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”.

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).

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.

Today, tool steel heat treatment is based on a simple premise, that to obtain the optimum performance from any given grade, every step of the heat treating process— including stress relief, preheating, austenitizing, quenching, deep freeze/cryogenic treatment and multiple tempers—must be done exactly correct. Absolute control of both process and equipment variability is one of many reasons why vacuum processing (Fig. 1) is popular in tool steel heat treatment ^[1].

The selection of any tool steel depends on a combination of factors including component design, application end use and performance expectation. For any given application (Fig. 2) the goal of heat-treating is to develop the ideal microstructure to help achieve the proper balance of desired properties (Table 1) namely hot (red) hardness, wear resistance, deep hardening and/or toughness.

The main components of a landing-gear structure are wheels and brakes, axles, bogie beams (a.k.a. truck beams), shock absorbers (a.k.a. shock struts), and drag and side braces. Primary design considerations on landing gear include maximum sink speed, spin up, spring back, lateral drift (on landing), towing, jacking, turning, braked roll, taxi, rebound, pivoting (main landing gear only), crashworthiness and fatigue.

Secondary loads include retraction/extension, aerodynamic loads, lock/unlock loads and emergency extension. In addition, nose landing-gear specific forces include dynamic breaking, nose-gear yaw and steering. Alloys used in these applications must have high strengths, normally between 260-300 ksi (1,792-2,068 MPa). By Carmine Filice, Daniel H. Herring, Paul Vanderpol