WO2014070006A1

WO2014070006A1 - Enhanced hardfacing alloy and a method for the deposition of such an alloy

Info

Publication number: WO2014070006A1
Application number: PCT/NL2013/050773
Authority: WO
Inventors: Ismail HEMMATI; Jeff Theo Marie DE HOSSON; Václav OCELIK
Original assignee: STICHTING MATERIALS INNOVATION INSTITUTE (M2I)
Current assignee: STICHTING MATERIALS INNOVATION INSTITUTE (M2I)
Priority date: 2012-10-30
Filing date: 2013-10-30
Publication date: 2014-05-08
Anticipated expiration: 2015-04-30
Also published as: NL2009730C2

Abstract

The invention relates to a nickel-basedalloy, comprising a nickel matrix, and alloying elements chromium, boron and silicon, whereby the Si:B ratio is between 1:2 and 3:1, the resulting Ni-Cr-B-Si alloy comprising 0.5-5.0 wt.% Nb, whereby the Nb:B ratio is between 1:2 and 2:1. The invention further relates to a method for manufacturing at least a first hardfacing layer on a substrate, comprising heating a nickel based alloy to above the solidus temperature to obtain an at least partially molten alloy; depositing the at least partially molten nickel based alloy on the substrate, the alloy comprising nickel, chromium, boron, silicon, and 0.5-5.0 wt.% Nb; and cooling the deposited alloy to at least below the solidus temperature for obtaining a layer with a microstructure comprising boride particles formed with at least one of chromium and niobium, a nickel in solid solution phase nucleated on and enclosing the boride particles and nickel-boron- silicon binary and ternary eutectic phases between the nickel solid solution dendrites. Furthermore, the invention relates to a substrate having a layer deposited thereon, the layer comprising the hardfacing nickel based alloy.In addition, the invention relates to a three-dimensional structure comprising the hardfacing nickel based alloy.

Description

Enhanced hardfacing alloy and a method for the deposition of such an alloy

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a Ni-base alloy for depositing a hardfacing layer, which preferably enhances the conventional Ni-Cr-B-Si alloys, in particular an outside coating for corrosion and wear protection. In addition, the invention relates to a method for manufacturing at least one such hardfacing layer and a 3 -dimensional structure. Description of the Related Art

The Ni-Cr-B-Si alloys are boride-containing nickel base hardfacing alloys with excellent wear and corrosion resistance in chemically-aggressive environments and high temperature working conditions. In some cases, additions of copper, molybdenum or zinc are made to improve the corrosion resistance.

These alloys have been originally developed for deposition of protective layers by thermal spraying techniques. Examples of the use of Ni-Cr-B-Si alloy layers are given in J. M. Miguel et. al, Tribology International (36), 2003 and E. Fernandez et. al., Wear (259), 2005. These documents describe examples for the application of Ni-Cr-B-Si alloys in chemical industry, petrochemical industry, glass mould industry and for valves, hot working punches, fan blades, mud purging elements in cement factories and piston rings or rollers in steel making.

Document GB 860733 describes nickel-silicon-boron alloys used for brazing steel parts. An example of such an alloy is described to contain 0.3 wt% C, 2.5 wt% Co, 2.7 wt% B, 3.2 wt% Si, 0.1 wt% Ti, 1.4 wt% Nb, 0.8 wt% Mn, 1.0 wt% Fe and the balance being nickel.

Document JP01092334 describes a nonmagnetic corrosion-resistant and wear- resistant alloy comprising Cr, B, Si, Mo, Nb, Fe, Mn and Ni. An example of such an alloy is described to contain 9 wt% Cr, 10 wt% Co, 3 wt% B, 2.9 wt% Si, 1.0 wt% Mn, 1.5 wt% Mo, 1.5 wt% Nb and the balance being nickel.

Use of a Ni-Cr-Fe-B-Si alloy for applying a layer on a substrate in order to make surface of the substrate wear and corrosion resistant is described in WO84/03306. In the mentioned publication the Ni-Cr-Fe-B-Si alloy is thermal-sprayed on a substrate. The alloy contains 18.0-35.0 wt.% Cr, 0.1-25.0 wt.% Fe, 0.5-4.5 wt.% B, 0.5-5.5 wt.% Si, 0.1-2.0 wt.% C, 0-15.0 wt.% Mo, 0-2.0 wt.% Nb, and the remainder Ni.

The laser deposition process is increasingly employed to produce dense and

metallurgically-bonded deposits with superior functional properties. A drawback for a reliable laser deposition of these Ni-Cr-B-Si alloys is their tendency to produce various microstructures from the same chemical composition. This phenomenon occurs because the composition of Ni-Cr-B-Si hardfacing alloys is very sensitive to the solidification conditions during the deposition process and fast cooling rates associated with laser processing. As a consequence, different kinds of microstructures are formed along the length and depth of a deposit with the same chemical composition. This will cause inhomogeneities in hardness, and therefore have its effect on the wear resistance of the layer.

High cracking tendency is another drawback of the Ni-Cr-B-Si alloys. This tendency becomes more pronounced during laser processing because of the high cooling rates associated with laser processing techniques. There have been a number of attempts to eliminate the cracking problem of Ni-Cr-B-Si coatings during laser deposition by either reducing the cooling rate of the deposits by preheating and postheating or increasing the toughness of the deposits by compositional modification. The latter case is more attractive from the practical and economic points of view, especially for cladding of large pieces. The purpose of compositional modification is to refine the microstructure which leads to a higher resistance against cracking under rapid cooling rates. The compositional modification should preferably be done in such a way that the hardness of the original composition is not reduced.

