Technical Field

The present invention relates to a process for coating a mandrel intended to be used for seamless pipe production.

Background Art

In the metallurgy sector, the rolling processes are well known for the formation of so-called “seamless” pipes, i.e. monolithic products made to obtain pipes without joints/welding points.

One of the most efficient technologies for the formation of the product is to use a steel bar (called “billet”), axially pre-drilled, and then fit a retained or floating mandrel. By passing the pipe between rollers which compress and stretch the material on the mandrel, it is possible to define the desired diameter and thickness.

The life cycle of a mandrel can vary depending on the material and dimensions of the pipes produced, but generally it is used for a limited period of time after which the outer surface is reduced by friction with the material or begins to present surface defects which can compromise its operation.

It is known in fact that in the production of seamless pipes the surface condition of the mandrel is of great importance. During the entire running time of the mandrel, the extreme working conditions at high and very high temperatures (even more than 1,200° C) give rise over time to cracks and problems in the outer surface of the mandrel, in addition to a natural reduction in thickness due to use, which considerably limit its service life. As a general rule, the life of the mandrel is generally limited to the formation of no more than about 1500 pipes after which it must be taken out of production, or reduced by turning, and if necessary chromium-plated, so that the mandrel can be reused for the production of pipes with a diameter smaller than the previous one.

In some cases, the mandrel may also be discarded at the end of an operating cycle and replaced with a new one.

The Applicant has noted that mandrel reconditioning systems of known type, although cost-effective compared to complete mandrel rejection, have great limitations inasmuch as they are dedicated only to reducing the mandrel diameter. Consequently, it is necessary to have several mandrels available in order to be able to make the best use of the rolling mill’s potential.

The Applicant has thus considered reconstructing the outer diameter through submerged arc welding by means of which highly erosion-resistant material is welded, which thus enables the mandrel to be reused to produce pipes of the same diameter.

Description of the Invention

The present invention therefore relates, in one of its first aspect, to a process for coating a mandrel in accordance with claim 1 in order to obtain a regenerated mandrel having a life span similar to the life span of a newly manufactured mandrel and having structural and functional characteristics such as to meet the above requirements and at the same time to overcome the drawbacks mentioned above with reference to the state of the art.

According to a further aspect, the present invention relates to a mandrel for use in the manufacture of seamless pipes in accordance with claim 9.

Brief Description of the Drawings

Further characteristics and advantages of the present invention will be more evident from the following detailed description of the preferred embodiment thereof, such description being presented merely by way of example and given with reference to the attached drawings. In the drawings:

- Figure 1 is a schematic view of the side section of a mandrel according to the invention;

- Figures 2 to 5 are cross-sectional views of the mandrel in Figure 1 during the coating process according to the invention.

Embodiments of the Invention

In the figures, reference numeral 1 globally indicates a mandrel according to the present invention.

The mandrel 1 extends along a longitudinal axis X-X and comprises, in one of its radial internal positions, a central body 2 preferably made of steel or steel alloy.

As mentioned above, the mandrels are used for the production of seamless (i.e., without welds) pipes in a rolling mill. After a period of use, the mandrels are usually taken out of production or reduced by turning in order to be reused for the production of pipes with a diameter smaller than the previous one. Differently, the process in accordance with the invention makes it possible to again use a mandrel for pipes of the same diameter or even larger than the original one.

In this case, the coating process first of all involves having a new or already used mandrel for the production of seamless pipes.

The mandrel 1 to be reconditioned is positioned on special rotary supports that put it in axial rotation. This way, preferably by means of a turning phase, it is possible to remove part of the outer surface of the mandrel 1 to remove any surface defects depending on its wear.

As shown in the example in Figures 2 and 3, turning is preferably carried out so as to reduce the diameter of the mandrel 1 up to a maximum of 30% of the initial diameter dl. After turning, the diameter d2 of the mandrel will be smaller than the initial diameter dl of the mandrel.

If necessary, after turning, a straightening phase can be carried out in order to eliminate any axial deformation of the mandrel 1. Such deformations can be corrected by machining the mandrel 1 on presses in themselves known to an expert in the sector.

After turning, the process comprises a heat treatment phase (normalization) to relieve the mandrel from any stress accumulated during use. Preferably, normalization is by heat treatment over a period of approximately 24 hours.

