The present invention relates to an exhaust valve spindle for an internal combustion engine, particularly a two-stroke crosshead engine, which exhaust valve spindle comprises a valve head with a base portion of an alloyed steel, and an outer facing forming the surface of the valve spindle towards a combustion chamber, which outer facing has been formed from particulate starting material of a hot-corrosion-resistant alloy being nickel- based, chromium-based or cobalt-based, where said particulate starting material has been united to a coherent layer.

US patent 6,173,702 describes a known exhaust valve spindle of this kind where the corrosion-resistant outer facing is provided onto the base portion by powder-metallurgy processing where particulate ma- terial of the corrosion-resistant alloy is placed in a mould on the base portion and unified with the latter in a HIP process (Hot Isostatic Pressure). The mould is evacuated in order to remove as much air or gas as possible. The HIP process is performed in a chamber that can be both heated and set under pressure. In order to utilise the chamber in an effi- cient manner it is filled with as many base portions or other parts as possible, and these objects are all subjected to the same HIP treatment within the chamber. When the treatment is initiated the chamber is heated and pressurised to HIP conditions, and these conditions are then maintained for the required period, typically at least 8 to 12 hours.

During the HIP treatment the pressure influences the particulate material as isostatic pressure (completely even pressure in all directions), and the volume of the particulate material is reduced evenly in all directions as it is compacted onto the base portion. In the resulting mi- crostructure of the outer facing the particles are seen to remain shaped as spheres with circular outlines when viewed in a ground and polished sample taken out from the completed valve spindle. In the drawings Figs. 1 and 10 are photographs from such samples.

The lower valve head surface has a large area and is therefore exposed to considerable heat stresses e.g. when the engine load is changed, and especially when the engine is started or stopped. The heat impact is heaviest at the centre of the surface, partly because the combustion gases have the highest temperatures near the middle of the combustion chamber, partly because the valve head is cooled near the outer rim where the seat area on the upper surface is in contact with the water-cooled stationary valve seat while the valve is closed. The colder peripheral material prevents thermal expansion of the hotter central material, and thus causes considerable heat stresses. The slowly varying, but large heat stresses caused by said thermal influences place very high demands for strength and quality of the outer facing.

The HIP process is known to provide a microstructure of high quality and fine coherency of the outer facing, but the HIP process is very time consuming, and the long process time at elevated temperature may also cause undesirable metallurgical processes, like diffusion of a component from the one alloy to the other alloy.

The present invention aims at obtaining high strength of the outer facing and at obtaining a microstructure in the outer facing with a strong structure, in particular near the transition zone to the base portion.

With a view to this the exhaust valve spindle according to the present invention is characterized in that at least at the transition zone to the base portion the particles in the particulate material of the outer facing have been deformed into oval or elongate shapes by shear strain caused by forging the outer facing and the base portion, and that the forged outer fac- ing has a density of at least 98.0 %.

The shear strain induced by forging causes displacement of powder particles in relation to other powder particles so that the particles rub against each other and penetrate oxide film layers that may be present on the surface of the particles. Any oxide film layer will be thin because the particulate material is typically manufactured by atomisation in an oxygen-free gas, however during storage some oxides will unavoidably form on the particles. The shear strain deforms particles into non-spherical shapes that may be characterised as oval or elongate shapes. During the forging the particulate material is compressed into a dense layer, and the particles unite into a coherent material bonded to the adjacent layer, which is the base portion when the particulate material is located directly on the base portion. A density of at least 98.0% can also be expressed as a porosity of at the most 2%.

The shear strain induced by the forging causes the particulate material to flow at least in radial directions, these radial directions being perpendicular to the axial direction of the exhaust valve spindle, and thus these radial directions being in parallel with the lower surface of the valve head and in parallel with the substantially planar transition zone between the material of the outer facing and the material of the base portion . The shear strain and the resulting radial movements within the material near the transition zone ensures creation of effective bonds between the materials and in conjunction with the extremely effective manner of rubbing particles against one another during the deformation, the resulting microstructure will have only a very low number of inner failure points within the microstructure. The binding together of the materials in the transition area thus has a strong microstructure and it is possible to completely dispense with geometrical locking between the outer facing and the base portion.

