NO337143B1 - Process for producing a nozzle for a fuel valve in a diesel engine and a nozzle - Google Patents

Abstract

In a mould (13) a corrosion-resistant first alloy (10) is arranged at least in an outer area which is to constitute the outer surface of a nozzle around the nozzle bores (4). A second alloy (11) is used in another area of the nozzle. The materials in the mould are treated by isostatic pressing into a consolidated material. The boundary area between the two alloys (10, 11) is free of cracks.

Description

Method of manufacturing a nozzle for a fuel valve in a diesel engine and a nozzle.

The present invention relates to a method for manufacturing a nozzle for a fuel valve in a diesel engine, in particular a two-stroke crosshead engine. A first material of a first alloy which is resistant to corrosion is arranged in a mold at least in an outer area which is to form the outer surface of the nozzle around the nozzle bores.

Such a method is known from WO 95/24286, which describes that after filling with the first material that makes up the nozzle as a whole, the mold is subjected to a HIP treatment which results in a nozzle with extremely good properties in terms of nozzle strength and resistance to corrosion. Furthermore, a very precise geometry is achieved around the nozzle bores, which results in a good atomization of fuel. In this HIP treatment (HIP: Hot Isostatic Pressing), a fine-grained powder is consolidated into the entire die blank at high pressure and high temperature and the resulting die blank maintains an isotropic extremely fine-grained alloy structure.

EP 982493 Al describes a fuel valve with a nozzle which extends far up into the valve housing past the primary valve seat and further past the slide. These parts of the fuel valve are made of steel to give the valve seat etc. the required hardness. The lower part of the nozzle is coated with a corrosion-protective layer by means of laser welding, plasma welding or thermal powder spraying, whereby completely or partially melted material binds to the steel. The material in the bonding area has properties that can cause the corrosion-resistant alloy to peel off, either in connection with hardening or after a certain period of operation. During operation, the nozzle is exposed to large thermal cyclic loads which bring with it a risk of degradation of the booklet of the corrosion-resistant alloy.

It is an aim of the present invention to produce a nozzle with a long service life. With regard to this, a first-mentioned method for producing a nozzle according to the invention is characterized in that a second material of a different alloy is also arranged in the mold in an inner area, and that the materials arranged in this way are processed by isostatic pressing into a consolidated (uniform ) die blank free of microcracks in the adhesion area between the first alloy and the second alloy.

Despite the fact that the use of a second alloy in the die results in a change in the structure of the die, and that a change in the material or structure normally affects the long life in a negative way, the life of the die is improved. Presumably, the improved longevity is achieved because the bond area between the two alloys is free of microcracks. The consolidation (unification) of the various materials by means of isostatic pressing produces a diffusion-dependent consolidation without any areas of adhesion, especially of the type known from the application of molten material to solid material. Even if a nozzle of the type known from EP 982493 A1 is heat treated to a more uniform hardness before use, microcracks will occur in a melt-based mixing area which is thin and very hard immediately after application and in the heat-affected zone of the steel. The diffusion-dependent consolidation according to the invention forms no melt-based mixing areas with an associated heat-affected zone. Avoiding microcracks in the transition between the two different materials removes a very important source of initiation of fatigue failure in the nozzle, which reduces in a considerable improvement the lifetime of the nozzle. The diffusion-based consolidation also results in an extremely low risk of bond failure between the two different materials.

Preferably, to achieve a further improvement in lifetime, the second material of the second alloy has a higher fatigue strength than the first corrosion resistant alloy in the finished die. The fatigue strength is important for nozzle life because, in addition to the usual thermally based loads on the nozzle, a considerable increase in fatigue load is expected to occur as a consequence of the use of higher injection pressures and faster pressure variations than applied until now, with the intention of achieving a more precise control of fuel injection in the diesel engine and thus a better combustion of the fuel and reduced formation of polluting compounds.

At least when the corrosion resistant alloy contains more than 0.6% Al, an oxygen preventing diffusion barrier is preferably used between the first material and the second material before the isostatic pressing. The oxygen-preventing diffusion barrier prevents diffusion of oxygen released from the second of the alloys into the first of the alloys and reaction with alloy components or unwanted Al impurities. Oxygen can, for example, be found in a dissolved state in the second alloy, or can be released by dissolving oxides in the second alloy during heating of the materials. Even very small amounts of oxygen of only a few ppm can lead to the precipitation of aluminum oxides and/or other undesirable precipitates in the interface region between the two alloys, with resultant weakening of the overall fatigue strength of the die.

