WO1995024286A1

WO1995024286A1 - A method of manufacturing a nozzle for a fuel valve, and a nozzle

Info

Publication number: WO1995024286A1
Application number: PCT/DK1995/000112
Authority: WO
Inventors: Harro Andreas Hoeg
Original assignee: Man B & W Diesel A/S
Priority date: 1994-03-10
Filing date: 1995-03-09
Publication date: 1995-09-14
Also published as: NO963760D0; HRP950114A2; JPH09509984A; EP0749365B1; EP0749365A1; RU2124417C1; KR970701605A; DE69502277D1; NO963760L; KR100324398B1; HRP950114B1; JP3355190B2; DE69502277T2; NO314170B1

Abstract

A nozzle (4) for a fuel valve (1) for an internal combustion engine is manufactured by exposing an isotropic finely grained powder in a form having substantially the desired external nozzle shape to a pressure of at least 800 bar and a temperature of at least 1000 °C for at least 1 hour, whereupon a flow passage with a number of nozzle holes (5) is machined in the nozzle blank thus HIP-treated. The nozzle may be HIP-treated from a cobalt-based alloy, such as Stellite 6, or a nickel-based alloy comprising 20-30 % of Cr, 4-8 % of Al and 0.2-0.55 % of C and possibly W, Hf, Nb, Mo, Si, Y and/or Fe in amounts of 1.8 %. The HIP-treated nozzle (4) has a substantially greater fatigue strength and is easy to machine into its final shape.

Description

A method of manufacturing a nozzle for a fuel valve, and a nozzle

The invention relates to a method of manufacturing a nozzle for a fuel valve for an internal combustion engine, particularly a large two-stroke engine, in which substantially isotropic, finely grained powder of such a composition that the finished nozzle possesses hot corrosion resistance is arranged in a form and is HIP- treated at a pressure of at least 800 bar and at a temperature of at least 1,000°C.

In each engine cycle, the nozzle of an internal combustion engine is exposed to a sudden pressure influence, when the internal bore of the nozzle at the opening of the valve is supplied with pressurized fuel which is sprayed out through the nozzle holes. In the area around the nozzle holes, the fuel may have quite a heavy erosive influence on the nozzle, which makes great demands on the strength of the nozzle material, particularly in large two-stroke engines fuelled by heavy fuel oil, the particle content of which has a highly erosive effect. As the nozzle projects a distance down into the combustion chamber, it is also affected by the changing temperatures in the chamber. In large two-stroke engines, the nozzle tip is in reality uncooled. Particularly the high temperature level at combustion makes great demands on the nozzle material, which must have a suitable strength at high temperatures and must furthermore be resistant to hot corrosion.

Prior art nozzles consist of a material which is resistant to hot corrosion and erosive influences from the fuel. Nozzles made of cast Stellite 6 are known. These nozzles are manufactured by precision casting, so- called investment casting, where a sand mould is created around a positive form of the nozzle in wax, which mould is baked simultaneously with melting out of the wax, whereupon the nozzle blank is cast.

In consideration of the strength properties of the cast nozzle, the casting must be cooled very quickly in order to obtain a sufficiently fine grain structure in the finished nozzle. The rapid cooling increases the risk of porosities and cold flow of the casting, i.e., a kind of lamination of the material without a proper complete metallurgical bonding between the individual layers. The lamination reduces the fatigue strength of the nozzle. Therefore, there is a limit to the number of nozzle holes which can be machined into the material of the nozzle, as the holes weaken the material and give rise to stress concentrations. As a too high level of stress in the known nozzles might lead to crack forma¬ tion in and subsequent ruptures of the nozzle and in the worst case to injection of concentrated jets of fuel directly towards the surface of the piston, known nozzles are not manufactured with closely adjacent nozzle holes. This limits the amount of fuel which can be injected per fuel valve per engine cycle.

In known nozzles of Stellite 6 with bored nozzle holes it has proved that at the opening in the central longitudinal bore of the nozzle, the nozzle holes have a very broken rim section, i.e., many small chips have been knocked off the hole rim. The uneven hole rim causes notch effects which reduce the fatigue strength of the nozzle.