In a preferred embodiment, the transition zone between the outer facing and the base portion extends along at least one straight plane in the area extending from inside the rim area of the valve head to the central area of the valve head. In the rim area the outer facing can have an enlargement that extends upwards and encloses the base portion along the periphery so that the outer facing in itself is pan-shaped with an upstanding wall at the periphery. At a distance from the rim the transition zone begins to extend along a straight plane towards the middle of the valve head, and this plane extension brings the advantage that there is no abrupt change of thickness or shape of the outer facing and thus there are no large local variations in how the particulate material in the outer facing is treated during the forging. Near the planar transition zone the particulate material is subjected to substantially the same treatment within a local area, and as the one or more planar areas extend from inside the rim area to the central area they cover the majority of the area of the valve head. It is possible that the outer facing is located directly onto the base portion. As an alternative, at least one buffer layer of an alloy is located in between the base portion and the outer facing. When such a buffer layer is used, the alloy of the buffer layer is of a third alloy having a composition dif- ferent from the alloyed steel of the base portion and different from the hot- corrosion-resistant alloy of the outer facing. The difference in composition means that the analysis of the alloy of the buffer layer differs in alloying components or in the amounts (in percentage of weight) of one or more of the alloying components. The buffer layer may e.g. be an alloyed steel with a different amount of carbon or different amounts of other components, such as chromium, iron or nickel. The term composition is thus to be taken to mean the analysis of the alloys. The location of the buffer layer in between the base portion of alloyed steel and the outer facing has the effect that the alloyed steel is directly in contact with only the material of the buffer layer and not with the corrosion-resistant alloy of the outer layer. The buffer layer acts to reduce or to prevent diffusion of alloying components from the outer facing to the base portion, and vice versa.

Preferably, the buffer layer is selected from the group comprising steel, austenitic steel, a nickel-based alloy, and an alloy which, apart from unavoidable impurities, is of Fe or Ni. These alloys are considered compatible both with the steel of the base portion and with the alloy of the outer facing.

In an embodiment the alloyed steel of the base portion is an austenitic, stainless steel. For years it has been accepted to make entire valve spindles out of the alloy NIMONIC 80 A (registered trade mark, Special Metals). A special alloy like this is, however, not so easily available as austenitic, stainless steel, and the stainless steel also has high strength and is considered all in all to perform very well, especially in two-stroke crosshead engines, provided the corrosion resistance can be improved above the capa- bilities of stainless steel on the surface facing the combustion chamber. The stainless steel, however, has a rather high content of carbon. The buffer layer absorbs diffused carbon, so that the advantages of utilizing stainless steel for the major part of the exhaust valve are not impaired by the high requirements for hot-corrosion resistance and for long-term ductility of the completed exhaust valve.

In a preferred embodiment the buffer layer has a thickness of at least 2 mm. This thickness is sufficient to ensure that carbon cannot diffuse across the buffer layer, even when the buffer layer is of an alloy exhibiting carbide formation abilities where the carbon diffusing into the layer can be converted into carbides and thus not cause an increase in the carbon- activity of the layer.

The present invention also relates to a method of manufacturing an exhaust valve spindle for an internal combustion engine, which exhaust valve spindle comprises a valve head with a base portion of an alloyed steel, and an outer facing forming the surface of the valve spindle towards a combustion chamber, which outer facing is formed from particulate starting material of a hot-corrosion-resistant alloy being nickel-based, chromium-based or cobalt-based.

According to the present invention the method is characterized in that, while the particulate material of the hot-corrosion-resistant alloy is held in an enclosure at the base portion the particulate material is heated to forging temperature and forged, whereby the particulate material is subjected to shear strain that deforms the particles into elongate or oval shapes, said forging compacting the particulate material to a density of at least 98.0% and uniting the outer facing with the base portion or with a buffer layer and the base portion.

The forging occurs very quickly in comparison to a HIP-treatment, and alloying components thus have only short time for diffusion from the one alloy to the adjacent alloy while the valve parts are at the elevated forging temperature. As described in the above, the forging presses the particulate material together and the shear strain moves particles in directions parallel with the transition zone and makes the particles in the particulate material rub against one another and merge. During the movement, rubbing and merging any oxide films initially present on the particles break up and clean alloy material from grains inside any one particle is brought into direct contact with clean alloy material from grains inside other particles, and the grains can thus effectively connect at the microstructure level.

