Nickel based wear protection for a DTH drilling assembly

Abstract

A down the hole (DTH) drilling assembly (2) comprising a casing (4); optionally an inner cylinder (20) that extends axially within the casing along at least part of length of the casing; and a piston (22) that extends axially within the casing and if present also within the inner cylinder; wherein the piston comprises an external surface (28); wherein, if present, the inner cylinder comprises an external surface (40) and an internal surface (42); characterized in that: at least part of the external surface of the piston or at least part of the internal surface of the inner cylinder is coated with a first wear protection layer (36); wherein the first wear protection layer is nickel-based; and at least part of the first wear protection layer is coated with a polymer-based sealant top layer (38).

Description

Field of invention

[0001] The present invention relates to a coating for a DTH drilling assembly to improve its lifetime, especially to improve the lifetime of the piston of the DTH drilling assembly.Background

[0002] Holes can be drilled in rock by means of various rock drilling assemblies. Drilling may be performed with a method of combining percussions and rotation. This type of drilling is called percussive drilling. Percussive drilling may be classified according to whether an impact device is outside the drill hole or in the drill hole during drilling. When the impact device is in the drill hole, the drilling is typically called down the hole (DTH) drilling. When the impact device is outside the drill hole, the drilling the is typically called top hammer (TH) drilling.

[0003] The piston is a vital part of the DTH drilling assembly. Typically, the piston is made of carburized steel. The piston slides backwards and forwards inside a casing and optionally also within an inner cylinder within the DTH hammer and strikes the drill bit to generate the energy that breaks the rock.

[0004] For a reliable DTH hammer, it is important that the piston does not fail prematurely. Common failures of the piston are related to galling, which is wear caused by adhesion between sliding surfaces. Therefore, the problem to be solved is how to improve the wear resistance and decrease galling of the piston for a DTH drilling assembly.Summary of the Invention

[0005] It is an objective of the present invention to provide a means to reduce galling within a DTH hammer assembly and to increase its lifetime. This objective is achieved by providing a down the hole (DTH) drilling assembly comprising a casing; optionally an inner cylinder that extends axially within the casing along at least part of length of the casing; and a piston that extends axially within casing and optionally also within the inner cylinder; wherein the piston comprises an external surface; wherein if present the inner cylinder comprises an external surface and an internal surface; wherein at least part of the external surface of the piston or at least part of the internal surface of the inner cylinder is coated with a first wear protection layer; wherein the first wear protection layer is nickel-based; and at least part of the first wear protection layer is coated with a polymer-based sealant top layer.

[0006] Advantageously, this coating combination improves the wear resistance of the piston or if present also the wear resistance of the inner cylinder which consequently increases its lifetime. Furthermore, the friction coefficient between the sliding surfaces is reduced, for example between the piston and inner cylinder or between the piston and the casing. This is favourable because then less heat is generated between the sliding surfaces, which reduces melting the sliding surfaces and therefore galling is reduced. Reducing galling will be beneficial for increasing the lifetime of the piston.

[0007] Additionally, the combination of the nickel-based wear protection layer with the polymer-based sealant top sealant results in fewer open cracks and flaws between the atmosphere and steel substrate and therefore improved resistance against corrosion is also achieved. This coating combination provides a corrosion protective layer on the piston and / or inner cylinder thus delaying radial crack propagation, hence delaying transversal fractures. Additionally, this coating can be applied in a relatively simple, inexpensive manner that does not pose any major health, safety, or environmental concerns.

[0008] In some examples at least part of the external surface of the piston is coated with a first wear protection layer; wherein the first wear protection layer is nickel-based; and at least part of the first wear protection layer is coated with a polymer-based sealant top layer and the internal surface of the inner cylinder is left uncoated. Advantageously, the friction coefficient between the piston and the surfaces the piston slides against, which would typically be the inner cylinder or the casing, is reduced. Furthermore, galling is reduced and so the lifetime of the piston is increased. This would be the preferred option for DTH drilling assemblies where the piston slides against both the inner cylinder and the casing. In some examples at least part of the internal surface of the inner cylinder is coated with a first wear protection layer; wherein the first wear protection layer is nickel-based; and at least part of the first wear protection layer is coated with a polymer-based sealant top layer and wherein the external surface of the piston is left uncoated. Advantageously, the friction coefficient between the inner cylinder and the piston that slides against it is reduced. Furthermore, galling is reduced and so the lifetime of the piston and the inner cylinder is increased.

