Maintenance procedures and safe working conditions are essential in maintaining vacuum furnaces as is proper preparation of components to be heat treated prior to charging to the furnace.

VAC AERO International operates more than a dozen vacuum furnaces in its own heat treating facilities. In addition, it has manufactured hundreds of vacuum furnaces for sale to the global market. Through these activities, the company has gained extensive experience in the care and maintenance of vacuum furnace equipment.

The assembly is a five-piece, tubular structure manufactured entirely from 4330V steel. It consists of a hollow tube approximately 1500mm long by 120mm in diameter, two fittings (upper and lower) that are TIG welded to each end of the tube and two backing rings that bridge the gaps between the fittings and the tube during the welding operation. For many years, VAC AERO has been performing welding and heat treating operations on a landing gear component for a popular turboprop aircraft. Because of the part design, the welding operation in particular is complex and challenging and often involves substantial re-work. by Jeff Pritchard CEO, VAC AERO International Inc.

By J.E. Pritchard, S. Rush, A. Kiela

VAC AERO International Inc.

Abstract

Application of sacrificial coatings has long been used to reduce rotor-shroud clearances in gas turbine engines. Materials normally used for these coatings include sintered metal-powder segments, sintered metal-fibre segments, metallic honeycomb (filled and unfilled), elastomers and thermally sprayed abradable coatings. Thermally sprayed coatings offer advantages over the other materials, including direct application, easy removal and repair, variety of coating materials and good performance. New abradable thermal spray coating materials have been developed for performance in industrial turbine engines at operating temperatures up to 980ºC. Results are presented from laboratory evaluations of these coatings by burner rig and hot abradability testing.

In recent years, the operating temperatures of land-based gas turbine engines have increased to improve efficiency.  As a result, greater demands are placed on the materials used in the manufacture of the engine components.

In particular, hot section turbine blades must function in a very severe operating environment.  The blades are usually manufactured from advanced nickel-based superalloys but these materials on their own are still not durable enough.  To enhance their durability, they are protected from hot corrosion and high temperature oxidation through the use of special coatings.  The coatings form adherent oxide layers that inhibit the blade material from directly interacting with potentially damaging elements within the combustion gases like oxygen, sulphur and other contaminants.  A popular approach involves coating the blades with an MCrAlY bond coat topped with a thermal barrier coating (TBC) overlay.

Carbon fibre-reinforced carbons (C/C for short) are used in modern vacuum or protective gas furnaces in the form of heating elements or charging systems. They are characterised by thermal shock resistance, absence of distortion, low mass and strength increase with rising temperature. These features enable users to operate their plants more effectively, minimise reject rates and therefore reduce the cost of production. C/C is thus a key element in many process optimisation steps and helps companies to improve their competitiveness. by Alexander Racek, SGL CARBON GmbH

Thermal spray processes like air plasma spray and High Velocity Oxygen Fuel (HVOF) are usually thought of as being used primarily for applying protective coatings to new parts. While new part applications do indeed constitute the majority of their use, there are also a wide variety of repair techniques that employ thermal spray processes. VAC AERO has been a leader in developing repairs for aircraft structural components and gas turbine engine parts using thermal spray processes. An example of a structural component repair involves a flap track from a popular turbo-prop aircraft.  As the wing flaps of this aircraft are extended and retracted during landing and take-off, rollers run along the surfaces of a series of components known as flap tracks.

There are a number of factors that influence the development of a brazing cycle. These include such things as base metal and braze alloy composition, mass of the assembly and joint design. 

However, each cycle is comprised of a number of common segments. The illustration below shows the typical profile for a vacuum brazing cycle. During the initial pumpdown, water vapour adsorbed by the parts and furnace is driven off. For most brazing applications, a pumpdown before heating to a vacuum level of 8 x 10-4 torr or better is recommended. A vacuum safety interlock should be programmed into the cycle to ensure this level is reached. After pumpdown, the initial heating rate should not exceed 15ºC (30ºF) per minute. Faster rates may cause paste braze alloy to spall off or distortion of the assembly. Heating continues to a stand off temperature at about 25ºC (50ºF) below the solidus temperature of the braze alloy. The load is then soaked at this temperature to ensure temperature uniformity and to allow vacuum levels to recover. A soak time of 30 minutes is usually sufficient, though the incorporation of a second vacuum safety interlock in the braze cycle program may be desirable.

This is the last in a series of four articles on Vacuum Brazing Tecniques. (read part 1) / (read part 2) / (read part 3)