In one example the particulate material in the enclosure is provided in a layer of substantially even thickness in the area extending from inside the rim area of the valve head to the central area of the valve head. When the layer of particulate material is of substantially even thickness in the enclosure and substantially even forging conditions are applied, the resulting outer facing will have a substantially even thickness. The transition zone between the outer facing and the base portion is thus extending along at a single straight plane in the radial direction of the valve head.

In another example the particulate material in the enclosure is provided in a layer of increasing thickness towards the longitudinal centre axis of the valve in the area extending from inside the rim area of the valve head to the central area of the valve head. As the lower surface of the valve head is usually a flat surface, the lower surface of the base portion can be made with a recessed central area that allows for larger thickness of the outer facing. The transition zone between the outer facing and the base portion may thus extend along several straight planes or the transition zone may have a slightly curved shape. The increased thickness of the outer facing at the central area of the lower surface of the valve head provides the exhaust valve with a long life in comparison to the spent amount of particulate material, because during use the highest temperature impact and the highest loss of material during operation occur at the central area of the lower surface of the valve head. If the exhaust valve spindle is for use in an engine where the loss of material is expected to be largest closer to the rim area or in other local areas the outer facing may alternatively have increased thickness in direction of the rim area or increased thickness at such local areas.

A further method according to the present invention is characterized in that the particulate material of the hot-corrosion-resistant alloy is hot sprayed in an inactive atmosphere to build a pre-formed part, that the preformed part and the base portion are heated to forging temperature and forged whereby the particulate material is subjected to shear strain that de- forms the particles into elongate or oval shapes, said forging compacting the particulate material to a density of at least 98.0% and uniting the outer facing with the base portion or with a buffer layer and the base portion.

With this method the particulate material is firstly shaped into a pre-formed part of sufficient stability of shape to allow the particulate mate- rial to be located on the base portion as a single body. It is even possible to spray the particulate material directly onto the base portion . In case the particulate material is without interconnected porosities it is possible to avoid using an enclosure. When an enclosure is used, it has to be machined off after completed forging . Although the particulate material in the pre-built part will have irregular shapes before the forging, the forging causes the same effects as described in connection with the first-mentioned method, but the resulting deformed particles will have quite irregular shapes.

Whatever method is utilized it is preferred that prior to the forging, the material of the outer facing is evacuated to a pressure of less than 1 x 10 ^"4 bar. The evacuation removes gasses from voids within the particulate material to be forged, and this facilitates the compression of the material . Although gasses present in the material of the outer facing are typically oxygen-free, such as inert gasses, it is still an advantage to have as little gas present as practically possible. Consequently it is preferred that the material of the outer facing is evacuated to a pressure of less than 1 x 10 ^"7 bar.

If a buffer layer of a third alloy is to be used, this third alloy has a composition different from the alloyed steel of the base portion (the first alloy) and different from the hot-corrosion-resistant alloy of the outer facing (the second alloy) . The third alloy is preferably applied to the surface of the base portion before the material of the outer facing is located at said surface of the base portion . The third alloy can alternatively be applied to the material of the outer facing . The surface of the base portion is, however, normally a regular and smooth surface onto which the third alloy can be applied in very well controlled manner, in particular well controlled as to the amount and to the spreading out of the material in an even manner.

It is possible to make the forging in more than one step in order to obtain the valve head with the outer facing . Following the forging step that unites the base portion and the outer facing, the united base portion and outer facing can be forged in at least one subsequent step in order to obtain the final shape. This may e.g . be advantageous if the base portion and the outer facing are having a cylindrical shape when united in the first forging step, following which the head portion of the valve is forged in a subsequent step. In order to reduce the diffusion across the transition zone the forging is preferably carried out in less than 10 minutes, and the base portion with the outer facing are cooled immediately after the forging.

In a further development of the method a valve shaft portion is friction welded onto the base portion after completed forging. One advantage of this is that the amount of material that has to be heated to forging temperature is smaller when the valve shaft portion is not present. Another advantage is that the base portion can be completely enclosed and supported in the die on the side facing away from the outer facing instead of having a shaft portion extending to that side.