It would therefore be desirable to provide a hardfacing Ni-Cr-B-Si alloy that alleviated the perceived inconveniences of the prior art.

BRIEF SUMMARY OF THE INVENTION

According to the invention in one aspect, there is provided a nickel-based alloy, comprising a nickel matrix, and alloying elements chromium, boron and silicon, whereby the resulting Ni-Cr-B-Si alloy comprises 0.5-5.0 wt.% Nb. In another aspect, the invention pertains to a hardfacing nickel based alloy, comprising nickel, chromium, boron and silicon, forming a Ni-Cr-B-Si alloy, whereby the Ni-Cr-B-Si alloy comprises Nb in a composition ratio of Nb:B in the range of 1 :1 to 2:1.

The high cracking tendency is eliminated by increasing the toughness of the deposits through compositional modification. Reducing the cracking tendency through compositional modification is desirable and convenient from practical and economic points of view, especially for cladding of large pieces. This compositional modification yields a refined microstructure, which in turn leads to a higher resistance against cracking under rapid cooling rates, but retains the hardness of the alloy as the phase constitution of the alloy is not considerably changed.

The improved microstructure of the modified alloy composition can

advantageously be employed in laser deposition of these alloys because the variations in cracking tendency, microstructure and hardness observed in the art are thus reduced or even avoided.

Different contents of Nb up to 15 wt.% were added to common Ni-Cr-B-Si alloys. Measuring the hardness values and observation of the microstructures revealed that addition of less than 5.0 wt.% Nb was enough for microstructural refinement and homogenization while preserving the hardness of the original composition. The optimum amount of Nb addition depends on the B content of the original composition. In most cases, a Nb content equal to the boron content, i.e. the Nb:B ratio is 1 : 1, is sufficient to refine and homogenize the microstructure. Another relevant ratio in this system is the Si:B ratio. Increasing this ratio will increase the toughness of the deposits at the expense of some hardness loss. The Si:B ratio can be between 1 :1 and 3:1 in this alloy system.

To optimize the effect of added Nb, it is preferred to maintain the composition ratio of Nb:B present in the alloy in the range of 1 : 1 and 2: 1. Nb has a high affinity for B and C. So during the solidification, Nb combines with B and/or C to produce Nb

borocarbides, borides and/or carbides. Adding too much Nb will produce excessive amounts of Nb borocarbides, borides and/or carbides, which is not desirable or necessary. At too high Nb-contents large Nb-rich particles will form, which will contribute to crack propagation. On the other hand, adding insufficient Nb may result in too low Nb borocarbide, boride and/or carbide levels, and therefore less refinement. The addition of Nb in small quantities (0.5 - 5.0 wt.%) to the conventional Ni-Cr-B-Si alloys will generate a homogenized microstructure resulting in homogenous properties of the deposits. The alloy preferably comprises at least 1.0 wt.% Nb and at most 4.0 wt.% Nb. More preferably at least 2.0 wt.% or at least 2.1 wt.% or at least 2.2 wt.% Nb, most preferably at least 2.5 or 3.0 wt.% Nb. An optimum was found around about 3.5 wt.% Nb. According to a preferred embodiment, the content of Nb is close to the boron content, i.e. an Nb:B ratio of 1.5 :1 - 1 :1.5, preferably about 1 : 1.

Preferably, the hardfacing alloy comprises 10.0-30.0 wt.% chromium, 2.0-6.0 wt.% silicon, 2.0-4.0 wt.% boron, 0-5.0 wt.% iron, 0-5.0 wt.% molybdenum, 0-3.0 wt.% copper, 0-1.0 wt.% carbon, 0-0.5 wt.% of one or more of other elements, such as P, Mn, Ti, V, Zr, Co including the usual impurities, and the balance being nickel, forming a Ni- Cr-B-Si alloy, whereby the Ni-Cr-B-Si alloy comprises 0.5-5.0 wt.% niobium. The amount of one or more of other elements is preferably 0-0.5 wt.% or at least 0.1 wt.% or 0.01 wt.%, more preferably 0-0.1 wt.%, most preferably at most 0.01 wt.%.

Preferably, the alloy comprises at least 2.0 wt.% Fe and at most 4.5 wt.% Fe. Most preferably, the alloy comprises 3.5 - 4.5 wt.% Fe. In addition, the alloy preferably comprises at least 1.5 wt.% Mo and at most 4.0 wt.% Mo. Most preferably, the alloy comprises 1.5 - 2.5 wt.% Mo. Furthermore, the alloy preferably comprises at least 1.5 wt.% Cu and at most 3.0 wt.% Cu. Most preferably, the alloy comprises 1.5 - 2.0 wt.% Cu. If present, the carbon content of the alloy is at least 0.001 wt.% C, preferably at least 0.01 wt.% C, more preferably at least 0.1 wt.% C, and at most 1.0 wt.% C. The carbon content will usually be low, as the presence of this element is not a requirement for the niobium to refine the microstructure. However, as it is difficult to remove all carbon from the Ni-Cr-B-Si alloy, low amounts will in practice be present. The present carbon will form carbides with Nb and Cr.

The alloy preferably comprises 3.0 - 5.0 wt.% Si, 10.0 - 20.0 wt.% Cr and 2.5 - 4.0 wt.% B, including the above mentioned elements, the balance being nickel. The quantities of Cr, B, Si and C can vary depending on the desired properties. Increasing the contents of Cr, B and C at the same time will produce more boride and carbide particles, so the hardness will go up but at the expense of some toughness loss. In the alloy system of interest, Ni will form several binary and ternary eutectics with B and Si. The ratio between Si and B will determine the type of the eutectic products. Increasing the Si:B ratio will reduce the hardness and increase the toughness. For alloys with an Si:B ratio of 3:1 or less, binary Ni-B eutectic phases will dominate the microstructure. For alloys with an Si:B ratio of more than 3: 1, the binary Ni-Si eutectic phases will be the dominant ones. All the remaining Ni, B and Si will end up in the ternary Ni-B-Si eutectic phases. Fe is added to reduce the cost of the alloy but its content is limited to less than 5.0 wt.% Fe. Cu and Mo are added to improve the corrosion resistance.

