Method and apparatus for detecting embrittlement of metal material

Abstract: On the basis of a finding that a magnetic hysteresis of a metal material, among magnetization characteristics changing with secular embrittlement of the metal material, shows a clear correspondence with the degree of embrittlement of the metal material, a change in such a magnetization characteristic is measured to estimate the degree of secular embrittlement of the metal material. In a typical embodiment, a superconducting quantum interference device (340) is used to detect the magnetization characteristic of a measuring object (11). According to the present invention, the degree of embrittlement of a metal material used in an environment of high temperatures can be quickly detected in a non-destructive fashion so that the danger of brittle fracture of the metal material can be reliably prevented.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to a method and apparatus for detecting degradation of a metal material. More particularly, this invention relates to a degradation measuring method and apparatus suitable for detecting embrittlement due to high-temperature aging, of practical plant components of a metal material such as a ferritic stainless steel used in an environment of high temperatures such as that encountered in a chemical plant and a nuclear power plant. DESCRIPTION OF THE PRIOR ART Methods of measuring embrittlement of a metal material are disclosed in, for example, JP-A-54-61981 and JP-A-61-28859. According to one of the prior art methods disclosed in JP-A-54-61981, a weld metal such as an austenitic stainless steel is decided to become brittle when the amount of δ-ferrite has decreased by more than 5 % of its initial value. Also, according to the prior art method disclosed in JP-A-61-28859, a change in a magnetic characteristic of a measuring object is measured to detect degradation of the measuring object. WO-A-8503577 describes testing ferromagnetic samples involving comparison with a standard use of a point on the hysteresis curve and embrittlement measurement are referred to. Further, JP-A-56-168545 discloses a method in which a magnetic characteristic of a measuring object is utilized to monitor the metallographic structure of the measuring object, and JP-A-59-108970 discloses a method of detecting a magnetic characteristic itself of a measuring object. As disclosed in JP-A-54-61981 cited above, it is already known that aging degradation occurs on a metal material, especially, a ferritic stainless steel, when the stainless steel is used for a long period of time at high temperatures. That is, at a relatively high temperature higher than about 600 °C, σ-embrittlement attributable to precipitation of a σ-phase occurs, while in a temperature range of from 400 °C to 500 °C, so called 475 °C-embrittlement occurs. However, this 475 °C-embrittlement may occur even when the ferritic stainless steel is used for a long period of time in a temperature range lower than 400 °C. Therefore, it is necessary to give sufficient consideration to the use of practical plant components of the ferritic stainless steel at high temperatures. However, the tendency to become brittle at temperatures lower than 500 °C was not taken into consideration in the prior art disclosure cited above, and the 475 °C-embrittlement could not be detected. Further, the initial amount of ferrite in practical welded parts differs depending on the position of the welds and tends to greatly fluctuate. Furthermore, because the number of welds in a practical plant is very large, it is difficult to monitor the initial amount of ferrite for each of all of the welds and all of equipments. Thus, the prior art method disclosed in JP-A-54-61981 could not be put into practical use in a practical plant because it was not applicable to parts where the initial amount of ferrite was unknown. On the other hand, an eddy current test method (referred to hereinafter as an ECT) is disclosed in, for example, JP-A-55-141653. According to this prior art method, an ECT value of a measuring object, which is, for example, an iron alloy, is compared with an ECT value of the measuring object measured before practical use or an ECT value of the same material as the material of the measuring object and subjected to heat treatment similar to initial heat treatment applied to the measuring object, and the degree of degradation of the iron alloy is decided depending on whether the result of comparison is positive or negative. However, a quantitative measurement or analysis could not be made because the decision is merely based on whether the result of comparison is positive or negative. In the prior art method disclosed in JP-A-61-28859, it is necessary to measure the initial value of a magnetic characteristic of a measuring object. Also, the prior art methods disclosed in JP-A-59-108970 and JP-A-56-168545 are merely directed to measurement of magnetic characteristics of metal materials and cannot be utilized for detection of the degree of embrittlement of the metal materials. DE-C-741194 discloses a method of measuring magnetically the mechanical hardness of ferromagnetic materials. SOVIET JOURNAL OF NONDESTRUCTIVE TESTING, vol. 10, no. 2, January 1975, p. 223-229 discloses an apparatus for nondestructive testing of mechanical properties of ferromagnetic materials. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and an apparatus which can detect, with high accuracy and in a non-destructive method, the degree of embrittlement of a practical plant member of a metal material such as a ferritic stainless steel used in an environment of high temperatures. The present invention is defined in claims 1 and 2. Magnetization characteristics of a metal material changing as a result of embrittlement due to aging of the metal material are measured to decide the degree of embrittlement of the metal material. The configuration of a magnetic hysteresis, which is representative of magnetization characteristics of a measuring object of a metal material, shows a clear correspondence with the degree of embrittlement of the metal material. Thus, by detecting a change in this magnetic characteristic, the degree of embrittlement of the measuring object of the metal material can be estimated. Further, by means of statistical data processing such as a regression analysis, the degree of embrittlement of the metal material can be estimated with a high rate of correlation. An exciting coil in the form of a coil of a superconducting material is be used for efficiently magnetizing the measuring object. The apparatus may be used for inspecting embrittlement of a metal material used in a nuclear reactor by means of magnetic diagnosis, and comprise cable means for suspending the inspecting apparatus from a crane located above a reactor container so that internal equipments of the reactor container can be inspected, and a fluidtight inspection driver including vacuum-actuated attachments for fixing the inspecting apparatus to the inner wall of a pressure vessel of the reactor, and means for actuating three-dimensional scanning movements of the inspecting apparatus. MEANS FOR APPLYING MAGNETIC FIELD An exciting coil is a typical example of means for applying a magnetic field to a measuring object. This exciting coil is in the form of a superconducting coil so that the measuring object can be efficiently magnetized to the level of magnetic saturation. MAGNETIC CHARACTERISTICS A magnetic hysteresis characteristic is a typical example of magnetic characteristics of a measuring object. For example, a plurality of parameters of a magnetic hysteresis loop (the area of the magnetic hysteresis loop, residual magnetic flux density and saturating magnetizing force) of a metal material are detected, and the relation between changes in the plural parameters of the magnetic hysteresis loop and embrittlement of the metal material is previously computed. An arithmetic data processing device is preferably used to carry out statistical data processing such as a regression analysis of those data so as to decide the degree of embrittlement of the measuring object. As another example, a plurality of parameters of a magnetic hysteresis loop (the area of the magnetic hysteresis loop, residual magnetic flux density and saturating magnetizing force) of a metal material are detected so as to normalize fluctuations of the parameters in an unused virgin state of the metal material, and the relation between changes in the plural normalized parameters and embrittlement of the metal material is previously computed. An arithmetic data processing device is preferably used to carry out processing of those data so as to decide the degree of embrittlement of the measuring object. UNUSED MATERIAL The term "unused material" is used in this specification to indicate that a specific metal material has not been used yet, that is, to indicate that the metal material is received in its as-received material. The initial amount of ferrite is, for example, the basis for identification that a metal material is received in an unused initial state. AUXILIARY EQUIPMENTS A demagnetizing device is an example of incidental equipments. Preferably, a measuring object is demagnetized, and, after the demagnetization, is magnetized while changing the magnetizing force to obtain a plurality of magnetic hysteresis loops for detecting changes in the plural parameters of each magnetic hysteresis loop i.e. the area of the magnetic hysteresis loop, residual magnetic flux density and saturating magnetizing force. An arithmetic data processing device is used to decide the degree of embrittlement of the metal material on the basis of the detected changes. Another example of the auxiliary equipments is an exciting device of feedback controlled type. Preferably, the maximum magnetic flux density of a magnetic hysteresis loop to be measured is selected, and an exciting device of feedback controlled type is provided so as to detect the magnetic hysteresis loop of a metal material by means of feedback control of the magnetizing force with high accuracy. The superconducting quantum interference device is preferably shielded by both a magnetic shield and a radiation shield. ARITHMETIC PROCESSING DEVICE An arithmetic data processing device preferably used in the present invention includes a data base storing data showing an exact correspondence between various patterns of magnetic hysteresis loops and the degree of degradation of a metal material. After normalization of a measured magnetic hysteresis loop of a measuring object, the pattern of the magnetic hysteresis loop most analogous to the measured magnetic hysteresis loop is read out from the data base, and the arithmetic data processing device decides the degree of degradation of the measuring object. More concretely, the material of the measuring object is magnetized to the level of magnetic saturation to obtain a plurality of magnetic hysteresis loops of different patterns, and data showing an exact correspondence between the patterns of the magnetic hysteresis loops and the degree of embrittlement of the measuring object are stored in the data base. After normalization of a measured magnetic hysteresis loop of the measuring object, the pattern of the magnetic hysteresis loop most analogous to the measured magnetic hysteresis loop is read out from the data base, and the arithmetic data processing device decides the degree of embrittlement of the measuring object. Another arithmetic data processing device preferably used in the present invention detects higher harmonic distortion components of a magnetic hysteresis loop of a metal material by the Fourier transformation and decides the degree of embrittlement of the metal material on the basis of a change in the values of the higher harmonic distortion components. Another preferable arithmetic data processing device makes frequency analysis of data measured by a superconducting quantum interference device to decide the degree of embrittlement of a measuring object on the basis of a change in Barkhausen noise components. CRITERION OF EMBRITTLEMENT The criterion of embrittlement will be described in detail later in the specification. For example, a decrease in the value of fracture toughness of a metal material due to a change in its metallographic structure can be conveniently used as the criterion of embrittlement. MAGNETIZATION SENSOR The magnetization sensor is a superconducting quantum interference device to accurately detect a change in the