The diffusion barrier limits or prevents harmful diffusion of oxygen so that the nozzle maintains a high fatigue strength. Obviously, the diffusion barrier can also be used in cases where the alloy contains less than or precisely 0.6% Al. A positive effect of the barrier can, for example, be achieved with alloys with an aluminum content in the range 0.1 to 0.5% Al. The barrier can also be used if the alloys actually used represent a risk for metallurgical processes that result in other undesirable precipitations, such as precipitation of intermediates, carbides or oxides other than aluminum oxides. The diffusion barrier has a preventing effect on the transport of alloy components other than oxygen from one material to the other, especially small elements such as C or boron can diffuse from an alloy with a high content of the element in its free form into the alloy with a low content of the element. The use of the diffusion barrier provides greater freedom to choose components in one alloy independently of the components in the other alloy. Limiting the carbon content of the second alloy can, for example, in connection with hardening the nozzle to a high hardness, constitute the advantage that the second material has a lower tendency to form hardened cracks in areas of the nozzle with a complex geometry, such as around the nozzle bores.

Such diffusion barriers can for example be of nickel, copper or a nickel alloy since both nickel and copper are suitable for forming a dense and stable layer in connection with the corrosion-resistant alloys suitable for use as nozzle materials. The alternative can be a cobalt layer, a cobalt alloy or a chrome layer.

Fabrication of the nozzle can be suitably simplified by placing the diffusion barrier on the inner surface of a prefabricated element of the first material or on the outer surface of a prefabricated element of the second material. The prefabricated element can act as a carrier of the diffusion barrier until all the material is placed in the mold and the isostatic pressing is carried out and the materials are consolidated. At the same time, the use of prefabricated elements enables the mold to be filled quickly and easily before pressing since the element is either stacked together with another prefabricated element or used as a holder for special materials that are filled around or into the element in the mold.

In the latter case, the prefabricated element is preferably made of the first material and filled with starting particulate material of the second alloy. In terms of manufacturing, this provides an advantageous opportunity for rapid filling of the mold. At the same time, the second material is produced by powder metallurgy and thus achieves the isotropic structure that results in particularly good fatigue properties.

When a prefabricated element is used in the first alloy, this is preferably cast or produced by powder metallurgy in a bowl-shaped or tubular wall which forms part of the mold used in the isostatic pressing. The use of such a prefabricated component as a mold simplifies or makes unnecessary the subsequent removal of the mold from the workpiece being manufactured, since the prefabricated component is part of the finished workpiece.

In another embodiment of the method according to the present invention, a preformed core element of the second material is placed in a mold in which powder of the first alloy is arranged before isostatic pressing is carried out. Because at least part of the second material is preformed, the core element can be used to control the placement of powder in the first alloy. The powder can be arranged directly adjacent to the core element without any danger of mixing powder from the first and the second alloy. The preformed core element also simplifies the arrangement of the particulate material precisely in the desired places.

The preformed element of the first material and/or the preformed element of the second material can for example be produced from particulate material such as by means of a CIP treatment, a HIP treatment, possibly followed by machining or extrusion or by means of sintering, with subsequent pressing so that the element has an advantageous isotropic structure. It is also possible to use an element in cast or forged material. The properties of these are improved by isostatic pressing.

The isostatic pressing is suitably a HIP treatment which results in the consolidation of materials by diffusion without any real grain growth, which makes it possible to maintain a fine-grained structure due to the fact that one or more of the materials are of a fine-grained starting material consolidated into a cohesive material without melting. The isostatic pressing can also be a CIP treatment in which the pressing takes place at a considerably lower temperature than in the HIP treatment.

In another aspect, the present invention further relates to a nozzle for fuel valves in a diesel engine, particularly a two-stroke crosshead engine having a central longitudinal channel communicating with a number of nozzle bores opening into the outer surface of the nozzle and wherein the nozzle is made of a first alloy which is resistant to corrosion at least in an outer area around the nozzle bore and is made of another alloy in an area different from said outer area.

With a view to improving the lifetime of the nozzle, also mentioned in connection with the description of the method above according to the first aspect of the invention, the nozzle is characterized in that the material in the bonding area between the first alloy and the second alloy has a structure that is free of microcracks.

