Tungsten is used in vacuum furnaces when there is a need for structural integrity at elevated temperature and/or in situations where other materials may degrade, such as when lower melting point eutectics are a concern. One example of its use in is roller rail assemblies in which graphite wheels are positioned between molybdenum rails using tungsten axles.

Tungsten (chemical symbol W) is a member of the family of refractory metal (Mo, Nb, Re, Ta, W) and has the highest melting point and vapor pressure of this group. Due to this unique property, it is commonly used as a material of construction in specific areas of vacuum furnace hot zones operating above 1315ºC (2400ºF). Tungsten can also be used for heating elements given that it has the highest duty temperature, typically 2800°C (5075°F). In practice, this rating is often downgraded as it is for all heating element material choices. Tungsten will become brittle, however, if exposed to oxygen or water vapor and is sensitive to changes in emissivity. In general, tungsten is resistant to corrosion below 60% relative humidity. By Dan Herring 

But carbon-steels present a unique situation to designers and brazing companies, because when being heated all the way up to copper-brazing temperature, the metals will actually go through a temperature-range where the steel will actually be contracting (shrinking) while being heated, and then do just the opposite when cooling, thus potentially causing distortion and/or fracturing of brazements during a high-temp brazing cycle. Such a scenario is illustrated in Fig. 1 where an automotive fuel rail brazement failed to braze properly, because some unique CTE problems associated with carbon-steels was not properly taken into account during the furnace brazing cycle. by Dan Kay 

Measurement of the amount of phases or constituents in metals and alloys is probably the most commonly performed quantitative microstructural test. The amount present is usually referred to as the volume fraction, although it is rarely expressed as a fraction but usually as a percentage. The volume fraction, or V_V, in stereological terms, is the volume per unit volume of the phase or constituent. However, there is no simple direct way to measure the volume fraction. Instead we measure the area fraction, A_A, a lineal fraction, L_L, or a point fraction, P_P, which can be measured and correlate with the volume fraction: V_V = A_A = L_L = P_P (1).

Areal analysis was first described by Delesse, a French geologist, in 1848. As the minerals were rather coarse in size, he could measure the area fraction of the grains of interest compared to the total two-dimensional area. As microstructures are rather fine in size, this is not a simple method to perform manually. Delesse suggested that a linear ratio of dimensions could also be used, but he thought that the accuracy would not be as good and did not try to develop a lineal analysis method. Rosiwal, a German geologist, was the first to publish a lineal fraction method in 1898 to assess the volume fraction. The point counting method to assess the volume fraction came much later and was proposed by Thompson in 1933, by Glagolev in 1933 and by Chalkey in 1943 – each working in different countries and different fields of science. By George Vander Voort

Metallographic techniques for cast irons are similar to those for steels; with the exception that graphite retention is a more challenging task. Recommended procedures to prepare cast irons are given. Colloidal silica is an excellent final polishing abrasive for many metals and alloys. However, for pearlitic cast iron grades, colloidal silica often produces small etch spots on the specimen surface. In this case, OP-AN alumina suspension yields excellent results, much better than standard alumina abrasive powders made by the calcination process. Examples of cast iron structures revealed using a variety of etchants is presented.

New concepts and new preparation materials have been introduced that enable metallographers to shorten the process while producing better, more consistent results. But first, the specimens must be sectioned. Many metallographers do not use a blade designed for metallography work, and the depth of damage will be much greater when production-type abrasive saws are used. So, as a first rule, produce a cut with the least possible amount of damage. If an automated device is used that holds a number of specimens rigidly (central force), then the first step must remove the sectioning damage on each specimen and bring all of the specimens in the holder to a common plane. This first step is often called “planar grinding.” SiC paper can be used for this step, although more than one sheet may be needed. Alternatively, the metallographer could use MD-Piano 120 or 220 (for specimens with hardness >150 HV) for the initial grind, followed (if desired) by MD-Piano 600 for a second grinding step. If the cast iron has a low hardness (<250 HV), one can planar grind with MD Primo 220. Alternatively, MD-Allegro could also be used to planar grind for specimens >150 HV hardness. If the hardness is <150 HV, MD-Largo can be used. By George Vander Voort

Vacuum furnace hot zones are manufactured using materials that can withstand temperatures in the range of 1315ºC (2400ºF) and higher. Of the various types of refractory metals in use, none is more common than molybdenum.