With reference to the example in Figure 4, advantageously, the process comprises the phase of applying a coating layer 3 on the central body 2 with a predetermined thickness si depending on the diameter of the mandrel. As an example, in a mandrel 1 with a diameter of 260 mm, the coating layer 3 can have a thickness si ranging between 5 mm and 40 mm, preferably about 13 mm. In one version, the phase of applying the coating layer 3 on the central body 2 is such that the diameter d3 of the mandrel after the application of the coating is at least equal to the diameter dl of the mandrel before turning.

In another version, the phase of applying the coating layer 3 on the central body 2 is such that the diameter d3 of the mandrel after the application of the coating is greater than the diameter dl of the mandrel before turning.

In yet another version, the phase of applying the coating layer 3 on the central body 2 is such that the diameter d3 of the mandrel after the application of the coating is smaller than the diameter dl of the mandrel before turning.

Preferably, the phase of applying the coating layer 3 is carried out by submerged arc welding.

In the remainder of the present description and in the subsequent claims “submerged arc welding” means an automated welding process for the deposition of a predetermined amount of material on a workpiece where the torch and the arcs (not shown) are partially submerged on a flux material designed to support the weld with sufficient conductivity to achieve an efficient current path between the mandrel and the wire torch.

Preferably, in the process according to the invention, the torch can operate by means of powder or wire application of material.

In another embodiment, welding can also be carried out using the well-known laser process.

In detail, the coating layer 3 is applied to the mandrel 1 while the latter is set in rotation and by means of the forward movement of one or more torches along the longitudinal direction X-X. Basically, the coating layer 3 is applied in a spiral on the central body 2 by suitably adjusting the step of forward movement of each torch longitudinally along X-X.

It should be pointed out that the duration of the phase of applying the coating layer 3 depends on the amount of torches used. For example, in a mandrel 1 with a length of approximately 12 meters, the coating application phase takes approximately 2 hours.

Preferably, the recommended chemical dosages for making the coating layer 3 are listed below: - from 0.06% to 0.09% carbon,

- from 0.001% to 0.006% sulfur,

- from 1.5% to 1.8% manganese,

- from 0.5% to 0.6% silicon,

- from 0.006% to 0.008% phosphorus,

- from 0.6% to 0.65% molybdenum,

- from 10% to 12% chromium,

- from 1% to 1.25% nickel.

In a preferred embodiment of the invention, carbon is present in a weight amount of about 0.16%.

In a preferred embodiment of the invention, sulfur is present in a weight amount of about 0.005%.

In a preferred embodiment of the invention, manganese is present in a weight amount of about 1.55%.

In a preferred embodiment of the invention, silicon is present in a weight amount of about 0.67%.

In a preferred embodiment of the invention, phosphorus is present in a weight amount of about 0.008%.

In a preferred embodiment of the invention, molybdenum is present in a weight amount of about 0.78%.

In a preferred embodiment of the invention, chromium is present in a weight amount of about 3.62%.

In a preferred embodiment of the invention, nickel is present in a weight amount of about 0.21%.

If necessary, a phase of surface stretching of the coating layer 3 can be carried out after the application of the coating layer 3.

Usefully, the process according to the invention can comprise a phase of surface peeling of the coating layer 3. This phase is preferably carried out after the stretching phase.

Following peeling, the process may comprise polishing the coating layer 3, preferably carried out by means of a brush polisher. Finally, advantageously, the process according to the invention ends with a phase of application of a chromium-plating layer 4 on top of the coating layer 3 (Figure 5).

Preferably, the chromium plating is carried out by means of the forward movement of the mandrel rotated inside a tank containing a (hexavalent or trivalent) chromium base. Preferably, the speed of forward movement of the mandrel during the chromium-plating phase is approximately lm/h.

As an example, in a mandrel 1 with a diameter of 269 cm the chromium-plating layer 4 can have a thickness S2 ranging between 0.045 mm and 0.14 mm, preferably of about 0.08 mm.

As can be seen from the present description, it has been ascertained how the described invention achieves the intended objects and in particular the fact is underlined that, by means of the coating process named by the applicant ToleroXTM, it is possible to regenerate a mandrel to be used again for the production of seamless pipes of potentially any diameter. The coating process thus developed also makes it possible to enormously extend the life span of the mandrels through the application of a coating layer which, thanks to its chemical composition, is extremely resistant and durable.