WO2018093265A1

WO2018093265A1 - Nickel-base alloy, coating of such an alloy, and method for manufacturing such a coating

Info

Publication number: WO2018093265A1
Application number: PCT/NL2017/050755
Authority: WO
Inventors: Ismail HEMMATI; Jeff Theo Marie DE HOSSON; Václav OCELIK
Original assignee: Stichting Materials Innovation Institute (M2I)
Priority date: 2016-11-21
Filing date: 2017-11-20
Publication date: 2018-05-24
Also published as: NL2017827B1

Abstract

The invention relates to a nickel-base alloy including nickel and at least alloying elements chromium, boron, silicon and niobium, the alloy including 4.0-5.0 wt% Nb, with 18 - 22 wt% Cr, 5.5 - 6.5 wt% Si, 1.0 - 1.5 wt% B, 5 - 8 wt% Fe, 0.5 - 2 wt% W, less than 0.5 wt% C, the balance being nickel, forming a modified Ni-Cr-B-Si alloy and having by weight percentage a Si:B ratio between 4: 1 to 6: 1. A method is disclosed for manufacturing a coating of the nickel-base alloy on a substrate, including: - heating a surface of the substrate; - adding a coating material locally to the heated area on the surface so as to melt the coating material in the heated area, and - cooling the surface at a cooling rate of at least 1000 °C/s such that the melted coating material solidifies and forms the coating.

Description

Nickel-base alloy, coating of such an alloy, and method for

manufacturing such a coating

Field of the invention

The present invention relates to a nickel-base alloy. Also, the invention relates to a coating of such an alloy. Moreover, the invention relates to a method for manufacturing such a coating.

Background

Nickel-base hardfacing alloys are used in many coating applications to enhance the corrosion/wear resistance of components for chemically and mechanically aggressive environments and/or high temperature operating conditions. The currently available Ni-base alloys for corrosion and wear applications belong to either families of Ni-Cr-Mo-C (such as Inconel 625 or Eatonite 5) or Ni-Cr-B-Si-C (such as various Colmonoy powders).

The high hardness of these alloys is based on formation of hard carbide or boride precipitates and eutectic phases in the niicrostructure of the deposits. However, the size and distribution of the same phases make the deposits brittle. These alloys have been originally developed for production techniques with low cooling rates after deposition including various thermal spray methods. Using newer technologies with rapid solidification such as laser cladding, the deposited coatings undergo much faster cooling rates. Hence, brittleness of the prior art Ni-base hardfacing alloys makes it difficult to get hard and crack-free coatings using laser cladding or similar technologies.

There have been some attempts to increase the toughness of these alloys by addition of refining elements such as Ti, Ta or V, usually at the expense of some hardness loss.

Because of the rapid cooling rates for example during laser cladding, coatings may contain cracks. During the solidification and the subsequent rapid cooling of the alloy, shrinkage of the melt pool and the constraint of the substrate on which the coating is formed, may produce tensile stresses in the coating layer up to several hundreds MPa. When combined with the low ductility of the coating layer, these tensile stresses can surpass the ultimate tensile strength (UTS) of the deposit and result in a rapid propagation of cracks in the coating layer. In this cracking process, both tensile stresses and ductility of the deposits play a role. Hence, the efforts to solve the cracking problem of the coatings from the prior art have been focused on either reducing the tensile stresses in the coating layers by preheating or postheating or increasing the ductility of the deposited alloys.

From prior art it is known that it is possible to considerably refine the hard particles in the Ni-Cr-B-Si-C alloy system while keeping the high hardness of the original composition by addition of few weight percent of Nb (up to 5 wt%). However, it was observed that refinement of the microstructure was unable to substantially enhance toughness of the deposits. The reason for this failure is that even the Nb- modified Ni-Cr-B-Si-C hardfacing alloys contain large quantities of hard Ni-B-Si eutectic structures which solidify as a continuous network and provide an easy route for crack propagation.

