Method for detecting freckle-type anomalies in a sample made of a nickel-based superalloy

Abstract

The invention relates to a method (101) for detecting freckle-type anomalies in a sample made of a nickel-based superalloy. The method (101) comprises a first heating (105) of the sample to a first temperature, a first cooling (107) of the sample at a first determined cooling rate, a second heating (109) of the sample to a second temperature, lower than the first temperature, a second cooling (111) of the sample and an identification (113) of at least one freckle-type anomaly, at the surface of said sample, corresponding to a determined local change in the appearance of said sample.

Description

[0001] Description

[0002] TITLE: METHOD FOR DETECTING FRECKLE-TYPE ANOMALIES IN A NICKEL-BASED SUPERALLOY SAMPLE

[0003] TECHNICAL FIELD

[0004] The invention relates to the field of monitoring the condition of materials, in particular intended for the manufacture of aircraft parts. It relates to a method for detecting freckle-type anomalies in a nickel-based superalloy sample.

[0005] PRIOR ART

[0006] The state of the art includes in particular the documents GENEREUX PD et al: "Characterization of Freckles in a High Strength Wrought Nickel Superalloy", proceedings of the ninth 2000 international symposium on superalloys; September 17 - 21, 2000, seven springs, pa, DEXIN MA et al: "Some New Observations on Freckle Formation in Directionally Solidified Superalloy Components", metallurgical and materials transactions b, springer-verlag, new york, vol. 43, no. 2, November 29, 2011, and EP-B1-2 510 131.

[0007] Superalloys, and particularly nickel-based superalloys, exhibit high-temperature mechanical strength properties (e.g., temperatures above 500°C) that make them commonly used materials for the manufacture of aircraft parts.

[0008] The hardening of nickel-based superalloys is obtained by precipitation of y' or y" phases in a y matrix of CFC structure (rich in nickel) in the presence of y'-gene elements such as titanium, aluminum and tantalum or y"-gene elements such as niobium. As non-limiting examples, commercial superalloys such as AD730®, Waspalloy®, Inco718® and René65® may be cited.

[0009] To achieve these properties, it is necessary to produce superalloys with a homogeneous structure, in other words without metallurgical anomalies in order to limit the areas where cracks can initiate. The production of superalloys is complex and involves at least two successive melting stages. In a first stage of vacuum induction melting (VIM), the raw materials are melted under vacuum and cast to obtain an ingot with a target and homogeneous composition. In a second stage, the ingot obtained in the previous stage serves as an electrode and is melted under high vacuum using the Vacuum Arc Remelting (VAR) method. Controlled solidification of the molten metal is achieved by cooling in a crucible and allows the solidification conditions to be controlled. The aim of these different stages is to homogenize the structure and reduce the number of metallurgical anomalies.

[0010] Thermomechanical treatments of the remelted ingot later allow the ingot to be converted into a billet, to further reduce the number of anomalies and to modify the structure by promoting the appearance of certain hardening phases in particular.

[0011] Despite a complex multi-stage development of melting, remelting and thermomechanical treatments, anomalies (called metallurgical insofar as they are linked to the manufacture of the superalloy) which correspond to localized defects in the structure (i.e. the arrangement of atoms) of the material in the finished product may remain.

[0012] These anomalies include in particular "freckle" type anomalies (denoted freckle without quotation marks in the following) which consist of areas too enriched in titanium compared to the matrix, favoring the precipitation of intermetallic phases of the q-NisTi type, undesirable because impossible to remove by thermomechanical treatment. Freckle type anomalies appear in the form of continuous alignment. They also have negative effects on the effective mechanical properties of the material.

[0013] However, when the material is intended for the manufacture of aircraft parts, it is subject to strict mechanical strength criteria. In other words, the material must guarantee certain mechanical properties when subjected to a fatigue test. For these reasons, it is useful to be able to detect the presence of freckle-type anomalies in such a material before its possible use for the manufacture of a part.

[0014] A known approach to detect anomalies is that of macrographic control (also called attack) which allows the detection of emerging anomalies of the freckle type but which is dependent on the surface condition of the sample under test and also dependent on the operator.

