MAGNETOMETRIC ANALYSIS TO EXAMINE CRITICAL TEMPERATURES AND STRUCTURAL STATE OF THE 13%-Cr STEELS

In physical metallurgy practice the analysis of phase and structural transformations often is made through the methods based on measurement of magnetic characteristics of metal body; in these methods  measurements of coercive forces, Curie points, the analysis of frequency dependence of magnetic properties and Barkhausen effect are used.  In this paper, the technique based on change of magnetic permeability  of the sample at its continuous cooling from temperatures above Curie point (Tc ), through the martensite transformation start temperature  (Ms ) to the final stages of transformation (Mf ) is applied. The method  essence consists in measurement of frequency fluctuations in oscillatory circuit based on a chain “L (inductance coil, as a magnetic-test  coil) – C (precision condenser)”. Explored metal sample played a magnetic core role in the measuring coil. For supervision of phase transformations effects, the sample was warmed up preliminary and then  quickly transferred to the coil. The main effects were associated with  the transition through the Curie point of ferrite, as well as with the  transformation of austenite into martensite. The measuring scheme has  allowed fixing Curie point of ferrite for various steels in the range of  580  –  780  °С with accuracy of 5  ºС, at the same time martensite transformations interval had extent not less than 100  ºС. It has been shown  that magnetometric analysis technique, based on deviation of magnetic  state of ferromagnetic phases in material at near Curie temperature,  allows to define quantity of δ-ferrite in the mixed structure (martensite  +  δ-ferrite) at its various morphology that is not always achievable by traditional metallographic methods. Magnetometric analysis  of the samples subjected to primary high-temperature quenching and  the subsequent heating on lower temperatures with cooling in the measuring coil unit, has allowed defining temperatures of Ac1 and Ac3 in  studied steels which were in the range of 760 – 1020 ºС. Determination of the point Ac1 for 15Cr13Mn5MoWVB steel (780  –  790  °С) allowed setting the temperature of its tempering after quenching equal  to 780  °С, closest to the temperature of Ac1 , that made it possible to  reduce the rate of decrease in hardness at the subsequent long ageing (up to 3000  hours) of this steel at 720  °С. The developed method  for determining the temperatures Ac1 and Ac3 in steels is additional  to similar methods based on measurements of thermal, dilatometric  and mechanical characteristics and makes it possible to make a more  reasonable choice of these temperatures. The proposed methodology in  the current hardware implementation is applicable only to those steels  where the austenite at its overcooling is sufficiently stable against  processes of decomposition by diffusion mechanism, but undergoes a complete transformation with the aid of the martensitic mechanism  upon reaching Ms and Mf  points (this means that the time of the incubation period at the level of “nose” of the S-shaped decomposition curve  should be not less than 5 minutes); for other steels, including austenitic, carbon and ordinary low alloy engineering steels this technique is  inapplicable.

1. Lifshits B.G., Kraposhin V.S., Linetskii Ya.L. Fizicheskie svoistva metallov i splavov [Physical properties of metals and alloys]. Moscow: Metallurgiya, 1980, 320 p. (In Russ.).

2. Fizicheskoe metallovedenie. Vyp. 1. Atomnoe stroenie metallov i splavov [Physical metal science. Issue 1. A nuclear structure of metals and alloys]. Kan R. ed. Moscow: Mir, 1967, 334 p. (In Russ.).

3. Metallovedenie i termicheskaya obrabotka stali. Sprav. izd. v 3-kh tomakh T. 1. Metody ispytanii i issledovanii [Metal science and heat  treatment of steel. Vol. 1. Methods of testing and researches]. Bernshtein M.L., Rakhshtadt A.G. eds. Moscow: Metallurgiya, 1985,  352 p. (In Russ.).

4. Fizicheskoe metallovedenie. Vyp. 2. Fazovye prevrashcheniya. Metallografiya [Physical metallurgical science. Issue 2. Phase transformations. Metallography]. Kan R. ed. Moscow: Mir, 1968, 492 p. (In Russ.).

