Last month we discussed Gas Molecules and Gas Flow and at the end of the article mentioned the term Conductance. This time we will talk a bit more about conductance in vacuum system piping and why it has to be taken into consideration in the design of a typical vacuum furnace or similar vacuum system.

Firstly though, we will discuss Throughput. Have you ever wondered why vacuum pipes and connections are of several different sizes on any vacuum system? I would suggest that most users don’t really give it any thought. It is what it is. So let’s look at the sections of a vacuum system and again try to visualize those gas molecules, which are so tiny we can’t see them, and understand the conditions at different places in the system. By Howard Tring

I am often asked about the differences between brazing and soldering.  Perhaps this is a good time to describe the two processes in more detail, so that readers can understand the significant differences between them.

There are some similarities between soldering and brazing, but many significant metallurgical differences.  They are both used to join metals together to form a bond between the metals being joined, but the bonding mechanisms are very different.  Let’s take a look at these two processes, and see how they compare. by Dan Kay

Decarburization occurs when carbon atoms at the steel surface interact with the furnace atmosphere and are removed from the steel as a gaseous phase (1-8). Carbon from the interior will then diffuse towards the surface, that is, carbon diffuses from a region of high concentration to a region of low concentration to continue the decarburization process and establish the maximum depth of decarburization (MAD).

Because the rate of carbon diffusion increases with temperature when the structure is fully austenitic, the MAD will increase as the temperature increases above the Ac₃. For temperatures in the two phase region, between the Ac₁ and Ac₃, the process is more complex. The diffusion rates of carbon in ferrite and in austenite are different and are influenced by temperature and composition. By George Vander Voort

A manufacturer of quartz products for the lighting industry was curious as to the origin of black “flakes” (particles) found on the outside and inside surfaces of their quartz tubes after heat treatment. These flakes appeared to be “fluffy bits of carbon”. The thought process to investigate this phenomenon presents a unique learning experience for us all.

One of the last steps of the quartz-production process is the heat treatment of the quartz tubes, which takes place in one of several vacuum furnaces at this manufacturer’s facility. The quartz tubes are heated under vacuum to 1050°C (1220°F) and held at temperature for several hours. This is followed by a quench with nitrogen. The vacuum furnaces in question have graphite heating elements, a combination ceramic fiber/felt insulation pack with a molybdenum hot face and stainless steel cold face and a graphite hearth. The quartz tubes themselves are placed onto graphite fixtures (racks). By Dan Herring

If you are a homeowner with a garden and a lawn, watering and mowing are regular tasks that need to be carried out. When you are watering your flower bed or vegetable patch at home you turn on the hose tap and see the water coming from the spray nozzle. You can then direct the water to the places where it is needed. Similarly, when mowing your lawn you can see what area you have already cut and can direct the mower to cut the next area of long grass. These tasks are made easier because you can see what you are doing. Trying to water or mow with your eyes covered would be much more difficult.

When you are evacuating a vacuum system, one of the biggest problems is that you can’t see what you are evacuating. Gas molecules are so small that you cannot see them. So how do you know when they have been moved out of the system and how do you know the best way to move enough of them to allow you to complete your particular process? In this article we will try to understand what gas molecules are and how they behave at different pressures in a vacuum system. If you have a mental picture of the molecules using your imagination, it can possibly help you to understand how your vacuum system works and solve problems with its operation if something goes wrong. By Howard Tring

IGO is a surface phenomenon that is most often associated with atmosphere gas carburizing (Fig. 1). The consequence of IGO (and the concentration gradients that develop during oxide formation) is that the material adjacent to the oxides has modified transformational behavior. Instead of forming martensite on quenching, steels with this condition develop non-martensitic transformation products (e.g. bainite, pearlite), which adversely affect mechanical properties (e.g. hardness, residual stress, bending fatigue)..

The rate of diffusion of oxygen into a steel surface is dependent on the oxygen potential of the furnace atmosphere and the process variables (i.e. the depth of oxide penetration is influenced by case depth, time at carburizing temperature, carbon potential and the chemical composition of the steel). During the carburization process, the oxygen atoms (which are about 35% smaller than the iron atoms) are released as a direct result of the presence of water vapor and carbon dioxide in the gas. Oxygen diffuses slowly into the steel surface (as does carbon and hydrogen, albeit more quickly) and migrates to the grain boundaries. Once in the steel, oxygen combines chemically with the elements already present (e.g. chromium, titanium, manganese) that have an affinity for oxygen. By Dan Herring

On any machine that has a rotating shaft there will be a shaft seal of one type or another. If the machine is a simple electric motor, for example, the seal may be used just to retain the lubricant in the bearings and to prevent dust and dirt from entering the bearing. This type of seal generally needs little or no maintenance for small motors from ¼ to perhaps 10 HP.

The shaft seals in any pump that the electric motor drives are ones that do need maintenance and replacement, whatever type of pump it is. If the pump moves liquids, such as a centrifugal water pump, it is important that the seal doesn’t leak although in some applications a small amount of leakage can be tolerated. If the liquid being pumped is hydraulic fluid, it may be at high pressure and the seal would be designed to withstand that pressure without failing. By Howard Tring

A common concern in the brazing field is the cleanliness of parts that are to be brazed.  Some people think that cleanliness can be achieved by merely heating the parts in a furnace or via a torch-flame, and those high-heat conditions will effectively “burn off” any surface contaminants and render the parts sufficiently brazeable. NOT TRUE!

Parts that are going to be brazed need to be thoroughly cleaned prior to assembly for brazing, which usually involves degreasing of parts and thorough drying. Then, once the parts have been cleaned, they need to be handled and assembled with clean hands in order to maintain that surface cleanliness. So one important question to answer is:  “Can I get my hands clean enough to adequately handle surfaces that will be brazed? by Dan Kay

Come visit Us at the ASM Heat Treating Expo, Booth 1818

We’re excited to be exhibiting at the ASM Heat Treating Society Conference and Expo and look forward to seeing you in Indianapolis, IN – September 16-18. Our sales representatives will be available to answer questions about how VAC AERO can help your business with exactly the solutions that you’re looking for. See you there!

Vacuum furnaces come in all shapes and sizes but common to each is that they require some type of vacuum vessel. Most vacuum vessels in the heat treating industry are cylindrical in shape and either horizontal or vertical in orientation. In general, vacuum vessel designs are made as small as possible so long as they don’t hamper the process being run in the vessel. There are several obvious reasons for this including the cost of the vessel itself as well as the vacuum hardware involved. In addition pumping systems become larger and operating costs tend to be higher (e.g. more gas is required for quenching).

Designs for vacuum chambers involve two distinct aspects; one structural and one related to the process application. The basic structural design falls in the realm of mechanical engineering following established industry guidelines, an example of which is the ASME Pressure Vessel Code for vacuum furnaces utilizing high pressure gas quenching (above 14.7 psig). The design must take into account both the external and internal forces acting on the vessel to prevent buckling (i.e. structural instability) and deal with overpressure issues, both major concerns from a strength of materials standpoint. By Dan Herring