Precipitation Hardening Stainless Steels – Applications, Properties, Fabrication Processes

2024-07-18

Precipitation hardening stainless steel creates a remarkable class of alloys that combines the advantageous properties of both martensitic and austenitic grades. These alloys undergo a specialized heat treatment process that significantly enhances their strength while preserving workability and ductility. One of the standout features is their ability to achieve tensile strengths ranging from 850 MPa to 1,700 MPa.

The high tensile strengths of precipitation hardening stainless steels come after a heat treatment process that leads to precipitation hardening of a martensitic or austenitic matrix. Hardening is achieved through the addition of one or more of the elements Copper, Aluminium, Titanium, Niobium, and Molybdenum.

Precipitation hardening involves a sequence of steps including: solution treatment to dissolve precipitates, rapid cooling to create a supersaturated solid solution, and aging to form finely dispersed precipitates. This results in increased strength, improved wear resistance, and heightened corrosion resistance. Precipitation-hardened stainless steels find extensive applications across diverse industries, from oil & gas to aerospace.

The most well known precipitation hardening steel is 17-4 PH. The name comes from the additions 17% Chromium and 4% Nickel. It also contains 4% Copper and 0.3% Niobium. 17-4 PH is also known as stainless steels grade 630.

Applications of precipitation hardening stainless steels

Due to the high strength of precipitation hardening stainless steels, most applications are in aerospace and other high-technology industries.

Applications include:

Gears
Valves and other engine components
High strength shafts
Turbine blades
Moulding dies
Nuclear waste casks

Properties of precipitation hardening stainless steels

Strength of precipitation hardening stainless steels

Yield strengths for precipitation-hardening stainless steels are 515 to 1415 MPa. Tensile strengths range from 860 to 1520 MPa. Elongations are 1 to 25%. Cold working before ageing can be used to facilitate even higher strengths.

Heat Treatment of precipitation hardening stainless steels

The key to the properties of precipitation hardening stainless steels lies in heat treatment.

After solution treatment or annealing of precipitation hardening stainless steels, a single low temperature “age hardening” stage is employed to achieve the required properties. As this treatment is carried out at a low temperature, no distortion occurs and there is only superficial discolouration. During the hardening process a slight decrease in size takes place. This shrinking is approximately 0.05% for condition H900 and 0.10% for H1150.

Corrosion Resistance of precipitation hardening stainless steels

Precipitation hardening stainless steels have moderate to good corrosion resistance in a range of environments. They have a better combination of strength and corrosion resistance than when compared with the heat treatable 400 series martensitic alloys. Corrosion resistance is similar to that found in grade 304 stainless steels.

In warm chloride environments, 17-4 PH is susceptible to pitting and crevice corrosion. When aged at 550°C or higher, 17-4 PH is highly resistant to stress corrosion cracking. Better stress corrosion cracking resistance comes with higher ageing temperatures.

Corrosion resistance is low in the solution treated (annealed) condition and it should not be used before heat treatment.

Heat Resistance of precipitation hardening stainless steels

17-4 PH has good oxidation resistance. In order to avoid reduction in mechanical properties, it should not be used over its precipitation hardening temperature. Prolonged exposure to 370-480°C should be avoided if ambient temperature toughness is critical.

Fabrication of precipitation hardening stainless steels

Fabrication of all stainless steels should be done only with tools dedicated to stainless steel materials or tooling and work surfaces must be thoroughly cleaned before use. These precautions are necessary to avoid cross contamination of stainless steels by easily corroded metals that may discolour the surface of the fabricated product.

Cold Working of precipitation hardening stainless steels

Cold forming such as rolling, bending and hydroforming can be performed on 17-4PH but only in the fully annealed condition. After cold working, stress corrosion resistance is improved by re-ageing at the precipitation hardening temperature.

Hot Working of precipitation hardening stainless steels

Hot working of 17-4 PH should be performed at 950°-1200°C. After hot working, full heat treatment is required. This involves annealing and cooling to room temperature or lower. Then the component needs to be precipitation hardened to achieve the required mechanical properties.

Machinability of precipitation hardening stainless steels

In the annealed condition, 17-4 PH has good machinability, similar to that of 304 stainless steels. After hardening heat treatment, machining is difficult but possible.

Carbide or high speed steel tools are normally used with standard lubrication. When strict tolerance limits are required, the dimensional changes due to heat treatment must be taken into account

Welding of precipitation hardening stainless steels

Precipitation hardening steels can be readily welded using procedures similar to those used for the 300 series of stainless steels.

Grade 17-4 PH can be successfully welded without preheating. Heat treating after welding can be used to give the weld metal the same properties as for the parent metal. The recommended grade of filler rods for welding 17-4 PH is 17-7 PH.

Lork Group is an supplier of industrial metals to the global aerospace, oil and gas, power generation, petrochemical, bio-medical, pharmaceutical, and other high tech manufacturing industries. Products of premium metal products including Duplex Stainless Steel，600 Series Stainless Steel and 300 Series Stainless Steel. We certify to most ASME, ASTM, AMS, and military specifications as well as most international standards certification.

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