EP-A-0 569 655 describes a nozzle consisting of a mechanically alloyed, dispersion strengthened nickel- based super alloy, i.e., a so-called ODS alloy (Oxide Dispersion Strengthened) . The mechanical alloying takes place in high energy mills, such as large ball mills, where powder and/or flaky starting material consisting of a dispersion component of yttrium oxide and a nickel- based alloy component are mechanically kneaded into a material with a homogeneous, very fine icrostructure. Then, in several stages, the material can be cold or hot forged into the desired shape and subsequently be heat treated to cause precipitation hardening. As a conse¬ quence of the dispersion strengthening with oxides, this known nozzle has a relatively high strength at very high temperatures. The manufacturing of these nozzles is very costly, and the forming of nozzle holes is difficult, as finely distributed, very hard yttrium oxides in the material render it difficult to machine.

Japanese patent application published under No. 1- 215942 describes a nozzle manufactured in the manner described in the introduction above from a sintered material of an intermetallic compound of TiAl and Ni₃Al, which is well known as an extremely hard particle- precipitated component in alloys. By manufacturing the nozzle from this intermetallic compound it is obvious that its resistance to wear becomes very high, but the machining of the nozzle blank to the finished shape becomes difficult and financially costly. Additionally, the HIP treatment can only take place during a short period of, for example, 30 min. , as the alloy changes character at longer holding periods. Therefore, the HIP treatment cannot be completed to a compact nozzle blank, but must be followed by forging the nozzle blank into the desired shape and machining it into a finished nozzle. The high-temperature resistance of the alloys Ni₃Al and TiAl is insufficient in case of use in engines operated on heavy fuel oil.

The uneven transition between the intersecting bores in the known nozzles also gives rise to an unfavourable course of flow when the fuel flows through the nozzle holes, as the fuel continues into the combustion chamber as a combined jet over a relatively long distance, which increases the heat influence on components at a longer distance from the nozzle and counteracts a rapid and fine distribution of the fuel, and thus affects the operation of the engine in an undesired manner.

The object of the present invention is to provide a simpler method of manufacturing a nozzle in a material which, on one hand, permits simple mechanical machining into an accurate desired geometrical shape which yields an improved injection of the fuel, and on the other hand has a relatively high strength at high temperatures.

In view of this object, the first mentioned method is characterized in that the form is of substantially the desired external nozzle shape, that the HIP treat- ment lasts for at least one hour at the pressure and temperature mentioned, and that a flow passage with a central longitudinal bore and a number of nozzle holes is bored into the nozzle blank so HIP-treated, whereby the hole edges of the nozzle holes at the transition to the central bore become sharp.

As a consequence of the small dimension of the nozzle blank, the mass is dense when the material in the form has been brought up to the desired values for temperature and pressure during the holding time of at least 1 hour. With this holding time, the necessary bonds between the powder grains have become established by diffusion so that the nozzle blank has a homogeneous structure. The finely grained, dense and homogeneous structure renders possible boring of the nozzle holes with sharp hole edges. Sharp hole edges at the transi¬ tion to the central bore, i.e., at the inlet to the nozzle holes, promote the distribution of the fuel jet sprayed out from the opposite ends of the nozzle holes at the external side of the nozzle. The finely grained powder is combined at the HIP (Hot Isdstatic Pressure) treatment of the nozzle into a strong, cohesive material without causing the powder to melt. The result of the non-melting is that, in a manner known per se, the material in the nozzle retains the isotropic structure of the finely grained powder with very small crystal grains. The fine grain size gives the material a high strength without at the same time imparting properties to the material which render difficult a mechanical machining.

The manufacturing of the nozzle is advantageously simple, as the HIP treatment can be carried out in a simple operation directly from the finely grained powder, and the nozzle holes can be bored into the blank with no complications and without any intermediate cumbersome and tool-demanding treatment. At the HIP treatment the material of the nozzle is exposed to substantially the same treatment in all cross-sections so that local variations in the material properties of the nozzle blank are avoided. As the material of the nozzle does not contain internal weakenings, the nozzle achieves a high fatigue strength seen in relation to the strength of a cast nozzle of a material with the same analysis. It is also an advantage that a relatively cheap, gas-atomized powder material is used at the HIP treat¬ ment, and that the HIP treatment can exploit the major part of an atomized powder charge.

