METHOD FOR DETECTING FRECKLE-TYPE ANOMALIES IN A NICKEL-BASED SUPERALLOY SAMPLE

Abstract

The invention relates to a method (101) for detecting freckle-type anomalies in a nickel-based superalloy sample. 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 predetermined 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 on the surface of said sample, corresponding to a predetermined local change in the appearance of said sample. Figure for the abstract: Fig. 1

Description

Title of the invention: METHOD FOR DETECTING FRECKLE-TYPE ANOMALIES IN A NICKEL-BASED SUPERALLOY SAMPLE technical field

[0001] The invention relates to the field of material condition testing, particularly for use in the manufacture of aircraft parts. It concerns a method for detecting freckle-type anomalies in a nickel-based superalloy sample. Prior art

[0002] Superalloys, and in particular nickel-based superalloys, exhibit high-temperature mechanical strength properties (for example, at temperatures above 500 °C) which make them commonly used materials for the manufacture of aircraft parts.

[0003] The hardening of nickel-based superalloys is achieved by precipitation of y' or y” phases in a y matrix with a nickel-rich FCC structure in the presence of y'-genes such as titanium, aluminum, and tantalum, or y”-genes such as niobium. Non-limiting examples include commercial superalloys such as AD730®, Waspalloy®, Inco718®, and René65®.

[0004] 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 crack initiation zones.

[0005] The production of superalloys is complex and involves at least two successive melting steps. In a first step, vacuum induction melting (VIM), the raw materials are melted under vacuum and poured to obtain an ingot with a target and homogeneous composition. In a second step, the ingot obtained in the previous step 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, allowing control of the solidification conditions. These various steps aim to homogenize the structure and reduce the number of metallurgical anomalies.

[0006] Thermo-mechanical treatments of the remelted ingot later allow the ingot to be converted into a billet, further reducing the number of anomalies and modifying the structure by promoting the appearance of certain hardening phases in particular.

[0007] Despite a complex multi-step process involving melting, remelting and thermomechanical treatments, anomalies (called metallurgical insofar as they are linked) occur. to the manufacture of the superalloy) which correspond to defects located in the structure (i.e. the arrangement of atoms) of the material in the finished product may persist.

[0008] These anomalies include, in particular, "freckle" type anomalies (referred to as "freckle" without quotation marks hereafter), which consist of areas excessively enriched in titanium relative to the matrix, promoting the precipitation of undesirable q-Ni3Ti type intermetallic phases, which cannot be removed by thermomechanical treatment. Freckle type anomalies appear as continuous alignments. They also have negative effects on the effective mechanical properties of the material.

[0009] 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.

[0010] 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.

[0011] A known approach for detecting anomalies is that of macrographic control (also called attack) which makes it possible to detect outflow anomalies of the freckle type but which is dependent on the surface condition of the sample under test and also dependent on the operator.

[0012] Another known approach used to detect anomalies is eddy current testing, which also allows the detection of surface anomalies (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 "doubt relief" concerning any anomalies detected. Summary of the invention

[0013] The present invention proposes a solution to these drawbacks.

[0014] Thus, an objective of the invention is to enable the detection of freckle-type metallurgical anomalies in a non-destructive manner and on samples with a variable surface state.

[0015] 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:

[0016] - a first heating of said sample to a first temperature, for a first duration;

[0017] - a first cooling of said sample at a first cooling rate determined division;

[0018] - a second heating of said sample to a second temperature, lower than the first temperature, for a second duration, greater than the first duration;

[0019] - a second cooling of said sample; and,

[0020] - an identification of at least one freckle-type anomaly on the surface of said sample, corresponding to a specific local modification of the appearance of said sample.

[0021] The method according to the invention may comprise one or more of the following features, taken individually or in combination with each other:

[0022] - a local modification of the appearance of the sample is a darker area and more matte than the appearance of the rest of said sample.

[0023] - the first heating and the second heating are carried out in a vacuum oven.

[0024] - the first temperature is greater than 1000°C and less than 1100°C, of preferably between 1051 and 1079°C.

[0025] - the second temperature is greater than 700°C and less than 800°C, preferably between 746°C and 774°C.

[0026] - the first cooling is achieved by immersing the sample in a flow argon and the second cooling is achieved by immersing the sample in open air.

[0027] - the first duration is greater than 20 minutes and less than 2 hours, preferably between 30 minutes and 1 hour and 30 minutes and the second duration is greater than 4 hours and less than 12 hours, preferably between 6 hours and 10 hours.

[0028] - the sample is a plate or a bar which is obtained from an operation comprising including: sawing, shot blasting, milling, wire cutting, polishing or attack with macrographic etching reagent.

[0029] - 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

[0030] The invention will be better understood with the aid of the following description, given solely by way of example and made with reference to the accompanying drawings in which:

[0031] [Fig.1] is a step diagram of an implementation method for detecting freckle-type anomalies in a nickel-based superalloy sample according to the invention;

[0032] Figure 2 illustrates an evolution of the temperature profile of a sample as a function of time during the implementation of the process according to the invention;

[0033] [Fig.3] is an example of a local modification of the appearance of a sample in a nickel-based superalloy corresponding to an anomaly detected by the process according to the invention; and,

[0034] [Fig.4] is a set of sample examples to which an implementation method of the process according to the invention has been applied. Description of the implementation methods

[0035] With reference to [Fig.1] and [Fig.2], we will now describe an implementation method of a process 101 for detecting freckle-type anomalies in a nickel-based superalloy sample 103.

