Institute of Fundamental Technological Research

In this work, an attempt at determination of mechanical properties by means of a method based on magnetic Barkhausen emission measurements was proposed. The specimens made of P91 steel were subjected to creep or plastic flow which were interrupted after a range of selected time periods in order to achieve specimens with an increasing level of strain. Subsequently, measurements of magnetic Barkhausen emission were carried out, and then static tensile tests were performed in order to check variations of basic mechanical parameters. It is shown that evident relationships between the yield point /ultimate tensile strength and some parameters of the Barkhausen emission exist.

mechanical properties, magnetic Barkhausen effect, plastic strain

Efekt Barkhausena (EB) kojarzony jest z impulsami napięcia indukowanymi w cewce detekcyjnej zbliżonej do magnesowanej płyty. Typowa sonda efektu Barkhausena składa się z elektromagnesu jarzmowego (typu C) i cewki detekcyjnej z rdzeniem ferrytowym [1]. Natężenie efektu Barkhausena zależy od mikrostruktury (wielkości ziarna, morfologii wydzieleń i dyslokacji) oraz od poziomu naprężeń [2]. Magnesowanie jest łatwiejsze, gdy kierunek pola magnetycznego sondy zgodny jest z kierunkiem działania naprężeń rozciągających i trudniejsze, gdy magnesowany jest materiał poddany działaniu naprężeń ściskających. Pomiar naprężeń z wykorzystaniem efektu Barkhausena ma wiele zalet: krótki czas pomiaru (kilkanaście sekund), względnie proste przygotowanie powierzchni (usunięcie warstwy tlenków szlifierką oscylacyjną z papierem ściernym o gradacji powyżej 100). W celu określenia składowych płaskiego stanu naprężenia konieczne jest wyznaczenie rozkładu kątowego natężenia EB [2, 3]. W artykule opisano wyniki badań stanu naprężenia za pomocą nowo opracowanego aparatu wyposażonego w sondę, która generuje w badanym elemencie namagnesowanie o skokowo zmiennym kierunku magnesowania. Aparat ten umożliwia automatyczny pomiar rozkładów kątowych natężenia EB. Badania te są realizowane w ramach projektu NCBiR nr PBS1/A9/14/2012 pt. Opracowanie magnetycznej metody oceny stanu naprężeń w materiałach konstrukcyjnych zwłaszcza anizotropowych. Zdaniem Autorów jest to rozwiązanie o lepszych walorach użytkowych niż proponowane w poprzednich opracowaniach [4, 5]. Anizotropowość właściwości magnetycznych (wynikająca np. z tekstury struktury ziaren) ujawnia się w eliptycznym, a nie kołowym rozkładzie kątowym natężenia EB. Oczywista jest konieczność uwzględnienia tej anizotropii przy badaniu stanu naprężenia tą technika. Poniżej podano przykłady badania stanu naprężenia nowym układem dla złącz spawanych wykonanych z praktycznie izotropowej magnetycznie stali oraz dla stali z wyraźną anizotropią magnetyczną.

The paper presents relationships between strain level generated by creep process and two parameters determined form non-destructive tests, i.e. acoustic birefringence and amplitude of magnetoacoustic emission for three kinds of steel: 40HNMA, P91 and 13HMF. Moreover, the relationships between prior deformation level and selected mechanical parameters resulting from the standard tensile tests subsequently carried out at room temperature were established. As a consequence, this enabled to formulate mutual relationships between these mechanical parameters and parameters obtained from the ultrasonic/magnetic investigations.

creep, damage, yield point, ultimate tensile stress, Young’s modulus, acoustic birefringence, magnetoacoustic emission

The paper presents the results of multiparameter analysis of Barkhausen noise (BN) signal properties. In addition to the commonly used quantifiers of the BN signal, such as amplitude, integral of the BN envelope or results of pulse count analysis, we propose an additional analysis based on the change in magnetizing current amplitude. As it turns out the character of the change of the BN signal (as a function of the plastic deformation level) measured for various magnetizing currents differs significantly. Being so, a comparison of the results obtained for at least two magnetizing intensities gives a much better description of a plastic deformation level. In addition to that we observe two monotonic changes in the BN signal properties - a systematic shift of the BN signal peak position and the increase in the frequency for which the maximum in the BN FFT spectra occurs.

Barkhausen effect, plastic deformation, nondestructive evaluation

Development of creep damage at elevated temperatures and structural degradation due to plastic deformation at room temperature were assessed using destructive and non-destructive methods in steels commonly applied in power plants (40HNMA, 13HMF and P91). As destructive methods the standard tension tests were carried out after every kind of prestraining. Subsequently, an evolution of the selected tension parameters was taken into account for damage identification. In order to assess a damage development during the creep and plastic deformation the tests for the steels were interrupted for a range of the selected strain magnitudes. The ultrasonic and magnetic techniques were used as the non-destructive methods for damage evaluation. The experimental programme also contained microscopic observations.

The correlation between magnetoacoustic emission intensity and mechanical Barkhausen effect intensity has been tested in the case of P91 steel having different microstructure states due to either plastic flow at room temperature or speeded-up creep, performed under stress 290 MPa at temperature 773 K. The plastic strain range for the first set of samples has been from 2.0% up to 10.5% and for the second one from 0.85% up to 10.0%. Both effects revealed analogous monotonous decrease of their intensities as a function of resulting plastic strain. The decrease was higher for plastic flow than for creep. Those results are well consistent with results of tensile tests of the as damaged samples.

Magnetic properties, Mechanical factors, Steel, Plastics, Creep, Testing, Temperature distribution, Microstructure, Performance evaluation, Stress

The paper is devoted to an analysis of creep damage at elevated temperatures and structural degradation due to plastic deformation at room temperature of selected steels commonly applied in power plants (40HNMA, 13HMF). The materials were tested in the as-received state, however, in the case of the 13HMF steel also after different periods of exploitation (76000h and 144000h at elevated temperature (813K) under internal pressure (14 bars)). Destructive and non-destructive testing methods were applied to assess material degradation. As destructive methods the standard tension tests were carried out after every kind of prestraining. Subsequently, an evolution of the selected tension parameters were taken into account for damage identification. In order to assess a damage development during the creep and plastic deformation the tests for both steels were interrupted for a range of the selected strain magnitudes. The ultrasonic and magnetic techniques were used as the non-destructive methods for damage evaluation. The last step of the experimental programme contained microscopic observations. A good correlation of mechanical and selected non-destructive parameters identifying damage was achieved for the tested steels. It gives very promising tool for degradation assessments appearing in pipelines at power stations.

Development of damage due to creep at elevated temperatures and fatigue at room temperature was assessed for power engineering steels using destructive and non-destructive methods. The experimental programme contained also microscopic observations to find feedback between macro- and micro-scopic parameters. Three different power engineering steels were tested, i.e. 40HNMA, 13HMF and P91

Development of damage due to creep at elevated temperatures and fatigue at room temperature was assessed for power engineering steels using destructive and nondestructive methods. The experimental programme contained also microscopic observations to find feedback between macro and microscopic parameters