TECHNICAL ARTICLE:

Introduction

The pierce-and-draw process is particularly suited for the production of thick-walled, seamless large-diameter tubes. These are used not only for power generation or oil exploration applications, but also as base components for machine-building purposes. The precise control of the whole process yields high-tech products.

“When service conditions become really tough, the seamless steel tube is nearly unbeatable", explains Wolfgang Grümmer, general manager (marketing & sales) of added value products, with Vallourec & Mannesmann Tubes GmbH. In the huge press hall of the V&M factory in Düsseldorf-Reisholz (Germany), the process is meticulously undertaken. A pit furnace cover slides aside, and a big dogging crane claws into the glistening heat, lifting out a huge steel block glowing bright red. The crane transfers the ingot to the huge press at the centre of the hall, where it is inserted into a round steel tool, which then slides under the press cylinder. With solemn slowness, a piercing mandrel - as wide as a man - comes down until it contacts the surface of the ingot. Bringing to bear all its might of 40MN (4,000 tonnes), the press then forces the mandrel deeper and deeper into the massive block.

Using 40MN, the press forces the mandrel deeper into the massive block

The sight of solid steel forced up around the piercing mandrel, like dough under the thumb of a pizzeria cook, exerts a very specific sort of fascination. The operation stops only inches before the ingot is fully pierced. When the tool is withdrawn, it leaves a hollow body resembling an oversized thimble, which is then transported to a horizontal press. There, the ‘thimble’ is fitted onto a mandrel bar acting as an internal tool, and then elongated by pushing it through several drawing dies, giving it a tubular shape.

The inner diameter is determined by the size of the mandrel bar, while the outer dimension depends on the diameter of the last drawing die used. In principle, any combination of diameter and wall thickness can be achieved. The length of the tube depends on the calculated charge weight, and thus on the amount of available material. The maximum length is limited to about 10.5m due to limitations of the stroke of the horizontal press.

An ingot with a temperature of around 1260°C

Solid steel is forced up around the piercing mandrel

When withdrawn, the tool leaves a hollow body

The mandrel pushes the workpiece through a drawing die

The ‘Brute Force’ of Powerful Machines

This pierce-and-draw forming process leads – just like forging – to an optimized, very homogenous crystalline structure. When compared to other processes frequently used to produce seamless tubes, one of its most noticeable advantages is that it is suitable for producing bigger parts. In Reisholz, ingots weighing as much as 27 tonnes can be processed into monolithic tubular bodies, far in excess of the 7 tonnes maximum ingot weight limit for the pilger mill of the Mannesmann type.

Input buffer: these ingots weigh up to 27 metric tons

Another special feature is the integrated ‘natural’ bottom seamlessly forged together with the cyclindrical wall, a great advantage in applications where the tube has to be fitted with a cover, such as pneumatic or hydraulic cyclinders, reactors for chemical processes or high-pressure gas reservoirs.

The advantage of this process is the nearly unlimited bandwidth of chemical compositions of the materials processed. This encompasses unalloyed as well as medium and high-alloy steels, duplex steels and austenitic grades. Additionally, the process can be used to produce tubes from nickel-base, copper or titanium alloys.

Large-diameter seamless tube from copper

Precise Computerized Control

“Don’t be fooled by the rugged appearance of our machinery: the outcome is a high-tech product,” states W. Grümmer. Quality assurance starts with the proper selection of materials, which calls for highly qualified specialists, and often the computer has already been active in order to optimise the production process by simulating the shaping process long before the ingot has actually been placed in the pit furnace to warm up.

The shaping process utilises finite-element computer simulation software

This care is dictated by the sheer order of magnitude of costs for such a tube, which make ‘right first time’ approaches an absolute must. Consequently, the quality management system of the production facility is certified according to the highest level, DIN ISO 9001. In case a customer needs additional support, a specialist team from the R&D department can investigate material selection or process requirement issues.

