WO2013113853A1

WO2013113853A1 - Method of laser cladding a rotation symmetric steel rolling mill with two layers; corresponding roll mill roll

Info

Publication number: WO2013113853A1
Application number: PCT/EP2013/051968
Authority: WO
Inventors: Cristian FEDORCIUC-ONISA; Samuel John LESTER; Nicholas Peter Longfield
Original assignee: Tata Steel UK Ltd
Current assignee: Tata Steel UK Ltd
Priority date: 2012-01-31
Filing date: 2013-01-31
Publication date: 2013-08-08
Anticipated expiration: 2014-07-31

Description

METHOD OF LASER CLADDING A ROTATION SYMMETRIC STEEL ROLLING MILL WITH TWO LAYERS ; CORRESPONDING ROLL MILL ROLL

This invention relates to a method for laser cladding rolling mill rolls and to mill rolls provided with a coating applied by said method.

The inherently harsh tribological conditions associated with rolling mill roll surface in service require very good resistance to abrasive wear, corrosion and thermal fatigue, in order for the rolling process to be performed without the need to stop and replace the roll with either a new or reconditioned one. So far this has been achieved with roll material containing high carbon and alloying elements content, resulting in an expensive, brittle, but relatively wear resistant roll .

Normally the work roll is used for a period of time after which it has to be replaced. The roll is taken out and reconditioned by machining the surface. If this is no longer possible, then a new work roll surface is applied by a thermal process like welding or cladding, in order to make further use of that work roll. Welding has been the technique of choice over the years, but the welding process is limited in the kinds of metals that can be applied. Laser cladding is an alternative to welding which is starting to be used more and more.

However, the high carbon content of cast iron leads to cracks within the Heat Affected Zone (HAZ) when laser cladding is used for hardfacing. These cracks then propagate through the coated layer upon cooling, resulting in a weak coat with limited functionality.

It is an object of this invention to provide a method for laser cladding of rolling mill rolls which provides a wear resistant, corrosion resistant and fatigue resistant coating onto the rolls.

According to a first aspect of the invention this object is reached by providing a method for cladding a rotation symmetric rolling mill roll iron or steel substrate with a wear and corrosion resistant metal coating, said coating comprising at least a first metal coating layer as an intermediate _ - layer and a second metal coating layer as a top layer, the method comprising the steps of:

a. cleaning the substrate by machining the surface of the substrate to be coated;

b. rotating the substrate around its axis of rotational symmetry;

c. forming a melt pool on the surface of the rotating substrate by means of a laser beam and applying the first coating layer by feeding a first powder material into the melt pool, wherein the first powder material is fed into the melt pool coaxially with the laser beam;

d . forming a melt pool on the surface of the substrate provided with the first coating layer by means of the laser beam and applying the second coating layer by feeding a second powder material into the melt pool, wherein the second powder material is fed into the melt pool coaxially with the laser beam.

wherein the top layer is a cobalt alloy, wherein the cobalt alloy comprises between 27 and 32% Cr, 4 to 6% W, 0.9 to 1.4% C, 1 to 2 % Fe, 1 to 3% Ni, up to 1% Mn, up to 1% Mo, up to 0.1% B, up to 0.01% O, up to 0.02% S, and balance Co and inevitable impurities; wherein the intermediate layer is a nickel alloy, wherein the nickel alloy comprises between 15 to 17% Mo, 14.5 to 16.5% Cr, 4 to 7% Fe, 3-4.5% W, up to 2% Mn, up to 0.1% C, up to 0.05% P, up to 0.02% S, up to 0.5% V and balance Ni and inevitable impurities. All percentages are in weight%.

