BACKGROUND

[0002] Roll wear is a key factor in improving mill output. During hot rolling of long steel products, the passform surface(s) of the roil becomes worn. Once worn, each passform must be re-ground and then the roll put back into service. This process is repeated until the passform depth reaches a point referred to as the 'scrap diameter' meaning that the roll can no longer be used. Referring to Fig. 1, the scrap diameter 12 of a roll 10 is approximately halfway between an outer diameter 1 and an inner diameter 16 with the remaining material 18 below the scrap diameter (scrap diameter to inner diameter) remaining unused.

[0003] Being virgin material, this represents a significant cost, so if this portion of the roll could be replaced with a lower cost recycled material then this would reduce the cost of a roll. For example, a standard 8" diameter roll uses approximately 25 kg of powder with the mass of unused material accounting for approximately 10.5 kg.

SUMMARY

[0004] In one aspect, a compound roll of the present invention includes a sintered inner core of a first cemented carbide and at least one sintered outer sleeve of a second cemented carbide disposed around the inner core. The at least one sintered outer sleeve and inner core each have a joining surface, wherein when the inner core and outer sleeve are assembled together each joining surface is brought into contact to form a bonding interface there between. When the assembled sintered inner core and sintered at least one outer sleeve are heated to a predetermined temperature, the inner core and outer sleeve are fused together at the bonding interface to form the compound roll. [00053 ⁵¹ another aspect, a method of forming a compound roll includes the steps of providing a sintered inner core formed of cemented carbide and providing at least one sintered outer sleeve formed of a second cemented carbide. The sintered inner core and sintered at least one outer sleeve are assembled, the at {east, one outer sleeve and inner core each having a joining surface, such that when the at least one sintered inner core and outer sleeve are assembled each joining surface is brought into contact to form a bonding interface (3) there between. The assembled sintered inner core and sintered at least one outer sleeve are fused together at the bonding interface (30) to form the compound roll.

[0006] in still another aspect, a compound roll of a mill includes a sintered inner core of a first cemented carbide and a sintered outer sleeve of a second cemented carbide disposed around and fused to the inner core, the first cemented carbide being different from the second cemented carbide.

[00071 One advantage of the compound roll of the present invention is the potential savings in maintenance costs for the rolling mill. Using a carbide material with a lower density for the inner core, even if it is a virgin grade, e.g. a 6% binder grade for the outer and a 10% or 15% binder for the inner, would reduce the overall mass of the roll, thereby reducing the load on the bearings in the mill and other driveline anciliaries,

[0008J The foregoing summary, as well as the following detailed description of the embodiments, will be better understood when read in conjunction with the appended drawings. It should be understood that the embodiments depicted are not limited to the precise arrangements and instrumentalities shown.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Fig. 1 is a cross-section of a known rolling mill roll.

[0010} Fig. 2 is a perspective view of a compound roll of the present disclosure.

[0011] Fig. 3 is a perspective vie w of the outer sleeve.

[00121 Fig, 4 is a perspective vie of the inner core.

[0013] Fig. 5 is a flow diagram illustrating the steps of the present method.

[0014] Fig. 6 is a cross-section of the compound roll after joinirig/fusing,

[0015] Figs. 7(a)-7(c) are SEM images of the compound roll according to the present disclosure after assembly and prior to fusing, [0016J Figs. 8(a)-8(b) are SEM images of the compound roll according to the present disclosure after fusing.

DETAILED DESCRIPTION

[00171 A compound roll of the present invention includes a sintered inner core of first cemented carbide and at least one sintered outer sleeve of a second cemented carbide disposed around the inner core. The at least one sintered outer sleeve and inner core each have a joining surface, wherein when the inner core and outer sleeve are assembled together each joining surface is brought into contact to form a bonding interface there between. When the assembled sintered inner core and sintered at least one outer sleeve are heated to a predetermined temperature, the inner core and outer sleeve are fused together at the bonding interface to form the compound roll.

|0018| As shown in Fig. 2, a compound roll 20 of the present invention includes an inner core 22 and at least one outer portion or sleeve 24. Core 22, when inserted within outer sleeve(s) 24, as will be described further herein, forms a multi-part or compound roll.

