Radiography (X-ray) is a means of looking at the inside of a braze joint without actually having to cut it apart to see it, as long as the appropriate conditions are met to allow its use. Radiography is heavily dependent on the thickness and mass of the part being radiographed. In order for x-ray to yield a useful image of any voids or inclusions in a brazed joint, the thickness of that void or inclusion should be at least 2% of the thickness of the metal through which those x-rays are being sent, in order for it to be visible in a radiograph, or, if using real-time radioscopy (RTR) on a TV screen or monitor.
a. 2%-Rule needs to be followed
|a) Two flat pieces of steel brazed together; b) Radiograph of assembly shown in (a); Section through areas shown in (b).|
The “2% Rule” in brazing is a very important guideline to follow, since it can effectively rule out radiography as a method for inspecting components that are too thick to be able to see any of the imperfections inside a braze joint.
As stated in the AWS Brazing Handbook (Fifth Edition, 2007), on page 173: “Radiographic Examination is widely used in the inspection of brazed joints. It should be noted, however, that in most cases, this examination method fails to detect discontinuities in brazed joints whose joint clearance is 2% or less of the joint cross section.”
For example, if a brazed joint is 0.002” (0.05 mm) thick, and it contains voids the same size, then for those voids to be seen on an x-ray image, the metal through which the x-rays are being sent should not be greater than about 0.100” (2.5 mm) thick. Here’s why: 2% is the same as 2/100 or 1/50. So if a 0.002” (0.05 mm) thick void in the joint is to be seen, then multiply the 0.002” (0.05 mm) by 50 to get the maximum metal thickness that can effectively be x-rayed in this manner, which is only about 0.100” (2.5 mm) thickness.
Shown on the right is an illustration from the AWS Brazing Manual (First Edition, 1955) showing the image of a high quality radiograph of a brazed joint, in which the joint clearance meets the 2% rule. The photo shows two carbon-steel sheets silver-brazed together in which the brazed joint is about 3% of the total metal thickness through which the x-rays are being sent. The voids (white spots) in the joint are clearly visible.
|Fig. 8.1 (p. 173) from AWS Brazing Handbook , 2007 ed.|
Shown on the left is an illustration taken from a later edition of the AWS Brazing Manual (Fifth Edition, 2007, Fig. 8.1, p. 173), showing how an x-ray image might be taken of a tubular assembly, and showing that the total thickness of the part through which the x-rays are sent is represented by the total thickness of the metal involved, not just the diametrical clearance of the part. Thus, the sum of the thicknesses of the two tube-walls through which the x-rays are being sent is the only thing that constitutes the total metal thickness of the part for purposes of calculating the 2% rule.
b. Radiography reveals nothing about metallurgical bonding by BFM
Radiography allows a person to see what is inside a brazed joint (if the 2% rule is met), but it cannot, under any circumstances, indicate if there is a metallurgical bond between the BFM and the base metals being brazed. This is a major limitation of this inspection technique. It is possible for a component to look good in an x-ray (nice BFM flow all the way through the joint, few if any voids, etc.) and still fail a leak-test because the BFM did not properly alloy with (bond to) the base-metals in a satisfactory manner. This can be especially true for wider-gap brazing joints in which gravity is playing a role in moving the BFM through the joint.
c. When to use Radiographic inspection
Radiographic inspection can be justified as a brazing inspection method, in my opinion, ONLY if each of the following items can be CLEARLY demonstrated:
1. No other inspection technique will give an acceptable level of confidence in the “quality” of the brazed joint.
2. The radiographs can be easily read and accurately interpreted.
3. Radiographs are taken of 100% of critical surfaces in the brazed joints.
4. The thickness of the brazed joint section being radiographed is greater than 2% of the total metal thickness through which the x-rays must travel.
5. The use of radiography does not interfere with the optimization of other braze-process variables, e.g., filler metal selection, part cost, etc.
Honeycomb panels, and large thin-walled combustion chambers are two examples of brazed components that routinely use radiography as a viable inspection technique. In both cases the x-rays pass through thin sheet metal cross-sections running perpendicular to the plane of the x-ray path. The intent is to locate areas of non-fill (voids) and other imperfections in the joint, not to see if the BFM is present in the joint. As mentioned earlier in this article, even if the BFM has flowed into the joint, x-rays cannot indicate if the BFM has alloyed with (bonded to) the base metals. Small diameter cylindrical joints are especially hard to interpret when x-rays must go through two walls of the tube, or must travel in the same direction as the braze joint.
IMPORTANT NOTE: X-ray interpretation is easiest when there is significant density difference between the base metal and the filler metal. However, inspection-techniques must NEVER EVER be the basis for selection of a brazing filler metal! The ONLY basis for the selection of a BFM should be the end-use service conditions of the part! BFM should never be selected solely because it can yield a more readable x-ray than another (more preferred for the service application) BFM.
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. Dan can be reached via e-mail at [email protected], and his website can be visited at: http://www.kaybrazing.com/
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