If pressure, temperature or holding time becomes lower or shorter than the values stated, sufficient bonding of the finely grained powder may not be obtained. The finely grained structure of the nozzle blank can be obtained independently of the actual composition of the alloy used. The HIP treatment can be carried out with a starting material having a powder grain size in the interval of from 0 to 1000 μm and a pressure in the ^■ interval of from 900 to 1100 bar and a temperature in the interval of from 1100 to 1200°C. In the alloys tested, these interval limits have proved to yield HIP- treated nozzle blanks with largely isotropic properties, i.e., uniform properties in all directions. For most alloys, pressures above 1100 bar and temperatures above 1200°C will cause a risk of increased grain growth and incipient melting of the material, which would destroy the very small crystal grain size in the powdery starting material. The lower limit of 900 bar and 1100°C and a holding time of at least 1 hour ensure for most alloys that the powder is bonded into a uniform body. Keeping the grain size, i.e., the largest outer dimen¬ sion of the powder, at 1000 μm at the most, ensures that the starting material has very fine crystal grains.

In a further development which is applicable in case of a nickel-based alloy with a high content of chromium, the method is characterized in that the nozzle blank comprises an austenitic nickel phase, and that after the mechanical machining the blank is subjected to heat treatment at a temperature in the interval of 550-1100°C, preferably 700-850°C, for a period of at least 5 hours, whereby a solid phase transformation takes place in which ferritic a phase is particle- precipitated in a very fine distribution in the austen¬ itic nickel phase. The desired high hardness of the alloy and the consequent low machinability is thus only created when the mechanical machining of the HIP-treated blank is finished.

The invention also relates to a nozzle for a fuel valve for an internal combustion engine, particularly a large two-stroke engine, with a central longitudinal bore and a number of nozzle holes positioned in the side wall of the nozzle and constituting, together with the longitudinal bore, a flow passage for pressurized fuel, which nozzle is made of a material which is resistant to hot corrosion and erosive and cavitational influences from the fuel. In a large two-stroke engine, the fuel used may be heavy fuel oil, which subjects the nozzle to substantial erosive influences from particle content, etc., in the fuel, and additionally, the frequently sulphurous oil results in a very corrosive environment in the combustion chamber. In large two-stroke engines, the nozzle is of large length and is substantially uncooled at its lower end.

In one embodiment, the nozzle according to the invention is characterized in that it is m3.de of a HIP- treated cobalt-based alloy comprising r.romium and tungsten, such as Stellite 6, the flow passage for the fuel being bored after the HIP treatment. At the HIP treatment of the nozzle material, the high resistance to the environment prevailing in the combustion chamber, which the cobalt-based alloy is known per se to have, is combined with a substantially improved fatigue strength and novel excellent properties when the material is mechanically machined. In an alternative embodiment, the nozzle is made of a HIP-treated, nickel-based alloy which, in percen¬ tage by weight and apart from generally occurring impurities, comprises from 20 to 30% of Cr, from 0 to 8% of W, from 4 to 8% of Al, from 0.2 to 0.55% of C, from 0 to 2% of Hf, from 0 to 1.5% of Nb, from 0 to 8% of Mo, from 0 to 1% of Si, from 0 to 1.5% of Y and from 0 to 5% of Fe.

This material has shown surprisingly good mechan¬ ical machinability and high fatigue strength, and resistance to both hot corrosion and erosive influences from the fuel. At boring of the nozzle holes, no chipping off of flakes at the ends of the bore was observed. Likewise, tests have shown that the knife-edge inlets to the nozzle holes are retained, even after very long periods of operation.

The Cr content of the alloy is important to the ability of the nozzle to resist hot corrosion, and additionally, the Cr content provides a solution- strengthening effect which, in addition to the fine grain structure, contributes to increasing the strength of the alloy. If desired, this effect can be enhanced by adding Mo and/or W to the alloy.

Together with Cr, Al forms a combined surface layer of Al₂0₃ and Cr₂0₃ which protects the nozzle against corrosion at high temperatures. The Al content further¬ more provides a γ' phase consisting of the inter etal Ni₃Al, which causes precipitation hardening of the alloy, but is a relatively brittle phase. Al contents of more than 8% involve a risk that the γ' phase becomes cohesive instead of being enclosed by a ductile austen- itic phase which secures the high fatigue strength and good machinability of the material. The Al content of the alloy can suitably be restricted to maximum 6 per cent, as most of the positive properties of Al have then been exploited without any risk of loss of strength owing to an incomplete enclosing of the γ' phase.

