Vacuum Heat Treating with Dan Herring Archive
by Joseph
The Use of Molybdenum in Vacuum Furnace Applications
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
by Joseph
Dry Lubricants for Vacuum Service
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
by Joseph
Tips for Selecting Vacuum Furnace Equipment – Part Three
We continue our discussion on the factors that must go into the decision making process during the acquisition phase of a vacuum furnace. Part One focused on how one goes about choosing the right furnace for the job and talked about the various choices for hot zones (e.g. insulation, heating elements, etc.). Part Two discussed pumping systems, controls and ancillary support items (e.g., grids/baskets/fixtures, water systems, features & options). It is now time to understand how your supplier partner will handle the project once an order is received, including project management, codes & standards, approvals, installation, commissioning and long-term support.
Order Processing; Okay, you have selected a vendor partner to supply your vacuum furnace, so what type of support should you now expect? To begin with, it is important to understand how your order will be handled internally by the company you have selected. The first task they face is to transfer the order from their sales team to their engineering team and, ultimately, to the manufacturing and service/support teams. After receipt of an order, your supplier partner will typically schedule an engineering “kickoff” meeting where the project is given to engineering and a project manager (or project engineer) is assigned. Out of that meeting will be generated the final equipment specification and this should be provided to the purchaser shortly (e.g., 1 – 2 weeks) after receipt of the purchase order for approval. By Dan Herring
Tips for Selecting Vacuum Furnace Equipment – Part One
Tips for Selecting Vacuum Furnace Equipment – Part Two
by Joseph
Tips for Selecting Vacuum Furnace Equipment – Part Two
We continue our discussion on the many factors that must go into the decision making process during the acquisition phase of a vacuum furnace. Part One focused on how one goes about choosing the right furnace for the job and talked about the various choices for hot zones (e.g. insulation, heating elements, etc.).
We now continue this discussion by looking at other common vacuum furnace features and options. Recall that the four common elements of any vacuum furnace are; Hot zone (c.f. Part One), Heating elements (c.f. Part One), Pumps and Controls. Once decisions have been made in these areas, other ancillary items (e.g. partial pressure control, loaders, etc.) must also be considered and will be talked about here as well. By Dan Herring
Tips for Selecting Vacuum Furnace Equipment – Part One
Tips for Selecting Vacuum Furnace Equipment – Part Three
by Joseph
Tips for Selecting Vacuum Furnace Equipment – Part One
Acquisition of a vacuum furnace represents a major capital equipment investment and one that creates a long-term relationship with your supplier partner. Thus the choice of what to buy and who to purchase it from requires careful planning and considerable up-front research.
You need to know when and how to apply vacuum technology, if it will be the most cost effective solution for what you need to do, what questions to ask and what information to provide. The process begins by understanding your specific needs and asking all the right questions. Is it more prudent to upgrade an older piece of equipment, purchase new or purchase used? Is it better to have one large furnace or two smaller ones? Is a batch solution best or is a continuous approach better? By Dan Herring
by Joseph
Vacuum Heat Treatment of Fasteners
Fastener applications are demanding. Whether fasteners are being used in the petrochemical industry, in medical or mining applications, for assembly of marine or nuclear components or in the aerospace, automotive or construction world, vacuum processing allows us to repeatedly achieve the highest quality and metallurgy.
Most fastener materials, including stainless steels and superalloy grades, benefit from or actually require vacuum processing for heat treatment instead of being run under protective atmospheres. In general, there are three main sets of applications that where vacuum heat treatment is used: processes that can be done in no other way than in vacuum; processes that can be done better in vacuum from a metallurgical standpoint or processes that can be done better in vacuum from an economic standpoint. By Dan Herring
by Joseph
Maintenance of Vacuum Furnaces – Part Three
We continue our discussion on the maintenance of vacuum furnaces. In Parts One and Two we talked about establishing a sound maintenance strategy, implementing that strategy and working on critical components of the vacuum system to return the unit to full operational status. Let’s continue this discussion.
General Maintenance – The following additional topics comprise the critical areas and systems that routinely require maintenance to ensure they operate correctly. Hot Zone Maintenance – After ensuring proper ventilation and following all safety guidelines with respect to asphyxiation and confined space entry, the interior of the hot zone should be inspected after every load. The bottom of the hot zone should be cleaned of all debris and foreign matter and the heating elements and heating element connections inspected for damage and tightness. Graphite heating elements can in some instances be patched and the damaged section replaced with a new element section. Molybdenum heating elements can be repaired although no more than three (3) repairs are recommended per element band. By Dan Herring.
by Joseph
Maintenance of Vacuum Furnaces – Part Two
We continue our discussion on the maintenance of vacuum furnaces. Part One talked about establishing a sound maintenance strategy and once this has been determined, the real maintenance work can begin. Let’s talk about the specifics on what, when, how and why we maintain certain critical components on our vacuum furnaces.
Vacuum furnaces come in all shapes and sizes and have many common features and operational/maintenance needs. However, it is important to understand the particular needs of the vacuum furnaces in your shop to have an effective maintenance program. When performing maintenance it is important to have a written plan defining the specific task to be performed, and the reason why a particular task is necessary (i.e. purpose of the task). A work order should be issued and the work signed off upon completion (which includes testing to ensure that the repair was successful). By Dan Herring
by Joseph
Maintenance of Vacuum Furnaces – Part One
Today, the maintenance of heat treatment equipment is a point of major emphasis and this is especially true for vacuum furnaces. Over the next few articles we will explore various aspects of vacuum furnace maintenance providing useful tips and practical techniques to simplify the work and make sure that it is done correctly. Let’s begin by understanding the importance of the role of maintenance, and more specifically, how planned preventative maintenance is helping to manage the overall cost of equipment operation.
Maintenance is a fact of life for heat treat equipment. In general, the cost of maintenance increases dramatically as the operating temperature increases and/or the process environment becomes more severe (e.g. carburizing versus hardening). This remains true in vacuum furnaces despite the fact that they are often operated below their maximum temperature ratings. As with all equipment, some styles and designs require more attention than others. It is interesting to note, however, that construction of heat treat equipment can often be classified as “heavy duty” or “light duty” by the amount of maintenance required. Of course, if any furnace is operated outside their design limitations, this almost always translates to a need for more extensive maintenance. By Dan Herring
by Joseph
Vapor Pressure and Evaporation in Vacuum Furnaces
Knowledge of vapor pressure and rates of evaporation of various materials is valuable information for those operating vacuum furnaces, whether we are heat treating or brazing at high temperature and low vacuum levels or dealing with outgassing at very low temperatures and pressures.
When we think about a solid or liquid in a sealed vessel, we find that, even at room temperature and atmospheric pressure, there are molecules that leave the surface and go into the gaseous phase. The gas phase thus formed is called a vapor. The process of forming a vapor is known as evaporation and the rate of evaporation is determined by the temperature of the substance involved. In time, some of the evaporated molecules will, in all likelihood in the course of random movement, strike and stick to the surface of the vessel. This process is known as condensation and the rate of condensation is determined by the concentration of gas molecules (that is, the pressure of the evacuated gas). Eventually, the number of molecules leaving the surface of the substance is equal to the number returning to it (that is, the evaporation rate equals the condensation rate) and we have dynamic equilibrium. The (partial) pressure at which this occurs is known as the vapor pressure of the substance.2 Below this pressure, surface evaporation occurs faster than condensation, while above it, surface evaporation is slower. By Dan Herring