15-5PH and 17-4PH Stainless Steel: How Precipitation Hardening Works
In our previous blog post, we examined annealing, a method for strengthening stainless steel with heat. However, annealing isn’t the only way to strengthen stainless steel by heating it. Another method is precipitation hardening, which we use to make 15-5PH and 17-4PH stainless steel. The “PH” in the names of those alloys stands for “precipitation hardening.” This hardening method strengthens and hardens the steel by heating and then rapidly cooling the metal to change the arrangement of the atoms to form a more stable structure.
Here is an overview of how precipitation hardening works for 15-5PH and 17-4PH stainless steel alloys.
Process
As with annealing, the process of precipitation hardening follows three steps:
- Solution Treatment (Austenitizing): The stainless steel is heated to a high temperature, typically 900-1150°F, depending on the specific grade. The steel is then held at this temperature to dissolve all alloying elements into a solid solution.
- Quenching: Next, the steel is rapidly cooled, usually by immersion in water, oil, or a specific quenching medium. Rapid cooling freezes the dissolved alloying elements in a supersaturated solid solution.
- Aging (Precipitation): The steel is then heated again to a lower temperature, typically between 900-1150°F, depending on the desired properties. It is held at this temperature for a specific period to allow fine precipitates of intermetallic compounds (e.g., martensite, carbides) to form within the steel matrix (arrangement of the atoms).
Effects
The reason for putting any stainless steel alloy through a hardening process is to change the properties of the metal. These are the primary effects of precipitation hardening on stainless steel:
- Increased Strength: Precipitation hardening significantly increases the strength and hardness of stainless steel compared to annealing.
- Improved Toughness: It also enhances toughness and resistance to fatigue and stress corrosion cracking.
- Dimensional Stability: PH stainless steels exhibit good dimensional stability after heat treatment.
Applications
Different applications require hardening by different methods. Precipitation hardening is used in applications where high strength, good corrosion resistance, and good mechanical properties at elevated temperatures are required. It is common in aerospace, defense, and high-performance industries where lightweight, strong, and durable materials are essential.
15-5PH and 17-PH Stainless Steel Alloys | Alliant Metals
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