Correlation between MBN and reverse transformation of martensite by heat treatments in AISI 304

ABSTRACT

Austenite reversion, α'-martensite → γR-austenite reverted phase transformation, in AISI 304 steel was investigated. All the specimens had been subjected to a one-hour austenitizing heat treatment at 1050 °C in vacuum and with air cooling. Then, they were laminated at –70 °C, with a resulting total reduction of 63%. Finally, the samples thus generated were individually subjected to thermal annealing treatments for 1 hour, with increasing temperatures between 300 °C and 950 °C. In addition to microstructural changes and Vickers microhardness, the reversal process was studied through magnetic measurements: magnetic saturation and magnetic Barkhausen Noise (MBN). These measurements were carried out at room temperature. A temporal analysis of the MBN signals was done and their RMS (Root Mean Square) values were calculated. A similar trend was observed for the RMS of the MBN and the α' martensite contents, beyond the intrinsic differences in the techniques used to perform the measurements. Both techniques allow observing the evolution of the content of α' martensite or ferromagnetic phase as the temperature of the thermal reversion treatment increases.

Magnetic Barkhausen Noise; Non-Destructive Testing; C steel; Welding

1. INTRODUCTION

Austenitic stainless steels (ASS) are conventionally used in various applications over a wide range of service temperatures. They have good corrosion resistance, adequate mechanical properties such as ductility, formability, toughness and an important hardening capacity by cold working. By applying cold deformation to these steels, a phase transformation is induced: from γ-austenite to α'-martensite (deformation induced martensite, DIM). The degree of this transformation depends on the composition of the material, the process temperature and the applied deformation [¹,²,³,⁴].

However, from annealing heat treatments the austenitic phase can be partially reverted. The amount of austenite reverted (γR) depends on whether the heat treatment temperature (TT) is above the austenization start (AS) temperature (at which the reverse transformation begins α' → γR). In theory, the reversion to the austenitic phase would be completed if the heat treatment temperature equaled the austenitization completion temperature (Af). This is due to the energy barrier imposed by interfacial energy and accumulated elastic stresses, which require additional superheating to promote the nucleation and growth of the austenitic phase ref. Other factors that also affect the degree of reversion are: duration of the annealing TT, chemical composition of the steel, type of deformation that caused the DIM and its percentage existing before the annealing TT [², ³]. The γ austenitic phase is paramagnetic and its crystal structure is face-centered cubic. On the other hand, the α' martensite phase is ferromagnetic and its crystallographic structure is body-centered cubic [¹,²,³,⁴]. From several years now, the reversion α' → γ_R, is used as a process to control the microstructure and thus obtain an ultrafine grain size in austenitic stainless steels [⁵, ⁶]; or to produce localized areas of austenite in a martensitic structure in order to achieve an increase in the mechanical strength and ductility [⁶]. In particular, in severe rolling deformation processes, α' martensite nucleates and grows within γ austenite grains. The morphology of α' is lath type. In turn, the laths tend to align parallel to each other in an area of the austenitic grain and each of them constitutes a martensite crystal with a high density of lattice defects. In this way, the austenitic grains are divided into several packets (group of laths with the same plane of habit) and each of these contains parallel blocks (groups of thin laths of equal orientation) [⁷, ⁸].

MBN is a Non-Destructive Testing technique, which is used in the evaluation of structures of ferromagnetic materials. It is based on a phenomenon of the same name that occurs in ferromagnetic materials, when they are subjected to slowly varying magnetic fields, generating changes in its magnetization that are manifested by discrete “jumps.” These jumps are caused by the movement of the magnetic domain walls (MDW) and are detected by placing a small receiving coil on the surface of the sample. The movement of MDW are determined by the characteristics of the material’s microstructure (precipitates, grain size, residual stresses, etc.). Therefore, the MBN allows the material to be characterized indirectly [⁹,¹⁰,¹¹]. This technique proved to be effective in the evaluation of the austenite-martensite phase transformation [¹¹,¹²,¹³,¹⁴,¹⁵,¹⁶,^{17 18}], so it was used to study the samples subjected to martensite-austenite reversion TTs.

