The hot zone is perhaps the most critical feature of a vacuum furnace in terms of its affect on furnace performance and operating cost. There are a variety of hot zone designs and the choice of a design should be based on a careful analysis of specific processing applications. Most vacuum-furnace hot zones consist of four major components: the heating elements and the details on which they are mounted; the insulation package (or heat shields); a surrounding structure that supports the heating elements and insulation package; and a hearth that supports the load during processing. By Jeff Pritchard - VAC AERO International Inc., Ontario, Canada

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)

Stretching the service life of a hot zone-one of the most expensive consumables in a vacuum furnace-is of prime importance to the furnace owner. Proper care and maintenance can slow down hot-zone deterioration, which directly translates to cost savings. Hot-zone service life is directly related to the type of heat treating process used in a vacuum furnace and to the severe thermal stresses imposed by continuous vacuum-furnace cycles. Distortion and contamination are the main factors causing a breakdown of hot-zone components. Distortion is inevitable because thermal cycling is inherent to the process. However, periodic inspection and remediation of hot-zone components together with thorough component cleaning can extend service life. By Bill Potts, Vac-Aero International Inc., Oakville, Ontario, Canada

All aviation gas turbine engines require regular overhaul to ensure continued safe operation. During engine overhaul, decisions must be made on whether to replace deteriorated individual parts and assemblies with new components, or to repair them.

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)

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

During construction, all welded joints in a vacuum furnace are inspected for hermetic integrity and should remain intact for the life of the furnace.

However, there are also a number of demountable connections that can be separated and reconnected for component changing or repair access reasons. These connections are generally sealed with a gasket sandwiched between two flanges. Though different gasket materials are used depending on the vacuum level required, most seals in a standard vacuum furnace incorporate an o-ring made of natural or synthetic rubber. Buna-N, silicone and Viton o-rings are readily available in many sizes.

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