For metallurgists, heat treatment is typically thought to be a well-understood practice. As a common process, it is used in both industrial and academic settings to change aspects of final components such as the metallic structure, stress state, and the sizes and compositions of precipitates. Because heat treatment is commonly used, it is quite intriguing when we stumble upon something new or surprising about this process.
Low Temperatures Produce Big Changes
My research group at Carnegie Mellon University (CMU) recently found that heat treatment can be leveraged to control inclusions in stainless steel. In addition to an intentional change in the metallic structure, we found that heat treatment also creates an unintentional, substantial change in the oxide inclusions. The inclusions first form by the oxygen-scavenging role of manganese and silicon, which are alloyed into the steel.
We found that heat treatment at fairly low temperatures (around 1100°C) then changes the chemical composition and shape of the inclusions. This happens because chromium in the steel internally reacts with the inclusions, changing them from round manganese-silicate to angular chromium-galaxite. Although these changing inclusions are a small fraction (a few tens of parts per million) of the total material, they are enough to change the mechanical behavior of the material substantially. It is known that oxide inclusions affect how readily stainless steel can be polished. An attractive surface finish is often the reason why stainless steel is chosen, so anything that might change the polishing response should be studied carefully.
Oxide inclusions are often assumed to be chemically stable during steel processing. In carbon and low-alloyed steels, however, oxide inclusions change in size, shape and chemistry while the steel is liquid. In stainless steels, we have now found such changes in the solid state during a heat treatment that is used routinely when reheating the steel before hot rolling. Similar changes are likely in other highly alloyed steels.
A Chance Observation
This research stemmed from a chance observation in a project designed to explore whether or not there are inclusion-based reactions in stainless steel. My lab at CMU, along with visiting researcher Ying Ren from the University of Science and Technology Beijing of China, was looking at reactions in liquid steel. Ren found that the inclusions changed in shape upon heat treating the steel (see an example of this in Figure 1). Because of this observation, my group decided to pursue this research further.
Full findings of this research can be found in a forthcoming paper in Metallurgical and Materials Transactions B (“Transformation of oxide inclusions in Type 304 stainless steels during heat treatment;” DOI: 10.1007/s11663-017-1007-8).
For an example of how inclusions change during industrial processing of liquid steel, see a paper in the AISTech 2017 Proceedings (pp. 2,693-2,706) (“Application of Kinetic Model for Industrial Scale Ladle Refining Process” by Deepoo Kumar, Kevin Ahlborg and Chris Pistorius). For more information on Pistorius’ research, visit his website or view a short video at www.industrialheating.com/Pistorius.
ASTM Standard A480: Standard Specification for General Requirements for Flat-Rolled Stainless and Heat-Resisting Steel Plate, Sheet, and Strip. ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428