The magnetic properties of steel after the gas nitriding process text.pdf2017/04/03  · The...

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170 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII The magnetic properties of steel after the gas nitriding process Jerzy Michalski 2*,1 , Hubert Fuks 3 , Sławomir Maksymilian Kaczmarek 3 , Grzegorz Leniec 3 , Barbara Kucharska 4 , Piotr Wach 1 1 Institute of Precision Mechanics, 2 Warsaw University of Life Sciences – SGGW, 3 West Pomeranian University of Technology in Szczecin, 4 Czestochowa University of Technology, * [email protected] The paper presents the relationship between the chemical composition of the steel balls, their size and parameters of the nitriding process and magnetic prop- erties defined in the study by EPR and SQUID. The balls made form non-alloyed AISI 1085 and alloyed AISI 420-C steels were investigated. At resonance spectra for both steels before the nitriding process, the resonance signal only existed for two magnetic induction values, 400 mT and 650 mT. This values are untypical because for magnetic centres in the form of iron ions, the expected EPR signal occurs usually in magnetic fields of approx. 340 mT, when the spectrometers operates in the so called microwave X band. This abnormality presumed that the resonance signals did not originate from isolated ions, but instead from magnetically more complex systems, e.g. Fe–Fe pairs or iron clusters, while the observed EPR line position is normally even lower and occurs for a magnetic induction below 200 mT. The fact, that the magnetic centres not had the nature of isolated Fe ions, additionally confirmed the abnormal in- crease in resonance signal intensity as a function of temperature, which is a behaviour inconsistent with the Curie-Weiss law. After nitriding the AISI 420-C steel in the Nx1021(580/600°C) process, the resonance line became distinctly wider and shifts towards greater magnetic induction values. After nitriding at a lower temperature (the Nx1025(490/520°C process), the resonance lines shifted towards greater magnetic induction values, but, at the same time, the line became more symmetric, closer to the shape of the Lorentz function derivative. The results obtained from measurements by the SQUID method, recording variations in magnetization as a function of temperature, confirm the untypical reinforcement of the magnetic conditions of the samples with the increase in temperature. For the samples tested, the magnetization was relatively weaker, when the tests were conducted in a stronger magnetic field. Key words: gas nitriding, magnetism of nitrides, steel balls. Inżynieria Materiałowa 4 (218) (2017) 170÷176 DOI 10.15199/28.2017.4.3 © Copyright SIGMA-NOT MATERIALS ENGINEERING 1. INTRODUCTION The formation of a nitrided layer on steel change its electrical (con- ductivity) and magnetic properties, among which the relative per- meability and magnetic coercive field strength are the most sensitive to phase and structural changes in nitrided steel. The diffusion layer at ambient temperature and up to a temperature of approx. 590°C is ferromagnetic. In contrast, the iron nitride surface layer built of the ε-Fe 2 N 1 – x and γʹ-Fe 4 N 1 – x phases is paramagnetic at the process temperature. The coercion and iron nitride saturation depend on the nitrogen concentration in the nitrides — with increasing nitrogen content, the coercion decreases, while the saturation increases [1]. Changes in the magnetic properties of unalloyed steel AISI 1085 and alloyed steel AISI 420-C before and after the controlled gas nitriding (CGN) process have been investigated in the manuscript. 2. TESTING METHODOLOGY AISI 1085 (C85) and AISI 420 (X20Cr13) steels in the form of balls, with chemical composition and diameters as shown in Ta- ble 1, were subjected to nitriding processes. The controlled gas nitriding (CGN) processes were realized in an Nx609 type industrial soaking furnace with working space dimen- sions of Ø600×900 mm. Test steel samples were nitrided in two- stage processes which differed in temperature, time and nitriding atmosphere nitrogen potential [2, 10]. Table 2 summarizes the basic parameters of the nitriding processes. After the nitriding processes, the steel balls were subjected to standard physical metallurgy and X-ray examinations. The nitrided layer structural examinations were conducted using a Zeiss Neo- phot 2 light microscope on Nital-etched microsections, and includ- ed also the thickness measurement of the nitride layers formed in the nitriding process. The use of the ball-shaped test samples allowed the accuracy of nitride layer thickness measurement to be increased. A schematic diagram of the iron nitride layer thickness assessment method, de- veloped and used in the Institute of Precision Mechanics, is shown in Figure 1. The iron nitride layer thickness was calculated using formula (1) resulting from the schematic diagram [3]. The phase composition as well as the thickness of the iron ni- tride layer on the test balls were determined by the X-ray diffraction method with a 3003TT Seifert 3003TT X-ray diffractometer using K α Co radiation and symmetric measurement geometry. The meas- urement parameters were as follows: voltage, 30 kV, current inten- sity 40 mA, step 2Dθ, 0.05°, counting time 5 s. The diffraction an- gle range of 43÷58° covered the main characteristic lines from the iron nitrides γʹ and ε and from the substrate, according to the stand- ards from the PDF4+ diffractometric database. The layer thickness Table 1. Chemical composition of steels used in the tests Tabela 1. Skład chemiczny badanych stali Grade AISI Sample No. Ø mm Element contents, wt % C Mn Si P S Ni Cr 1085 89 2.5 0.85 0.9 0.3 0.04 0.05 420-C 88 90 2.381 1.0 0.47 0.9 1.0 0.04 0.03 0.9 13.5 Table 2. The basic parameters of the nitriding processes Tabela 2. Podstawowe parametry procesów azotowania Process No. Stage I Stage II Inlet atmosphere T °C t h N p atm 0,5 T °C t h N p atm 0,5 Nx1021 580 5 3.20 600 11 0.50 NH 3 /NH 3zd Nx1025 490 4 12.5 520 20 0.50 NH 3 /NH 3zd

