Vacaero

Manufacturers of heat treating and brazing vacuum furnaces and controls, complete hot zone and vacuum furnace retrofits, thermal spray coatings, plasma, HVOF and paint coating services.

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Measuring Inclusion Content by ASTM E 1245

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table-1-wsOver the past forty-plus years, steelmakers have introduced improved practices for reducing the inclusion content of steels. The success of these practice changes can be monitored in a variety of ways. Chemical analysis of the bulk sulfur and oxygen contents provides a relatively simple means to assess the impact of these changes. However, microscopical test methods are still needed to assess the nature of the inclusions present.

Traditional chart-based measurement methods have wide acceptability, and their data are well understood by both purchaser and producer for heat acceptance purposes. These methods do have inherent weaknesses which limit their usefulness for quality control, SPC, and database applications. Image analysis-based chart measurements are an improvement over manually generated chart ratings, but the data still does not lend itself to databases and statistical comparisons. ASTM Committee E-4 on Metallography has developed a stereologically-based image analysis standard test method, E 1245, which provides the necessary data in a form which is easily databased and analyzed statistically. The presentation will describe E 1245 and show how data from different heats and melting practices can be compared statistically to ascertain valid test differences.

Mechanical Booster Pumps for Vacuum Systems

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In many vacuum systems, especially those where the chamber is large, has a large internal surface area and the chamber load adds extra surface area, the pump down can be slowed substantially when the chamber pressure drops to the range where the water vapor molecules on the surface desorb and have to be pumped away.

Pressure and temperature determine when this vapor desorbs, but at ambient temperature around seventy two degrees Fahrenheit or twenty degrees Centigrade the vapor desorbs from about 50 Torr down to about 0.1 Torr. The vapor pressure of water at ambient temperature is about 18 Torr, so that is where maximum desorption may occur.

Dimensional Changes After Heat Treatment

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One of the questions all Heat Treaters are asked is, “How much, if at all, will my part change (i.e. shrink or grow) during heat treatment?” While the heat treater would love to be able to give a precise answer to this question, in most situations volumetric size change during heat treatment cannot be accurately predicted, at least not accurately enough to allow for final machining and/or grinding to close tolerances prior to heat treatment.

Experimental work has been done on many materials to show the effects of heat treatment on size change. As one might expect, the effects are different for every material grade. For example, an 80 mm (3.15”) cube of D-2 tool steel (Fig. 2) reveals growth (0.08%) in one dimension and shrinkage in the other two dimensions as a result of vacuum hardening. This graph demonstrates how knowing the part orientation from the mill-supplied bar is important when trying to plan for size change during heat treatment. By Dan Herring, THE HERRING GROUP Inc., and Patrick McKenna, Nevada Heat Treating, Inc.

New Considerations when Buying a Vacuum Furnace

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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.

 

Hot Zone Selection for Vacuum Brazing of Superalloys

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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

Vacuum Gauges Used on Vacuum Furnaces

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Vacuum gauges measure the pressure readings in the range from atmospheric pressure down to some lower pressure approaching absolute zero, which is not attainable. Some gauges read the complete range with low resolution and others can only read a portion of the range but with better resolution, usually used for the lower pressures.

There are three groups of vacuum gauges based on the method of operation, mechanical, thermal conductivity and ionization. For this discussion we will only talk about the thermal conductivity and ionization gauges because purely mechanical vacuum gauges are generally not used on vacuum furnaces.

Grain Size Measurements by the Triple Point Count Method

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Aside from the well-known grain size measurement techniques using either the planimetric methods of Jeffries or Saltykov, or the intercept method of Heyn, Hilliard and Abrams, one can measure the grain size through a count of grain-boundary triple point intersections within a known area through the use of Euler’s law. This technique has rarely been used but it should be possible to do such a count by image analysis. In general, measurements based on point counts (0 dimensional) are less subject to errors than lineal measurements (one-dimensional) which are less subject to error than areal measurements (two-dimensional). By George Vander Voort