Heater_guy Your problem is a complicated one to reply in a discussion forum. Yet, you could think of the following to improve quality of testing, brazing practices, leak tightness points of view. 1. Vacuum leak test - is a more surer way to ensure that the tube supplied did not have any pin hole leaks in ERW supply for Incoloy 800 or S.S 304. While swaging you are drawing from 12.7 mm to 11.0 mm which is approximately 15% reduction in OD and consequently reduction in wall thickness will also come down to approxiamtely 0.6 mm. 2. Alternatively you can think of doing ET (Eddy Current testing) of the full length of ERW Incoloy 800 tubes before and after swaging to ensure that the reduced wall thickness and OD did not affect the ERW weld quality. 3. Silver brazing and the joint configuration between the tube and tube sheet will have to be assessed. What is the joint design chosen for this will have a bearing in leak tightness between the brazed tube and tube sheet? 4. For bending the tube (Incoloy 800), you should not exceed the tube bending or pipe bending tolerances . Chances are that the already swaged ERW tube is further stressed due to bending tolerance exceeding the permissible limits. A cold worked ERW Incoloy 800 ( 15% due to swaging, and an additional 8-10% for bending to heater coil geometry) may be the problem for the leaks you experience. 5. Again for leak test, the 6- 25 Kg applied pressure test is one way of ensuring that no leak is possible before you put in service. Also, you could lookat pneumatic leak or helium leak test which is more sensitive than even the pressure test from tube to tube sheet brazed joint, leaks in the already cold worked ERW tubes. 6. For Annealing Incoloy 800 - you should be sure of attaining temperature of 980-1000 deg cent for mill annealing or 1150-1175 deg cent for solution annealing. Which annealing process you adopted - for the anneal to relieve the stresses due to cold working (15% reduction due to swaging) and another 10% due to bending ? 7. If tube leak test does not reveal any drop in pressure or leak, you should not have any problem in operation at 200-400 deg cent for Incoloy 800 tube in heater service. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd India Dec 17, 2005 E-mail: nishi@vsnl.com >I look after the manufacture of electric heating elements at >the firm i am employed in. > >We manufacture heaters for various applications using >various sheath materials like S.S. 304, S.S. 316 and Incoloy >800. > >As a part of the manufacturing process, we use Incoloy 800 >ERW Tubes of 12.7 mm diameter and 0.7 mm wall thickness. >These are filled with insulation etc. and reduced to 11 mm >diameter by a swaging process. > >Subsequently, the tubes are annealed and bent into a hair >pin shape. > >In order to test the tube for cracks etc, we leave the tubes >immersed in water for a period of 4-5 hours. We test >insulation resistance before and after this test and assume >a crack or a defect in case the insulation resistance drops. > >Subsequently, these tubes are assembled onto a tube sheet >and brazed with 43% silver alloy to the tube sheet. > >After assembling the complete heater onto the tube sheet, it >is left to cool. It is then inserted in a vessel to check >the leak tightness of the welded joint (Tube to tube sheet). >These checks are carried out at about 30 - 35 degrees >Celsius of water and a pressure that can be anything from 6 >kg per sq. cm to 25 kg per sq. cm. > >Upon completion of the test, we have found many tubes which >show a considerable drop in insulation resistance subsequent >to the test, suggesting a breach in the tube somewhere. The >place of failure is not always the same, as we test it by >passing current between the heater sheath and the live >terminal. The defective area becomes hot indicating the >trouble spot. > >We anneal the Incoloy tubes much more than S.S. 304 and use >either a gas annealing or a electric annealing process >before bending. > >I need help with the following: > >1. What is causing these defects? > >2. Why do these defects show up under pressure and not >before during the immersion test > >3. What is the most reliable way to identify the trouble >spot. The tubes we use are over 5 metres in length > >4. Could the problem be with tube metallurgy (mill supplied) > >5. If not, then what steps can we take to ensure that the >problem is not repeated > >6. If the tubes tested donot display the problem, what is >the reliability that they will when the heaters is in >service at 200 - 400 degrees Celsius, continuously? > >The problem does not present itself with S.S. Tubes

Frances Ling Incoloy 800 in tube form can be / is used for U bends. Bending radius - min shd be atleast 1.5 D. For small dia tubes with "U" bend 3.5 D to 5 D wd give better crack resistant bending. Incoloy 800 is either mill annealed at 980-1000 deg cent or solution annelaed at 1130-1160 deg cent depending on the carbon content specified and supplied. With Solution annealing at 1150 deg cent your crack problem may not occur - i guess. Tooling : is a decision depends on the radius of the tube to be bent for a specific application. To me it appears a 5 D "U" bend wd give less crack problem and a tooling to suit this will find useful application. Trust this helps u C.V.Srinivasan Nishi Engineers Pvt Ltd India June 09,2006 E-mail:nishi@vsnl.com >We are also manufacturing electric heating elements with >Incoloy 800 tube and have experienced high cracking rate >(20% or up) during U-bend. > >To find the difference between cracking and non-cracking >tubing, we did Rockwell B test on both. The result shows the >non-cracking tube has an average hardness of 42.15RB and >cracking tube has an average of 53RB. > >Can anyone help on the following: >1, Will this material in tube form fit for U-bend generally? >2, To achieve a better annealing result, what is the >suggested recipe for annealing. >3, Any tooling consideration, like radius range, for U-bend? > >Thanks! >