Transcript of The magnetic properties of steel after the gas nitriding process text.pdf2017/04/03  · The...

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170 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

The magnetic properties of steel after the gas nitriding process

Jerzy Michalski2*,1, Hubert Fuks3, Sławomir Maksymilian Kaczmarek3, Grzegorz Leniec3, Barbara Kucharska4, Piotr Wach1

1Institute of Precision Mechanics, 2Warsaw University of Life Sciences – SGGW, 3West Pomeranian University of Technology in Szczecin, 4Czestochowa University of Technology, *[email protected]

The paper presents the relationship between the chemical composition of the steel balls, their size and parameters of the nitriding process and magnetic prop-erties defined in the study by EPR and SQUID. The balls made form non-alloyed AISI 1085 and alloyed AISI 420-C steels were investigated. At resonance spectra for both steels before the nitriding process, the resonance signal only existed for two magnetic induction values, 400 mT and 650 mT. This values are untypical because for magnetic centres in the form of iron ions, the expected EPR signal occurs usually in magnetic fields of approx. 340 mT, when the spectrometers operates in the so called microwave X band. This abnormality presumed that the resonance signals did not originate from isolated ions, but instead from magnetically more complex systems, e.g. Fe–Fe pairs or iron clusters, while the observed EPR line position is normally even lower and occurs for a magnetic induction below 200 mT. The fact, that the magnetic centres not had the nature of isolated Fe ions, additionally confirmed the abnormal in-crease in resonance signal intensity as a function of temperature, which is a behaviour inconsistent with the Curie-Weiss law. After nitriding the AISI 420-C steel in the Nx1021(580/600°C) process, the resonance line became distinctly wider and shifts towards greater magnetic induction values. After nitriding at a lower temperature (the Nx1025(490/520°C process), the resonance lines shifted towards greater magnetic induction values, but, at the same time, the line became more symmetric, closer to the shape of the Lorentz function derivative. The results obtained from measurements by the SQUID method, recording variations in magnetization as a function of temperature, confirm the untypical reinforcement of the magnetic conditions of the samples with the increase in temperature. For the samples tested, the magnetization was relatively weaker, when the tests were conducted in a stronger magnetic field.

Key words: gas nitriding, magnetism of nitrides, steel balls.

