Our specialized process for seamless tubing manufacturing begins with either an extruded hollow tube or a solid bar drilled to our exacting specifications. The material is then reduced in size a number of times through various cold working techniques until it reaches the specific size, tolerances, and temper required by our customer. After each cold working cycle the tubes are cut, cleaned and heat treated in preparation for the next cold working step.
At Superior Tube, we employ multiple combinations of value-added steps to produce the highest quality tubing to meet your requirements. Seamless tubes can be created in a variety of different sizes and shapes from a wide range of alloys. A brief description of the operations we use during our production is offered for your information.
Pilgering reduces the size of the tube across three dimensions - outside diameter (OD), internal diameter (ID) and wall thickness. We roll a die set with a tapered groove across the outside of the tube while supporting the inside diameter using a taper-matched, hardened steel mandrel. In a single cold working step, we can achieve a significant reduction in cross-sectional area while minimizing material loss and, most importantly, improving the material’s microstructure. Seamless tubes constitute the majority of volume processed by tube reducing or pilgering.
Like pilgering, tube rolling also uses compression to reduce the size of seamless tubes. However, while pilgering uses a pair of grooved, tapered dies to work the metal over a tapered mandrel, tube rolling utilizes one or two sets of rolls with constant cross-section grooves on the circumference of the tube. Generally, the process employs a cylindrical mandrel with little or no taper. The rolls are driven by rack and pinion gears of different radii along profiled cams, completing multiple 360º rotations around the tube.
Tube rolling is a very precise method of reducing very thin walled and/or smaller diameter tubes, and is often used as the final cold working step. By rolling, we’re able to achieve exceptional control over dimensional tolerances and surface finish while also minimizing material loss and improving the metal’s microstructure. Because it uses compression, tube rolling is well suited to processing unique metals like titanium and zirconium alloys.
Our tube rolling capabilities include both classic 3-roll (single roll set) tube rollers and an advanced 6-roll (2 roll sets) approach that Superior Tube developed and patented.
Typically used as the first form of size reduction for seamless tubes, cold drawing reduces the diameter by pulling the tube through a die that is smaller than the tube. In order to fit the tube into the die, one end is ‘swaged’ or ‘tagged’ thereby reducing the diameter of the leading end before drawing. Next, the narrowed end is passed through the die and clamped to a drawing trolley which pulls the tube through the die. After drawing the ‘tag’ is cropped from the tube end prior to cleaning.
Superior Tube employs three types of cold drawing techniques:
This is the simplest of the three drawing methods, as there is no tooling to support the ID surface. The tube is drawn through a die made of polished tool steel or industrial diamond, thereby reducing its inside and outside diameters. Our specialized lubrication and application techniques, combined with our proprietary die profiles, enable the OD surface to become smoother as the tube is drawn. Since the inside diameter is not constrained, the wall thickness of the tube will normally increase during drawing, and the ID surface finish will normally become rougher during a sink draw.
Rod drawing is our most commonly used cold draw method, primarily for intermediate or in-process drawing stages, where both the outside diameter and wall thickness are reduced at the same time.
The tube is loaded over a hardened steel mandrel rod and both are then drawn through a die. This squeezes the tube onto the rod, reducing the outside diameter and thinning the wall simultaneously. The die and mandrel determine the size of the drawn tube, which is then slightly expanded by applying pressure to the outside of the tube so that the rod can be removed. Since larger reductions in cross-sectional area can be achieved by rod drawing, this method is used for mid-process stages to reduce tube sizes prior to the final drawing cycle.
This type of drawing is used to achieve the best possible surface finish and the greatest control over both dimensions and final temper. The outside diameter and wall thickness of the tube are both reduced during plug drawing, as the tube travels through a die and over a stationary plug/mandrel made of high grade tool steel. The plug or mandrel has a polished surface and is attached to a fixed back rod, which is carefully positioned within the drawing die. The tube is loaded over the mandrel/back rod. As the tube passes through the die, the burnishing action of the metal flowing over the stationary plug imparts a high tolerance surface finish inside the tube.
When properly lubricated and prepared, the ID will show very few flaws and finishes of 16 RMS or better can be achieved. Plug drawing is normally chosen for the final draw stage because it achieves a high quality surface finish, exceptional dimensional control, and positive influence on tensile strength requirements.
Our seamless tubes can be produced in almost any profile. For further details please contact our product teams.
