Steel part for an aircraft and method for manufacturing same

Abstract

The invention relates to a method for manufacturing a steel part (10) for an aircraft, the method comprising the welding of at least two steel elements (12, 14) to one another so as to form this part (10), the application of a resistance treatment to the part (10) which comprises austenitization (30) followed by quenching (32) and at least one step of tempering (34), and the local application, to a weld bead (16) between the at least two elements (12, 14), of a softening treatment comprising heating the weld bead (16) to a softening temperature lower than the temperature Ac1 at which the steel starts to transform from ferrite into austenite.

Description

Aircraft steel part and its manufacturing process Technical field

[0001] This disclosure relates to the field of metallurgy, and more particularly to a method for manufacturing a steel part for aircraft. This disclosure also relates to a steel part obtained by this method. Such a part can be used for various applications, for example for any type of aircraft such as an airplane, a helicopter, a drone, etc., in particular in landing gear. Prior art

[0002] Aircraft landing gear structural parts, for example, can be manufactured from forged steel parts that are forged, giving them an overall shape (or preform), and then machined. These parts then undergo one or more heat treatments in their entirety, allowing them to achieve the desired mechanical characteristics.

[0003] In order to reduce manufacturing costs, it was considered to form such parts not from a single billet but by assembling several elements. However, the certification of the parts thus obtained posed a problem, because the assembly areas constituted weak points without knowing how to treat them properly.

[0004] There is therefore a need for a new type of process for manufacturing a steel part. Statement of the invention

[0005] To this end, the present disclosure relates to a method for manufacturing a steel part for an aircraft, comprising the welding of at least two steel elements together so as to form said part, the application to the part of a resistance treatment comprising austenitization followed by quenching and at least one tempering, then the local application, to a weld bead between the at least two elements, of a softening treatment comprising heating the weld bead to a softening temperature below the Acl temperature at which the steel begins to transform from ferrite to austenite.

[0006] The process is particularly applicable to low-alloy steel parts with a tensile strength Rm greater than or equal to 1800 MPa. It is also particularly useful for forged steel parts. A forged steel part is a steel part in which at least one of the elements has undergone at least one forging step.

[0007] Each steel element is welded to at least one other steel element so that together, the steel elements form the steel part. Hereinafter, for the sake of brevity, we will simply speak of element and part to designate a steel element and the steel part. Furthermore, hereinafter, and unless otherwise indicated, by "a" or "the" element or other, we mean "at least one" or "the at least one" or even "each" element or other. Conversely, the generic use of the plural can include the singular.

[0008] Machining of at least one element can optionally be carried out before resistance treatment: this facilitates machining, because resistance treatment makes the element more difficult to machine. Furthermore, machining can be carried out before welding, because it is easier to handle an isolated element than the entire part. In either case, we speak of pre-machining.

[0009] Welding two elements together results in the formation of a weld bead at the interface between these two elements.

[0010] Resistance treatment, so named because of its ability to give the part very high mechanical strength, is a heat treatment that includes austenitization, then quenching, and then at least one tempering. Resistance treatment is applied to the part, that is, at least to the elements welded together. Resistance treatment can be applied to the entire part.

[0011] Austenitization involves heating the part to a temperature above the Acl temperature at which the steel begins to transform from ferrite to austenite, or even to a temperature above the Ac3 temperature at which the steel ends to transform from ferrite to austenite.

[0012] Quenching, carried out for example in air, water, oil or in a polymer bath, allows the steel to be cooled very quickly, for example to a temperature substantially equal to room temperature, or more generally less than 100°C. Quenching increases the strength of the steel.

[0013] Tempering may involve heating the part to a temperature below the Acl temperature. Tempering allows the trade-off between strength and ductility of the part to be adjusted.

[0014] The resistance treatment may further include a cryogenic treatment, for example after quenching and / or before tempering. This treatment includes maintaining the part at a temperature less than or equal to 0°C.

[0015] Softening treatment, so named because of its ability to increase toughness and resistance to crack propagation, involves heating the weld bead to a softening temperature. The fact that the softening temperature is lower than the Acl temperature prevents austenitization of the weld bead, which would otherwise negate the benefits of the strength treatment. In contrast to strength treatment, which is a global treatment of the part, softening treatment is a local treatment that primarily, if not exclusively, targets the weld bead and the heat-affected zone of the weld.

[0016] Welding, strength treatment, and softening treatment are performed in this order, so that the strength treatment also benefits the weld bead, and the softening treatment further improves the properties of the weld bead. In other words, the strength treatment normalizes the microstructure of the weld bead before softening to achieve optimal mechanical properties in the weld bead after softening.

