The most important materials used in man’s history have lent their names to the periods concerned, namely the stone, copper, bronze, and iron ages. Although it would be an exaggeration to call modern times the ‘graphite age’, graphite has become an indispensable and reliable material for the manufacture of modern high-performance products made from metal and ceramics. Graphite-based composites, in particular, are gaining constantly in importance. In the high-temperature applications field, these composites include carbon fiber-reinforced carbon (C/C for carbon composites) and carbon fiber-reinforced silicon carbide (C/SiC), as well as rigid and flexible felts based on pitch or viscose fiber. Carbon fiber-reinforced carbons (C/C for short) are used in modern vacuum or protective gas furnaces in the form of heating elements or charging systems. They are characterized by thermal shock resistance, the absence of distortion, low mass and strength increase with rising temperature. These features enable users to operate their plants more effectively, minimize reject rates and therefore reduce the cost of production. CIC is thus a key element in many process optimization steps and helps companies to improve their competitiveness.

Towards the end of the 19th century, carbon fibers based on pyrolyzed bamboo were used to manufacture incandescent filaments. In the 1950s, fibers with aligned crystalline structures were first produced in Great Britain. The starting materials now used for the production of carbon fibers are viscose (rayon), PAN (polyacrylonitrile) and pitch. Among these raw materials, the one with the poorest electrical and thermal conductivity is viscose. It is therefore often used as a thermal insulation material in the form of felts. Felts can be used up to a temperature of 2700°C. Pitch materials cost less than PAN, but the subsequent treatment required makes the pitch more expensive. by Alexander Racek, SGL CARBON GmbH

Applications involving vacuum heat-treating are typically performed for one of the following reasons:

All of the common (and several uncommon) heat treatment processes can be run in vacuum, from annealing and brazing to sintering and tempering. Many companies that currently outsource vacuum heat-treating ask themselves if they would be better served by setting up this capability in-house. Others who already have an in-house heat treat department wonder if switching to vacuum processing will offer them a competitive edge. This article will help address these questions.

Vacuum furnaces are typically characterized by their method of loading, horizontal or vertical, as well as if there is internal load movement, being classified as either batch or continuous (i.e. multi-chamber) types (Fig. 1 – 2). The various furnaces sizes, production capabilities, and feature configurations are almost endless and detailed extensively elsewhere ^{[1], [2]}. Since most vacuum furnaces have a life expectancy of 40 – 50 years, decisions as to what to purchase, and from whom, become very important.

Cold wall vacuum furnaces dominate the vacuum furnace industry and are available in a variety of sizes and workload configurations. In comparison to hot wall furnace designs in which the process is run inside a vacuum-tight retort, the cold wall design runs the process inside a sealed vacuum vessel. As such, the furnace shell must not only be suitable for vacuum service but for operation at positive pressure.

Before 1960, metallurgical heat-treatment processes in vacuum equipment were limited to specialized applications and relatively simple processes such as the annealing of copper wire. The introduction of industrial cold-wall furnaces increased the number of applications considerably because of the possibility to quench the charge by forced gas circulation (at pressures slightly below atmospheric). By the early 1970’s, cold-wall furnaces with oil quenching capability were introduced and by the mid-1980’s high-pressure gas quenching designs became available. Since the 1980’s the quench pressures have steadily advanced, from sub-atmospheric to as high as 25 bar, with 5 and 6 bar, 10 bar, and 20 bar dominating the industry.

Vacuum furnaces can be either hot wall or cold wall designs. There are notable differences between the two, and each has specific advantages and limitations. In this and our next article, we will examine these differences and describe which applications are best suited for each.

Early in the history of vacuum furnaces, hot wall designs were the only ones available. These furnaces, just as the ones today, utilize a retort (aka muffle) into which the load is placed. The retort is then sealed. A pump evacuates the retort and the process runs either under vacuum or at a specific (negative or positive) pressure once the initial vacuum level is reached. The advantages of using a retort include the fact that the furnace surrounding the retort can be gas-fired or electrically heated and that the retort can be rapidly pumped down. Hot wall designs are generally less expensive to manufacture than their cold wall counterparts. In addition, since the volume inside the retort is relatively small, the pumps are smaller and it takes less time to reach the required vacuum level for the process than a cold wall furnace of comparable size.

During the steelmaking process, while the molten steel is still in the ladle and before it is poured, the steel must be degassed in order to: (1) reduce/eliminate dissolved gases, especially hydrogen and nitrogen; (2) reduce dissolved carbon (to improve ductility); and (3) to promote preferential oxidation of dissolved carbon (over chromium) when refining stainless steel grades.

In the steel smelting process, unwanted gases are dissolved in the liquid, which could produce any number of imperfections and defects. A common method used to remove these undesired gases is vacuum degassing. The process is done after the molten steel has left the furnace and before being poured into ingots or introduced into a continuous caster. Throughout the early part of the 1900s, the steelmaking industry was plagued by poor quality steel This was due to the fact that during the production process, hydrogen and nitrogen gases were present in the molten steel. During cooling and subsequent solidification, the dissolved hydrogen caused pinholes and porosity in the final product since hydrogen has low solubility in steel at ambient temperature. The result was the release of hydrogen during solidification. Even the presence of a few parts per million of hydrogen gas causes defects and loss of yield strength.

The development of particle accelerators was previously discussed (see Part 1 and 2 on Vacuum Technology at CERN). The use of the large hadron collider (LHC) located at CERN with its enormous, multiple vacuum systems is intended to investigate the fundamental particles of the universe and one of the most exciting in recent years being the search for the Higgs boson. Here we also take a closer look into the technologies used in the LHC, discussing some of the other discoveries it has led to, as well as reviewing past contributors to the field of vacuum technology whose innovations made the LHC possible.

The Standard Model of Particle Physics – Developed in its current form in the 1970’s, the so-called “Standard Model” of particle physics is a theory of fundamental particles and how they interact. It integrated all that was known about subatomic particles at the time and predicted the existence of additional particles as well. The Standard Model describes three of the four known fundamental forces in the universe – electromagnetic forces, weak nuclear, and strong nuclear interactions.

Vacuum furnaces can use a variety of different gases during the processing cycle in partial pressure operation, for backfilling to atmospheric pressure at the end of the processing cycle and for cooling/quenching. The most common of these gases (in order of frequency of use) are nitrogen, argon, hydrogen, and helium. Other common gases include various hydrocarbons and ammonia (for vacuum carburizing/carbonitriding) and specialty gases such as neon (for certain electronics applications).

In vacuum heat treatment nitrogen is used primarily for cooling/quenching, as a partial pressure gas and for backfilling to atmospheric pressure at the end of the heat treating cycle. A common misconception, however, is that nitrogen gas is a true inert gas. It is not and under the wrong circumstances, it can react with the surface of the material being heat treated with deleterious effects. Nitrogen has a gaseous specific gravity of 0.967 and as such is slightly lighter than air (whose specific gravity is 1.0) and has a boiling point of -195°C (–320.5°F) at atmospheric pressure. It is colorless, odorless and tasteless. Commercially, nitrogen is produced by a variety of air separation processes, including cryogenic liquefaction and distillation, adsorption separation and membrane separation (Special note: membrane separation technology typically does not produce a gas with high enough purity for vacuum use).