At Cr contents of less than 20%, the nozzle cannot resist the corrosive influences at high temperatures. It may be possible to add more than 30% of Cr to the alloy, but this would not result in any noticeably improved resistance to high temperature corrosion. On the contrary, high Cr contents will impair the mechan¬ ical machinability of the nozzle, and so, preferably, the alloy comprises 24% of Cr at the most. The possible Fe content of the alloy is kept at a maximum of 5% to prevent deterioration of the corrosion properties of the nozzle.

The finely grained structure of the powder used as a starting material at the HIP treatment is provided by pressure atomization of melted material into a relative¬ ly cold gas, where the atomized drops are subjected to quenching during simultaneous formation of extremely small crystal grains in the material. The quenching also results in an extremely small distance between the dendritic branches of the crystal grains. The Si content of the alloy of up to 1% does not impart any special advantages to the finished nozzle, but has a deoxidizing effect during the powder production so that pollution of the powder with undesired oxides is avoided. Alterna¬ tively, other deoxidizing components may be used in small amounts.

The C content of the alloy is kept at a maximum of 0.55% to prevent precipitation of needle and plate- shaped carbides which may lower the ductility of the alloy. At a C content of less than 0.2%, the alloy does not achieve the hardness necessary to resist the erosive influences from the fuel. Addition of up to 2% of Hf may modify unfortunate carbide precipitations into having more rounded shapes. Addition of Nb in amounts of up to 1.5% may result in a finer precipitation of metal carbides, which presumably imparts greater ductility to the alloy.

The corrosion resistance of the nozzle at high temperatures can be improved by the addition of Y in amounts of up to 1.5%. Further addition of Y does not result in further improvement.

When the nozzle is intended for applications where the erosive influence is great, the C content is preferably at least 0.35% _.in consideration of the hardness of the alloy.

In a further alternative embodiment, the nozzle is made of a HIP-treated, nickel-based alloy which, in percentage by weight and apart from generally occurring impurities, comprises from 40 to 50% of Cr, from 0 to 0.55% of C, less than 1.0% of Si, from 0 to 5.0% of Mn, less than 1.0% of Mo, from 0 to less than 0.5% of B, from 0 to 8.0% of Al, from 0 to 1.5% of Ti, from 0 to 0.2% of Zr, from 0 to 3.0% of Nb, maximum 0.01% of O, maximum 0.03% of N, maximum 2% of Hf, maximum 1.5% of Y, a combined content of Co and Fe of maximum 5.0% and the balance Ni.

This material has a high fatigue strength and extremely high resistance to both hot corrosion and erosive influences from the fuel.

The Cr content of the alloy is important to the ability of the nozzle to resist hot corrosion, and the Cr content further has a solution-strengthening effect which, in addition to the fine grain structure, contrib¬ utes to increasing the strength of the alloy. If desired, this effect can be enhanced by the addition of Mo and/or W to the alloy.

Together with Cr, Al forms a combined surface layer of Al₂0₃ and Cr₂0₃ which protects the nozzle against corrosion at high temperatures. The Al content further¬ more provides a γ' phase consisting of the intermetal Ni₃Al, which causes precipitation hardening of the alloy, but is a relatively brittle phase. Preferably, the Al content is higher than 2.5% to obtain suitable amounts of the desired surface layer. Al contents of more than 8% involve a risk of the formation of a β phase which reduces the ductility of the alloy at room temperature and reduces the strength of the alloy at high temperatures. The Al content of the alloy can suitably be restricted to maximum 6%, as most of the positive properties of Al have then been exploited without any risk of loss of strength owing to unsuitable •structural components. The possible Fe content of the alloy is kept at a maximum of 5% to prevent deterioration of the corrosion properties of the nozzle. Fe and Co are both impurities in the alloy, and it is desired to limit their combined content to a maximum of 5.0%. The finely grained structure of the powder used as a starting material at the HIP treatment is provided by pressure atomization of melted material into a relative¬ ly cold gas, where the atomized drops are subjected to quenching during simultaneous formation of extremely small crystal grains in the material. The quenching also results in an extremely small distance between the dendritic branches of the crystal grains. The Si content of the alloy of up to 1% does not impart any special advantages to the finished nozzle, but has a deoxidizing effect during the powder production so that pollution of the powder with undesired oxides is avoided. Alterna¬ tively, other deoxidizing components may be used in small amounts, such as Ti or Mn. Mn is not quite such an efficient deoxidizing agent, and it is desired to restrict the amount thereof to a maximum of 5% in order not to dilute the effective components in the finished alloy. By the addition of Ti, for example in amounts of at least 0.5%, the risk of formation of so-called prior particle boundaries (PPB) may be increased, particularly if the alloy comprises C and impurities from O and N, for which reason, simultaneously with Ti, an addition of HF of about 0.5% to the alloy is preferably made to counteract this tendency.

