by William Sandford
These systems require very high-quality weld joints, with emphasis placed on obtaining a smooth inside surface to avoid contaminant traps and on all the normal structural requirements for welded joints.
Stainless steel, such as 316L, is used to provide corrosion resistance against attack by the process chemicals or the cleaning procedures. Most of the tubing used in high-purity or sanitary systems is mechanically polished and/or electropolished to obtain a clean, smooth inside diameter (ID) suitable for the application.
The cutting procedures and weld end preparation techniques must consider all aspects of obtaining a high-quality weld joint without contaminating the ID, introducing heat-affected zones (HAZs), 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; and
- burr-free or with a minimum burr that is consistent and uniform to allow consumption by the welding procedure.
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.
The following discussion of tolerances is made up of general recommendations for high-quality weld joints required for high-purity and sanitary systems.
- Flatness should be within .0003″ (.008mm) which will leave a maximum gap of .0006″ (.0015mm) when two ends are fitted for welding.
- Squareness 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 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 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.
When deburring is allowed, care must be taken to avoid generating chamfers on either the ID or OD of the tube, which can cause excessive suck back in the weld zone.
Cutting (Severing) Procedure Options
Part Off Cutting. This is for very flat and square ends with precise bevel angles. Achieving a burr-free cut on the ID of the tube is difficult without a second operation. Two basic groups of tools cut with part off tools: stationary engine lathes and portable pipe lathes. Stationary engine lathes, while very precise with flexible cutting procedures, are only suitable for short lengths of tubing, because the tubing must turn and setup time is high. Single operation cuts without deburring are suitable for some applications. Tool bits are available for various materials.
They are not generally suitable for use inside clean rooms due to the multiple lubrication points and floor space required. Portable pipe lathes dedicated to cutting tubing, clamp the tubing with OD collets, orbit the cutting tools around the tube, and can be used for bent tubing if some straight length for mounting is available. Setup time is low, and single operation cuts without deburring are suitable for some applications.
Tool bits are available for various materials. Optional cutting modules for end plunge cuts, similar to dedicated end squaring tools, are available to provide almost burr-free ends. Lathes can be set up with sealed bearings, dry lubrication coatings, and totally enclosed electric motors for operation inside of clean rooms. Machine chips are long streamers for easy cleanup, which is especially important in clean room operations.
Orbiting Mill Cutting. This is generally done with portable machines which plunge a milling cutter through the wall of the tube and then finish the cut by orbiting the cutter around the tube. It has a very fast cut sequence, and one type of milling cutter will function on most materials. The machines must be kept in good shape and used with trained operators to minimize lack of flatness where the mill cutter enters and exits the cut. Mill cutters generate small chips which are difficult to confine in clean room operations. They can be set up with sealed bearings, dry lubricants, and totally enclosed electric motors for use in clean rooms, provided the chips are acceptably captured.
Abrasive Cutoff Wheels. These are very fast, low-cost cutting systems which can be used on most materials. The ends are not suitable for welding as cut, and fine grit particles stick to the ID of the tubing. The initial cutoff position must allow sufficient length for machining burrs and HAZ off the ends before welding. Abrasive cutoff wheels are used where the ID contamination may be mechanically cleaned and the ends are finished with a second operation. They are not suitable for clean room operations.
Band Saw Cutting. This type of system is very fast and flexible. It can be used on most materials when equipped with variable-speed drives and when the blade is selected for the material. As with the abrasive cutoff wheel, the “as cut” ends are not suitable for welding, and fine grit particles stick to the ID of the tubing. Band saw cutting is used when the ID may be washed clean and the ends are to be finished by a second operation. The fine cutting swarf generated by band saws makes them unsuitable for most clean room operations.
Circular Saw Cutting. Similar in performance and usage with band saw cutting, circular saw cutting results in very flat and square ends, but leaves an unevenly distributed burr between the OD and ID of the tubing.
Chipless Roller Cutting. Low-cost, hand-powered chipless roller cutters are available. They can be used on most materials, with the possible exception of some titanium tubing with wall thicknesses in excess of .100″ (2.54mm). This type of cutting is suitable for use in clean rooms without modifications. The ends are not suitable for welding as cut. With chipless roller cutting, there is some concern for Brinelling effects of guide rollers in conventional hand cutters. This may require an excessive amount of end cutting during final end preparation.
Single and Double Die Shear Cutting. These systems have very high production rates. In many situations, the ends are suitable for welding as is. This type of cutting is generally used for manufacture of fixed lengths of tubing required in high production operations. It is not suitable for use in clean rooms because of space, complexity, and multiple lubrication points.
Thermal Cutting. Thermal cutting is not used extensively for generated autogenous weld end preparations because of HAZ effects and the degree of precision required. The small size of most of the tubing being cut makes it difficult to avoid cutting two walls simultaneously. This affects squareness of the cut that can be achieved. Some tubing more than 6″ (152.4mm) in diameter is being cut with plasma cutters when the HAZ effect is not considered detrimental. However, the finish preparation must be a second operation for a high quality weld. Orbital precision laser cutting may become competitive for fabrication work, but it is now limited to high-volume production work on fixed length, thin-wall tube cutting operations. Thermal cutting procedures are not suitable for clean room operations.
End Squaring Tools and Procedures
In general, higher quality ends for autogenous welding can be generated with dedicated end squaring tools as opposed to cutting tools. The possible exceptions are the stationary lathe and specialty honing operations.
With cutting or severing operations, the cut must be made from the OD of the tubing. As a consequence, some burr is pushed ahead of the cutting tools, leaving a burr on the ID of the tube.
The burr can be minimized, but not eliminated, with specially designed tool bits and speed and feed adjustments relative to the material. With orbiting mill cutters, the cutting wheel position can be set to pull the chips to the outside after the initial plunge through the wall is made.
This will minimize the ID burr, except where the initial through-wall plunge cut was made, and leave more burr on the outside of the tube, causing a possible variance in the arc length of an automatic tube welder.
Dedicated tube end squaring tools are portable or bench tools which mount with OD collets on open-ended tube and address the tube with axially-fed tool bits. The tools are capable of meeting the tolerance defined for optimum autogenous weld end preparations on almost all tubing materials.
Tubing materials and surface treatments change the cutting characteristics. Therefore, tool bits, feeds, and speeds must match the material to obtain optimum end surface finishes, burr-free conditions, and tool bit life.
The machining characteristics of annealed and electropolished stainless steel tubing are very different from as-drawn tubing. These differences require that procedures be changed to adjust to the optimum burr-free conditions.
It is not enough to specify the process controls for just the material designation without including the heat treatment and surface treatment.
Equipment and operation sequencing must be compatible with the job specifications and provide for a smooth flow through the fabrication process. Equipment designed specifically for the job can be placed at the point of need to reduce time and costs by minimizing extra handling, multiple cleaning operations, weld rejects, and rework time. Tri Tool equipment is available for cutting and end squaring tubing to the most exacting specifications.