奥氏体304不锈钢磁检测法探头设计及实验研究【测控技术与仪器专业资料】
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Journal of Nondestructive Evaluation, Vol. 20, No. 3, September 2001 (q 2001)Observation of Changes in Magnetic Images Due to a Strainor Fatigue Progress in Austenite Stainless Steels using aScanning Hall-Sensor MicroscopeA. Oota,1,* K. Miyake,1D. Sugiyama,2and H. Aoki2Received September 14, 1999; Revised September 15, 2001Using a scanning Hall-sensor microscope with an active Hall element of area 50mm 3 50mm, wemeasured two-dimensional magnetic images of spontaneous magnetization on a surface of a numberof 304 stainless plates in a paramagnetic austenite phase. Stainless plates with a yield point of 31kg/mm2were subjected to a strain or pulling-fatigue process at room temperature. In a strain state,most plates show the same progressive change in magnetic images resulting from a strain-inducedmartensite transformation, but others produce additional and/or different change although the originremains unsolved. A pulling-fatigue process at a stress amplitude of 28 kg/mm2also produces somedifferences in the images from the strain process, at the early stage of fatigue progress. The studyshould provide a new horizon to elucidate the complex destructive-progress in strain or fatigueprocess of304 stainlesssteels that areused asstructural materials inelectric powerstations, chemicalplants and medical equipment.KEYWORDS:ScanningHall-sensormicroscope;magneticimages;spontaneousmagnetization;non-destructiveevaluation; stainless steel; austenite phase; strain-induced martensite transformation; fatigue process.tum interference devices (SQUID)(2,3)and micro-HallI. INTRODUCTIONsensors(5,6)provides new and promising techniques forNDE of magnetic materials, because of many advantagesInvoking of social securities and environmentalover conventional techniques such as eddy current, ultra-problems demands higher levels of reliability and stabil-sound and x-ray imaging, etc. Magnetic imaging tech-ity for structural materials such as iron and stainlessnique using a SQUID has enabled a non-destructivesteels supporting infra-structuring in industries. Muchdetection of Lu ders bands in mild steels subjected toeffort has been made to develop non-destructive evalua-a strain process.(3)A scanning Hall-sensor microscopetion(NDE) techniquesto investigatethe destructivepro-(SHM) with an active Hall element of area 50mm 3gress under fatigue and/or strain processes in struc-50mm has also enabled a magnetic detection of smalltural materials.(14)cracks (,10 mm long and ,0.1 mm wide) in mild steelsRecent progress in magnetometers utilizing highly-caused by a fatigue process,(6)so that SHM can be usedsensitivemagneticsensorssuchassuperconductingquan-as a simple, economic and conventional tool for NDE ofmagnetic materials. We present here the magnetic imagesof spontaneous magnetization on a surface of 304 stain-* Corresponding author. E-mail: ootaeee.tut.ac.jpless steels subjected to strain or fatigue process at room1Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichitemperature observed using this microscope, and the441-8580, Japan.influence of the destructive progress on the images of2Technical Research and Development Laboratories, Topy IndustriesLtd., Akemi-cho, Toyohashi 441-8510, Japan.304 stainless steels.870195-9298/01/0900-0087/0 q 2001 Plenum Publishing Corporation88Oota, Miyake, Sugiyama, and Aoki2. EXPERIMENTALprogressive way. Measurements were repeated afterobserving the samples to higher levels and reducing theload to zero. In addition, a number of the plates A were2.1 Stainless Platessubjected to a pulling-fatigue test using a conventionalhydraulic-servo fatigue testing machine at a stress ampli-Commercial 304 stainless steels with a yield pointof 31 kg/mm2and a tensile strength of 68 kg/mm2weretude of 28 kg/mm2and a frequency of 29.2 Hz, to eluci-date a destructive progress under a fatigue process.machinedintoa3.8mmthickplate.Theyhavethefollow-ing chemical composition except for iron: Cr, 18.2 wt%;Magnetic measurements were made as a parameter ofstress cycles in a progressive manner by stopping theNi, 8.3 wt%; C, 0.05 wt%; Si, 0.43 wt%; Mn, 0.89 wt%;P, 0.03 wt%; S, 0.05 wt%. As can be seen from Fig.machine and taking away the plate, until micro-cracksappeared in the sample.1, two types of stainless plates A and B with differentgeometries were used for the study. There are notches onboth sides in the former, but not in the latter.2.2 MeasurementsToinduceaplasticdeformationatroomtemperature,Magnetic images of spontaneous magnetizationanumberofstainlessplatesoftypesAandBwereuniaxi-were measured on a surface of stainless plates under zeroally strained using a conventional Amsler-type testingexternal fields using SHM with an active area 50mm 3machine at a strain rate of approximately 0.001 s21along50 mm, of which the details have been published else-the length of the sample. Special attention was paid towhere.