The B content has surprisingly turned out to be of importance for the achievement, by the nickel alloy with the high Cr content, of a high ductility advantageous to the fatigue strength. Already at such a small amount as 0.05%, B causes the solidification of the melted material to change from cellular solidification into dendritic solidification where the dendritic branches interlock and produce a geometrical locking of the structural components. B is largely insoluble in the γ and the a phases, and it is presumed that the solidifi¬ cation involves an eutectic with a number of borides. Larger contents of B may cause precipitation of the well-known and undesired low-melting eutectics of no great strength.

At heat treatment with holding periods of more than 1 hour, preferably more than 5 hours, the alloy is exposed to a solid phase transformation, where chromium- comprising ferritic a phase is precipitated in the austenitic nickel phase as very finely distributed precipitates. Nb influences the solid phase transform¬ ation into yielding globular precipitation rather than lamellar precipitation, which increases the ductility of the alloy.

The C content of the alloy is maintained at a maximum of 0.55% in order to counteract precipitation of needle and plate-shaped carbides which may reduce the ductility of the alloy. The addition of up to 2% of Hf may modify unfortunate carbide precipitates into having more rounded shapes and may at the same time relieve a possible Nb content from being incorporated in the carbide formation. The addition of Nb in amounts of up to 3.0% may result in a finer precipitation of metal carbides, which is presumed to impart greater ductility to the alloy, and at the same time a residual amount of free Nb will be present to influence the solid phase transformation. In an embodiment where the hardness of the alloy has primarily been obtained by means of the solid phase transformation, the C content is maximum 0.1%, and the Hf content is less than 0.5%, as there is no need for a large excess of carbide modifiers. Holding times at the HIP treatment or at a subsequent heat treatment at a temperature of above 550°C, preferably in the interval of 700-850°C may in this case be longer than 5 hours, so that there is time for the diffusion at the trans- formation to take place.

The corrosion resistance of the nozzle at high temperatures may be improved

the addition of Y in amounts of up to 1.5%. Addition of more Y does not result in further improvement . In a preferred embodiment, the alloy comprises maximum 0.45% of Al, maximum 0.1% of C, and maximum 0.1% of Ti . With this composition, the precipitation of carbide networks, borides and/or intermetals, such as Ni₃Al (γ') , in the basic matrix of the alloy is substan- tially suppressed, and therefore, after the HIP treat¬ ment the alloy will have a high ductility and low hardness so that the HIP-treated blank may be machined to the desired geometry with no problems. The finished blank is then subjected to a heat treatment at a temperature in the interval of 550-1100°C, preferably 700-850°C, for a period of at least 5 hours. At the heat treatment, a solid phase transformation takes place, whereby ferritic a phase is particle-precipitated in a very fine distribution in the austenitic nickel phase γ, whereby the alloy hardens and gets the desired high hardness which gives the nozzle good wear resistance. The phase precipitates are so finely distributed that the microhardness of the matrix is largely evenly increased, which promotes both wear and hot corrosion resistances. The holding time of the heat treatment may also be longer, such as at least 20 or at least 40-50 hours .

In a further embodiment, the alloy comprises at least 45% of Cr and from 0.15 to 0.40% of B, preferably maximum 0.25% of B. The upper limit of 0.4% of B suitably ensures that at the solidification of the alloy the amount of hardness-increasing borides does not exceed a level where the alloy is embrittled, and the lower limit of 0.15% is suitable for a Cr content of 45%.

In a further embodiment, the alloy comprises from 1.0 to 2.0% of free Nb. The advantageous change of the hardening mechanism into globular precipitation is strengthened if the free Nb content is at least 1.0%, and for financial reasons, the content of the relatively costly Nb may suitably be limited to the 2.0%, as a higher content of Nb usually does not substantially improve the properties of the alloy.