[0009] In some examples at least part of the external surface of the piston and at least part of the internal surface of the inner cylinder is coated with a first wear protection layer; wherein the first wear protection layer is nickel-based; and at least part of the first wear protection layer is coated with a polymer-based sealant top layer. Advantageously, if both surfaces that are sliding against each other are coated the friction coefficient between the piston and inner cylinder is even further reduced. Furthermore, galling is reduced and so the lifetime of the piston, inner cylinder and DTH hammer assembly is increased.

[0010] In some examples the thickness of first wear protection layer is between 5 - 200 µm. Advantageously, this thickness provides the optimal balance between providing sufficient wear protection without adding excessive costs or destabilizing the coating.

[0011] In some examples the thickness of the polymer-based sealant top layer is between 0.1 - 10 µm. Advantageously, this provides the optimal balance between having sufficient thickness to be effective whilst not adding unnecessary cost or adding the risk that streaks are formed as the coating dries. The polymer-based sealant top layer may also penetrate the cracks therefore providing extra adhesion and corrosion protection.

[0012] In some examples the first wear protection layer is a nickel phosphorous alloy. Advantageously, nickel phosphides provide a hard coating and therefore increased wear resistance.

[0013] In some examples the phosphorus content of the nickel phosphorus alloy is 6-9 wt%. Advantageously, this phosphorus content provides increased wear resistance without undesirable brittleness.

[0014] In some examples the first wear protection layer and the polymer-based sealant top layer are located on an external sliding surface of the piston. Advantageously, this provides wear protection on the surface that is most exposed to galling from sliding against the casing and / or inner cylinder.

[0015] In some examples the fiction coefficient between the polymer-based sealant top layer and the surface against which it slides is between 0.05 - 0.15. Advantageously, the low friction coating means that less heat is generated between the sliding surfaces, which consequently reduces melting of the sliding surfaces and therefore decreases galling.

[0016] Another aspect of the present invention relates to a method for providing a wear protection coating on a piston and / or an inner cylinder for DTH drilling assembly as described hereinbefore or hereinafter comprising the steps of: depositing a first wear protection layer comprising nickel on at least part of the external surface of the piston and / or at least part of the internal surface of the inner cylinder; depositing a polymer-based sealant top layer on top of the first wear protection layer; heat treating the coated piston and / or inner cylinder.

[0017] Advantageously, this method provides a means to provide a highly effective wear protection to the surface of the piston and / or inner cylinder. This method does not pose any major health, safety, or environmental issues either.

[0018] In some examples the first wear protection layer is applied using an electroless nickel bath. Advantageously, this method is quick and less harmful to the environment compared to alternative methods of providing wear and corrosion resistant coatings. A further benefit of this method is that it can provide a coating having a more uniform thickness with high reproducibility.

[0019] In some examples the polymer-based sealant top layer is sprayed on. Advantageously, this method provides a coating having a uniform thickness.

[0020] In some examples the heat treatment is conducted at a temperature of between 150 - 300°C for 30 - 120 minutes. Advantageously, this temperature range provides the balance between having a high enough temperature and heating time that a hard and wear resistant surface is formed and not having too high a temperature and time that case hardened properties of the steel substrate are adversely affected.

[0021] In some examples the surface of the piston and / or inner cylinder is shot peened or blasted before the nickel-based wear protection layer is applied. Advantageously, this step provides improved adhesion of the protection layer to the surface of the drill component.Brief description of drawings

[0022] A specific implementation of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 is a schematic drawing of a DTH drilling assembly. Figure 2 is schematic drawing of the coating on an external surface of a piston. Figure 3 is a schematic drawing of the coating on an internal surface of an inner cylinder. Detailed description

[0023] Figure 1 shows a cross section of a down the hole (DTH) hammer assembly 2 comprising a substantially hollow cylindrical casing 4 having an axially rearward end 6 (attachment end) and an axially forward end 8 (cutting end). A top sub 10 is at least partially accommodated within the rearward of casing 4 whilst a drill bit 12 (cutting head) is at least partially accommodated within the forward end of the casing 4. The drill bit 12 comprises a bit shaft 14 (shank) and a drill head 16 having a plurality of wear resistant mining inserts (buttons) 18.

