A Flattening tests evaluate the tube's ductility and ability to withstand deformation without cracking. A ring section is compressed between two parallel plates until a specified height is reached; the sample must show no cracks or imperfections. Flaring tests involve expanding the tube end with a conical mandrel to a specified angle. These tests are particularly important for tubes that will be cold-formed, bent, or flanged during installation, ensuring they can be worked without failure.

A Transverse cracking results primarily from poor billet heating, where uneven temperature causes hot spots or insufficient soaking, leading to irregular deformation during piercing. Uneven wall thickness in the hollow shell from misaligned plugs or worn rolls creates localized stress. Sudden water quenching of hot tubes induces local embrittlement. Surface defects like straight grooves or folds from worn tooling also act as crack initiation points. Prevention requires strict heating control, precise tool alignment, regular equipment maintenance, and continuous wall thickness monitoring.

A Billets should be heated strictly to the upper limit of the specified temperature range, ensuring complete soaking throughout. In sloped-bottom furnaces, billets must be turned frequently for uniform heating, with the door closed promptly after each operation to minimize heat loss. The rolling rhythm must be controlled so billets are heated thoroughly without any "black core" remaining. This prevents the black helix defect that can occur during piercing, ensuring uniform metal flow and significantly reducing transverse cracking risk.

A Accurate wall thickness requires correctly setting piercing and rolling parameters, using precision tools, and ensuring uniform deformation of the billet. The centering position must be adjusted so the plug aligns precisely with the billet center, avoiding piercing eccentricity. Guide rollers must firmly grip the plug bar to prevent sagging. A regular equipment maintenance schedule keeps all machinery in optimal condition. Operators should frequently measure shell wall thickness and make immediate adjustments upon detecting deviation.

A Sudden water cooling creates severe local thermal stresses that can cause embrittlement and cracking. The production rhythm should be coordinated so the tube stays on the table for minimal time before rolling. Final rolling temperature must remain above 850°C to ensure a uniform internal structure and minimal residual stress. Cooling water pressure should be moderate to prevent leakage onto hot tubes. Any localized rapid cooling can create cold-brittle spots that crack under subsequent mechanical working.

A Straight grooves and rolling folds are surface defects caused by worn or damaged tooling. They act as stress concentrators during subsequent drawing or service loading. Under tensile stress, these grooves can open into cracks. Rolling folds may also trap oxide or debris, creating weak zones in the tube wall. Regular inspection and timely replacement of worn tooling, along with frequent checks of the hollow shell surface quality, prevent these defects from propagating into more serious cracking problems.

A Rockwell hardness (HRC, HRB) measures the depth of penetration of a diamond cone or steel ball indenter under a specific load. The result is read directly from the machine dial, making it fast and convenient for production-floor testing. Vickers hardness (HV) uses a diamond pyramid indenter and requires measuring the diagonal length of the resulting indentation under a microscope. Vickers is more versatile for thin-walled tubes and case-hardened layers but is slower. Rockwell is preferred for routine QC of heat-treated tubes due to its simplicity.

A Hardness is not a simple physical property but a composite indicator reflecting the material's elastic stiffness, plastic flow resistance, strength, and toughness. Higher hardness generally correlates with higher strength and wear resistance but lower ductility. For cold-drawn tubes, hardness testing verifies that the work-hardening from drawing, or the softening from annealing, has produced the desired balance of properties. It is widely used because it is quick, non-destructive to the tube's function, and correlates well with tensile strength for quality control purposes.

A A high-precision cold-drawn tube is a steel tube processed at room temperature through cold drawing, cold rolling, or cold bending. This cold working method enables fast production, high output, and the creation of various cross-sections without damaging any coating. The process induces significant plastic deformation, which raises the steel's yield point. Compared to hot-rolled tubes, cold-drawn tubes achieve far tighter dimensional tolerances and superior surface quality. They can be manufactured with outer diameters as small as 6 mm and wall thicknesses down to 0.25 mm, making them ideal for precision applications.

A They are classified by production process (cold-drawn vs. hot-rolled) and by application or material. Common categories include general-purpose steel tubes, low and medium pressure boiler tubes, alloy steel tubes, stainless steel tubes, petroleum cracking tubes, and thick-walled tubes. Based on wall thickness, they also include carbon steel thin-walled tubes, alloy thin-walled tubes, and stainless steel thin-walled tubes. Special shaped tubes (profile tubes) form another category. Each classification serves different industrial needs, with thin-walled tubes capable of outer diameters as small as 5 mm and wall thicknesses under 0.25 mm.