for large scale PM tool and high-speed steel powder production have been submitted in all relevant countries world wide [6]. The three gas jet system controlling the primary and secondary atomisation steps generates a comparatively fine powder with a mean particle size around 60 µm and a small fraction of large particles.
The resulting powder is favourable for fully dense tool material due to low levels of crack-initiating carbide clusters, which results in extended tool life .

[1]. A Fölzer, C Tornberg, Advances in Processing Technology for Powder-Metallurgical
Tool Steels and High Speed Steels Giving Excellent Cleanliness and Homogeneity.
Materials Science Forum Vols. 426 - 432 (2003), pp 4167 - 4172
[2]. S Marsoner, R Ebner, M Panzenböck, W Liebfahrt and F Jeglitsch: Fatigue Behaviour
of High-strength Powder Metallurgically Processed Tool Steels, A. F. Blom (Ed.): Fatigue
2002, Proceedings of the Eighth International Fatigue Congress, Stockholm, Sweden, 2002
[3]. C Tornberg, "Particle size prediction in an atomization system", Advances in Powder
Metallurgy & Particulate Materials, MPIF Dec 1992 Vol. 1, pp 137-150.
[4]. A J Yule & J Dunkley, Atomization of Melts for Powder Production and Spray
Deposition, pp 30-46, Clarendon Press, Oxford 1994.
[5]. S P Mates et al, "An alternative view of close-coupled gas atomization of liquid metals",
Advances in Powder Metallurgy & Particulate Materials - 2002, MPIF, Vol. 3 pp 178 - 187.
[6]. Patent application EP 1022078.

This article was taken from Fine tool steel powders using novel free-fall gas atomisation technology, a paper by Claes Tornberg and Andreas Fölzer of Böhler Uddeholm Powder Technology, Austria.