EP3473734A1 - Method for treating a steel - Google Patents

Abstract

The invention relates to a process for treating a steel comprising, in percentages by weight, 0.2% to 0.33% of carbon, 4% to 8% of cobalt, 7% to 11% of nickel, 0.8% at 3% chromium, 0.5% to 2.5% molybdenum, 0.5% to 5.9% tungsten, 0.05% to 0.2% vanadium, and at most 0.02% of titanium, the remainder being iron and unavoidable impurities, the process comprising at least: a solution heat treatment (E2) of the steel at a temperature of between 950 ° C. and 1100 ° C., a quenching treatment (E3) of the steel, carried out after the solution heat treatment, the placement of the steel in a cryogenic chamber after the quenching treatment, cooling (E5) of the inside of the cryogenic chamber in which the steel is present, this cooling being carried out up to a treatment temperature (Tc) of less than or equal to -73 ° C, and the cryogenic treatment (E6) of the steel during which the treatment temperature is maintained in the chamber, the time separating the end of the quenching treatment from the beginning of the cryogenic treatment being less than or equal to 4 hours.

Description

The present invention relates to a method of treating a steel including a cryogenic treatment, the steel may be intended for the constitution of parts of aircraft landing gear.

Background of the invention

The patent US 9,051,635 discloses a steel having good mechanical properties associated with good resistance to stress corrosion.

This steel comprising, in percentages by weight, 0.2% to 0.33% of carbon, 4% to 8% of cobalt, 7% to 11% of nickel, 0.8% to 3% of chromium, 0.5% at 2.5% molybdenum, 0.5% to 5.9% tungsten, 0.05% to 0.2% vanadium, and at most 0.02% titanium, the remainder being iron and unavoidable impurities, is - in the aforementioned patent - treated by the following succession of steps without precision on the time separating each of these steps: heat treatment of solution dissolution of the steel, quenching treatment, immersion in the liquid nitrogen and income treatment.

Such steel is a material of interest for the constitution of parts of aircraft landing gear, in particular.

However, the inventors have found that there remains a significant dispersion of mechanical properties within batches of steel parts subjected to the treatment disclosed in the patent. US 9,051,635 . It is desirable to reduce the dispersion of these mechanical properties in order to optimize the dimensioning curves and, consequently, to improve the performances of the parts formed by this steel, for example by lightening them, by extending their service life or by increasing the stresses to which they may be exposed.

In addition, the immersion in liquid nitrogen carried out in the document US 9,051,635 is not a suitable step for a steel treatment on an industrial scale. It remains, therefore, desirable to have processes for treating this steel that are more compatible with a treatment on an industrial scale.

We also know the publication Wang et al. "Austenite layer and precipitation in high Co-Ni maraging steel" (Micron 57 (2014) 112-116 ) which discloses a treatment of a steel in which the latter is immersed after quenching in a cryogenic bath at -73 ° C.

Object and summary of the invention

• un traitement thermique de mise en solution de l'acier à une température comprise entre 950°C et 1100°C,
• un traitement de trempe de l'acier, réalisé après le traitement thermique de mise en solution,
• le placement de l'acier dans une enceinte cryogénique après le traitement de trempe,
• le refroidissement de l'intérieur de l'enceinte cryogénique dans laquelle l'acier est présent, ce refroidissement étant réalisé jusqu'à une température de traitement inférieure ou égale à -73°C, et
• le traitement cryogénique de l'acier pendant lequel la température de traitement est maintenue dans l'enceinte, la durée séparant la fin du traitement de trempe du début du traitement cryogénique étant inférieure ou égale à 4 heures.

According to a first aspect, the invention relates to a method of treating a steel comprising, in percentages by weight, 0.2% to 0.33% of carbon, 4% to 8% of cobalt, 7% to 11% of nickel, 0.8% to 3% chromium, 0.5% to 2.5% molybdenum, 0.5% to 5.9% tungsten, 0.05% to 0.2% vanadium, and plus 0.02% titanium, the remainder being iron and unavoidable impurities, the process comprising at least:

• a heat treatment for dissolving the steel at a temperature of between 950 ° C. and 1100 ° C.,
• a quenching treatment of the steel, carried out after the solution heat treatment,
• placing the steel in a cryogenic chamber after quenching treatment,
• cooling the inside of the cryogenic chamber in which the steel is present, this cooling being carried out up to a treatment temperature of less than or equal to -73 ° C, and
• the cryogenic treatment of the steel during which the treatment temperature is maintained in the chamber, the time separating the end of the quenching treatment from the beginning of the cryogenic treatment being less than or equal to 4 hours.