[0015] Another known approach used to detect anomalies is eddy current testing, which also allows the detection of anomalies that emerge (or even those underlying the surface), but it also has a very high sensitivity to the surface condition of the sample and necessarily involves carrying out a complex additional step of "removing doubt" concerning any anomalies detected.

[0016] SUMMARY OF THE INVENTION

[0017] The present invention provides a solution to these drawbacks.

[0018] Thus, one objective of the invention is to enable the detection of freckle-type metallurgical anomalies in a non-destructive manner and on samples having a variable surface condition.

[0019] To this end, the invention according to a first aspect relates to a method for detecting freckle-type anomalies in a nickel-based superalloy sample, said method comprising the following steps:

[0020] - a first heating of said sample to a first temperature, for a first duration;

[0021] - a first cooling of said sample at a first determined cooling rate;

[0022] - a second heating of said sample to a second temperature, lower than the first temperature, for a second duration, greater than the first duration;

[0023] - a second cooling of said sample; and, - an identification of at least one freckle-type anomaly, on the surface of said sample, corresponding to a determined local modification of the appearance of said sample.

[0024] This means that the sample is held at a first temperature for a first time, then cooled at a first rate, before being held at a second temperature for a second time, then cooled again, resulting in a local change in the appearance of the sample that makes it easy to identify one or more freckle-type anomalies.

[0025] The method according to the invention may comprise one or more of the following characteristics, taken in isolation from one another or in combination with one another:

[0026] - a local modification of the appearance of the sample is an area that is darker and more matte than the appearance of the rest of the sample.

[0027] - the first heating and the second heating are carried out in a vacuum furnace.

[0028] - the first temperature is greater than 1000°C and less than 1100°C, preferably between 1051 and 1079°C.

[0029] - the second temperature is greater than 700°C and less than 800°C, preferably between 746°C and 774°C.

[0030] - the first cooling is carried out by immersing the sample in a flow of argon and the second cooling is carried out by immersing the sample in the open air.

[0031] - the first duration is greater than 20 minutes and less than 2 hours, preferably between 30 minutes and 1 hour 30 minutes and the second duration is greater than 4 hours and less than 12 hours, preferably between 6 hours and 10 hours.

[0032] - the sample is a plate or bar which results from an operation including: sawing, shot blasting, milling, wire cutting, polishing or attack with a macrographic etching reagent.

[0033] - the first cooling rate is greater than 50°C / min and less than 100°C / min, preferably between 70°C / min and 90°C / min. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention will be better understood with the aid of the following description, given solely by way of example and with reference to the appended drawings in which: Figure 1 is a diagram of steps of an embodiment of a method for detecting freckle-type anomalies in a nickel-based superalloy sample according to the invention;

[0035] Figure 2 illustrates an evolution of the temperature profile of a sample as a function of time during the implementation of the method according to the invention; Figure 3 is an example of a local modification of the appearance of a nickel-based superalloy sample corresponding to an anomaly detected by the method according to the invention; Figure 4 is a set of examples of samples to which an embodiment of the method according to the invention has been applied; and, Figure 5 is a schematic representation of examples of samples subjected to the method for detecting freckle-type anomalies according to the invention.

[0036] DESCRIPTION OF EMBODIMENTS

[0037] With reference to Figure 1 and Figure 2, we will now describe an embodiment of a method 101 for detecting freckle-type anomalies in a sample 103 made of nickel-based superalloy.

[0038] Examples of Sample 103 (nickel-based superalloy) are shown in Figure 3 and Figure 4 which will be described in more detail later.

[0039] Such a sample may be, for example, a plate or a bar whose material, i.e. the nickel-based superalloy in which the sample is formed, is intended for the manufacture of an aircraft part (such as, for example, a turbine disk). It may in particular be obtained from a hot forging step from a billet such as, for example, rolling or die-stamping. It may also be, in a particular embodiment, as illustrated in FIG. 5, a wafer 503, taken (i.e. a longitudinal portion) from a billet 501 or a billet 505. Advantageously, the method thus makes it possible to test the entire section of a billet or billet before an operation, such as die-stamping for example, modifies this section.