[0024] In some examples an inner cylinder 20 extends axially within casing 4 and an elongate substantially cylindrical piston 22 extends axially within the cylinder 20 and the casing 4 and, the piston 22 is capable of shuttling back and forth along a central longitudinal axis 24 extending through the assembly 2. In other examples, there is no inner cylinder and so the piston 22 extends axially and slides within the casing 4. Pressurised fluid is delivered to assembly 2 via a drill string (not shown) that is coupled to the top sub 10. Fluid passages are arranged inside the hammer assembly 2 to drive the piston's 22 motion. A driver sub 26 (alternatively termed a drive chuck) is positioned towards the forward end the assembly 2 surrounding the bit shaft 14.

[0025] In some examples the drill piston 22 is hollow and has an external surface 28 and an internal surface 30. Alternative DTH hammer assemblies 2 may have a solid piston 22 that has an external surface 28 but does not have an internal surface.

[0026] The piston 22 has an external sliding surface 32 that slides against the internal surface 42 of the inner cylinder 20 and / or against the internal surface of the casing 4. Typically, hollow pistons 22 are longer and thus slide against both the internal surface 42 of the inner cylinder 20 and the internal surface of the casing 4. Typically, solid pistons are shorter and therefore only slide against the internal surface 42 of the inner cylinder 20. The piston 22 also has a striking surface 34 that is in contact with the drill bit 12. If there is no inner cylinder, then the piston 11 will slide only against the internal surface 42 of the casing 4.

[0027] Figure 2 and figure 3 shows that at least a part of the external surface 28 of the piston 22 or at least part of the internal surface 42 of the inner cylinder 20 respectively is coated with a first wear protection layer 36 wherein the first wear protection layer 36 is nickel-based. At least part of the first wear protection layer 36 is coated with a polymer-based sealant top layer 38. Areas of the external surface 28 of the piston 22 or internal surface 42 of the inner cylinder 20 that are not coated with the first wear protection layer 36 may also be coated with the polymer-based sealant top layer 38. Part or all of the first wear protection layer 36 could be coated with the polymer-based sealant top layer 38.

[0028] In some examples at least part of the external surface 28 of the piston 22 is coated as described hereinabove and hereinbelow and the internal surface 42 of the inner cylinder 20 is left uncoated or is not present at all. In some examples at least part of the internal surface 42 of the inner cylinder 20 is coated as described hereinabove and hereinbelow and the external surface 28 of the piston 22 is left uncoated. In some examples both at least part of the external surface 28 of the piston 22 and at least part of the internal surface 42 of the inner cylinder 20 are coated as described hereinabove and hereinbelow.

[0029] "Nickel-based" means that the major component on the first wear protection layer 36, in both mass % and atomic %, is nickel, for example >50% nickel, for example >70% nickel, for example >80% nickel, for example >90% nickel.

[0030] "Polymer-based" means that the major component of the sealant top layer 38, in both mass % and atomic %, is polymer, for example >50% polymer, for example >70% polymer, for example >80% polymer, for example >90% polymer.

[0031] If the piston 22 has an internal surface 30 in some examples at least part of the internal surface 30 of the piston 22 may also be coated with the first wear protection layer 36 and / or the polymer-based sealant top layer 38. At least part or all of the internal surface 30 may be coated with only first wear protection layer 36. At least part or all of the internal surface 30 may be coated with only the polymer-based sealant top layer 38. At least part or all of the internal surface 30 may be coated with both the first wear protection layer 36 and the polymer-based sealant top layer 38. Any combination of these options is possible.

[0032] In some examples the thickness of first wear protection layer 36 is between 5 - 200 µm, for example between 5 - 150 µm, for example between 7-100 µm, for example between 10-50 µm.