In particular, the invention is remarkable in that the time between the end of the quenching treatment and the start of the cryogenic treatment is limited.

The inventors have found, in recent studies, that the residual austenite content could vary rather lightly within a batch of steel parts described above having undergone cryogenic treatment. This variation, however slight, has a significant influence on the dispersion of the mechanical properties within the batch of treated parts. The identification of these slight variations in the residual austenite content has been made possible through the use of specific and sufficiently precise measurement (synchrotron X-ray diffraction and sigmameter). Once the origin of the dispersion of the mechanical properties was identified, the inventors studied the influence of the conditions of reaching the cryogenic treatment temperature and found that the time separating the end of the quench from the start of the cryogenic treatment had a influence on the residual austenite content. Thus, by limiting this period as described above, the invention advantageously makes it possible to better control the residual austenite content obtained in the treated steel and, consequently, to reduce the dispersion of the mechanical properties. In addition, the inventors have found that limiting this time advantageously makes it possible to improve the stress corrosion resistance of the steel. The fact of carrying out the cryogenic treatment in a cooled cryogenic chamber after loading of the steel advantageously makes it possible to make the process compatible with a treatment on an industrial scale. In addition, the invention provides a gradual cooling of the cryogenic chamber in which the steel is placed which also participates in controlling the residual austenite content with respect to the case where the steel is directly immersed in a cryogenic bath. This last case results indeed in an undesirable phenomenon of heating which does not make it possible to control correctly the cooling, and which has the consequence of not allowing to control correctly the residual austenite content.

In an exemplary embodiment, the time separating the end of the quenching treatment from the beginning of the cryogenic treatment is less than or equal to 2 hours, preferably 1 hour.

The fact of imposing such a time between quenching and the beginning of the cryogenic treatment advantageously makes it possible to further reduce the dispersion of the mechanical properties obtained for the treated steel.

In an exemplary embodiment, the duration of the cryogenic treatment is greater than or equal to 1 hour.

In an exemplary embodiment, the method further comprises a treatment of income of the steel produced after the cryogenic treatment.

In an exemplary embodiment, the treated steel is a part of a landing gear of an aircraft. The piece can for example be an axle, the balance or part of the balance, as the pivot axis of the balance.

In an exemplary embodiment, the steel constitutes a piece having a mass greater than or equal to 10 kg. The mass of the piece may be greater than or equal to 40 kg, for example greater than or equal to 100 kg.

The invention is particularly advantageous in the case of the treatment of a workpiece having a significant mass, and therefore a substantial size, insofar as the undesirable phenomenon of calefaction mentioned above occurs regardless of the size of the workpiece but is all the more important as the size of the room is large.

In an exemplary embodiment, the steel is cooled to a quenching temperature of less than or equal to 71 ° C during quenching treatment.

The fact of having a reduced quenching end temperature advantageously contributes to further reducing the residual austenite content.

Brief description of the drawings

• la figure 1 représente, de manière schématique, l'évolution de la température imposée à l'acier, selon un exemple de procédé de traitement selon l'invention, et
• la figure 2 est un résultat d'essai comparatif montrant l'influence de la durée séparant la fin du traitement de trempe du début du traitement cryogénique sur la teneur en austénite résiduelle.

Other features and advantages of the invention will emerge from the following description, given in a non-limiting manner, with reference to the appended figures in which:

• the figure 1 represents, schematically, the evolution of the temperature imposed on the steel, according to an example of a treatment method according to the invention, and
• the figure 2 is a comparative test result showing the influence of the time separating the end of the quenching treatment from the start of the cryogenic treatment on the residual austenite content.

Detailed description of embodiments

The treated steel comprises, in percentages by weight, 0.2% to 0.33% of carbon, 4% to 8% of cobalt, 7% to 11% of nickel, 0.8% to 3% of chromium, 0, 5% to 2.5% molybdenum, 0.5% to 5.9% tungsten, 0.05% to 0.2% vanadium, and at most 0.02% titanium, the balance being iron and unavoidable impurities.

