NDTnet - May 1996, Vol.1 No.05

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Technical developments in the production and processing of tube

by Wire/Tube Press Office Bernhard Kunzelmann

Behind the collective term 'lube" you will find an impressive variety of products for the most diverse applications. Basically speaking, a tube is a hollow profile of mostly circular cross-section designed to act as either a conveying or structural element. The term covers all sizes, from the smallest to the largest diameter, made of a host of materials - from steels and nonferrous metals to concrete, ceramic and plastic, of which the main representatives are PVC (polyvinylchloride) and PE (polyethylene). Production methods include extrusion and winding.

Plastics such as PE are also used to coat steel tubes. The extrusion winding and extrusion hose methods are used, for example, to prolong the life and increase the reliability of tubes, particularly when they are to be laid underground or outdoors. Epoxy resins or cement mortar are preferred, on the other hand, to protect the inside of steel tubes from aggressive media, depending on whether the medium is a liquid or a gas.

Steel tube, seamless or welded

The breakthrough in the production of seamless steel tube was achieved 110 years ago, i.e. in 1886, by the Mannesmann brothers with their development of the cross-roll pilger-step process. Seamless tubes are not only produced by this and other hot rolling methods, but also by cold-rolling. For the production of welded tube, on the other hand, the options are longitudinal seam welding and spiral tube welding.

Total tube output fluctuates with regional economic trends, but the long-term picture is undoubtedly one of growth. Global tube consumption has increased by approximately 4.5% per annum over the past 25 years . In statistical terms of the world population, an average 17 kg of steel tube is now being produced per person per year. And considering that many countries are still in the process of developing their infrastructures and industries, there is every reason to assume that the growth phase is by no means over.

Conventional continuous tube rolling mills are extremely complex installations requiring an immense investment that can only be paid back if the high capacities are well utilized. There is a rising demand world-wide, however, for smaller-capacity plants with a correspondingly lower investment burden. This demand is met by plants for cross-roll piercing elongation (CPE), which are based on the push bench method. This solution uses a cross-rolling mill rather than a piercing press as piercing unit, and the hollow block is flanged after the piercing. CPE plants are suitable for tube diameters of up to 9 5/8" and annual capacities of a maximum 350,000 tons.

By contrast, the MRK process extends the limits of the conventional continuous tube rolling mill with free-running bar in the direction of larger diameters and longer tube billet lengths. Using a fixed mandrel with controlled adjustment facility, the MRK process is able to produce a tube billet length of 48 m and maximum tube diameters of 7 5/8" or 14" (depending on the version) instead of the customary 35 m billet length and 7" tube diameter.

Hot pilger rolling, the oldest rolling method for seamless steel tube, has managed to defend its position"ion in rolling technology and mechanical engineering through continuous further development and regular incorporation of the state of the art. The same applies for the other classical hot rolling methods, too, such as plug rolling and the Assel process. Assel mills are particularly well suited for the production of tube needing to meet high requirements on wall thickness precision. Modern Assel mills are notable for the precision and reproducibility of their high-speed roll adjustment, fast roll change, automatic tube end inspection and computer-based process control and optimization. Operational reliability, a vital factor in the production of thin-walled tube, is assured above all by the highly sophisticated process control technology.

Modern tube welding systems are extremely productive thanks to their high production speeds. Fast changeover facilities contribute to their efficiency by shortening plant stoppages for resetting purposes. Contour forming systems permit the process strip to be shaped by adjustable roller systems and with far less tool changing. Good reproducibility of setting parameters, fewer tools for changing and smaller dimensions of the rolling plant all help to reduce the set-up work for pass and strip thickness changes, which is a great advantage.

The requirements needing to be met by longitudinally welded large- size tubes are always higher. Various tube forming techniques are available for different product, production and efficiency criteria. Machines of modular design enable adaptation to special applications. Computer_controlled roll adjustment systems shorten conversion and set-up times, improve product quality and permit reproducible line settings for longitudinal tube welding systems. Advances with the computer

The output of seamless or welded tubes generally decreases with the diameter of tube produced directly on the lines. It is better, therefore, to keep the starting diameter of the production line as large as possible and to wait until the downstream reducing and stretch-reducing mills to produce the finial diameter. While the reducing process decreases the outer diameter with practically no change of the wall thickness, stretch- reducing decreases both values. Reducing and stretch-reducing mills generally work with three-high rolling stands because of their technological advantages. The state of the art for such hot forming plants includes individual, electronically controlled, hydrostatic stand drives, electronic program controllers and automatic stand changing facilities.

Impetus for significant further developments in the stretch-reducing process is being generated by the computer and its possibilities. One forward-pointing development is the use of process management systems that work with advanced real-time simulation. Measurement and control functions are performed on the basis of system-integrated process models, which store all the complex know-how about the production process in the form of a mathematical-physical model. The production flow is adapted to changing production conditions within fractions of a second, thus ensuring a constant quality of product with a high rate of productivity. "Intelligent" systems of this type are able to learn from the rolling operation and further improve the accuracy of the process control on the basis of this experience. What is more, they can also monitor the process flow, identify faults, and either rectify production trouble on their own or at least issue timely alarms. With these concepts it is possible in practice to reduce material losses drastically, to maintain the desired wall thickness automatically and to cut the wall thickness scatter range by more than 50%.