Several authors have studied the transformation changes from austenite to martensite phases in austenitic steels with MBN. KLÉBER and BARROSO [¹⁵] showed different shot peening conditions that were applied to an AISI 304L austenitic stainless steel to transform austenite into α' martensite at different depths. In the case of cold-worked martensite formation for AISI 304: RAO et al. [¹⁶], AMITRAVA et al. [¹⁷], and O’SULLIVAN et al. [¹⁸], MESZAROS and PROHÁSZKA [¹²] and AHMADZADE-BEIRAKI et al [¹¹], studied the change in magnetization during martensitic transformation in samples that were cold deformed, measuring the MBN after deformation of the specimens. NEYRA ASTUDILLO et al [¹³], studied the change in magnetization during the martensitic transformation in online cold-deformed samples. However, very few authors have studied the reverse transformation of martensite to austenite in AISI 304: MESZAROS and PROHÁSZKA [¹²], O’SULLIVAN et al., [¹⁴], studied the reversion of α' -martensite to austenite during heat treatment and observed that the magnetic recovery and hardness processes start at different temperatures. This suggests that the MBN measurement may have limitations in studying the reverse transformation process due to the influence of residual stresses and changes in magnetic properties during heat treatment [¹⁴].

2. EXPERIMENTAL DEVELOPMENT

2.1. Materials

For the development of this work, a series of 9 AISI 304 austenitic stainless steel samples that had been previously processed in the IAMEND Group, were studied [¹⁹]. All the specimens were subjected to a one-hour austenitizing TT at 1050 °C in vacuum and later air-cooling. They were then rolled at –70 °C (200 K), with a resulting total reduction of 63%. The samples were rolled in a STANAT rolling mill with 120 mm diameter cylinders at a speed of 127 rpm, maintaining between passes at predetermined temperatures using thermostatically controlled baths or coolers [¹⁹, ²⁰]. Finally, the samples thus formed (65 × 30 × 2.3 mm³) individually subjected to annealing TT for 1 hour, with temperatures between 300^oC and 950 °C, in order to recover partially the γR phase. A set of specimens with different contents of α'-martensite was then obtained. The reversion TTs were made in a tubular oven, under vacuum and with air-cooling [¹,²,³, ¹⁴, ¹⁹]. The chemical composition of this AISI 304 steel was determined by Energy Dispersive Spectroscopy (EDX) tests, is detailed in Table 1.

Table 1
Chemical composition of AISI 304 (% weight).

Fe	C	Si	Mn	P	S	Cr	Ni	Mo
Balance	0.04	0.68	1.67	0.0022	0.005	18.8	9.2	0.08

2.2. α'martensite content from magnetic saturation and Vickers microhardness measurements

The α'-martensite phase induced by cold deformation of AISI 304 series stainless steels exhibit a ferromagnetic behaviour. Then, it is possible to quantify its content through tests in which an external saturation magnetic field was applied. The IAMEND Group carried out the characterization of the specimens according to % α'-martensite and Vickers microhardness prior to the present work [³]. Those results are now compared with the new MBN measurements.

Magnetic saturation measurements were carried out in two vibrating sample magnetometers: 1) Versalab model, Quantum Design brand, belonging to the Laboratory of Electrical and Magnetic Properties, Department of Condensed Matter, Research and Applications Management, CAC, CNEA at room temperature and applying a maximum magnetic field of 30 kOe. However, at a field of 20 kOe, saturation was already achieve in the samples. 2) LakeShore 7400 model, from the Low Temperature Laboratory, Department of Physics, Faculty of Exact and Natural Sciences, University of Buenos Aires, at room temperature and with a maximum magnetic field of 20 kOe.

From the quotient between the measured values of the specific saturation magnetization for each sample and that of the specific saturation magnetization calculated for a sample with 100% martensite, the content (%) of the α'-martensite phase was obtained [¹⁶,⁷,¹⁸]. Vickers microhardness tests were carried out to observe how this magnitude varies with respect to the temperature of the heat treatments.

A Leitz Wetzlar model Durimet microdurometer was used, with a load of 0.2 kgf and an indenter with a pramidal diamond tip. Table 2 indicates the values of the α'-martensite phase content calculated from the magnetic saturation and Vickers microhardness measurements as a function of the temperatures of the heat treatments to which the samples were subjected.

Table 2
Values of α'-martensite content and Vickers microhardness for AISI 304 samples. The martensite phase α' was calculated from the ratio between the specific saturation magnetization values for each sample and the specific saturation magnetization calculated for a sample with 100% martensite.

AISI 304 SAMPLE	TT TEMPERATURE (º C)	MARTENSITE (% m/m)	VICKERS MICROHARDNESS (HV0.2)
M2	300	81 ± 4	540 ± 10
M3	350	81 ± 4	560 ± 10
M4	400	83 ± 4	580 ± 10
M5	500	65 ± 3	550 ± 10
M6	600	56 ± 3	520 ± 10
M7	700	22 ± 1	490 ± 10
M8	800	26 ± 1	470 ± 10
M9	850	12.4 ± 0.6	430 ± 10
M11	950	1.4 ± 1	250 ± 10

2.3. Microstructure characterization

According to SALGADO LARA [³], in AISI 304 steel samples the α'→γ reversion process begins for temperatures between 415 °C and 445 °C.

