One of the most widely used charts in the field of brazing is the strength vs. clearance chart created from work done in the Handy & Harman laboratories in Fairfield, Connecticut back in the 1930’s. This chart is shown below, in Fig. 1:

Notice that as the joint clearance gets tighter and tighter (moving from right to left along the bottom axis), the tensile strength (as shown on the vertical axis on the left-side of the chart) gets higher and higher. Although there is a lot of experience with this over the years, and general acceptance of this information is widespread, it must be pointed out that this chart is very specific only to the actual testing performed in making this particular chart, and may not be identical to tests performed by others using similar materials or conditions. But the general principal of increased joint strength with tighter gaps can be accepted. by Dan Kay

Aside from the well-known grain size measurement techniques using either the planimetric methods of Jeffries or Saltykov, or the intercept method of Heyn, Hilliard and Abrams, one can measure the grain size through a count of grain-boundary triple point intersections within a known area through the use of Euler’s law. This technique has rarely been used but it should be possible to do such a count by image analysis. In general, measurements based on point counts (0 dimensional) are less subject to errors than lineal measurements (one-dimensional) which are less subject to error than areal measurements (two-dimensional). By George Vander Voort

During vacuum brazing with nickel-based brazing filler metals (BFMs), it is possible to hold the brazed parts at brazing temperature long enough for the BFM to solidify completely while being held at brazing temperature! The key is “diffusion”, and involves tiny interstitial atoms in the BFM.

Isothermal solidification can be a very useful brazing process for some brazing filler metals (BFMs), and can result in a significant increase in the re-melt temperature of the BFM in that brazed joint. To better understand the process, let’s first examine the component parts of the phrase “isothermal solidification”. “Iso” essentially means “ equal, or the same”, and “thermal” of course refers to temperature. So we’re looking at a BFM solidification process in which that solidification takes place while the furnace is being held at the same, steady temperature! Although that may sound strange, there’s some real logic to it. Isothermal solidification (we’ll refer to it as ITS in this article) depends a lot on the diffusion capabilities of various components of the BFM while that BFM is being held at the brazing temperature. by Dan Kay

Vacuum gauges all measure the pressure readings in the range from atmospheric pressure down to some lower pressure approaching absolute zero pressure, which is not attainable. Some gauges read the complete range and others can only read a portion of the range, usually used for very low pressures.

If you have a typical vacuum furnace it is normal to have at least three electronic vacuum gauge heads mounted on the system to monitor the level of vacuum at selected positions. These gauge heads send signals back to the controls system and the vacuum readings are used to ensure that the vacuum pumps are working correctly and that the process chamber is at the correct low pressure (vacuum) for the specific process. To many casual observers the readings and names of the measuring units being used are like a foreign language, and they may well be because many names were derived in Europe. Let’s take a look at the different vacuum measurement units in use around the world and where the names came from. By Howard Tring

If a specimen has been cold worked, or it did not recrystallize after hot working, the grains will not be equiaxed and extra care must be taken when assessing the specimen’s grain size. Always test a longitudinally oriented plane first to determine if the grains are, or are not, equiaxed.

A low-carbon sheet steel was tested in the as-received condition (reportedly annealed), and after cold reductions in thickness of 12, 30 and 70%. Above 70% reduction, it can be quite difficult to reveal the ferrite grain boundaries well enough to get a precise measurement. Measurements were made on the three principal planes using the Jeffries planimetric method, the Abrams three-circle intercept method and the intercept method using directed parallel test lines. By George Vander Voort

Most powder used in the manufacture of brazing filler metal (BFM), to be used in either its pure powder form, or blended to make a brazing paste, is initially produced by a gas-atomizing process.

This process begins with molten metal that is poured through an atomizing nozzle at the top of a tall atomizing tank, in which high-pressure/high-velocity inert gas hits the molten stream, blasting it into billions of droplets which then cool into individual tiny particles of powder as they fall down to the bottom of the tall atomizing tank, where the powder will then be collected for further processing. by Dan kay

The two previous articles covered methods for measuring grain size that have been incorporated into ASTM E112 for many years. The Jeffries planimetric method was introduced into standard E2 in 1917 – Committee E-4’s first standard. Zay Jeffries was a founding member of the committee and had published several articles about the method, which he learned from his PhD advisor, Albert Sauveur, the dean of American metallographers.

This method is precise, but a bit slow for production work because the grains must be marked off as they are counted manually. The method, however, can be modified for image analysis work. The second method was the Heyn intercept method, which was developed in Germany in 1903 and was mentioned briefly in ASTM E2, but not described in detail, when published in 1917. The intercept method was later modified by John Hilliard and then by Halle Abrams. The Abrams three-circle intercept method is used in production work as the intercepts (or intersections) do not need to be marked off on a template when counted. But, the writer recently has introduced the Saltykov rectangle to E112 as it can yield accurate grain size measurements down to fewer counts per field than the other two methods. As with the Jeffries method, the Saltykov method does require marking of the grains for accurate counting, although it, too, can be used by image analysis. By George Vander Voort

In simplest terms, backstreaming is the movement of pumping fluid back into the vacuum furnace chamber, that is, oil vapor molecules attempt to reverse course and move up and back toward the vacuum vessel, opposite to the direction of the desired gas flow.

Backstreaming is not limited to the pumps themselves, but encompass the entire pumping system (e.g. plumbing, valves, baffles and traps). The oil type and characteristics play a role as well. In all cases, the result of backstreaming, namely the contamination of the work chamber or workload, is totally unacceptable and often catastrophic. Backstreaming is often due to: Incorrect start-up or shutdown procedures – the far most common operator mistake as far as the writer is concerned; Exceeding maximum throughput capacity for long periods of time; Exceeding the critical discharge pressure in the foreline. By Dan Herring

“It was only a tiny drop of water, now and then,” lamented the home owner. “How was I to know that all those little drops would add up to a huge water bill?” The same can be said of a heat treat furnace that is always down for this reason or that. Avoiding the hidden costs associated with equipment downtime is the key to saving money.

Maximizing furnace productivity requires a proactive approach, which must continue throughout a unit’s operational lifetime. This requires careful planning and anticipation of problems. The process should begin even before the purchase of a piece of equipment by matching equipment and supplier capabilities with production and process needs. Buying good, well-built, high-quality equipment and operating and maintaining it properly will avoid most hidden costs. By Daniel Herring

Oakville, Ontario, January 7, 2013 – The new column, written by vacuum pump expert Howard Tring will be published monthly and will offer helpful vacuum pumping tips and techniques to a worldwide audience of individuals using vacuum methods and vacuum equipment. The purpose of this column is to inform, educate and answer questions for commercial…

Read More