Method for obtaining rolling mill rolls with a coating of tungsten carbide alloy, and resulting roll

Abstract

The method for obtaining rolling mill rolls with a coating of tungsten carbide or the alloy thereof, wherein the coating is a single layer and is carried out by high velocity thermal spraying is disclosed.

Claims

1. A method for obtaining rolling mill rolls with a coating of tungsten carbide alloy, characterised in that the following operational phases or steps are defined therein: a) degreasing the surface of the mill roll c) heating the surface of the mill rolls with a diameter greater than 500 mm between 40-50° C. and the mill rolls with a diameter of less than 500 mm between 80-100° C. d) coating the mill roll with a tungsten carbide alloy comprising molybdenum borides or chromium carbides by means of thermal spraying wherein a powder of a tungsten carbide alloy is melted exhibiting a powder granulometry comprised between 30 μm and 15 μm in a combustion chamber, the molten material is transported to a spraying gun by means of a carrier gas and is sprayed on the mill roll by means of the gun with a supply flow at values comprised between 1 and 8 kg/h, wherein: thermal spraying is high velocity fuel-air thermal spraying wherein the fuel is materialized in propane with a pressure comprised between 593 and 614 kPa, air is applied at a pressure comprised between 552 and 579 kPa, nitrogen and hydrogen are applied at a pressure of the order of 496 kPa, using a nitrogen flow rate of the order of 23 l/min, and a hydrogen flow rate of the order of 15 l/min, setting the pressure of the combustion chamber at about 496 kPa, the carrier gas is nitrogen, with a flow rate comprised between 20 l/min and 30 l/min, the propane pressure is at least 14 kPa above air pressure, the distance between the gun and the mill roll to be coated is comprised between 19 cm and 26 cm, the transverse speed of the spraying gun is comprised between 2 and 3 mm/s; having envisaged that the linear movement of the mill roll moves at a speed of between 2000 mm/s for diameters less than 500 mm, 2500 mm/s for diameters comprised between 200 and 500 mm and 3000 mm/s for diameters less than 200 mm.

2. The method for obtaining rolling mill rolls with a coating of tungsten carbide alloy according to claim 1, characterised in that a final polishing step using diamond abrasive is additionally established.

3. The method for obtaining rolling mill rolls with a coating of tungsten carbide alloy according to claim 1, characterised in that a phase or step b) for activating the surface by blasting is additionally established.

4. The method for obtaining rolling mill rolls with a coating of tungsten carbide alloy according to claim 1, characterised in that two additional operational steps or phases are established; a step e) wherein a surface cleaning process is carried out by means of heating, and a step f) wherein the roughness peaks are rounded by spraying glass beads.

5. The method for obtaining rolling mill rolls with a coating of tungsten carbide alloy according to claim 1, characterised in that in step f) air is applied at a pressure comprised between 483 and 552 kPa, while the combustion chamber will be kept at a pressure of the order of 262 kPa, with the particularity that the glass microspheres exhibit a composition based on sodium-calcium glass without free silica and chemically neutral, with a spherical and regular shape, non-porous, with a hardness comprised between 48-50 HRc and a granulometry comprised between 45-90 μm.

6. A rolling mill roll with a coating of tungsten carbide alloy, characterised in that the coating is a single layer, with a thickness comprised between 0.003 mm and 0.020 mm, and which affects 100% of the work surface, and has a gas permeability comprised between 0% and 0.1%, and that the tungsten carbide alloy comprises molybdenum borides or chromium carbides.

Description

DESCRIPTION OF THE DRAWINGS

(1) As a complement to the description that will be provided herein, and for the purpose of helping to make the features of the invention more readily understandable, according to a preferred practical exemplary embodiment thereof, said description is accompanied by a set of drawings constituting an integral part thereof in which, by way of illustration and not limitation, the following is represented:

(2) FIG. 1A shows a cross section of a chromium coating currently in use.

(3) FIG. 1B shows the cross section of a typical tungsten carbide coating applied by means of high velocity oxygen fuel spraying technology.

(4) FIG. 10 is the cross section of the tungsten carbide coating applied by means of high velocity air fuel spraying technology object of the invention.

PREFERRED EMBODIMENT OF THE INVENTION

(5) In accordance with any of the variant embodiments of the method of the invention, the following operational steps are established in all of them: a) degreasing the surface of the mill roll; c) heating the surface of the mill roll; d) coating the mill roll with a tungsten carbide alloy or alloys thereof by means of thermal spraying wherein a powder of a tungsten carbide alloy is melted exhibiting a powder granulometry comprised between 30 μm and 5 μm in a combustion chamber, the molten material is transported to a spraying gun by means of a carrier gas and is sprayed on the mill roll by means of the gun with a supply flow at values comprised between 1 and 8 kg/h.

(6) Preferably step a) is carried out by using cold solvents or vapor phase.

(7) Preferably, the temperature of step c) should be similar to the temperature that the mill roll will reach during the thermal spraying process, and it will be a function of the mass of the mill roll to be coated. In particular, for mill rolls with a diameter greater than 500 mm, the temperature must be comprised between 40-50° C. In particular for mill rolls with a diameter of less than 500 mm, the temperature should be 80-100° C. In particular, the heating is carried out with a combustion flame.

(8) When the rolling mill roll to be obtained is intended to have a smooth finish, (0.2 to 0.4 microns), a final polishing operation is carried out, in the same spraying machine and/or external machine. The polishing is carried out in a revolution machine using a diamond abrasive to reduce the roughness that comes out of the spraying and achieve the specified values.

