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Discolored Work in a Vacuum Furnace – The Heat Treat Community Answers the Clarion Call

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Question: Staining of titanium parts after vacuum heat treatment following a gas quench, your thoughts on possible causes and remedies? 

In response to this question, phenomenal suggestions by everyone! A wealth of great information here. So, what else could be happening? Let The Doctor add a few thoughts to the discussion. First, the fact that the discoloration (staining) is brown in coloration suggests that the oxide is forming on the part surface during cooling when the temperature is in the range of (approximate) 245ºC – 270ºC (475ºF – 520ºF). This is supported by the fact that the oxidation does not occur “during natural cooling” (which we assume to mean cooling under vacuum). Second, the fact that the discoloration is more evident at the bottom of the load suggests the phenomenon is (gas exposure) time dependent, that is, the longer the parts take to cool through the critical range, the greater the chance for discoloration. Third, a “steel-copper-stainless steel” test will be helpful in isolating if it is a water or air leak. Leaks in heat exchangers have been known to “open up” during the cooling cycle when exposed to hot gases and close up again at room temperature. The writer has personally experienced this – the solution being the replacement of the old heat exchanger at which time the problem went away. Fifth, look in all the places suggested by those who responded, but remember to make only one change at a time and evaluate its impact in order to find then correct the problem. Finally, as an unabashed promotion of my books on Vacuum Heat Treatment (Volume II of which comes out this fall), there are a number of sections that discuss this very issue in considerable detail covering subjects such as “Vacuum Furnace Contamination and Cleanup Cycles”, “Leaks External to the Vacuum Furnace Proper” and “Factors Affecting Performance: Discolored Work” to name a few.

Vacuum Pump Oil: The “Circulatory System” of the Vacuum Furnace

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Just as vacuum pumps can be considered the heart of the vacuum furnace, so too can the oil be thought of as its circulatory system. The selection and properties of the oil are critical to proper furnace operation. Pump oil serves different purposes in different types of pumps, and even has different functions within the same pump. In addition to lubrication, it helps provide the seal on rotary vane and other wet pumps, and serves as the media to propel the pumped gas via kinetic action in diffusion pumps.

Oil Formulations – Different pump oil formulations are specifically designed for different pump applications and careful consideration must be given to the oil selection. Typical motor oil, for example, is not sufficiently refined for use in a vacuum pump, has insufficient resistance to chemical attack, and contains additives that may be detrimental to the process being performed in the vacuum furnace. In addition, the viscosity must be considered. Lower viscosity oils are used for lower operating temperatures, and for smaller pumps, and medium viscosity oils are used for medium to large pumps. Temperature resistance is also critical, as many pumps operate at high temperatures, and the oil must be rated for these temperatures. Many of the oils used in vacuum pumps are not traditional oils at all, but made of silicone or other non-hydrocarbon fluids.

Coating Gas Turbine Engine Blades Using HVOF

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In recent years, the operating temperatures of land-based gas turbine engines have increased to improve efficiency.  As a result, greater demands are placed on the materials used in the manufacture of the engine components.

In particular, hot section turbine blades must function in a very severe operating environment.  The blades are usually manufactured from advanced nickel-based superalloys but these materials on their own are still not durable enough.  To enhance their durability, they are protected from hot corrosion and high-temperature oxidation through the use of special coatings.  The coatings form adherent oxide layers that inhibit the blade material from directly interacting with potentially damaging elements within the combustion gases like oxygen, sulphur, and other contaminants.  A popular approach involves coating the blades with an MCrAlY bond coat topped with a thermal barrier coating (TBC) overlay. BY JEFF PRITCHARD

Vacuum Science and Engineering

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We study vacuum science and vacuum engineering in order to better understand the role vacuum technology plays in creating useful engineered products (Fig. 1, Table 1). Manufacturing as we know it, and research and development as we have come to depend upon it, would not exist without the creation and control of the vacuum “atmosphere”.. 

Vacuum techniques are important in both the industrial setting and for the scientific community, whether it be in heat treatment or high-energy physics. At the heart of vacuum processing for manufacturing is the modern vacuum furnace. Ever since the introduction of the electric light at the beginning of the 20th century, society and manufacturing have been linked to advances in vacuum science and engineering. Examples include the development of modern computers to advanced transportation systems; the very fabric of modern society depends on vacuum technology.

Dry Pumps: Claw Pumps

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Every industrial vacuum furnace system uses a primary (aka mechanical) pump, which is also commonly referred to as a “backing” pump when either used in series with a booster pump, or used with both a booster and secondary (“high vacuum”) pump combination, such as a diffusion pump. These primary pumps are further divided into “wet” pumps (e.g., oil sealed rotary vane style or liquid ring pumps) or “dry” pumps (e.g., claw, hook or screw). Of the dry primary pumps, the most common types are the claw pump and the screw pump. 

Dry pumps are being increasing popular as an alternative to oil sealed rotary vane pumps for many medium and high vacuum applications (e.g., in low-pressure vacuum carburizing where fine granular soot is carried from the process into the pump). Designers and users of vacuum furnaces must have a good understanding of how claw and screw pumps operate. This includes the principles of operation, pump design, sealing, operating characteristics, features, purging, and ancillary devices.

Overview of Common Vacuum Furnace Equipment

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Vacuum furnaces are available in numerous styles and sizes and come in both standard and custom configurations. They are designed to process an almost limitless number of both semi-finished component parts as well as raw materials using a diverse set of thermal processes in equipment available from a wide variety of different equipment manufacturers located around the world.

The intent here is to provide a brief overview of some of the more common designs and applications found throughout the heat-treatment industry. The hope is that the reader will come away with an understanding that there is a vacuum furnace solution to virtually any design, application or specification encountered.

Why Vacuum?

Historically, vacuum heat-treating applications were classified in one of four areas, namely processes that could be:

  • Performed using no technology other than vacuum
  • Done better in a vacuum from a metallurgical perspective
  • Done better in a vacuum from an economic standpoint
  • Done better in a vacuum from a surface enhancement perspective

Metallurgy for the Vacuum Heat Treater

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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.