WO2022234220A1

WO2022234220A1 - Method for forging a part made of maraging steel

Info

Publication number: WO2022234220A1
Application number: PCT/FR2022/050809
Authority: WO
Inventors: Maxime Eric VINCENT; Laurent Ferrer
Original assignee: Safran Aircraft Engines
Priority date: 2021-05-05
Filing date: 2022-04-28
Publication date: 2022-11-10
Also published as: FR3122667A1; FR3122667B1

Abstract

The invention relates to a method for forging a part from a maraging steel ingot, characterized in that said method, further to conventional forging steps, comprises a recrystallizing step selected from: - a step of recrystallizing by heating to a temperature higher than the temperature Ac3 marking the end of transformation of ferrite to austenite, and a temperature lower than the precipitate redissolution temperature; and/or - a ferritic annealing step, the ferritic annealing step being performed: - either before the coldest temperature of the part is lower than the temperature Ar1, by holding the part at a temperature between Ar1 and Ar3, where Ar1 is the temperature marking the end of transformation of austenite to ferrite and Ar3 is the temperature marking the start of transformation of austenite to ferrite, with the proviso that these temperatures are measured in a cooling cycle; - or, after the hottest temperature of the part is lower than Ms, by holding the part at a temperature between Ac1 and Ac3, where Ms is the martensite formation temperature, Ac1 is the temperature marking the start of transformation of ferrite to austenite and Ac3 is the temperature marking the end of transformation of ferrite to austenite, with the proviso that Ac1 and Ac3 are the temperatures measured in a heating cycle.

Description

PROCESS FOR FORGING A PIECE OF MARAGING STEEL

Technical area

The invention relates to the field of preparation of metal parts for the aeronautical industry and more specifically to forging processes for very high-strength steels.

Prior technique

Very high resistance steels of the maraging type are particularly used for dimensioned fatigue parts, for example in turbomachines, and in particular for compressor and turbine shafts. Improving the fatigue resistance of these steels represents a considerable potential gain for the service life of parts made from these steels.

Conventionally, maraging steel parts are made from ingots, themselves obtained by vacuum arc remelting processes, or by vacuum induction melting. However, it is observed that the conventional forging processes making it possible to transform the ingot into a desired part can cause the appearance of zones which are not completely recrystallized, which has an unfavorable impact on the ductility of the steels. In particular, recrystallization heterogeneities are observed which appear when the ingots are deformed at rational strain rates greater than or equal to 1 s ¹ .

These heterogeneities are not desirable, because they are at the origin on the one hand of a significant dispersion in the results of tensile tests in fatigue, and because they are also at the origin of ultrasonic background noises which exceed the usual minimum thresholds, which reduces the ability of ultrasonic characterization techniques to detect possible defects.

For at least these two reasons, it is necessary to succeed in improving the forging processes in order to obtain parts whose mechanical properties are improved, and in particular whose dispersion of the results of tensile tests is reduced, or even whose ultrasound background is reduced.

Disclosure of Invention To meet this need, the inventors propose a process for forging a part from a maraging steel ingot comprising the following steps:

- a first step of forging the ingot carried out at a rational deformation rate of less than 1 s ¹ until a deformation greater than or equal to 1 at a temperature between 1150° C. and 1250° C.; followed

- a second forging step carried out at a rational deformation rate greater than or equal to 1 s ¹ and up to a deformation greater than or equal to 0.5 at a temperature between 900°C and 1050°C, characterized in that the method further comprises at least one stage of recrystallization of the part after the second heating stage, the recrystallization stage being chosen from:

- a step of recrystallization by heating to a temperature above the Ac3 temperature at the end of transformation of the ferrite into austenite, and a temperature below the temperature at which the precipitates are redissolved; and or

- a ferritic annealing step, the ferritic annealing step being carried out:

- either, before the coldest temperature of the part is lower than the Arl temperature, by maintaining the part at a temperature between Arl and Ar3, where Arl is the end temperature of the transformation of austenite into ferrite and Ar3 the start temperature of transformation of austenite into ferrite, it being understood that these temperatures are measured in a cooling cycle;

- or, after the hottest part temperature is below Ms, by heating the part to a temperature between Acl and Ac3, where Ms is the martensite formation temperature, Acl is the start temperature transformation of ferrite into austenite and Ac3 the end temperature of transformation of ferrite into austenite, it being understood that Ac1 and Ac3 are the temperatures measured in a heating cycle.