Therefore, in samples heat-treated up to 400 °C it was consider that this process has not started.

In agreement to the literature, the preliminary microstructure at the beginning of the reversion is composed of elongated grains oriented to the rolling direction corresponding to the previous austenite and inside blocks and packets of martensite (ferromagnetic phase) containing a large number of crystallographic defects [³, ¹²]. Figure 1 represents the microstructure for sample M4, whose reversion TT temperature was 400 °C. Etched with Aqua Regia and Methanol to reveal martensite structure [⁸].

Figure 1
Sample M4. Microstructure of elongated grains due to rolling deformation. Martensite is distributed in blocks and packets. Etched with Aqua Regia and Methanol to reveal martensite structure. 1000x magnification.

Figure 2 shows a micrograph corresponding to the sample M7, heat-treated at a temperature of 700 °C. According to this, the reversion process has already begun, noticing a structure similar to that of the Figure 1, with defined grain boundaries still containing a large amount of α'-martensite. Electrochemical polished, etched with oxalic acid to reveal austenitic grain boundaries [⁸].

Figure 2
Sample M7. Austenitic grains deformed by rolling and few martensite. Electrochemical polished, etched with oxalic acid to reveal austenitic grain boundaries. 100x magnification.

2.4. Magnetic Barkhausen Noise

The MBN tests were carried out at the ICES Laboratory. To produce the variable excitation magnetic field, a 1 V/10 Hz sinusoidal source was applied to a U-shaped yoke. The MBN sensor coil was located in the center of the yoke (see Figures 3b and 3c). Signals were recorded on a PicoScope 5444b oscilloscope. The sampling rate was 2.976 Msamples/s. Figure 3 shows: a) the experimental measurement system and b) the yoke over the sample and c) the MBN sensor coil.

Figure 3
Experimental set-up. a) MBN measurement system. b) yoke with excitation coil front view; c) yoke bottom view with the MBN sensor coil between the legs of the yoke.

2.5. Analysis of MBN signals

For each test, 10 recordings were taken containing the excitation current and the MBN signal information collected by the sensing coil. The signals were processed using MATLAB routines to calculate the RMS values of a signal cycle and the half-cycle envelope function.

3. RESULTS AND DISCUSSION

The results correspond to the temporal and quantitative analysis of the MBN signals in the inverse transformation of AISI 304, for different TT temperatures.

3.1. MBN signal analysis

Figure 4 shows a decrease in the amplitude of MBN signals with increasing TTs temperatures of the α' → γ reversion. For samples with TTs starting above 400 °C, the MBN signal turned to be very weak due to the decrease in the content of the α› magnetic phase as a consequence of the reversion process. Therefore, for temperatures above 400 °C, the amplitude scale of the graphs was magnified to visualize and distinguish any variations. In all cases above 700 °C, a nearly constant signal was observed which could be electronic noise.

Figure 4
MBN signals in AISI 304 steel samples for different reversion heat treatments temperatures.

Figure 5 a) shows the envelope function of MBN vs. time for each of the specimens studied. In Figure 5b) the superimposed MBN envelope functions for samples heat treated above 400 °C are illustrated with an enlargement in the vertical axis. Above this temperature the amplitude of the MBN signal decreases notably in accordance with the decrease in the content of the α' magnetic phase due to the reverse transformation α' → γ_R. At temperatures greater than or equal to 700 °C, the observed signals could be due to electronic noise.

Figure 5
MBN envelope function vs. time for AISI 304 steel specimens with reversal TTs in the range between (300–950) °C: a) Full scale b) Magnified scale to visualize low voltages signals for TTs above 400 °C.

Plots for samples heat treated from 300 to 400 °C show an increase in the number of peaks and a decrease in the amplitude of the MBN envelopes with increasing TT temperatures. Furthermore, it is observed that the time to reach the maximum of the envelope function increases, shifting to the right, indicating that a higher field (current) is needed to generate MBN. This could be because due to the rolling process the samples contain a large amount of martensite, presenting a high density of dislocations within the laminate of the material structure, which can affect the hardness, strength and other mechanical properties. Dislocations can act as anchoring points for the domain walls, affecting the motion of the magnetic domains and therefore the magnetic response of the material [⁹].

The grain refinement of reverted austenite can occur in two ways: If the reversion occurs via a non-diffusional mechanism, the resulting austenite is denoted as γR-austenite. In this type of reversion, the α' martensite laths will become γR-austenite through a cooperative movement of atoms and will have a high density of dislocations within the laths resulting from the previous deformation of the martensite, in the early stages of annealing, the boundaries between the γR austenite laths rapidly disappear; the dislocation rearrangement within the laths then forms cells that transform into subgrains. Finally, the coalescence of these subgrains forms recrystallized grains. If the reversion occurs via a diffusional mechanism, the new γR austenite grains nucleate randomly between the martensite laths and grow equiaxed [²¹, ²²].