[0017] By virtue of the fact that the part undergoes a resistance treatment and then the weld bead undergoes a local softening treatment, the non-welded parts retain their good mechanical properties related to the resistance treatment and can be certified by the same certification methods as those used previously. In addition, the softening treatment ensures that the weld bead has good resistance to crack propagation, which makes it possible to certify the parts of the part containing the weld bead by methods taking into account criteria such as crack propagation. Thus, the proposed manufacturing process makes it possible to reduce the cost of manufacturing of steel parts while ensuring their good mechanical properties and their suitability for certification.

[0018] In some embodiments, during the softening treatment, an area of ​​the part comprising the weld bead is heated more than an area of ​​the part remote from the weld bead. Thus, the softening treatment is local, so as not to degrade the mechanical strength or the yield strength in remote areas that do not require it. The remote area may not be heated at all, and at most see its temperature increase by thermal conduction from the area comprising the weld bead. Optionally, an insulation or even cooling system may be put in place to limit the increase in temperature of the remote area.

[0019] In some embodiments, at least one of the elements comprises a locally thicker section in the area comprising the weld bead. This makes it possible to maintain a margin to ensure that despite the softening, the maximum stresses reached in said area do not exceed the strength level of this area. In any event, the fact that the softening treatment is local avoids impacting the entire element.

[0020] In some embodiments, the softening temperature is lower than the Acl temperature by at most 150°C. In other words, the softening temperature is greater than or equal to Acl-150°C and less than Acl. In this temperature range, the resistance to crack propagation can be significantly increased.

[0021] In some embodiments, the resistance treatment comprises, after the tempering, a second tempering. The initial tempering may also be referred to as the first tempering. The second tempering may or may not be the same as the first tempering.

[0022] In some embodiments, the manufacturing method includes, after the strength treatment, finish machining. Finish machining allows the part to be given its final shape with great precision. Being more difficult to implement due to the greater strength of the part, finish machining can remove less material from the part than pre-machining, which precedes welding.

[0023] In some embodiments, the heating of the weld bead is carried out by conduction or induction. Conduction can be implemented from a heating resistor or a heating mat. The heating resistor or the heating mat can be positioned on the weld bead, and can be limited to the area to be heated, to the exclusion of other areas of the part (whose boundary condition can be an ambient temperature). Induction can be implemented using one or more turns arranged at a distance from the part and which, traversed by alternating currents, create a magnetic field which heats the part locally. While conduction heating is simple to implement, induction heating makes it easy to adapt to complex part geometries.Different heating modes, including for example the two above-mentioned, can further be combined, for example to improve temperature uniformity.

[0024] In some embodiments, the weld comprises at least one of electron beam welding, laser welding, and friction welding.

[0025] In some embodiments, the welding of the at least two elements is carried out without filler metal. The elements are therefore joined to each other without adding material, and heating their common interface allows them to be welded. This results in good continuity of chemical composition and mechanical properties obtained at the end of the resistance treatment.

[0026] In some embodiments, the steel has a mass composition of: 0.38 to 0.45% C, 0.60 to 0.90% Mn, 1.45 to 1.80% Si, 1.65 to 2.00% Ni, 0.70 to 0.95% Cr, 0.35 to 0.50% Mo, 0.05 to 0.10% V, 0.35% or less Cu, 0.01% or less P, 0.0010% or less S, 0.0080% or less Ti, 0.0050% or less Nb, and the remainder Fe and unavoidable impurities. Such steel, symbolized 40NiSiCrMo7 according to European standards, is known under the AISI reference "300M", or the AMS reference "6417-6419". The steel can also have the composition defined by the reference "AMS 6257". It has high mechanical characteristics, good resistance to shocks and fatigue effects, and is suitable for aircraft parts, particularly landing gear.

[0027] Unavoidable impurities are defined as elements that are not intentionally added to the composition and are brought with other elements. Although the above composition gives the maximum content of certain impurities such as copper, phosphorus or sulfur, other impurities may be present such as antimony, with a maximum mass content of 0.01%.

[0028] In some embodiments, the manufacturing method further comprises certifying the steel part by a first method for the elements and by a second, different method for the weld between the elements. The weld between the elements may refer to the weld bead and possibly the directly adjacent area. According to one example, the first method aims to verify that the element is dimensioned to withstand given loads and achieve a given fatigue life: for example, the element may be dimensioned, among other things, to withstand a limit load case achievable in the life cycle of the element and for which the element must not undergo plastic deformation, an ultimate load case which induces a higher stress than in the limit load case and for which the element must not break, and / or a fatigue life cycle with a predetermined life.According to one example, the second method is intended to verify that the weld bead is sized to tolerate damage: for example, the weld bead may be sized, among other things, to resist crack propagation long enough to allow crack detection and maintenance, to provide multiple force paths to provide redundancy in force transmission, and / or to have a shape capable of limiting crack propagation. Certification, in particular the first method and / or the second method, may include physical tests performed on the steel part. These physical tests include, for example, fatigue crack propagation resistance tests. In some embodiments, these physical tests are performed according to ASTM E647, for example ASTM E647-23a.