(6)The SHM was equipped with a micro-Hall sen-the environment around the machine so as not to magne-sor on a movable x-y stage using a stepping motor andtize the sample during the test. After reducing the loada sample holder facing the sensor. The Hall sensor (ARE-to zero, the plates in a strain state were subjected toPOC: HHP-VP) consisting of epitaxial GaAs films wasmagnetic measurements as a parameter of strain in aused to measure magnetic fields normal to the samplesurface. The sensitivity was 3.39 mV/G at a Hall currentof 10mA. Hall voltages were measured using a nano-voltmeter. The Hall sensor on the x-y stage was scannedat a finite step (minimum step being 0.05mm) in a two-dimensional grid pattern, at a fixed distance (,0.5mm)from the sample surface. The two-dimensional distribu-tions of magnetic fields from the sample were obtainedthrough a conversion procedure from voltages to mag-netic fields by a personal computer. Special attention waspaid to a drift of null voltages causing a zero-point shiftof magnetic fields during the whole measuring run. Thescannedareasonasurfaceofstainlessplatesareindicatedin Fig. 1.3. RESULTS AND DISCUSSIONA large number of 304 stainless plates A and Bwere subjected to a strain or fatigue process at roomtemperature, andthe magnetic images dueto spontaneousmagnetizationweremeasuredonthesurfaceofthesampleusing SHM. In this section, we present the magneticimages ofthe plates A andB subjected toa strain process,and then move onto the results for the plates A subjectedto a pulling-fatigue process.Fig. 1. The geometry and dimensions in mm for 304 stainless plates3.1 Change in Magnetic Images for Stainless Plateswith (a) type A having notches at both ends and (b) type B withoutnotches. Note that a load line for application of a strain or a pullingA and B Under a Strain Progressfatigue is directed to the horizontal direction in the figures. Also shownMost of the plates A exhibited similar changes inare scanned areas on a sample surface using a scanning Hall-sensormicroscope.the magnetic images under a strain progress. As can beChanges in Magnetic Images Due to a Strain or Fatigue Progress in Austenite Stainless Steels89(a)(b)(c)Fig. 2. Magnetic images of spontaneous magnetization on a surface of stainless plate A subjected to a strain progress: (a) unstrained; (b) 2.5%-strained; (c) 7.1%-strained; (d) 11.0%-strained. Note that a load line for the strain is directed parallel to the y-axis. Also shown in the frame is thegray scale corresponding to approximate values of the magnetic field in Gauss to guide eyes for a reader.90Oota, Miyake, Sugiyama, and Aoki(d)Fig. 2Continued.seen from Fig. 2, the images of unstrained plate show aclear boundary line with the shape of the notches at bothends. An application of plastic strain causes significantchange in the images after loading and converts them tothe distributions with two peaks, positioning at aroundthe notches along a direction at a finite angle from theload line along which the sample was strained. In spiteofnomovementsofpeakpositionsintheimages,adegreeof magnitude for the change in the images becomes moresignificant with an increase in the plastic strain.In an attempt to interpret the change in the imagesin a strain state, it is worth mentioning that 304 stainlesssteels undergo a transformation from the paramagneticaustenite phase to the ferromagnetic martensite phaseunder a plastic strain at room temperature,(710)which iscalled the strain-induced martensite transformation. Toaddress thistransformation, the value ofVickers hardnessHv at a load level of 5 kg was measured from place toplace on the surface of the plate. The result shows astrong correlation between the distribution of Hv and themagnetic images for the strained plate A shown in Fig.2. Incidentally, the value of Hv for the 11%-strained plateshowsamaximumof320360aroundtheplacesatwhichtwo peaks appear in the images as shown in Fig. 2(d).The same correlation also stands between the distributionof magnetic permeability and the magnetic images on asurface of the strained plate A. Furthermore, as shownin Fig. 3, there are many slip lines in microstructuresnear the notches of the strained plate A, in comparisonwith the unstrained sample. All these facts suggest thatthe change in the magnetic images shown in Fig. 2 can beascribed to the strain-induced martensite transformation.Fig. 3. Microstructures of stainless plate A near a notch when it was:In contrast to the result mentioned above, several(a) unstrained; (b) 11.0%-strained. Note that the sample is the same asthat shown in Fig. 2.(but not exceptional number) of the plates A showedChanges in Magnetic Images Due to a Strain or Fatigue Progress in Austenite Stainless Steels91different and anomalous changes in the images under atrigger for circular patterns) in a strain progress, andrevealed the complexity in the destructive progress of astrain progress, although the imagesin an unstrained stateshowed no notable difference from Fig. 2(a). Such exam-strain process, accompanied by the strain-induced mar-tensite transformation.ples are shown for the 5.5%-strained plate in Fig. 4(a)and also for the 0.7%-strained plate in Fig. 4(b). AsIn comparison, it is worth mentioning the resultsfor the plates B, where there is relatively more evencan be seen from the figures, the latter shows additionalcircular patterns in the images between