Steel aircraft component and its manufacturing process

Abstract

A method for manufacturing a steel aircraft part (10), comprising welding at least two steel elements (12, 14) together to form said part (10), applying to the part (10) a strength treatment comprising austenitizing (30) followed by quenching (32) and at least one tempering (34), and applying locally 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 Ac1 temperature at which the steel begins to transform from ferrite to austenite. Figure for the abstract: Fig. 1

Description

Title of the invention: Steel part for aircraft and its manufacturing process technical field

[0001] This description relates to the field of metallurgy, and more particularly to a process for manufacturing a steel part for an aircraft. This description also relates to a steel part obtained by this process. 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., particularly in landing gear. Previous technique

[0002] Aircraft landing gear structural components, 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 to achieve the desired mechanical properties.

[0003] In order to reduce manufacturing costs, it was considered to form such parts not from a single blank but by assembling several elements. However, the certification of the parts thus obtained posed a problem, as the assembly areas constituted points of weakness for which there was no known suitable treatment.

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

[0005] To this end, the present exposition relates to a method for manufacturing a steel part for aircraft, comprising welding at least two steel elements together so as to form said part, applying to the part a strength treatment comprising austenitizing followed by quenching and at least one tempering, and applying locally, to a weld bead between the at least two elements, a softening treatment comprising heating the weld bead to a softening temperature lower than the Acl temperature of the beginning of the transformation of ferrite into austenite of the steel.

[0006] The process is particularly applicable to low-alloy steel parts having a tensile strength Rm greater than or equal to 1800 MPa. It also finds a particularly interesting application for forged steel parts. A forged steel part is defined as a steel part in which at least one element has undergone at least one forging step.

[0007] Each steel element is welded to at least one other steel element in order to Together, the steel elements form the steel part. For the sake of brevity, we will henceforth simply refer to an element and a part to designate a steel element and the steel part. Furthermore, unless otherwise indicated, "an" or "the" element or other means "at least one" or "each" element or other. Conversely, the generic use of the plural can include the singular.

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

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

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

[0011] Austenitizing includes heating the part to a temperature above the Acl temperature at the start of the transformation of ferrite into austenite in steel, or even to a temperature above the Ac3 temperature at the end of the transformation of ferrite into austenite in steel.

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

[0013] The heating process may include heating the part to a temperature lower than the Acl temperature. The heating process allows for adjusting the trade-off between the resistance and ductility of the part.

[0014] The strength treatment may also include cryogenic treatment, for example after quenching and / or before tempering. This treatment includes maintaining the part at a temperature of 0°C or lower.

[0015] The 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 the strength treatment, which is a global treatment of the part, the Softening treatment is a local treatment that targets essentially, if not exclusively, the weld bead and the area heat-affected by the welding.

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

[0017] Thanks to the fact that the part undergoes a strength treatment followed by a localized softening treatment of the weld bead, the unwelded areas retain their good mechanical properties related to the strength treatment and can be certified using the same certification methods as previously employed. Furthermore, the softening treatment ensures good resistance to crack propagation in the weld bead, which allows the parts of the component containing the weld bead to be certified using methods that take into account criteria such as crack propagation. Thus, the proposed manufacturing process reduces the manufacturing cost of steel parts while ensuring their good mechanical properties and their suitability for certification.

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

[0019] In certain embodiments, at least one of the elements comprises a locally thicker section in the area containing the weld bead. This allows for a margin to ensure that, despite the softening, the maximum stresses reached in said area do not exceed the strength level of that area. In any case, the fact that the softening treatment is localized prevents it from affecting the entire element.

[0020] In certain 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. Within this temperature range, the resistance to crack propagation can be significantly augmented.

[0021] In some embodiments, the resistance treatment includes, after the initial income, a second income. The initial income may also be called the first income. The second income may or may not be identical to the first income.

[0022] In some embodiments, the manufacturing process includes, after strength treatment, a finishing machining operation. The finishing machining allows the part to be given its final shape with high precision. Being more difficult to implement due to the greater strength of the part, the finishing machining can remove less material from the part than the preliminary machining, which precedes welding.

[0023] In certain embodiments, the weld bead is heated by conduction or induction. Conduction can be implemented using a heating element or a heating mat. The heating element or heating mat can be positioned on the weld bead and can be limited to the area to be heated, excluding other areas of the workpiece (the boundary condition of which may be an ambient temperature). Induction can be implemented using one or more coils arranged at a distance from the workpiece, which, when traversed by alternating currents, create a magnetic field that heats the workpiece locally. While conduction heating is simple to implement, induction heating allows for easy adaptation to complex workpiece geometries.Different heating methods, including for example the two mentioned above, can also be combined, for example to improve temperature uniformity.