The first two stages of the forging process considered alone correspond to a conventional forging process, and it is observed that these stages, and particularly the second forging stage, can create significant recrystallization heterogeneities in the part obtained directly after forging. In particular, significant recrystallization heterogeneities are often observed between the inside and the outside of the ingot. However, the inventors have observed that the recrystallization step makes it possible on the one hand to significantly improve the mechanical properties of the parts obtained, and in particular their tensile strength and on the other hand to reduce the background noise of the parts obtained during ultrasonic testing.

It is understood that the "resolution temperature of the precipitates" is understood with respect to the precipitates having an influence on the mobility of the grain boundaries in the maraging steel.

As will be explained in the detailed description of the invention, the temperatures Acl, Ac3, Ar3, Arl and Ms referred to in the present application are understood with their usual meaning in the field of steels and are a function of the contents exact in addition elements chosen for the alloy.

These values can be measured for a given alloy by plotting constant cooling rate thermograms, to obtain an accurate phase diagram for the specific steel chosen.

Without wishing to be bound by theory, the inventors believe that recrystallization makes it possible to reduce the quantity of non-recrystallized zones and thus to allow better homogenization of the conditions between the core and the periphery of the initial ingot, so as to reduce the non-recrystallized zones. recrystallized, responsible for the aforementioned inconveniences.

In addition, the proposed method makes it possible to improve the parts obtained after forging while keeping the steps of the forging method unchanged with respect to the methods of the prior art. This results in a forging process whose implementation is facilitated, because it does not require reviewing the existing installations but only adding a recrystallization step as indicated.

Within the meaning of the invention, a maraging steel is a steel having high strength and hardness, while maintaining good ductility, in the usual sense of the terms “maraging steel” in the field of steels. The adjective “Maraging” is a portmanteau word born from the contraction in English of “martensitic aging”, that is to say maturing of martensite, in relation to the process of preparation of these steels. In one embodiment, the maraging steel ingot was obtained by vacuum arc remelting.

In one embodiment, the deformation obtained after the first step is between 1.0 and 2.5, preferably between 1.0 and 2.0.

Within the meaning of the invention, the deformation e obtained during a process step corresponds to the natural logarithm of the ratio between the surface of the cross section of the part obtained after deformation and the surface of the cross section of the initial part. In other words, the deformation e can be measured by the following formula: e = In (S _fj n/Sjnit) where Sinit is the surface of the transverse section of the part before the deformation and S _fin the surface of the section transverse part after deformation.

In one embodiment, the deformation obtained after the second step is greater than 0.5, for example between 1.0 and 2.0.

In one embodiment, the first step is performed in a hydraulic press.

In one embodiment, the second step can be carried out in a 4-hammer type forging machine or in a rolling mill.

In one embodiment, the recrystallization step is only a heating recrystallization step.

In one embodiment, the duration of the stage of recrystallization by heating can be chosen between 30 minutes and 4 hours.

In one embodiment, the recrystallization step is only a ferritic annealing step, carried out directly after the second forging step.

In such an embodiment, the duration of the ferritic annealing step depends on the exact composition of the alloy.

The precise determination of the optimal duration of such a step can be made, for example by reading the time-temperature-transformation graph or by reading of the transformation graph in continuous cooling obtained specifically for the chosen steel.

