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 vacuum heat treatment is used by both captive and commercial heat treaters and spans such diverse markets as Aerospace, Automotive, Electronics, Optics, Housewares, Industrial Products, Tool & Die, Military/Defense and Farm Implement to name a few. Most processes run in the temperatures range of 175°C – 730°C (350°F – 1350°F). Special applications extend these ranges down to as low as 120°C (250°F) and up to as high as 925°C (1700°F), but this is unusual. Temperature uniformity in dedicated furnaces is considered excellent throughout the standard temperature ranges listed. By Dan Herring

The movement of gases is an important and interesting subject but one often dismissed as a topic best left to scientists. However, the Heat Treater needs to know something about the basic nature (theory) of gases and in particular how they behave in vacuum. The main difficulty is that too much theory tends to become a distraction. Our focus here will be to better understand what goes on inside a vacuum furnace.

One definition of a gas is that it is simply a collection of molecules in constant motion (Fig. 2). The higher the temperature, the faster these molecules move, and as one might expect, the motion of gas molecules stops or dramatically slows down at or near absolute zero (0°K). As molecules speed up with an increase in temperature, there is an increase in their kinetic energy (or energy of motion). Molecular collisions occur between molecules and if contained, these molecular collisions against the walls of their container result in a pressure rise (which always occurs in a closed container when a gas is heated). In other words, pressure is simply the force per unit area that a gas exerts on the walls of its container. By Dan Herring

Highly distortion prone gearing (Fig. 1) was the subject of an investigation into the dimensional changes which result from utilizing either oil or high pressure gas quenching following a low pressure vacuum carburizing process. For comparative purposes, the gears in question were also atmosphere gas carburized and plug quenched, which is standard practice for these geometries. Full production loads (Fig. 2) were run using two (2) different carburizing methods (atmosphere, vacuum) in combination with free quenching in either oil at 75°C (165°F) or high pressure gas (nitrogen) at 11 bar.

Gears were taken from multiple locations throughout each load for analysis. Parts for metallurgical evaluation were selected from the center of each load. Multiple areas on each part were then analyzed for microstructure, case depth, and hardness (surface, profile, core). Dimensional checks (out of round, gear tooth profiles) were conducted on the gears before and after heat treatment. For brevity, only a portion of the complete test program is presented here (see Reference 4 for more detail). By Dan Herring

Over the years, many people have asked if we could recommend good books on the subject of Vacuum Heat Treatment. The following list includes books that we have found particularly useful  with respect to the scientific and practical aspects of vacuum, heat treatment, metallurgy and material science. Enjoy..

As you can tell from the list, some books are classics which have stood the test of time, some are relative newcomers, but all share the common trait that they are used each and every day by those of us who work in the fields of vacuum, heat treatment and metallurgy. The readers are encouraged to offer suggestions as to their favorite and most useful texts. By Dan Herring

A residual gas analyzer or RGA for short is a compact mass spectrometer, designed for use either in the laboratory or out on the shop floor. These devices are often mounted for in-situ use on a vacuum furnace. RGA’s are typically designed for process control and contamination monitoring in vacuum systems.

Applications for residual gas analyzers include distinguishing leaks from outgassing, fingerprinting the process background, detecting helium and determining the effectiveness of gas line purging. A typical RGA gas analysis can reveal how much of a particular species is present either in the vacuum vessel or in the pump manifold. RGAs are used in most cases to monitor the quality of the vacuum and easily detect minute traces of impurities in the low-pressure gas environment. These impurities can be measured down to 10^-14 Torr levels, possessing sub-ppm detectability in the absence of background interferences. By Dan Herring

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. By Dan Herring

The history of vacuum technology is a fascinating one. It seems to have begun in ancient Greece when the philosopher Democritus (circa 460 to 375 B.C.) proposed that the world was made up of tiny particles that he called atoms (atomos, Greek: undividable). Democritus proposed that empty space (in other words, in modern terminology, a vacuum) existed between the atoms, which moved according to the general laws of mechanics. Democritus, together with his teacher Leucippus, may be considered as the inventors of the concept of a vacuum. Our modern view of physics is heavily influenced by the ideas of Democritus.

However, it was the thinking of Aristotle (384 – 322 B.C.) that dominated the scientific community up until the 16th century. Aristotle denied the existence of a vacuum as it conflicted with the idea that the universe was comprised of countless individual particles. According to Aristotle, nature consisted of the four basic elements namely water, earth, air, and fire.  In fact, the word vacuum comes to us from the Latin word “vacuus” meaning empty or “vacare” meaning “to be empty”. By Dan Herring

A manufacturer of quartz products for the lighting industry was curious as to the origin of black “flakes” (particles) found on the outside and inside surfaces of their quartz tubes after heat treatment. These flakes appeared to be “fluffy bits of carbon”. The thought process to investigate this phenomenon presents a unique learning experience for us all.

One of the last steps of the quartz-production process is the heat treatment of the quartz tubes, which takes place in one of several vacuum furnaces at this manufacturer’s facility. The quartz tubes are heated under vacuum to 1050°C (1220°F) and held at temperature for several hours. This is followed by a quench with nitrogen. The vacuum furnaces in question have graphite heating elements, a combination ceramic fiber/felt insulation pack with a molybdenum hot face and stainless steel cold face and a graphite hearth. The quartz tubes themselves are placed onto graphite fixtures (racks). By Dan Herring

IGO is a surface phenomenon that is most often associated with atmosphere gas carburizing (Fig. 1). The consequence of IGO (and the concentration gradients that develop during oxide formation) is that the material adjacent to the oxides has modified transformational behavior. Instead of forming martensite on quenching, steels with this condition develop non-martensitic transformation products (e.g. bainite, pearlite), which adversely affect mechanical properties (e.g. hardness, residual stress, bending fatigue)..

The rate of diffusion of oxygen into a steel surface is dependent on the oxygen potential of the furnace atmosphere and the process variables (i.e. the depth of oxide penetration is influenced by case depth, time at carburizing temperature, carbon potential and the chemical composition of the steel). During the carburization process, the oxygen atoms (which are about 35% smaller than the iron atoms) are released as a direct result of the presence of water vapor and carbon dioxide in the gas. Oxygen diffuses slowly into the steel surface (as does carbon and hydrogen, albeit more quickly) and migrates to the grain boundaries. Once in the steel, oxygen combines chemically with the elements already present (e.g. chromium, titanium, manganese) that have an affinity for oxygen. By Dan Herring