We’ve been looking at the leak-up rate of vacuum furnaces in my last two articles, and this month’s article looks at another type of vacuum leak, that isn’t really a leak at all!

This kind of “leak that isn’t a real leak” is actually known as a “virtual leak”, and represents the outgassing of substances that have condensed onto the inner walls of the vacuum furnace during prior furnace runs. Then, when those walls get very hot during subsequent furnace runs, the condensed substances on the walls may volatilize and outgas once again into the vacuum chamber, the effects of which often appear to be very similar to an actual furnace leak. by Dan kay

Rzeszow, Poland, (September 21, 2012) – Pratt & Whitney (United Technologies, East Hartford, CT) has qualified Vac-Aero Rzeszow (Plant 2), as a source of PWA 11-17 and -22 heat treatment processing for nonrotor applications. This new approval has now been added to the many other heat treating and coating accreditations that VAC AERO Poland has…

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Grain size is probably the most frequent microstructural measurement due to its influence on properties and behavior/service performance. Grain size can be determined by several methods. Chart comparison ratings are probably the most often performed, as this method is fast and simple. But its accuracy is at best ± 1 G value.

An ASTM E-4 interlaboratory round robin test using Plate I of ASTM E 112 showed that chart ratings were biased with the rating being ½ to 1 G value coarser than the actual measured grain size. Similar studies have not been conducted with Plates II or III. Actual measurements of grain size are done by either the planimetric or the intercept methods, as defined in E 112. These are unbiased methods, as long as the grain boundaries were properly delineated by the etchant. Experience has shown that measuring the grain size of BCC metals is much easier than measuring the grain size of FCC metals and alloys that contain annealing twins. ASTM E 112 has two comparison charts for such metals; Plate II for specimens that exhibit a so-called “flat” etch appearance and Plate III for those that exhibit a grain contrast etchant response. Plate III was developed using copper specimens and the images are at 75X, while the other E 112 charts are at 100X. To further confuse the issue, Plate III expresses grain size in terms of d, the average grain diameter, calculated by taking the square root of the average grain area (which is the reciprocal of the number of grains per mm²), rather than as an ASTM grain size number, G. By George Vander Voort

As mentioned in last months article, all vacuum furnaces will leak air into their vacuum chambers over time, causing the pressure in the vacuum chamber to go back up. Thus the term “leak-up rate” is used to determine just how fast that vacuum chamber is allowing air back in.

Of course, air represents the presence of oxygen entering the furnace, which is not good for brazing. The air will also contain moisture, which then becomes another source of oxygen. Moisture itself can become an adherent problem on the metal surfaces being brazed, with adsorbed moisture often being very difficult to drive off the metal surface (can be quite a problem in aluminum vacuum brazing). by Dan kay

Cold working is well known to change the properties of metals and alloys. Deformation increases the strength of metals but usually reduces it toughness and leads to anisotropy of properties, that is, directionality. Hot working also produces similar affects, the microstructural results after hot work with low finishing temperatures may appear to be the same as from cold working.

Hot rolling of shapes, plate or bar, for example, elongates the nonmetallic inclusions in the deformation direction, which will reduce the isotropy of mechanical properties. Hot working can also lead to segregation being elongated parallel to the deformation direction, which also reduces isotropy. Reducing the finishing temperature, that is, the temperature of the steel at the last deformation pass, will promote “banding” – parallel alignment of the constituents into layers, such as alternate bands of ferrite and pearlite. This also promotes anisotropy of mechanical properties, chiefly toughness and ductility. Strength is not usually affected to a significant degree by banding, compared to toughness and ductility. By George Vander Voort

We continue our discussion of ways to improve vacuum performance by understanding how to maximize the operation of our vacuum systems. Tip #5: Diffusion Bonding

Diffusion bonding is a solid-state joining process involving both vacancy and interstitial diffusion of atoms between surfaces in intimate contact with one another. The process is capable of intentionally bonding together a wide range of metal and/or ceramic part combinations, forming strong metallurgical bonds. Applications include hot compacting of oxides, nitrides, borides, carbides, sulfides and their mixtures to near theoretical densities as well as sintering ceramics and powder metals. By Dan Herring

Come Visit Us at FNA 2012 – Booth 110

We’re excited to head down to Nashville for the Furnaces North America Conference and Expo.  If you plan on attending the show, please be sure to stop by our booth number 110. Our sales reps will be available to answer questions about how VAC AERO can help your business with exactly the vacuum processing solutions that you’re looking for. See you there!

All vacuum furnaces are leaky! That’s right, all vacuum furnaces can leak outside-air into the vacuum chamber, which could potentially cause some problems with your brazing, since an air-leak represents an influx of oxygen into the furnace.

Fortunately, the problem is usually quite controllable, and regular furnace inspections can usually keep those leaks completely under control. Leaks most typically occur through some of the sealing-surfaces in the furnace, the most common leak-source being the O-ring seal-surface in the furnace door itself. As the door is opened and closed everyday, the light coating of vacuum-grease on the door seal and on the O-ring can pick up dust and dirt, which, if not properly removed regularly, might begin to initiate small holes/cracks in the O-ring seal, which can eventually open up enough to allow air to start to leak into the furnace during furnace operation. by Dan kay

We continue our discussion of ways to improve vacuum performance by understanding how to maximize the operation of our vacuum systems. Tip #4: Maintain Your O-Ring Seals

O-rings are an integral part of any successful vacuum system, however O-rings lose their elastic nature over time and eventually crack (Fig. 1) leading to air infiltration into the vacuum chamber. When replacing O-rings it is critical to use the correct type, normally specific by the original equipment manufacturer. By Dan Herring

We continue our discussion of ways to improve vacuum performance by understanding how to maximize the operation of our vacuum systems. Tip #2: Select The Right Vacuum Level for the Job

When selecting the right vacuum level, one must ask oneself, “What type of vacuum level does my application require?” For example, while many vacuum systems using modern high speed pumps can, in a clean, dry, empty and outgassed chamber, reach vacuum levels as low as the 10^-9 torr range, we must ask ourselves, “Is this level of vacuum really necessary?” Remember, either too low or too high a vacuum level could result in undesirable surface conditions – from vaporization of elemental constituents to oxidation of surfaces causing rework or even scrap. By Dan Herring