In an exemplary embodiment, the treated steel comprises, in percentages by weight, 0.25% to 0.31% of carbon, 6.8% to 8% of cobalt, 9.3% to 10.5% of nickel, 0.8% to 2.6% chromium, 0.9% to 2.1% molybdenum, 0.7% to 2% tungsten, 0.05% to 0.12% vanadium, and not more than 0.015% % titanium, the rest being iron and unavoidable impurities.

In an exemplary embodiment, the treated steel comprises, in percentages by weight, 0.29% to 0.31% of carbon, 6.8% to 7.2% of cobalt, 9.8% to 10.2% of nickel, 0.8% to 2.6% chromium, 0.9% to 2.1% molybdenum, 0.7% to 1.4% tungsten, 0.05% to 0.12% vanadium, and at most 0.015% titanium, the balance being iron and unavoidable impurities.

The process is initiated by a rise ramp (step E1) to a temperature Ts, between 950 ° C and 1100 ° C.

The temperature Ts is then maintained in order to carry out the heat treating solution of the steel (step E2). The duration of the E2 solution heat treatment of the steel may be greater than or equal to 1 hour, and for example be between 1 hour and 2 hours.

A quenching treatment of the steel (step E3) is then carried out after the dissolution treatment E2. This quenching treatment E3 consists of rapid cooling of the steel by immersion in a cooling fluid such as water or oil. During the quenching treatment E3, the steel is cooled to a quenching end temperature Ta. This end of quenching temperature Ta is, in the example illustrated, equal to the ambient temperature (20 ° C.), but it is not beyond the scope of the invention if this is different from the ambient temperature, and is for example, higher than room temperature. The quenching end temperature Ta may be less than or equal to 71 ° C., preferably less than or equal to 50 ° C. In particular, the quenching temperature Ta may be between 16 ° C and 71 ° C. At the end of the quenching treatment E3, there is no longer any cooling of the steel by heat exchange between the latter and the aforementioned cooling fluid used for quenching. The temperature of the steel is equal, at that time, to the end of quenching temperature Ta.

If necessary, a first intermediate step (step E4) during which the steel is maintained in an environment at room temperature Ta can be performed after quenching E3 and before placing the steel in the cryogenic chamber. As a variant, it would be possible to dispense with this first intermediate step E4 and directly place the steel in the cryogenic chamber after the quenching treatment E3. Of course, when it is carried out, this first intermediate step E4 is of limited duration so that the time separating the end of the quenching treatment E3 from the beginning of the cryogenic treatment also remains limited, as indicated above.

Once the steel placed in the cryogenic chamber, a cooling of the interior of the enclosure is then achieved (step E5).

This cooling comprises a temperature ramp down to the treatment temperature Tc which is less than or equal to -73 ° C. The rate of cooling imposed during this ramp down temperature may be greater than or equal to 0.5 ° C / minute, for example greater than or equal to 1.5 ° C / minute, or even greater than or equal to 2.5 ° C / minute, or even greater than or equal to 5 ° C / minute. This cooling rate may furthermore be less than or equal to 4 ° C./minute. This imposed cooling rate can be substantially constant. It is not beyond the scope of the invention if the cooling rate varies during the cooling E5, the cooling E5 can thus comprise a first temperature decrease at a first cooling rate, then a second temperature decrease at a second cooling speed. cooling different from the first, for example less than this.

As mentioned above, in the invention limits the time between the end of the quenching treatment E3, corresponding to the moment when the quenching temperature Ta is reached, from the beginning of the cryogenic treatment E6, corresponding to the moment when the quenching temperature Tc treatment is reached. This duration corresponds to the duration after the end of the hardening E3 during which the steel is at a temperature above the treatment temperature Tc. Limiting this duration makes it possible to limit the residual austenite content. The figure 2 is a comparative test result made on a Ferrium® M54 steel by implementing a quenching temperature Ta of 20 ° C and a treatment temperature Tc of -73 ° C. In this test, the time between the end of the quenching treatment and the start of the cryogenic treatment was varied. It is found that the longer this duration increases the residual austenite content in the steel increases. The inventors have also carried out a comparative test in order to determine the influence of the time separating the end of the quenching treatment from the beginning of the cryogenic treatment on the resistance to stress corrosion of the steel. In this test, the stress corrosion resistance of the steel was measured in the following manner: an initial crack of the specimen is made which is wetted with a solution of NaCl, then a constant stress is imposed on the sample and after 1000h the K1SCC is determined. After 1000h, the specimen is statically broken, which makes it possible to determine its K1SCC sound from the initial crack size and with the value of the load. K1SCC is a parameter known to those skilled in the art quantifying the resistance to stress corrosion. The test carried out has shown that the stress corrosion resistance of the steel is significantly better when the steel has been treated by limiting the time separating the end of the quenching treatment from the beginning of the cryogenic treatment to 2 hours than in the case except invention where this duration is 8 hours.

