Stainless steel 17-7PH - S17700 - 1.4568 - AISI 631 - X7CrNiAl17-7

General Introduction to 17-7PH

The 17-7PH is one of the most formable precipitation-hardening (PH) stainless-steel alloys. Austenitic in condition A, it is therefore less magnetic than other PH grades and can be easily formed.

This grade hardens to high strength levels using relatively low heat-treatment temperatures (compared with conventional quenches), which minimizes distortion. Once heat treated, this alloy offers outstanding mechanical properties up to 482 °C (900 °F), although the most stringent standards generally recommend use up to 316 °C (600 °F). Beyond that, over-ageing can occur.

Heat Treatments

The typical heat-treatment process for 17-7PH involves several steps :

1. Austenitic conditioning, typically ~1065 °C
2. Controlled cooling transforming the austenite into martensite
3. Precipitation hardening, heating to the target temperature

To obtain the highest mechanical properties, the alloy is transformed into martensite by cold reduction (work hardening) in condition C. After fabrication, a single low-temperature heat treatment is carried out to reach condition CH 900, which provides the maximum strengths of 17-7PH.

Whereas 17-4PH steel, mainly martensitic, hardens by precipitation thanks to the addition of copper, 17-7PH, semi-austenitic, becomes martensitic only after thermal transformation (or cold work) before ageing. Its precipitation hardening is achieved thanks to aluminium.

Mechanical and Chemical Properties

Before heat treatment, 17-7PH attains a minimum tensile strength of 1034 MPa, a yield strength above 1275 MPa and a hardness of 20.5 HRC.

This alloy provides good corrosion resistance in moderately aggressive environments (humidity, weak acids, alkaline solutions) and industrial atmospheres. It remains sensitive to corrosion in marine environments or in the presence of strong acids.

Machinability, Formability and Weldability

The machinability of 17-7PH in condition A is rated at 75 % of B1112. As with many stainless steels, the use of a chip breaker is recommended : long chips complicate machining and can damage the machine. Low cutting speeds and constant feeds limit heating, work hardening and tool wear.

Like steel 301, 17-7PH has excellent formability. Because of its work-hardening rate, an intermediate anneal may be necessary to restore ductility during severe forming operations.

The weldability of 17-7PH is compatible with conventional processes (arc, resistance). An inert shielding gas (argon) is required to limit oxidation of the aluminium contained in the alloy.

Industrial Applications

In the aerospace industry, 17-7PH is commonly used for components requiring reliable elasticity or springback. It is found in the manufacture of flat springs, precision parts, or spring washers (e.g. : Belleville, Grower). wave springs (wave springs), valued for their compactness, fully benefit from the CH 900 treatment that provides the alloy’s maximum mechanical strength.