**1. Introduction**

Research is now being conducted in many scientific centres on methods that will enable the determination of the effect of cold plastic strain [1–4], mechanical fatigue [5–7], heat treatment [8], and creep [9] on the state and electromagnetic properties of austenitic steels. The applied diagnostic signals are the quantities describing the magnetic hysteresis loop, the eddy currents, the Barkhausen noise parameters, and the changes in the anisotropy of electromagnetic properties. The variations in these parameters result from the state of the microstructure, the grain size, and the impact of the dislocation density on the material electromagnetic properties.

Austenitic steels are widely used materials, and the strain-induced martensite transformation occurring in them, depending on the chemical composition, the magnitude of the rolling reduction, and the deformation temperature can have both favourable effects

**Citation:** Roskosz, M.; Fryczowski, K.; Tuz, L.; Wu, J.; Schabowicz, K.; Logo ´n, D. Analysis of the Possibility of Plastic Deformation Characterisation in X2CrNi18-9 Steel Using Measurements of Electromagnetic Parameters. *Materials* **2021**, *14*, 2904. https:// doi.org/10.3390/ma14112904

Academic Editors: Francesco Iacoviello and Raffaele Landolfo

Received: 9 April 2021 Accepted: 26 May 2021 Published: 28 May 2021

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causing the material strengthening (a higher yield point or an increase in tensile strength) and unfavourable consequences causing a decrease in corrosion resistance and the appearance of the ferromagnetic phase [10–15]. Under the influence of cold plastic strain, a change occurs in the dislocation structure. As a result, metastable austenite undergoes a partial transformation into martensite ε and ferromagnetic martensite α' with a body-centred cubic lattice [12,13].

Novotný et al. [1] introduced a novel application of magneto-optical films. At the magnetic field sensitivity of 100 A/m, coercivity can be mapped with a resolution as high as 50 μm. Promising results were obtained for austenitic steel by applying the magneto-optical method to indicate critically degraded (plastically deformed) locations.

O'Sullivan et al. [2] characterised work hardening of an austenitic stainless steel grade (SS404) using non-destructive magnetic measurement techniques, including measurements of the magnetic Barkhausen noise, the ferromagnetic phase, and coercivity. It was found that the material work-hardening was caused by the dislocation density rather than by the α- -martensite phase. The coercivity measurement proved to be a useful non-destructive quantitative method for characterizing work hardening in relation to the degree of plastic deformation.

In [3], the authors investigated selected phase transformations of the AISI 304 austenitic steel. The Barkhausen noise, coercivity, and ferrite content were measured to identify changes in the strain-induced α- -martensite phase due to cold rolling and elongation. The research proved that it was possible to study the mechanism of austenite transformation into the α phase, and the reverse transformation of the αphase into austenite.

In [4], the authors presented experimental studies on the amount of transformed martensite by measuring the continuous change in impedance during plastic deformation on specimens made of the 304 steel grade. The specimens were cores of a prototype solenoidal coil, which was subjected to compressive load.

In [5], the authors investigated specimens of the chromium-nickel steel used to make the generator retaining rings and the generator rotor shrouding. The specimens were subjected to fatigue and static loads. The austenite instability became apparent after plastic deformation (increase in the material permeability by about 0.1 μr). Magnetic measurements based on austenite instability detection in mechanical and thermal correlations are an alternative to ultrasonic wave attenuation tests. Moreover, they give a more complete picture of the wear degree of the retaining ring (material degradation evaluation).

Vincent et al. [6] investigated the low-cycle fatigue (LCF) of steel 304L and the influence of the strain-induced α- -martensite on the magnetic Barkhausen noise (BN). It was shown that the variations of the martensite content induced by LCF could be related to and characterised by the BN. The number of cycles had an effect on the α- -martensite phase, and the αpeak was clearly visible in the BN signal envelope.

In [7], AISI 31 austenitic stainless steel samples were subjected to fatigue testing. The effect of fatigue on the accumulation of damage and changes in the content of the α- martensite was investigated. The obtained results showed the possibility of assessing the fatigue state of the AISI 31 steel using acoustic nonlinearity measurements and magnetic coercivity.

In [8], the authors investigated the relationship between the eddy current output signal and the surface hardness of a martensitic AISI 410 stainless steel sample in terms of impedance and inductance. They also examined the effects of different quenching temperatures on the steel surface hardness.

Augustyniak et al. [9] tested samples of 347, 321, and 304 austenitic steels taken from service-aged power plant boiler tubing. The accumulation of damage due to the creep process was proportional to the concentration of the created magnetic oxide layer. Simultaneously, a magnetic ferrite phase also formed in the grains and at grain boundaries under the scale layer. The content of the ferrite-phase layer was proportional to the initial creep-related damage. These changes were related to the eddy current signal.

This paper is focused on the analysis of the possibility of characterizing the plastic strain ratio in specimens made of X2CrNi18-9 steel by measuring the residual magnetic field, the impedance components in in-series LCR circuits, and the Barkhausen noise. The same measurement quantities were used to characterise the active stress state in [16]. Additionally, metallographic tests were performed to observe the structural changes occurring due to deformation during static tensile testing.
