WO2018150080A1

WO2018150080A1 - Method of coating piston engine component and piston engine component

Info

Publication number: WO2018150080A1
Application number: PCT/FI2017/050111
Authority: WO
Inventors: Aulis Silvonen; Edoardo HLEDE; Jyrki SUUTALA; Mattia PUJATTI; Mikko Turunen
Original assignee: Wartsila Finland Oy
Current assignee: Wartsila Finland Oy
Priority date: 2017-02-20
Filing date: 2017-02-20
Publication date: 2018-08-23
Anticipated expiration: 2019-08-20

Abstract

In the method of coating a piston engine component (8, 9, 2), a coating matrix of a cobalt- or nickel-based alloy with solid lubricating particles embedded in the coating matrix is applied onto a surface of the component (8, 9, 12) using a cold metal transfer process.

Description

Method of coating piston engine component and piston engine component

Technical field of the invention

The present invention relates to a method of coating a piston engine component in accordance with claim 1 . The invention also concerns a piston engine component as defined in the other independent claim.

Background of the invention

Gas exchange valves of piston engines are subjected to harsh conditions. Temperatures in combustion chambers are high and exhaust gases can be corrosive. A gas exchange valve comprises a valve stem and a valve head. The valve head has a contact surface, which cooperates with a valve seat. The contact surfaces of the valve heads and the valve seats are subjected to wear caused by friction between the surfaces. Wear of the contact surfaces can lead to leakages, which can cause various problems, such as incomplete combustion, power loss and increased emissions. Wear of the contact surfaces is increased when cleaner fuels with worse lubricating properties are used.

Wear problems have been addressed by different coatings. One known technique is to apply a coating on a valve head or a valve seat using Plasma Transferred Arc (PTA) welding technique. A problem with the PTA welding is that it is difficult to achieve coatings with uniform composition. In addition, the PTA welding is not suitable for certain materials having limited weldability.

Summary of the invention An object of the present invention is to provide an improved method of coating a piston engine component. The characterizing features of the method according to the invention are given in claim 1 . Another object of the invention is to provide an improved piston engine component. The characterizing features of the piston engine component are given in the other independent claim. In the method according to the invention, a coating matrix of a cobalt- or nickel- based alloy with solid lubricating particles embedded in the coating matrix is applied onto a surface of the component using a cold metal transfer process.

The piston engine component according to the invention has a coating depos- ited on the component by a cold metal transfer welding process, the coating comprising a coating matrix made of a cobalt- or nickel-based alloy and solid lubricating particles embedded in the coating matrix.

With the method according to the invention, coatings with uniform composition and distribution of the solid lubricating particles can be achieved with mini- mized dilution of the parent material. A piston engine component according to the invention has a coating with uniform composition and distribution of the solid lubricating particles.

According to an embodiment of the invention, the solid lubricating particles comprise sulfides, sulfates, silicides and/or fluorides. According to an embodiment of the invention, hard carbides are embedded in the coating matrix.

According to an embodiment of the invention, the coating material is introduced into the coating process in the form of a flux-cored wire, where the solid lubricants are mixed in the core part of the wire. The solid lubricating particles can comprise a nickel- or cobalt-coating. The coating reduces evaporation and disintegration of the compounds during the coating process.

According to an embodiment of the invention, additional heat is introduced into the coating process simultaneously with the cold metal transfer. Certain mate- rials with limited weldability may require additional heating for preventing cracking. Additional heat can also increase the deposition rate of the coating.

According to an embodiment of the invention, the additional heat is introduced into the process by means of induction and/or laser.

The piston engine component can be a gas exchange valve or a valve seat for a gas exchange valve. Brief description of the drawings

Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which Fig. 1 shows a simplified cross-sectional view of a cylinder head and a cylinder of a piston engine,

Fig. 2 shows an enlarged view of contact surfaces between a gas exchange valve and a valve seat, and

Fig. 3 shows a schematic cross-sectional view of CMT welding equipment.

Description of embodiments of the invention

Figure 1 shows a simplified cross-sectional view of a cylinder head 1 and a cylinder of a piston engine. The engine is a large internal combustion engine, such as a main or an auxiliary engine of a ship or an engine that is used at a power plant for producing electricity. The engine comprises a plurality of cylinders. Each cylinder is formed by a cylinder liner 2, which is arranged in an engine block 3. The cylinder liner 2, the cylinder head 1 and a piston 4 delimit a combustion chamber 5. The piston 4 is arranged to move in a reciprocating manner in the cylinder. The engine comprises an intake duct 6 for introducing air into the combustion chamber 5 and an exhaust duct 7 for discharging exhaust gases from the combustion chamber 5. Each cylinder of the engine is provided with at least one intake valve 8 and with at least one exhaust valve 9. The intake valve 8 is used for opening and closing fluid communication between the intake duct 6 and the combustion chamber 5. The exhaust valve 9 is used for opening and closing fluid communication between the combustion chamber 5 and the exhaust duct 7. The intake valve 8 and the exhaust valve 9 are hereinafter also referred to as gas exchange valves 8, 9. The gas exchange valves 8, 9 can be operated by means of a camshaft, hydraulic actuators, electro-hydraulic actuators or using any other similar means. In the example of figure 1 , the engine is a gas engine, where gaseous fuel is introduced into the intake duct 6 by means of a gas admission valve 1 1 and ig- nited in the combustion chamber 5 by means of a spark plug 10. However, the present invention could also be used in connection with any other type of piston engines. The engine could thus be a compression ignition engine and it could be operated using any gaseous or liquid fuel or a combination of gase- ous and liquid fuels, such as gaseous main fuel and liquid pilot fuel.