The method according to the invention applies a crack-free laser deposited second coating layer onto the iron or steel substrate onto an intermediate layer which is also laser deposited. After step c and before step d of the method the first coating layer solidifies. Although it has practical advantages to finish the first coating layer before starting the deposition of the second coating layer, there is no technical need to complete the first coating layer before starting with step d . It may be advantageous to complete the first coating layer (step c) before starting with the second coating layer (step d), but the deposition of the second coating layer may also start before step c is fully complete, e.g . by using a second coaxial laser simultaneously with the first coaxial laser. The rotation of the _ _ substrate around its axis of rotational symmetry and the machining can be performed on a CNC-type machine. Step c and d can also be performed while the substrate is still mounted in this machine. The machine can also provide the basis for the laser or lasers and move the laser(s) along the rotating substrate at a preselected speed while providing the first and/or second coating layer onto the substrate.

The intermediate layer prevents cracking of the base of the second coating layer, but it may also increase its wear resistance. When the second coating layer is being applied, the untempered martensite within the HAZ of the substrate becomes tempered, thus reducing the risk of cracking in service. For this process no preheating is needed . Nevertheless, the process and the resulting clad substrate may benefit from preheating the substrate, particularly when the substrate acts as an effective heat sink for the heat introduced into the substrate by the cladding process. Larger rolling mill rolls may benefit more from preheating than smaller rolls. Although by no means limited to it, the method according to the invention could be performed using a laser equipped with a coaxial nozzle such as the one disclosed in EP0574580. The nozzle may be inclined with respect to the target surface and the angle between the laser beam and the perpendicular of the surface. Preferably said angle is between 0 and 45°. Excellent results were obtained with an angle of inclination between 20 and 35°.

The thickness of the second coating layer (i.e. the top layer) is between about 0.5 to 5 mm, and preferably between 0.7 and 2 mm.

It was found that this method can be beneficially used for cast steel substrates, which are prone to cracking, but improvements are also obtained with cast iron substrates. By optimising the laser deposition process in terms of feeding rate, speed, power, nozzle inclination angle a stable and consistent deposition process is obtained, such that the top layer is intact, and the intermediate layer performs its function as described above. For the sake of avoiding any misunderstanding, the rolling mill rolls may relate to the work rolls or to the backup (or support) rolls, such as those in a 4-high rolling stand . The rolling mill rolls may _ _ relate to rolls for rolling long products, but also to rolls for rolling flat or strip products.

The top layer is a cobalt alloy wherein the cobalt alloy comprises between 27 and 32% Cr, 4 to 6% W, 0.9 to 1.4% C, 1 to 2 % Fe, 1 to 3% Ni, up to 1% Mn, up to 1% Mo, up to 0.1% B, up to 0.01% O, up to 0.02% S, and balance Co and inevitable impurities. Cobalt alloys are known to be wear and corrosion resistant. However, the application of cobalt alloys directly on the iron or steel substrate will lead to cracking of the cobalt layer. The application of at least an intermediate layer according to the invention prevents this cracking of the top layer.

The intermediate layer is a nickel alloy wherein the nickel alloy comprises between 15 to 17% Mo, 14.5 to 16.5% Cr, 4 to 7% Fe, 3-4.5% W, up to 2% Mn, up to 0.1% C, up to 0.05% P, up to 0.02% S, up to 0.5% V and balance Ni and inevitable impurities. The inventors found that an alloy layer consisting of predominantly nickel is sufficiently ductile to accommodate the stresses resulting from the thermal impact of the laser cladding treatment, provides good adhesion to both the substrate and the top layer in case of a two layer metal coating system, or to the layer(s) below the intermediate layer and/or to the layer(s) on top of the intermediate layer. Although the system consisting of two layers functions very well, there may be reasons to use one or more additional layers between the intermediate layer and the substrate, or the intermediate layer and the top layer.

In an embodiment the nickel alloy comprises between 8 to 10% Mo, 20 to 24% Cr, 4 to 6% Fe, between 3 to 4.5% of (Nb+Ta), up to 1% Co, up to 1% Mn, up to 0.1% C, up to 0.05% P, up to 0.02% S, up to 0.5% V, and balance Ni and inevitable impurities.