[0029] The mixed certification of the steel part, by two methods applied to two different areas, makes it possible to better meet the certification standards: this mixed or hybrid certification adapts to the changes in the part, thanks to the use of a second method for welding between the elements, while relying on the experience accumulated with the first method for steel elements, the manufacture of which involves controlled methods.

[0030] This disclosure also relates to a steel part for aircraft, in particular for aircraft landing gear, produced by the manufacturing process described above. The part may have characteristics which result from all or part of the characteristics of the process presented above.

[0031] In certain embodiments, the toughness of an area of ​​the part comprising the weld bead is greater than or equal to 80 MPaVm, preferably 90 MPaVm, more preferably 100 MPaVm. The resistance of this area to crack propagation is therefore very good. The toughness can be measured in a manner known per se to those skilled in the art, for example according to standard ASTM E399-22. Brief description of the drawings

[0032] Other characteristics and advantages of the subject of the present disclosure will emerge from the following description of embodiments, given as non-limiting examples, with reference to the appended figures.

[0033] Figure 1 schematically represents steps of a manufacturing method according to one embodiment.

[0034] Figure 2 is a time-temperature graph illustrating an example of a resistance treatment. Detailed description

[0035] A method of manufacturing a steel part 10 for an aircraft according to one embodiment is described with reference to Figures 1 and 2. In Figure 1, two steel elements, for example forged, are shown schematically, namely a first element 12 and a second element 14 (collectively designated as the elements 12, 14), intended to be assembled to each other in order to form the part 10. However, more elements can be provided.

[0036] Elements 12, 14 are here of identical composition. Alternatively, elements of different compositions may be provided, provided that their difference in composition remains compatible with the welding assembly which will be described later.

[0037] More specifically, elements 12, 14 are here made of 300M steel, the composition of which was given previously. This steel has an Acl temperature at which ferrite to austenite begins to transform of 725°C and an Ac3 temperature at which ferrite to austenite ends to transform of 870°C.

[0038] The elements 12, 14 may be supplied in the normalized, tempered condition. According to one example, the normalization comprises the austenitization of the steel and then its cooling in air in order to recrystallize the grain resulting from the hot thermomechanical transformation and to redissolve the cementite in order to then control its distribution. According to one example, the tempering comprises the heating of the steel to a temperature generally between 650 and 720°C, in any case below the Acl temperature, in order to improve its machinability. In the normalized, tempered condition, the elements may have a tensile strength Rm of approximately 1100 MPa.

[0039] At least one of the elements 12, 14 may be pre-machined, for example if the element in question is initially supplied in blank form. The bar shape illustrated in Figure 1 is purely schematic.

[0040] As indicated previously, the manufacturing method comprises welding the elements 12, 14 together so as to form the part 10. The welding operation leads to the creation of a weld bead 16 at the welded interface between the first element 12 and the second element 14.

[0041] For example, the welding can be carried out by electron beam, by laser and / or by friction, in particular by inertial rotary friction. These welding methods are, in themselves, known, and will not be described further. More generally, the welding can be carried out without filler metal, which guarantees good control of the composition of the weld bead 16.

[0042] After welding, a resistance treatment is applied to the part 10. As shown schematically in FIG. 1, the resistance treatment may be applied to the entire part 10, for example by placing the entire part 10 in suitable thermal equipment 20. At the very least, the resistance treatment is applied at least to the first element 12, the second element 14 and the weld bead 16, preferably simultaneously.

[0043] The resistance treatment is more particularly illustrated in Figure 2, which represents, as a function of time, the temperatures to which the part 10 is subjected. Figure 2 is schematic, so that the temperatures and times represented may not be to scale or consistent from one heat treatment to another. The origin of the ordinates is the ambient temperature, typically between 20 and 25°C.

[0044] The resistance treatment comprises austenitization 30: the part 10 is maintained at a temperature above the Acl temperature, for example above 800°C for 300M steel, for a period of 25 minutes to 9 hours so as to transform the ferrite of the elements 12, 14 and of the weld bead 16 into austenite. In the present description, the temperatures given designate the temperatures of the enclosures in which the component to be treated is maintained.