From 700 °C onwards, a clear MBN signal is not observed, even though in Table 2 it can be seen that there is a lower percentage of martensite. This could be due to the heat treatments, the martensitic phase γ' decreases, increasing the amount of austenitic phase γR (non-magnetic) and consequently reducing the amount of magnetic domains.

3.2. Quantitative analysis

Figure 6 shows the dependence of the RMS value of MBN, the martensite content, and the Vickers microhardness as a function temperature to which the samples were subjected. Figure 6a) illustrates the effect of the TT temperatures on the α'martensite content. In the temperature range of TTs between 300 °C and 400 °C, the contents of α' do not present significant variations according to the indicated uncertainty values. For temperatures above 400 °C and up to 700 °C the martensite content decreases rapidly due to the α' → γ_R transformation as reported in Salgado’s thesis work [³], in relation to the start and end temperatures of reversion and their mechanisms. Beyond 700 °C the martensite content decreases more gently due to a change in the reversion mechanism that becomes diffusional [²³, ²⁴].

Figure 6
Comparison of the MBN RMS value with a) martensite content and b) microhardness value, for different TT temperatures for AISI 304.

The variation of the RMS value of MBN with respect to the temperatures of the TTs presents a similar trend to that of the α' content. This is the reason why the MBN technique is effective in the characterization and quantification of the ferromagnetic phase α' in all the specimens. However, both curves present significant decreases over different temperature ranges of the TTs. In particular, the RMS values decline abruptly between 350 °C and 500 °C. For the martensite contents calculated by the magnetic saturation technique using magnetometers, the significant decrease occurred in a more extended TT temperature ranges. In b) the Vickers microhardness increases in the range of reversion TT temperatures between 300 °C and 400 °C, reaching its maximum value (510 ± 20 HV0.2) at the end of that interval. The hardening of the material for samples with temperatures of TTs up to 400 °C could be associated with a combination of the different mechanisms studied by LEE et al. [²⁴], for this stainless steel deformed by cold rolling. These mechanisms being the redistribution of C atoms towards the dislocations in the martensite and whose interaction increases the hardness in the α' phase. The segregation of C atoms towards dislocations and microtwins in the austenite deformed during TT produce a hardening of the γ phase, and the formation of an additional amount of α' due to the precipitation of carbides that also favors the increase in hardness. For TT temperatures greater than 400 °C. The graph shows two well-differentiated regions in the decreasing of microhardness. Up to 700 °C a slight decrease is caused by the decrease in the α' content due to the martensite reversion process. Beyond this temperature, the decrease is abrupt and coincides with a slight decrease in the content of α' due to reversion by a diffusional mechanism and also with a reduction of crystalline defects in the structure of the material.

4. CONCLUSION

The MBN technique was used to characterize a set of AISI 304 austenitic stainless steel samples deformed by cold rolling and subjected to different 1 hour TTs to recover the austenitic phase. The RMS values of MBN were compared with the α' content values obtained from magnetic saturation measurements and with the Vickers microhardness of the specimens. A similar trend was observed for the RMS of MBN and α' contents, beyond the intrinsic differences in the techniques used to perform the measurements. Both techniques allow observing the evolution of the content of α' martensite (ferromagnetic phase) as the temperature of the reversion TT increases. The MBN technique was effective and presents a good correlation with the martensite contents calculated from measurements using magnetic saturation and microhardness, carried out prior to the present work. However, the MBN technique seems to be more limited than the magnetic saturation technique, since the latter allows greater discrimination in the detection of the magnetic response of the material starting at a temperature of 500 °C. For temperatures of the reversal TTs lower than 400 °C (prior to the start of the reversal process), high values of hardness and martensite content are observed, which correspond to high RMS values of MBN.

From 400 °C there is a significant decrease in the martensite content and microhardness, which correlates with the decrease in the MBN signal due to the reversion process. The MBN can be affected by physical parameters such as grain boundaries, vacancies, inclusions or dislocations, since they act as barriers to the movement of the MDW and this is revealed in the appearance of a greater number of peaks in their corresponding envelope function.

5. ACKNOWLEDGMENTS

Research associated with the ICES (International Center of Earth Sciences) Project, was endorsed and funded by the National Atomic Energy Commission.

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Publication Dates

• Publication in this collection
  30 Mar 2026
• Date of issue
  2026

History

• Received
  21 July 2025
• Accepted
  05 Feb 2026