Cold pilger rolling and drawing on bull-blocks or continuous drawing machines are among the most frequently used cold-forming methods. A major factor behind the efficiency of the cold pilger rolling process are its large cross-sectional reductions. Ongoing further developments have enabled its performance to be improved at regular intervals. One example is the horizontal mass compensation system that enables far higher stroke rates on smaller types of rolling mill. Another is the continuous-type cold pilger rolling mill, where it is no longer necessary to stop the line in order to load a new pierced billet. Together with in- line coils, these continuous-type cold pilger rolling mills have also led to a considerable increase in possible tube weights for the production of copper tube. High tube weights, on the other hand, enable the drawing speeds of the follow-up bull-blocks to be used to optimum effect.

Continuous bull blocks are designed to take hot-produced, drawn or rolled tubes in straight lengths or coiled form and to draw them to final diameter in several passes. Thin-walled tube that was produced on an extruder and is suitable for direct drawing on a continuous bull block can reach a length of just 40 to 80 meters. Utilization is better if a larger tube in terms of diameter and wall thickness is first produced on an extruder, cross-rolling mill or continuous hollow caster and is subsequently reduced on a cold pilger rolling line. Tube lengths of between 100 and 250 meters are then possible and the microstructure is improved simultaneously.

Double the capacity The productivity of tube production processes has been just about doubled in the past 25 years. Annual capacities of seamless tube now lie at around 500,000 tons. Among other things this has had repercussions for the finishing equipment which follows the rolling mill. A modern finishing line is characterized not only by a high degree of automation for higher output but also by an effective quality assurance concept as well as protective measures against noise and dust in order to improve working conditions for the personnel still required. Integrated tube tracking systems, which record the tubes at the various processing stations, are an essential part of any automation system. Powerful process> computers, on the other hand, are not absolutely necessary for the automation system. With proper planning and optimum software, the work can be done adequately well by small computers. Plant simulations enable the efficiency and functionality of such complex finishing lines to be tested early in the planning and design phases.

Spray compacting is a process for the near-net-size production of tube. It is based on the rapid solidification of liquid metal directly in tube shape without any hot forming whatsoever. This is done by atomizing a current of molten metal and catching the metal spray on a rotating and forward-moving collector. The advantages of this process are the small number of operations, a higher material output, and the possibility of processing not only non-ferrous metals but also high-alloy steels with a strong inclination to segregate during solidification as well as such alloys that are hard to process because of their high resistance to heat and strong inclination to hardening.

In the production of spun iron tube, additional rationalization Potential can be tapped by automatic cycle lines for the simultaneous processing of neighbouring tube sizes in one operation. The system works by scanning the tube size and adjusting the processing machines automatically to the respective nominal width. This largely dispenses with the need for any elaborate pre-sorting and intermediate storage of the tubes. New bending process For many applications it is necessary for tubes to be worked into a specific geometrical shape. A recently introduced alternative to form-fit bending with rollers, edges or dies is end-controlled free bending. In this forming process the tubes are fixed at their ends in clamping devices, which are moved in ways that are calculated on a PC. Without any tool changing whatsoever, this process is able to produce a large number of different bent shapes with" considerable precision. Rapid changeover to new shapes makes this method particularly interesting for small series in which set-up times are a critical economic factor.

Flexibility, automation, set-up time minimization, complete processing and improved product quality are the aims of further developments on conventional tube bending machines. As regards control systems, the spectrum ranges from stored-program controllers and freely programmable controllers for individual machines to production control systems of networked computers. Movable bending heads or the Arrangement of two independent bending heads on one machine permit both right-hand and left-hand bends, i.e. a free choice of bending direction. In conjunction with programmable bending sequences it is also possible to perform even complex bending operations automatically. lnterlinking the bending machine with handling systems and additional processing units transforms it into a bending center.

Automatic measurement and correction systems for the bending angle increases machine output and the quality of finished parts. The purpose of these systems is the automatic compensation of tube spring-back. They are either integrated in the bending machine itself or are designed as separate measuring stations for interlinking with the machine. They work with the most modern technology, e.g. video and laser systems.

The direct joining together (interlocking) of a tapered tube end and a flared tube end is an economical process provided the ends are formed accurately in the die. A good fit may be prevented, however, by stress-induced deformations if the formed tube ends have to be cut. With optimized dies it has been possible for some time now to produce low-stress tube ends, complete with the slots required for joining, without any undesirable deformations of the tube cross-section.

A process that has attracted great attention of late, particularly from the automobile industry, is the liquid bulge forming of tubular workpieces. It is capable of producing even complex components with a liquid medium, e.g. oil or water, subjected to hydrostatic internal pressure combined with additional mechanical forces acting on the workplace ends. The process is an alternative to the conventional production of intricately shaped hollow parts in sheet-metal, where weld seams could be a drawback. Liquid bulge forming results in load- compatible fibre flow lines and enables strain values of over 80%, i.e. nearly double those of conventional processes, without any intermediate annealing.

©copyright Rolf Diederichs 1.May 1996, info@ndt.net
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