For example, in the case of ferritic annealing, the duration may be greater than or equal to one hour, or even greater than or equal to 8.0 hours and less than or equal to 72.0 hours, or even less than or equal to 9.6 hours .

In one embodiment, the duration of a ferritic annealing step can be between 8.0 hours and 9.6 hours.

In one embodiment where a step of recrystallization by heating is carried out, the temperature of the step of recrystallization by heating can be between the Ac3 temperature and the temperature at which the precipitates blocking the mobility of the grain boundaries are put back into solution.

In this embodiment, the stage of recrystallization by heating may have a duration greater than or equal to 10 minutes, or even greater than or equal to one hour and less than or equal to 5 hours, or even less than or equal to 1.2 hours.

In the particular case where the carbon content of the maraging steel is greater than 0.05% by mass, the forging process may further comprise, after the step of recrystallization by heating, an additional heat treatment comprising heating to a temperature between 150°C and Acl + 75°C and for a duration between 1.0 hour and 4.0 hours, preferably followed by air cooling.

This additional heat treatment makes it possible to reduce the risks of the appearance of cracking defects known as “splits” in the part obtained at the end of the process.

The choice of the applied heating treatment(s) may depend on the specific alloying elements contained in the maraging steel under consideration.

In particular, a stage of recrystallization by ferritic annealing is preferred for maraging steels having a carbon content greater than 0.05% by mass of carbon, since it may be suitable on its own, without additional heat treatment and since it therefore makes it possible to obtain the advantages of the process in a simplified manner. In addition, the ferritic annealing step is also advantageous for maraging steels having an individual content of vanadium, molybdenum, niobium and/or tungsten greater than or equal to 5%, since the latter may exhibit carbide precipitation. Ferritic annealing then makes it possible to finely homogenize the precipitation of these carbides, leading to homogeneous mechanical properties throughout the part.

The step of recrystallization by heating is itself particularly preferred when the carbon content of the maraging steel is less than or equal to 0.05% by mass of carbon, or when the time before the recrystallization step is greater than or equal to 12 o'clock.

In addition, this step has the advantage of being able to be carried out directly in the forge furnaces, which then facilitates the implementation of the method.

In one embodiment, the recrystallization step comprises a first step of recrystallization by heating, then a second step of ferritic annealing.

The combination of these two steps makes it possible to obtain an even better recrystallization of the part obtained after forging.

In one embodiment, the steel of the ingot comprises, in mass percentages:

- a carbon content of between 0.0% and 0.30%;

- a cobalt content of between 5.0% and 8.5%;

- a chromium content of between 0.0% and 5.0%;

- an aluminum content of between 0.0% and 2.0%;

- a molybdenum content of between 1.0% and 5.5%;

- a nickel content of between 10.5% and 19.0%;

- optionally vanadium in a content less than or equal to 0.30%;

- optionally niobium in a content less than or equal to 0.10%;

- optionally boron in a content less than or equal to 60 ppm;

- optionally silicon in a content less than or equal to 0.10%;

- optionally manganese in a content less than or equal to 0.10%;

- optionally calcium in a content less than or equal to 600 ppm; - optionally rare earths in a content less than or equal to 500 ppm;

- optionally titanium in a content less than or equal to 0.50%;

- optionally oxygen in a content less than or equal to 50 ppm;

- optionally nitrogen in a content less than or equal to 100 ppm;

- optionally sulfur in a content less than or equal to 50 ppm;

- optionally copper in a content less than or equal to 1.0%;

- optionally phosphorus in a content less than or equal to 500 ppm;

- optionally hydrogen in a content less than or equal to 5 ppm;

- optionally zirconium in a content less than or equal to 400 ppm; the rest being iron and unavoidable impurities.

In particular embodiments, the maraging steel ingots can be chosen from alloys with the trade name Maraging 250 or ML 340, the compositions of which are described respectively in the documents EP 3156151 and FR 2885142, for example, the content of which is introduced here. by reference.