The base Ni-Cr-B-Si alloys are usually produced from constituents with a purity level of about 99.9% and by inert gas atomization. Therefore, the impurity levels are relatively low, between 0-0.5 wt.%, preferably 0.1 wt.% or less. These ranges also incorporate conventional impurities introduced before, during or after manufacturing of the alloy.

Preferably, the alloy comprises boride and carbide particles formed with at least one of chromium or niobium. During solidification, three groups of phases are generated, being boride and carbide particles, the nickel solid solution and the eutectic phases. The microstructure of the alloy is refined by the formation of smaller strengthening particles upon solidification from the melt, such as the boride and carbide particles, than in commonly used Ni-Cr-B-Si alloys. During the solidification, Nb-rich precipitates form first and act as the nucleation sites for the Cr borides and/or carbides which form at lower temperatures during solidification. This results in a decrease of the size of the Cr boride and/or carbide particles and thus a refined microstructure compared to the conventional Ni-Cr-B-Si alloys.

According to an aspect of the invention, the alloy has a microstructure comprising boride particles formed with one of chromium and niobium. In addition, the

microstructure can comprise a nickel in solid solution phase nucleated on and enclosing the boride particles. Furthermore, the microstructure can comprise nickel-boron-silicon binary and ternary eutectic phases between the dendrites of Ni solid solution. The fractions of each phase are usually 40-50 vol.% for Ni in solid solution, 10-20 vol.% for Cr borides, 0-5 vol. % Nb-rich particles, in most cases less than 2 vol.%. and the balance being the eutectic phases. In order to observe the phases present in the microstructure, several methods may be used, such as optical microscopy, Scanning Electron Microscopy (SEM) or

Transmission Electron Microscopy (TEM). The constituent phases can be identified using X-ray Diffraction (XRD), Energy Dispersive Spectroscopy (EDS), Electron Backscatter Diffraction (EBSD), diffraction in Transmission Electron Microscope (TEM) or their combinations.

The microstructure is obtained after solidifying the alloy from a melt and cooling to at least 1273 Kelvin with a cooling rate between about 10² to 10⁴ Kelvin per second (K/s). Boride carbide and/or borocarbide particles are the first phases to form during solidification. This is followed by solidification of the Ni solid solution around the boride, carbide and/or borocarbide particles. Finally, different binary and ternary eutectic phases form at the latest stage of the solidification. All three phases can be found in the equilibrium phase diagrams. The high cooling rates will introduce kinematic effects and will act through the competitive nucleation and growth mechanisms.

The alloy can comprise nickel in solid solution, which comprises chromium, and, if present in the alloy, molybdenum, iron and silicon. The nickel solid solution phase comprises part of Cr, Mo, Fe and Si, as far as these elements are not consumed in the boride, carbide and/or borocarbide particles that formed at an earlier stage. The nickel solid solution phase appears in dendrites when formed from the liquid melt. The Ni dendrites surround the boride, carbide and/or borocarbide particles and the eutectic phases will finally form a continuous network between them.

Cu and Mo do not significantly influence the three phase formation steps from liquid to solid including carbide/boride/borocarbide precipitation, solidification of nickel solid solution and formation of binary and/or ternary eutectic phases. Mo will either be included in the boride, carbide and/or borocarbide particles or in the Ni solid solution. Cu is usually dissolved in the Ni solid solution.

According to another aspect, the alloy has a hardness between 7.3* 10³ and 8.9* 10³ MPa (-750-900 hardness Vickers), preferably between 8.0*10³ and 8.6* 10³ MPa (-825- 875 HV), most preferably about 8.3*10³ MPa (-850 HV). The hardness of Ni-Cr-B-Si- Nb laser-deposited layers is not compromised by the Nb addition. In addition, the hardness is more homogeneous throughout the layer, i.e. has relatively small variations along the measured surface. The hardness of the original composition without Nb lies between 600-900 HV with relatively large variations along the measured surface.

Preserving the high hardness of the original composition is important because these alloys are used in hardfacing applications in which the hardness of the deposit plays a significant role. The alloy can be manufactured in the form of powder or wire and consumed to make deposits using various deposition methods.

The invention also relates to a method for manufacturing at least a first hardfacing layer on a substrate, comprising:

- heating a nickel based alloy to above the solidus temperature to obtain an at least partially molten alloy;

- depositing the at least partially molten nickel based alloy on the substrate, the alloy comprising nickel, chromium, boron, silicon, and 0.5-5.0 wt.% Nb;

- cooling the deposited alloy to at least below the solidus temperature for obtaining a layer with a microstructure comprising boride particles formed with at least one of chromium and niobium, a nickel in solid solution phase nucleated on and enclosing the boride particles and nickel-boron-silicon binary and ternary eutectic phases between the nickel solid solution dendrites. The nickel solid solution phase comprises part of the alloying elements content of the composition.