[0040] Furthermore, the sample may be obtained from any of the following operations: sawing, shot blasting, wire cutting, milling, polishing, or micrographic etching.

[0041] In other words, the detection method according to the invention can be applied to a sample from any stage corresponding to the manufacture or testing of a nickel-based superalloy without prior surface preparation and, consequently, with any surface condition (after sawing, shot blasting, etc.) on the sole condition that the surface in question is flat.

[0042] As a non-limiting example, the bars shown in Figure 4 have dimensions of 60 millimeters, 15 millimeters and 15 millimeters.

[0043] In the following, an anomaly refers to a metallurgical anomaly of the freckle type and the method therefore makes it possible to detect any freckle present on the surface of a sample.

[0044] In the method 101, step 105 consists of heating the sample 103 to a temperature T1, for a duration d1. This means that the sample 103 is maintained at the temperature T1 for the duration d1. In particular, the temperature must be sufficiently high, and for a sufficiently long duration, to cause the transformation of one or more constituents of the sample 105 into a solid solution. Step 105 is therefore a dissolution of a sample made of a nickel-based superalloy.

[0045] For example, the temperature Tl may be greater than 1000°C and less than 1100°C and more precisely between 1051 and 1079°C while the duration dl may be greater than 20 minutes and less than 2 hours and more precisely between 30 minutes and 1 hour 30 minutes in particular so as to guarantee this dissolution. Step 107 then consists of cooling the sample 103 at a determined cooling rate vl. The cooling rate vl is said to be determined in the sense that the conditions causing this cooling are sufficiently controlled to obtain a desired cooling rate.

[0046] As a non-limiting example, this cooling can be carried out by immersing the sample 103 in a flow of argon. Thus, the sweeping of the surface of the sample by the argon gas flow causes rapid cooling thereof.

[0047] Also by way of non-limiting example, this cooling rate vl (for example obtained under the effect of an argon flow) may be greater than 50°C / min and less than 100°C / min and preferably between 70°C / min and 90°C / min.

[0048] This cooling step 107, the purpose of which is a rapid drop in the temperature of the sample, therefore corresponds to a quenching of the sample 103 in nickel-based superalloy.

[0049] Step 109 then consists of heating the sample 103 again. Unlike step 105, this heating is done at a temperature T2, lower than the temperature T1 and for a duration d2 greater than the duration d1 so that it is not a solution treatment but an annealing. This means that the sample 103 is maintained at the temperature T2, which is lower than the temperature T1, for the duration d2, which is greater than the duration d1.

[0050] For example, the temperature T2 can be greater than 700°C and less than 800°C and more precisely between 746°C and 774°C while the duration d2 can be greater than 4 hours and less than 12 hours and more precisely between 6 hours and 10 hours.

[0051] In a particular embodiment, the heating of step 105 and the heating of step 109 may both be carried out in a vacuum furnace, in order to avoid surface oxidation phenomena.

[0052] Step 111 then consists of a second cooling of the sample 103. In the case of this second cooling, the cooling conditions do not require precise control to obtain a determined speed and do not require rapid cooling. Thus, this second cooling can be carried out by immersing the sample 103 in the open air, that is to say by placing the sample in contact with the ambient air to obtain the reduction of its temperature.

[0053] Finally, step 113 consists of identifying at least one freckle-type anomaly on the surface of the sample 103. This anomaly is identifiable to the extent that it corresponds to a determined local modification of the appearance of the sample 103 in the area comprising the anomaly.

[0054] Thus, in a particular implementation mode, an operator can identify one or more anomalies, visible on the surface of the sample 103, for example by using a binocular magnifying glass.

[0055] In another embodiment, one or more anomalies may be identified by acquiring and processing digital images of the surface of the sample that has been subjected to steps 105 to 111 of the detection method.

[0056] Figure 3 shows an enlargement of a portion of the surface of a sample in which an example of anomaly 201 (surrounded by dotted lines) detected using the method of the invention is visible.

