Peculiarities of magnetic phase analysis in 30Ni steel

Abstract

This work is focused on the magnetic determination of martensite phase contents and characteristic temperatures in a Fe-30Ni steel alloy. To achieve this, magnetic saturation measurements and thermomagnetic measurements are applied. Due to the presence of ferromagnetic austenite in the alloy's microstructure, an own lab calibration for magnetic saturation measurements has been carried out, and is being described herein. The calibration, revised in 2017 using XRD analyses, is subsequently applied to determine the martensite phase contents of tensile test samples of Fe-30Ni, tested at temperatures from −40 °C to 120 °C.

Full text

1
Peculiarities of magnetic phase analysis in
30Ni steel
Sandig, E. Frank
Institut für Eisen- und Stahltechnologie, sandigf@iest.tu-freiberg.de
Hauser, Michael
Institut für Eisen- und Stahltechnologie, michael.hauser@iest.tu-freiberg.de
Weiß, Andreas
Institut für Eisen- und Stahltechnologie, weiss@iest.tu-freiberg.de
Volkova, Olena
Institut für Eisen- und Stahltechnologie, volkova@iest.tu-freiberg.de
Diese Arbeit konzentriert sich auf die magnetische Bestimmung von Phasenanteilen und charakteristi-
schen Temperaturen eines Fe-30Ni-Stahls. Um dies zu erreichen wurden magnetische Sättigungsmes-
sungen sowie thermomagnetische Messungen angewandt. Auf Grund des Vorliegens ferromagneti-
schen Austenits im Gefüge der Legierung wurde eine eigene Kalibrierung durchgeführt. Diese wird im
vorliegenden Beitag beschrieben. Die Kalibrierung wurde 2017 mittels XRD-Messungen überarbeitet.
Anschließend kam sie für die Bestimmung von Martensitgehalten von bei −40 °C bis 120 °C im Zug-
versuch geprüften Rundproben zur Anwendung.
This work is focused on the magnetic determination of martensite phase fractions and characteristic
temperatures in a Fe-30Ni steel alloy. To achieve this, magnetic saturation measurements and thermo-
magnetic measurements are applied. Due to the presence of ferromagnetic austenite in the alloy’s mi-
crostructure, an own lab calibration for magnetic saturation measurements has been carried out, and is
being described herein. The calibration, revised in 2017 using XRD analyses, is subsequently applied
to determine the martensite phase contents of tensile test samples of Fe-30Ni, tested at temperatures
from −40 °C to 120 °C.
1 Introduction
In the course of research of the Collaborative Research Center 799 (CRC799) at TU Bergakademie
Freiberg, various metastable austenitic steels have been developed and investigated. Those alloys exhi-
bit deformation induced phase transformation and/or twinning under external load. Due to this so-cal-
led Transformation Induced Plasticity (TRIP)/Twinning Induced Plasticity (TWIP) effect, metastable
steels achieve substantially higher strength and ductility values in the as-cast state than comparable
steel alloys without TRIP/TWIP effect [1–12].
This work is focused on magnetic investigation methods for a Fe-30Ni alloy. This iron-based alloy
with around 30 wt-% of Ni was one considered model alloy during thermodynamic-mechanical model-
ling of the TRIP effect in high-alloyed steels from CRC799 [13]. This alloy is fully austenitic at room
temperature due to its nickel content [14, 15]. More detailed information on and results from the mag-
netic investigation of the TRIP effect in the Fe-30Ni steel may be found in our previous works [16–
20]. In the following sections, the term “deformation-induced martensite formation” refers to all indu-
ced martensite formation under external load. It consists of stress-induced martensite formation in the
range of elastic deformation and strain-induced martensite formation in the range of plastic deforma-
tion [6].

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2 Magnetic saturation measurement
Magnetic measurements of phase contents can be carried out very fast (few seconds per sample) and
only minor preparation of samples is required. In contrast, quantitative analyses of LOM images may
take hours per sample, which also require more effort in preparation. Additionally, metallographic
means tend to clearly overestimate the martensite volume content [13, 21, 22]. Therefore, magnetic
measurements are often preferable, although always requiring additional verification, as they are indi-
rect measurement methods. The measurement is based on magnetically saturating the sample in a mag-
netic field of H = 300 kA m−1 and correlating the measured magnetic dipole moments j [μWb cm] ver-
sus the alloy’s ferromagnetic phase content. It is frequently applied to examine martensite contents of
metastable austenitic steels [3–5, 11, 13, 21, 23]. Samples used in this work are hot rolled Fe-30Ni ma-
terial, ø12 mm × 3.5 mm and pieces taken from the uniform elongation part of round tensile test samp-
les, ø6 mm × 2.5 mm.
2.1 Calibration
For the calibration of the magnetic saturation measurement device (MSAT), samples of Fe-30Ni mate-
rial with martensite contents of 0 vol-% (three), 3 vol-%, 37 vol-%, 64 vol-%, 70 vol-% and 71 vol-%
have been examined. The α´-martensite phase content VM of each sample, determined by means of X-
ray diffraction (XRD) with subsequent Rietveld analyses, has been plotted versus the apparent ferro-
magnetic phase content VFa . This value is given by the MSAT based on its factory calibration, which
assumes alloys with one ferromagnetic and one paramagnetic phase. A linear regression has been car-
ried out regarding the calibration samples. Figure 1 shows the resulting diagram. Subsequently, all re-
sults from MSAT measurements of samples from athermal and deformation-induced α´-martensite for-
mation have been calibrated using the regression line from figure 1. The MSAT calibration exploits a
small difference in the magnetic moments of the present phases (around 0.2 μB to 0.5 μB at room tem-
perature [15]). This difference is represented by the slope of the calibration curve. The positiveness of
the slope indicates that martensite shows a higher magnetic moment (at room temperature) than auste-
nite, as reported by in literature [15].
Figure 1. Calibration of MSAT measurements based on XRD/Rietveld analyses. The diagram
shows martensite content V M versus apparent ferromagnetic content V Fa , each in vol-%.
2.2 Results
The above calibration has been applied for two sets of samples. One series has been deep-cooled. The
second series represents all samples investigated with quasi-static tensile tests until fracture in a tem-
perature range from −40 °C to 120 °C. As to be seen from figure 2, martensite contents increase both
with deep cooling and plastic deformation. While a sample which has been exposed to liquid nitrogen
at −196 °C became martensitic to 74 vol-%, the microstructure of another sample, cooled down to −80

