Radiography is often used by some companies for inspection of their brazed components, but its use is sometimes not appropriate, especially when the parts being inspected are thick. Radiography should be used as a brazing inspection method, in my opinion, ONLY if each of the following points can be clearly demonstrated:
- No other inspection technique will give an acceptable level of confidence in the “quality” of the brazed joint.
- The radiographs can be easily read and accurately interpreted.
- Radiographs are taken of 100% of critical surfaces in the brazed joints.
- 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.
- It does not interfere with the optimization of other braze process variables, e.g., filler-metal selection, part cost, etc.
For example, if a brazed joint is 0.002 inch (0.05 mm) thick and it contains voids the same size, then the metal through which the X-rays are being sent should not be greater than about 0.100 inch (2.5 mm) thick for those voids to be seen on an X-ray image.
Here’s why: 2% is the same as 2/100 or 1/50. So, if a 0.002-inch-thick void in the joint is to be seen, multiply the 0.002 inch by 50 to get the maximum metal thickness that can effectively be X-rayed in this manner, which is only about 0.100 inch (2.5 mm) thickness.
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 direction of the X-ray path. The intent is to locate areas of nonfill (voids) and other imperfections in the joint, not to see if the brazing filler metal (BFM) is present in the joint. 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 if the X-rays must travel in the same direction as the braze joint.
Although X-ray interpretation is easiest when there is significant density difference between the base metal and the filler metal, the reader should be strongly cautioned that inspection techniques must never be the primary 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 (the chosen BFM) is much more dense than another, even though the less-dense BFM might be preferred to use when the end-use conditions are taken into account.
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.