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. READ MORE

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

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. READ MORE

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

Most base metals typically brazed in vacuum furnaces have a natural oxide “coating” that can inhibit the flow of brazing filler metals. 

The oxides of the less reactive metals like iron, nickel and cobalt tend to dissociate (break down) under low pressure and high temperature. Therefore, alloys such as the 300 and 400 series stainless steels, carbon steels and many tool steels can be successfully brazed in vacuum at relatively high pressures (1 to 50 microns). READ MORE

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

All furnace equipment used for heat treating should be properly instrumented and periodically tested for uniformity.

The temperature uniformity within the furnace must be regularly surveyed. The frequency of surveying is largely dependent on the type of equipment in use and its previous history in accuracy and reliability. Exact survey frequencies should be determined from applicable processing specifications. However, quarterly temperature uniformity surveys are fairly standard. The purpose of the uniformity survey is to determine the range of temperatures present at different locations in the furnace under normal operating conditions. A furnace is normally qualified through an initial comprehensive survey. READ MORE

In any heat treating cycle, there are two important considerations concerning temperature: the temperature of the furnace hot zone which is generating the heat input, and the temperature of the actual workload.

Heating by direct radiation, the main heating mechanism in vacuum, tends to be a slower process than other heating mechanisms such as convection or conduction. As a result, there are times in the heat treating cycle, particularly during heat up, when the load will be at a lower temperature than the furnace hot zone. This is known as temperature lag. READ MORE

Good fixturing and loading practices are essential elements in achieving proper heat treating results and long equipment life.

Fixture materials and design must be appropriate for the processing application.  Maintenance of fixtures is equally important.  The possibility of reactions between the workpieces and baskets or fixtures must also be considered.  High temperature sintering of the workpieces to themselves or the fixtures can occur.  Eutectic melting can also occur when certain chemical compositions come into contact at high temperature.  Selection of a fixture material is influenced by cost, service environment and compatibility with the workpiece and furnace hearth. READ MORE

The Ultimate Cleaning Treatment For Superalloy Repair.

All 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. The ultimate decision depends on both technical and commercial criteria. That is, is there a technically sound repair available and, if so, is it economically favorable compared to the cost of a replacement component? Sometimes lack of availability of replacement components may override purely economic criteria. READ MORE 

Once a good fixture design has been developed, careful consideration should next be given to the loading of the workpieces.

Heating in a vacuum depends mostly on the transfer of energy through radiation from the elements to the load.  For uniform heating and cooling, it is important that the workpieces are not shielded by one another.  Pieces within the load should be evenly spaced to ensure even exposure to radiation.  The size, shape and high temperature strength of the workpiece should also be considered during loading.  Alloys with complex shapes and relatively low strength at heat treating temperatures may distort during processing.  In some cases, it may be necessary to support these components with specially designed fixtures. READ MORE

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

The heat treatment of landing gear is a complex operation requiring precise control of time, temperature, and carbon control. Understanding the interaction of quenching, racking, and distortion contributes to reduced distortion and residual stress. Arguably, landing gear has perhaps the most stringent requirements for performance. They must perform under severe loading con­ditions and in many different envi­ronments. They have complex shapes and thick sections. Alloys used in these applications must have high strengths between 260 to 300 ksi (1,792 to 2,068 MPa) and excellent fracture toughness (up to100 ksi in.1/2, or 110 MPa×m0.5). To achieve these design and per­formance goals, heat treatments have been developed to extract the optimum performance for these alloys. By D. Scott MacKenzie, Houghton International Inc. Valley Forge, PA