Handling and Segregation of a High Alloyed and Nitrided Tool Steel Powder

Abstract: The demand on powder metallurgical steel is increasing due to their excellent mechanical properties, to obtain these properties the metal powder fractions need to be evenly distributed through out the body. This report presents Uddeholms AB:s high alloyed and nitrided tool steel powder STEEL-X’s powder fraction distribution through out the process and recommendations how to improve the powder fraction distribution in the future.
From the manufacturing of powder to the point when the powder is to be HIPed, the powder undergoes several handling steps (including, inter alia, powder vessel change, nitriding process and transportation), which promotes a change in the powder fraction distribution. This change in powder fraction distribution is called segregation. There are different types of segregation, for example the most common type; sieving segregation. Sieving segregation can occur when the bulk is subjected to for example vibrations (e.g. transportation).
To investigate how the powder fraction distribution changes, powder samples have been taken, with two different sampling probes, from the bulk in its container. The samples have been analyzed by sieving, and the results have been plotted in histograms. To investigate how the powder acts during filling and discharge, experiments on small replicas of the powder vessels have been performed and a novel filling technique have been performed. Experiments with small scale mixers have been performed to investigate if a powder mixer can improve the powder fraction distribution, varying the parameters speed and time in order.
The results show that there is a variation in the powder fraction distribution, mainly in the smaller fractions, in the powder vessels. The variation in the vessels also differentiate depending on which sampling probe that have been used. Furthermore, the powder fraction distribution is good in the powder vessels. The powder filling experiments showed that the current filling technique is destructive for the powder fraction distribution, whilst the novel filling technique shows promising results with regard to minimizing segregation during filling. If the right mixing parameters are used, mixing showed to have a positive effect on the powder fraction distribution. However, gas bubbling may be sufficient to obtain a good powder fraction distribution, but more research needs to be performed.
To acquire an ideal powder fraction distribution in a HIP-capsule more research needs to be made, especially about the filling process. To really obtain a statistical foundation how the powder fraction distributes in the bulk, samples from several different places needs to be taken.