2.1. Ultra-high strength and corrosion resistant steels

Three steels were considered for this research paper; ultra-high strength steels AerMet100 and 300M, and the more recently developed, corrosion-resistant high strength steel MLX17. Such examples typically witness an improved fracture toughness through the addition or control of non-metallic inclusions [9]. All alloys were received in their normalised condition and heat treated as recommended by the manufacturers to achieve the desired properties for landing gear application. Once heat treated, alloys were mounted in Bakelite, ground and polished to a final stage of OPS Colloidal Silica in preparation for etching. Etching was performed utilising the methods and solutions shown in Table 1.

Etching procedure for all steel alloys assessed.

Latrobe Lescalloy® 300M steel is a medium carbon, modified 4340 steel with added silicon allowing for use at high temperatures. Its most commonly used in its quenched and tempered martensitic condition where it is at its highest strength and toughness. The addition of silicon also acts to retard the coarsening of the cementite phase, allowing the 300M to be tempered at much lower temperatures, thus avoiding the loss of strength [10]. Its composition is shown in Table 2.

Chemical composition of 300M (%wt).

300M undergoes VIM-VAR processing, providing it with higher strength than 4340, but similar fracture toughness. As recommended, 300M underwent the following heat treatment: 927 °C for 1 h → 870 °C for 1 h → oil quench to room temperature → double temper at 300 °C for 2 h each → air cool to room temperature. The heat treatment resulted in the martensitic structure as shown in Figure 1.

Optical micrograph of 300M microstructure.

CarTech's® AerMet100 is considered a high alloy steel due to it containing large amounts of Cr, Ni and Co. It is a martensitic steel with an Fe–Ni lath structure which is strengthened by M₂C (M = Cr, Mo and Fe) carbides [11]. Its chemical composition is shown in Table 3.

Chemical composition of AerMet100 (%wt).

It receives a similar VIM-VAR process as 300M, which gives it a clean, inclusion-free matrix. AerMet100 also receives a cryogenic treatment after treatment after quenching to room temperature following austenitising. This is to ensure the majority of the retained austenite is transformed to martensite. The addition of Co increases M₂C carbide nucleation as well as increasing particle reinforcement [12]. Heat treatment of AerMet100 was carried out as follows: 899 °C for 1 h → air cool to room temperature → 885 °C for 1 h → quench to room temperature over 1–2 h → refrigerate at -73 °C for 1 h → air heat to room temperature → temper at 482 °C for 5 h → air cool to room temperature. This resulted in a fully martensitic structure as shown in Figure 2.

Optical micrograph of AerMet100 microstructure.

MLX17 is a corrosion resistant high strength steel developed by Aubert & Duval. Its high corrosion resistance is attributed to its comparably high %weight of Cr and Ni and it is precipitation hardened where Al and Ti are hardening additions [4]. The chemical composition of MLX17 is shown in Table 4.

Chemical composition of MLX17 (%wt).

MLX17 is a high purity alloy, manufactured through vacuum primary melting and consumable electrode remelting. It was received in its cryogenic solution treated condition, however, to achieve optimum hardness values, ageing was carried out at 510 °C for 7 h followed by an air cool at room temperature. The heat treatment resulted in a martensitic structure as shown in Figure 3.

Optical micrograph of MLX17 microstructure.

Since MLX17 is a more recently developed alloy, little research has been conducted into its baseline mechanical properties, hence a key aim of this paper is to further the current understanding of its monotonic behaviour under various loading conditions whilst comparing its properties to existing alloys.