This article is written for vacuum pumps such as the oil sealed rotary piston pumps used on many heat treating and vacuum furnace applications. The same information would also apply to the oil diffusion holding pump if it is used. This pump may be either a vee belt driven pump or a direct drive pump. The holding pump is used to keep the oil diffusion pump evacuated below the critical backing pressure when the main pump is in roughing mode.

All mechanical vacuum pumps need maintenance and the pump manufacturer usually lists the basic checks needed in the pump operation manual. This will vary with the application that the pump is used on but, at a minimum, will include the following: check oil level daily or weekly, depending on the application and use, change oil and check the shaft seal area for leaks every 6 months and inspect the exhaust valves and gas ballast valve seals every 12 months. By Howard Tring

Burlington, Ontario, November 18, 2014 – VAC AERO has recently shipped four complete replacement hot zones for various sized furnaces to be delivered to a North American manufacturer. The hot zones feature unitized construction for easy removal and maintenance. The lightweight design’s low thermal mass enables faster quenching and longer life. The heat shield package consists of three layers of half inch thick graphite felt and an inner reflective layer of graphoil bonded carbon composite that provides added protection and enhanced reflectivity. The graphite felt does not shrink which minimizes heat leakage and contributes to stable uniformity throughout its useful life. The heat shield package is supported by a 14 ga. stainless steel assembly that uniformly distributes the quenching gas to the workload. Some ongoing improvements on VAC AERO’s hot zones include improved durability, thermal efficiency and extended hot zone life.

Grain size measurement by electron backscattered diffraction (EBSD) has several unique advantages over the traditional measurement of etched specimens by the light optical microscopy (LOM) approach as defined in ASTM E 112. This is most evident when trying to measure the grain size of twinned face-centered cubic (FCC) metals where two major problems are encountered.

First, in many cases, it is difficult to reveal a very high percentage of the grain boundaries by etching. Secondly, nearly all etchants for twinned FCC metals do reveal the twin boundaries and one must ignore the twin boundaries when measuring the grain size by LOM. The notable exception to this experience is electrolytic etching of the 300 series of austenitic stainless steels where Bell and Sonon’s aqueous 60% nitric acid ^[1], using a platinum cathode and a voltage no greater than 1.5 V DC will reveal nearly 100% of the grain boundaries and virtually none of the twin boundaries. Another significant problem that affects EBSD results somewhat more than LOM etching results is the greater difficulty in preparing the highly ductile FCC metals to the perfection needed to get a very high percentage of the pixels to be indexable. Specimen preparation ^[2-4] is a very critical step in getting a very high percentage of indexable pixels in the field of view. This is not a trivial matter. By George Vander Voort

There are many vacuum-brazing shops out there that still believe it is necessary to try to use the strongest vacuum possible for brazing if they expect to get good results.

Such thinking is erroneous, and has led many shops to actually see “worse” results (increased void content of joints, increased discoloration on furnace walls, etc.) than they would have seen if they had merely used a “weaker” (less strong) vacuum. Many people today still like to use some of the older vacuum terminology, such as “soft vacuum”, “rough vacuum”, “hard vacuum”, etc., and some of those same people still believe that a “very hard vacuum” is always necessary for effective brazing. IMPORTANT NOTE #1: Good brazing does not necessarily require a very hard vacuum! How “hard” a vacuum is necessary for good brazing? Just “hard” enough to reduce the amount of oxygen present in the chamber to the level that the number of oxygen atoms remaining in the hot-zone of the furnace is not sufficient to cause damaging surface oxidation on the faying surfaces of the metals being brazed. by Dan Kay

We continue our discussion on the maintenance of vacuum furnaces. Part One talked about establishing a sound maintenance strategy and once this has been determined, the real maintenance work can begin. Let’s talk about the specifics on what, when, how and why we maintain certain critical components on our vacuum furnaces.

Vacuum furnaces come in all shapes and sizes and have many common features and operational/maintenance needs. However, it is important to understand the particular needs of the vacuum furnaces in your shop to have an effective maintenance program. When performing maintenance it is important to have a written plan defining the specific task to be performed, and the reason why a particular task is necessary (i.e. purpose of the task). A work order should be issued and the work signed off upon completion (which includes testing to ensure that the repair was successful). By Dan Herring

Today, the maintenance of heat treatment equipment is a point of major emphasis and this is especially true for vacuum furnaces. Over the next few articles we will explore various aspects of vacuum furnace maintenance providing useful tips and practical techniques to simplify the work and make sure that it is done correctly. Let’s begin by understanding the importance of the role of maintenance, and more specifically, how planned preventative maintenance is helping to manage the overall cost of equipment operation.

Maintenance is a fact of life for heat treat equipment. In general, the cost of maintenance increases dramatically as the operating temperature increases and/or the process environment becomes more severe (e.g. carburizing versus hardening). This remains true in vacuum furnaces despite the fact that they are often operated below their maximum temperature ratings. As with all equipment, some styles and designs require more attention than others. It is interesting to note, however, that construction of heat treat equipment can often be classified as “heavy duty” or “light duty” by the amount of maintenance required. Of course, if any furnace is operated outside their design limitations, this almost always translates to a need for more extensive maintenance. By Dan Herring

This article discusses inlet filters that are used on oil sealed mechanical medium vacuum pumps such as rotary vane and rotary piston pumps typically used on vacuum furnaces and, for smaller pumps used for many laboratory and light industrial applications. One of the downsides of any trap is that it will eventually require servicing. Many vacuum system operators prefer not to use traps for that reason. If the correct traps are used and maintenance is planned, the downtime and service costs can be kept in line.

