Michael L. Schmidt

Preoxidation prior to gas carburizing: Theory and its effect on pyrowear ® 53 alloy

The absorption of carbon during gas carburizing of steel components is reviewed based on thermodynamic and kinetic considerations. The effects of chromium and silicon on carbon absorption are reviewed based on the thermodynamics of passive film formation in the presence of a carburizing gas. Finally, the merits associated with the use of preoxidation treatments prior to gas carburizing are discussed and the results of an in-depth analysis performed on carbon absorption in Pyrowear®1 53 alloy are presented based on preoxidation temperature.Maximum carbon absorption was observed when using a preoxidation temperature of approximately 700/927° C (1292/1700° F) prior to carburizing Pyrowear 53 alloy at 927° C (1700° F) for 7.5 hr at a carbon potential of 1.16-1.19 using a 40% N2, 40% H2, and 20% CO endothermic carrier gas and vaporized methanol. This effect is believed to be related to the thickness of the oxide layer and to the roughness of the oxide/metal interface.

Mechanical Behavior of Two High Strength Alloy Steels Under Conditions of Cyclic Tension

The results of a recent study aimed at understanding the conjoint influence of load ratio and microstructure on the high cycle fatigue properties and resultant fracture behavior of two high strength alloy steels is presented and discussed. Both the chosen alloy steels, i.e., 300M and Tenax™ 310 have much better strength and ductility properties to offer in comparison with the other competing high strength steels having near similar chemical composition. Test specimens were precision machined from the as-provided stock of each steel. The machined specimens were deformed in both uniaxial tension and cyclic fatigue under conditions of stress control. The test specimens of each alloy steel were cyclically deformed over a range of maximum stress at two different load ratios and the number of cycles to failure recorded. The specific influence of load ratio on cyclic fatigue life is presented and discussed keeping in mind the maximum stress used during cyclic deformation. The fatigue fracture surfaces were examined in a scanning electron microscope to establish the macroscopic mode and to concurrently characterize the intrinsic features on the fracture surface. The conjoint influence of nature of loading, maximum stress, and microstructure on cyclic fatigue life is discussed.

Development of Custom 465® Corrosion-Resisting Steel for Landing Gear Applications

AbstractExisting high-strength low-alloy steels have been in place on landing gear for many years owing to their superior strength and cost performance. However, there have been major advances in improving the strength of high-performance corrosion-resisting steels. These materials have superior environmental robustness and remove the need for harmful protective coatings such as chromates and cadmium now on the list for removal under REACH legislation. A UK government-funded collaborative project is underway targeting a refined specification Custom 465® precipitation hardened stainless steel to replace the current material on Airbus A320 family aircraft main landing gear, a main fitting component developed by Messier-Bugatti-Dowty. This is a collaborative project between Airbus, Messier-Bugatti-Dowty, and Carpenter Technology Corporation. An extensive series of coupon tests on four production Heats of the material have been conducted, to obtain a full range of mechanical, fatigue, and corrosion properties. Custom 465® is an excellent replacement to the current material, with comparable tensile strength and fracture toughness, better ductility, and very good general corrosion and stress corrosion cracking resistance. Fatigue performance is the only significant area of deficit with respect to incumbent materials, fatigue initiation being often related to carbo-titanium-nitride particles and cleavage zones.