Study of non-metallic inclusions in Alloy 825
Steel is a vital component in today’s society. Without it it’s impossible for the current society to work. Of course this places demands of cheap and high quality steel on the steel producers. It is with this in mind that Hans Kellner Högskolan Dalarna performed this study on non-metallic inclusions (NMI) in Alloy 825 in cooperation with Sandvik Materials Technology AB and Kungliga Tekniska Högskolan.
There are many parameters that affect the quality of produced steels and alloys. One of them is amount of non-metallic inclusions, their size, morphology and composition. Large size inclusions (> 11µm) are the most common causes for a crack and fracture formation at high temperatures and low stresses for superalloys, to where Alloy 825 belongs. Small size inclusions (< 10 µm) do not influence the cracking and fracturing in these steel grades. However, small size inclusions affect the grain size to a larger extent. Therefore, it is important to be able to predict and control the size and number of non-metallic inclusions during production.
This study is therefore concentrated on the ladle treatment of Alloy 825 and the NMI’s there. What types they are, where they come from and how they behave during stirring.
For this several trial have been performed during the production of Alloy 825 at Sandvik as well as lab-scale trials for FeTi70R at KTH.
An example of how the ladle may look like during stirring is seen in the image below.
Currently the existing large size inclusions during the ladle treatment and their origin have been found. There were five different types with two spherical and three clusters. Inclusions were from the slag, titanium nitrides, spinel and from FeTi70R. The behavior of existing inclusions in FeTi70R alloy when they enter the melt has also been investigated. Further investigations with both 2D and 3D analysis have been done for spinel clusters and TiN clusters regarding the stirrings influence on agglomeration, number density, morphology and size. The image below show an example of a TiN cluster observed during this investigation.