In order to obtain a hardfacing layer with a homogenized and refined microstructure on a substrate, resulting in a homogeneous hardness across the hardfacing layer, a Ni-Cr- B-Si alloy comprising 0.5-5.0 wt.% Nb is deposited on the substrate, while before or during deposition, the alloy comprising Nb is heated to above the solidus temperature or higher to obtain at least a semi-solid or semi-molten alloy with the molten alloy and the crystalline alloy co-existing. Upon deposition, the alloy then forms a hardfacing layer. The alloy can either be semi-solid, or be fully melted, upon heating to either at least the solidus or liquidus temperature, such that it spreads over the substrate. Subsequently to the deposition, the layer is cooled to at least below the solidus temperature to obtain a hardfacing layer.

According to an embodiment, the method comprises: - heating the alloy to at least above its liquidus temperature to obtain a fully molten alloy; and

- solidifying the molten alloy after depositing on the substrate, thereby obtaining the first layer with a microstructure comprising boride particles formed with at least one of chromium and niobium, a nickel in solid solution phase nucleated on and enclosing the boride particles and nickel-boron-silicon binary and ternary eutectic phases between the nickel solid solution dendrites.

- cooling the layer to below at least the eutectic temperature.

In order to be able to obtain a microstructure comprising boride particles formed with at least one of chromium and niobium and nickel-boron-silicon binary and ternary eutectic phases, the alloy has to be heated to at least above the liquidus temperature. When the deposition of the alloy is performed by cladding, the alloy is deposited on the substrate as a melt, which is subsequently cooled with cooling rates between 10² and 10⁴ Kelvin per second.

According to another embodiment of the method of the invention, depositing the nickel based alloy on the substrate comprises:

- injecting the Ni-Cr-B-Si alloy without any Nb additions in powder form into an irradiating focused beam to obtain a melt;

- injecting a niobium-containing powder into the irradiating focused beam, wherein the Nb-containing powder is provided in an amount sufficient to render the Ni-Cr-B-Si alloy comprising 0.5-5.0 wt.% niobium, according to the invention; and

- depositing the Ni-Cr-B-Si alloy melt comprising 0.5-5.0 wt.% niobium onto the substrate forming a first track of deposited alloy. The alloy according to the invention is thus in situ prepared, such that the nucleants, i.e. boride and/or borocarbide particles, can be very small.

Depositing the alloy in the molten state on the substrate in the shape of a track can be done in various ways. One such deposition method is to inject a Ni-Cr-B-Si alloy without Nb in powder form into an irradiating focused beam, such as a laser beam for laser cladding, a plasma for plasma welding or electron beam for electron beam melting, in order to obtain a molten Ni-Cr-B-Si alloy (without Nb). A niobium-containing powder is injected into the same irradiating focused beam simultaneously with powdered Ni-Cr- B-Si alloy, in order to obtain a Nb-containing melt. The amount of niobium powder is sufficient to form an Ni-Cr-B-Si alloy comprising 0.5-5.0 wt.% Nb from the separate Ni- Cr-B-Si melt and the Nb melt. Alternatively, the Ni-based alloy comprising Nb can be pre-produced using processes such as casting or powder atomization. In such a case, the original Ni-Cr-B-Si composition is prealloyed with Nb and can be used directly as a powder or a wire without any further additions.

This Ni-based alloy comprising Nb is deposited onto the substrate forming a first deposited track of material, such as a cladding or welding track. The Nb-containing powder can be pure Nb powder or a powder comprising Nb and other components compatible with the alloy chemistry. In the case of thermal spraying, the deposit will be heat treated after deposition to fuse the deposit and form a dense hardfacing layer.

According to yet another embodiment of the method, depositing the nickel based alloy on the substrate comprises:

- providing the Ni-Cr-B-Si alloy comprising 0.5-5.0 wt.% Nb, in wire form;

- feeding the wire into an irradiating focused beam, such that the Ni-Cr-B-Si alloy comprising 0.5-5.0 wt.% Nb melts;

- depositing the melt onto the substrate forming a first track of the deposited alloy. Another deposition method is to feed a Ni-Cr-B-Si alloy comprising 0.5-5.0 wt.%

Nb in wire form into an irradiating focused beam, such as a laser beam used for laser cladding or a plasma used for plasma welding. The alloy will melt under the influence of the focused beam or plasma. This melt is then deposited onto the substrate forming a first track of deposited material, such as a welding or cladding track.

According to an aspect of the method, the Ni-Cr-B-Si alloy comprises 10.0-30.0 wt.% chromium, 2.0-6.0 wt.% silicon, 2.0-4.0 wt.% boron, 0-5.0 wt.% iron, 0-5.0 wt.% molybdenum, 0-3.0 wt.% copper, 0-1.0 wt.% carbon, and the balance being nickel.

Depending on the method for depositing the alloy onto a substrate, as described above, the alloy can comprise 0.5-5.0 wt.% Nb as well. The Nb can also be added during the deposition of the layer, for example as a powder in the melt of Ni-Cr-B-Si.

According to an embodiment of the method, the melt has a temperature of at least 1273 K. In order to obtain at least a partial melt, i.e. at least a mixture of liquid and solid alloy (semi-solid), the alloy can be heated to a temperature of at least 1273 K, depending on the composition. Preferably, the melt is cooled with a cooling rate of 10²-l 0⁴ Kelvin per second (K/s).

According to a further embodiment of the method, the melt has a liquidus temperature of at least 1773 K. Usually, the solidus of a Ni-Cr-B-Si alloy lies above 1273 K, such that the melt has a temperature well above 1273 K. The eutectic temperature of these alloys usually lies around 1373 K. In addition, when the alloy is deposited with cladding and/or welding techniques, the substrate which is usually steel, can be melted as well in order to obtain a good bonding. Using laser or plasma deposition, the focused beam used for these processes will form a melt pool on the substrate. Steels, irrespective of their composition, usually melt around 1773 K. So, the Ni-Cr-B-Si melt, with or without 0.5-5.0 wt.% Nb, will have a temperature at least equal to the liquidus of the nickel based alloy.