The forged valve head, or the complete valve with valve head and shaft, can optionally be subjected to a final heat treatment, such as tempering or annealing. The heat treatment may cause diffusion of alloying components at transition zones and can strengthen the metallurgical bonding between materials.

Examples of embodiments according to the present invention are in the following described in more detail with reference to the highly schematic drawings, on which

Fig. 1 is a photograph in a microscope of a ground and polished sample taken out from a valve spindle where an outer facing has been provided by a prior art HIP treatment,

Fig. 2 illustrates a cross-sectional part view of an exhaust valve spindle in form of an exhaust valve according to the invention,

Figs. 3 and 4 are schematic illustrations of forging of a valve head according to the present invention,

Fig. 5 is an illustration of a valve head and a valve shaft according to the present invention,

Figs. 6 and 7 are photographs in a microscope of a ground and polished sample taken out from a valve spindle where an outer facing has been provided according to the present invention,

Figs. 8 and 9 are top view and side view, respectively, of a test sample,

Fig. 10 is a photograph in a microscope of a ground and polished sample taken out from a valve spindle where an outer facing has been provided by a prior art HIP treatment, and

Fig. 11 illustrates particulate material held in an enclosure prior to forging in accordance with the method of the present invention.

In Fig. 1 and in Fig. 10 the sample has been taken from a HIP compacted particulate material and the circular shapes from cut through particles can be seen. This shows that the particles retain their spherical shape during the compacting. It is a typical sign of the HIP compacting that the particles are spherical, and this is a result of the isostatic pressure applied during compacting. The isostatic pressure makes the par- ticulate material shrink in a manner where the particles are not moved around within the material during the process. It is a very orderly process where the mutual positions among particles are maintained. In order to more clearly discern the microstructure of the prior art three circles have been added to the photograph in Fig. 10 so as to outline three of the particles appearing in the photograph.

Fig. 2 illustrates in schematic form an exhaust valve spindle 1 for an exhaust valve for a two-stroke crosshead engine. The left half of the valve is seen from the exterior, and the right half of the valve is seen in cross-section visualising an example of the location and extent of an outer facing 5. The valve spindle comprises a valve shaft 2, of which only a lower section is seen in Fig. 2, and a valve head 3 with a base portion 4 and the outer facing 5. The axial direction of the exhaust valve is illustrated by arrow A pointing in direction of the line C extending at the centre of the shaft, and arrow R illustrates a radial direction being perpendicular to the axial direction.

A valve seat 6 at the upper surface of the valve head 3 is manufactured in a hot-corrosion-resistant alloy suitable for counteracting the formation of dent marks on the sealing surface of the seat. Such seating alloys are well-known and described, inter alia, in Applicant's WO 97/47862 which is hereby incorporated by reference in relation to seating alloys.

The outer facing 5 on the valve head is a layer of hot-corrosion resistant material counteracting the burning off of material from the exhaust valve and forming a surface 7 facing downwards towards the com- bustion chamber, when the exhaust valve spindle is mounted in the engine. The hot-corrosion resistant material is formed of particulate starting material of an alloy which is nickel-based, chromium-based or cobalt-based . When in operation in the engine the exhaust valve spindle is moved, at suitable times of the engine cycle, between a closed position, where the valve seat 6 abuts a stationary valve seat in the exhaust valve housing (not shown), and an open position, where the exhaust valve spindle 1 has been moved downward and the valve seat 6 is at a distance from the stationary valve seat. The exhaust valve spindle 1 to- gether with the cylinder liner and a cylinder cover (not shown) define the combustion chamber of the internal combustion engine and are thus exposed to the hot and aggressive environment occurring at combustion .

The internal combustion engine utilizing the exhaust valve spindles may be a four-stroke engine or a two-stroke crosshead engine. The two-stroke engine may be of the make MAN Diesel, such as of the type MC or ME, or may be of the make Wartsila, such as of the type RTA of RTA-flex, or may be of the make Mitsubishi. For such two-stroke cross- head eng ines the diameter of the piston may range from 250 to 1100 mm, and the outer diameter of the valve head may range from 120 to 600 mm, and typically be at least 170 mm .