°C in a cryostat, contains less than one third of α´-martensite. Thus no Mf temperature has been found
in the given temperature range. The maximum martensiter content achieved by deformation is 70 vol-
% at −40 °C. No martensite formation has been found at deformation temperatures of 40 °C and abo-
ve, as well as quenching temperatures of −40 °C and above. This has been confirmed in comparison
measurements of different samples with XRD. The original approach of calibration based on
LOM/EPQ quantification had indicated a fully martensitic structure quenched below −160 °C or defor-
med below −20 °C [19].
Figure 2. Comparison of athermal and maximum deformation-induced α´-martensite formation.
Martensitic phase contents from both processes in vol-% versus temperature.
3 Thermomagnetic measurement
The thermomagnetic measurement is suitable for the evaluation of temperature dependent magnetic
properties of metallic samples in a temperature range from −140 °C to 1000 °C. It offers the possibili-
ty of comparison measurements for DSC or dilatometry results. By this procedure, changes in magnet-
ic behaviour are measured; instead of thermal or geometric changes associated with the magnetic
transformations. The samples, solid hot-rolled cylinders around ø5 mm × 10 mm, may be heated in-
ductively or cooled with nitrogen gas. The measurement device provides separate V-t and T-t curves
for each experiment. Discontinuities in the V-t curve are evidence for magnetic transformations or for-
mation of phases with different magnetic properties in the sample material, respectively. For the time
at which a certain discontinuity occurs, a characteristic temperature may be read from the T-t curve.
Figure 3 illustrates the experimental set-up schematically.

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Figure 3. Experimental set-up a) and full bridge compensator circuit b) for the determination of
martensite start temperature Ms in the thermomagnetic measurement device.
For the evaluation, the curves have been plotted in one combined diagram (figure 4). The martensite
start temperature for the formation of α´-martensite, Ms, has been determined via cooling down to
−100 °C at 0.26 K s−1 to be around −42 °C by the tangent onset method. After the primary coil is swit-
ched on, the voltage first slowly increases with falling temperature. This means that the magnetic mo-
ment of the austenite increases, as known for Fe-Ni alloys from literature [15]. At −42 °C, the voltage
starts to decrease, while at the same instant the decrease of the sample temperature is slowed down to
arou nd 0.20 K s−1 , without changing the target cooling rate. This indicates a phase transition within
the sample. The drop of voltage Umag in this case is around 70 mV, which means a rather small and
slow change in the magnetic moments. The fact that voltage decreases with rising martensite content
means that at low temperatures the magnetic moment of martensite is lower than that of austenite. The
calibration had yielded that the opposite is true at room temperature. Thus the temperature dependence
of the two phases’s respective magnetic moments has to be substantially different. The martensite con-
tent of the sample from Ms measurement was less than 30 vol-% after cooling down to −100 °C, which
fits the results from figure 2. The declining slope after 310 ms indicates that the martensite formation
may halt before reaching 100 vol-%, i.e. the material would not show a Mf temperature, as indicated
above.
Figure 4. Thermomagnetic measurement of the martensite start point Ms : voltage and temperatu-
re are plotted versus time. Ms is determined via a tangent method on the voltage development.
4 Summary
The Fe-30Ni alloy, an austenitic steel of the Invar-type with 30 wt-% nickel, exhibits α´-martensite
formation. This martensitic transformation can be evoked by deep cooling as athermal α´-martensite
formation or by application of external load as deformation-induced α´-martensite formation. Near the
Ms point of −42 °C, nearly 70 vol-% of deformation-induced α´-martensite may form due to maximum
uni-axial tensile load. Fe-30Ni exhibits a particular magnetic behaviour. Due to its high Nickel con-
tent, the alloy is ferromagnetic at room temperature despite its austenitic structure. To conclude from
the difference in the magnetic moments of austenite and α´-martensite to the martensite content in the
material, a calibration of the MSAT measurements has been carried out.

5 Acknowledgments
The authors would like to acknowledge the funding of this work by the German Research Foundation
as part of the Collaborative Research Centre 799: TRIP Matrix Composite (CRC799), as well as the
support through realisation and evaluation of XRD measurements by Dr.-Ing. Christian Schimpf and
Dipl.-Ing. Christiane Ullrich, Institute of Materials Science, TU Bergakademie Freiberg, Germany.
We particularly acknowledge the support through realisation of and counsel regarding the ferromagne-
tic measurements by Prof. Dr. Ferenc Tranta, Miskolc University, Hungary.
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