According to an aspect of the method, the microstructure comprises at least one of niobium boride or niobium carbide particles. Upon solidification of the melt, the first phases to crystallize are carbide and boride particles, in particular niobium borocarbides. The Nb borocarbides will act as nucleation sites for Cr boride and carbide particles. This will refine the Cr boride and carbide particles, i.e. make them smaller in size, in relation to carbide and boride particles comprising chromium, in commonly used Ni-Cr-B-Si alloys without the addition of Nb.

In addition, the microstructure comprises a nickel solid solution phase comprising chromium, and, if present in the alloy, molybdenum, iron, silicon and carbon. After the formation of the boride and/or carbide particles, the liquid alloy crystallizes to form a nickel solid solution phase. A solid solution is a solid-state solution of one or more solutes, i.e. the alloying elements, in a solvent, i.e. nickel. Such a mixture is considered a solution when the crystal structure of nickel remains unchanged by addition of the alloying elements, and when the mixture remains in a single homogeneous phase. In this phase, the alloying elements, such as chromium, molybdenum, iron , silicon and carbon, are either incorporated into the crystal lattice of nickel substitutionally, i.e. by replacing a nickel atom in the lattice, or interstitially, by fitting into the spaces between nickel atoms. Both of these types of solid solution affect the properties of the alloy by distorting the crystal lattice and modifying the physical and electrical homogeneity of the pure nickel. Upon cooling below the eutectic temperature, the eutectic phases will form between the nickel solid solution dendrites.

According to an embodiment, the method comprises depositing a second track of material on the substrate, whereby the second track of material at least partly overlaps the first track of material, thereby partially melting the first track of material and forming a first layer on the substrate upon solidification of the second track and the partially melted first track.

In order to form a layer on the substrate, a number of tracks of material, i.e.

cladding or welding tracks, are deposited on the substrate, whereby the tracks at least partly overlap each other. By partly overlapping of the tracks, a continuous layer across the surface of the substrate can be obtained. In case the tracks overlap completely, i.e. the tracks are layered on top of each other, a three-dimensional structure that at least extends from the surface of the substrate in an outward direction, can be built by depositing track upon track. In addition, due to high temperature of the second track of material, the first track will be partially remelted. The resulting layer or structure will have a homogeneous and refined microstructure with a relatively constant hardness across the surface of the layer or structure.

According to a further embodiment, the method comprises depositing a second layer on the first layer, thereby forming a three-dimensional structure on the substrate. In order to build the three-dimensional structure, several layers composed of one or more tracks can be overlaid. In such a case, the three-dimensional structure extends along the surface of the substrate as well as from the surface of the substrate in an outward direction. In addition, the method may comprise removing the substrate, leaving the layer or the three-dimensional structure as a result. The resulting layer or three-dimensional structure can then be used as an individual product made of the hardfacing layer. Good examples are bushes used in heavy wear applications

Preferably, the method comprises heating the substrate to at least 500 K before depositing the layer on the substrate. Heating the substrate before the deposition of the layer prevents the layer from cracking during cooling. The preheating temperature depends on several factors including the type of the substrate, volume of the deposit and the deposition rate and can be as high as 973 K. The substrate may be kept at the preheating temperature during the deposition process.

Furthermore, the invention relates to a substrate having a layer deposited thereon, whereby the layer comprises a nickel alloy comprising 10.0-30.0 wt.% chromium, 2.0-6.0 wt.% silicon, 2.0-4.0 wt.% boron, 0-5.0 wt.% iron, 0-5.0 wt.% molybdenum, 0-3.0 wt.% copper, 0-1.0 wt.% carbon, 0-0.5 wt.% of one or more of other elements including the usual impurities, and the balance being nickel; and 0.5-5.0 wt.% niobium, whereby the alloy has a microstructure comprising boride particles formed with at least one of chromium and niobium, a nickel in solid solution phase nucleated on and enclosing the boride particles and nickel-boron-silicon binary and ternary eutectic phases between nickel solid solution dendrites. In addition, the alloy comprises 1.5-5.0 wt.% niobium, preferably 3.0 wt.% niobium. According to an aspect, the layer is deposited on the substrate by laser cladding, such that cladding tracks are identifiable.

Furthermore, the invention relates to a three-dimensional structure manufactured according to the method described above, comprising a hardfacing nickel based alloy, the alloy comprising nickel, chromium, boron and silicon and 0.5-5.0 wt.% Nb.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure la shows a substrate with deposited thereon a clad layer comprising a Ni-Cr- B-Si alloy without Nb.

Figure lb shows a cross section along II-II of the substrate and the layer of fig. la, showing the cladding tracks.

Figure 2 shows various microstructures produced in the same Ni-Cr-B-Si alloy without Nb forming the tracks of fig. lb.

Figure 3 shows the microstructural changes along the tracks of fig. lb.

Figure 4 shows the microstructure of a track comprising a Ni-Cr-B-Si alloy comprising Nb.

Figure 5 shows the graph of hardness values for coatings deposited from alloys modified with different amounts of Nb addition.

Figure 6 shows the differences between the scale of the constituent phases in the microstructure of the Ni-Cr-B-Si alloy with and without Nb. Figure 7 shows a phase map of a Ni-Cr-B-Si alloy microstructure after solidifying from the melt.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Figure 1 a shows a substrate 1 with thereon deposited a clad layer 2 comprising a Ni-Cr- B-Si alloy without Nb. Cladding is the covering of one material with another. It can be performed in a number of ways, including welding, for example Plasma Transferred Arc welding (PTA), and laser cladding.