Inżynieria Materiałowa 4 (218) (2017) 170÷176DOI 10.15199/28.2017.4.3© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTION

The formation of a nitrided layer on steel change its electrical (con-ductivity) and magnetic properties, among which the relative per-meability and magnetic coercive field strength are the most sensitive to phase and structural changes in nitrided steel. The diffusion layer at ambient temperature and up to a temperature of approx. 590°C is ferromagnetic. In contrast, the iron nitride surface layer built of the ε-Fe2N1 – x and γʹ-Fe4N1 – x phases is paramagnetic at the process temperature. The coercion and iron nitride saturation depend on the nitrogen concentration in the nitrides — with increasing nitrogen content, the coercion decreases, while the saturation increases [1].

Changes in the magnetic properties of unalloyed steel AISI 1085 and alloyed steel AISI 420-C before and after the controlled gas nitriding (CGN) process have been investigated in the manuscript.

2. TESTING METHODOLOGY

AISI 1085 (C85) and AISI 420 (X20Cr13) steels in the form of balls, with chemical composition and diameters as shown in Ta-ble 1, were subjected to nitriding processes.

The controlled gas nitriding (CGN) processes were realized in an Nx609 type industrial soaking furnace with working space dimen-sions of Ø600×900 mm. Test steel samples were nitrided in two-stage processes which differed in temperature, time and nitriding atmosphere nitrogen potential [2, 10]. Table 2 summarizes the basic parameters of the nitriding processes.

After the nitriding processes, the steel balls were subjected to standard physical metallurgy and X-ray examinations. The nitrided layer structural examinations were conducted using a Zeiss Neo-phot 2 light microscope on Nital-etched microsections, and includ-ed also the thickness measurement of the nitride layers formed in the nitriding process.

The use of the ball-shaped test samples allowed the accuracy of nitride layer thickness measurement to be increased. A schematic

diagram of the iron nitride layer thickness assessment method, de-veloped and used in the Institute of Precision Mechanics, is shown in Figure 1. The iron nitride layer thickness was calculated using formula (1) resulting from the schematic diagram [3].

The phase composition as well as the thickness of the iron ni-tride layer on the test balls were determined by the X-ray diffraction method with a 3003TT Seifert 3003TT X-ray diffractometer using KαCo radiation and symmetric measurement geometry. The meas-urement parameters were as follows: voltage, 30 kV, current inten-sity 40 mA, step 2Dθ, 0.05°, counting time 5 s. The diffraction an-gle range of 43÷58° covered the main characteristic lines from the iron nitrides γʹ and ε and from the substrate, according to the stand-ards from the PDF4+ diffractometric database. The layer thickness

Table 1. Chemical composition of steels used in the testsTabela 1. Skład chemiczny badanych stali

GradeAISI

Sample No.

Ømm

Element contents, wt %

C Mn Si P S Ni Cr

1085 89 2.5 0.85 0.9 0.3 0.04 0.05 — —

420-C 88 90

2.3811.0 0.47 0.9 1.0 0.04 0.03 0.9 13.5

Table 2. The basic parameters of the nitriding processesTabela 2. Podstawowe parametry procesów azotowania

Process No.

Stage I Stage IIInlet

atmosphereT°C

th

Npatm0,5

T°C

th

Npatm0,5

Nx1021 580 5 3.20 600 11 0.50 NH3/NH3zd

Nx1025 490 4 12.5 520 20 0.50 NH3/NH3zd

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was estimated by comparing the total intensity of reflections from the steel substrate (as determined by describing them with p-Voigt curves in the RafleX program) according to relationship (2) [4, 5].

g R R g g rmp mp mp= − + − ⋅( )2 2’ ’

(1)

where: gmp – actual iron nitride layer thickness as measured on the ball/cylinder diameter, gʹmp – iron nitride layer thickness as ob-served on the microsection, R – sample diameter after the process with the iron nitride layer, Rʹ – sample diameter with the iron nitride layer thickness subtracted, r – radius of the outer circle on the ball microsection, rʹ – radius of the inner circle on the ball microsec-tion, h – distance from the nitrided sample centre to the microsec-tion surface.

g I

Isin2

ln 0= ⋅∗

(2)

where: I0 – intensity of the reflection from the substrate, I – intensity of the reflection from the substrate with the nitride layer, μ* – mass layer absorption coefficient, ρ – layer density, q – reflection angle.