[0036] Examples of samples 103 (in nickel-based superalloy) are shown in [Fig.3] and [Fig.4] which will be described in more detail later.

[0037] Such a sample may be, for example, a plate or bar whose material, i.e. the nickel-based superalloy from 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 produced by a hot forging step from a billet, such as rolling or die forging.

[0038] In addition, the sample can be obtained from any operation including sawing, shot blasting, wire cutting, milling, polishing or micrographic etching reagent attack.

[0039] 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.

[0040] By way of non-limiting example, the bars shown in [Fig.4] have dimensions of 60 millimeters, 15 millimeters and 15 millimeters.

[0041] In what follows, an anomaly refers to a freckle-type metallurgical anomaly and the process therefore makes it possible to detect the freckle possibly present on the surface of a sample.

[0042] In process 101, step 105 consists of heating sample 103 to a temperature Tl for a duration dl. In particular, the temperature must be sufficiently high, and for a sufficiently long time, to cause the transformation of one or more constituents of sample 105 into a solid solution. Step 105 is therefore a dissolution of a sample in nickel-based superalloy.

[0043] For example, the temperature Tl can be greater than 1000°C and less than 1100°C, and more precisely between 1051 and 1079°C, while the duration dl can be greater than 20 minutes and less than 2 hours, and more precisely between 30 minutes and 1 hour and 30 minutes in order to guarantee this solution.

[0044] Step 107 then consists of cooling the sample 103 to a predetermined cooling rate vl. The cooling rate vl is said to be predetermined in the sense that the conditions resulting in this cooling are sufficiently controlled to obtain a desired cooling rate.

[0045] By way of non-limiting example, this cooling can be achieved by immersing the sample 103 in a flow of argon. Thus, the sweeping of the sample surface by the argon gas flow results in its rapid cooling.

[0046] By way of non-limiting example also, this cooling rate vl (for example obtained under the effect of an argon flow) can be greater than 50°C / min and less than 100°C / min and preferably between 70°C / min and 90°C / min.

[0047] 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.

[0048] Step 109 then consists of a further heating of the sample 103. Unlike step 105, this heating is done at a temperature T2, lower than the temperature Tl and for a duration d2 greater than the duration dl so that it is not a dissolving but a tempering.

[0049] 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.

[0050] In a particular embodiment, the heating of step 105 and the heating of step 109 can both be carried out in a vacuum oven, in order to avoid surface oxidation phenomena.

[0051] 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 achieved by immersing sample 103 in open air, that is, by placing the sample in contact with ambient air to obtain a drop in its temperature.

[0052] Finally, step 113 consists of identifying at least one freckle-type anomaly on the surface of sample 103. This anomaly is identifiable insofar as it corresponds to a determined local modification of the appearance of sample 103 in the area including the anomaly.

[0053] Thus, in a particular embodiment, an operator can identify one or several anomalies, visible on the surface of sample 103, for example using a binocular magnifying glass.

[0054] In another embodiment, one or more anomalies can be identified through the acquisition and processing of digital images of the surface of the sample having been subjected to steps 105 to 111 of the detection process.

[0055] Fig. 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 by means of the method of the invention is visible.

[0056] In particular, in the example shown, the local change in 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.

[0057] Generally, a person skilled in the art will be able to distinguish such a local change in appearance from a possible stain or any other change that would not be related to an anomaly.

[0058] More specifically, the matrix of sample 103, that is to say the three-dimensional arrangement of the atoms of the superalloy, gives sample 103 a general homogeneous appearance except at the level of one or more anomalies revealed following the successive heating and cooling of process 101.

[0059] Figure 4 shows three examples of samples 103 for which freckle-type anomalies have been revealed, i.e. made apparent by steps 105 to 111 of the process and whose identification is here symbolized by the solid-lined ellipses surrounding said anomalies 201.

[0060] In this case, these are bar-type samples whose surface condition differs insofar as the upper part of the figure shows a sample 103 from a wire cutting operation (we speak of wire cutting ass), the middle part of the figure shows a sample 103 from a milling operation (we speak of milling ass) and the lower part of the figure shows a sample 103 from polishing and attacking with the macrographic etching reagent.

[0061] These images therefore illustrate the ability of process 101 to detect anomalies regardless of the surface condition of the sample subjected to said process.

[0062] Furthermore, thanks to the process according to the invention, it is possible to detect anomalies in a nickel-based superalloy sample without using any chemical product and without resorting to destructive testing methods such as eddy currents.

[0063] The method according to the invention makes it possible to reveal freckle-type anomalies consisting of positive segregations (local concentration of Ti). However, although not tested, the invention could find application in revealing other segregation-type anomalies such as negative anomalies (local titanium depletion or local deficiency in the y' phase) of the White Spot type. In this case, the anomaly is not breakthrough and would require detection to be carried out on a sample section taken in the plane of the anomaly.

Claims

Claims

1. Method (101) for detecting freckle-type anomalies of a sample (103) made 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) to 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. The method (101) of claim 1, wherein a local modification of the appearance (201) of the sample (103) is an area that is darker and more matte than the appearance of the remainder of said sample (103).

3. A method (101) according to any preceding claim, 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. A method (101) according to any preceding claim, wherein 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 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. A method (101) according to any preceding claim,

9. wherein the sample (103) is a plate or bar which is produced by an operation included among: sawing, shot blasting, milling, wire cutting, polishing or attack with a macrographic etching reagent. 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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