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Brazing-inspectionbefore, during and after brazing.SOLUTIONS with Effective, Practical Advice
Brazing-inspection includes all the operations that must be carried out to assure compliance with drawing and service requirements. Brazements are often complex constructions. Not all brazed joints may be inspectable once the parts are completed. Therefore Brazing Inspection has to perform partial controls at definite steps in the process. Parts must be initially designed for inspectability, and all manufacturing and Brazing-inspection stages must be programmed well before starting fabrication.
Inspection operations must be carried out thoroughly and professionally to assure compliance with drawing and service requirements. A previous page on Brazing outlines the definitions of the processes and presents the advantages and limitations that help producing successful applications. A certain familiarity with the processes assists in understanding Brazing-inspection requirements. As it is by now clear to all, much of the strength properties of brazed joints depend upon creating and maintaining the proper capillary clearances between elements to be brazed. See Brazing Joint Design. Therefore one of the first and most important Brazing-inspection steps, well before starting the joining process, consists in verifying that the right fabrication tolerances are observed and realized in practice even before assembling. Much of the brazing success depends, as is widely known, on cleanliness and freedom from surface oxides. Therefore Brazing inspection has to determine that suitable means and procedures are used, and that no contaminants are permitted to come in contact with the materials. The assembly procedures may be complex and demanding, and the use of proper and clean fixtures may need attention and fussiness. The placement of the proper amount of brazing filler metal and of flux in all joints may be quite exacting. The time permitted from the end of cleaning to the start of heating is limited because of the need to avoid that oxides form again. All these requirements must be assured for successful brazing. The Brazing-inspection function catering to the fulfillment of all assembly requirements must be performed by a responsible and experienced worker or inspector, to avoid oversight that cannot be easily repaired after brazing. Then, whatever the heating method, the brazing process itself must be controlled by proper automatic means to assure that all parameters be at any time within the tolerances established in the development stage. See Brazing Heating.
If the process is manual, however, its control is in the very hands of the brazer whose skill and experience are the best assurance of success. Once the parts are finally brazed, Brazing-inspection is called to verify that all joints are correctly made to meet all requirements. The search is for discontinuities in the joint, meaning lack of metallurgically bonded filler metal or presence of cracks or voids, caused by entrapment of flux or gas. The importance of the discontinuities that were found has to be interpreted against the criteria set up by applicable Codes or by drawing requirements. For simple joints, visual inspection may verify the proper filler flow at the opposite side from the emplacement of the filler metal. Liquid penetrant inspection (see Welding Inspection) expands the sensitivity of visual examination and may display fine cracks open to the surface, invisible to the naked eye. Leak test or pressure tests may be required for more demanding applications. Proof tests, consisting in submitting the joints to loads exceeding those to be sustained in service, may be required. However Brazing-inspection should make sure that the test itself does not compromise joint integrity. Occasionally a test piece may be tested to destruction, to expose the brazing filler metal in all parts of the joint. Crack are not admitted but isolated voids may be tolerated if not open to the surface. The quantity and distribution of accepted internal voids and the method to determine them, (usually by radiography or ultrasonic testing, see Welding Inspection) must be established by the designer according to function and importance of the application. Less common techniques like thermographic examination, laser holography or other specialized methods were developed to meet exceptional requirements not readily covered by regular Brazing-inspection. Ultrasonic Inspection (see Welding Inspection) is regularly used for inspecting brazed assemblies. An Article on Nondestructive Acoustic Cross-Sectioning was published (7) in Issue 135 of Practical Welding Letter for October 2014. An Article on Measuring Heating Rates in Vacuum Furnaces was published (3) in Issue 140 of Practical Welding Letter for April 2015. An Article on Aim for Small, Concave Braze Fillets was published (3) in Issue 148 of Practical Welding Letter for December 2015.
An Article on Resistance Brazing Filler Metal was published (4) in Issue 167 of Practical Welding Letter for July 2017. The most important function of Brazing-inspection is not to reject defective parts but to keep every stage of brazing within the limits that were established, in the development stage, as necessary to meet the requirements of successful operation.
Scorpion
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