During laser cladding a powdered or wired material is deposited by melting the powder or wire material (feedstock) and consolidating by use of a laser in order to coat part of the substrate or fabricate a three-dimensional shape. The latter method is also called additive manufacturing technology. Laser cladding is often used to improve mechanical properties or increase corrosion resistance, repair worn out parts, and fabricate metal matrix composites.

The powder feedstock used in laser cladding is injected into the system by either coaxial or lateral nozzles. The laser beam melts the metal feedstock along with a part of the substrate and forms a melt pool. The melt pool is formed on a substrate. Moving the substrate and the laser relative to each other allows the melt pool to solidify and thus produce a track of solid metal. This process is repeated to create multiple solidified tracks on the substrate that at least partly overlap, such that a layer is created on the substrate.

Figure la shows a substrate 1 with laser clad thereon a layer 2 comprising five tracks 3-7 of a commonly used Ni-Cr-B-Si alloy. The composition of this alloy comprises Ni-3B-4Si-0.7C-13Cr-4Fe-l .9Mo-l.8Cu; thereby excluding Nb. Figure lb shows a cross section along Π-ΙΙ of such a coated substrate 1, showing the tracks 3-7. Track 3 is the first track that has been clad on the substrate. Subsequently, track 4 is clad on the substrate, partially overlapping track 3. Tracks 5-7 are clad subsequently in a similar manner.

Figure 2 shows various microstructures of the Ni-Cr-B-Si alloy forming the tracks of fig. lb. Fig. 2a to 2d show Scanning Electron Microscopy (SEM) microstructural images for the first track 3, the second track 4, the third track 5 and the fourth and fifth tracks 6, 7, respectively. As the scale of the SEM micrographs is the same, a comparison leads to the observation that the microstructure of every track is considerably different from the previous and subsequent ones. This leads to the observation that the

microstructure, and therefore the properties, across the layer are not homogeneous.

Figure 3 shows the microstructural changes along the tracks 5-7 of fig. lb. Fig. 3a shows the microstructural changes from the bottom of track 7 in the direction of the free surface of the layer 2. It is observed that the microstructure gradually changes with increasing distance from the substrate. Fig. 3b shows the abrupt microstructural change from track 5 to track 6. Track 5 shows a much coarser microstructure than track 6, which was clad onto the substrate after cladding track 5.

Figure 4 shows the microstructure of a track 9 comprising a Ni-Cr-B-Si alloy with Nb. The track 9 has been applied by using laser cladding. The track 9 forms part of a layer 8 of the Ni-Cr-B-Si alloy with Nb on the substrate 1. The composition of the Ni-Cr- B-Si-Nb alloy is Ni-13Cr-3B-4.6Si-0.7C-2.5Nb-6Fe-2.5Mo-2.5Cu, all in wt.%. As shown in fig. 4, the microstructure of the track 9 is homogeneous, in contrast to the variations of the microstructure with increasing distance from the substrate 1 for the Ni-Cr-B-Si alloy shown in fig. 3a and the multiple microstructures of individual tracks as shown in fig. 2. The indented squares 19 in the microstructure are the result of numerous Vickers Hardness measurements.

Figure 5 shows the hardness values of the modified alloys with different amounts of Nb addition. The composition of the base alloy to which Nb is added is: Ni-16.5Cr-3.6B- 4.8Si-0.55C-3Fe-3.5Mo-2.1Cu, all wt.%. As can be seen, the hardness of the layer 8 lies around 775-825 HV for all alloys with added Nb. The hardness decreases dramatically with the transition from layer 8 to substrate 1. In order to achieve a hardfacing layer with a more homogeneous hardness across the surface of the layer, i.e. relatively small hardness differences between individual measurements, and with less costs, an addition of less than 5.0 wt.% Nb to a commonly used Ni-Cr-B-Si alloy is sufficient, according to fig. 5. In comparison, the hardness of the commonly used Ni-Cr-B-Si alloy varies between 600-900 HV across the surface of the layer, showing a heterogeneous hardness, i.e. relatively large differences in hardness between individual hardness measurements, due to a heterogeneous microstructure.

Figure 6 shows the differences between the microstructure of the Ni-Cr-B-Si alloy and the microstructure of the Ni-Cr-B-Si-Nb alloy. As shown in fig. 6, the microstructure of the layer 2 is much coarser than the microstructure of the track 9 of layer 8. The refinement of the microstructure is due to the addition of Nb and the formation of Nb borides and/or borocarbides upon solidification which act as nucleation sites for chromium boride and/or carbide particles and make the chromium-rich particles smaller.

Figure 7 shows a phase map of a Ni-Cr-B-Si alloy microstructure after

solidification from the melt. The solidified microstructure comprises a number of phases, of which chromium boride particles 16 and chromium carbide 15 are the first to form upon cooling the melt below the liquidus temperature. Subsequently, the phase of nickel solid solution 17 starts to form. This phase contains all elements dissolved in Ni to their solid solubility level. Only B has a very limited solubility in Ni according to the Ni-B phase diagram. At the latest stage of the solidification, binary and ternary eutectic phases 18 start to form as a consequence of a number of eutectic reactions in the Ni-B, Ni-Si and Ni-Si-B phase diagrams. The phase formations of the investigated alloy system do not follow the Ni-Cr phase diagram, although Cr is one of the main alloying elements.

