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

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

Many brazing shops use graphite fixtures on which to set parts that are to be brazed. Graphite fixtures have excellent thermal stability, enabling them to be used again and again through many brazing cycles. This high thermal stability is often coupled with low cost compared with some metals used for making fixtures (such as the high nickel/chromium or moly- alloy type fixtures).

If you are using graphite fixtures, or intend to consider doing so, it is VERY important to remember that graphite is carbon, and pure carbon likes to react chemically/metallurgically with metals containing iron (such as steel), to form low-melting eutectic compositions at temperatures just under 2100°F/1150°C, as shown in the iron-carbon (Fe-C) phase diagram illustrated in Fig. 1. This temperature is often lower than some of the brazing temperatures being used in many brazing shops today! by Dan Kay

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

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

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

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

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

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

For a number of years I have been encouraging people to re-think the ramp-rates they use for their vacuum brazing cycles.  Many brazing shops using rather high ramp-rates during heating claim that “this is the way we’ve always done it”.  Perhaps it’s time to re-think this. From a metallurgical point of view, too-rapid a heating rate can lead to stresses and strains in the metal assemblies being heated, which can often lead to distortion of parts during their heat-up, and can even lead to parts-failure (I’ve seen this too many times).

I have recommended to a number of brazing shops that they slow down their heating ramp rates (and I’ve seen excellent results), using the following guideline: Heat the parts at the fastest rate that will allow you to bring all the parts (assemblies) up to brazing temperature without the need for any holds (for temperature-equalization) on the way up. by Dan kay