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.

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.

Vacuum level as indicated by vacuum gauge readings is not always a true indication of the actual conditions within a vacuum furnace. It is possible to have two identical furnaces operating at the same pressure but producing entirely different results on heat treating.

The reason for this can be explained by the relative leak tightness or leak rate of each furnace. Most furnaces are equipped with pumping systems sufficient to overcome reasonably significant leaks. On the furnace with the higher leak rate, air would be leaking continuously into the furnace resulting in a higher residual oxygen content than in the leak tight furnace. The higher oxygen content would adversely affect the heat treating results. Therefore, leak rate checks as prescribed by the furnace manufacturer should be performed at regular intervals to prevent this problem.

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

Whenever the door to a vacuum furnace chamber is open, humidity from the air will enter the chamber and condense in a very thin film on the chamber walls or be absorbed into the hot zone materials.

When the chamber is subsequently evacuated (before heating) and the furnace internals are exposed to this lower pressure, “outgassing” of the entrapped moisture will occur. If sufficient moisture has been entrapped (such as in very humid environments), the outgassing effect will slow the pumpdown process and may even give the appearance of a malfunction in the pumping system. Eventually, the outgassed moisture will be pulled out of the chamber by the pumping system and evacuation rates will improve. This same effect will be apparent when oily or contaminated workloads are placed in the furnace. It may be more pronounced in furnaces with graphite-based hot zone insulation materials.

This is the third in a series of four articles on Vacuum Furnace Maintenance. (read part 1) / (read part 2)

Clean, oxide-free surfaces are essential to achieve sound brazed joints. 

Uniform capillary action will occur only when all grease, oil, dirt and oxides have been removed from both the braze alloy and base metal prior to brazing. The length of time that cleaning remains effective depends on the material involved, atmospheric conditions, storage techniques and the amount of handling that may be involved. It is recommended that brazing be performed as soon as possible after the material has been cleaned. The selection of a cleaning technique depends on the nature of the contaminant, the base metals involved and the joint design. The same cleaning practices used for vacuum heat treating (ie. manual cleaning, vapour degreasing, etc.) are applicable to vacuum brazing.

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

Brazing involves the joining of two or more base metal components by melting a thin layer of filler metal into the space between them. 

Bonding results from the intimate contact produced by the dissolution of a small amount of base metal into the filler metal, without melting of the base metal. Brazing differs from welding, in which the joint is formed through melting of the base metal. Brazing is similar to soldering but, by definition, is performed at higher temperatures. In brazing, the filler metal can be placed within the joint as a foil, or placed over the joint in the form of paste or wire. Joint clearances must be very carefully controlled and usually do not exceed .12mm (.005"). Capillary action draws the molten filler metal into the joint and holds it there. The base metal components must be designed to enhance the capillary action. Brazing is a process that has been well adapted to vacuum heating methods.

This is the second in a series of four articles on Vacuum Brazing Tecniques. (read part 1)

As with any piece of equipment, proper maintenance at regular intervals is essential for long service life and trouble free operation.

The mechanical components in a vacuum furnace require standard maintenance practices (ie. cleaning, lubrication, etc.). However, successful use of a vacuum furnace depends on the purity and reliability of its vacuum. Additional maintenance activities are required to ensure good vacuum levels in the system. Leaks in joints and contamination of furnace internals will greatly affect operating vacuum levels and the quality of the processing.  Leaks are the most time-consuming and troublesome of the maintenance items.

This is the first in a series of four articles on Vacuum Furnace Maintenance due to appear over the next three months.