The nitrided steel balls were examined by the electron para-magnetic resonance (EPR) method. For this purpose, the balls were placed in the resonance chamber of an EPR spectrometer. The Bruker ELEXSYS E 500 CW spectrometer operated within an X microwave band of approx. 9.4 GHz, being standard for this type of devices, and with a modulation of 100 kHz. The magnetic field was varied in the range from 0 to 1400 mT.

The temperature during the magnetic tests was controlled with an Oxford Instruments ESP continuous flow nitrogen-helium cry-ostat within the temperature range of 4÷300 K and 80÷300 K, re-spectively, using the respective helium and nitrogen medium.

gmp – actual iron nitride layer thickness as measured on the ball/cylinder diameter,

g’mp – iron nitride layer thickness as observed on the microsection,

R – sample diameter after the process with the iron nitride layer,

R’ – sample diameter with the iron nitride layer thickness subtracted,

r – radius of the outer circle on the ball microsection,

r’ – radius of the inner circle on the ball microsection,

h – distance from the nitrided sample centre to the microsection surface

Fig. 1. A schematic of a microsection on a ball-shaped sample (the grey surfaces depict the iron nitride layer)Rys. 1. Schemat szlifu metalograficznego na próbce w kształcie kuli (po‑wierzchnie szare obrazują warstwą azotków żelaza)

Fig. 2. Microstructure of nitrided layer obtained on 2.5 mm-diameter AISI1085 steel balls – sample no. 89 in the Nx1021 process (a) and in the Nx1025 process (b) Rys. 2. Mikrostruktury warstw azotowanych uzyskane na kulkach ze stali AISI1085 o średnicy 2,5 mm – pr.89 w procesie Nx1021(a) i w procesie Nx1025 (b)

Fig. 2. Microstructure of nitrided layer obtained on 2.5 mm-diameter AISI 1085 steel balls – sample No. 89 in the Nx1021 process (a) and in the Nx1025 process (b)Rys. 2. Mikrostruktura warstw azotowanych uzyskana na kulkach ze stali AISI 1085 o średnicy 2,5 mm – pr. 89 w procesie Nx1021 (a) i w procesie Nx1025 (b)

3. TESTING RESULTS

3.1. Iron nitride layer thicknessThe microstructure of nitrided layers obtained on the test steel balls is shown in Figures 2 and 3. The thicknesses of iron nitride layers produced on steel AISI 1085 in the Nx1021 (580/600°C) and Nx1025 (490/520°C) processes amounted to, respectively, 20.0 µm and 8.1 µm (Fig. 2). For the layer obtained in the Nx1025 process, the same thickness was determined in X-ray measure-ments (Fig. 4b). The iron nitride layer obtained from the Nx1021 process was thicker than the thickness from the measurement

a) b)

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172 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

using KαCo radiation, which was approx. 15 μm at a sample sur-face incidence angle of 23÷25°. For such layers (with a thickness 15 μm), the X-ray method of determining their thickness using KαCo is inefficient [6].

On the AISI 420-C steel, an iron nitride layer was only obtained from the Nx1021 (580/600°C) process. This layer had an uneven thickness in the range from 8.5 to 16.5 µm (Fig. 3a). In the Nx1025 (490/520°C) process, an enrichment of the steel subsurface layer with nitrogen down to a depth of 70 µm only occurred. (Fig. 3b).

When comparing Figures 2 and 3, a distinct influence of the steel chemistry on the thickness of forming iron nitrides can be seen.

The reduction in the kinetics of growth of iron nitride thickness on the AISI 420-C steel results from the presence of chromium (13.5 wt %) in the steel. As a nitride-forming element, chromium strongly bonds nitrogen in the steel surface layer, thus effectively reducing the formation of an iron nitride layer [7].

