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Solution and age: Nickel alloys

Solution treatment is the heating of an alloy to a suitable temperature, holding it at that temperature long enough to cause one or more constituents to enter into a solid solution and then cooling it rapidly enough to hold these constituents in solution. Subsequent precipitation heat treatments allow controlled release of these constituents either naturally (at room temperature) or artificially (at higher temperatures).

There are a multitude of cast and wrought nickel-based alloys that can have various desirable characteristics enhanced by either solution treating or by solution treating and precipitation age hardening. Characteristics such as room temperature and/or elevated temperature mechanical strength, corrosion resistance and oxidation resistance are typically enhanced by such heat treatments.

The properties of heat treatable nickel-based alloys can be enhanced by selection of appropriate heat treating parameters. The use of solution treating alone or solution treating followed by precipitation age hardening is commonly used with nickel-based alloys.

Solution treatment

  • During manufacturing processing, most materials can be work-hardened, limiting the ability to further process the material. In-process solution treating (stress relieving) can reduce this work hardened condition to allow further processing.
  • Manufacturing processes, such as brazing, welding or coating may have an undesirable impact on material properties which may be reversed through solution treating prior to further downstream processing.
  • Manufacturing processes may result in the premature start of the final precipitation age hardening process, which can be reversed through re-solution treating prior to further processing.
  • Many nickel-based alloys develop their desired properties solely through the solution treating process. They are considered solid solution strengthened alloys. Examples of these are Hastelloy X, INCO 625 and HA 230.

Precipitation age hardening

  • Development of the final material properties required to satisfy part specific design criteria typically requires the material (casting/wrought material) to be subjected to a long lower temperature heat treat cycle to develop an alloy specific microstructure through precipitation age hardening.
  • Typically, this step is performed at or near the end of the manufacturing process, as the heat treat process results in a significant increase in material hardness and there is a predictable amount of size change that occurs (shrinkage) that must be accounted for. Machining costs can increase dramatically if post-age hardened machining is required.
  • Typical examples of materials are: INCO 718, INCO 738, Mar-M-247, Waspaloy and C263.

  • Solution treating is typically performed at temperatures ranging from 1800 to 2450°F in vacuum, followed by rapid gas fan cooling to room temperature. Many materials have specific cooling rates that must be achieved to ensure the proper metallurgical microstructure is achieved in the final product.
  • Precipitation age hardening is typically performed at temperatures ranging from 1000 to 2080°F in vacuum, inert atmosphere or air for hold times ranging from 2 hours to 40+ hours depending on exact material and metallurgical microstructure specified. Multiple steps (with decreasing temperatures at each step) can also be necessary to achieve the final desired results.
  • Applicable industrial specifications would include SAE AMS 2773 and AMS 2774.