DESCRIPTION

The present invention relates to a composite roller. In particular, it relates to composite rollers applicable to all industrial processes of cold-rolling, forming, but also hot-rolling and to all sub- processes related to them, such as for example scaling, straightening, stretching at the yield point or beyond, for towing or simply for guiding materials being processed.

The composite rollers comprise at least one insert made of sintered material surrounded at least partially by a copper alloy support.

Usually tungsten carbide is used as insert.

Due to its excellent mechanical properties, particularly its high wear resistance, it is used in processing where high superficial hardness and the maintenance of such a characteristic even at high temperatures, i.e. in presence of high contact pressures, are required.

Tungsten carbide combines high hardness, which distinguishes it, with good toughness.

This makes it particularly suited to work in environments where both of these characteristics are required.

Given the high cost of tungsten carbide, in many cases this precious material is used in the form of an insert, which is mounted on a support usually made of steel.

The fixing of the insert to the support is usually carried out by interference fit, and/or in conjunction with fittings such as ring nuts and/or flanges suitable for partially/totally counteracting the movement of the insert with respect to the support.

Considering that the cutting of notches on hard metal, such as for keyways/tongue seats or fixing holes significantly increases the percentage of initiations of cracks and therefore the risk of breakages of the insert, fixing by interference is considered a good compromise between efficiency and economy on a large number of applications, particularly those which are subject to low temperature differences.

In environments where the temperature difference is more substantial, the problem of fixing the insert to the support by interference can cause breakage of the insert, since the coefficient of expansion of the support typically is much greater than that of the insert.

It should also not be forgotten that the vibrations negatively affect the stability of the inserts on their supports, with the risk of loosenings and therefore catastrophic breakages.

In order to overcome these difficulties, some have replaced the technology of the traditional fit with a technology that foresees the embedding of the hard metal part in a casting usually made of cast iron.

One of the advantages of this solution is that of being able to insert hard metal inserts even on very small supports (which is difficult to realize with the traditional techniques). Thus, even rollers of small dimensions which were built entirely of hard metal can be built with only the untreated part in the expensive material.

In this case, the bond between the two materials is ensured by a layer, the thickness of which varies on average between 1 and 5 mm, wherein the two materials weld together. This entails that the sintered material and the molten material of cast iron must be subjected together to the temperature rise (approximately 1200 °C) necessary for the welding of the two materials and furthermore entails the necessity of having to exert pressure on the molten cast iron to improve the welding between the materials. For this reason, in many cases, centrifuges are used.

Another problem derives from the fact that, in many cases, in order to avoid premature solidification of the cast or to favour the formation of chemical bonds between the components of the casting and the insert, it is necessary to raise the casting temperature of the casting, bringing it even above 1500 °C. Many cases even fall within the fusion range of sintered carbide and others go far beyond 1600 °C. However, all the processes described above are not easy to carry out and in any case require accurate control of the temperatures of the various processes.

Further problems are encountered in the cooling phase: given the inhomogeneity of shrinkages, harmful stresses can form inside the piece. The effects of these stresses are accentuated by the high interpenetration of the two materials, which is also favoured by the fact that the temperature of the cast falls within the fusion range of sintered carbide. Such stresses can be limited by choosing particular compositions of cast iron or by carrying out stress relieving heat treatments, thus with the necessity of having to subject the compound to further treatments.

Furthermore, the application of this solution, although overcoming some of the difficulties mentioned above at low rotation speeds, is not suitable for high speeds, given the poor dynamic characteristics of cast iron.

Another flaw of cast iron is that it is a particularly abrasive material that severely limits the cutting speed of the tools and causes premature wear of the same. Therefore, the processing of this material proves to be slow and expensive, severely restricting its convenience in large-scale industrial production applications.

The aim of the present invention is that of providing a composite roller which overcomes the problems of the prior art.

In accordance with the present invention, these and further aims are achieved by means of a composite roller comprising at least one insert made of sintered material, characterized in that it is surrounded at least partially by a copper alloy support comprising: Cu, Fe, Ni, Al, Zn.

Further characteristics of the invention are described in the dependent claims.

The advantages of this solution are different compared to the solutions of the prior art.

In order to solve the inconveniences of the prior art, the Applicant has thought of using alloys for the casting, which contain elements that are capable of forming chemical bonds with the components of hard metal at significantly lower temperatures than those of the known methods.

It is also known that zinc tends to alloy particularly efficiently with cobalt used as alloying element in hard metal. In fact, zinc itself is used to dissolve cobalt in the recycling process of hard metal itself.

In accordance with the present invention, alloys that can be used in the preferential form are industrial copper alloys (aluminium bronze) containing sufficient quantities of zinc: the presence of zinc in significant quantities allows the formation of a superficial chemical bond with the cobalt present in the sintered element (HM): the cobalt transfers from the surface of the sintered element into the molten alloy and the zinc migrates inside the sintered element through the micro-channels previously filled by the cobalt. Such bonds begin to form at the temperature of 700 °C, well below the casting temperature of the alloy. The Zn-Co bonding phenomena, albeit superficial so as not to alter the characteristics of the sintered element and not to produce notch effects, contribute efficiently to anchorage between the insert and the molten support.