The iron nitride layers obtained on the AISI 1085 unalloyed steel both in the Nx1021 (580/600°C) process and the Nx1025 (490/520°C) process were built of the γʹ-Fe4N1 – x phase. In the case of steels nitrided in the Nx1025 process, in addition to the diffrac-tion lines characteristic of the γʹ phase, a line from the steel sub-strate was also recorded (Fig. 4a). No lines from the substrate were

a) b)

Fig. 3. Microstructure of nitrided layer obtained on 2.381 mm-diameter AISI420-C steel balls – sample no. 88 in the Nx1021 process (a) and in the Nx1025 process (b) Rys. 3. Mikrostruktury warstw azotowanych uzyskane na kulkach ze stali AISI420 o średnicy 2,381 mm – pr.88 w procesie Nx1021(a) i w procesie Nx1025 (b)

a)

Fig. 3. Microstructure of nitrided layer obtained on 2.381 mm-diameter AISI 420-C steel balls – sample No. 88 in the Nx1021 process (a) and in the Nx1025 process (b)Rys. 3. Mikrostruktura warstw azotowanych uzyskana na kulkach ze stali AISI 420 o średnicy 2,381 mm – pr. 88 w procesie Nx1021 (a) i w procesie Nx1025 (b)

Fig. 4. Diffraction patterns from spherical samples with an iron nitride layer obtained on 2.5 mm-diameter AISI1085 steel balls – sample no. 89 – in the Nx1021 process (a) and in the Nx1025 process (b) Rys. 4. Dyfraktogramy z powierzchni próbek sferycznych z warstwą azotków żelaza uzyskaną na kulkach ze stali AISI1085 o średnicy 2,5 mm ‑ pr.89 w procesie Nx1021 (a) i procesie 1025 (b)

b)

Fig. 4. Diffraction patterns from spherical samples with an iron nitride layer obtained on 2.5 mm-diameter AISI 1085 steel balls – sample No. 89 – in the Nx1021 process (a) and in the Nx1025 process (b)Rys. 4. Dyfraktogramy z powierzchni próbek sferycznych z warstwą azotków żelaza uzyskaną na kulkach ze stali AISI 1085 o średnicy 2,5 mm – pr. 89 w procesie Nx1021 (a) i procesie 1025 (b)

a) b)

b)a)

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recorded for the 20 μm-thick layer obtained in the Nx1021 process, because, as has already been mentioned, the layer thickness was greater than the KαCo radiation penetration depth.

The layers formed on the AISI 420-C alloy steel in the Nx1021 (580/600°C) process were built of a mixture of γʹ-Fe4N1 – x and ε-Fe2N1 – x phases. The γʹ and ε phase volumetric fractions of the iron nitride layer were, respectively, 57% and 43% (Fig. 5a).

In the Nx1025 (490/520°C) process, no iron nitride layer was formed on the AISI 420-C alloy steel (Fig. 3b); only the line charac-teristic of the substrate was recorded in the diffraction pattern (Fig. 4b). The widening of the substrate line suggests the saturation of the steel structure with nitrogen [4, 8].

3.2. Magnetic testing results

Figure 6a shows a juxtaposition of resonance spectra for test balls of steel AISI 1085 and steel AISI 420-C before the nitriding pro-cess. As can be seen in the figure, the resonance signal only exists for two magnetic induction values, 400 mT and 650 mT, for both

the AISI 1085 unalloyed steel and the AISI 420-C alloy steel The resonance lines have generally an asymmetric shape, which might be due to the conditions of conducting the EPR experiment and the formation of eddy currents in the test samples. As Figure 6a shows, the EPR signal of the 1.0 mm-diameter AISI 420-C sample (sample No. 90) is the closest to the symmetrical one. This might result from the fact that this ball had a diameter nearly 2.5 times smaller than that of sample No. 88.

The position of the resonance lines of the test steels subjected to nitriding is untypical. For magnetic centres in the form of iron ions, the expected EPR signal occurs usually in magnetic fields of approx. 340 mT, when the spectrometers operates in the so called microwave X band [9]. Whereas, for the steel test samples, the EPR signal occurs for the inductances 400 and 650 mT. It can be pre-sumed that the resonance signals does not originate from isolated ions, but instead from magnetically more complex systems. This abnormality occurs usually when the resonance signal comes from more complex magnetic centres, e.g. Fe–Fe pairs or iron clusters, while the observed EPR line position is normally even lower and

Fig. 5. Diffraction patterns from spherical samples with an iron nitride layer obtained on 2.381 mm-diameter AISI420-C steel balls – sample no. 88 – in the Nx1021 process (a) and in the 1025 process (b) Rys. 5. Dyfraktogramy z powierzchni próbek z warstwą azotków żelaza uzyskaną na kulkach ze stali AISI420‑C o średnicy 2,381 mm ‑ pr.88 w procesie Nx1021 (a) i procesie 1025 (b)

a) b)