In an embodiment of the invention the laser used for cladding the alloy layers is a coaxial laser, particularly a high power diode laser, a Nd :YAG- laser, a C0₂, a fibre laser or fibre disk laser.

In an embodiment of the invention the speed of the laser beam in relation to the rotating substrate is between 0.5 to 3 m/min. _ _

In an embodiment of the invention the melt pool of a track overlaps the adjacent melt pool, preferably wherein the overlap between adjacent tracks is between 10 to 60%, more preferably between 30 and 50%. The inventors found that the risk of crack formation increased with increasing degrees of overlap. Overlap of 70% or more appeared to result in unfavourable results, whereas overlap of less than 10% results in a coating layer which is does not have a consistent thickness. When the laser forms a melt pool and the substrate moves underneath the laser, a line of molten and subsequently solidified metal remains behind. This line is a track. When a second track is made, the degree of overlap between the first and the second track can is a variable. When there is no overlap (0%) then the second track is immediately adjacent to the first. 50% overlap means that 50% of the first track is remelted and 50% of new metal is melted. The tracks on the rotation symmetric iron or steel substrate may be made as parallel circles, or as a spiral wherein the laser moves in a direction perpendicular to the tracks thereby covering the surface of the cylindrical surface. The sideways speed in relation to the speed of the cladding determines the degree of overlap between subsequent tracks.

In an embodiment of the invention particles, particularly hard particles, are embedded in the top layer. These layers provide the top layer with added functionality, the nature of which depends on the nature of the particles. By adding hard particles to the top layer such as carbides (WC, Cr₃C₂ or TiC), the wear resistance can be further improved. These particles are mixed with the metal powder prior to the addition of the powder to the melt pool.

Although the method can be used to provide each iron or steel substrate with a wear resistant, corrosion resistant and fatigue resistant coating, the method according to the invention has proven particularly useful in cladding rolling mill roll surfaces of an iron or steel substrate as these surfaces are subjected to particularly harsh tribological conditions particularly in case of hot rolling, but also in case of cold or warm rolling .

The work roll for a rolling mill thus obtained has a wear resistance outer layer, which will increase the life of the roll, allowing the rolling process to - - be performed for longer periods of time without the need to stop and replace the roll, therefore leading to increased productivity. Also the invention enables to provide relatively cheap roll grades (such as HiCr- steel, HiCr-iron, AIC, ACS or SGA) with a coating to outperform the more expensive (semi-)HSS grades.

In an embodiment of the invention the rolling mill roll is an Alloy Cast Steel roll (ACS). These typically have a composition of C 0.50 - 1.50%, Si 0.30 - 0.60 %, Mn 0.60 - 0.90 %, Cr 0.75 - 1.2 %, Ni < 1.00 %, Mo < 0.40, the remainder iron and unavoidable impurities.

In an embodiment the rolling mill roll is an alloy cast steel with a carbon content of between 0.5 to 1.8%, preferably of 0.5 to 1.5%, preferably of at most 1.2%, preferably of between 0.5 to 0.8%, more preferably of 0.7 to 0.8% C. These substrates become increasingly prone to cracking as a result of welding, and have proven to be excellently treatable by the process according to the invention.

The laser deposited layer can be machined or removed using ceramic tools, so that the base roll material can be reused for subsequent coatings, thereby substantially increasing the lifetime of the base roll material . Also the surface of the deposited layer can be machined to give it the desired texture or roughness or to give it the required dimensions.

According to a second aspect of the invention a rotation symmetric iron or steel rolling mill rollsubstrate provided with the metal coating according to the invention is provided.

The invention is now further explained by means of the following non- limitative examples.

