The purchase of a vacuum furnace involves a considerable capital investment.  As a result, the question of buying a used furnace at a lower cost than a new furnace is a fairly common one.  However, there are a number of potential issues with used equipment that should underscore the warning “buyer beware”. To begin with, good used vacuum furnaces are a rare commodity.  When they do appear on the market, they don’t last long.  Many of the best are purchased through industry networking and never reach the general market.  Still, there are numerous dealers of used furnace equipment with inventories posted on their websites.

Most used furnaces are sold on an as-is, where-is basis with no manufacturer’s warranty.  If a decent used candidate is located, there are a few very important items to investigate before purchasing.  One of the biggest and most difficult to detect problems with used furnaces is the condition of the water jacket in the vacuum chamber.  The life of a properly maintained vacuum chamber can be well over twenty years.  However, in situations where the furnace cooling system has been connected to an untreated water supply, water jacket blockages from mineral build-up can begin to appear in as little as three years.  Beyond dissecting the vacuum chamber, there are no fool-proof methods for detecting blockages.  Ultrasonic testing is sometimes used but can be expensive and unreliable.  The presence of blistered or discolored paint on the outside of the chamber is a good indication of hot spots due to blockage.  Perhaps the best approach is to avoid altogether used furnaces more than twenty years old.  If water jacket blockage problems arise after purchase, the only sure solution is re-lining the chamber at considerable time and expense. BY JEFF PRITCHARD

Component parts come in all shapes and sizes. To meet this demand vacuum furnaces have been designed to accommodate many standard workload configurations. Despite the almost limitless choices, some common sense rules apply. It is important to recognize that loading arrangements generally fall into two classes: weight limited and volume limited. In either case, when loading parts in furnace baskets or onto racks the goal is often to maximize loading efficiency. One must also be concerned with proper part spacing, that is, how parts are situated within the load for optimal heat transfer (e.g. line of sight heating), support and stability of the load at temperature, temperature uniformity, and heat extraction during quenching so as to achieve the desired metallurgical properties and minimize distortion.

For maximum fuel efficiency, many gas turbine engine designs depend on sacrificial coatings to tighten internal clearances between moving parts. An extra gap of .005″ between the rotating blades and the engine casing can increase fuel consumption by as much as 0.5%. As fuel comprises more than half of direct operating costs, this waste can be significant. Engine efficiency largely depends on close clearance between blades and casing. Clearance can be affected by a number of engine operating variables, including casing expansion and contraction, loading due to maneuvering, thrust, gust, stall, vibration and manufacturing tolerances. An industrial turbine engine manufacturer (“OEM”) was experiencing unsatisfactory results with the ring segment coatings used to maintain rotor-shroud clearance. Because of poor abradability, the coatings cause excessive wear on the tips of the turbine blades. The OEM and VAC AERO agreed to work cooperatively to develop an improved abradable coating for these applications.

Materials normally used in sacrificial coatings for gas path seals include sintered metal-powder segments, sintered metal-fiber 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 proposed for performance in industrial turbine engines at operating temperatures up to 980°C.

An investigation of a variety of these new materials was under taken in order to quantify their performance during cyclic oxidation burner rig testing, hot corrosion burner rig testing and hot abradability rig testing as compared to existing abradable coating technology. Based on the results of these tests it was concluded that a significantly improved coating for abradable seals in industrial gas turbine engines was developed. This new coating can prevent excessive blade tip wear between 24,000-hour inspection intervals at operating temperatures up to 980°C. The successful coating consists of a specially heat treated MCrAIY bond coat, applied by HVOF, covered with a proprietary abradable top coat, applied by air plasma spray. By Jeff Pritchard, Scott Rush and A. Kiela

Lubricants for vacuum service are a diverse family of highly formulated products. The types of lubricants for vacuum service fall into three general categories: (a) wet, organic or silicone-compound based oils and greases, (b) dry lubricants including PTFE (Teflon^®) and metal dichalcogenide compounds (e.g. molybdenum disulfide, tungsten diselenide) and (c) metal on metal combinations.

The choice of lubricant depends on a number of considerations that are highly dependent on the specific end-use applications including; operating temperature and vapor pressure, the presence or absence of sliding or rolling motion, the presence or absence of reactive species (e.g. plasma), loading characteristics and frequency of usage. Lubricants are used in a vacuum system for three primary reasons: (1) in an “O” ring sealed system to help minimize externally applied forces on the ring material, (2) between moving surfaces to reduce the coefficient of friction, minimize wear and/or reduce/eliminate the formation of particular matter and (3) as anti-seizing agents where mating surfaces are likely to seize.

VAC AERO provides custom furnace designs and process capabilities to suit your specific requirements.

In addition to standard vacuum heat treating and brazing furnaces, VAC AERO also manufactures custom vacuum systems for a wide variety of special processing applications. VAC AERO’s skilled team of engineers works closely with the customer from concept through design, final manufacture and installation. VAC AERO designs and builds standard Gas Quench furnaces for maximum durability, reliability and trouble-free operation and provides an outstanding level of training, field and technical support based on decades of commercial heat treating experience.

With recent advances in technology, the vacuum furnace purchaser faces an array of new considerations when buying furnace equipment.  Innovations in hot zone materials, control packages and quenching systems can offer many operating and performance improvements, but can also affect the price of the equipment.  As both a heat treater and vacuum furnace manufacturer, VAC AERO can provide informed answers to cost/value questions affecting difficult purchase decisions.

Hot Zone Materials: Lightweight, curved graphite elements are becoming increasingly popular for vacuum furnaces.  These elements have a lower thermal mass than old-style graphite rod or bar elements.  Compared to molybdenum strip elements, the curved graphite element is more durable and has better resistance to operating hazards like accidental breakage or braze alloy spill. Graphite insulation materials also exhibit excellent performance in vacuum furnaces.  Thousands of hours of operating service have confirmed that graphite felt insulation is suitable for almost all high vacuum applications, including brazing of advanced superalloys.  Less expensive and easier to maintain than graphite board or all-metal shielding, felt insulation can be combined with a reflective carbon composite hot face to maximize hot zone life, even in high-pressure gas quench furnaces.

 

The Thermal Processing Divisions of VAC AERO International have provided repair services for damaged components from land-based and aerospace gas turbine engines. Engine manufacturers, operators, overhaul centers and commercial airlines are just a few of the customers that depend on these services. Many hot section engine components are fabricated from nickel-based superalloys. These materials cannot be repaired by traditional techniques, such as welding, without causing significant reductions in mechanical properties. As a result, VAC AERO developed proprietary vacuum brazing techniques to repair cracks, wear, and other service-induced damage.

The extent of damage to the engine components is often severe. Therefore, the brazing process involves the use of large amounts of brazing filler metal to make the necessary repairs. When subject to high temperature under vacuum, volatile metallic and organic constituents vaporize from the filler metal. While a portion of these volatiles is removed from the furnace chamber by the vacuum pumping system, the balance tends to condense within the chamber, much of it depositing on the hot zone insulation. In addition, excess molten braze alloy occasionally drips from the workload onto the heating elements and insulation, despite the use of drip trays. These deposits can have a detrimental effect on the performance of the furnace. As a manufacturer and user of vacuum furnaces, VAC AERO needed a hot zone design to withstand these aggressive brazing applications. BY JEFF PRITCHARD