The installation useful for the implementation of the method is known per se. Such an installation comprises a cryogenic chamber connected to a cooling fluid reservoir and a control system configured to control the rate of introduction of the cooling fluid into the chamber, and its evacuation rate to outside of it. The cooling fluid can be introduced inside the chamber in the gaseous state. In this case, the cooling fluid can be vaporized outside the enclosure and the cooling fluid in the gaseous state thus generated can be introduced inside the enclosure through at least one port injection. Due to the control of these flow rates of introduction and evacuation, it is possible to obtain the desired cooling rate, which contributes to having the desired duration between the end of the quench E3 and the beginning of the cryogenic treatment E6. This control of the flow rates of introduction and evacuation also makes it possible to maintain the treatment temperature Tc during the cryogenic treatment. As an example of a cryogenic installation that can be used, mention may be made of the Linde Gas VF TES fluid type liquid nitrogen installation.

A temperature stabilization plateau, during which the treatment temperature Tc is maintained, is then carried out to effect the cryogenic treatment of the steel (step E6). The duration of the cryogenic treatment E6 is predetermined, and may be greater than or equal to 1 hour, and for example be between 1 hour and 2 hours.

Once the cryogenic treatment E6 has been carried out, a progressive rise in temperature to room temperature Ta can be carried out (step E7).

Then, if desired, a second intermediate step E8 can be carried out during which the steel is maintained in an environment at room temperature Ta.

A rise ramp can then be carried out (step E9) to a tempering temperature Tr, for example between 465 ° C. and 550 ° C.

A temperature stabilization plateau, at the tempering temperature Tr, is then performed in order to perform the heat treatment of income (step E10). The duration of the income treatment can be greater than or equal to 4 hours, and for example be between 4 hours and 32 hours.

The steel can then be cooled, for example by keeping it in an environment at room temperature.

The expression "understood between ... and ..." must be understood as including boundaries.

Claims (8)

Process for treating a steel comprising, in percentages by weight, 0.2% to 0.33% of carbon, 4% to 8% of cobalt, 7% to 11% of nickel, 0.8% to 3% of chromium , 0.5% to 2.5% of molybdenum, 0.5% to 5.9% of tungsten, 0.05% to 0.2% of vanadium, and at most 0.02% of titanium, the balance being consisting of iron and unavoidable impurities, the process comprising at least: a solution heat treatment (E2) of the steel at a temperature of between 950 ° C. and 1100 ° C., a quenching treatment (E3) of the steel, carried out after the solution heat treatment, the placement of the steel in a cryogenic chamber after the quenching treatment, cooling (E5) of the inside of the cryogenic chamber in which the steel is present, this cooling being carried out up to a treatment temperature (Tc) of less than or equal to -73 ° C, and the cryogenic treatment (E6) of the steel during which the treatment temperature is maintained in the chamber, the time separating the end of the quenching treatment from the beginning of the cryogenic treatment being less than or equal to 4 hours. A process according to claim 1, wherein the time between the end of the quenching treatment and the start of the cryogenic treatment is less than or equal to 2 hours. A process according to claim 2, wherein the time between the end of the quenching treatment and the start of the cryogenic treatment is less than or equal to 1 hour. A process as claimed in any one of claims 1 to 3 wherein the duration of the cryogenic treatment (E6) is greater than or equal to 1 hour. The method of any one of claims 1 to 4, further comprising a tempering treatment (E10) of the steel made after the cryogenic treatment (E6). A method as claimed in any one of claims 1 to 5 wherein the treated steel constitutes a part of an aircraft landing gear. Process according to any one of claims 1 to 6, wherein the steel constitutes a piece having a mass greater than or equal to 10 kg. A process according to any one of claims 1 to 7, wherein the steel is cooled to a quenching end temperature (Ta) of less than or equal to 71 ° C during quenching treatment (E3).

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