[0019J Inner core 20 can be made of a first cemented carbide or cermet that is of a lower cost cemented carbide, i.e., as compared to the cost of a roll of solid virgin carbide, for example, recycled cemented carbide or a cemented carbide with a lower density, i.e., as compared to the density of a solid virgin carbide. This cemented carbide can have up to 100 wt % of recycled carbide. Although not illustrated, it should be appreciated that the core can be solid or of any shape and is not limited to a cylindrical or ring shape as shown.

f 0020] Outer sleeve 24 is made of a second cemented carbide or cermet, for example, virgin cemented carbide. As described herein, virgin cemented carbide refers to carbide that does don't include and reclaimed carbide. Recycled cemented carbide refers to cemented carbides that are recycled by metallurgical or chemical means, e.g., a zinc recovering process, electrolytic recovery and, extraction or oxidation, which are known to one skilled person in the art. Although only one outer sleeve is shown, the compound roll can includes a plurality of sleeves. [0021] Thus, inner core 22 and outer sleeve 24 can be made from different grade cemented carbide of compacts of liquid phase materials, which include low melting phase components and high melting phase components. Cemented carbide normally has a hard phase comprising tungsten carbide and of one or more carbides, nitrides or carbonitrides of titanium, chromium, vanadium, tantalum, niobium bonded by a metallic phase binder typically cobalt, nickel, iron in varying proportions. The binder may be in the range of about 6 wt-% to 30 wt-%. Each of the first and second carbides can be tungsten carbide bonded with a binder.

[0022] Cemented carbides exist in different grades. The grades depend on the composition of the cemented carbides and on the grain size. For example, a high quality grade is a cemented carbide with a quaniifiably greater performance and reliability in a given application compared with a Sow quality grade, it should be appreciated that other materials for the cores and sleeve(s) are contemplated by the present disclosure, for example, cermets. Thus, the first and second carbides can be made of different cemented carbide.

[O023J Outer sleeve 24 can be pressed from ready to press powder (RTP) of cemented carbide using a grade having a binder that consists of Co only, with no additions of recycled cemented carbide. The average starting, i.e., as provided before treatment, WC grain size of the obtained sintered sleeve can range from about 5.0 to about 8.0 μτη. As with the outer sleeve, inner core 22 can be pressed from RTP of cemented carbide using a mixed binder grade that consisted of Co/Ni/Cr along with recycled RTF of cemented carbide (for example, about 25 wt% of the cemented carbide was recycled). As with the outer sleeve, the average starting WC grain size of the inner core can range from about 5.0-about 8.0 μιη.

[0024] Thus, compound roll 20 includes two parts - inner core 22 of lower cost cemented carbide and outer sleeve 24 of a higher grade cemented carbide or virgin cemented carbide. It should be appreciated that a plurality of outer sleeves can be provided and the present disclosure is not limited to two parts to form the compound roll. As will be further described herein, to create the compound roll, each of a sintered outer sleeve of virgin quality cemented carbide and a sintered inner core of recycled cemented carbide are fused together. [0025] Referring to Figs. 3 and 4, outer sleeve 24 has an inner joining surface 26 and inner core 22 includes an outer joining surface 28. As will be described further herein, sintered inner co e 22 and at least one outer sleeve 24 are fused together at joining surfaces 26, 28 to form a unitary body 32 , the first and second cemented carbides each have different melting points, wherein the assembled sintered at least one inner core and sintered at least one outer sleeve are heated to a temperature intermediate or lower than the melting point of the lower sintering temperature of the sintered core and sleeve to fuse the same at the bonding interface 30 to form the unitary body 32 (Fig, 2). A unitary body is defined as a singular integral body.

[0026] According to a method 40 of the present invention illustrated in Fig. 5, the core 22 and sleeve(s) 24 of the compound roll are formed separately and provided in step 42, according to the particular desired end use. As set forth above, inner core 22 can be pressed from RTF of cemented carbide using a mixed binder grade that consisted of Co Ni/Cr along with recycled RTF of cemented carbide and outer sleeve 24 can be pressed from (RTF) of cemented carbide using a grade having a binder that consists of Co only, with no additions of recycled cemented carbide.

[0027J Each is then sintered for obtaining substantially the foil density and hardness thereof in step 44. The core and sleeve(s) can be sintered in either a vacuum furnace or sinter-HIP furnace at their respective temperatures, for example, 1350 °C - 1520 "C, as known.

[0028J in step 46, the core and sleeve(s) are ground or machined at the mating/joining surfaces 26, 28 to provide a sufficiently smooth finish therebetween to encourage interference fit during assembly. For example, inner core 22 was placed in liquid nitrogen for approximately 30 seconds in order to shrink it and allow it to be pressed into the outer sleeve, thus employing the phenomenon of thermal expansion to make a strong joint at interface 30.

[002 J The sintered core and sleeve(s) can also be cleaned, for example, in an acetone wash to provide a clean, interface at the joining surfaces. Thereafter, the individual parts are assembled into the compound roll in step 48. The assembly of step 48 involves bringing the core and s!eeve(s) into contact in an assembled relationship, with inner core 22 being positioned within outer sleeve 24, such that the first bonding or joining surface 26 is in mating engagement with second bonding or joining surface 28 (Fig. 2) to form unitary body 32. See Fig. 6. Thereafter, as described herein, the step of heating the members in the assembled relationship is employed to fuse the members together.