[0033] In some examples the thickness of the polymer-based sealant top layer 38 is between 0.1 - 10 µm, for example between 0.1 - 5 µm.

[0034] The first wear protection layer 36 could be nickel alloyed with sulphur, phosphorus, boron, or any other suitable element. Preferably, the first wear protection layer 36 is a nickel phosphorous alloy. Preferably, the phosphorus content of the nickel phosphorus alloy is 6-9 wt%.

[0035] For example, the polymer could be a Teflon or a fluorinated polymer (e.g., Polytetrafluoroethylene (PTFE)) or a non-fluorinated crystalline polymer (e.g., Polyether ether ketone (PEEK)) or a high-density polymer (e.g., Ultra-high-molecular-weight polyethylene (UHMWPE)).

[0036] In some examples at least part of the outer surface 42 of the inner cylinder 20 may also be coated with the first wear protection layer 36 and / or the polymer-based sealant top layer 38. At least part or all of the external surface 42 may be coated with only first wear protection layer 36. At least part or all of the external surface 42 may be coated with only the polymer-based sealant top layer 38. At least part or all of the external surface 42 may be coated with both the first wear protection layer 36 and the polymer-based sealant top layer 38. Any combination of these options is possible.

[0037] The first wear protection layer 36 and the polymer-based sealant top layer 38 could be located on all regions of the external surface 28 of the piston 22 or all regions of the internal surface 42 of the inner cylinder 20. Alternatively, the first wear protection layer 36 and the polymer-based sealant top layer 38 may be only located on selected areas of the external surface 28 of the piston 22 or selected areas of the internal surface 42 of the inner cylinder 20, for example on the external sliding surface 28. In some examples the first wear protection layer 36 and the polymer-based sealant top layer 38 are located the striking surface 34 of the piston 22. In other examples the striking surface 34 of the piston 22 is left uncoated.

[0038] In some examples the friction coefficient between the polymer-based sealant top layer 38 and the surface against which it slides is between 0.05 - 0.15, for example between 0.1 - 0.13. For example, this could be between the piston 22 and the inner cylinder 20 or between the piston 22 and the casing 4.

[0039] Another aspect of the present application relates to a method for providing wear protection on a piston 22 and or inner cylinder 20 of the DTH drilling assembly comprising the steps of: depositing a first wear protection layer 36 comprising nickel on at least part of the external surface 28 of the piston 22 and / or the internal surface 42 of the inner cylinder 20; depositing a polymer-based sealant top layer 38 on top of the first wear protection layer 36; heat treating the coated piston 22 and / or inner cylinder 20.

[0040] In some examples the first wear protection layer 36 is applied using an electroless nickel bath. The nickel or nickel-based alloy adheres to the substrate, the substrate being the metal of the drill component, i.e., the piston and / or the inner cylinder. In electroless nickel plating the object reacts to the plating bath chemistry, creating a uniform and smooth, layer with very little surface porosity. The even deposition makes it an ideal choice for complex, non-line of sight, geometries and often eliminates the need for grinding after plating. Alternatively, the first wear protection layer 36 could be applied via electroplating or any other suitable method.

[0041] In some examples the polymer-based sealant top layer 38 is sprayed on. Alternatively, the polymer-based sealant top layer 38 could be applied through dipping or any other suitable method.

[0042] In some examples the heat treatment is conducted at a temperature of between 150 - 300°C for 30 - 120 minutes. The purpose of the heating step is to enable the formation of Ni-P intermetallic phases, improve the adhesion to the substrate and to cure the polymer-based sealant top layer 38. Typically, the heat treatment is conducted in a furnace.

[0043] In some examples the heat treatment is conducted at a temperature of between 150 - 300°C, for example between 225 - 260°C. In some examples the heat treatment is conducted for between 30 - 120 minutes, for example between 45 - 75 minutes.

[0044] In some examples the external surface 28 of the piston 22 is shot peened or blasted before the nickel-based wear protection layer 36 is applied. In some examples the internal surface 42 of the inner cylinder 20 is shot peened or blasted before the nickel-based wear protection layer 36 is applied.