According to another of its aspects, the invention relates to the preparation of a compressor shaft or a turbine shaft in maraging steel for a turbomachine, comprising a step of forging an ingot as described above.

Brief description of the drawings

[Fig. 1] Figure 1 schematically represents an iron-carbon phase diagram.

[Fig. 2] FIG. 2 gives a comparative representation of the dispersion obtained for tests carried out with maraging steel characterization specimens obtained either with a forging process of the invention or with a process of the prior art.

[Fig. 3] Figure 3 gives a comparative representation of the breaking stress as a function of the number of fatigue cycles for maraging steel characterization specimens obtained either with a forging process of the invention or a process of the prior art .

Description of embodiments The invention is now described by means of figures which relate only to certain embodiments of the invention and should not be construed in a limiting manner.

As described above, the invention relates to a process for forging a part from a maraging steel ingot comprising the following steps:

- a first step of forging the ingot carried out at a rational deformation rate of less than 1 s-1 until a deformation greater than or equal to 1 at a temperature between 1150° C. and 1250° C.; followed

- a second forging step carried out at a rational deformation rate greater than or equal to 1 s-1 and up to a deformation greater than or equal to 0.5 at a temperature between 900°C and 1050°C, characterized in that the method further comprises at least one stage of recrystallization of the part after the second heating stage, the recrystallization stage being chosen from:

- a ferritic annealing step carried out:

- or, after the hottest part temperature is below Ms, heating the part to a temperature between Acl and Ac3, where Ms is the martensite formation temperature, Acl is the start temperature of martensite transformation of ferrite into austenite and Ac3 the end temperature of transformation of ferrite into austenite, it being understood that Ac1 and Ac3 are the temperatures measured in a heating cycle. The Acl, Ac3, Ar3, Arl and Ms temperatures referred to in this application are understood with their usual meanings in the field of steels, and are explained below in relation to Figure 1.

Figure 1 represents an iron-carbon phase diagram, ie it indicates, according to the carbon content 200 and the temperature 100, the phase in which the alloy is located. The domains 101, 102, 103, 104, 105 represent the different domains of predominance of the phases of the alloy and the straight lines the boundaries between these different domains.

The phase domains are shown schematically in FIG. 1 and no quantitative conclusions should be drawn from this figure, in particular, no importance should be attached to the relative scale between these different phase domains.

The phase a of ferrite alone corresponds to the domain 101 is relatively restricted, and the phase g of austenite alone corresponds to the domain 104. The domain 105, which is not of interest for the invention, is reached at higher contents of carbon and corresponds to a phase composed of a mixture of austenite and Fe3C.

The temperature Ms, identified by the marker 111 on the vertical axis of the temperatures 100, is the temperature of separation between the martensitic phase, domain 102, composed of a phase a and a phase of Fe ₃ C noted below a+Fe ₃ C and a phase consisting of a mixture of phases a and g, domain 103, subsequently denoted a+g.

It is known that when the diagram is traversed in cooling 11, or in heating, 12, the phase changes of the steel are not strictly observed at the separation temperatures between the phase domains.

Thus, in cooling 11, the change between phases g, domain 104, and a+g, domain 103, takes place at a temperature denoted Ar3 and marked 22 in figure 1 a little lower than the separation between the corresponding domains, and the same for the change between the phases a+g, domain 103, and a+Fe ₃ C, domain 102, which takes place at the temperature Arl marked 21. On the contrary, in heating 12, the change between the phases a+Fe3C, domain 102, and a+g, domain 103, takes place at the temperature Acl marked 31 a little higher than the theoretical temperature of the phase diagram, and likewise for the phase change between phases a+g, domain 103, and g, domain 104, which takes place at a temperature denoted Ac3 and marked 22 in FIG. 1 a little higher than the separation between the corresponding domains 103 and 104.

