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Precipitation hardening: Stainless steels

Precipitation heat treatments strengthen materials by allowing the controlled release of constituents to form precipitate clusters which significantly enhance the strength of the component.

There are a multitude of cast and wrought stainless steel 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 and corrosion resistance are typically enhanced by such heat treatments.

The properties of precipitation hardenable stainless steels 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 precipitation hardenable stainless steels.

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 laser/flame cutting 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.
  • It is not recommended that solution treated only material be placed in service due to the presence of untempered martensite which can lead to brittle failures and undesirable losses in corrosion resistance. Examples of these are 15-5PH, 17-4PH and Ph13-8Mo.

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; the process is called 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: 15-5PH, 17-4PH, 17-7PH and Maraging 250.
  • Typical heat treat designations are H900 or H1075; where H900 indicates aging at 900°F for one hour and H1075 indicates ageing at 1075°F for 4 hours. In all cases, it is important to start with material that is in the solution treated condition or more commonly called Condition A.

  • Solution treating is typically performed at temperatures ranging from 1700 to 1950°F in vacuum, followed by rapid gas fan cooling to room temperature. Typically, the material must be cooled to below 90°F (or lower, in some cases) prior to additional processing.
  • A group of meta-stable alloys (17-7PH for example) require several intermediate processing steps after solution treating to achieve the desired properties before precipitation age hardening. These steps include an austenite conditioning treatment and a freeze to ensure complete austenite to martensite transformation.
  • Precipitation age hardening is typically performed at temperatures ranging from 900 to 1150°F in vacuum, inert atmosphere or air for hold times ranging from 1 hour to 4 hours depending on exact material and characteristics specified.
  • Applicable industrial specifications would include SAE AMS 2759/3.