It is an object of the invention to overcome the disadvantages from the prior art. The present invention discloses an alloy intended for (laser) clad coating that advantageously solves the problem from the prior art.

Summary of the invention

The object is achieved by a nickel base alloy in accordance with claim 1, comprising nickel and at least alloying elements chromium, boron, silicon and niobium, the alloy comprising 4.0-5.0 wt% Nb, with 18 - 22 wt% Cr, 5.5 - 6.5 wt% Si, 1.0 - 1.5 wt% B, 5 - 8 wt% Fe, 0.5 - 2 wt% W, less than 0.5 wt% C, the balance being nickel, forming a modified Ni-Cr-B-Si alloy and having by weight percentage a Si:B ratio between 4: 1 to 6: 1.

The nickel-base alloy as defined by claim 1 has a favorable combination of hardness, toughness and corrosion resistance.

The nickel-base alloy is basically a modified Ni-Cr-B-Si system. In this alloy system, the main strengthening phases are hard but very fine Ni-base intermetallic and eutectic phases.

A limited quantity of boron can be added to produce submicron chromium borides which further increase the hardness but are not essential components of the system as before. j

According to an aspect, the invention relates to a nickel-base alloy as described above wherein the alloy has a microstructure comprising Ni dendrites as dendritic substructure and interdendritic eutectics as eutectic substructure; the dendritic substructure partitioning the eutectic substructures into separated regions of the microstructure.

According to an aspect, the invention relates to a nickel-base alloy as described above wherein the dendritic substructure comprises elongated chromium boride particles which are decorated with Ni-base intermetallic particles.

According to an aspect, the invention relates to a nickel-base alloy as described above wherein the eutectic substructure comprises one or more Ni-base binary and/or ternary eutectic phases.

According to an aspect, the invention relates to a nickel-base alloy as described above wherein the Ni-base intermetallic particles are surrounded by a Ni solid solution.

According to an aspect the invention relates to a hardfacing alloy coating on a substrate wherein the hardfacing alloy is a nickel-base alloy as described above.

According to an aspect, the invention relates to a hardfacing alloy coating as described above, wherein the substrate is a steel product.

According to an aspect, the invention relates to a steel product comprising a coating of a nickel-base alloy as described above.

Additionally, the invention relates to a method for manufacturing a coating of a nickel-base alloy on a substrate, the alloy comprising nickel and at least alloying elements chromium, boron, silicon and niobium, the alloy comprising 4.0-5.0 wt% Nb, with 18 - 22 wt% Cr, 5.5 - 6.5 wt% Si, 1.0 - 1.5 wt% B, 5 - 8 wt% Fe, 0.5 - 2 wt% W, less than 0.5 wt% C, the balance being nickel, forming a modified Ni-Cr-B-Si alloy and having by weight percentage a Si:B ratio between 4: 1-6: 1, the method comprising:

- heating a surface of the substrate;

- adding a coating material locally to the heated area on the surface of the substrate so as to melt the coating material in the heated area;

- cooling the surface of the substrate at a cooling rate of at least 1000 °C/s such that the melted coating material solidifies and forms the coating.

In an embodiment, the surface of the substrate is heated by directing a laser beam to an area on a surface of a substrate for heating the surface in the laser irradiated area using the laser beam as heat source; and the cooling of the surface of the substrate at at least 1000 °C/s is established by moving the laser irradiated area relative to the substrate so that the heat source is moved from the melted material.

In an embodiment, the method as described above comprises a step wherein the coating material is added as a powder to the laser irradiated area.

In an embodiment, the method as described above comprises a step, wherein the powder is added coaxially to the laser beam.

In an embodiment, the method as described above comprises a step, wherein the powder is added to the laser beam in a sideway direction.

In an embodiment, the powder is a pre-alloyed powder containing at least Nb, Ni, Cr, Si and B.