From these dimensions it is clear that the surfaces of the exhaust valve spindles facing the combustion chamber have large areas, which give rise to large heat stresses in the outer facing 5 and in the interface areas between the outer facing and the base portion, respectively. In the embodiments of the present invention the outer facing 5 is strong ly connected to the base portion 4 in the planar area extending from the centre of the exhaust valve spindle towards the rim area of the valve head.

The exhaust valve spindle 1 can also be exploited in smaller engines, for example four-stroke engines of the medium or high-speed type, but the exhaust valve spindle is especially applicable in the two- stroke crosshead engines, which are large engines where the loads are heavy and the requirement of continuous operation without failure is dominant.

In one embodiment the outer facing 5 is applied directly onto the surface of the base portion 4. In another embodiment of the exhaust valve spindle, the one from which the specimens photographed in Figs. 6 and 7 have been taken, a buffer layer 9 is located between base portion 4 and outer facing 5. The buffer layer 9 may apart from unavoidable im- purities be a layer of substantially pure nickel applied to the surface of the base portion. The nickel layer can be applied to the surface in different manners, such as being provided as particulate material placed on top of the base portion. The nickel layer can also be provided in a separate procedure prior to placing the particulate material of the outer fac- ing on top of the buffer layer. In such a separate procedure the base portion can be placed in a galvanic bath and the nickel be deposited by nickel electroplating forming a layer having a thickness in the range from 30 to 150 Mm, preferably 30 to 70 Mm . The electroplated layer has the advantage of being a very dense layer of pure nickel . The electroplated layer has a metallurgical bonding to the base portion.

In another embodiment the buffer layer is, apart from unavoidable impurities, of Fe. One advantage of making the buffer layer of pure, or almost pure, iron or nickel is that the buffer layer has no or only very small amounts of carbide formants. When this is the case, the formation of car- bides in the buffer layer is suppressed, and diffusion of carbon into the buffer layer increase the carbon-activity in the buffer layer and thus further diffusion of carbon into the layer will be resisted. Carbon only has very small solubility in iron and nickel. As an example, the solubility of carbon in nickel at a temperature of 500°C is less than 0.1 % by weight, so when even small amounts of carbon has diffused into the buffer layer, the buffer layer will obtain a carbon-activity of 100% and thus virtually prevent further diffusion of carbon into the layer.

As another example the buffer layer 9 may be of steel or aus- tenitic steel. The buffer layer may be a plate of steel. As a more specific example, base portion 4 is of forged valve steel (SNCrW - Alloy 1 in Table 1), the outer facing 5 is of Alloy 671, and the plate of steel is of the alloy W.-No. 1.4332 selected from the alloys of Table 2. As another example the buffer layer 9 can be provided as particulate material of alloy UNS S31603 and the outer facing 5 be of particulate material of Alloy 671. The base portion 4 is of forged steel. In this case, both the particulate material of the buffer layer and the particulate material of the outer facing are united into a coherent material on the base portion 4 during the forging.

As an alternative embodiment the buffer layer can be of a nickel- based alloy. An alloy of this type is particularly suitable for binding well with the alloy of the outer facing, and it may have a content of chromium that is considerably lower than the outer facing, such as a chromium content of less than 25 % by weight, such as the alloy IN 625 having from 20 to 23 % chromium, the alloy INCOLOY 600 having from 19 to 23% chromium, or the alloy IN 718 having 10 to 25 % chromium, or the alloy NIMONIC Alloy 105 having about 15% chromium, or the alloy Rene 220 having from 10 to 25 % chromium. The buffer layer may also be of a more nickel-rich alloy as nickel in larger amounts has a tendency to prevent diffusion of carbon.

The particulate material may be manufactured in several different manners which are well-known in the art. The particulate materials may, for example, have been manufactured by atomisation of a liquid jet of a melted alloy of the desired composition into a chamber with an inactive atmosphere, whereby the material is quenched and solidifies as par- tides with the very fine dendritic structure. The particulate material may also be called a powder.

The particulate material may alternatively be manufactured by atomisation of a liquid jet of a melted alloy of the desired composition into a chamber with an inactive atmosphere, where the spray of atom- ised particles are directed so as to hit and deposit on a solid part. The solid part may be cooled and in this instance the particles build up a preformed part which is separate from the solid part. The particles may alternatively bind to the solid part, and a base portion 4 be used as such, so that the pre-formed part is bonded directly to the base portion.

