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.
There are three types of cold drawing techniques,sink drawing ,rod drawing and plug drawing.
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.
Like pilgering for hot roll pipes, 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 most metals such as carbon steel,stainless steel,titanium alloy etc.
Our tube rolling capabilities include both classic 3-roll (single roll set) tube rollers.
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 1000°) according different metal grades 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.
We can also process normalizing with adding hydrogen to make tube surface more smoother and bright.
Our closely controlled annealing and heat treat processes are audited regularly by our customers from different application fields.
Drawing and annealing generally results in some degree of bowing, producing a slight bend in the tubing. At our mill 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 0.8/1000mm, 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.
NON-DESTRUCTIVE TESTING AND INSPECTION
All lots of tubing will be inspected 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
- 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- Elongation- 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.