Technical Bulletins
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TECHNICAL BULLETIN 99001

April 28, 1999

NOTICE: THE FOLLOWING CONTENTS OF THIS BULLETIN CONTAIN CRITICAL INFORMATION CONCERNING THE PROPER PROCEDURE FOR MACHINING EXPLOSION WELDED TITANIUM OR ZIRCONIUM CLAD STEEL OR STAINLESS STEEL PLATES AND TUBE SHEET BLANKS

Background: Recently one of our clients attempted to machine and drill two Titanium Grade 1 clad 304L stainless steel tube sheet blanks simultaneously.  They stacked the blanks on top of one another with the top blank inverted so the titanium clads were face to face.  During the course of the drilling operation, it was ultimately noticed that the titanium clad on the upper (inverted) blank had delaminated from the stainless steel. Subsequent analysis of the failed weld revealed the cause of the failure was not related to the quality of the explosion weld but due to improper machining practice.

It has often been claimed by those knowledgeable with the process that an explosion weld is always stronger than the parent metals; therefore, failure will always occur in the weaker of the parent metals.  While this is true for most metals combinations, it is typically not true for titanium and zirconium clad steels and stainless steels.  In these systems, tensile and/or peeling forces at or along the explosion weld or bond line have a higher potential to cause the weld to fail at stress levels somewhat below the strength levels of the parent metals. Further, failure will occur with little or no strain or deformation of the parent metals.

In the instance of the above cited failure, it had been assumed that by placing the titanium clads face to face, the upper titanium clad would be supported by the one below it which was not the case.  By the applicable Specification to which explosion clad tube sheet blanks are manufactured, an out of flatness tolerance of up to 0.125" (1/8") is allowed across the diameter of the blank; therefore, the mating clad surfaces of the blanks could have as much as 0.250" (1/4") spacing between them.  This obviously means the upper clad cannot be expected to be supported by the lower one in all areas.  Steels and, stainless steels in particular, require significant force to be applied on the drill during the drilling operation.  As the drill passes from the steel or stainless steel through the explosion weld interface into the titanium, the drilling force can be sufficient to cause localized failure of the explosion weld around the hole.  When adjacent holes are subsequently drilled, the fracture over the unsupported clad area grows by peeling failure which occurs at a lower stress level than tensile fracture which occurred around the initial hole.  The failure area, therefore, continues to easily grow over the entire area where the inverted clad is not supported and allowed to move away.

Recommended Machining Procedures:  In order to assure that you do not experience such a failure, we recommend the following machining and drilling practice:

     -Unless dictated otherwise by extenuating circumstances, always drill and machine titanium or zirconium clad steel or stainless steel tube sheet blanks and plates so that the drilling or machining progresses from the titanium or zirconium into the steel or stainless steel.  Typically the steel backer is much thicker and provides more rigid support and resistance to movement as the cutting progresses through the weld interface.

     -Always be sure to use sharp drills and cutting tools (inserts) when cutting through the explosion weld interface.

     -Keep cutting depths and feed rates at reasonable levels to avoid high load and deformation stresses when passing through the weld interface.

     -If you must drill or machine in the reverse direction from the steel backer into the titanium or zirconium clad, make absolutely sure the total surface of the clad is fully supported.  This can best be accomplished by first machining the surface of the clad perfectly flat and then placing it in direct contact with an equally flat mating surface that has either likewise been machined flat or started out perfectly flat.  Then use sharp drills and cutting tools with light passes and cutting rates when cutting through the explosion welds interface.  Please keep in mind that even with the above precautions, you still have a somewhat  higher risk of delaminating the clad during machining.

Technical Advisory Services:  Remember that part of Regal Technology Corporation's commitment to producing quality explosion clad products is providing our customers with complete technical support to assure optimum performance of your explosion clad material in its application.  This includes technical advisory support for post explosion welding processing; therefore, if you have any questions, concerns or problems associated with processing the clad material we have supplied to you, please contact us directly.  We will be more than happy to work with you.