Swagelok Co., well known for its products in fluid systems, is now commercializing an interesting process for treating the surface of austenitic stainless steel that makes the material act like much more expensive alloys.
According to the first patent issued for the development, the surface of the metal is activated and is then carburized at a temperature that is low enough that carbides are not formed. In effect, the enrichment of carbon on the surface makes the stainless harder. This is generally done by a process called case hardening, which is done at higher temperatures that promote formation of carbides. The beauty of the new Swagelok process, called SAT12, is that corrosion resistance of the stainless is not compromised by the hardening process.
“Hardness and corrosion resistance are known to be tradeoffs,” says Swagelok. “Typically, when carbon enters the crystal structure, chromium moves in from surrounding areas to form chromium carbides.” This reduces corrosion resistance, the reason stainless is often used. Meanwhile, the hardness for a thin layer around a treated part is about four to five times greater than the hardness for bulk 316L.
Hardness can be increased through additional carburization treatments. Hardness depths of 10 µm to 50 µm have been reached.
According to Swagelok, the process “enables ordinary stainless steel to adopt certain performance characteristics of expensive alloys like Hastelloy or titanium.”
Swagelok, based in Solon, OH, is not a steel producer, but has done extensive research on metal treatment. In fact, its partner for the development and commercialization of the process is the Case Western Reserve University Center for Surface Analysis and Materials, just renamed the Swagelok Center. Swagelok and CWRU received a $5.5 million grant from the State of Ohio to develop the new surface hardening process.
Swagelok has been using the process internally since 2000 to improve the operation of the rear ferrule on a tube fitting. SAT12 makes the ferrule harder without compromising its corrosion resistance. Earlier this year, Swagelok Technical Services Co. was formed to commercialize SAT12. The company is considering licensing, but a specific go-to-market plan is still under development.
Potential markets are commercial pumps and circulation systems; fasteners; bearings, clutch plates and other industrial parts subject to heavy wear; military applications; aerospace, automotive; and medical devices, such as hip implants.
Key Properties of SAT12-Treated Stainless
Hardness: In 316L, SAT12 results in a surface hardness that is between three and four times harder than the core of the material. The Vickers hardness at the surface typically exceeds 1000 HV, as compared to 300 HV at the core. Other alloys, including superaustenitic grade, duplex grade, ferritic grade, martensitic grade, Ni-base alloys and Ni-Co-Cr alloys, have been treated with SAT12 as well, with case hardening between two and six times that of the core.
Corrosion resistance: In tests for Electrochemical Pitting Potential (Ep) in neutral chloride solution, treated 316L specimens averaged 945 mV, compared with 344 mV for untreated material. In polarized crevice corrosion tests in neutral chloride solution at room temperature, treated specimens showed no damage after one week, whereas untreated specimens showed damage within hours. (There has been no testing at elevated temperatures.) Treated 316L in these tests showed crevice-corrosion behavior comparable to that of aerospace titanium-base alloys or Hastelloy C-22, a nickel-base alloy.
Fatigue Resistance: In fatigue tests, 316 stainless steel treated with SAT12 showed an order of magnitude improvement over untreated specimens at the same maximum stress level in totally reversed fatigue cycle testing (R = -1). Maximum design stress for infinite life improved by approximately 50 percent, from 30 ksi to 45 ksi. SAT12 reduced surface imperfections from becoming sources of fatigue failure. Indeed, the onset of fatigue cracks in the SAT12-treated specimen occurred beneath the surface.
Wear Resistance: In pin-on-disk ball tests, SAT12 significantly mitigated galling and the rates of wear. Treated specimens of 316 stainless steel reduced wear by about 100 times, as compared to untreated specimens.
Erosion Resistance: Cavitation tests up to six hours in duration, with a vibratory horn and mercury as the dense liquid medium, showed that treatment of 316 stainless steel specimens reduced weight loss by 5.5 times.
The front and back ferrules of Swagelok’s tube fitting are shown magnified in profile. The company has used the SAT12 process to case harden the back ferrule (shaded in dark blue) since 2000.
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