magnetism. When a metal material is used for a long period of time in an environment of a high temperature, a change generally occurs in its internal structure, resulting in a decreased mechanical strength. Especially, in the case of a ferritic stainless steel, its mechanical strength decreases markedly with the increase in the period of aging heat treatment at a high temperature. The inventors made researches and studies on the tendency toward embrittlement of a metal material such as a ferritic stainless steel due to heating at high temperatures. As a result of the researches and studies, the inventors found that, with the progress of aging at high temperatures, the electromagnetic characteristics such as the electrical resistivity ρ and permeability µ of the metal material were subjected to a change, and the mechanical properties such as the hardness and metallographic structure of the metal material were also subjected to a change. Especially, the inventors found that the tendency toward brittleness of the metal material shows a clear correspondence with a change in the magnetization characteristic of the metal material. The inventors measured a magnetic hysteresis of the metal material in an unused virgin state and that of the metal material in a state treated at a high temperature. As a result of the above measurements, the inventors found that the area of the magnetic hysteresis loop (the magnetic hysteresis loss) and the residual magnetic flux density were subjected to a change which was dependent on the degree of brittleness of the measuring object. Thus, when such a phenomenon is utilized, the progress of the tendency toward brittleness of a metal material, such as, a ferritic stainless steel, can be detected with high accuracy. According to the present invention, the degree of brittleness of a metal material used at high temperatures can be detected quickly in a non-destructive way. Therefore, a rupture failure of such a metal material can be prevented before it occurs, and the safety of a practical plant member of such a metal material can be improved. BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a schematic sectional view of part of an embodiment of the present invention utilizing a superconducting system. DESCRIPTION OF THE PREFERRED EMBODIMENT The FIGURE shows the preferred embodiment, and the measuring object can be acquired in the form of a small sample 11. The superconducting exciting coil 23 is placed in the coil container 110 of a non-magnetic material filled with the coolant 130 so that the coil 23 can be maintained in its superconducting state. The coil container 110 is thermally insulated from the ambient atmosphere by the thermal insulating member 120. The coil container 110 containing the superconducting exciting coil 23 is formed at its central area with a pair of spaced holes, so that the sample 11 and a reference sample 12 can be inserted into the respective holes from the outside. The superconducting exciting coil 23 establishes a uniform magnetic field in the zone where the samples 11 and 12 are placed. The differential pickup coil 330 associated with the superconducting quantum interference device 340 is wound around the acquired sample 11 and reference sample 12 in the two holes, and the superconducting quantum interference device 340, which is magnetically shielded, makes magnetic measurement. Thus, the difference between the magnetization characteristics of the acquired sample 11 and reference sample 12 can be detected. According to the embodiment shown in the Figure, a change in the magnetization characteristic of the metal material can be detected with high accuracy, so that a distortion of the magnetic field can be detected with high accuracy thereby greatly improving the accuracy of evaluation of the degree of embrittlement of the metal material.

Claims: Apparatus for evaluating the degree of embrittlement of a small test sample (11) taken from a part of a measuring object, comprising means for applying a magnetic field to the test sample, and a magnetic characteristic measuring device for measuring a magnetic characteristic of the sample, characterized by a fluid-tight thermally insulated container (110) provided with a pair of spaced holes for inserting the test sample (11) and a reference sample (12), an superconducting exciting coil (23) disposed in said container (110) for establishing an uniform magnetic field, a magnetization sensor (330, 340) in form of a superconducting quantum interference device (340) having a pickup coil (330) of the differential type disposed inside the exciting coil (23) in said container (110) and defining spaces therewithin, which include said spaced holes, and a recirculating cooling system (130) for cooling the superconducting system. Apparatus according to claim 1, characterized in that the exciting coil (23) is of a feedback-controlled type for detecting the magnetic hysteresis loop of the test sample (11) by feedback control of the magnetizing force. Method of detecting the embrittlement of a measuring object in form of a small test sample, using the apparatus of claim 1, comprising the steps of placing the test sample (11) and a reference sample (12) in the container (110) containing the superconducting exciting coil (23) and a coolant (130), establishing a uniform magnetic field by said superconducting exciting coil (23) in the container zone where the samples (11 and 12) are placed, measuring the magnetization characteristics, namely the area of the magnetic hysteresis loop, the residual magnetic flux density and/or the saturating magnetization force of the test sample (11) and the reference sample (12) by using the differential pickup coil (330) of the magnetic shielded superconducting quantum interference device (340), detecting the difference between said magnetic characteristics of the test sample (11) and the reference sample (12) and estimating the degree of embrittlement of the probe (11) on the basis of said difference.

Inventors: ENOMOTO KUNIO, HASEGAWA KUNIO, HAYASHI MAKOTO, OTAKA MASAHIRO, SHIMIZU TASUKU, TAKAKU KAZUO, ENOMOTO, KUNIO, HASEGAWA, KUNIO, HAYASHI, MAKOTO, OTAKA, MASAHIRO, SHIMIZU, TASUKU, TAKAKU, KAZUO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: G01N27
IPC codes: G01N27