Preparing Tube Ends for Superior High Purity Welding Results

Autogenous welding (fusion welding without the use of filler metal) is used to join the tubing used for most high-purity and sanitary tubing systems in numerous industries.

This type of welding requires very high precision weld joints, with the primary emphasis on maintaining the smooth inside surfaces to guarantee there are no potential contaminant points.

Durable alloys of stainless steel such as 316L are used to provide corrosion resistance against attack by either petrochemicals being processed, or routine stringent cleaning procedures. Tubing used in high-purity or sanitary systems is mechanically polished or passivated (electro-polished) to obtain an ultraclean, smooth inside diameter critical to system quality.

The top priority of all cutting procedures and weld end prep techniques are to achieve a high quality weld joint without contaminating the ID, or introducing heat-affected zones (HAZ), carbon contamination, or introducing any surface scratches which can either be corrosion or stress riser induction points.

Ultimately, to achieve acceptable autogenous welds, the weld end preparations must be:

Square to the run of the tube
Cut with zero degree bevel angle
Either burr-free or with a minimum burr that is consistent and uniform to allow consumption by the welding procedure.

Figure 1 illustrates an ideal autogenous weld joint. There is zero gap between the ends at any point, and the tube run remains straight.

Specifications for the exact tolerances to be applied to tubing for fusion weld end preparations have not been universally accepted and are, in general, established by in-house welding procedures and project determined tolerances.

The following discussion of tolerances is made up of general recommendations for high quality weld joints required for high purity and sanitary systems.

Flatness (Figure 2) should be within .0003” (.008mm) when two ends are fitted for welding.
Squareness (Figure 3a) should be within .002” (.05mm) per diameter inch or 1/10 degree as measured from within two to four diameters of the end.

Significant out-of-squareness (Figure 3b) can cause poor fit-ups, forcing the use of cold springing to make the final joints, or, in the case of automatic tube welding, the alignment clamps can pull the joint apart on one side.

Bevel angle (Figure 4) should be zero degrees plus or minus 1/4 degree. With a .065” (1.65mm) wall thickness tubing, the maximum gap either on the ID or the outside diameter (OD) would be .0006” (.0015mm).
Allowable burr depends on the welding procedure and the specific application requirements.

When it is not permissible to touch or mark the surface of the tube in any way, the burr must not exceed the amount that the welding procedure can consume and not cause drop-through or require excessive HAZ causing heat on either side of the weld joint. Generally, the burr must be less than .010” (.25mm) per side on the inside or outside of the tube wall.

Call any time to speak with a TRI TOOL technical or orbital welding representative for complete assistance with any of your tube squaring, severing or welding requirements.

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