Sophisticated automation and data acquisition sensors can be found at unexpected places - for instance on the main cylinder of the giant press, which has been fitted with a precise measurement system to ensure the piercing mandrel is exactly in line with the axis of the ingot. This has proven to be a rewarding investment, since every millimetre of deviation means that, in consequence, 2mm of wall thickness have to be chipped off. The plant is currently able to produce tubes with inner diameters of between 220 and 1,220mm, outer diameters up to 1,500mm and wall thickness ranging between 18 and 270mm.

Well-Dosed Heat Treatment

After this hot forming process, the head and foot discard scrap. The unusable end section at the open end of the tube and, if required the bottom, are sawed off. The next step is heat treatment in order to attain the material properties required for the specific application. This is achieved by heating in a state-of-the-art bogie hearth furnace with subsequent controlled cooling.

Heat treatment performed in a bogie hearth furnace

For this treatment, the engineers can choose from among different options ranging from air cooling through water-spray cooling to dipping into a water basin. The next step is annealing treatment. Afterwards, the tubes are straightened on a press. For specific applications, such as high-pressure gas reservoirs, the open end of the tube can be heated and then necked into the shape of a bottle.

Depth-drilling machine for inside machining of tubes (15 m long)

A Wide Variety of Machining Operations

“All our tubes are machined, since this is a prerequisite for quality control,” discloses W. Grümmer. Defect-detection systems require smooth, machined surfaces. The factory thus has many plants of different types for the machining of large tubes and is ready to carry out additional operations according to specific customer requirements. This may include preparatory treatments, such as straightening, sawing to size and rough grinding as well as precision machining, such as turning on a lathe, fine grinding, boring or depth drilling.

Further options include plants for shot peening, band grinding or horizontal drilling, as well as milling machines. The wide variety of plants offers additional advantages since it becomes possible to select the most economical approach to achieve the result required by the customer. For instance, if an inside finishing is required, turning on a lathe may be a better option than boring, and in the case of outside grinding there are important differences between lengthways or circular operation.

Adding Value Instead of Adding Costs

Many of the seamless, thick-walled tubes processed in the factory are intended to be used as components for the machine and plant-building industry, such as hydraulic cylinders, axle drive shaft, pulley or cylinder linings.

Modern CNC machining system

“The awareness of our customer’s intentions enables us to save him costs. This is why we prefer to talk to him about his application instead of selling him ‘semis’ according to catalogue,” says W. Grümmer. Many of the machining operations the customer intends to perform after receiving a ‘semi’ can be performed just as well on the equipment available in the Reisholz works. The deeper this treatment reaches downstream into the value generation chain of the customer, the greater the potential advantage for both parties.

With such bulky and heavy objects, even the ‘simple’ task of fixing them up on a lathe already generates tangible costs. The cost advantage becomes even more apparent if a customer does not possess the equipment to perform such tasks, but sources them from outside. This leads to a whole string of cost-driving activities – identification and assessment of qualified suppliers, transportation and double quality control assurance.

Lifting 1,000 metric tons for up to 3m within seconds: The pneumatic
cylinder made in Reisholz, which helped salvage the ‘Kursk’ Russian submarine

In contrast, the factory in Reisholz is fully equipped with the complete range of state-of-the-art machining and finishing equipment and can perform all necessary tasks at lower costs than a customer who may have just one lathe able to accept such big work-pieces. Furthermore, although nothing looks simpler than a tube, reality tells a different story. Achieving optimised results in terms of quality and costs calls for in-depth knowledge of numerous problems and ‘knacks’.

This already starts with the definition of raw-piece dimensions and of an elaborate treatment scheme taking into account the different machining options available. Even seemingly marginal aspects, such as the proper machining strategy or the agreement of a surface quality above usual standards, which spares the customer additional preparatory operations, may lead to cost reductions in an order of magnitude between some hundred, some thousand or even some ten thousands of Euros.

Photos: by Klaus Vollrath and Vallourec & Mannesman Tubes

Klaus Vollrath, Bergmannstr. 7J, D-44651 Herne, Germany
Fax: +49 2325-62996
E-mail: KVollrath@t-online.de
Website: www.t-online.de/home/KVollrath