Suitable materials for the base portion 4 comprise stainless steels. Examples of such materials are given in the following Table 1. The W.-No. is the German standard number for the alloy. The percentages stated are percentages by weight.

Table 1 W.-No. C Si Mn Cr Ni Other Balance

Alloy 1 0.25% 1.4% 1.3% 20% 9% 3% W Fe

- 0.35% 2.5% 0.8% 11.5% - 1% Mo Fe

1.4873 0.43% 2.3% 1.2% 18% 9% 1% W Fe

1.4718 0.45% 3.2% 0.4% 9% - - Fe

1.4871 0.52% - 9% 20.8% 3.9% 0.45% N Fe

1.4747 0.81% 2% - 19.5% 1.4% - Fe

Suitable materials for the optional buffer layer comprise steels as exemplified in the following Table 2. The W.-No. is the German standard number for the alloy. The percentages stated are percentages by weight.

Table 2

Another suitable material for the buffer layer is the alloy UNS S31603 comprising 0.5 - 1.0 % Mn, 16.5 - 18% Cr, 11.5 - 14% Ni, 2.5 - 3.0% Mo, 0 - 0.1% N, 0 - 0.025% O, 0 - 0.03 % C, and the balance Fe. When the buffer layer is of plate material then there are normally not any requirements to the contents of nitrogen and oxygen. However, when the buffer layer is of particulate material then it is preferred that the content of nitrogen is at the most 0.1% and preferred that the con- tent of oxygen is at the most 0.03%.

Suitable materials for the outer facing are well known in the art of exhaust valves, and examples are Stellite 6, an alloy of the type 50% Cr and 50% Ni, an alloy of the type IN 657 comprising 48-52% Cr, 1.4- 1.7% Nb, at the most 0.1% C, at the most 0.16% Ti, at the most 0.2% C+N, at the most 0.5% Si, at the most 1.0% Fe, at the most 0.3% Mg, and a balance of Ni. Another example is an alloy having the composition 40 to 51% Cr, from 0 to 0.1% C, less than 1.0% Si, from 0 to 5.0% Mn, less than 1.0% Mo, from 0.05% to less than 0.5% B, from 0 to 1.0% Al, from 0 to 1.5% Ti, from 0 to 0.2% Zr, from 0.5 to 3.0% Nb, an aggregate content of Co and Fe of maximum 5.0%, maximum 0.2% 0, maximum 0.3% N, and the balance Ni. Other suitable facing alloys for use as the outer facing are given in the article "Review of operating experience with current valve materials", published in 1990 in the book "Diesel engine combustion chamber materials for heavy fuel operation" from The Institute of Marine Engineers, London.

The forging is prepared by placing the base portion 4 of the valve head at the forging location and applying the buffer layer 9, if any, to the surface of the base portion. The particulate material of the outer facing 5 can be provided in several different manners. In one example illustrated in Fig. 11, the outer facing is provided as particulate material held in an enclosure 12 at the base portion 4 while the base portion with enclosure and the particulate material is arranged in a die part as a preparation for the forging. The arrangement of the enclosure 12 and the particulate material on the base portion can be effected in several different manners. The enclosure may be welded onto the base portion and be provided with a pipe stud, which is used for filling particulate material into the enclosure and then used to connect vacuum equipment and then closed off prior to forging. Alternatively, the enclosure 12 is fixed to the base portion 4 after the particulate material has been deposited within the enclosure. This fixing may be by use of welding, or as another example by vacuum brazing. As a further alternative the enclosure is fixed to the base portion, and subsequently the particulate material is filled into the enclosure, and finally brazing is performed. When using vacuum brazing the enclosure may be cup-shaped and provided with a coarse threading at the inside, which threading matches external treading on the base portion. Solder is provided on the threading. Then the heating and fixing may take place in a vacuum oven. In another example the particulate material of the outer facing 8 is provided as a pre-formed part that is located on the base portion 4 as illustrated in Fig. 3. The formation of such a pre-formed part is described in further detail in the following.