In an alternative embodiment, the method as described above comprises a step of premixing of Nb powder with a powder of an alloy containing at least Ni-Cr-B-Si including:

- delivering the Nb powder from a first powder feed source to a cyclone;

- delivering the Ni-Cr-B-Si powder from a second powder feed source to the cyclone;

- mixing the Nb powder and the Ni-Cr-B-Si powder in the cyclone using an inert gas;

- feeding the mixed powder to the laser beam by a flow of the inert gas from the cyclone towards the laser beam.

In an embodiment, the method as described above comprises a step of preheating the substrate to a predetermined temperature between about 100 and about 300 °C, before at least the step of heating the surface of the substrate.

Advantageous embodiments are further defined by the dependent claims.

Brief description of drawings

The invention will be explained in more detail below with reference to drawings in which illustrative embodiments thereof are shown. The drawings are intended exclusively for illustrative purposes and not to restrict the inventive concept as defined by the claims.

In the following figures, the same reference numerals refer to similar or identical components in each of the figures.

Figures la, lb, lc show images of the microstructure of a laser cladded alloy according to the invention; Figures 2a, 2b, 2c show schematically laser cladding methods for depositing an alloy according to the invention;

Figure 3 shows a diagram of the hardness of a laser deposited layer of an alloy according to the invention as a function of depth in the layer and a comparison to the hardness of some other Ni base hardfacing alloys;

Figure 4 compares the wear rates in slurry erosion testing of coatings deposited from an alloy according to the invention with the wear rate of similar coatings deposited from some other Ni base hardfacing alloys.

Figure 5 shows a diagram for corrosion resistance of a laser deposited layer of an alloy according to the invention and a comparison to the corrosion resistance of some other Ni- base hardfacing alloys.

Detailed description of embodiments

The Ni-base alloys according to the invention comprise nickel and alloying elements chromium, boron, silicon and niobium, wherein the alloy comprises 18-22 % Cr, 5.5-6.5 % Si, 1.0-1.5 % B and 4.0-5.0 % Nb (numbers in weight percent).

Further, the ratio of Si :B is between 4: 1 and 6: 1 .

In Ni-base alloys according to the invention, the main strengthening phases are Ni-base intermetallic and Ni-base eutectic phases. Additionally, the alloys can comprise chromium borides which form due to the presence of boron in the alloy composition.

Figures la - lc show representative images of the microstructure of the modified Ni-Cr-B-Si alloy deposit produced by laser cladding.

In the Scanning Electron Microscopy (SEM) image of figure la the deposited layer of Ni-base alloy is shown to comprise two different types of microstructures: a eutectic structure Ml and a dendritic structure M2. Each of these microstructures is shown in higher magnifications in figure lb and figure lc.

The dendritic microstaicture M2 shown in figure lb comprises a tree-like structure of tiny elongated chromium boride particles (dark) which are decorated with Ni-base intermetallic particles of Nb Ni2Si (bright) which has been identified by X-ray diffraction. These particles have an average size of less than 100 nm.

Within the dendritic structure M2 the chromium boride rods are covered with Ni- base intermetallic particles that are surrounded by layers of a tough Ni solid solution phase. In figure lc the Ni-base eutectic microstructure Ml is shown in more detail. The eutectic structure comprises one or more Ni-base binary and/or ternary eutectic phases.

In the Ni-base alloys according to the invention, the dendritic microstructure M2 separates the eutectic structures Ml, thus preventing the eutectic staictures Ml from forming a continuous network. It is considered that by breaking up the continuous network of the eutectic phases, potential paths for crack propagation are removed and toughness of the Ni-base alloy is enhanced.

Figures 2a, 2b, 2c show schematically laser cladding methods for depositing an alloy according to the invention.

In laser cladding, a coating of an alloy is deposited on the surface of a substrate.

In many applications, the substrate will be the surface of a steel product although other metal substrates are suitable as well.

A laser beam is directed towards the substrate surface to locally heat the surface.

At the location where the laser beam strikes the substrate surface, a feedstock of Ni- base alloy in the form of a powder or wire is provided and exposed to the laser beam.