Degreasing – cleaning
As part of our efforts to develop and implement more environmentally friendly processes, Superior Tube eliminated the use of TriChlorethylene (TCE) as a degreasing agent in 2008, replacing it with an equally effective but safer solvent called n-Propyl Bromide (nPB). Due to increasing concerns and the risks associated with the use of TCE, we invested significant effort in searching for a safer solvent that would not compromise the exceptional product quality upon which we have built our reputation.
While our switch to nPB has been very positive for our business, we recognize that our responsibility to protect the environment is an on-going effort, and we are continuing our development efforts to further improve the safety and reduce the environmental impact of our tube cleaning processes.
Annealing is used to soften the metal before further cold working or fabrication processes, and improves the overall metallurgical microstructure of the tube. During tube reduction or cold drawing, it can become hard and somewhat brittle. To be able to draw the tube again, stresses formed during cold working need to be removed to return the material to its normal state.
During annealing the tube is heated to a controlled temperature (up to 2100°F) and soak time. Through this process the tube remains in shape, but the grains in the structure of the tube reform into a regular unstressed pattern. The resulting annealed tube is softer and suitable for redrawing.
Our closely controlled annealing and heat treat processes are audited regularly by our nuclear, medical, and aerospace customers.
Drawing and annealing generally results in some degree of bowing, producing a slight bend in the tubing. At Superior Tube we use multiple roll mechanical straighteners in the first stage of finishing. The straightener applies pressure and flex to the product in order to remove bends or bows, resulting in a straightness level of 0.010” per foot, or better. Straightening can introduce slight changes to the size and mechanical properties of the tubing, so these aspects are very carefully controlled during the process.
OD belt polishing / grinding (optional)
When specified by our customers, we can polish or grind the outside diameter of their tubes using multiple head belt grinding machines. Polishing is done to improve the cosmetic appearance of the surface, providing a bright, uniform surface finish. Belt grinding is used to remove a small amount of metal from the OD in order to eliminate minor flaws and imperfections, which may result from cold working. Both polishing and grinding reduce the final outside diameter and wall dimensions, so precise controls are used to ensure a uniform size with an extremely high quality surface finish.
ID grit blasting (optional)
Grit blasting is used to condition the ID surface, similar to the way grinding conditions the OD surface. An abrasive medium is forced through the tube using compressed air, resulting in a uniform surface finish free of minor cold working flaws. Grit blasting can alter both the ID size and wall thickness, so once again we use precise controls to ensure a uniform size. An extremely high quality finish is achieved.
Non destructive testing and inspection
Superior Tube inspects all lots of tubing before shipment to ensure that every aspect of the customer’s order has been met. Our in-house inspection capabilities include:
- Alloy identity verification (Positive Material Identification, or PMI)
- Visual inspection
- Dimensional inspection using non-contact techniques such as laser micrometers and air gages whenever possible
- Surface roughness testing
- Immersion ultrasonic testing to exacting standards
- Encircling coil Eddy Current testing
- Dye penetrant examination
- Hydrostatic pressure testing
- Burst pressure testing
- Destructive mechanical and metallurgical testing:
- Yield Strength
- Ultimate Tensile Strength
- Rockwell and Knoop Hardness testing
- Metallographic analysis for grain size, structure, micro-cleanliness, and flaws
- Manipulation testing such as bending, flattening, and flaring
- We also work with several qualified independent labs that can perform additional metallurgical testing not listed above, if required by our customers
All shipments include a full Test Certificate showing all required test results and verifying compliance with the order requirements.
Stamping / labeling
All products will be identified in accordance with you requirements. Stamps usually include information such as our lot number, raw material heat number, dimensions, material and other pertinent information. For many products this information is marked in removable ink along the length of the tubing in the lot. For those products that are too small or too thin, the information can be included on material tags or box labels.
Straight lengths of seamless tubing can be individually sleeved in paper or plastic and/or packaged in our unique cardboard spacers to ensure no damage occurs during transit. Coiled seamless tubes are horizontally packed using paper or cardboard separators to prevent tube-to-tube contact. All tubing is packed in either wooden crates lined with wax craft paper, tri-wall clay-coated reinforced cases, or standard cardboard boxes. Plastic end caps that prevent foreign material from getting inside the tubes are also available upon request.
When requested, we can arrange for shipment and delivery to your dock, or ship your tubes to a third party and, if needed, can arrange for expedited shipments. We currently export to over 30 countries all over the world and use Incoterms® as our international framework for shipping.