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Home > Services > SAT12 Surface Hardening Service > Technology
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 SAT12 Technology
The technology behind the service is a patented method of heat treating austenitic stainless steels. In effect, the Swagelok SAT12 surface hardening process enables ordinary stainless steel to adopt certain performance characteristics of expensive alloys, like Hastelloy® or titanium.
The SAT12 service has been performed on finished components without distortion or change of dimension, including components with complex shapes and structures. The technology can be extended to other important alloys besides austenitic stainless steels.
SAT12 treatment diffuses carbon atoms into the austenitic, or face-centered cubic (FCC), crystal structure of the stainless steel. That such a process could occur without compromise to the corrosion resistance of stainless steel runs contrary to knowledge in the field of metallurgy. It is generally understood that one cannot carburize austenitic stainless steels without compromising the steel’s corrosion-resistant qualities. Hardness and corrosion resistance are known to be tradeoffs.
Typically, when carbon enters the crystal structure, chromium moves in from surrounding areas to form chromium carbides. The result is increased hardness, but the downside is that chromium has been depleted in the surrounding areas and these areas are now less resistant to corrosive attack. Stainless steels are corrosion resistant because a passive oxide layer forms on the surface when it is first exposed to the atmosphere. This layer, which is a chromium-rich oxide about 20Å to 60Å thick, acts as a barrier, slowing down the corrosion rate. However, if this layer is compromised, it loses its corrosion-resistant effect. Stainless steel is still mostly iron, and it will rust without the passive oxide layer.
The SAT12 treatment creates a set of conditions that enables unit cells to draw in carbon atoms without forming chromium carbides. First, the stainless steel is placed in an atmosphere of carbon-assisted gaseous HCl, which removes the passive oxide layer. If this layer is not removed, the carbon diffusion into the surface will be blocked. Then, the stainless steel is heated in a mixture of gases containing carbon at a low temperature for several hours. Carburization begins to occur at a temperature high enough to promote carbon atom diffusion but low enough to prevent carbide formation. Additional heating procedures follow. The result is extremely high or colossal carbon supersaturation. At the end of the process, the amount of carbon in the interstices of the alloy lattice structure has been elevated in excess of 12 atomic percent. In other words, carbon levels are approximately 80,000 times higher than the concentration that thermodynamics and kinetics suggest would be possible. In terms of hardness, the carbon behaves like a carbide, but it is not one. The carbon atom locates in the interstitial space of the unit cell, expanding the crystal lattice and increasing its hardness.
The hardness is limited to a conformal surface layer at least 25µm thick. Close to the surface, a hardness of ≈HV1200 has been measured – comparable to 70 Rockwell C, which describes hardness in the range of tool steel, and about four to five times the hardness for bulk 316L stainless steel. The thickness of the hardened layer can be increased further by additional carburization treatments. Hardness depths of 10µm to 50µm have been routinely achieved.
"Arguably, the SAT12 process is
one of the most significant
breakthroughs in materials
science in many decades, as well
as one with many extraordinary
technological implications."
Arthur Heuer, Ph.D., University Professor
and Kyocera Professor of Materials Science
and Engineering at Case Western University.
More Information About SAT12
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