Keyword

Eliminating "holds" in heating rates during vacuum-brazing cycles

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.

Let’s look at an example of how this might be done. Figure 1 represents a temperature chart for a typical vacuum brazing cycle, such as might be used when brazing at a temperature of approximately 1975°F/1080°C with a high-temperature nickel-based brazing filler metal (BFM) like AMS 4777 (AWS A5.8, Class BNi-2). In the absence of any information to the contrary, I always recommend brazing at a temperature approximately 100F/50C above the published liquidus of the BFM being used. This will insure that the BFM will indeed become fully molten, and will have enough extra thermal energy to flow well and spread out by capillary action.

furnace-chart-sm-3
Fig. 1 - Typical furnace chart showing two (2) built-in temperature-stabilization holds, one at about 1250°F (675°C) for 45-minutes, and another at about 1750F (950C) for 30-minutes, with a hold at brazing temperature for an additional 30-minutes at about 1975°F (1080°C). Brazing is with AMS 4777/BNi-2 (solidus temp = 1780°F/970°C, liquidus = 1830°F/1000°C).

The chart shows two (2) temperature-stabilization holds built into that cycle, and represents the temperature being measured by the main furnace thermocouple (TC) which extends slightly into the top of the furnace hot-zone.

Total cycle time in the example shown in Fig. 1 is about 3.5 hours, more than a third of which (36%) is merely the time spent in two (2) temperature-stabilization holds during the heating portion of the furnace cycle. To hopefully eliminate those two holds, a straight line is drawn on the chart from the start of the heating cycle up to the end of the “time-at-brazing-temperature” line, to determine what heating-rate would hopefully get you up to brazing temp without the need for any stabilization holds. In this case, that straight line represents a heating-rate of approximately 12°F (6°C) per minute, and connects with the end-point of the "time-at-temp" brazing line, i.e., at a point where cooling normally begins, since the slower heating-rate of only 12°F/6°C per minute all the way up to brazing temp should be able to eliminate the need for any "hold at brazing temp". Here’s why:

Time at brazing temperature.

I am concerned primarily with the total time the brazed-assemblies are above the solidus-temp of the brazing filler metal (BFM), because the BFM will start to melt and flow by capillary-action as soon as the BFM is heated to a temp above its solidus temperature.

IMPORTANT: It is important that this temp be measured within the coldest section of the coldest assembly being brazed in the load. It should be observed that the chart in Fig. 1 does NOT show any secondary temperature curves, i.e., there are no load-TC measurements shown on the chart.  This was done intentionally, so that the new recommended furnace-heating ramp-rate with no holds could clearly be seen in comparison to the previously used rapid heating rate that have the built-in holds.  Normally there would be additional temperature lines on the chart representing the temperatures recorded by other TC's placed throughout the furnace load, but they have been excluded from the chart for clarity.   Obviously, placing numerous TC’s throughout the load itself would tell the brazing shop personnel what is happening throughout the actual load, since that cannot be determined merely by looking at the furnace-TC readings. I always strongly recommend the use of at least three (3) load TC’s or more in every braze load.

Assuming proper thermocoupling of the furnace (including the load), the TOTAL amount of time above the solidus temp of the BFM, i.e., from the time the TC within the coldest part crosses that solidus on the way up to brazing temp, until it again crosses that solidus temp on the way down from brazing temp, should represent the “total capillary flow time for the BFM". And, considering the slow heating ramp-rate being recommended in this example, the total time above the BFM’s solidus temp may actually be quite long. Therefore, as soon as the load-TC in the coldest assembly in the furnace-load reaches the brazing set-point temp (1975F/1080C), slow cooling can immediately commence (no extended hold is necessary) --- unless there are particular metallurgical reasons for extending the hold (such as special boron diffusion requirement for isothermal solidification, etc.).

IMPORTANT REMINDER: Therefore, to effectively accomplish what I am recommending in this article, i.e., to develop an effective brazing cycle that can eliminate the need for stabilization holds on the way up to brazing temperature, it is essential that TC’s be well placed throughout the furnace load during the development/test phases of this work. TC’s must be connected to the thinnest (most easily heated) section of ONE part (assembly) as well as within the thickest (slowest heating) section of the same assembly, so that the temperature-differential (i.e., delta-T, or Δ-T) within that ONE assembly can be determined, because distortion of parts is the result of the Δ-T within a single component, not just the Δ-T of the total load (when many assemblies are being brazed at the same time).  Thus, for proper brazing cycles, TC’s should be throughout the entire furnace, in the hottest and coolest section of the total load, as well as having at least one of the assemblies in the coolest part of the load thermocoupled to show the temps of the hottest and coldest portions of that part/assembly.

yield-curves-sm
Fig. 2 - Each curve represents the temperature difference (Δ-T) within a single brazed component at which thermal stresses equal the yield point of various metals, based on tests using thin sheet rigidly attached at its ends to a thicker member of the same metal (just as in a heat-exchanger for example). For two attached members having similar cross-sectional thicknesses, use approximately double the values shown. (Chart taken from the article “Control of Distortion During the Furnace Cycle”, written by Cliff Tennenhouse (AWS Welding Journal, October 1971).

Will slow heating-rates eliminate distortion of parts?

Heating must always be done at a rate that does not exceed an allowable Δ-T for the specific metal being brazed in any given furnace load, as shown in Fig. 2.

The curves shown in Fig. 2 represent the allowable Δ-T’s for a variety of different base-metals, beyond which distortion (yielding) of that base-metal will occur. To avoid distortion (yielding) of a given metal, find on the chart the particular metal you are brazing, and then be sure that the brazing furnace always operates to the left of that metal’s particular curve. Operating to the right of that curve will probably result in yielding of that particular base-metal, causing it to stretch, distort, etc..

EFFECT ON TOTAL FURNACE-BRAZING CYCLE TIME

Remember --- at the start of this article I suggested that you find the FASTEST heating rate that will allow you to eliminate all “holds” during furnace heat-up. In the example used in this article, a heating rate of 12°F/6°C per minute was determined by drawing the straight line on the production temperature-chart shown in Fig. 1. If, after that test-run has been completed using the 12°F/6°C heating rate, the parts look great, and meet all internal testing criteria, then another furnace run should be attempted using a slightly faster heating rates (such as perhaps 20°F/10°C) to see if that higher rate will also allow good results (again, with no holds during heat-up). Such testing should continue until you determine the fastest heating rate you can use that will allow you to not only eliminate all holds on the way up to brazing temperature, and also prevent any distortion of the parts/assemblies.

By taking the time to do this test work to determine an optimum heating-rate for your parts, please understand that even though this optimized heating rate will be slower than that which you are currently using, the total furnace cycle time may actually be shorter than previously, since the time for all the stabilization-holds has been eliminated.

BENEFITS OF SLOWER HEATING RATES THAT ELIMINATE “HOLDS”:

  1. Lower thermal-stressing of metals, since the temperature differentials within the parts/assemblies are kept to a minimum;
  2. Total cycle times can sometimes be SHORTER than cycles using rapid heating rates combined with their lengthy built-in intermediate holds for temperature-stabilization;
  3. The much slower heating rate should allow the system to more effectively handle any outgassing of materials during heat-up (such as BFM binders, etc.) without causing any vacuum-pump shutdowns and equalization-holds that often occur when high heating-rate cycles are used. During rapid heating-rate cycles, the outgassing temp of BFM binders may be reached so fast that sudden and severe outgassing surges may occur, causing automatic reaction by furnace to stop heating, hold, and try to recover its vacuum-level before proceeding further. Much slower heat-up rates may allow much more gradual outgassing, eliminating surges, and allowing the furnace to continue steadily up through that temperature region with no holds required for “recovery”.

CONCLUSIONS

It’s time for many brazing shops to break the paradigm of using rapid heating-rates (just because “we’ve always done it that way, and the people who developed those rates must have known what they were doing…..” etc. Instead, it’s time to clearly reconsider the advantages of using a much slower heating rate, and all the advantages which that can offer.

The goal for all of us should be to "think outside the box", to thoroughly examine past-practices, in order to achieve better brazing, while at the same time trying to reduce process-costs, whenever and wherever possible.


DAN KAY BRAZING SEMINARS: 3-Day seminars provide intensive training in "The Fundamentals of Brazing", covering the brazing of materials ranging from aluminum to titanium to ceramics! FIND OUT MORE

Dan Kay - Tel: (860) 651-5595 - Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. CLICK HERE to contact Dan Kay.

To View a listing of all Dan’s articles please click here

Read Dan Kay's Biography

© Copyright Dan Kay 2012

Log in to your account or