The properties of the Ni-Cr-B-Si alloys (with or without Nb) mostly depend on the type of the borides/carbides and the ratio between these particles, Ni solid solution and binary and ternary eutectic phases. Addition of Nb refines and homogenizes the microstructure of the commonly used Ni-Cr-B-Si alloys. Example:

To produce a deposit with the composition and microstructure as described in this invention, any Ni-Cr-B-Si alloy powder with the composition in the ranges mentioned in the invention could be selected. In this example, the alloy with nominal composition of Ni-16.5Cr-3.6B-4.8Si-0.55C-3Fe-3.5Mo-2.1Cu, all wt.%, was used in the form of metallic powder. The particle size of the alloy powder ranges between 50 and 150 μπι. Pure Nb powder with a similar particle size was used as the second powder component. A powder feeding system comprising two independent powder feeders, argon as a carrier gas, a mixing cyclone and a coaxial or side clad powder nozzle, was used to deliver and to mix the two powders together just before the powder stream entered a high power laser beam. A possible variant is to use only a single powder feeder with mechanically pre- mixed Ni based alloy and Nb powder, but in this case the amount of Nb in the final coating cannot be varied.

Mixed powder particles driven by the carrier gas formed a powder stream after leaving the coaxial or side laser cladding nozzle. This powder stream entered a high power laser beam with the power density of about 8.3- 12.5 10 3 W/cm 2 formed by defocusing of fiber laser beam with the wavelength of about 1 μιη. Particles inside the powder stream were quickly heated up and melted inside this laser beam due to the absorption of radiation and finally they joined the melt pool formed at the top of the moving carbon steel substrate by mixing of a small amount of molten substrate material and the delivered molten particles from the powder stream. During this continuous process, the part of the melt pool quickly solidified upon moving out of the laser beam and formed the required microstructure. The speed of the substrate, the laser power density and the amount of delivered mixture of both powders were carefully tuned to obtain a regular single track of clad material metallurgically bonded to the substrate. Continuous coating was formed by a side overlapping of individual laser tracks, as Fig. 1 clearly demonstrates.

The microstructure and properties of the processed deposits were characterized using various microscopy techniques, including SEM, TEM, EDS and hardness measurements.

List of parts

1. Substrate

2. Layer of Ni-Cr-B-Si alloy

3. First track in layer 2

4. Second track in layer 2

5. Third track in layer 2

6. Fourth track in layer 2

7. Fifth track in layer 2

8. Layer of Ni-Cr-B-Si-Nb alloy

9. Track of layer 8

10. Ni-Cr-B-Si alloy with 2.15 wt.% Nb

11. Ni-Cr-B-Si alloy with 4.29 wt.% Nb

12. Ni-Cr-B-Si alloy with 5.49 wt.% Nb

13. Ni-Cr-B-Si alloy with 6.91 wt.% Nb

14. Ni-Cr-B-Si alloy with 9.21 wt.% Nb

15. Chromium carbide phase (Cr₇C3)

16. CrsBs phase

17. Nickel solid solution

18. Binary and ternary eutectic phases

19. Hardness indentations

Claims

1. Nickel-based alloy, comprising a nickel matrix, and alloying elements chromium, boron and silicon, whereby the Si:B ratio is between 1 :2 and 3 :1 , the resulting Ni-Cr-B-Si alloy comprising 0.5-5.0 wt.% Nb, whereby the Nb:B ratio is between 1 :2 and 2:1.

2. Alloy according to claim 1, comprising

10.0-30.0 wt.% chromium,

2.0-6.0 wt.% silicon,

2.0-4.0 wt.% boron, whereby the Si:B ratio is between 1 :2 and 3:1 ,

0-5.0 wt.% iron,

0-5.0 wt.% molybdenum,

0-3.0 wt.% copper,

0-1.0 wt.% carbon,

0-0.5 wt.% of one or more of other elements, and the balance being nickel, forming an Ni-Cr-B-Si alloy, whereby the Ni-Cr-B-Si alloy comprises 0.5-5.0 wt.% niobium, and whereby the Nb:B ratio is between 1 :2 and 2: 1. 3. Alloy according to claim 1 or 2, whereby the Nb:B composition ratio is 1 :1.5 to 1.5: 1.

4. Alloy according to any of the preceding claims, whereby the Si:B composition ratio is 1 :2.5 to 3.5: 1 .

5. Alloy according to any of the preceding claims, wherein the alloy has a

microstructure comprising boride particles formed with one of chromium and niobium.

6. Alloy according to claim 5, wherein the microstructure comprises a nickel in solid solution phase (17) nucleated on and enclosing the boride particles.

7. Alloy according to claim 6, wherein the microstructure comprises nickel-boron- silicon binary and ternary eutectic phases (18) between the dendrites of Ni solid solution.

8. Alloy according to any one of the preceding claims, comprising 3.0-4.0 wt.% niobium, preferably about 3.5 wt.% niobium.

9. Alloy according to any one of the preceding claims, whereby the alloy is manufactured as a powder or wire. 10. Method for manufacturing at least a first hardfacing layer on a substrate (1), comprising:

- cooling the deposited alloy to at least below the solidus temperature for obtaining a layer (8) with a microstructure comprising boride particles formed with at least one of chromium and niobium, a nickel in solid solution phase (17) nucleated on and enclosing the boride particles and nickel-boron-silicon binary and ternary eutectic phases (18) between the nickel solid solution dendrites.

11. Method according to claim 10, comprising:

- heating the alloy to at least above its liquidus temperature to obtain a fully molten alloy; and

- solidifying the molten alloy after depositing on the substrate, thereby obtaining the first layer with a microstructure comprising boride particles formed with at least one of chromium and niobium, a nickel in solid solution phase nucleated on and enclosing the boride particles and nickel-boron-silicon binary and ternary eutectic phases between the nickel solid solution dendrites;

- cooling the layer to at least below the eutectic temperature.