With regard to the effects and advantages achieved by the nozzle according to the invention, please see the description above in connection with the description of the method according to the first aspect of the invention.

In a preferred embodiment, the second alloy has a higher fatigue strength than the corrosion-resistant first alloy, which contributes to further achieving a long service life as mentioned above.

In one embodiment, an oxygen preventing diffusion barrier is arranged in the nozzle between the first alloy and the second alloy. The diffusion barrier makes it possible to determine the analysis of the first corrosion-resistant alloy on the basis of the desired properties and prepare conditions for this alloy without having to take into account whether the components of this alloy may interact negatively with the components of the second alloy. Similarly, the analysis of the second alloy can be determined without having to take into account the components of the first alloy.

The second alloy can be made by powder metallurgy, resulting in improved properties compared to a material made only by melting to the desired element. In a preferred embodiment, the first alloy is a nickel-based alloy and the second alloy is a steel-based alloy. The steel-based alloy used inside the nozzle provides high strength in the area of the nozzle, which has a complex geometry because a number of nozzle bores are located within a small area and are cut into a central longitudinal channel at different angles.

Nickel-based alloys are very susceptible to carbide formation and therefore only have a limited carbon content, for example up to 0.6% by weight. Common steel-based alloys with high fatigue strengths usually have a high carbon content of up to several percent by weight. If necessary, the steel-based alloy can be selected so that it has less free carbon at the operating temperature of the nozzle than the nickel-based alloy. The carbide-forming alloy elements in the steel-based alloy can, for example, be selected so that the dissolution temperature of the carbides is well above the operating temperature of the die and also above the HIP temperature when the die is produced by a HIP process. Release of free carbon by dissolution of carbides is thus avoided. Furthermore, strong carbide formers can be selected as alloying components, helping to capture and bind any free carbon.

The nozzle is suitably designed as a separate unit located in the fuel valve in extension of the stem bore containing the fuel valve's primary valve seat. With this design, the nozzle is not or only to a minimal extent affected by rather large stresses that occur in the primary valve seat. When replacing the nozzle, the replacement can also be limited to a smaller part mainly consisting of the part of the fuel valve that extends into the combustion chamber.

In a further embodiment, the second alloy comprises more than 70% of the total mass of the die, which can also be a cost advantage in addition to a strength advantage if the second alloy is cheaper than the first alloy.

The invention will now be described below in more detail with reference to the very schematic drawing in which

Fig. 1 shows a longitudinal section through the nozzle mounted at the lower end of the fuel valve, and Figures 2-7 show cross-sections through various powder-filled forms for use in HIP treatment of the nozzle. Fig. 1 shows a nozzle, generally designated as 1 in a fuel valve in an internal combustion engine, which may be a four-stroke engine, but which is preferably a two-stroke crosshead engine with more than one fuel valve in each cylinder. The latter engine typically has very strict requirements for a long service life of the nozzle, partly because the engines are often run on heavy fuel oil which can also contain sulphur.

The nozzle extends through a central hole at the end of a valve housing 2, whose annular surface 3 can be pressed against a corresponding support surface in a cylinder liner or in a cylinder liner indicated by a dashed line, so that the nozzle tip with the nozzle bores 4 extends into the combustion chamber A and can inject fuel when the fuel valve is open. The fuel valve has a valve slide 5 with a nozzle needle 6 and a valve seat 7 located, in the valve model shown, at the lower end of the sliding path 8. The sliding path is pressed down against an upwardly facing surface of the nozzle 1.

The nozzle has a longitudinal center channel 9 from which the nozzle bores 4 lead out to the outer surface of the nozzle. The nozzle is built up of a first material of a first alloy 10 which is resistant to corrosion and of a second material of a second alloy 11. The first alloy forms at least the outer area of the nozzle in the area around the nozzle holes and can stretch upward and form the outer surface of the nozzle over the entire part of the nozzle extending from the valve body 2.