The investigated substrates were traditional Spheroidal Graphite Acicular (SGA) rolls (also known as Bainitic Nodular Cast Iron Rolls) and Alloy Cast Steel rolls (ACS). The latter typically have a composition of C 0.50 - 1.50%, Si 0.30 - 0.60 %, Mn 0.60 - 0.90 %, Cr 0.75 - 1.15 %, Ni < 1.00 %, Mo 0.20 - 0.40. The ACS of the example had a carbon content of 0.75%

These substrates were provided clad with a coating layer consisting of two metal layers. The nickel alloy layer was deposited on the substrate after _ _ cleaning the substrate by machining and the roll was placed on a CNC machine allowing the roll to be rotated around its axis. The powder to be applied to the substrate was loaded in a hopper and fed coaxially to the laser beam into the melt pool through a nozzle in the laser head .

The feeding rate of the powder was varied between 26 and 35 g/min and the power of the C0₂ laser was 3 kW, with a 5 mm focus spot and 12 mm stand-off distance. The nozzle was inclined with respect to the target surface and the angle between the laser beam and the perpendicular of the surface is 30°. The travel speed of the laser head relative to the rotating substrate is 0.9 m/min, with an overlap of 40% between adjacent tracks.

The composition of the nickel alloy layer that was applied as the intermediate layer between the substrate and the top layer was consistent with the composition of Hastelloy^® C276 or C276 MLC, a trademark of Haynes International, Inc.

The second metal layer that was applied as the top layer on the intermediate layer was consistent with the composition of Stellite^® 6, a trademark of Deloro Stellite Holdings Corporation.

A micrograph of the coating system is shown in Figure 1 where both metal layers have a thickness of about 1 mm and the base material is cast steel.

Wear tests at elevated temperatures (figure 2) confirmed the increased wear resistance of the coating system against the alloy cast steel base material. The ratio of wear as volume loss between various coating systems is given as a function of the temperature (in °C) of the surfaces. These values were determined by performing tests using a rotating disc provided with the desired coating at a disc rotation speed of 30s^"1 and for 1000 revolutions. An M2 tool steel counterpart was pressed against the surface with a 30 N force and the temperature of the surface was varied from room temperature to 600°C. It is clearly visible that the coating system according to the invention (denominated as "Co base" in figure 2) has a 70 times lower weight loss at 20°C than ACS (top curve, open circles) and about 3 times lower weight loss at 20°C than SGA (diamonds). The system also outperforms high chromium rolls (HiCr, - - closed circles). Thermal fatigue tests were carried out on a disc-on-disc tribometer which showed that the top coating performs at least as well as current graphitic steel base roll materials.

Figure 3 shows that the hardness of the top layer is almost twice the hardness of the base material (Figure 3). The resulting laser clad alloy cast steel retains the ductility of the core whilst offering a wear and corrosion resistant surface layer.

Experiments with additions of hard particles to the top layer revealed that up to 10% in volume of hard particles, particularly WC-particles, could be added without the formation of cracks in the top layer.

The research leading to these results has received funding from the European Union's Research Fund for Coal and Steel (RFCS) research programme under grant agreement RFSR-CT-2010-00009.

Claims

Method for cladding a rotation symmetric iron or steel rolling mill roll substrate with a wear and corrosion resistant metal coating said coating comprising at least a first metal coating layer as an intermediate layer and a crack-free second metal coating layer as a top layer, the method comprising the steps of:

a. cleaning the substrate by machining the surface of the substrate;

b. rotating the substrate around its axis of rotational symmetry; c. forming a melt pool on the surface of the rotating substrate by means of a laser beam and applying the first coating layer by feeding a first powder material into the melt pool, wherein the first powder material is fed into the melt pool coaxially with the laser beam;

d . forming a melt pool on the surface of the substrate provided with the first coating layer by means of the laser beam and applying the second coating layer by feeding a second powder material into the melt pool, wherein the second powder material is fed into the melt pool coaxially with the laser beam;