As a consequence of the high fatigue strength of the HIP-treated nozzles according to the invention, a number of nozzle holes may be provided closer to each other than has been possible previously. The pressure of the fuel acts on the central bore of the nozzle with an excess pressure which produces tensile stresses in the nozzle material. The higher fatigue strength of the nozzle permits an increase of the tensile stress level and thus an advantageously higher injection pressure, which can be used for injection of a larger fuel amount during an engine cycle. The method and the nozzle according to the invention thus render it possible to manufacture engines with a higher cylinder output .

The invention will now be explained in further detail below with reference to the drawing, in which

Fig. 1 is a longitudinal section through a nozzle mounted in a fuel valve, Figs. 2 and 3 are photos of nozzle holes in two different prior art nozzles, and

Figs. 4-6 are corresponding photos of nozzle holes in a nozzle according to the invention. Fig. 1 shows the lower end of a fuel valve 1 having a housing 2 for mounting in a cylinder cover, not shown, in such a manner that an annular, inclined surface 3 at the lower end of the housing is pressed into abutment against a corresponding surface on the cover. A nozzle 4 passes through a central hole in the housing 2 and projects down into the combustion chamber so that nozzle holes 5 in the side wall of the nozzle are located a suitable distance down in the combustion chamber. At levels below the inclined surface 3, the nozzle is substantially uncooled, and therefore the nozzle tip with the holes 5 is heated to a high temperature by the hot gases in the combustion chamber.

The nozzle has a central bore 6 extending from a flow passage 7 in the fuel valve to the nozzle tip at a lower level than the nozzle holes 5. In the nozzle, the bore 6 and the holes 5 form a flow passage for the fuel, which may be oil or gas.

When the nozzle is intended for a two-stroke engine with a number of valves per cylinder, each fuel valve 1 is normally positioned near the vertical side wall of the combustion chamber. In that case, the fuel has to be injected in a fan-shaped cloud directed towards the middle of the combustion chamber, which means that the nozzle holes 5 are all formed in one side of the nozzle, and that the longitudinal axes of the nozzle holes form an angle of maximum 100° with each other. When two or three fuel valves per cylinder are used, the spherical angle is often limited to less than 80°. The nozzle holes 5 are bored through the side wall of the nozzle to the central bore 6. The holes can also be produced in another manner, for example by spark machining, but boring is preferred because it is a rapid and simple mechanical machining.

Two different prior art nozzles of cast Stellite 6 have been examined by means of an endoscope known from, for example, gastric examinations of humans. By means of the endoscope, photos have been taken of the openings of the nozzle holes into the central bore. A photo of the central bore of each nozzle has been shown in Figs. 2 and 3, respectively. All the way round the rim of the nozzle holes 5, flakes have been chipped off from the side wall of the central bore 6, so that the transition between the two intersecting bores is uneven and rough. A nozzle according to the invention has been manufactured by HIP treatment of an isotropic finely grained powder of Stellite 6, where the powder grains are less than 300 μm. Stellite 6 has an approximate analysis of 1.14% of C, 1.06% of Si, 28.5% of Cr, 0.43% of Fe, 4.65% of W and the balance Co. The HIP treatment was carried out at a temperature between 1100 and 1200°C and a pressure between 900 and 1100 bar, and with a holding time of 2 hours. The central bore 6 was bored into the HIP-treated blank, whereupon the nozzle holes 5 were bored from the outside to the central bore. The nozzle was examined by means of the endoscope, which showed smooth hole edges at the openings of the nozzle holes in the central bore. Thus, the HIP-treated Stellite 6 has a substantially better machinability than cast Stellite 6. The smoother hole edges cause smaller stress concentrations in the nozzle.

From a nickel-based alloy with the approximate analysis of 23% of Cr, 7% of W, 5.6% of Al, 1% of Si, 0.5% of C and 0.4% of Y, all in percentage by weight, a HIP-treated nozzle was manufactured in the same manner as above. The endoscope examination of the nozzle has been shown in Figs. 4-6, where it can be seen that the edges of the nozzle holes at the openings into the central bore 6 are sharp and without chippings . The HIP-treated nozzles have then been tested by operating tests in a trial engine, which showed that both types of HIP-treated nozzles have greater resis¬ tance to hot corrosion and the formation of microcracks than the known cast nozzles of Stellite 6. In the area between the two mutually closest nozzle holes, a few very small cracks in the material were observed in the HIP-treated nozzle of Stellite 6, while the HIP-treated, nickel-based nozzle was completely crack-free.