The first material of a first alloy which is resistant to corrosion can be made from the particle starting material or it can be made for example by casting. Examples of suitable alloys for use as the first alloy are nickel-based alloys which may, for example by weight percentage and excluding generally present impurities, comprise from 15 to 30% Cr, from 0.02 to

0.55% C and optionally one or more of the following components: From 0 to 15% W, from 0 to 8% Al, from 0 to 5% Ti, from 0 to 20% Co, from 0 to 2% Hf, from 0 to 1% Nb, from 0 to 20% Co, from 0 to 2% Hf, from 0 to 5% Nb

and/or Ta, from 0 to 35% Mo, from 0 to 10% Si, from 0

to 1.5% Y, from 0 to 1.5% Y and from 0 to 20% Fe. The alloy may contain unavoidable impurities and the remaining is nickel. A typical example of such an alloy has the following analysis: 23% Cr, 7% W, 5.6% Al, 1% Si,

0.5% C and 0.4% Y. This HIP-treated alloy has a fatigue strength aA of approximately ± 450 MPa.

The nickel-based alloys may also be of the type which, by weight percentage and excluding impurities generally present, comprise from 35 to 60% Cr, from 0.02 to 0.55% C, and optionally one or more of the following components: from 0 to less than 1.0% Si, from 0 to 5.0% Mn, from 0 to 5.0% Mn, from 0 to 5.0% Mo and/or W, from 0 to less than 0.5% B , from 0 to 8.0% Al, from 0 to 1.5% Ti, from 0 to 0.2% Zr, from 0 to 3.0% Nb, from 0 to a maximum of 2% Hf, from 0 to 1% N, from 0 to a maximum of 1.5% Y and a total amount of Co and Fe of a maximum of 5.0%. The alloy may contain unavoidable impurities and what remains is Ni. This material has a high fatigue strength and extremely high resistance to both hot corrosion and erosion effects from the fuel.

Other examples of alloys for use as the first corrosion-resistant alloy material are shown in Table 1.

The thermal expansion coefficient indicated above is the average linear thermal expansion coefficient for heating from 20°C to 500°C, i.e. it is relevant for 500°C, preferably the first alloy has approximately the same thermal expansion coefficient as the second alloy. Alternatively, the first alloy may suitably have a higher coefficient of thermal expansion than the second alloy so that compressive stresses arising in the center region 1 of the die in connection with cooling from the HIP temperature to 20°C.

It is also possible to use cobalt-based alloys, such as Celsit 50-P, but in the HIP-treated condition they can only achieve fatigue strengths with aA of approximately ± 150 MPa. For this reason, they are not preferred materials.

As alloy materials for the second alloy, steel-based alloys are preferred, such as the tool steel AISI H13 with the analysis 0.4% C, 1.0% Si, 0.4% Mn, 5.2% Cr, 1% V, 1.3% Mo and residual Fe, or the tool steel AISI H19 with the analysis 0.45% C, 0.4% Si, 0.4% Mn, 4.5% Co, 4.5% Cr, 0.5% Mo, 2% V , 4.5% W and the remaining Fe, or the tool steels CPM1V and CPM3V from Crucible Research, USA, CPM1V contains 0.5% C, 4.5% Cr, 1% V, 2.75% Mo, 2% W, 0.4% Si, 0.5% Mn and the remaining Fe, and CPM1V contains 0.8% C, 7.5% Cr, 2.5% V, 1.3% Mo, 0.9% Si, 0 .4% Mn and the remaining Fe.

The tool steels can be produced by powder metallurgy as a fine-grained isotropic powder with an extremely fine structure so that the formation of carbide networks is avoided despite high percentages of added alloying components. By pressure atomizing molten alloy into a cold atmosphere, the carbides become extremely small and uniformly distributed.

Other examples of alloy materials for the second alloy are mentioned in Table 2, in which the thermal expansion coefficients are given in the same way as in Table 1.

For a particular tool steel, the fatigue strength can be regulated by heat treatment given to the die blank. After isostatic pressing of the die blank, the outer and inner geometry of the die blank is complete. This typically means that the longitudinal center channel and nozzle bores are machined into the workpiece and that the outer surface of the workpiece can also be turned or ground to its final shape.

Once the geometric machining is complete, the die blank may be subjected to a heat treatment in which the second alloy is hardened to a suitable hardness. The hardening can, for example, be carried out at a temperature in the range from 1000 °C to 1100 °C with a heating time of from 10 to 40 min. Then the final heat treatment is carried out in the form of one or more tempering treatments, and the tempering is important for the final fatigue strength of the finished die. The tempering can, for example, be carried out with a warm-up time of 2 hours at a temperature in the range from 450 °C to 600 °C. Preferably, a double or triple tempering with 2 or 3 periods of two hours is used. The tool steels mentioned may have fatigue strengths aA of approximately ± 500-900 MPa in the finished die. Preferably, the fatigue strength aA is at least ± 750 MPa. At the same time, the tool steels advantageously have high wear resistance and hardness.

