Stainless Character of a Martensitic Grade with Structural Hardening and High Mechanical Strength; Improved Durability Applied to the Aerospace Market

Martensitic stainless steels owe their high strength to ageing heat treatments, during which the precipitation of hardening phases takes place. The temperature of these treatments is around 450-550°C and the nature of the precipitates varies according to the steel’s chemical composition.

This specific microstructure induces structural hardening and thus properties that meet the requirements of sometimes critical aeronautical structural parts. With a range of mechanical strength (Rm) between 1,500 and 2,000 MPa and a toughness (K_IC) from 50 to 140 MPa√m, these steels are suitable for severe conditions in service.

In this context, Aubert&Duval has been developing the MLX^®19 grade, a promising alternative to 300M, generally used for certain landing gear parts, with stainless properties. After development work on the VIM (vacuum induction melting) double VAR (vacuum arc remelting) process, followed by solution and ageing heat treatment optimization, the targeted properties for use have been achieved.

During ageing, the duplex structural hardening obtained by the precipitation of NiAl and Ni₃Ti is combined with the appearance of reversion austenite, resulting in an Rm/K_IC balance of around 1,930 MPa and 52 MPa√m (compared with approximately 2,000 MPa and 60 MPa√m for 300M).

These optimized mechanical properties are coupled with the grade's inherent stainless properties, enabled by an appropriate chemical composition. As a result, stress corrosion resistance, a dimensional property for the targeted applications, is such that K_ISCC > 40 MPa√m at open-circuit potential (OCP), in the case of a rising step load (RSL) testing.

The advantage of this grade over the currently used solutions for these applications lies in its stainless property. Indeed, the grade's resistance to corrosion, mainly due to its higher chromium content (i.e. 10% compared with 0.8% for 300M), offers different advantages: i) increased service life; ii) reduced need for coatings; and iii) reduced maintenance