[0045] After austenitization, the part 10 undergoes quenching 32: it is immersed in a much colder medium (air, water, oil, polymers, etc.) to cool, typically at an average speed greater than or equal to 0.8°C / s over the range 800°C-100°C. According to a particular embodiment, during quenching, the part can be cooled rapidly, for example at an average speed greater than or equal to 0.8°C / s, to a temperature between 150°C and 300°C, then maintained at this temperature for a period of at least 10 minutes. Following quenching, the temperature of the part 10 is again increased to subject the part 10 to a first tempering 34. The temperature of the part 10 during tempering 34 remains below the temperature Acl. For example, during the first tempering 34, the part 10 can be maintained at a temperature between 290°C and 310°C for a period of between 2 hours and 12 hours.The first tempering 34 may be followed by a second tempering 36, identical or different from the first tempering 34. Between the first tempering 34 and the second tempering 36, the temperature of the part 10 may have dropped substantially to ambient temperature.

[0046] At the end of the resistance treatment, the entire part 10, including the elements 12, 14 and the weld bead 16, is in a homogeneous state, with mechanical characteristics of very high resistance both static and fatigue. However, the resistance to crack propagation may be relatively low. Although this is not a difficulty for the elements 12, 14, for which a great deal of experience has been accumulated, this may pose a problem for the certification of the weld bead 16.

[0047] To this end, the manufacturing method comprises the local application, to the weld bead 16, of a softening treatment comprising heating the weld bead 16 to a softening temperature lower than the temperature Acl. However, in order to sufficiently improve the resistance to crack propagation, the softening temperature may be relatively close to the temperature Acl, for example greater than or equal to Acl-150°C (for example, greater than or equal to 600°C). Preferably, the softening temperature may be between Acl-125°C and Acl-25°C. The control of the softening temperature may be carried out with reference to prior tests on parts instrumented with temperature sensors, for example thermocouples and / or pyrometers.The weld bead 16 can be maintained at the softening temperature for a period of between 5 minutes and 15 hours, depending on the expected duration of softening: the higher the temperature, the faster the softening.

[0048] As illustrated in Figure 1, local equipment 22 may be provided to heat an area of ​​the part 10 comprising the weld bead 16 more than an area of ​​the part 10 remote from the weld bead 16. The local equipment 22 may comprise a heater of the heating resistor or heating mat type, which is placed on the area comprising the weld bead 16 in order to heat the part 10 by conduction. Alternatively or in addition, the local equipment 22 may comprise a conductive coil, positioned for example coaxially around the part 10 and in line with the weld bead 16; by being traversed by alternating currents, the coil creates a local magnetic field which induces currents in the part 10 in order to heat it by induction.

[0049] Although Figure 1 illustrates the case of local equipment 22 placed around the part 10, the local equipment 22 may, alternatively or additionally, be placed inside the part 10 if necessary, for example when the part 10 is hollow. Furthermore, as an alternative to the local equipment 22, the softening treatment may be applied locally by placing the part 10 in a furnace, including at least one area not comprising the weld bead, while ensuring localized cooling of said at least one area of ​​the part 10.

[0050] The manufacturing process may, if necessary, include further machining steps following the softening treatment. This finishing machining generally concerns operations which require a precision incompatible with carrying them out before the resistance treatment and / or the softening treatment.

[0051] The part 10 thus obtained can be used for an aircraft, for example for a landing gear. Thanks to the fact that a softening treatment is added to the other treatments provided, the toughness of an area of ​​the part 10 comprising the weld bead 16 is increased, which has the effect of reducing the propagation of cracks in the weld bead 16. For example, the toughness of an area of ​​the part comprising the weld bead 16 can be greater than or equal to 80 MPa.Vm, preferably 90 MPa.Vm, more preferably 100 MPa.Vm.

[0052] The manufacturing method may further comprise certifying the part 10. A first certification method 24 may be used to certify the elements 12, 14, while a second certification method 26, different from the first method 24, may be used to certify the weld bead 16.

[0053] According to one example, the first method 24 may comprise verifying that the elements 12, 14 are sized to withstand a limit load case, an ultimate load case and / or a certain fatigue life cycle, as detailed previously. For the limit load case, the elements 12, 14 must not undergo plastic deformation that could prevent safe operation. The ultimate load case is based on forces that do not occur during normal operation and that are greater, for example by 50%, than those used to size the limit load case. Finally, the fatigue life cycle may be predetermined, for example defined by the aircraft manufacturer. Typically, a landing gear component must be able to withstand 60,000 landings or takeoffs. Thus, the first method 24 may comprise physical tests.