5. Byeon J.W., Kwun S.I. Effect of thermal exposure of 2.25Cr-1Mo  steel on magnetic Barkhausen noise. J. Korean Phys. Soc. 2004,  vol.  45, pp. 733–737. 

6. Buttle  D.J.,  Briggs  G.A.D.,  Jakubovics  J.P.,  Little  E.A.,  Scruby  C.B., Busse G., Sayers C.M., Green R.E. Magnetoacoustic and  Barkhausen emission in ferromagnetic materials [and discussion].  Philos. Trans. R. Soc. Lond. A. 1986, vol. 320, pp. 363–378.

7. Nakai N., Furuya Y., Obata M. Effect of carbide particle morphology and prior austenite grain size on Barkhausen noise in 0.4 C–5Cr– Mo–V hot-work tool steel. Mater. Trans. 1989, vol. 30, pp. 197–199. 

8. Moorthy V., Vaidyanathan S., Raj B., Jayakumar T., Kashyap B. Insight into the microstructural characterization of ferritic steels using  micromagnetic parameters. Metall. Mater. Trans. A. 2000, vol.  31A,  pp. 1053–1065.

9. Moorthy V., Vaidyanathan S., Jayakumar T., Raj B. On the influence  of tempered microstructures on magnetic Barkhausen emission in  ferritic steels. Philos. Mag. A. 1998, vol. 77, pp. 1499–1514.

10. Byeon  J.W.,  Kwun  S.I.  Magnetic  nondestructive  evaluation  of  thermally degraded 2.25Cr–1Mo steel. Mater. Lett. 2003, vol. 58,  pp.  94–98.

11. Jiles D.C. Magnetic properties and microstructure of AISI 1000 series carbon steels. J. Phys. D. 1988, vol. 21, p. 1186.

12. Kim C.S., Kwun S.I. Influence of Precipitate and Martensite Lath  on the Magnetic Properties in Creep Damaged 11Cr-3.45W Steel.  Mater. Trans. 2007, vol. 48, pp. 3028–3030. 

13. Yamaura S., Furuya Y., Watanabe T. The effect of grain boundary  microstructure on Barkhausen noise in ferromagnetic materials.  Acta Mater. 2001, vol. 49, pp. 3019–3027.

14. Dickinson S.J., Binns R., Yin W., Davis C., Peyton A.J. The development of a multifrequency electromagnetic instrument for monitoring the phase transformation of hot strip steel. IEEE T. Instrum. Meas. 2007, vol. 56, pp. 879–886.

15. Hao X., Yin W., Strangwood M., Peyton A., Morris P., Davis C.  Characterization of decarburization of steels using a multifrequency  electromagnetic sensor: experiment and modeling. Metall. Mater. Trans. A. 2009, vol. 40A, pp. 745–756.

16. Hao X.J., Yin W., Strangwood M., Peyton A.J., Morris P.F., Davis  C.L. Off-line measurement of decarburization of steels using  a  multifrequency  electromagnetic  sensor.  Scripta Mater.  2008,  vol.  58, pp. 1033–1036.

17. Liu J., Strangwood M., Davis C. L., Peyton A. J. Magnetic evaluation of microstructure changes in 9Cr-1Mo and 2.25Cr-1Mo steels  using  electromagnetic  sensors.  Metall. Mater. Trans. A.  2013,  vol.  44A, pp. 5897–5909.

18. Rachkov V.I., Obraztsov S.M., Solov’ev V.A., Belomyttsev M.Yu.,  Mikhailov M.A., Chizhikov K.E. Optimization of the chemical composition of ferrite-martensite steel to increase short-time mechanical  properties. Atomic Eenergy. 2013, vol. 115, no. 1, pp. 26–31.

19. Livshits B.G. Metallografiya [Metallography]. Moscow: Metallurgiya, 1990, 236 p. (In Russ.).