Hard chromium plating has traditionally been used on aircraft airframe components to provide corrosion protection and wear resistance. Chromium plating has been identified by governmental regulatory agencies as a health hazard due to the presence of hexavalent chromium within the plating solutions. This has prompted major airframe and landing gear manufacturers to search for alternative processes to chromium plating which do not pose a health threat to the process operators. The leading candidate for hard chromium replacement is tungsten carbide coating applied by the HVOF (High Velocity Oxygen Fuel) thermal spray process. There are many practical considerations to account for when implementing a change of this magnitude.
Transient liquid phase bonding, also known as diffusion brazing, is a metal joining process that produces a high quality bond through the isothermal solidification of a melting point depressant-rich interlayer. The advantage of using diffusion brazing over conventional processes is that the microstructural properties in the fusion zone are very similar to those of the base metal. The process is broken down into four distinct stages; heating, dissolution, isothermal solidification and homogenization. The rate-controlling stage of the process is the isothermal solidification stage which may take hours or even days.
VAC AERO’s latest advancement in vacuum furnace controls is a hybrid system based on the Honeywell HC900 controller for mechanical and thermal functions integrated with Honeywell Plantscape Vista for supervisory control and data acquisition (SCADA). Operator interface is provided through a large, colour touchscreen. Using this system, process information is accessible by operators at the furnace and by process engineers via the network. This provides comprehensive control and monitoring capabilities for higher productivity, reduced costs and increased quality.
Vacuum heat treatment offers an alternative method to traditional salt-bath and controlledatmosphere furnace hardening techniques for high strength steels, such as AISI 4340M and 300M. However, heat treaters must be prudent when choosing between oil and gas quenching for vacuum hardening of high strength steels, because each process has its advantages and disadvantages. High strength steel alloys, such as 4340M, 300M, and others, are most commonly used in the manufacture of landing gear components. These alloys are hardened and tempered to produce ultimate tensile strengths exceeding 280 ksi (1,930 MPa). By Jeff Pritchard and Scott Rush – VAC AERO International Inc., Oakville, Ontario, Canada
High strength steels based on the 43XX series are commonly used for aircraft landing gear components because of their desirable mechanical properties; these alloys are usually heat treated by quenching in oil from the hardening temperature, then tempered. During hardening, internal stresses created from the thermal shock of direct oil quenching almost always cause some degree of distortion in the heat treated component. Interrupted quenching techniques – such as ausbay quenching offer good potential for reducing quench distortion. However, due to the difficulty of machining these alloys in the hardened condition, machining allowances must be minimized. By Jeff Pritchard of VAC AERO International Inc., Oakville, Ontario, Canada.