Prior to forging the base portion 4 with the particulate material of the outer facing 5, and possibly the buffer layer 9 and the enclosure 12, are heated to forging temperature which preferably is in the range from a temperature of 950°C to 1100°C. The heated parts are introduced into a forging press having a lower die part 10 and an upper die part 11 and a drive mechanism which may be mechanically driven or hydraulically driven. The drive of the forging press displaces the one die part towards the other die part, and the material held within the die part is mechanically deformed during this displacement. The forces required in order to perform the forging depend on the size of the valve head. For a valve head diameter of about 490 mm an efficient forging operation may be carried out with a forging press capable of delivering a compressive force in the range of about 250 to 400 MN. For smaller diameter valve heads the utilized compressive forces may be smaller, such as 35 MN for an exhaust valve having a valve head diameter of 150 mm. The forging operation is preferably carried out within 10 minutes, and more preferably within 3 minutes. During the forging the particulate material of the outer facing 5 is compacted, typically so that the thickness of the outer facing is reduced to from 30 to 70% of the initial thickness of the particulate material. If a dense pre-formed part is used the density can be rather high prior to the forging, and in this instance the thickness of the outer facing may be reduced to from 30 to 95% of the initial thickness of the particulate material. The particulate material is reduced in thickness so that the resulting density of the outer facing is at least 98.0%. When compacted to this degree by using forging, the particulate material has obtained a suitable density. It is of course more preferable to further compact the particulate material, such as to a density of at least 99.0% or even better to a density of at least 99.5%, and most preferable to compact it to 100% density.

During the forging the particulate material is subjected to shear strain that makes the particles shift position and deforms the material . Strain is a geometrical measure of deformation representing the relative displacement between particles in the material . The shear strain causes particles to shift place and it deforms particles when the particles interact. Shear strain is acting parallel to the surface affected by the forging . The forging affects the surface 7 in the axial direction of the exhaust valve spind le, wh ich is perpend icu la r to th is su rface, a nd the shear strain thus acts in parallel with this surface, which is in the radial direction of the exhaust valve spindle. During the compacting of the outer layer, the shear strain displaces particles in the rad ial d irection and causes the particles to rub against each other and force the particles to be deformed into non-spherical shapes, such as oblong shapes, oval shapes or irregular shapes. When forging is completed the forged valve head is removed from the dies and air cooled or cooled in another manner.

It is preferred that the amount of effective strain in the material of the outer facing is at least 0.3. The effective strain is calculated in the traditional manner disclosed in basic textbooks, such as in "Manufacturing engineering and technology" by Kalpakjian and Schmid, 5 ^th edition, Prentice Hall, year 2006, or i n "Formelsa mg l ing I Ha l lfasthetslare" by Gert Hedner, publication 104, Royal Swedish Technical University, Stockholm, year 1978 on pages 222-223. Even more preferably the effective strain is at least 0.4. This ensures a very effective and strong bonding between the particles of the outer facing and the material of the base portion or the buffer layer.

A first method of manufacturing the exhaust valve spindle is described in the following . The exhaust valve spindle has a valve head with a base portion of alloyed steel, and an outer facing forming the surface of the valve spindle towards a combustion chamber. The base portion of alloyed steel is prepared, such as by forging the material into suitable shape. A particulate starting material for forming the outer facing is prepared . The material is of a hot-corrosion-resistant alloy. The particulate starting material is enclosed within an enclosure the inside of which has the shape of the outer facing . The enclosure is in other words prepared for being removed after the valve head has been forged. While the particulate material of the hot- corrosion-resistant alloy is held in the enclosure at the base portion, the particulate material and the base portion are heated to forging temperature and positioned within one of the die parts, typically the lower die part 10. Then the material is forged, whereby the particulate material is subjected to shear strain that deforms the particles into elongate or oval shapes. At the same time the particulate material is compacted to a density of at least 98.0% and united to the base portion or to a buffer layer and the base portion.

A second method of manufacturing the exhaust valve spindle is to hot spray the particulate material of the hot-corrosion-resistant alloy to build a pre-formed part. The preformed part can be formed directly on the base portion during the spray procedure or it can be formed separately and be located on the base portion and be heated to forging temperature. Then the pre-formed part and the base portion, and optionally also the buffer layer, can be forged into an exhaust valve part. During forging the particulate material is subjected to shear strain that deforms the particles into elongate or oval shapes, said forging compacts the particulate material to a density of at least 98.0% and unites the outer facing with the base portion or with a buffer layer and the base portion.

