The basic equations defining (see equations 1 and 2) the Knoop (HK) and Vickers (HV) hardness, where the applied force is multiplied by a geometric constant and then divided by the long diagonal squared or the mean diagonal squared, respectively, cause an inherent problem in measuring small indents, that is diagonals ≤20 µm in length.  Figure 1 shows the calculated relationship between the diagonal and load and the resulting hardness for Knoop indents while Figure 2 shows this relationship for Vickers indents. As the test load decreases, and the hardness rises, the slope of the curves for diagonal versus hardness becomes nearly vertical. Hence, in this region, small variations in diagonal measurements will result in large hardness variations.

If we assume that the repeatability of the diagonal measurement by the average user is about ±0.5 µm, which is quite reasonable, and we add and subtract this value from the long diagonal length or the mean diagonal length, we can then calculate two hardness values. The difference between these values is ΔHK and ΔHV, shown in Figures 3 and 4. From these two figures, we can see how the steepness of the slopes shown in Figures 1 and 2 will affect the possible range of obtainable hardness values as a function of the diagonal length and test force for a relatively small measurement imprecision, ±0.5 µm. These figures show that the problem is greater for the Vickers indenter than for the Knoop indenter for the same diagonal length and test force. For the same specimen and the same test force, the long diagonal of the Knoop indent is 2.7 times greater than the mean of the Vickers’ diagonals, as shown in Figure 5. By George Vander Voort

In last month’s article, we looked at the use of titanium-“getters” when vacuum-brazing high-temperature base-metals that are very sensitive to oxidation.  In this month’s article, let’s look at how magnesium (Mg) is used as a “getter” when vacuum-brazing at temperatures of only about 1000-1100°F (540-600°C), as needed for joining aluminum base metals.

Magnesium (Mg), often referred to simply as “mag”, can be highly effective at gettering both oxygen and moisture that may be present in a vacuum-furnace atmosphere being used in aluminum-brazing operations. Aluminum (Al) reacts readily with oxygen to instantly form a tenacious Al-oxide layer on its surface.  This Al-oxide layer is very stable, and, if mechanically removed, will quickly re-form.  Thus, in real life, a layer of aluminum-oxide will constantly be present on the aluminum surface before, during, and after aluminum brazing.  Dealing with that oxide layer has proven to be a challenge to many brazing shops over the years. by Dan Kay

This article continues the series of Five Reasons that vacuum is used in science and industry. The first “To Provide a Working Force” was published in December 2013. In addition to writing about the reason itself I will try to give one or more examples of the reason in practical applications. If you know of other applications that fit the reason, I will be pleased to hear from you.

2. To Remove Active & Reactive Constituents.

For many vacuum applications the process is a cyclic one. The workload is placed in the chamber, the chamber is evacuated and the process takes place. The chamber is then let back up to atmospheric pressure and the process workload is removed. These cycles can be as short as a few seconds in the case of coating a small communications crystal (Fig.1) to give it a specific frequency, or it can take several hours in the case of a heat treating cycle that may include evacuation, heating, soaking, cooling and eventually back to atmospheric pressure. In this type of cycle the main “constituent” being removed is “air”. By Howard Tring

Burlington, Ontario, February 18, 2014 – VAC AERO has recently commissioned a horizontal vacuum furnace at a major South Korean university metallurgical research center.

The VAH5058 HV-8 furnace chamber is 36” wide X 36” high X 58” long with a hot zone comprised of high efficiency graphite felt, carbon composite and curved graphite elements. It is equipped with a gas quench system capable of quickly cooling the workload from processing temperatures, at quench pressures of up to eight bar. The furnace operating system is based on VAC AERO’s versatile HC900 interactive hybrid control package with SCADA and complete network integration capabilities and remote monitoring and control.

March 9, 1862 marks the date when the USS Monitor and the CSS Virginia (formerly the USS Merrimack) fought an indecisive naval battle at Hampton Roads that changed naval warfare from wood and sails to iron and steam. The USS Monitor sunk off the Outer Banks of North Carolina during a storm on December 31, 1862 but its remains were discovered in 1973. The wreck site, the Monitor National Marine Sanctuary, is managed by the National Oceanic and Atmospheric Administration (NOAA).

The Confederates began construction of an “ironclad” ship at the Gosport Yard of Hampton Roads in 1861. This was well known to the Union Navy Department. The US Army had actually launched ironclad gunboats in the summer of 1861 to patrol the Mississippi River; but none were available in the east to counter the Virginia. On August 3, 1861, Gideon Wells (Secretary of the Navy) requested design proposals for ironclad warships. Swedish inventor John Ericsson had designed an ironclad in 1854 for Napoleon III that incorporated a revolving cupola turret. Cornelius Bushnell promoted this design to Abraham Lincoln. By George Vander Voort

Vacuum brazing is a growing industry, with more and more companies entering it each year, due primarily to the bright, clean, as-brazed component surfaces resulting from brazing in a vacuum environment, which, when conducted properly, allows brazed components to be used immediately, with no additional cleaning operations needed after brazing.

Of course, that assumes that the vacuum furnace is clean and tight, with a minimal leak-up rate.  Leak-up rate?  What?  Do vacuum furnaces leak?  Yes, every vacuum furnace, unfortunately, is leaky!  There are many fittings, connections, seals, etc., on each vacuum furnace, and it is very important that all such seals and connections be as leak-tight as possible.  Otherwise, air will leak into the furnace through any of those potential leak-paths and the pressure inside the furnace will start to go back up toward atmospheric. This “leak-up” rate must be measured for each vacuum-brazing furnace, and that information made available to brazing personnel prior to starting any vacuum brazing cycle. by Dan Kay