Fig. 5. Diffraction patterns from spherical samples with an iron nitride layer obtained on 2.381 mm-diameter AISI 420-C steel balls – sample No. 88 – in the Nx1021 process (a) and in the 1025 process (b)Rys. 5. Dyfraktogramy z powierzchni próbek z warstwą azotków żelaza uzyskaną na kulkach ze stali AISI 420‑C o średnicy 2,381 mm – pr. 88 w procesie Nx1021 (a) i procesie 1025 (b)

a) b)

a) b)

Fig. 6. The resonance spectra of steel balls before the nitriding process (a). The temperature relationship of EPR signal intensity on the example of 2.381 mm-diameter AISI 420-C steel ball (pr. 88) (b)Rys. 6. Widma rezonansowe kulek stalowych przed procesem azotowania (a). Temperaturowa zależność intensywności sygnału EPR na przykładzie kulki ze stali AISI 420-C o średnicy 2,381 mm (pr. 88) (b)

b)

b)

a)

a)

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174 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

occurs for a magnetic induction below 200 mT [9]. It can therefore be stated that the magnetic centres do not have the nature of isolated Fe ions, which additionally confirms the abnormal increase in reso-nance signal intensity as a function of temperature (Fig. 6b), which is a behaviour inconsistent with the Curie-Weiss law.

After nitriding the AISI 420-C steel in the Nx1021 (580/600°C) process, the resonance line becomes distinctly wider and shifts to-wards greater magnetic induction values (Fig. 7). After nitriding at a lower temperature (the Nx1025 (490/520°C) process), the reso-nance lines shift towards greater magnetic induction values, but, at the same time, the line becomes more symmetric, closer to the shape of the Lorentz function derivative.

The shift in EPR signal position results from the changes in lo-cally noticeable magnetic fields between magnetic objects, as a con-sequence of which the resonance conditions change. The width of

a) b)

a) b)

Fig. 7. Comparison of the EPR spectra of selected balls before (a88, a89) (a) and after nitriding in the processes: Nx1021 (n88.21, n89.21) and Nx1025 (n88.25, n89.25) (b)Rys. 7. Porównanie widm EPR wybranych kulek przed (a88, a89) (a) oraz po azotowaniu w procesach Nx1021 ( n88.21, n89.21) i Nx1025 (n88.25, n89.25) (b)

b) a)

Fig. 8. Magnetization characteristics as a function of temperature for AISI 420-C steel balls with a diameter of 1.0 mm (No. 90) and 2.381 mm (No. 88) after the Nx1025(490/520°C) processRys. 8. Charakterystyki namagnesowania w funkcji temperatury dla kulek ze stali AISI 420‑C o średnicy 1,0 mm (pr. 90) i 2,381 mm (pr. 88) po procesie Nx1025 (490/520°C)

magnetic lines, on the other hand, is associated with the strength of relaxation processes, when the supplied microwave energy is absorbed by the magnetic centres and then released to the environ-ment through the spin–spin or spin–lattice relaxation mechanism.

The results obtained from measurements by the SQUID method, recording variations in magnetization as a function of temperature, as shown in Figure 8a, confirm the untypical reinforcement of the magnetic conditions of the samples with the increase in tempera-ture. For the samples tested, the magnetization was relatively weak-er, when the tests were conducted in a stronger magnetic field, as shown as an example in Figure 8a. A deviation is the behaviour of the AISI 420-C steel nitrided in the Nx1025 (490/520°C) process – specimen No. 88, for which a stronger magnetization was found with strong magnetic fields (Fig. 8b); this different behaviour may explain the absence of the surface iron nitride layers (Fig. 3a).

b)

b)

a)

a)

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4. CONCLUSIONSThe influence of the chemical composition of steel and con-trolled gas nitriding parameters on the thickness of formed iron nitrides layer. Layers on AISI 1085 steel were thicker and mono-phase (γʹ-Fe4N1 – x) in both two-stage processes 580/600°C and 490/520°°C. The presence of chromium in the steel AISI 420-C (13.5 wt %) and a lower nitriding temperature reduced growth ki-netics of the iron nitride layer.