[0030] Prior art assembly of carbide members includes co-pressing, sintering either two green pieces or one green piece to one smtered piece, but these techniques carry risks from the different shrinkage levels and the higher sintering temperatures needed, which may result in tensile stresses in the outer leading to the possibility of cracking. The present sinter-fusing technique utilizes a temperature just above the WC-Co eutectic and a short isothermal hold compared to that required for a green body.

[0031] Referring again to Fig, 5, in step 50, the assembled smtered core and sleeve(s) are joined by fusing the same at a temperature low enough so that minimal grain growth occurs at bonding interface 30. The assembled sintered core and sleeve(s) are placed in a standard sintering furnace and subject to a vacuum or gas atmosphere, without the application of external pressure. For example, the assembled compound roll is brought from room temperature to the fusing temperature of about 1350 °C, at about 10 "C per min with about a 15 minute dwell time at the top temperature. This fusing temperature and heat cycle is lower than the original sintering temperatures and heating cycles of the cemented carbide having the lowest melting temperature. For example, of about 1340°C to about 1360°C for about 10 to about 30 minutes, and more preferable about 1350°C for about 15 minutes.

[0032] This step takes the already dense and hard pieces of carbide and puts them back into the sintering furnace. But, instead of getting shrinkage and more density, as in the original sintering operation, the core and sleeve(s) remains essentially the same in physical properties.

[0033 j As shown in Fig 6, boundary interface 30 is located between the inner core and outer sleeve and occurs at fusion. The parts are fused at a temperature lower than the melting point of cemented carbide having the lowest original sintering temperature of the core and sleeve(s). This lower temperature and shorter time enables the fusing to proceed by short range diffusion of the binder metals across interface 30 and minimal grain growth is induced in the microstructures but a unitary body 32 is formed. [0034] EXAMPLE

[0035] The compound roll consisted of an outer sleeve in a 6% Co binder grade and an inner core of a recycled 30% Ni/Co/Cr binder grade. Both the core and sleeve were pressed on a SQt manually operated press (Sealey Lid, Suffolk, UK) using single-ended pressing and sintered at their respective temperatures, 1360 °C for the inner core and 1510 °C for the outer sleeve.

[0036] Table ί shows the dimensions and properties of inner core 22 ("inner") and outer sleeve 24 ("outer") before and after sintering. As can be seen, the weight of the core and sleeve before and after sintering is relatively unchanged. The inner core shrunk slightly and the outer sleeve expanded slightly, but the overall thickness changed by a minimal amount. He increased slightly due to grain coarsening. Accordingly, there is physical contact between the core and sleeve to enable fusing.

Ϊ0037] Table 1

Weight Thickness Diameter He

(g) (mm) (mm) (kA/m)

inner Outer

Before 106.665 7.718 7.718 36.01 4.1 1

After 106.675 6.551 8.052 36.58 4.35 [0038] Both the core and sleeve were cut by EDM and initially ground to a locational interference fit according to ISO 286-2: 1988 ISO system of limits and fits. The inner core was placed in liquid nitrogen for about 30s and then pressed into the outer sleeve employing the phenomenon of thermal expansion to make a strong joint, for example, after shrink fitting (shrink fit tolerance + 20 μηι). The applied tolerance was gradually reduced towards a locational transition fit and the process repeated until the inner core could be positioned within the outer sleeve.

[00391 Referring to Figs. 7(a) -7(c), SEM micrographs show the joint lines 26, 28 between the two members prior to fusing treatment. As can be seen, the core and sleeve are joined together and the same are fused to form a unitary compound roll.

[0O4OJ Figs. 8(a) and 8(b) are SEM micrographs showing the boundary zone 30 between the core and sleeve after the fusing treatment. As can be seen some grain coarsening has taken place, more so for the inner core material, but not preferentially at the interface of the joining surfaces 26, 28 between the core and outer sleeve.

1ΘΘ4Ί] it should be appreciated that different grade materials can be joined to optimize local properties of the compound roll. Thus, for example, wear resistance, toughness, brazability, friction coefficient and/or cubic boron nitride (cBN) content of a material can be chosen. Moreover, a cobalt or grain-size mismatch can also be chosen to induce binder metal fusing and consequent density changes, This can induce compressive stresses at the surfaces to provide a toughening effect. For example, fatigue resistance could be increased if one or more layers of decreasing Co content/ Co mean free path/WC grain size are incorporated.

[0042] Although the present embodiment(s) has been described in relation to particular aspects thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. it is preferred therefore, that the present embodiment(s) be limited not by the specific disclosure herein, but only by the appended claims.