The popularity and widespread use of molybdenum in vacuum furnaces is due to the wide range of properties that it exhibits, namely: high melting point, 2620ºC (4748ºF), low vapor pressure, high strength at elevated temperature, low thermal expansion, high thermal conductivity, high elastic modulus, high corrosion resistance, and elevated recrystallization temperature, between 800º – 1200ºC (1470º – 2190ºF). Mechanical properties of molybdenum are influenced by purity, type and composition of any alloying elements and by microstructure. Properties such as strength, ductility, creep resistance and machinability are enhanced by additions of alloys such as titanium, zirconium, hafnium, carbon and potassium along with rare earth element (La, Y, Ce) oxides. By Dan Herring 

As mentioned in a previous blog-article, magnesium (Mg), often referred to simply as “mag”, is a highly effective “getter” that is used when vacuum-brazing aluminum. Because Mg is very effective at gettering (reacting with and removing) both oxygen and moisture that may be present in a vacuum-furnace atmosphere during aluminum-brazing operations, it can effectively prevent (or minimize) the reaction of these elements with aluminum, thus allowing aluminum-brazing to occur.

However, magnesium is a highly combustible metal, and when it condenses on the walls of a vacuum-furnace during aluminum brazing operations, extreme caution must be exercised in removing the condensed mag from the furnace walls during subsequent furnace clean-up, so that no sparks are generated which could cause rapid ignition of the condensed magnesium, resulting in explosive combustion, and even death. To prevent this, coating the walls of the vacuum furnace with a “non-stick” surface, may be highly effective. by Dan Kay 

Resistance to gas flow through components that make up a vacuum system has a considerable effect on the pumping speed and pressure obtainable within the system. Any pipe or component that gas has to flow through is a hindrance to the flow of that gas, i.e. it offers resistance to the flow. It occurs in the roughing line, the foreline and the high vacuum piping and affects the amount of time taken to evacuate a vacuum chamber to its required base or process pressure.

For example, if a large vacuum chamber is connected to its vacuum pump using a long small bore pipe the gas flow down the pipe will be difficult and the gas will be removed from the chamber very slowly. There is a high resistance to gas flow and the conductance of that small bore pipe is low. Let’s first look at a simple vacuum system, using a single mechanical vacuum pump, the vacuum chamber if often mounted right above the inlet to the vacuum pump. By Howard Tring

Copper and its alloys are among the most malleable metals and alloys in existence. Cartridge brass, Cu – 30% Zn, has been used for many years to produce cartridge cases for ammunition due to its superior cold forming characteristics. This article shows the microstructure and hardness of cartridge brass from the fully annealed to the heavily cold worked condition. Then, it illustrates the influence of annealing temperature and time on removing the effect of the cold work and returning the alloy to a very low hardness annealed structure.

Cartridge brass, Cu – 30% Zn, is a single-phase Cu-based alloy where the addition of zinc increases the strength of copper by solid solution strengthening. The maximum solubility of zinc in copper at ambient temperature is slightly above 30% Zn. Higher levels of Zn, for example, 40% Zn, produce two phased α-β brass which is less malleable than the single phase, α-Cu cartridge brass. Cartridge brass, as the name states, has been used for many years to make cartridges for bullets due to its excellent formability and good cold formed mechanical properties. As an example, Figure 1 shows the microstructure of the starting cup with an annealed α-Cu grain structure, exhibiting annealing twins, used to cold form cartridge cases. Figure 2 shows the firing pin end of a formed 338 caliber cartridge case revealing a heavily cold worked microstructure. Color etching is far more effective than black & white etching to reveal the complete grain structure and deformation. Comparisons of color vs. B&W etching will be presented later. By George Vander Voort

Lubricants in vacuum applications include wet and dry lubricant types (Table 1), greases and oils. So-called “wet” lubricants tend to stay wet on the surface to which they are applied, while dry lubricants go on wet but dry as they are applied. In general solid particulates do not stick to dry lubricants but they do not tend to last as long as wet lubricants and as such need to be reapplied. By contrast, greases adhere better than oils and tend to last longer. Oil is preferred where the lubricant needs to be circulated.

The major disadvantage of conventional liquid lubricants is that they have relatively high vapor pressures (= 1.3 x 10-4 Pa at room temperature) and surface diffusion coefficients (= 1 x 10-8 cm2/s) with low surface tensions (in the order of 18 – 30 dyne/cm) and can volatilize or creep away from areas of mechanical contact resulting in high friction, wear or mechanical seizure. In addition, their volatility can cause issue with achieving proper vacuum levels and/or depositing on component part surfaces. The presence of other gaseous species in a vacuum environment (e.g., water vapor, oxygen, carbonaceous gases) can cause the force of adhesion between metal surfaces joined by liquid lubricants to be so strong that the joined areas can only be separated by fracture. By Dan Herring 

Please note that a braze fillet is actually a casting along the outside of a braze joint that simply shows that the brazing filler metal (BFM) has melted and flowed along the edge of a braze joint. However, it doesn’t tell you if the BFM has adequately penetrated the joint. Caution is therefore strongly advised to anyone attempting to merely use the size of a braze-fillet as an inspection criteria for judging the overall quality of a braze joint. by Dan Kay