It has been found that the nitriding process results in a change in the magnetic properties of the AISI 1085 unalloyed steel and the AISI 420-C alloyed steel. The temperature relationship of the EPR and SQUID signals has shown that the magnetic objects in the balls made of these steels have a complex nature, and the magnetic property changes, as examined by the EPR method, depend on the nitriding degree of the examined steels.

The obtained results enable one to conclude that the fast and non-destructive EPR magnetic resonance method can be used for the identification and characterization of steels after the nitriding process.

ACKNOWLEDGEMENT

The project is financed/co-financed by the National Research and Development Centre jointly with the National Science Centre in the years 2015÷2018 under the programme TANGO („NITRO-COR” 1/268920/NCBR/15).

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176 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

Właściwości magnetyczne stali po procesie azotowania gazowego

Jerzy Michalski2*,1, Hubert Fuks3, Sławomir Maksymilian Kaczmarek3, Grzegorz Leniec3, Barbara Kucharska4, Piotr Wach1

1Instytut Mechaniki Precyzyjnej, 2Szkoła Główna Gospodarstwa Wiejskiego w Warszawie – SGGW, 3Zachodniopomorski Uniwersytet Technologiczny w Szczecinie, 4Politechnika Częstochowska, *[email protected]

Inżynieria Materiałowa 4 (218) (2017) 170÷176DOI 10.15199/28.2017.4.3© Copyright SIGMA-NOT MATERIALS ENGINEERING

Słowa kluczowe: azotowanie gazowe, warstwa azotowana, właściwości magnetyczne, rezonans magnetyczny EPR, skład fazowy, mikrostruktura.

1. CEL PRACY

Podczas tworzenia się warstwy azotowanej na stali zmianom ule-gają jej właściwości elektryczne (przewodność) oraz ma gnetyczne, wśród których najbardziej czułe na zmiany fazowe i strukturalne azotowanej stali są względna przenikalność magnetyczna i natę-żenie pola koercji magnetycznej.

W pracy podjęto się zbadania relacji pomiędzy składem che-micznym, rozmiarem i parametrami procesu azotowania stalowych kulek a ich właściwościami magnetycznymi.

2. MATERIAŁ I METODYKA BADAŃ

Procesom azotowania poddano stale AISI 1085 (C85) i AISI 420 (X20Cr13) w postaci kul o średnicach 1,0, 2,381 i 2,5 mm (tab. 1). Regulowane azotowanie gazowe (RAG) stalowych kulek wy-konano w przemysłowym piecu wgłębnym typu Nx609 w dwóch dwustopniowych procesach: Nx1021 (580/600°C) i Nx1025 490/520°C) (Tab. 2). Po azotowaniu stalowe kulki poddano stan-dardowym badaniom metaloznawczym (mikroskop Neophot 2) wraz z pomiarem grubości warstwy azotków metodą opracowaną w Instytucie Mechaniki Precyzyjnej (rys. 1). Przeprowadzono ba-dania rentgenowskie (dyfraktometr Seifert 3003TT) obejmujące analizę fazową warstw azotków i również pomiar ich grubości oraz badaniom magnetycznym (spektrometr ELEXSYS E 500 CW, krio-stat ESP) rezonansowym EPR oraz w funkcji temperatury SQUID.

3. WYNIKI I ICH DYSKUSJA

Grubość warstwy azotków uzyskana na stali AISI 1085 w procesach Nx1021 i Nx1025 wynosiła odpowiednio 20,0 µm i 8,1 µm (rys. 2). Warstwy były zbudowane z azotków żelaza γʹ-Fe4N1 – x (rys. 4).

Na stali AISI 420-C warstwę azotków (o nierównomiernej grubości w zakresie 8,5÷16,5 µm) uzyskano tylko w procesie Nx1021 (rys. 3a). Warstwa była zbudowana z mieszaniny azotków γʹ-Fe4N1 – x i ε-Fe2N1 – x w proporcjach objętościowych 57% i 43% (rys. 5a). W procesie Nx1025 (490/520°C) wystąpiło jedynie wzbo-gacenie strefy przypowierzchniowej stali w azot do głębokości 70 µm (rys. 3b i 5b).

