The role of materials science is to study, develop, design, and perform processes that transform raw materials into useful engineering products intended to improve the quality of our lives. It is said by many that material science is the foundation upon which today’s technology is based and that real-world applications would not be possible without the materials scientist. The discipline has expanded to encompass materials for many highly specialized product applications.

The industrial revolution thrust metals into the forefront of technology, and they have stayed there ever since becoming the very foundation on which our modern society is built. One cannot envision a life where our transportation and communications systems, buildings and infrastructure, industrial machines and tools, and safety/convenience devices that are not an integral part of our daily lives. Metallurgy is that part of materials science and materials engineering which studies the physical and chemical behavior of metallic elements, intermetallic compounds and their alloys. This definition is all-encompassing and includes the study of processes run in furnaces and ovens, the forging and rolling of metals, foundry operations, electrolytic refining, creation and use of metal powders, welding, heat treatment and much more. One might say that metallurgy is the application of knowledge of materials while materials science focuses on the theory of the structure, properties, processing, and performance of engineering materials.

One of the most critical components on any vacuum furnace is the pressure relief valve. While its function is clear, the fact that it needs to be inspected – and tested is either not well understood or simply ignored. Normally positioned atop a vacuum furnace it is in an area that is not always conducive to maintenance, and complicated in many instances by the fact that only the manufacturer can service them. 

What is a Pressure Relief Valve? A pressure relief valve is a safety device designed to protect a vacuum furnace from over-pressurization. An overpressure event refers to any condition that would cause the pressure to increase beyond the specified design pressure (the so-called maximum allowable working pressure or MAWP). The pressure relief valve is an integral part of the safety system provided on most vacuum furnaces. Vacuum vessels, including evacuated chambers and associated piping, pose a potential hazard to personnel and the equipment itself from collapse, rupture, or implosion.

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.

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.

Besides catastrophic damage due to mechanical abuse, eutectic melting or braze alloy spillage, vacuum furnace hot zones will deteriorate over time as a result of the repeated thermal cycling to which they are exposed.