Process Simplifies Bright Age-Hardening for Stainless Steels

Oct. 26, 2011
Annealing improves ductility, but maintaining the valuable finish of stainless alloys takes some extra effort
Solar Amospheres uses a 10-bar vacuum furnace at its commercial heat-treating operation in Western Pennsylvania to anneal stainless products without damaging the bright finish, in a simple, closed-loop process. The same plant is the site of a 20-bar horizontal vacuum furnace (shown here), installed earlier this year. The 40-in. wide, 50-in. deep, 36-in. high, 3,500-lb load chamber was custom-designed and built by affiliate Solar Manufacturing. The furnace is able to increase core hardnesses for large cross-sections (e.g., gears), and the commercial heat-treater explained that the furnace’s excessive pressure and high-speed gas velocities simulate oil quenching. But, using inert gas as an alternative to oil minimizes distortion and is a cleaner and greener process.

Heat-treating demands different techniques and capabilities that are determined according to the alloy of a product and the demands of the application it is designed to serve. Annealing is a way to improve an alloy’s ductility by relieving internal stresses and creating a more homogenous grain structure. But, for some stainless steel grades, the heating and cooling requirements for annealing can be tricky. If the temperature changes are not managed properly, the smooth, bright finish that is one of the virtues of stainless steel may be ruined. With the high material and design costs involved with such products — valves, gears, couplings, even some medical instruments and utensils, just to list some of the castings that may be affected this way — there is an opening for a new approach.

Commercial vacuum heat-treater Solar Atmospheres developed an advanced gas-cooling technology that it introduced recently, which makes it possible to offer a two-step heat-treating process to anneal and bright age-harden various precipitation hardenable (PH) grades of stainless steel in a vacuum furnace without breaking the vacuum.

PH stainless steels 17-4 and 15-5 must be cooled to below 80°F from the annealing temperature for complete martensitic transformation prior to age hardening. In a similar way, PH stainless steels 13-8 Mo and 17-7 and 15-7 Mo Condition TH 1050 are required to be cooled from the annealing temperature to below 60°F relatively quickly prior to aging. Once PH stainless steel has been exposed to an air environment after annealing, it is especially difficult to preserve its “brightness,” or to stop oxidization from creating a gold/blue shade during age hardening.

Solar Atmospheres reports its gas cooling technology makes it possible to anneal PH stainless steel products, cool them to below 60°F, and age harden them, all under vacuum conditions in a single heat-treat batch in a vacuum furnace. It adds that the results are “consistently clean, bright, non-discolored parts that meet specification processing and material property requirements.”

“Part of our mission at Solar Atmospheres is to continually look forward in the area of technology,” explained corporate v.p. Roger Jones. “This innovative development underscores our commitment to continually raising the bar to provide stateof- the-art services. Our annealing customers can rely on consistently clean, bright, non-discolored parts that meet specification processing and material property requirements.”

As explained by a source, the closed-loop process requires only a vacuum furnace, a vacuum chamber, and a gas-cooling system. It starts with solution annealing or austenite destabilizing (depending on the alloy) at elevated temperatures. This is followed by an inert gas, positive-pressure quench to the furnace ambient temperature of 80°F or 60°F (again, according to the alloy.)

The next step is to stabilize the work at the requisite ambient temperature. Following this, the work is heated to an aging temperature and held there for the period necessary to complete the precipitation hardening process, also determined by the alloy type and condition. Solar Atmospheres uses state-of-the-art microprocessors for cycle control, optimization, and repeatability.

The process is being done in a 10-bar vacuum furnace at Solar Atmosphere’s Hermitage, PA, plant, near Pittsburgh. The group has plants in Souderton, PA (near Philadelphia) and Fontana, CA, too, where it conducts brazing, carburizing, and nitriding in more than 50 furnaces ranging from lab size to the world’s commercial largest vacuum heat-treating furnace.

About the Author

Robert Brooks | Content Director

Robert Brooks has been a business-to-business reporter, writer, editor, and columnist for more than 20 years, specializing in the primary metal and basic manufacturing industries. His work has covered a wide range of topics, including process technology, resource development, material selection, product design, workforce development, and industrial market strategies, among others. Currently, he specializes in subjects related to metal component and product design, development, and manufacturing — including castings, forgings, machined parts, and fabrications.

Brooks is a graduate of Kenyon College (B.A. English, Political Science) and Emory University (M.A. English.)