wherein the top layer is a cobalt alloy, wherein the cobalt alloy comprises between 27 and 32% Cr, 4 to 6% W, 0.9 to 1.4% C, 1 to 2 % Fe, 1 to 3% Ni, up to 1% Mn, up to 1% Mo, up to 0.1% B, up to 0.01% O, up to 0.02% S, and balance Co and inevitable impurities; wherein the intermediate layer is a nickel alloy, wherein the nickel alloy comprises between 15 to 17% Mo, 14.5 to 16.5% Cr, 4 to 7% Fe, 3-4.5% W, up to 2% Mn, up to 0.1% C, up to 0.05% P, up to 0.02% S, up to 0.5% V and balance Ni and inevitable impurities.

Method according to claim 1 wherein the nickel alloy comprises between 8 to 10% Mo, 20 to 24% Cr, 4 to 6% Fe, between 3 to 4.5% of (Nb+Ta), up to 1% Co, up to 1% Mn, up to 0.1% C, up to

0.05% P, up to 0.02% S, up to 0.5% V, and balance Ni and inevitable impurities.

Method according to claim 1 or 2 wherein the nickel content of the nickel alloy is between 55 and 60%.

Method according to any one of claims 1 to 3 wherein the laser is a coaxial laser, particularly a high power diode laser, a Nd :YAG-laser, a C0₂, a fibre laser or fibre disk laser.

Method according to any one of claims 1 to 4 wherein the speed of the laser beam in relation to the rotating substrate is between 0.5 to 3 m/min.

Method according to any one of claims 1 to 5 wherein the melt pool of a track overlaps the adjacent melt pool, preferably wherein the overlap between adjacent tracks is between 10 to 60%.

Method according to any one of claims 1 to 6 wherein hard particles are embedded in the top layer.

Method according to any one of claims 1 to 7 wherein up to 10% in volume of hard particles are added to the top layer.

Method according to any one of claims 1 to 8 wherein the feeding rate of the powder is between 26 and 35 g/min.

Method according to any one of claims 1 to 9 wherein the nozzle is inclined with respect to the target surface, and wherein preferably the angle between the laser beam and the perpendicular of the surface is between 10 and 45°, preferably between 20 and 35, more preferably 30°.

Method according to any one of claims 1 to 10 wherein the rolling mill roll is an alloy cast steel with a carbon content of between 0.5 to 1.8%, preferably of between 0.5 to 1.5, more preferably of between 0.5 to 0.8%.

Rolling mill roll substrate provided with a wear and corrosion resistant metal coating said coating comprising at least a first metal coating layer as an intermediate layer and a crack-free second metal coating layer as a top layer, the method comprising the steps of: a. cleaning the substrate by machining the surface of the substrate;

13. Rolling mill roll substrate provided with a wear and corrosion resistant metal coating said coating comprising at least a first metal coating layer as an intermediate layer and a crack-free second metal coating layer as a top layer according to claim 12 wherein the nickel allow comprises between 8 to 10% Mo, 20 to 24% Cr, 4 to 6% Fe, between 3 to 4.5% of (Nb+Ta), up to 1% Co, up to 1% Mn, up to 0.1% C, up to 0.05% P, up to 0.02% S, up to 0.5% V, and balance Ni and inevitable impurities.

14. Rolling mill roll substrate provided with a wear and corrosion resistant metal coating said coating comprising at least a first metal coating layer as an intermediate layer and a crack-free second metal coating layer as a top layer according to claim 12 or 13 wherein hard particles are embedded in the top layer, preferably wherein up to 10% in volume of hard particles are added to the top layer

Rolling mill roll substrate provided with a wear and corrosion resistant metal coating said coating comprising at least a first metal coating layer as an intermediate layer and a crack-free second metal coating layer as a top layer according to claim 12 to 15 wherein the rolling mill roll is an alloy cast steel with a carbon content of between 0.5 to 1.8, preferably of between 0.5 to 1.5% and more preferably of between 0.5 to 0.8%.