For both the first and the second alloy, the powder suitably used has a size in the range of 0 to 1000 µm.

Examples will now be given to show how the die blank can be treated with isostatic pressing.

Fig. 2 shows a preformed core mold 12, which is inserted into a mold 13, built up of a bottom plate 14, a side wall 15, a cover 16 and a filling nozzle 17. The core element is made in the second alloy and is made in advance from powder sintered together and cold-pressed into a so-called green blank.

The core element can also be made by high-speed powder pressing or by a CIP or HIP process. The core element can also be made by conventional methods for tool steel or can be an ESR (Electro Slag Refined) recast. The core element has a head part 18 with a significantly larger diameter than the main part 19 which extends upwards into the mold 13. The main part extends over an area which lies around the center channel 9 in the finished nozzle. The main part can end at a distance from the nozzle bores, or as shown in fig. 1, it can extend into the nozzle so that it encloses the entire center channel. In the latter case, the die bores pass through the second alloy in an area around the center channel and through the first alloy in an outer area of the die.

The side wall 15 fits around the head part 18 and tapers upwards to a smaller diameter. Around the main body, the sidewall is at a distance substantially corresponding to the desired thickness of the corrosion resistant first alloy. The circular bottom plate 14 is welded along its entire circumference to the lower edge of the side wall 15. Similarly, the cover 16 is welded to the upper edge of the side wall and the filling nozzle is welded to the upper surface of the cover.

The fine-grained powder of the first alloy is filled through the filling nozzle 17 into the cavity around the Core element 12 and the mold with powder is vibrated, re-filling with more powder is carried out if necessary, the mold is made airless and closed in a pressure-tight manner in the filling nozzle.

The mold is then placed in an oven and the oven chamber is pumped up with an inert gas, such as argon, to a pressure of approximately 200

bar and heated to a temperature in the range from 1000 to 1300 °C, typically 1150 °C. Simultaneously with the heating, the pressure in the furnace chamber is increased to approximately 900 to 1100 bar. Temperature

and pressure is maintained over a period of from 4 to 8 hours, during which time the materials inside the mold consolidate into a dense, pore-free body.

After cooling, the mold is removed from the HIP-treated workpiece. The form used can for example be made of steel or glass. In the latter case, the welds mentioned above will consist of melt heating of the glass.

The annealed die blank is then machined to its final shape, for example by turning and drilling, after which the blank can be stress-relieved, hardened and annealed. As a result of the HIP treatment at a high temperature, the heat treatments of the die can be carried out without significant grain growth in the material, provided that the heat treatment takes place at a lower temperature than the HIP temperature. This is an advantage since larger grains result in lower fatigue strength.

The lifetime of the nozzle according to the invention is long, partly because there are no microcracks in the transition area between the two materials. A microcrack is a crack in a single crystal grain or a crack that extends through several crystal grains. Microcracks can typically range in size from 0.05 mm to 0.5 mm. All cracks smaller than 0.05 mm can be ignored. When microcracks are found in a nozzle, there will be many cracks in the nozzle. The reason for this is that cracking is caused by the heat-affected zones mentioned above, or that bond failure affects a large area and not just a few grains. The presence of microcracks can thus be determined by examining a few stress-exposed bonding areas between the two alloys in the die. If the most exposed areas are free of microcracks, the entire nozzle can be assumed to be free of microcracks in the adhesion area.

For convenience, the following description of the other examples uses the same reference numerals used above for details having the same functions.

Fig. 3 shows a design of the mold 13 which is suitable for producing many die blanks in the same shape. The side wall 15 is here circularly cylindrical with an inner diameter that corresponds to the outer diameter of the head part 18.

The thickness of the first alloy is controlled by placing a circular cylindrical filler tube 21 around the main part before filling the tube with powder. By making the side wall 15 longer than shown in the figure, a first core element 12 and an associated diffusion barrier 24 and a filler pipe 21 can be placed inside the side wall before fitting the cover 16, after which the powder is filled in the filler pipe to its upper edge. Then the next core element with tube 21, barrier 24 and powder can be placed on top of the first and so on until the side wall 15 is filled, after which the entire mold is closed off by fitting the cover 16. Then the HIP treatment can be carried out as described above.