[0045] When a particular feature, or a particular combination of features, is described in connection with an embodiment or example, it is submitted that it is within the knowledge of one skilled in the art to implement such feature, or combination of features, in connection with other embodiments whether or not explicitly described.

Claims

1. A down the hole (DTH) drilling assembly (2) comprising a casing (4); optionally an inner cylinder (20) that extends axially within the casing (4) along at least part of length of the casing; and a piston (22) that extends axially within the casing (4) and if present also within the inner cylinder (20); wherein the piston (22) comprises an external surface (28); wherein, if present, the inner cylinder (20) comprises an external surface (40) and an internal surface (42); characterized in that: at least part of the external surface (28) of the piston (22) or at least part of the internal surface (42) of the inner cylinder (20) is coated with a first wear protection layer (36); wherein the first wear protection layer (36) is nickel-based; and at least part of the first wear protection layer (36) is coated with a polymer-based sealant top layer (38).

2. The DTH drilling assembly (2) according to claim 1 wherein the at least part of the external surface (28) of the piston (22) is coated with a first wear protection layer (36); wherein the first wear protection layer (36) is nickel-based; and at least part of the first wear protection layer (36) is coated with a polymer-based sealant top layer (38) and wherein the internal surface (42) of the inner cylinder (20) is uncoated.

3. The DTH drilling assembly (2) according to claim 1 wherein at least part of the internal surface (42) of the inner cylinder (20) is coated with a first wear protection layer (36); wherein the first wear protection layer (36) is nickel-based; and at least part of the first wear protection layer (36) is coated with a polymer-based sealant top layer (38) and wherein the external surface (40) of the piston (22) is uncoated.

4. The DTH drilling assembly (2) according to claim 1 wherein at least part of the external surface (28) of the piston (22) and at least part of the internal surface (42) of the inner cylinder (20) is coated with a first wear protection layer (36); wherein the first wear protection layer (36) is nickel-based; and at least part of the first wear protection layer (36) is coated with a polymer-based sealant top layer (38).

5. The DTH drilling assembly (2) according to any of the previous claims wherein the thickness of first wear protection layer (36) is between 5 - 200 µm.

6. The DTH drilling assembly (2) according to any of the previous claims wherein the thickness of the polymer-based sealant top layer (38) is between 0.1 - 10 µm.

7. The DTH drilling assembly (2) according to any of the previous claims wherein the first wear protection layer (36) is a nickel phosphorous alloy.

8. The DTH drilling assembly (2) according to any of the previous claims wherein the phosphorus content of the nickel phosphorus alloy is 6 - 9 wt%.

9. The DTH drilling assembly (2) according to any of claims 1 - 2 or 4 - 8 wherein the first wear protection layer (36) and the polymer-based sealant top layer (38) are located on an external sliding surface (32) of the piston (22).

10. The DTH drilling assembly (2) according to any of the previous claims wherein the friction coefficient between the polymer-based sealant top layer (38) and the surface against which is slides is between 0.05 - 0.15.

11. A method for providing wear protection on a piston (22) and / or inner cylinder (20) for a DTH hammer assembly (2) according to any of claims 1-10 comprising the steps of: - depositing a first wear protection layer (36) comprising nickel on at least part of the external surface (28) of the piston (22) and / or on at least part of the internal surface (42) of the inner cylinder (20); - depositing a polymer-based sealant top layer (38) on top of the first wear protection layer (36); - heat treating the coated piston (22) and / or coated inner cylinder (20).

12. The method according to claim 11 wherein the first wear protection layer (36) is applied using an electroless nickel bath.

13. The method according to claim 11 or 12 wherein the polymer-based sealant top layer (38) is sprayed on.

14. The method according to any of claims 11 - 13 wherein the heat treatment is conducted at a temperature of between 150 - 300°C for 30 - 120 minutes.

15. The method according to any of claims 11 - 14 wherein the external surface (28) of the piston (22) and / or the internal surface (42) of the inner cylinder (20) is shot peened or blasted before the nickel-based wear protection layer (36) is applied.

Images