[0057] In particular, in the example shown, the local modification of the appearance of the sample caused by the presence of the anomaly 201 is a darker and more matte area than the appearance of the rest of the sample 103.

[0058] Generally speaking, a person skilled in the art will be able to distinguish such a local modification from the appearance of a possible stain or any other modification which would not be linked to an anomaly.

[0059] More precisely, the matrix of the sample 103, i.e. the three-dimensional arrangement of the atoms of the superalloy, gives the sample 103 a generally homogeneous appearance except at the level of one or more anomalies revealed following the successive heating and cooling of the process 101. Figure 4 shows three examples of samples 103 for which freckle-type anomalies have been revealed, i.e. made apparent by the steps 105 to 111 of the process and the identification of which is here symbolized by the solid line ellipses surrounding said anomalies 201.In this case, these are bar-type samples whose surface condition differs in that the upper part of the figure shows a sample 103 resulting from a wire cutting operation (we speak of wire cutting stock), the central part of the figure shows a sample 103 resulting from a milling operation (we speak of milling stock) and the lower part of the figure shows a sample 103 resulting from polishing and macrographic etching operations.

[0060] These images therefore illustrate the ability of the process 101 to detect anomalies regardless of the surface condition of the sample subjected to said process.

[0061] Furthermore, thanks to the method according to the invention, it is possible to detect anomalies in a nickel-based superalloy sample without using chemicals and without resorting to destructive testing means such as eddy currents.

[0062] The method according to the invention makes it possible to reveal freckle-type anomalies which consist of positive segregations (local concentration of Ti). However, and although not tested, the invention could find an application in the revelation of other anomalies by segregation such as negative anomalies (local depletion of titanium or local deficiency in y' phase) of the White Spot type. In this case, the anomaly is non-transparent and would require detection to be carried out on a sample section made in the plane of the anomaly.

Claims

CLAIMS 1. Method (101) for detecting freckle-type anomalies of a sample (103) of nickel-based superalloy, said method (101) comprising the following steps: - a first heating (105) of said sample (103) to a first temperature (Tl), for a first duration (dl); - a first cooling (107) of said sample (103) at a first determined cooling rate (vl); - a second heating (109) of said sample (103) at a second temperature (T2), lower than the first temperature (Tl), for a second duration (d2), greater than the first duration (dl); - a second cooling (111) of said sample (103); and, - an identification (113) of at least one freckle-type anomaly (201), on the surface of said sample (103), corresponding to a determined local modification (201) of the appearance of said sample (103).

2. Method (101) according to claim 1, wherein a local modification of the appearance (201) of the sample (103) is an area darker and more matte than the appearance of the rest of said sample (103).

3. Method (101) according to any one of the preceding claims, wherein the first heating (105) and the second heating (109) are carried out in a vacuum furnace.

4. Method (101) according to any one of the preceding claims, wherein the first temperature (Tl) is greater than 1000°C and less than 1100°C, preferably between 1051 and 1079°C.

5. Method (101) according to any one of the preceding claims, wherein the second temperature (T2) is greater than 700°C and less than 800°C, preferably between 746°C and 774°C.

6. Method (101) according to any one of the preceding claims, in which the first cooling (107) is carried out by immersing the sample (103) in a flow of argon and the second cooling (111) is carried out by immersing the sample (103) in the open air.

7. Method (101) according to any one of the preceding claims, wherein the first duration (d1) is greater than 20 minutes and less than 2 hours, preferably between 30 minutes and 1 hour 30 minutes and the second duration (d2) is greater than 4 hours and less than 12 hours, preferably between 6 hours and 10 hours.

8. Method (101) according to any one of the preceding claims, in which the sample (103) is a plate or a bar which is obtained from an operation included among: sawing, shot blasting, milling, wire cutting, polishing or attack with a macrographic etching reagent.

9. Method (101) according to any one of the preceding claims, wherein the first cooling rate (vl) is greater than 50°C / min and less than 100°C / min, preferably between 70°C / min and 90°C / min.

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