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

[0025] In certain embodiments, the welding of at least two elements is carried out without filler metal. The elements are therefore butted together without the addition of any material, and heating their common interface allows them to be welded. This results in good continuity of chemical composition and mechanical properties obtained after the strength treatment.

[0026] In certain embodiments, the steel has the following mass composition: 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, the remainder being Fe and unavoidable impurities. This type of steel, designated 40NiSiCrMo7 according to European standards, is known by the AISI reference "300M", or the AMS reference "6417-6419". The steel can also have the composition defined by the reference "AMS 6257". It exhibits high mechanical properties, good resistance to impact and fatigue, and is well-suited for aircraft components, particularly landing gear.

[0027] Unavoidable impurities are defined as elements that are not intentionally added to the composition and are introduced along 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 certain embodiments, the manufacturing process further includes certification of 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. For example, the first method aims to verify that the element is designed to withstand given loads and achieve a given fatigue life: for example, the element may be designed, among other things, to withstand a limit load case attainable during the element's life cycle and for which the element must not undergo plastic deformation, an ultimate load case that 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 lifespan.According to one example, the second method aims to verify that the weld bead is sized to tolerate damage: for example, the weld bead may be sized, among other things, to resist the propagation of a crack long enough to allow the detection and maintenance of the crack, to offer several paths of force in order to ensure redundancy in the transmission of forces, and / or to have a shape suitable for limiting the propagation of cracks.

[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 evolutions of 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 the steel elements, the manufacture of which involves controlled methods.

[0030] This description also relates to a steel part for an aircraft, in particular for an aircraft landing gear, manufactured using the process described above. The part may have characteristics that derive from all or part of the process characteristics 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 MPa^m, preferably 90 MPa^m, and even more preferably 100 MPa^m. The resistance of this area to crack propagation is therefore very good. Brief description of the drawings

[0032] Other features and advantages of the object of this presentation will become apparent from the following description of embodiments, given by way of non-limiting examples, with reference to the attached figures.

[0033] Fig. 1 schematically represents steps in a manufacturing process according to one embodiment.

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

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

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

[0037] More specifically, elements 12 and 14 are here made of 300M steel, the composition of which has been given previously. This steel has a ferrite-to-austenite transformation start temperature (Ac1) of 725°C and a ferrite-to-austenite transformation end temperature (Ac3) of 870°C.

[0038] Elements 12 and 14 can be supplied in the normalized tempered condition. For example, normalization involves austenitizing the steel and then air-cooling it to recrystallize the grain resulting from the hot thermomechanical transformation and to resuspend the cementite in order to control its distribution. For example, tempering involves heating the steel to a temperature generally between 650 and 720°C, but in any case below the Acl temperature, to improve its machinability. In the normalized tempered condition, the elements can have a tensile strength Rm of approximately 1100 MPa.

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

[0040] As previously stated, the manufacturing process includes welding the elements 12, 14 together 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, welding can be carried out by electron beam, laser, and / or friction, particularly by inertial rotary friction. These welding processes are, in themselves, known and will not be described further. More generally, welding can be carried out without filler metal, which ensures good control of the weld bead composition.

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

[0043] The resistance treatment is illustrated more particularly in [Fig. 2], which shows, as a function of time, the temperatures to which part 10 is subjected. [Fig. 2] is schematic, so the temperatures and times shown 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 strength treatment includes austenitizing 30: the part 10 is held 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 the weld bead 16 into austenite. In this description, the temperatures given refer to the temperatures of the chambers in which the component to be treated is held.

[0045] After austenitizing, the part 10 undergoes quenching 32: it is immersed in a much colder medium (air, water, oil, polymers, etc.) to cool down, typically at an average rate 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 rapidly cooled, for example at an average rate greater than or equal to 0.8°C / s, to a temperature between 150°C and 300°C, and then held at this temperature for a period of at least 10 minutes. Following quenching, the temperature of the part 10 is raised again to subject the part 10 to a first tempering 34. The temperature of the part 10 during tempering 34 remains below the Acl temperature. For example, during the first income 34, room 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 income 34 may be followed by a second income 36, identical or different from the first income 34. Between the first income 34 and the second income 36, the temperature of room 10 may have dropped substantially back to ambient temperature.