The diffusion barrier 24 is placed on the outer surface of the main part 19 and on the upward facing side of the head part 18. The barrier can be a layer, for example applied to the preformed core element by means of electrolytic deposition or by means of other surface application methods, such as plating. The barrier can, for example, be made of nickel, copper, cobalt or nickel phosphor. Alternatively, the layer may be applied by spraying or by placing a thin foil of the desired material, typically pure metal, such as nickel, cobalt or copper, around the body 19. The barrier suitably has a thickness in the range of from 5 to 400 µm, preferably from 10 to 100 µm.

Fig. 4 shows another design in which the side wall 15 is circularly cylindrical and welded at its lower edge to a shoulder on the head part 18. This design makes the shape cheaper. In connection with the final machining of the die after HIP treatment, the weld with the associated heat-affected zone is removed by turning. The preformed core element can, for example, be made by a preceding HIP treatment or a CIP treatment (cold isostatic pressing), but then no welds can be made on the material. Fig. 5 shows a design in which a circular cylindrical tube wall 22 in the first alloy is placed around the main part 19. The inner diameter of the tube is slightly larger than the outer diameter of the main part so that the diffusion barrier 24 is not destroyed during assembly. The wall 22 can be produced by powder metallurgy, but can also be produced as a normal tube, preferably a seamless tube. After powder of the first alloy 10 is filled into the wall, the HIP treatment is carried out as described above with the exception that the wall 22 is retained on the core element and becomes part of the die blank. Fig. 6 shows a further design in which the wall 22 of the first alloy is cup-shaped and provided with the diffusion barrier 24. After fitting the cover 16, the mold is filled with powder of the second alloy and the HIP treatment is carried out as described above with the exception of the wall 22 remaining on the HIP fabricated inner portion of the die blank.

It is also possible that both the corrosion-resistant first alloy in the form of the wall 22 and the second alloy 11 and the diffusion barrier 24 are produced as two prefabricated parts with the barrier placed on either one or the other part, and that the parts are introduced into each other as shown in fig . 6, after which the cover 16 is mounted, the mold is emptied of air and closed, after which the HIP treatment is carried out as described above. In this case, the HIP treatment will not involve a real consolidation of the particulate material since the parts are already prefabricated as mentioned above, but the HIP treatment causes the parts to consolidate into a coherent blank by diffusion bonding at the interfaces.

Fig. 7 shows a further embodiment in which the mold is divided on the inside by means of a separating plate 23 which extends in the space between the first and the second alloy. The partition plate 23 can be made of the first alloy or the second alloy. It is also possible that the partition plate 23 is the oxygen preventing diffusion barrier made of a third material. Since the separator is relatively thick, the components from the first alloy cannot diffuse into the second alloy.

The bottom plate is also provided with a filling nozzle 17 for filling powder. First, a powder is filled through the associated filling nozzle, after which air is exhausted and the nozzle is closed. Then the mold is turned upside down, and the second powder is filled through the second nozzle, after which air is drawn out of the second chamber. The HIP treatment is then carried out as described above.

The nozzle can have other designs than the one shown in fig. 1. It is possible for the valve slide to carry a second closing part which closes the nozzle bores 4 from the fuel channel 9. The second closing part can advantageously be made of tool steel since it slides along the inner surface of the longitudinal channel which can also be made of tool steel. This is an exploitation of the fact that two tool steels slide well along each other. It is also possible to place the primary valve seat down in the nozzle, resulting in a minimum volume in the fuel channel below the valve seat. It is also possible that the valve bores are directed, not only to one side of the nozzle, but instead to both one and the other side or distributed along the entire circumference of the nozzle.

The preformed core member of the second alloy and/or the bowl-shaped or tubular wall of the first alloy may be fabricated in advance from material that is not based on particle starting material, such as cast or forged material. It is possible to control the temperature during the HIP treatment so that the other material is hardened and/or tanned or annealed during the HIP treatment or in direct connection therewith, thereby avoiding a step in the subsequent heat treatment.