The hot spraying of particulate material can occur by supplying a spray dryer nozzle with molten alloy and spraying the alloy as atomised particles onto a base portion 4 where the particles partly unite, but remain in a non-dense condition. The base portion with the hot spray applied pre-formed part is heated to forging temperature, and placed in one of the dies as mentioned in the above description, and then forged to a dense condition.

It is preferred that the particulate material prepared for the outer facing is evacuated prior to the forging, in order to reduce the amount of oxygen present to the particles. In this manner the formation of oxide films on the particles is counteracted.

At the forging the outer facing 5 is compressed to smaller thickness, such as to about 25% less thickness in comparison to the initial thickness. At the same time the density of the material in the outer fac- ing increases from about 65% to close to 100%. It is preferred that the resulting density is at least 98.0%

The valve head 3 produced by any of the above mentioned methods is a valve head having an outer facing 5 at the surface directed towards the combustion chamber. The valve head may have a valve shaft, if the base portion 4 of the valve head 3 is formed integrally with a valve shaft 2, and alternatively the valve head may also be manufactured without a valve shaft 2 if that is considered more convenient. In the latter case the valve shaft has to be mounted onto the valve head af- ter completed manufacturing of the valve head. Fig. 5 illustrates a completed valve head and a valve shaft 2. These two parts can be joined by friction welding in well-known manner. At such friction welding the one part, typically the valve head, is held fixed, and the other part, like the valve shaft, is firstly rotated and then moved axially into abutment with the valve head, so that these two parts friction weld together into a single exhaust valve spindle.

The strong microstructure obtained causes a strong bonding of the materials in the transition area. According to the present invention this bonding may be tested. In order to test the strength against tearing apart the materials in shear loading, a special test specimen is prepared on basis of a sample cut out of an exhaust valve. The test specimen is shaped as illustrated in Figs. 8 and 9. The test specimen has a width w = 9.0 mm, a length I = 40.0 mm, a distance d = 25.4 mm between centres of pulling holes, a thickness t = 3.5 mm of the base portion, and a thick- ness T of the outer facing. The thickness of the outer facing is measured and set at thickness T. Then a groove gl, g2 through the entire material is cut from either side in a width of at least 2 mm and with such a mutual separation in the length direction that the resulting overlap with a binding together of the layers is less than the measured thickness t of the outer facing.

Eight examples have been carried out, and the results are presented in Table 3. It is clearly seen that the shear strength obtained is at a high level. The level is corresponding to the shear strength of a solid material. The bonding obtained in accordance with the present invention thus causes no weakening of the material .

Table 3

In a further embodiment, the particulate material of the hot- corrosion resistant alloy is mixed with particles of isolating material, like the ceramic material Zirconia (Zr0 ₂ ). The isolating material may have a higher concentration near the outer surface of the outer facing, and preferably there is no isolating material in the transition zone between the outer facing and the base portion. The particulate material of the outer facing may include from 5 to 60% by weight of isolating material, but preferably the amount of isolating material does not exceed 40% by weight of the outer facing.

It is possible to combine details of the above-mentioned embodiments into other embodiments within the scope of the patent claims. It is furthermore possible within the scope of the patent claims to make variations in the details of the above-described embodiments. The valve seat 6 may e.g. be of the same alloy as the valve head, the buffer layer 9 may end at the valve seat 6 and have an oblique or a vertical extent at the largest diameter (in the area below the valve seat 6).

Any of the above-mentioned embodiments can be subjected to a final heat treatment, such as tempering or annealing. The heat treatment may e.g. have a duration in the range from 2 to 6 hours and take place at a temperature in the range from 800 to 1050 °C. Other temperatures are also possible.

The exhaust valve spindle is an important engine part, and infor- mation for documentation of the identity and possibly manufacturing details of the specific exhaust valve spindle may be stored in a tag embedded in the exhaust valve spindle. The tag is preferably of remotely write and readable RFID-type, preferably even containing individual authenticating data providing traceability. A specific spindle may be provided with more than one tag, if desirable. The tag may be located at a location within the exhaust valve spindle, where it is adequately shielded from heat and other tag-harming parameters.