The phase diagram represented is that of iron-carbon and the real steels include other elements of additions, which are the causes of the phenomenon of hysteresis described above, that is to say of the difference which can be observed between the temperatures Acl and Arl or Ac3 and Ar3. Thus, the temperatures Acl, Ac3, Ar3, Arl and Ms depend on the exact composition chosen for the alloy.

The recrystallization step corresponds to a step of maintaining the alloy at a temperature either in the g-phase domain 104 in the case of a recrystallization step by heating, or maintaining it in the a-phase domain 103 +g in the case of ferritic annealing.

In these two cases, the alloy has, thanks to the heating, sufficient thermal energy to allow the recrystallization of non-recrystallized zones.

In one embodiment, the recrystallization step can include both heat treatment and ferritic annealing.

For example, for the Maraging 250 alloy the temperatures Acl, Ac3, Ar3, Arl and Ms are as follows: Acl = 655°C, Ac3 = 775°C and Ms = 150°C. These temperatures are given as an indication and are accompanied by an inherent uncertainty in the possible differences in composition in a Maraging 250 alloy. These temperatures depend in fact on the exact composition of the Maraging 250 alloy considered.

By convention, the Arl and Ar3 values of a maraging steel can be considered to be between Acl-20°C and Acl-5°C for Arl and between Ac3-20°C and Ac3-5°C for Ar3.

For the ML 340 alloy, the temperatures Acl, Ac3, Ar3, Arl and Ms are as follows: Acl = 610°C, Ac3 = 860°C and Ms = 175°C. These temperatures are given as indicative and are accompanied by an inherent uncertainty in the possible differences in composition in an ML 340 alloy. These temperatures depend in fact on the exact composition of the ML 340 alloy considered.

The inventors have found that the optimum temperature of a ferritic annealing step for a maraging steel can be determined by the following calculation:

[Math. 1]

where Trecuit is the optimum temperature of a ferritic annealing step and Ni, Cr, Mo, Co, V and C are, expressed as a percentage, the mass concentrations in the alloy of the elements indicated.

In one embodiment, the temperature of a ferritic annealing step can be chosen within a temperature range ranging from 20° C. below the optimum temperature of a ferritic annealing step determined above to 50° C. -above. This makes it possible to obtain optimum annealing of the part obtained and thus to reduce the heterogeneities thereof.

For example, the optimum annealing temperature that can be calculated using the equation above is 650°C for the ML 340 alloy and 670°C for the maraging 250 alloy. Thus for a part in ML 340 a ferritic anneal is ideally carried out at a temperature between 630°C and 700°C. For a 250 maraging part, ferritic annealing is ideally carried out at a temperature between 650°C and 720°C.

The inventors have found that the preferred temperature of a stage of recrystallization by heating for a maraging steel can be determined by the following formula:

[Math. 2] where Trecrist is the preferred recrystallization temperature and C, V and Mo are, expressed in percentages, the mass concentrations in the alloy of the elements indicated.

In a particular embodiment, the recrystallization treatment by heating is carried out at a temperature between Ac3+25° C. and Trecrist, the preferred recrystallization temperature determined by the above formula.

In a still more preferred embodiment, the optimum temperature of a recrystallization step by heating for a maraging steel can be determined by the following formula:

[Math. 3]

Preferably, the recrystallization treatment is carried out at a temperature between Ac3+25° C. and Trecrist2, the optimum recrystallization temperature determined by the above formula. This makes it possible to obtain an even further improved recrystallization of the part obtained and thus to reduce its heterogeneities.

For example, the optimum recrystallization temperature determined using the above equation is 985°C for a ML 340 part and 890°C for a Maraging 250 part. Thus, a recrystallization step by heating can ideally be carried out between 885°C and 985°C for a part in ML 340 and between 800°C and 890°C for a part in Maraging 250.

