Over the years the brazing and welding industries have noticed something strange that sometimes happens when joining 304-stainless steel assemblies for a wide variety of applications exposed to outdoor weather. They noticed that sometime after the weldment or brazement was placed in service in situations where the stainless-assembly was exposed to moisture (such as in outdoor applications for automotive, aerospace, and tooling applications, etc.), the stainless steel started rusting, as if it were made from a regular carbon-steel rather than stainless-steel.

In the weldments, as shown in Fig. 1, the rust was limited to a rust-band up to about a half inch wide (a centimeter or more), located about that same distance away from, and parallel to, each side of the weld (i.e., along both sides of the weld). In the brazements, which has been furnace-brazed, the rusting was more general, generally spread over the entire exposed surface of the furnace brazed component. by Dan Kay

One of the questions all Heat Treaters are asked is, “How much, if at all, will my part change (i.e. shrink or grow) during heat treatment?” While the heat treater would love to be able to give a precise answer to this question, in most situations volumetric size change during heat treatment cannot be accurately predicted, at least not accurately enough to allow for final machining and/or grinding to close tolerances prior to heat treatment.

Experimental work has been done on many materials to show the effects of heat treatment on size change. As one might expect, the effects are different for every material grade. For example, an 80 mm (3.15”) cube of D-2 tool steel (Fig. 2) reveals growth (0.08%) in one dimension and shrinkage in the other two dimensions as a result of vacuum hardening. This graph demonstrates how knowing the part orientation from the mill-supplied bar is important when trying to plan for size change during heat treatment. By Dan Herring, THE HERRING GROUP Inc., and Patrick McKenna, Nevada Heat Treating, Inc.

It is surprising to hear people misuse several common metallurgical terms when they are trying to describe braze-prep or braze-inspection criteria. Two of these misused terms that I’ll talk about in this article are “Passivation” and “Defect”.

Passivation vs. Pickling – There is some confusion in the brazing industry regarding the correct use of the term “passivation” (when “pickling” is actually meant) when it comes to preparing metal surfaces for brazing.  The two terms, “passivation” and “pickling”, have completely opposite meanings, and thus, these two terms need to be clarified so that brazing personnel can use these two metallurgical terms properly.. by Dan Kay

VAC AERO vacuum furnace manufacturing is part of the VAC AERO International group of companies with over 50 years of technical experience specialising in design and manufacture of a wide range of advanced vacuum furnace systems, hot zones and controls for the processing of metals, ceramics, specialty alloys, and engineered materials. VAC AERO offers a wide range of…

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In many vacuum systems, especially those where the chamber is large, has a large internal surface area and the chamber load adds extra surface area, the pump down can be slowed substantially when the chamber pressure drops to the range where the water vapor molecules on the surface desorb and have to be pumped away.

Pressure and temperature determine when this vapor desorbs, but at ambient temperature around seventy two degrees Fahrenheit or twenty degrees Centigrade the vapor desorbs from about 50 Torr down to about 0.1 Torr. The vapor pressure of water at ambient temperature is about 18 Torr, so that is where maximum desorption may occur. By Howard Tring

Meteorites have fascinated mankind for centuries. Indeed, more than two dozen meteorites have been venerated by Indian tribes, aborigines, Arabs and other ancient peoples. The study of meteorites is part of the overall study of the origin of our solar system. There was a recent meteor explosion over the city of Chelyabinsk with up to 1000 injuries. Think what the damage would have been like if it hit a major city. Some asteroids are exceptionally large, and when they strike earth, they can make an immense crater. Some of these, as in Figure 1, are in arid climates and can be seen today. Such an impact near the Yucatan Peninsula has been claimed to have caused the extinction of dinosaurs.

There are three basic types of meteorites: stones, stoney-irons and iron. The classification of meteorites is a complex subject. For the iron meteorites, classification is based upon chemical composition, macrostructure and microstructure. Basically, iron meteorites “fall” (no pun intended!) into three categories – hexahedrites, octahedrites and ataxites. Some, however, do not fully fit the requirements of these groups and are classed as anomalous. By George Vander Voort

Monitoring the actual quality of the brazing atmosphere inside a vacuum-furnace during brazing cycles is very important, and is not hard to do. When vacuum brazing, you have to wait until the brazed parts are removed from the furnace at the end of the brazing cycle in order to see if everything was actually okay during that brazing cycle. If, when opening the furnace after a brazing cycle, you see that the parts you were trying to braze are discolored or poorly brazed, then how can you determine exactly what went wrong during that cycle, and how can you know when the brazing problem actually occurred (did it happen during heating, or during cooling, etc.)? Also, how do you determine whether the poor braze results are caused by physical problems with the furnace itself, or if they might be related to the brazed-component’s base-metal (parent-metal) composition, or perhaps with the brazing filler metal (BFM)? by Dan Kay

All of the common (and several uncommon) heat treatment processes can be run in vacuum, from annealing and brazing to sintering and tempering. Many companies that currently outsource vacuum heat-treating ask themselves if they would be better served by setting up this capability in-house. Others who already have an in-house heat treat department wonder if switching to vacuum processing will offer them a competitive edge. This article will help address these questions.

Vacuum furnaces are typically characterized by their method of loading, horizontal or vertical, as well as if there is internal load movement, being classified as either batch or continuous (i.e. multi-chamber) types. The various furnaces sizes, production capabilities and feature configurations are almost endless and detailed extensively elsewhere. Since most vacuum furnaces have a life expectancy of 40 – 50 years, decisions as to what to purchase, and from whom, become very important. By Dan Herring

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

There are three groups of vacuum gauges based on the method of operation, mechanical, thermal conductivity and ionization. For this discussion we will only talk about the thermal conductivity and ionization gauges because purely mechanical vacuum gauges are generally not used on vacuum furnaces. By Howard Tring

Finding leaks in vacuum furnaces is a task that few of us cherish but all of us know is important and necessary. Leaks are a problem experienced by almost every vacuum user. Leaks can occur suddenly or develop slowly over time but in either case they are damaging both to product quality and to furnace internal components.

In extreme cases, the problem is obvious: the furnace will not pump down and/or the hot zone (or heating elements) shows obvious signs of attack. Tiny leaks, however, are more common often going undetected because of the pumping systems ability to overcome them. However, even small leaks can cause continuous and sometimes catastrophic damage. Thus, routine leak checking and leak repair should be a part of any good vacuum furnace maintenance program. By Dan Herring