The laser beam melts the feedstock along with a part of the substrate and forms a melt pool on the substrate. Moving the substrate and the laser relative to each other allows the melt pool to cool down, solidify and move 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 of the Ni-base alloy is created on the substrate. For obtaining an alloy with the described microstructure, a cooling rate of at least 1000 °C/s is required.

Figure 2a schematically shows a coating 2 of the Ni-base alloy on a substrate 1 produced by laser cladding. Tracks 3, 4, 5, 6, 7 of the alloy formed by repetitive deposition using the moving laser beam are schematically shown on the surface.

Figures 2b and 2c show configurations of a laser cladding set-up with powder injection to deliver the coating material into the melt pool created by the laser beam.

Two configurations can be used for addition of the coating material: coaxial powder injection or side powder injection. In the first configuration as shown in Figure 2b, the laser cladding set-up 10 comprises a conically shaped wall 12 of an enclosed space 15. The laser cladding set-up and the substrates may have a relative movement as indicated schematically by arrows 17. Within the wall 12 of the laser cladding set-up 10, feeding conduits 13, 14 are arranged for feeding the coating material powder. During the deposition process, a focused laser beam 1 1 passes through the enclosed space 15 towards an outlet A and to the substrate surface. In a same direction a gas stream B is flowing towards the outlet A. The gas stream B comprises a shielding gas. The shielding gas may be an inert gas such as argon, nitrogen, helium or a mixture of these.

Through the feeding conduits 13, 14, powdered coating material is added from the same direction as the laser beam to provide a flow of coating material powder towards the substrate surface into the melt pool area irradiated by the laser beam.

The method has more geometrical freedom to clad on substrates with complicated geometrical shapes, i.e. on a non-flat, curved or jagged surface.

In the second configuration shown in figure 2c, a side feeding conduit 20 is positioned adjacent to the outlet A and arranged to provide a flow of coating material powder towards the substrate surface 1 into the area irradiated by the laser beam.

Alternatively, the coating material can be added to the melt pool created by the laser beam in the form of a wire. In that case, the above-mentioned alloy should be cast and formed into wires with a suitable diameter.

According to an aspect of the invention, the laser cladding method comprises premixing of Nb powder with a prior art Ni-Cr-B-Si alloy powder during the deposition process. To perform this, Nb powder can be injected simultaneously with the Ni-Cr-B- Si alloy powder following these steps:

- The intended Ni-Cr-B-Si and Nb powders are put in different hoppers of a powder feeder;

- The two powders are delivered by inert carrier gas into a cyclone which is positioned before the feeding conduit(s). The powders are mixed in the cyclone using a carrier gas, preferably using an inert gas such as argon or other inert gas as known to the skilled in the art.

- The mixed powders are injected under the laser beam and into the melt pool using a flow of an inert carrier gas. Injection can be done using either the first or second configuration by a coaxial nozzle 13, 14 for coaxial powder injection or a side nozzle 20 for sideways powder injection.

It is noted that alternatively, a pre-alloyed powder can be used that contains at least Nb, Ni, Cr, Si and B. According to an aspect of the invention, the laser cladding method comprises a preheating step in which the substrate is preheated to a predetermined temperature between 100 °C and 300 °C. By preheating the substrate before creating the coating layer 2 of the Ni-base alloy, the cooling rate and hence the thermal stresses during solidification and subsequent cooling are reduced. The substrate could be preheated for example by an electric furnace or heating blankets before deposition of the coating or by an induction head (not shown) attached to the laser cladding set-up 10 during the deposition process. Accordingly, the probability of creation of cracks in the coating layer 2 is significantly reduced, allowing the production of substantially crack-free coating layers 2. It is noted that the cracking tendency also depends on the size, geometry and thickness of the deposited layer. The preheating temperature should be adjusted according to size of the substrate. Preheating at higher temperatures is needed for deposition on larger substrates.

For a small area of deposited Nickel base alloy coating of about 1 mm thick, crack-free coatings can be obtained even after deposition on substrates at room temperature.