There are four types of inlet filters used on vacuum pumps used in laboratories and in light industrial applications: foreline traps, catchpots, dust traps and vapor traps. The first, foreline traps, are used to prevent contamination coming out of the vacuum pump; and the other three are used to prevent contaminants from entering the vacuum pump. Foreline traps – This type of trap is to prevent oil vapor that moves out of the pump inlet under low pressure conditions when the gas is in molecular flow. That would be at a pressure lower than about 0.1 Torr or 100 microns. The ultimate vacuum of an oil sealed vacuum pump is reached when the hot oil in the pump starts to evaporate. Under these conditions some molecules of oil vapor will backstream from the pump inlet toward the vacuum system. Although back streaming of oil vapor occurs in larger pumps as well, it can be more critical in smaller vacuum systems where the piping is shorter. Instruments such as mass spectrometers, electron microscopes and ultra-high vacuum systems can be contaminated if oil vapor reaches them so most of these instruments use foreline traps. If these instruments become contaminated it can take several days to clean them out and return them to operation. By Howard Tring

Over the years, ASTM Committee E-4 on Metallography has conducted interlaboratory test programs to evaluate the precision and bias associated with measurements of microstructure using proposed and existing test methods. ASTM decided in the late 1970s that all test methods that generated numerical data must have a precision and bias section defining the repeatability and reproducibility of the method. Defining bias associated with a test method is difficult unless there is an absolute known value for the quantity being measured and this is not possible when microstructural features are being measured. This paper shows the results for an interlaboratory test using Method A, “worst field” ratings of inclusions in steels by ASTM E-45. The results from 9 people who were reported to be qualified, regular users of the method revealed consistent problems of misclassification of inclusions types and a wide range of severity ratings for each specimen.

ASTM E45 was created in 1942 and was based on an earlier (1, 2) chart developed by Jernkontoret in Sweden. The charts were designed to determine the size, distribution, number and types of indigenous inclusions (naturally occurring particles that form before or during solidification due to limited solid solubility for O and S) in steels. Originally, E45 included 3 charts, Plates I, II and III, but now there are two, Plates 1r and II. Plate 1r replaced Plates I and III after these charts were measured (3) and corrected in the creating of the image analysis method for making E45 JK inclusion ratings (4, 5) published as E1122 in 1992, which was incorporated into E45 in 2006. The JK chart, the original Plate I, categorized indigenous inclusions as: sulfides (type A), aluminates (type B), silicates (type C) and globular oxides (type D), although the classification was stated to be only by morphology. There were thin and thick categories of each based on their thickness (or diameter for the D types) and the severity ratings varied in whole increments from 1 to 5. Plate III was similar but the severity limits were in 0.5 increments from 0.5 to 2.5. By George Vander Voort

Over the years, several different temperatures have been used to define the concept of brazing. When the American Welding Society (AWS) published its first Brazing manual back in 1955, brazing was officially defined using 800F as the liquidus temperature of a brazing filler metal (BFM), above which temperature a joining process using that BFM would be defined as “brazing” (see Fig. 1). If the liquidus temperature of the filler metal was lower than 800°F, a joining process using such a filler metal would be called “soldering”. 

First of all then, let’s define what we mean by the “liquidus” temperature of a BFM. When any BFM is heated, it will reach a temperature at which it will start to melt. Below that temperature the BFM will remain solid, but once it crosses that temperature it will start to melt. That temperature is called the “solidus” temperature of the BFM. Then, as heating continues and more and more of the BFM melts, it will finally reach a point where all the BFM has finally melted, and become completely liquid. It will be said to have crossed the “liquidus temperature” for that BFM. Technically, the “liquidus” temperature is determined by, and defined as, the temperature at which a molten BFM begins to solidify upon cooling from its fully-molten state. But for our purposes here in this article, I will merely assume that when a BFM crosses its liquidus temperature during heating, it will become fully liquid (molten). Liquation is not being considered. by Dan Kay

This article completes the series of Five Main Reasons that vacuum is used in science and industry; To provide a working force, to remove active and reactive constituents, to remove trapped and dissolved gases, to decrease thermal transfer and finally to increase the mean free path to a useful dimension.

The article printed back in January this year talked about solid, liquid and gas states of matter. The following is a short excerpt from that article. “In a gas the atoms and molecules are generally much further apart than in solids and liquids. In air at atmospheric pressure and room temperature the actual space occupied by atoms and molecules is about 0.01 per cent or one ten thousandth of the volume. The equivalent for solid copper is about 74 percent or close to three quarters. (So much for being called a “solid”). In air the molecules are in constant random movement, typically in a straight line, and the interatomic forces have little effect due to the space between the molecules. The moving molecules will constantly collide with other molecules and then move away in a different direction. These collisions occur about 10,000,000,000 times per second at atmospheric pressure.”. By Howard Tring