Each gas exchange valve 8, 9 comprises a valve head 8a, 9a and a valve stem 8b, 9b. The valve head 8a, 9a cooperates with a valve seat 12. The valve seat 12 can be an integral part of the cylinder head 1 . Alternatively, the valve seat 12 can be a separate insert arranged in the cylinder head 1 . Figure 2 shows an enlarged view of the exhaust valve 9 of figure 1 . However, the valve of figure 2 could also be an intake valve 8. When the gas exchange valve 8, 9 is closed, a contact surface 13 of the gas exchange valve 8, 9 lies against a contact surface 14 of the valve seat 12. Fluid communication between the combustion chamber 5 and the intake duct 6 or the exhaust duct 7 is thus pre- vented.

The opening and closing movement of each gas exchange valve 8, 9 is repeated once during each engine cycle and sometimes even twice, for instance if the exhaust valve 9 is opened for exhaust gas recirculation. This causes wear of the contact surfaces 13, 14 of the valve seat 12 and the gas exchange valves 8, 9. The wear problem is prominent in particular when clean fuels with weak lubricating properties are used. A known method for reducing the wear of the contact surfaces 13, 14 is to apply a coating on the contact surfaces 13, 14. A known method for coating the contact surfaces is the use of a Plasma Transferred Arc (PTA) welding technique. In the PTA welding, the coating is deposited on a work piece by means of a plasma arc that is formed between the work piece and an electrode. The coating material can be introduced onto the work piece via the plasma torch used in the welding process or alternatively it can be applied on the work piece as slurry. A problem with the PTA welding is that it is difficult to achieve coatings with uniform composition. In addi- tion, the PTA welding is not suitable for certain materials, such as superalloys, having limited weldability. Furthermore, the heat input of the PTA welding can deform the work piece and change the properties of the coating.

In the coating method according to the invention, the coating is applied on a piston engine component using cold metal transfer (CMT) welding. The coating comprises a coating matrix, which is made of a cobalt- or nickel-based alloy. Solid lubricating particles are embedded in the coating matrix. The solid lubricating particles can be sulfides, sulfates, silicides and/or fluorides. Some examples of suitable materials for the solid lubricating particles are CaF₂, WS₂ and MoS₂. In addition to the solid lubricating particles, hard carbides can be embedded in the coating matrix. The hard carbides increase the hardness of the coating and also contribute to the lubricating effect. The solid lubricating particles can comprise a nickel- or cobalt-coating. The coating can be applied to the particles electrolytically. The coating reduces evaporation and disintegration of the particles during the CMT welding. The component can be a gas exchange valve 8, 9 or a valve seat 12. CMT welding is an arc welding process. Figure 3 shows a simplified view of CMT welding equipment. In CMT welding, a consumable wire 15 acts both as an electrode and filler metal. According to an embodiment of the invention, the wire 15 is a flux-cored wire, where the solid lubricants are mixed in the core part of the wire 15. The wire 15 is fed by a wire feeding device 16. An electric arc is formed between the electrode 15 and the work piece, which can be a gas exchange valve 8, 9 or a valve seat 12. The heat produced by the electric arc melts the work piece 12 such that a weld pool is formed. The wire 15 moves towards the weld pool when the electric arc is present. As the wire 15 touches the weld pool, the electric arc is extinguished and the welding current is reduced. The wire 15 starts moving backwards, which helps in removing a blob that is formed at the end of the wire 15. The short circuit current is kept low. When the wire 15 is separated from the blob, electric arc is formed again and the process is repeated. In the CMT process, the temperature is signifi- cantly lower than in conventional welding processes. The solid lubricants have a low melting point, and the lower heat input of the CMT welding reduces evaporation of the lubricating materials. Also deformations of the work piece are reduced. A coating with uniform distribution of the solid lubricating particles is thus achieved. The CMT process is also suitable for coating super alloys and other materials with limited weldability.

Additional heat can be introduced into the coating process simultaneously with the cold metal transfer. For example induction and/or laser can be used for introducing the additional heat. The need for additional heating depends on the materials used. In some cases, the additional heating can be used for increas- ing the coating deposition rate. The additional heat may also be needed for certain materials with limited weldability to prevent cracking. However, in many cases the coating process can be performed also without any additional heating.

It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims.

Claims

1 . A method of coating a piston engine component (8, 9, 12), wherein a coating matrix of a cobalt- or nickel-based alloy with solid lubricating particles embedded in the coating matrix is applied onto a surface of the component (8, 9, 12) using a cold metal transfer process.

2. A method according to claim 1 , wherein the solid lubricating particles comprise sulfides, sulfates, silicides and/or fluorides.

3. A method according to claim 1 or 2, wherein hard carbides are embedded in the coating matrix.

4. A method according to any of claims 1 to 3, wherein the coating material is introduced into the coating process in the form of a flux-cored wire (15), where the solid lubricants are mixed in the core part of the wire (15).

5. A method according to any of the preceding claims, wherein the solid lubricating particles comprise a nickel- or cobalt-coating.

6. A method according to any of the preceding claims, wherein additional heat is introduced into the coating process simultaneously with the cold metal transfer.

7. A method according to claim 6, wherein the additional heat is introduced into the process by means of induction and/or laser.

8. A piston engine component (8, 9, 12) having a coating deposited on the component (8, 9, 12) by a cold metal transfer welding process, the coating comprising a coating matrix made of a cobalt- or nickel-based alloy and solid lubricating particles embedded in the coating matrix.

9. A component (8, 9, 12) according to claim 8, wherein the solid lubricating particles comprise sulfides, sulfates, silicides and/or fluorides.

10. A component (8, 9, 12) according to claim 8 or 9, wherein the coating comprises hard carbides.

1 1 . A component (8, 9, 12) according to any of claims 8 to 10, wherein the component is a gas exchange valve (8, 9) or a valve seat (12) for a gas ex- change valve (8, 9).