W widmach rezonansowych kulek ze stali przed procesem azotowania sygnał rezonansowy występował dla dwóch wartości indukcji magnetycznej 400 mT i 650 mT zarówno w przypadku stali niestopowej AISI 1085, jak i stopowej AISI 420-C (rys. 6a). Położenie linii rezonansowych stali poddanych azotowaniu było nietypowe. Dla centrów magnetycznych w postaci jonów żelaza oczekiwany sygnał EPR zwykle występuje w polach magnetycz-nych ok. 340 mT, gdy spektrometr pracuje w tzw. mikrofalowym paśmie X. Tymczasem dla badanych kulek stalowych sygnał EPR

występował dla indukcyjności 400 i 650 mT. To pozwala sądzić, że sygnały rezonansowe nie pochodziły od izolowanych jonów Fe, lecz od układów bardziej złożonych w sensie magnetycznym. Nie-typowość taka występuje zazwyczaj, gdy sygnał rezonansowy po-chodzi od bardziej złożonych centrów magnetycznych, np. par Fe–Fe lub klasterów żelaza, a obserwowana pozycja linii EPR zwykle jest jeszcze niższa i występuje dla indukcji magnetycznej poniżej 200 mT. Można więc było stwierdzić, że centra magnetyczne nie miały charakteru izolowanych jonów Fe, co dodatkowo potwierdził nietypowy wzrost intensywności sygnału rezonansowego w funkcji temperatury i co jest zachowaniem niezgodnym z prawem Curie--Weissa (rys. 6b).

Dla azotowanych kulek ze stali AISI 420-C w procesie Nx1021 (580/600°C) linia rezonansowa stała się wyraźnie szersza i przesu-nięta w stronę większych wartości indukcji magnetycznej (rys. 7). Po azotowaniu w niższej temperaturze (proces Nx1025 (490/520°C) linie rezonansowe przesunęły się również w stronę większych war-tości indukcji magnetycznej, ale jednocześnie linia stała się bardziej symetryczna, zbliżona do kształtu pochodnej funkcji lorentzowskiej.

Przesuwanie położenia sygnału EPR wynika ze zmian lokalnie odczuwalnych pól magnetycznych pomiędzy obiektami magnetycz-nymi, w wyniku czego zmianie ulegają warunki rezonansu. Szero-kość linii rezonansowych jest natomiast związana z siłą procesów relaksacyjnych, gdy dostarczona energia mikrofali jest pochłaniana przez centra magnetyczne, a następnie oddawana do otoczenia po-przez mechanizm relaksacyjny typu spin–spin lub spin–sieć.

Wyniki uzyskane z pomiarów metodą SQUID, rejestrujące zmia-ny namagnesowania w funkcji temperatury, potwierdziły nietypowe wzmocnienie stanu magnetycznego próbek wraz ze wzrostem tempe-ratury. Dla badanych próbek namagnesowanie było relatywnie słab-sze, gdy badania prowadzono w silniejszym polu magnetycznym.

4. PODSUMOWANIE

W pracy wykazano wpływ składu chemicznego stali oraz parame-trów regulowanego azotowania gazowego na grubość tworzących się warstw azotków żelaza. Warstwy na stali AISI 1085 były grubsze i jednofazowe (γʹ-Fe4N1 – x w obu procesach azotowania 580/600°C i 490/520°C. Obecność chromu w stali AISI 420-C (13,5% mas.) oraz niższa temperatura azotowania ograniczają kinetykę wzrostu warstw azotków żelaza. Wykazano, że azotowanie powoduje zmia-ny właściwości magnetycznych stali zarówno niestopowej AISI 1085, jak i stopowej AISI 420-C, które zależą od stopnia naazoto-wania stali. Temperaturowe zależności sygnału EPR oraz SQUID potwierdziły, że obiekty magnetyczne w badanych kulkach mają charakter złożony. Szybka i nieniszcząca metoda rezonansu magne-tycznego EPR może być wykorzystana do identyfikacji i charakte-ryzowania stali po procesie azotowania.