Operating tests with a nozzle of cast Stellite 6 and with correspondingly closely adjacent nozzle holes as in the HIP-treated nozzle showed major through cracks and several small cracks in the material. The compara¬ tive tests thus showed that the nozzle of HIP-treated Stellite 6 has a substantially improved fatigue strength.

HIP-treated nozzles have also been manufactured in the cobalt-based alloy Celsit 50-P with the approximate analysis showing 2% of C, 28% of Cr, 6.5% of Ni, 10% of W, 3.7% of Mo, 1.6% of Cu and the balance Co. Operating tests with these nozzles showed that the fatigue strength and the resistance to hot corrosion were on a par with nozzles of HIP-treated Stellite 6.

Comparative machinability tests have been made of HIP-treated alloys of Stellite 6 and of the above nickel-based material. Holes were bored in plate-shaped blanks, and the character of the hole edge on the back of the plate was examined, which gave the same results as in the above nozzles, viz., that the hole edge in the HIP-treated plates was unbroken in the plates of Stellite 6 and sharp-edged in the plates of nickel-based material .

The mechanical properties of the nozzle materials have been examined by means of the above plates and by means of round rod-shaped blanks of cast Stellite 6 and the HIP-treated nickel-based alloy. The result of this is shown in the below Table 1. Measurement of hardness and tensile tests have been carried out in a completely conventional manner. Additionally, the rod-shaped blanks were exposed to fatigue tests where each blank was set up at both ends and subjected to pulsating longitudinal tensile loads at a magnitude of P ± P, viz., a tensile force varying between 0 and 2P. The blanks were sub¬ jected to 10 million cycles. If the blank did not exhibit a rupture, the load P was increased by 10% and the 10 million cycles were repeated. If a rupture occurred, a new blank was set up and the load P was reduced by 10%, whereupon the process continued as described above. After testing of a number of blanks of each material composition, the fatigue strength σ_A was determined as the lower stress load which just did not cause a rupture after 10⁷ cycles. Table 1 shows that cast Stellite 6 had a fatigue strength of σ_A = + 150 N/mm², while the HIP-treated nickel-based alloy had a fatigue strength of σ_A = + 275 N/mm².

Table 1 Mechanical properties of nozzle materials

R , (N/mm²) R_β (N/mm²) A (%) HV 20 σ_A(N/mm²)

Cast 900 540 1 400 ± 150 Stelli te 6

HIP- treated 1200 760 2. 5 440

Stelli te 6

HIP- treated 1060 910 1 . 4 425 ± 215

Ni alloy

As seen, the HIP-treated nozzle material is both substantially stronger and substantially more ductile than the cast nozzle material, just as the HIP-treated material has a substantially improved fatigue strength.

Claims

P A T E N T C L A I M S

1. A method of manufacturing a nozzle (4) for a fuel valve (1) for an internal combustion engine, particularly a large two-stroke engine, in which substantially isotropic, finely grained powder of such a composition that the finished nozzle possesses hot corrosion resistance is arranged in a form and is HIP- treated at a pressure of at least 800 bar and at a temperature of at least 1,000°C, c h a r a c t e r- i z e d in that the form is of substantially the desired external nozzle shape, that the HIP treatment lasts for at least one hour at the pressure and tempera¬ ture mentioned, and that a flow passage with a central longitudinal bore (6) and a number of nozzle holes (5) is bored into the blank so HIP-treated, whereby the hole edges of the nozzle holes at the transition to the central bore become sharp.

2. A method according to claim 1, c h a r a c¬ t e r i z e d in that the nozzle blank comprises an austenitic nickel phase, and that after the mechanical machining the blank is subjected to heat treatment at a temperature in the interval of 550-1100°C, preferably 700-850°C, for a period of at least 5 hours, whereby a solid phase transformation takes place in which ferritic a phase is particle-precipitated in a very fine dis¬ tribution in the austenitic nickel phase.