Additionally, it is observed that the cracking tendency of the coating layers 2 deposited from the Nickel base alloy of the invention was lower than that of existing alloys such as "Eatonite 5".

Figure 3 shows a plot of the hardness of the coating layer 2 as a function of the position in the layer. Further, Figure 3 shows a comparison between the hardness of the coating layers deposited from a Nickel base alloy according to the invention with the hardness of the laser-deposited Inconel 625 and Eatonite 5 coatings.

It can be seen that hardness of the Nickel base alloy of the invention is significantly higher than that of Inconel 625 or Eatonite 5. In these samples, the coating layers have a thickness of about 1 mm. The position of interface between the coating layer(s) and the substrate are indicated schematically by arrow A2.

The coating layer 2 of the Nickel base alloy according to the invention has a Vickers hardness 30 of about 650 HV0.5. The Eatonite 5 layer has a hardness 32 of about 520 HV0.5 and the Inconel 625 layer has a hardness 34 of about 300 - 320 HV0.5.

In correspondence with the observed Vickers hardness, it is also observed that the Nickel base alloy according to the invention has a better erosion wear resistance against the slurry of silica sand and water than the Inconel 625 or Eatonite 5 coatings as shown in Figure 4.

A pot-type slurry erosion tester containing slurry consisting of water and silica sand particles was used to evaluate the erosion resistance of the coatings. Circular specimens with a diameter of 10 mm and thickness of 6 mm were machined from the clad layers and their erosion rate was measured in comparison with C22 steel reference samples after rotating in the slurry for 2 hours. For each coating, duplicate samples were tested and the amount of worn material per unit time was measured and averaged. Figure 4 presents the erosion wear rate of laser clad coatings deposited from the selected Ni-base alloys according to the invention (36) as well as those from Inconel 625 (38) and Eatonite 5 (40). The numbers show the wear rate in comparison to the wear rate of the C22 reference specimens. It can be seen that the coatings from the alloy according to the invention perform better in comparison to the commercial alloys, i.e., loose less material during the erosion wear test.

Figure 5a shows the results of corrosion resistance measurements for coatings deposited from the Nickel base alloy, Eatonite 5 and Inconel 625. The results relate to Bode impedance plots for samples exposed to seawater for 4 weeks. In the Bode plots, impedance at low frequency side increased for all samples and the capacitive behavior was more significant. The impedance data for the coating deposited from the alloy of this invention could be modeled using equivalent circuits shown in Fig. 5c, where Rei is the electrolyte resistance, Qdi the constant phase element for the double layer, Re the charge transfer resistance, W₀ the Warburg element and Q_c the constant phase element for the adsorption and diffusion layer.

For Eatonite 5 and Inconel 625 it is found that the results are best matched by the equivalent circuit in figure 5b. For the Nickel base alloy according to the invention the equivalent circuit of figure 5c best matches the Bode plot. The equivalent circuits imply that the passive layer on the coatings deposited from the Nickel base alloy is better than the passive layer on the Eatonite 5 or Inconel 625 layers. Hence, the Nickel base alloy possesses better passivation properties and thus an improved corrosion resistance.

In view of the higher hardness value, the higher erosion wear resistance and higher corrosion resistance, the coating layer of the Nickel base alloy according to the invention may be advantageously applied in offshore, marine and civil engineering industries as a coating layer suitable for use in such aggressive environments. In particular, hydraulic pistons as used in many applications in off-shore, marine, dredging and civil engineering industries could benefit from the application of a coating layer of a nickel base alloy according to the invention.