Figure 2 shows experimental results obtained with maraging steel characterization specimens stressed in tension. The tensile tests were carried out at 20°C. Curve 3 is obtained with test pieces from the prior art, while curve 4 with test pieces obtained by the forging process described above comprising the recrystallization step. Figure 2 presents for these two sets of specimens, the number of results 1 as a function of the observed elongation at break 2.

Also reported in Figure 2 are the mean values Mi and M2 of the results obtained for distributions 3 and 4 respectively, as well as the standard deviations of the distributions +/-3oi for distribution 3 and +/-3q ₂ for distribution 4 obtained by tensile tests of the specimens of the two distributions.

FIG. 2 shows that the dispersion of the values, in particular -3q ₂ for the specimens obtained according to the forging process described, is less important than the dispersion for the specimens of the prior art -3oi.

Thus, the forging process of the invention makes it possible, on the one hand, to increase the average value of the tensile strength of the specimens, but above all, it makes it possible to increase the value M-3 o, i.e. i.e. the tensile strength of the weakest specimens of the sample of specimens tested.

These two results make it possible to conclude that the forging process proposed makes it possible to manufacture specimens of maraging steel that are stronger than those of the prior art.

Figure 3 represents experimental results of cyclic fatigue tests, i.e. the number of cycles N that it took to break a steel characterization specimen by subjecting it to a cyclic stress oscillating periodically between two values of constraints a minimum value and a maximum value C _max . FIG. 3 illustrates the maximum stress that had to be imposed to break characterization specimens of maraging steel prepared according to the prior art, represented by curve 315, and characterization specimens manufactured by the method of the invention, represented by curve 325.

Figure 3 includes cyclic fatigue test results for parts obtained according to one embodiment in which the imposed deformation C _max is between 0.72% and 0.80%, with a frequency of stress cycles between 0 .25 Hz and 2 Hz and at a temperature between 20°C and 200°C.

In FIG. 3 are also represented the envelopes presenting the statistical distribution of the tests around the curves 315 and 325. These envelopes are respectively represented by the curves 314 and 316 on the one hand and 324 and 326 on the other hand.

The envelopes correspond to the average curve increased or decreased by three times the statistical difference o determined from the distribution of the experimental results around the average curves, noted +/-3oi around curve 315 and +/- 3O ₂ around curve 325.

FIG. 3, and in particular the comparison of curves 315 and 325, illustrates the strength gain permitted by a forging method of the invention compared to a forging method of the prior art. The comparison of the lower envelopes of the curves, 314 and 324, also shows that the least resistant specimens are substantially closer to the average values in the case of a process of the invention than in the case of a process of the art. prior.

Claims

[Claim 1] A method of forging a part from a maraging steel ingot comprising the following steps:

- a ferritic annealing step carried out:

- or, after the hottest part temperature is below Ms, heating the part to a temperature between Acl and Ac3, where Ms is the martensite formation temperature, Acl is the start temperature of martensite transformation of ferrite into austenite and Ac3 the end temperature of transformation of ferrite into austenite, it being understood that Ac1 and Ac3 are the temperatures measured in a heating cycle.

[Claim 2] The forging process according to claim 1, wherein the strain obtained after the first step is between 1.0 and 2.5.

[Claim 3] Forging process according to one of Claims 1 or 2, in which the deformation obtained after the second step is between 1.0 and

2.0.

[Claim 4] A forging process according to any one of claims 1 to 3, wherein the first step is carried out in a hydraulic press.

[Claim 5] A forging process according to any one of claims 1 to 4, wherein the second step is carried out in a 4-hammer type forging machine or in a rolling mill.

[Claim 6] Forging process according to one of Claims 1 to 5, in which the step of recrystallization by heating has a duration of between 30 minutes and 4 hours.

[Claim 7] Forging process according to one of Claims 1 to 6, in which the ferritic annealing step has a duration of between 8.0 hours and 9.6 hours.

[Claim 8] Forging process according to any one of Claims 1 to 7, in which the steel of the ingot comprises, in mass percentages: