brazing q+a december layout 1 11/13/13 1:36 pm page 20 ... · shapiro...

Q: We make connections be-tween copper and steel tubingon our assembly line usingmanual torch brazing. We arehand feeding a silver bearingbraze alloy (38% silver) andusing a white flux. After havinga high leak rate on these joints,we identified significant differ-ences between brazers in theheating process. In order toapply more consistent heating,we have been trying inductionheating using a hand-held in-duction unit. The only othervariable we changed is that weuse a preform ring rather thanhand feeding brazing rod. Theleak rate has not dropped aswe anticipated. We obviouslyare missing something. Canyou offer suggestions as to howwe can evaluate this?

A: Induction heating is a good choice forbrazing as long as the joints you are mak-ing are of a consistent nature. It is a veryuniform and consistent heating method,but it requires the parts being brazed tobe similarly uniform and consistent.

Manual torch brazing gets a bad rapbecause of the difficulty in having it ap-plied the same way by a group of opera-tors, but it does have some benefits. Tu-bular joints like those found in refrigera-tion and air-conditioning systems featurea great deal of variation. These are notthought of as precision braze joints. Inaddition to the wide variation in heatingyou mention, joint clearances vary, jointinsertion depths vary, tube thicknessvaries, and cleanliness is usually an issueof some sort. Sometimes nitrogen is usedas a purge to keep interior surfaces oxidefree; sometimes a vapor flux is aspiratedinto the fuel gas stream to help keep ex-terior surfaces clean along with myriadmethods to apply alloy and flux. A skilledoperator with a torch, properly set for gaspressures and flame condition, can adjustto the conditions presented and make agood braze joint.

The word skilled is the issue. In previ-ous columns we’ve addressed the issue of

training. On some joints, rather than bat-tle the training issue it seems easier tolook at another process. Hand-held in-duction is a logical place to look. Figure1 shows a typical setup and Fig. 2 shows aclose-up view of the tubular joint beingbrazed.

If you are going to pursue this option,you will be addressing the variable ofheating but, to make it work successfully,you must make sure you address the oth-ers. The fact that you are brazing dissim-ilar metals (steel and copper) adds someadditional considerations. The variablesthat need attention are:

1. Joint clearance

2. Joint depth

3. Cleanliness

4. Braze alloy and flux

5. Thermal conductivity

6. Thermal expansion

7. Heating process

Take a look at Fig. 3. It shows a typicaltubing braze joint. It highlights what youare facing. You can see that the joint clear-ance varies greatly. Typically, these typesof joints have a tremendous range of clear-ances. The maximum strength of a silverbrazed joint is achieved at a radial joint

clearance of 0.0015 in. (0.038 mm). This isalso a good clearance size for capillaryflow to pull the braze alloy into the joint.Try to keep the radial clearance between0.001 and 0.002 in. (0.025–0.05mm).

The next thing to notice in Fig. 3 is thejoint depth. This example shows an ex-treme amount of overlap. It is common tosee this in production, and it is not nor-mally controlled very well. As mentionedin October’s column, we usually recom-mend an overlap of 3T to 6T, where T isthe thickness of the thinner of the twopieces. For practical purposes, a jointdepth of 0.200 in. on these types of jointsis all you need. This then needs to be con-trolled throughout the joining process.Various types of end forms can be incor-porated to help ensure the depth.

The problems with the conditionshown here start with the fact that it re-quires an excessive amount of braze alloyto fill this joint. It is difficult to heat sucha deep joint and achieve proper capillaryflow throughout. It also takes longer toheat because of the extra mass involved.In short, it takes longer to make thesejoints, requires more thermal energy, and more braze alloy (in this case, more silver).

The cleanliness of components re-quired for induction brazing is the sameas for torch brazing. While a torch can beadjusted to be oxidizing, neutral, or re-ducing in nature, an induction processsimply heats the parts. One feature thatinduction offers is that you can flood the

BRAZINGQ&A BY TIM P. HIRTHE

Fig. 1 — Tubular joint in a hand-held induction coil.Fig. 2 — Close-up viewof the joint.

DECEMBER 201320

BRAZING Q+A December_Layout 1 11/13/13 1:36 PM Page 20

21WELDING JOURNAL

joint during heating with an inert gas tooffer some protection from oxidation. Inboth torch and induction heating, you canprotect the inside of the tubes by purgingwith nitrogen.

The induction process allows you touse a preform ring. It eliminates the over-feeding of braze alloy rod that normallyoccurs during manual torch brazing. Thisoffers excellent control over the quantityof braze alloy. There is a good optionavailable that incorporates the flux intothe braze ring. These come in a variety ofcross sections and flux contents. They areavailable in the materials you are using.The braze alloy conforms to AWS A5.8BAg-34 and the flux conforms to AWSA5.31 FB3A. In your application, you willwant the ring to be in good physical con-tact with the outside of the inner tube andin contact with the edge of the outer tube— Fig. 2.

When heating this combination ofbase metals, you will need to consider thedifferences in materials. The main differ-ences are in thermal expansion and ther-mal conductivity. Values for both materi-als, taken from the AWS Brazing Hand-book, 5th Edition, are as follows: 1020Steel coefficient of thermal expansion is6.5× 10–6 in./in./°F (11.7 × 10–6mm/mm/°C). Copper coefficient of ther-mal expansion is 9.4 × 10–6 in./in./°F(17.0 × 10–6 mm/mm/°C). 1020 Steelthermal conductivity is 357 (Btu)-(in./h)/(ft2)(°F) [51 W/m·K]. Copperthermal conductivity is 2689 (Btu)-(in./h)/(ft2)(°F) [388 W/m·K].

It can be seen from these data, thecopper will expand more than the steel

and the steel will tend to hold the heatwhile the copper will tend to draw it away.With induction, a ferrous part will heatfaster than a nonferrous one. Since thesteel is the outer member of the joint,when placed into the induction coil, thesteel will heat quickly. The inner coppermember will be heated mainly by con-duction from the steel part. The powersupply will need to be adjusted to allowthe parts to heat uniformly. You will haveno problem getting the steel hot. Youneed to be sure the copper is up to braz-ing temperature to ensure a good bond.In addition, the induction coil will need

to be placed so that the braze alloy isdrawn into the joint. Molten braze alloytravels toward the hottest section of thejoint area, so place the coil accordingly.

Making the switch to induction heat-ing can be achieved successfully, but it re-quires a strong attention to detail. As Imentioned previously, manual torchbrazing gets a bad reputation because ofthe difficulty in getting a group of opera-tors on the same page. Likewise, induc-tion brazing gets a bad reputation for itslack of flexibility. To make it work, youhave to make sure you have all of the vari-ables under control.♦

Fig. 3 — Tubular joint in a hand-held in-duction coil.

This column is written sequentially by TIM P. HIRTHE, ALEXANDER E.SHAPIRO, and DAN KAY. Hirthe and Shapiro are members of and Kay is an advisorto the C3 Committee on Brazing and Soldering. All three have contributed to the 5thedition of AWS Brazing Handbook.

Hirthe ([email protected]) currently serves as a vice chair of the Brazing & Sol-dering Manufacturers Committee (BSMC) and owns his own consulting business.

Shapiro ([email protected]) is brazing products manager at Tita-nium Brazing, Inc., Columbus, Ohio.

Kay ([email protected]), with 40 years of experience in the industry, operateshis own brazing training and consulting business.

Readers are requested to post their questions for use in this column on the BrazingForum section of the BSMC Web site www.brazingandsoldering.com.

For info, go to www.aws.org/ad-index

BRAZING Q+A December_Layout 1 11/13/13 1:36 PM Page 21

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