both notches,distribution of stress under a plastic strain in both thewidth and length directions because of the sampletogether with the signals ascribed to the martensite trans-formationmentionedabove.Ontheotherhand,theformergeometry. Figure 5 shows change in the images duringa strain progress. Note that in order to check the repro-has further anomalies in the images that are made of afew circular patterns, masking the signals resulting fromducibility, we took the magnetic images from severalplates B due to a strain process and obtained nearly thethe martensite transformation around the notches. Toexamine the origin of the anomalies, the plates A withsame results from them. As can be seen from Fig. 5,an application of plastic strain to the plates B producestheimagesshowninFig.4weresubjectedtoconventionalmetallurgical studies by an X-ray diffraction analysis andthe complexity in the images that are made of manycircularpatterns,wherethenumberofpatternsincreasesa scanning electron microscope with energy dispersiveX-ray spectroscopy. However, the results show neitherwith a strain progress. Although there are many sliplines in microstructures ascribed to the martensite trans-any difference from the plate A shown in Fig. 2 nor anyevidence of uneven distribution of composition in theformation in some places on the strained plate B, theinfluence of this transformation on the images stillplate.Alltrialsresultedinfailuresinexplainingtheoriginof the anomalies in the images (i.e., the kernel and/or theremains unsolved.Fig. 4. Different magnetic images of spontaneous magnetization on a surface of stainlessplate A subjected to a strain process: (a) 5.5%-strained; (b) 0.7%-strained. Note that there isno notable difference in the images for unstrained state from the result shown in Fig. 2(a).Also shown in the frame is the gray scale corresponding to approximate values of the magneticfield in Gauss to guide eyes for a reader.92Oota, Miyake, Sugiyama, and Aoki(c)(a)Fig. 5Continued.of 28 kg/mm2and a frequency of 29.2 Hz. Note that thesample showed small cracks (,5mm long and ,0.1mmwide) near the top of the notch (i.e., x,12mm andy,12mm in Fig. 6(d) after stress cycle N of 61,300.The result in the virgin state exhibits a clear boundaryline of the sample including shapes of notches in themiddle of the plate. A fatigue progress causes changesin the images, although the stress amplitude is less thanthe yield point (530 kg/mm2). In comparison with theresults for a strain process (see Fig. 2), the images havea single peak at near the top of the notch. An increasein N enhances the peak amplitude in the images (i.e., thestray field strength) and finally leads to small cracks(,5mm long and ,0.1mm wide) near the notch for N5 61,300. Note that the place at which the cracks occurcorrespondstotheplaceatwhichtheimageshaveasingle(b)peak near the notch. This fact suggests the possibility toFig. 5. Magnetic images of spontaneous magnetization on a surface offoresee the places of strain localization at which micro-stainless plate B subjected to a strained progress: (a) unstrained; (b)cracks occur in the plate ina fatigue progress. In addition,0.8%-strained; (c) 13.8%-strained. Note that a load line for the strainthere is a strong meandering line at around x,30 mmis directed parallel to the y-axis. Also shown in the frame is the grayalongthey-axisintheimagesattheinitialstageoffatiguescale corresponding to approximate values of the magnetic field inGauss to guide eyes for a gress (i.e., low cycle of N 5 20,000). This line neversmears out even when small cracks occur at N 5 61,300.Note that such a line does not appear in the images underastrainprogressmentionedinsection3.1.Sinceapulling-3.2. Change in Magnetic Images for Stainless Platesfatigue test is made with a load level below a yield point,A Under a Pulling-Fatigue Progressthefatigue progressshouldaffect microstructureswithoutmacroscopic deformation and produce localized micro-Figure 6 shows progressive change in magneticimages caused by a fatigue progress in stainless platesstrains.Thismaycausesomedifferenceinthemechanismof martensite transformation so that it leads to differentA, subjected to a pulling fatigue test at a stress amplitudeChanges in Magnetic Images Due to a Strain or Fatigue Progress in Austenite Stainless Steels93(a)(b)(c)Fig. 6. Magnetic images of spontaneous magnetization on a surface of stainless plate A subjected to a pulling-fatigue test at: (a) virgin state; (b)20,000 cycles; (c) 60,000 cycles, (d) 61,300 cycles. Note that a load line for a pulling fatigue test is directed parallel to the y-axis. Also shown inthe frame is the gray scale corresponding to approximate values of the magnetic field in Gauss to guide eyes for a reader.94Oota, Miyake, Sugiyama, and Aoki(d)Fig. 6Continued.changesin theimages inthe destructiveprogress betweenACKNOWLEDGMENTa strain process and a fatigue process.WewouldliketothankDr.K.Kawanoforhistechni-cal assistance in measurements and also for valuable dis-cussion.4. SUMMARYREFERENCESWe succeeded in visualizing complex changes in1. D. E. Bray and D. McBride (eds), Non-d
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