[0046] After the strength treatment, the entire part 10, including elements 12, 14 and the weld bead 16, is in a homogeneous state, with very high static and fatigue strength. However, the resistance to crack propagation may be relatively low. While this is not a problem for elements 12, 14, for which considerable experience has been accumulated, it may pose a problem for the certification of the weld bead 16.

[0047] To this end, the manufacturing process includes 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 Acl temperature. However, in order to sufficiently improve resistance to crack propagation, the softening temperature may be relatively close to the Acl temperature, for example, greater than or equal to Acl-150°C. Preferably, the softening temperature may be between Acl-125°C and Acl-25°C. The softening temperature may be controlled by reference to prior tests on instrumented parts 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 planned softening time: the higher the temperature, the faster the softening.

[0048] As illustrated in [Fig. 1], a local device 22 can be provided to heat an area of ​​the part 10 containing the weld bead 16 more than an area of ​​the part 10 located away from the weld bead 16. The local device 22 may include a heating element such as a heating resistor or a heating mat, which is placed over the area containing the weld bead 16 in order to heat the part 10 by conduction. Alternatively or in addition, the local device 22 may include a conductive loop, positioned, for example, coaxially around the part 10 and directly above the weld bead 16; when alternating currents flow through the loop, it creates a local magnetic field which induces currents in the part 10 in order to heat it by induction.

[0049] Although [Fig.1] illustrates the case of a local equipment 22 placed around the part 10, the local equipment 22 can, alternatively or in addition, be placed inside the part 10 where appropriate, for example when the part 10 is hollow.

[0050] The manufacturing process may, if necessary, include further machining steps following the softening treatment. This finishing machining generally concerns operations that require a level of precision incompatible with performing them before the strength treatment and / or the softening treatment.

[0051] The resulting part 10 can be used for an aircraft, for example for landing gear. Thanks to the fact that a softening treatment is added to the others In the planned treatments, the toughness of an area of ​​the part 10 including 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 including the weld bead 16 may be greater than or equal to 80 MPa.^m, preferably 90 MPa.^m, and even more preferably 100 MPa.^m.

[0052] The manufacturing process may further include the certification of part 10. A first method 24 of certification may be used to certify the elements 12, 14, while a second method 26 of certification, 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 include verifying that the elements 12, 14 are designed 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 stresses that do not occur during normal operation and that are greater, for example, by 50%, than those used to design 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 takeoffs or landings.

[0054] In practice, the first method 24 can be validated by comparison with 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 include verifying that the weld bead 16 is dimensioned to tolerate damage. Thanks to the softening treatment, not only does the weld bead 16 limit the propagation of cracks within it (at the same stress, the crack propagates over a shorter length), but it also helps to stop the propagation of cracks that would have initiated in one of the elements 12, 14.

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

[0057] In a first example, the two elements 12, 14 are hollow cylinders of revolution made 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 two elements 12, 14 are welded by electron beam welding. Austenitizing involves holding the part 10 in a furnace at a temperature of 870°C for 45 minutes. The part 10 is then oil-quenched to 30°C and tempered twice, holding at 300°C for two hours.

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

[0059] During tests, a toughness of between 90 MPa'Vm and 130 MPa'Vm is measured depending on the dimensions of the part and the effective 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% of C, 0.30% to 0.60% of Mn, 0.10% to 0.40% of Si, 3.60% to 4.10% of Ni, 1.60% to 2.00% of Cr, 0.25% to 0.40% of Mo, 0.035% or less of P, 0.0035% or less of S, according to the same parameters.

[0061] Although the present description refers to specific 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

Demands

1. A method for manufacturing a steel aircraft part (10), comprising welding at least two steel elements (12, 14) together to form said part (10), applying to the part (10) a strength treatment comprising austenitizing (30) followed by quenching (32) and at least one tempering (34), and then applying locally, 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 Acl temperature 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 part (10) comprising the weld bead (16) is heated more than an area of ​​the part (10) away from the weld bead (16).

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

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

5. A 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 resistance 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 an electron beam weld, a laser weld and a friction weld.

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

8. A manufacturing process according to any one of claims 1 to 7, wherein the steel has the following mass composition: 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, the remainder being Fe and inevitable impurities.

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

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

11. 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 MPa^m, preferably 90 MPa^m, preferably even more than 100 MPa^m.

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