Details of different designs and examples can be combined into new designs. It is also possible to mix powders of the first alloy or the second alloy from a number of powder sizes, and powders from a number of different metallic alloys may also be used, which may be of the types mentioned above. It is also possible to mix in ceramic powder to achieve an insulating effect. The ceramic powder can, for example, be placed in a layer at a short distance from the tip of the nozzle. Then the ceramic powder is covered by powder of the first corrosion-resistant alloy. Graded mixtures of different powders can also be used. Furthermore, it is possible, in connection with filling the mold before the HIP treatment, to place a mesh of ceramic material in the powder of the first alloy, for example in the powder which lies above the main part 19 in fig. 2. The powder of the first alloy is placed on top of the mesh, so that the outer part of the nozzle is made of a corrosion-resistant material.

Claims (19)

1. Method for manufacturing a nozzle (1) for fuel valves in a diesel engine, in particular a two-stroke crosshead engine, wherein a first material of a corrosion-resistant first alloy (10) is arranged in a mold (13), at least in an outer area which will form the outer surface of the nozzle around the nozzle bores (4), characterized in that a second material of a second alloy (11) is also arranged in the mold (13) in an inner area, and that the materials thus arranged are processed by isostatic pressing to a consolidated die blank free of microcracks in the boundary region between the first alloy (10) and the second alloy (11). 2. Method according to claim 1, characterized in that the second material of the second alloy (11) has a higher fatigue strength than the first corrosion-resistant alloy (10) in the finished die (1). 3. Method according to claim 1 or 2, characterized in that at least when the corrosion-resistant alloy (10) comprises more than 0.6% Al, an oxygen-preventing diffusion barrier (24) is used between the first material and the second material before isostatic pressing. 4. Method according to claim 3, characterized in that nickel, copper or a nickel alloy is used as the diffusion barrier (24). 5. Method according to claim 3 or 4, characterized in that the diffusion barrier (24) is placed on the inside of a prefabricated part of the first material or on the outside of a prefabricated part of the second material. 6. Method according to one of claims 1 to 5, characterized in that a prefabricated part of the first material is filled with particulate starting material of the second alloy (11), preferably by means of high-speed powder pressing. 7. Method according to claim 6, characterized in that the prefabricated part of the first alloy (10) is cast or produced by powder metallurgy in a cup-shaped or tubular wall (22) which forms part of the mold (13) used in the isostatic pressing. 8. Method according to one of claims 1 to 5, characterized in that a preformed core element (12) of the second material is placed in a mold (13) in which particulate starting material of the first alloy (10) is arranged before isostatic pressing is performed. 9. Method according to one of claims 6 to 8, characterized in that the preformed part of the first material and/or the preformed part in the second material is produced from particulate starting material, for example by means of high-speed powder pressing, a CIP treatment, a HIP treatment, optionally followed by machining or extrusion, or by means of sintering followed by pressing. 10. Method according to one of claims 1 to 9, characterized in that the isostatic pressing is a HIP treatment. 11. Nozzle (1) for a fuel valve in a diesel engine, in particular a two-stroke crosshead engine, having a longitudinal center channel (9) communicating with a number of nozzle bore rings (4) opening into the outer surface of the nozzle, where the nozzle (1) is made of a corrosion-resistant first alloy (10) at least in an outer area around the nozzle bores (4), and is made of a second alloy (11) in an area different from said outer area, characterized in that the material in the boundary area between the first alloy (10) and the second alloy (11) has a structure free of microcracks. 12. Nozzle according to claim 11, characterized in that the second alloy (11) has a higher fatigue strength than the corrosion-resistant first alloy (10). 13. Nozzle according to claim 11 or 12, characterized in that an oxygen preventing diffusion barrier (24) is provided in the nozzle between the first alloy (10) and the second alloy (11). 14. Nozzle according to one of claims 11 to 13, characterized in that the second alloy (11) is made by powder metallurgy. 15. Nozzle according to one of claims 11 to 14, characterized in that the first alloy (10) is a nickel-based alloy and the second alloy (11) is a steel-based alloy. 16. Nozzle according to one of claims 11 to 15, characterized in that the nozzle (1) is a separate unit placed in the fuel valve in extension of a spindle feedthrough comprising a primary valve seat (7) of the fuel valve. 17. Nozzle according to claim 16, characterized in that the second alloy (11) constitutes more than 70% of the total mass of the nozzle (1) • 18. Nozzle according to one of claims 11 to 17, characterized in that the fatigue strength aA of the second alloy (11) is at least ± 750 MPa. 19. Nozzle according to one of claims 11 to 18, characterized in that insulating ceramic material covered by the first alloy (10) is arranged in the nozzle (1).