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In this month's article on differential expansion of metals, I will explore the effect that thermal expansion has on joint clearance, and thus, on brazed joint strength and quality of 1018 carbon-steel. For anyone doing high-temperature brazing of carbon-steel components using a high-temperature brazing filler metal (BFM) such as pure copper, it's an important concept, and needs to be understood. By Dan Kay
Next Month: In my next article, we'll take a closer look at thermocouples -- which kind you should use, and how it should be connected to the parts in your furnace load, where they should be placed in the furnace, etc.
Last Updated on Sunday, 02 February 2014 22:10
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In this article I will explore the effect that thermal expansion has on joint clearance, and thus, on brazed joint strength and quality. It's an important concept, and although it is well known in the brazing world, many folks today still do not take this topic seriously enough when designing brazed assemblies. This article is based on one I wrote many years ago for an in-house brazing publication at a brazing filler metal supplier, and will be written in two parts. Next month's segment will look more closely at polymorphic metals, such as carbon steels, and will attempt to explain why they exhibit their very strange thermal expansion curves. By Dan Kay
Next Month: In my next article, I'll examine the thermal expansion curve for 1018 carbon steel to see why there are strange "reversals" in the thermal expansion curve for that alloy (and for similar metals).
Last Updated on Sunday, 02 February 2014 22:10
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Brazers commonly encounter voids in brazed joints and often wonder where they come from and how to avoid them in future brazements. Some common sources of voids in braze joints are: surface contamination, base metal and brazing filler-metal (BFM) constituents, brazing methods/temperatures used and poor joint fitup. The first three items listed above can often result in gas bubbles being formed in brazed joints. Such gas-bubble voids will usually try to form in spherical shape as they move through a joint. The "rounded" edges of such bubble-voids can often be clearly seen in cross-section photomicrographs of brazed joints, especially under high magnification. The inside surfaces of a bubble-void will often appear "clean" or "shiny" as well. By Dan Kay
For successful brazing to occur, the joints to be brazed have to be designed properly, and then properly manufactured to attain and maintain those shapes and dimensions. This second article looks at joint clearance considerations in more detail. Joint clearances must be close together and parallel. The amount of clearance between the faying surfaces (the mating surfaces inside a joint being brazed) should ideally be kept small, on the order of about 0.000"-- 0.002" (0.000-0.050 mm) total, so that capillary action can most effectively pull the molten brazing filler metal (BFM) completely into and throughout a braze-joint. By Dan Kay
Next Month: In next month's article we'll address some additional factors in joint design, specifically the topic of "differential metal expansion". All metals expand at different rates when heated, and since braze-joint clearances are calculated based on expected clearances at brazing temperature, we need to know how to properly optimize brazing of different metals in the same assembly.
Last Updated on Sunday, 02 February 2014 21:59
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There are basically two types of joint designs used in brazing: butt-joints and lap-joints. All other joint designs are modifications of these two. There are a number of important considerations when designing such joints in order to insure proper service life. This article looks at just a few of those considerations. By Dan Kay
Next Month: Next month I'll discuss the issue of braze gap clearance for some different base-metal and brazing filler metal (BFM) combinations, and how that affects joint strength and hermeticity. In succeeding months we'll address issues such as dissimilar metal brazing and how that affects joint design.
Last Updated on Sunday, 02 February 2014 21:57
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Good brazing depends on the ability of capillary action to draw the molten brazing filler metal (BFM) in all directions throughout the joint being brazed, either vertically or horizontally. A unique Vertical Capillary Test Specimen (VCTS) was developed to help brazers find out the maximum gap-thicknesses that capillary action can fill in their particular brazing furnaces. By Dan Kay
Next Month: Next month I'll discuss some important design issues regarding braze gap clearances and configurations, and in succeeding months we'll address issues of dissimilar metal brazing, and joint strength and its optimization in more detail.
Last Updated on Sunday, 02 February 2014 22:18
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Read more: How to Determine Braze Gap-Clearances for Your Furnace
The effective use of "metallurgical fixturing", instead of a lot of dead-weights, to effectively "load" parts with enough pressure to keep braze joints close together for effective brazing is described in detail.
The use of heavy weights on top of parts being brazed is a common practice. Its purpose is to load the top of the assembly with enough weight so as to insure that the components of the assembly will be pressed together sufficiently to keep the joints from opening up during furnace brazing. This should then insure that good capillary action of the brazing filler metal (BFM) into those joints will occur during the furnace cycle. But in real-life brazing, such use of dead-weights can lead to extended cycle times, and fail to so what it was supposed to do. This article explains why. By Dan Kay
Last Updated on Sunday, 02 February 2014 21:58
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Read more: Are the use of dead-weights for fixturing a recommended practice?