Automatic Eddy current Testing of Double- Wall Brazed Tubes ( Bundy Tubes )

Bundy tubes are double – wall and brazed tubes that should meet the highest quality and consistency specification . Due to the special manufacturing process , these tubes are almost guaranteed as leak-proof and also suitable for sustaining very high pressures ( over 1000 bar ) . The main application areas are : Refrigerator and air-conditioning industries , where the operational safety plays a major role such as for heat exchangers , and above all in automotive industries for oil and fuel lines and most importantly for cars brake lines that is an exclusive type of tubes that is acceptable for this important application .The processing industries increasingly require the guarantee of consistently high quality . This applies for example to the automotive industries . Nowadays by increasing the speed of the cars and using the light materials as well as due to high competition between car manufacturers , the demand for the highest quality especially for the Safety parts is required . The failure of even below 1 ppm so called Zero Defect policy is considered as a standard requirement . Such demand can be achieved only by a very accurate and reliable NDT methods on 100% of products . Only by an automatic suitable NDT method Human Factor can be eliminated . That means all decisions are made automatically . This requires high reliability of the test systems in which the defect detection and sorting takes place automatically . By implementing such automatic test process, the manufacturer saves considerably on personnel cost as well .In this paper the investigation and experimental test results in 2 Iranian plants who are producing double wall brazed tubes for cars break lines is reported . The typical diameter of the tubes are 4.75 mm that is made from a steel strip band with 0.35 mm thickness and 5- 7 micron of copper coating. First both edges of the band is sharpened and then wound twice i.e 720 deg. and then brazed in motion through a furnace at temperature of 1090 deg. C . The eddy current testing is executed online after cooling on 100% of the products by using an encircling test coil incorporating both Absolute and Differential windings . The defective areas are marked and sorted . Experiments indicated that all typical defects like : Unbrazing or weak brazing on outside or inside seams or in inner layer , holes, cracks, copper spots and dimensional variations would be detected by high reliability . It was also proven that by 100% NDT , almost all other quality requirements of the standard ASTM A 254 consisting : Tensile test, Hydrotest , crush test , Bend test , would be fulfilled by high reliability . Additionally , by real time testing during the production a condition monitoring of the production line is also executed that helps to reduce the scraps rate considerably .

Keywords:

Double-wall Brased tubes , Bundy tubes , Car Brake Tubes , Eddy Current Testing , Car Safety parts ,