The surface of hydraulic piston rods is one of the most important parts of any hydraulic cylinder. As hydraulic pistons usually work in harsh environments, piston rods are exposed to chemical attacks (from the water or other liquids) and different types of wear (e.g. abrasion or erosion). To extend the lifetime of the piston rods, their surface can be protected with a coating layer of the nickel base alloy of the invention applied by Laser Cladding. As illustrated above, the nickel base alloy coatings 2 of the invention have distinctive advantages over their predecessors such as higher corrosion and wear resistance. In addition, the laser cladding technique produces pore-free coatings with high integrity, metallurgical bonding to the substrate and superior functional properties in comparison to the coatings deposited by competing

technologies such as plasma spray or High Velocity Oxy-Fuel (HVOF). As a result, the laser clad Nickel base alloy coating deposited on the hydraulic piston rods is expected to have a higher lifetime which reduces the overall cost of using the hydraulic piston.

The invention has been described with reference to a preferred embodiment. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims.

Claims

1. A nickel-base alloy comprising nickel and at least alloying elements chromium, boron, silicon and niobium, the alloy comprising 4.0-5.0 wt% Nb, with 18 - 22 wt% Cr, 5.5 - 6.5 wt% Si, 1.0 - 1.5 wt% B, 5 - 8 wt% Fe, 0.5 - 2 wt% W, less than 0.5 wt% C, the balance being nickel, forming a modified Ni-Cr-B-Si alloy and having by weight percentage a Si:B ratio between 4: 1 to 6: 1.

2. The nickel-base alloy according to claim 1, wherein the alloy has a

microstructure comprising Ni dendrites as dendritic substructure and interdendritic eutectics as eutectic substructure; the dendritic substnicture partitioning the eutectic substructures into separated regions of the microstructure.

3. The nickel-base alloy according to claim 2, wherein the dendritic substructure comprises elongated chromium boride particles which are decorated with Ni-base intermetallic particles.

4. The nickel-base alloy according to claim 2 or claim 3, wherein the eutectic substructure comprises one or more Ni-base binary and/or ternary eutectic phases.

5. The nickel-base alloy according to claim 3 or claim 4, wherein the Ni-base intermetallic particles are surrounded by a Ni solid solution.

6. A hardfacing alloy coating on a substrate wherein the hardfacing alloy is a nickel- base alloy according to any one of the preceding claims 1 - 5.

7. The hardfacing alloy coating according to claim 6, wherein the substrate is a steel product.

8. A steel product comprising a coating of a nickel-base alloy in accordance with any one of claims 1 - 5.

9. A method for manufacturing a coating of a nickel-base alloy on a substrate, the alloy comprising nickel and at least alloying elements chromium, boron, silicon and niobium, the alloy comprising 4.0-5.0 wt% Nb, with 18 - 22 wt% Cr, 5.5 - 6.5 wt% Si, 1.0 - 1.5 wt% B, 5 - 8 wt% Fe, 0.5 - 2 wt% W, less than 0.5 wt% C, the balance being nickel, forming a modified Ni-Cr-B-Si alloy and having by weight percentage a Si:B ratio between 4: 1-6: 1, the method comprising:

- heating a surface of the substrate;

10. The method according to claim 9, wherein the heated area is heated by irradiation by a laser beam.

1 1. The method according to claim 9 or claim 10, wherein the coating material is added as a powder to the heated area.

12. The method according to claim 10 and claim 11, wherein the powder is added coaxially to the laser beam.

13. The method according to claim 10 and claim 1 1, wherein the powder is added to the laser beam in a sideway direction.

14. The method according to any one of the preceding claims 11 - 13, comprising a step of premixing of Nb powder with a powder of an alloy containing at least Ni-Cr-B- Si including:

- delivering Nb powder from a first powder feed source to a cyclone;

- delivering Ni-Cr-B-Si powder from a second powder feed source to the cyclone; - mixing the Nb powder and the Ni-Cr-B-Si powder in the cyclone using argon gas;

- feeding the mixed powder to the heated area by a flow of the argon gas from the cyclone towards the laser beam.

15. The method according to claim 11 - 13, wherein the powder is a pre-alloyed powder that contains at least Nb, Ni, Cr, Si and B.

16. The method according to any one of the preceding claims 9 - 15, comprising a step of preheating the substrate to a predetermined temperature between ~100°C and ~300°C, before at least the step of heating the surface.