(9) It is an operation that is carried out dry (without coolant) with the following parameters: rotation revolutions of the mill roll: 50-100 rpm Travel speed of the abrasive strip: 20-60 cm/min Type of abrasive: diamond Abrasive grit size: 150-250 μm Strip type: fiber with copper inserts for natural cooling.

(10) According to a second variant embodiment of the invention, to obtain a rough finish the method comprises a step b) for activating the surface by means of technical blasting. This step removes the remains of oils and fats. This blasting operation will be carried out with controlled roughness in hot or cold with the same spraying gun using an aluminium oxide.

(11) The blasting step ensures a coating adhesion of over 80%. The ASTM-B571 Standard is used for the qualitative determination of the adhesion of the coating.

(12) More specifically, in step d) thermal spraying is materialised in high velocity air fuel thermal spraying. Preferably, the air pressure is in a range comprised between 586 and 621 kPa. In particular, the fuel is propane. Preferably, the propane pressure is in a range comprised between 600 and 634 kPa; furthermore, in particular, the propane pressure is at least 14 kPa above air pressure.

(13) Preferably, the carrier gas is nitrogen. Preferably, the nitrogen flow rate is comprised between 20 l/min and 30 l/min. More preferably, between 23 l/min and 24 l/min.

(14) Preferably, spraying by gun is carried out in the presence of hydrogen. Preferably, the hydrogen flow rate is comprised between 30 l/min and 40 l/min. More preferably, between 33 l/min and 36 l/min.

(15) Preferably, the distance between the gun and the mill roll to be coated is comprised between 19 cm and 26 cm. Preferably, the powder has a granulometry comprised between 30 μm and 15 μm. Preferably, the supply flow is comprised between 4-8 Kg/h.

(16) Preferably, the transverse speed of the spraying gun is comprised between 2 and 3 mm/s. When several layers of coating are applied, these speeds cannot be used since it causes the part to overheat, inducing thermal stress. Preferably, the linear movement of the mill roll in mm/s is at a speed of between 2000 and 3000 mm/s. In particular, 2000 mm/s is used for diameters less than 500 mm and 2500 mm/s is used for diameters comprised between 200 mm and 500 mm and 3000 mm/s is used for diameters less than 200 mm.

(17) A second option when intending to obtain a rough finish, in which cleaning is important, consists of omitting step b) described above, so that after step d) two new steps are added.

(18) In a step e). a carbide fines cleaning process is carried out by means of a reducing flame, introducing hydrogen in the combustion. If hydrogen is not introduced, the flame would be oxidising and would cause partial oxidation of the coating, causing the following situations: Decarburisation: decrease in the content (%) of the useful Carbide phase, WC, another undesirable very brittle W.sub.2C phase being formed, reducing the wear resistance of the coating. Increase in natural porosity due to the higher oxygen content in the combustion.

(19) In this step, air pressure will preferably be comprised between 552 and 579 kPa, propane pressure between 593 and 614 kPa, while those of nitrogen, hydrogen and the combustion chamber will be set around 496 kPa, using a nitrogen flow rate of the order of 23 l/min, and a hydrogen flow rate of the order of 15 l/min.

(20) In a step f), the peaks are rounded by means of a spraying gun without using a flame, only the set air pressure. The spraying material are glass beads of lower hardness than the coating so as not to erode it. The pressure of the bundle of glass beads causes a plastic deformation in the peaks of the roughness, causing the rounding. Roughness tends to decrease, so it is necessary to start from a roughness that is 10% higher in order to achieve the target roughness.

(21) In this step, air pressure will preferably be comprised between 483 and 552 kPa, while the combustion chamber will be kept at a pressure of the order of 262 kPa.

(22) As for the glass microspheres, they will have a composition based on sodium-calcium glass without free silica and chemically neutral, with a spherical and regular shape, non-porous, with a hardness comprised between 48-50 HRc and a granulometry comprised between 45-90 μm.

(23) From this process, it is possible to reduce the dirt on the strip, obtaining a profile of the coating with more rounded peaks, which friction less with the sheet metal, reducing the number of iron fines on the strip.

(24) In any of the three cases described above, a rolling mill roll is obtained with a coating of tungsten carbide alloys wherein the coating is a single layer, with a thickness comprised between 0.003 mm and 0.020 mm, affecting 100% of the work surface.

(25) Preferably, the permeability of the coating is in a range between 0% and 0.1%.

(26) The alloy is preferably selected from: WC—CoCr, WC—NiCr, WC—Co, WC—Ni, WC—CrC—Ni, WC—CrC—Co, tungsten carbide and molybdenum boride alloy (e.g., WC—Mo B Ni Co Cr Fe).

(27) Preferably, the tungsten carbide alloy comprises chromium carbides.

(28) Preferably, the tungsten carbide alloy comprises molybdenum borides. These alloys with MoB in the composition exhibit a superior non-stick property than WC alloys.

(29) The coating layer has final properties such as those described below:

(30) TABLE-US-00001 Thickness, mm 0.003-0.012 mm Hardness, Hv 1300-1600 Hv Permeability, %  <0.1% Young's modulus, GPa ~450 GPa Adhesion, MPa >   80     No. of passes:    1    

(31) Having thus adequately described the nature of the present invention, as well as how to put it into practice, it must be noted that, within its essential nature, the invention may be carried out according to other embodiments differing in detail from that set out by way of example, which the protection sought would equally cover, provided that the fundamental principle thereof is not altered, changed or modified.