This type of alloys, in addition to ensuring outstanding anchorage, ensures above all outstanding workability and tool life in industrial production processes.

Further advantages derive from the fact that the high mechanical characteristics make it ideal for use in high-speed applications.

A significant advantage is, without doubt, linked to the outstanding thermal exchange capacity of these copper alloys: it is known that the thermal conductivity of copper is much higher than that of steel and cast iron. This capacity allows better heat removal by the roller while it is working.

The anchorage of the HM to the molten support is ensured by the volumetric shrinkage during solidification, which allows strong adhesion and containment of the ring. The shrinkage is such as to guarantee, on the one hand, the containment and to avoid, on the other hand, excessive internal stressing of the ring due to solidification. Such anchorage is also favoured by semi-spheroidal or semi-ellipsoidal notches realized in the HM ring.

The characteristics and the advantages of the present invention will become apparent from the following detailed description of one of its practical embodiments, illustrated by way of non-limiting example in the united drawings, wherein:

figure 1 schematically shows a perspective view of a composite roller in accordance with a first embodiment of the present invention;

figure 2 schematically shows a sectional view of a composite roller in accordance with a first embodiment of the present invention; figure 3 schematically shows a sectional view of a composite roller in accordance with a second embodiment of the present invention; figure 4 schematically shows a sectional view of an insert in accordance with a first embodiment of the present invention;

figure 5 schematically shows a sectional view of an insert in accordance with a second embodiment of the present invention; figure 6 schematically shows a sectional view of an insert in accordance with a third embodiment of the present invention.

With reference to the attached figures, a composite roller in accordance with the present invention, comprising a support 10 constituted by an alloy that surrounds at least partially an insert 11 made of sintered material.

The alloy of the support 10 is a copper alloy of the aluminium bronze type containing sufficient quantities of zinc.

In particular, it preferably and mainly comprises Copper (Cu), Iron (Fe), Nickel (Ni), Aluminium (Al), Zinc (Zi).

It preferably comprises:

Cu in a quantity to make up to 100%,

Fe in a quantity between 0.3% and 7%,

Ni in a quantity between 1.5% and 19%,

Al in a quantity between 8% and 15.6%,

Zn in a quantity between 7% and 10%.

Even more preferably, it comprises:

Cu in a quantity to make up to 100%,

Fe in a quantity between 0.3% and 7%,

Ni in a quantity between 1.5% and 19%,

Al in a quantity between 8 % and 15.6%, Zn in a quantity between 7% and 10%,

Pb in a quantity between 0% and 0.15%,

Sn in a quantity between 0% and 0.15%,

Mn in a quantity between 0% and 1 %,

Si in a quantity between 0% and 1.1 %.

Such copper alloy has a casting temperature of less than 1100 °C and presents the following characteristics: tensile strength at break R between 550 and 700 N/mm ² , yield strength Rp0.2 between 210 and 280 N/mm ² , Brinell hardness HB between 180 and 240.

The insert 11 made of sintered material is made of tungsten carbide, which preferably comprises:

WC in a quantity between 85% and 90%,

Co in a quantity between 15% and 10%.

In an embodiment example, a roller with a tungsten carbide insert having WC = 90% and Co = 10% was realized. The support contained Cu = 62.8%, Fe = 4%, Ni = 14%, Al = 11 %, Zn = 8%, Si = 0.2%.

The working temperature was approximately 900 °C and a tensile strength at break R of 650 N/mm ² , a yield strength Rp0.2 of 240 N/mm ² and a Brinell hardness HB of 195 were achieved. In order to further improve the grip of the support 10 on the HM insert 11 , a preferential embodiment foresees the alternative or additional embodiment of annular notches 12 on the inside diameter of the insert and/or on the wall of the same in the pressing phase.

The form of the section of the notches 12 is preferably that of a semi-sphere or a semi-ellipsoid in order to avoid cracks and initiations of cracks.

Further improvements for the grip and the containment of the insert 11 consist in inclining the side surface of the insert 11 and forming a cone, so as to improve the grip and ensure a better contrast to the centrifugal force.

The insert 11 has a toroidal form with the section having different forms. Its external side can have different types of profiles depending on the use of the same, and it can for example be a pointed insert 13, a flat insert 14 or a concave insert 15.

Normally, it is symmetrical with respect to a transverse central axis and its sidewalls preferably have an inclination.

The sidewalls are constituted by two rectilinear crop-ends 20 and 21 , of equal or different lengths, divided by the intersection point 22. In particular, the crop-end 20 is parallel to the transverse central axis, whereas the crop-end 21 is inclined, with respect to the transverse central axis, at an angle β between 3° and 10°.

For the embodiment of a roller in accordance with the present invention, the copper alloy is melted in a die in which the insert has been prearranged.