12. Method according to claim 10 or 11 , wherein depositing the nickel based alloy on the substrate comprises:

- injecting a Ni-Cr-B-Si alloy in powder form into an irradiating focused beam, preferably a laser beam, to obtain a melt;

- injecting a niobium-containing powder into the irradiating focused beam, such that the Ni-Cr-B-Si alloy powder and the niobium-containing powder melt to form a Ni- Cr-B-Si alloy melt comprising 0.5-5.0 wt.% niobium;

- depositing the Ni-Cr-B-Si alloy melt comprising 0.5-5.0 wt.% niobium onto the substrate forming a first track of deposited alloy.

13. Method according to claim 10 or 11 , wherein depositing the nickel-based alloy on the substrate comprises:

- providing the Ni-Cr-B-Si alloy comprising 0.5-5.0 wt.% Nb, in wire form;

- feeding the wire into an irradiating focused beam, preferably a laser beam, such that the Ni-Cr-B-Si alloy comprising 0.5-5.0 wt.% Nb melts;

- depositing the melt onto the substrate forming a first track (9) of deposited alloy.

14. Method according to any of claims 10-13, whereby the Ni-Cr-B-Si alloy comprises:

10.0-30.0 wt.% chromium,

2.0-6.0 wt.% silicon,

2.0-4.0 wt.% boron, whereby the Si:B ratio is between 1 :2 and 3:1 ,

0-5.0 wt.% iron,

0-5.0 wt.% molybdenum,

0-3.0 wt.% copper,

0-1.0 wt.% carbon,

0-0.5 wt.% of one or more of other elements including the usual impurities, and the balance being nickel, whereby the Nb:B ratio is between 1 :2 and 2:1.

15. Method according to any of claims 11 -14, whereby the melt has a temperature of at least 1273 K.

16. Method according to any of claims 11 -14, whereby the melt has a temperature between 1723 K and 1823 K.

17. Method according to any of claims 11 -16, whereby the melt has a temperature of at least 1773 K.

18. Method according to any of claims 10-17, whereby the melt is cooled with a cooling rate between 10²-10⁴ Kelvin per second (K/s). 19. Method according to any one of claims 12 to 18, comprising depositing a second track of material on the substrate, whereby the second track of material at least partly overlaps the first track of material, thereby partially melting the first track of material and forming a first layer on the substrate upon solidification of the second track and the partially melted first track.

20. Method according to claim 10 or 19, comprising depositing a second layer on the first layer, thereby forming a three-dimensional structure on the substrate.

21. Method according to any of claims 8-18, comprising heating the substrate up to at least 500 K before depositing the layer on the substrate.

22. Method according to claim 19-21, comprising removing the substrate, to obtain the layer or the three-dimensional structure. 23. Substrate (1) having a layer (8) deposited thereon, the layer comprising a hardfacing nickel based alloy, the alloy comprising nickel, chromium, boron and silicon and 0.5-5.0 wt.% Nb, whereby the Nb:B ratio is between 1 :2 and 2: 1.

24. Substrate according to claim 23, whereby the layer comprises a nickel alloy comprising:

10.0-30.0 wt.% chromium,

2.0-6.0 wt.% silicon,

2.0-4.0 wt.% boron, whereby the Si:B ratio is between 1 :2 and 3:1 ,

0-5.0 wt.% iron,

0-5.0 wt.% molybdenum,

0-3.0 wt.% copper,

0-1.0 wt.% carbon,

0-0.5 wt.% of one or more of other elements, and the balance being nickel, and 0.5- 5.0 wt.% niobium, whereby the Nb:B ratio is between 1 :2 and 2: 1, and whereby the alloy has a microstructure comprising boride particles formed with at least one of chromium and niobium, a nickel in solid solution phase (17) nucleated on and enclosing the boride particles and nickel-boron-silicon binary and ternary eutectic phases (18) between the nickel solid solution dendrites.

25. Substrate according to claim 23 or 24, whereby the deposited alloy comprises 3.0- 4.0 wt.% niobium, preferably about 3.5 wt.% niobium.

26. Substrate according to any of claim 22-25, whereby the layer has a hardness between 7.3*10³ and 8.9* 10³ MPa (-750-900 hardness Vickers) throughout the surface of the layer, preferably between 8.0* 10³ and 8.6* 10³ MPa (-825-875 HV), most preferably about 8.3*10³ MPa (-850 HV).

27. Substrate obtainable by the method according to any one of claims 10 - 21.

28. Three-dimensional structure obtainable by the method of claim 22, comprising a hardfacing nickel based alloy, the alloy comprising nickel, chromium, boron and silicon and 0.5-5.0 wt.% Nb, whereby the Nb:B ratio is between 1 :2 and 2: 1.

29. Structure according to claim 28, comprising a nickel alloy containing

10.0-30.0 wt.% chromium,

2.0-6.0 wt.% silicon,

2.0-4.0 wt.% boron, whereby the Si:B ratio is between 1 :2 and 3:1 , 0-5.0 wt.% iron,

0-5.0 wt.% molybdenum,

0-3.0 wt.% copper,

0-1.0 wt.% carbon,

0-0.5 wt.% of one or more of other elements, and the balance being nickel, and 0.5- 5.0 wt.% niobium, whereby the Nb:B ratio is between 1 :2 and 2: 1, and whereby the alloy has a microstructure comprising boride particles formed with at least one of chromium and niobium, a nickel in solid solution phase (17) nucleated on and enclosing the boride particles and nickel-boron-silicon binary and ternary eutectic phases (18) between nickel solid solution dendrites.