In principle, rolled rings are large, rotationally symmetrical forgings with a component weight of 300 tons and more, which are also used in tank and apparatus construction, especially for large pressure vessels, such as those used in the construction of nuclear reactors. The established process for manufacturing such components for pressure vessels is forging on large forging presses. There, forgings are then produced as cylindrical and thick-walled pipe sections — the so-called shells — by partial forging. The dimensions of such shells are typically in the range of approx. 8 m outside diameter and lengths of approx. 3 to 5 m with blank weights of up to 300 tons.
Conventional manufacturing: Large open-die forging presses
The forging process used to manufacture such components - known as forging over a mandrel - is carried out on large open-die forging presses, which can have a maximum pressing force of 70 MN to 150 MN depending on the size of the workpieces to be manufactured. The required blank is produced from a starting block corresponding to the raw weight of the shells to be produced (including material additions, e.g. for scale and, if necessary, slug waste). Alternatively, if metallurgically suitable, work pieces with a hole already prepared during casting, so-called hollow ingots, can also be used.
Once the necessary blank has been produced - this could also be described as a very thick-walled pipe section in terms of type and shape - it is then forged to the desired dimensions using suitable equipment in a large forging press over a mandrel.
Depending on the size and weight of the workpiece and the equipment used to handle the workpiece (the workpiece must be rotated incrementally after each press stroke) this process takes a very long time (many minutes to even hours cumulatively). In consequence of this one or more intermediate heating of the workpiece may also be necessary, as it loses temperature due to the continuous radiation and, if necessary, temperature ranges required for the processed material must be maintained for forging.
Advantages of ring rolling compared to forging over a mandrel
The process of forging over a mandrel involves geometric inaccuracies due to the process, which are caused by the partial forging itself and the limited possibility of measuring the geometries during the forging process. Consequently, this means that more material has to be considered as starting weight in order to achieve a forged shape that can be used to produce the required final geometry after tempering and machining.
In principle, the ring rolling process is initially better suited to producing rotationally symmetrical workpieces with a sufficient internal bore more precisely than is possible by forging on an open-die forging press. If the desired geometries are also suitable in principle for production using ring rolling, the process of forging over a mandrel can be at least partially substituted by the process of radial-axial ring rolling.
In the specific case of the "production of shells" considered here, it makes sense both from a metallurgical point of view and from an economic point of view (influencing the required machine size) to first produce a pre-forged workpiece (to achieve a high degree of forging) — the so-called ring blank — before this is then rolled to the desired final dimensions on a suitable radial-axial ring rolling machine in a final forging process. The required size of the ring rolling machine (and therefore the level of investment in such a machine) is significantly influenced by the initial wall thickness of the ring blank to be formed. As a result, the product to be manufactured can save up to several tonnes of input material per workpiece by substituting the final forging process in the press with a ring rolling process, depending on the size of the workpiece.
The technical limitation of using this process for the very large shells mentioned above was that suitable ring rolling machines capable of rolling workpieces weighing 300 tonnes and ring heights (=shot length) of up to 5 meters had not yet been built.
Design, construction, and commissioning of the most powerful ring rolling machine in the world
A customer request for the production of very large rings (a maximum rollable ring outer diameter of up to 16 m and a maximum rollable ring height of up to 5 m with ring weights of up to around 300 tonnes) required a ring rolling machine of unprecedented size and performance data. The necessary performance data determined by means of suitable process simulations available from the machine manufacturer resulted in required rolling forces of a maximum radial 50 MN and a maximum axial 12.5 MN. The installed motor power of all drives totaled approx. 12 MW. The manufacturer rose to this technical challenge and the result is a machine with the designation Radial-Axial Ring Rolling Machine Type RAW 5000/1250-16000/5000 — currently the largest and most powerful ring rolling machine in the world.
With the realization and successful commissioning at the customer's site, a new giant in the forging industry has been created that is unrivaled worldwide and will set new technical standards in many areas. With a total machine weight of approx. 6600 tonnes, an overall length of approx. 55 m, and a height above floor level of approx. 16.6 m, a globally unique machine has been created that will represent the "top of the world" in ring rolling for years to come.
