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Archives for June 2018

Low Temperature Vacuum Processing

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Low-temperature vacuum heat treatment offers unique advantages to a variety of industries including Aerospace, Automotive, Electronics, Household Appliances, Machine Tools and Tool and Die as well as Commercial Heat Treaters who must serve all of these customers. Low-temperature heat treatments that involve a vacuum purge at the onset of the cycle have become increasingly popular throughout the industry. These operations are conducted in vacuum furnaces and furnaces that employ a vacuum purge prior to the beginning of the heat process with parts placed inside a special vacuum tight vessel or in a retort. Processing being run using these methods take advantage of a highly controlled environment designed to minimize surface interactions.

Low-temperature processing can be batch or continuous, either as stand-alone units or “modules” incorporated into a continuous vacuum furnace system. The following is a basic description of the operation of a typical batch vacuum furnace. Once a workload has been positioned into the unit, an outer door is closed and the vacuum process can commence. A mechanical vacuum pump, optionally equipped with a blower, produces a vacuum level as low as 1.3 x 10-3 mbar (1 x 10-3torr), a common vacuum level being under 1.3 x 10-1 mbar (1 x 10-1 torr). This is normally achieved in 10 – 20 minutes depending on the size of the pumping system and the nature of any contamination present on the workload. In some instances, a double pump-down sequence is initiated once an initial vacuum level lower than 6.7 x 10-1 mbar (5 x 10-1 torr) is reached. Once the desired final vacuum level is reached, the unit is backfilled in the range of 667 mbar (500 torr) to 3.4 x 10-2 bar (0.5 psig) positive pressure with an inert gas such as nitrogen, argon or nitrogen/hydrogen (3% maximum) and heating begins. After reaching setpoint and soaking at temperature, a cooling cycle is initiated, typically with hot gases circulated through an internal or external heat exchanger to accelerate the process.

Five Main Reasons for using Vacuum

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In this article, we’re going to take a step away from vacuum pumps and systems and write about general applications that use vacuum in the process. There may be some applications you have heard about and some, hopefully, that may be new to you. Whenever a vacuum (a pressure lower than the surrounding atmospheric pressure) is used in a process it will generally fall into one of the Five Main Reasons for using Vacuum. In some cases, a process may use vacuum for two of the five reasons. This month I will discuss the first of these reasons, in no specific order.

First, a short explanation of the two vacuum measuring units used in this article. We know that standard atmospheric pressure is 14.7 lbs. in-2 and that your real life atmospheric pressure varies up and down a few percentage points from the standard depending on a) weather conditions in your area and, b) your altitude above sea level. Remember that “low” pressure is the same as “high” vacuum, and conversely “low” vacuum is the same as “high” pressure – but still below atmospheric pressure in the vacuum industry. I have written a small number and read many technical articles about vacuum and it is very difficult to ensure that these terms are consistent throughout. In the example below we also have to understand that the vacuum measuring units read in opposite directions and we have to change one of them. Does that sound confusing? Yes, it is.

Metallography of Iron-Nickel Meteorites

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Meteorites have fascinated mankind for centuries. Indeed, more than two dozen meteorites have been venerated by Indian tribes, aborigines, Arabs and other ancient peoples. The study of meteorites is part of the overall study of the origin of our solar system. There was a recent meteor explosion over the city of Chelyabinsk with up to 1000 injuries. Think what the damage would have been like if it hit a major city. Some asteroids are exceptionally large, and when they strike earth, they can make an immense crater. Some of these, as in Figure 1, are in arid climates and can be seen today. Such an impact near the Yucatan Peninsula has been claimed to have caused the extinction of dinosaurs.

There are three basic types of meteorites: stones, stoney-irons, and iron. The classification of meteorites is a complex subject. For the iron meteorites, classification is based upon chemical composition, macrostructure, and microstructure. Basically, iron meteorites “fall” (no pun intended!) into three categories – hexahedrites, octahedrites, and ataxites. Some, however, do not fully fit the requirements of these groups and are classed as anomalous. Displays of meteorites in museums generally consist of large, solid chunks of iron meteorites and of etched slices, as shown in Figures 2 to 6. These slices are ground smooth and then etched with a strong acid solution that brings out the growth structure. The octahedrites are commonly exhibited in this manner because they undergo a solid-state phase transformation where the kamacite (ferrite) nucleates and grows along the octahedral planes of the parent taenite (austenite) phase producing a beautiful etched pattern.

Vacuum Furnace Heat Exchangers: Design, Function & Factors Affecting Performance

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In the first part of this article, we focus on heat exchangers used in vacuum gas quenching furnace systems (Fig. 1) and examining their design and function. In the second part, we review the similarities and differences between internal and external types of heat exchangers as well as the advantages and limitations of each design.

In vacuum processing, the load being heated in the furnace is rapidly cooled at the end of the heat cycle to impart desired physical properties in a process referred to as quenching. Although the metallurgical reasons for quenching vary depending on the process used, in all cases the goal is to quickly cool the load. Gas quenching involves introducing an inert gas (i.e. nitrogen, argon, or helium) into the furnace and rapidly circulating it through the heating chamber under pressure to remove the stored thermal energy from the load. While being circulated, the gas is forced through a heat exchanger to remove its heat.

There are several interesting and important factors that play a role in the design of a heat exchanger used for quenching and cooling of workloads in a vacuum furnace. In order to understand the function of the heat exchanger in the quenching process, we first want to review the fundamentals of the heat exchanger operation.

A heat exchanger is a device that transfers heat from one fluid (liquid or gas) to another fluid (liquid or gas) without the two fluids coming in direct contact. The type of heat exchanger typically used in a vacuum furnace is the finned tube type. Heat is first transferred from the hot gas to the fins and then to the tubes by convection, then through the tube wall by conduction and finally from the tube interior to the cold fluid inside the tube, again by convection. The efficiency of a heat exchanger is highly dependent on the mode of heat transfer, with convection being the dominant form of heat transfer in fluids. However, the conductivity of the materials must also be considered.
The basic equation (Equation 1) governing convection heat transfer is Newton’s law of cooling.