In the world of precision metal parts, aesthetics can be everything. For example, metalworking professionals expect precipitation hardenable stainless steel to look bright silver. “When they hear ‘stainless steel’ and then see something other than silver, the immediate question in their minds is ‘is this really stainless steel?’ ” said Don Jordan, vice president of R&D and corporate metallurgist for Solar Atmospheres, which specializes in vacuum heat treating.
The process to anneal and bright age-harden PH stainless steels in a vacuum furnace typically requires the heat treater to cool the metal below the ambient temperature after annealing by placing it in a refrigerator before the age-hardening step, Jordan explained. That requires exposing stainless to air, which causes it to discolor—oxidize—to a golden blue hue during age hardening. “You’re breaking the vacuum,” he said, adding that the discoloration doesn’t negatively impact material properties but people interpret that it does.
Courtesy of Solar Atmospheres
Solar Atmospheres’ heat-treat process to anneal and bright age-harden PH stainless steels avoids discoloring the metal.
To prevent oxidization, Solar Atmospheres developed a two-step process using advanced gas-cooling technology to anneal and bright age-harden PH stainless in a vacuum furnace without breaking vacuum. The metal is heated to 1,900° F, cooled to 80° F for 17-4 and 15-5 stainless and below 60° F for the 17-7 and 15-7 Mo alloys, and then age-hardened at 900° F or higher depending on the desired end condition, which establishes the mechanical properties. Jordan noted that the typical hardness for 17-4 is 44 to 47 HRC for the H900 condition.
Before Solar Atmospheres developed this process, if the hardened stainless parts could not look blue, an end user would have to remove the oxide layer chemically or mechanically, such as with light glass beading or tumbling.
For more information about Solar Atmospheres, Souderton, Pa., call (215) 721-1502 or visit www. solaratm.com.
Related Glossary Terms
- age hardening
age hardening
Hardening of a heat-treated material that occurs slowly at room temperature and more rapidly at higher temperatures. Usually follows rapid cooling or cold working.
- alloys
alloys
Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.
- annealing
annealing
Softening a metal by heating it to and holding it at a controlled temperature, then cooling it at a controlled rate. Also performed to produce simultaneously desired changes in other properties or in the microstructure. The purposes of such changes include improvement of machinability, facilitation of cold work, improvement of mechanical or electrical properties and increase in stability of dimensions. Types of annealing include blue, black, box, bright, full, intermediate, isothermal, quench and recrystallization.
- hardening
hardening
Process of increasing the surface hardness of a part. It is accomplished by heating a piece of steel to a temperature within or above its critical range and then cooling (or quenching) it rapidly. In any heat-treatment operation, the rate of heating is important. Heat flows from the exterior to the interior of steel at a definite rate. If the steel is heated too quickly, the outside becomes hotter than the inside and the desired uniform structure cannot be obtained. If a piece is irregular in shape, a slow heating rate is essential to prevent warping and cracking. The heavier the section, the longer the heating time must be to achieve uniform results. Even after the correct temperature has been reached, the piece should be held at the temperature for a sufficient period of time to permit its thickest section to attain a uniform temperature. See workhardening.
- hardness
hardness
Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.
- mechanical properties
mechanical properties
Properties of a material that reveal its elastic and inelastic behavior when force is applied, thereby indicating its suitability for mechanical applications; for example, modulus of elasticity, tensile strength, elongation, hardness and fatigue limit.
- metalworking
metalworking
Any manufacturing process in which metal is processed or machined such that the workpiece is given a new shape. Broadly defined, the term includes processes such as design and layout, heat-treating, material handling and inspection.
- stainless steels
stainless steels
Stainless steels possess high strength, heat resistance, excellent workability and erosion resistance. Four general classes have been developed to cover a range of mechanical and physical properties for particular applications. The four classes are: the austenitic types of the chromium-nickel-manganese 200 series and the chromium-nickel 300 series; the martensitic types of the chromium, hardenable 400 series; the chromium, nonhardenable 400-series ferritic types; and the precipitation-hardening type of chromium-nickel alloys with additional elements that are hardenable by solution treating and aging.
Author
Alan holds a bachelor’s degree in journalism from Southern Illinois University Carbondale. Including his 20 years at CTE, Alan has more than 30 years of trade journalism experience.
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