3. A nozzle (4) for a fuel valve (1) for an internal combustion engine, particularly a large two- stroke engine, with a central longitudinal bore (6) and a number of nozzle holes (5) positioned in the side wall of the nozzle and constituting, together with the longi¬ tudinal bore, a flow passage for pressurized fuel, which nozzle (4) is made of a material which is resistant to hot corrosion and erosive influences from the fuel, c h a r a c t e r i z e d in that the nozzle (4) is made of a HIP-treated cobalt-based alloy comprising chromium and tungsten, such as Stellite 6, the flow passage for the fuel being bored after the HIP treat- ment.

4. A nozzle (4) for a fuel valve (1) for an internal combustion engine, particularly a large two- stroke engine, with a central longitudinal bore (6) and a number of nozzle holes (5) positioned in the side wall of the nozzle and constituting, together with the longi¬ tudinal bore, a flow passage for pressurized fuel, which nozzle (4) is made of a material which is resistant to hot corrosion and erosive influences from the fuel, c h a r a c t e r i z e d in that the nozzle (4) is made of a HIP-treated nickel-based alloy which, in percentage by weight and apart from generally occurring impurities, comprises from 20 to 30% of Cr, from 0 to 8% of W, from 4 to 8% of Al, from 0.2 to 0.55% of C, from 0 to 2% of Hf, from 0 to 1.5% of Nb, from 0 to 8% of Mo, from 0 to 1% of Si, from 0 to 1.5% of Y and from 0 to 5% of Fe.

5. A nozzle according to claim 4, c h a r a c¬ t e r i z e d in that the alloy comprises maximum 6% of Al.

6. A nozzle according to claim 4, c h a r a c¬ t e r i z e d in that the alloy comprises from 0.35 to 0.55% of C.

7. A nozzle according to claim 4, c h a r a c¬ t e r i z e d in that the alloy comprises maximum 24% of Cr.

8. A nozzle (4) for a fuel valve (1) for an internal combustion engine, particularly a large two- stroke engine, with a central longitudinal bore (6) and a number of nozzle holes (5) positioned in the side wall of the nozzle and constituting, together with the longi- tudinal bore, a flow passage for pressurized fuel, which nozzle (4) is made of a material which is resistant to hot corrosion and erosive influences from the fuel, c h a r a c t e r i z e d in that the nozzle (4) is made of a HIP-treated, nickel-based alloy which, in percentage by weight and apart from generally occurring impurities, comprises from 40 to 50% of Cr, from 0 to 0.55% of C, less than 1.0% of Si, from 0 to 5.0% of Mn, less than 1.0% of Mo, from 0 to less than 0.5% of B, from 0 to 8.0% of Al, from 0 to 1.5% of Ti, from 0 to 0.2% of Zr, from 0 to 3.0% of Nb, maximum 0.01% of 0, maximum 0.03% of N, maximum 2% of Hf, maximum 1.5% of Y, a combined content of Co and Fe of maximum 5.0% and the balance Ni.

9. A nozzle according to claim 8, c h a r a c¬ t e r i z e d in that the alloy comprises maximum 6% of Al.

10. A nozzle according to claim 8 or 9, c h a r¬ a c t e r i z e d in that the alloy comprises at least 2.5% of Al.

11. A nozzle according to any one of claims 8-10, c h a r a c t e r i z e d in that the alloy comprises maximum 0.1% of C and maximum 0.5% of Hf.

12. A nozzle according to claim 8, c h a r a c- t e r i z e d in that the alloy comprises maximum 0.45% of Al, maximum 0.1% of C and maximum 0.1% of Ti.

13. A nozzle according to claim 8, c h a r a c¬ t e r i z e d in that the alloy comprises at least 0.5% of Ti and preferably at least 0.5% of Hf.

14. A nozzle according to any one of claims 8-13, c h a r a c t e r i z e d in that the alloy comprises at least 45% of Cr and from 0.15 to 0.40% of B, prefer¬ ably maximum 0.25% of B.

15. A nozzle according to any one of claims 8-14, c h a r a c t e r i z e d in that the alloy comprises from 1.0 to 2.0% of free Nb.

16. A nozzle according to any one of claims 3-15, c h a r a c t e r i z e d in that the nozzle has six, seven or more nozzle holes (5) all having their longi¬ tudinal axes positioned within a spherical angle of maximum 100° and preferably maximum 80°.