[0054] In practice, the first method 24 can be validated by comparison with the experience acquired on similar elements 12, 14, which promotes the valorization of experience and accelerates certifications.

[0055] According to one example, the second method 26 may comprise verifying that the weld bead 16 is sized to tolerate damage. Thanks to the softening treatment, not only does the weld bead 16 limit the propagation of cracks within it (at iso-stress, the crack propagates by a shorter length), but in addition, it helps to stop the propagation of cracks which would have initiated in one of the elements 12, 14. Thus, the second method 26 may comprise physical tests.

[0056] Concrete examples of the implementation of the manufacturing process are detailed below.

[0057] In a first example, the two elements 12, 14 are hollow revolution cylinders of forged 300M steel, with a mass composition of 0.415% C, 0.79% Mn, 1.68% Si, 1.76% Ni, 0.81% Cr, 0.39% Mo, 0.068% V, 0.07% Cu, 0.005% P, 0.0006% S. The welding of the two elements 12, 14 is carried out by electron beam. The austenitization comprises maintaining the part 10 in a furnace at a temperature of 870° for 45 minutes. The part 10 is quenched in oil up to 30°C then tempered twice with a maintenance at 300°C for two hours.

[0058] Then, preferably after the part has returned to room temperature, a heating resistor is applied to the weld bead 16 so as to maintain the weld bead at 635°C for one hour.

[0059] During testing, a toughness between 90 MPaVm and 130 MPaVm is measured depending on the dimensions of the part and the actual softening temperatures.

[0060] In a second example, the same process can be implemented with the 35NCD16 alloy (AFNOR standard), with a mass composition of 0.32% to 0.39% C, 0.30% to 0.60% Mn, 0.10% to 0.40% Si, 3.60% to 4.10% Ni, 1.60% to 2.00% Cr, 0.25% to 0.40% Mo, 0.035% or less P, 0.0035% or less S, according to the same parameters.

[0061] Although the present description refers to specific exemplary embodiments, modifications may be made to these examples without departing from the general scope of the invention as defined by the claims. Furthermore, individual features of the various embodiments illustrated or mentioned may be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

Claims

CLAIMS

1. A method of manufacturing a steel part (10) for an aircraft, comprising welding at least two steel elements (12, 14) together so as to form said part (10), applying to the part (10) a resistance treatment comprising austenitization (30) followed by quenching (32) and at least one tempering (34), then locally applying, to a weld bead (16) between the at least two elements (12, 14), a softening treatment comprising heating the weld bead (16) to a softening temperature lower than the temperature Acl at which the steel begins to transform from ferrite to austenite.

2. A manufacturing method according to claim 1, wherein during the softening treatment, an area of ​​the workpiece (10) including the weld bead (16) is heated more than an area of ​​the workpiece (10) remote from the weld bead (16).

3. A manufacturing method according to claim 1 or 2, wherein the softening temperature is lower than the Acl temperature by at most 150°C.

4. A manufacturing method according to any one of claims 1 to 3, wherein the resistance treatment comprises, after the tempering (34), a second tempering (36).

5. Manufacturing method according to any one of claims 1 to 4, wherein the heating of the weld bead (16) is carried out by conduction, in particular from a heating resistor or a heating mat, or by induction.

6. A manufacturing method according to any one of claims 1 to 5, wherein the welding comprises at least one of electron beam welding, laser welding and friction welding.

7. Manufacturing method according to any one of claims 1 to 6, in which the welding of the at least two elements is carried out without filler metal.

8. Manufacturing method according to any one of claims 1 to 7, in which the steel has the mass composition: from 0.38 to 0.45% of C, from 0.60 to 0.90% of Mn, from 1.45 to 1.80% of Si, from 1.65 to 2.00% Ni, 0.70 to 0.95% Cr, 0.35 to 0.50% Mo, 0.05 to 0.10% V, 0.35% or less Cu, 0.01% or less P, 0.0010% or less S, 0.0080% or less Ti, 0.0050% or less Nb, the remainder being Fe and unavoidable impurities.

9. A manufacturing method according to any one of claims 1 to 8, further comprising certifying the steel part (10) by a first method (24) for the elements (12, 14) and by a second, different method (26) for the weld between the elements (12, 14).

10. Steel part (10) for aircraft, in particular for aircraft landing gear, produced by the manufacturing method of any one of claims 1 to 9.

11. A steel part (10) according to claim 10, wherein the toughness of an area of ​​the part comprising the weld bead (16) is greater than or equal to 80 MPaVm, preferably 90 MPaVm, more preferably 100 MPaVm.

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