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Materials Science and Engineering A283 (2000) 65-69

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Mo¨ssbauer study of the retained austenitic phase in

multiphase steels

A. Mijovilovich ^a,*, A. Gonc¸alves Vieira ^b, R. Paniago ^a, H.D. Pfannes ^a,

B. Mendonc¸a Gonzalez ^b

^a Departamento de Fı´sica, Universidade Federal de Minas Gerais, C.P. 702, 30123-970 Belo Horizonte, Brazil ^b Escola de Engenharia, Universidade Federal de Minas Gerais, Rua Espı´rito Santo 35, 30160-030 Belo Horizonte, Brazil

Received 31 May 1999; received in revised form 21 December 1999

Abstract

Samples of steels with composition 0.30%C-1.5%Mn-1.5%Si-0.5%Al-0.5%Mo (wt.%) were subjected to different thermomechan-ical treatments to produce ferrite/pearlite/bainite (FPB), spheroidized (ESF) and martensite (MAR) microstructures. Subsequently they underwent a two stage annealing to obtain a final structure comprising of ferrite, bainite, martensite and austenite. The samples were studied by means of Mo¨ssbauer spectroscopy (transmission and conversion electron Mo¨ssbauer spectrocopy (CEMS)), X-ray diffraction (XRD), and metallographic analysis. Austenite contents were found to be the same for all samples except for the spheroidized sample annealed at 750°C that showed an increase of the austenite with increasing temperature of the treatment. Mo¨ssbauer spectroscopy and quantitative XRD analysis exhibited significant discrepancies ascribed to texture effects. It is shown that the thermal treatment was successful in retaining significant quantities of the austenite phase for steels of this composition. © 2000 Elsevier Science S.A. All rights reserved.

Keywords: Multiphase steel; Mo¨ssbauer; Austenite; Martensite; Bainite

1. Introduction

In order to enhance the ductility in high-strength steels it was shown that it is necessary to increase the content of their retained austenite. Alloys with high ductility and excellent levels of mechanical strength can be obtained by the transformation of austenite to martensite during plastic deformation (i.e. trip: trans­formation induced plasticity effect) [1]. Matsumura et al. [2] increased the content of retained austenite in an alloy of C-Mn-Si by a two stage thermal treatment: an annealing followed by a quick quenching to the range of temperatures for the bainitic transformation. The amount of retained austenite increased with in­creasing content of Mn and Si in the alloy [3].

It is usual to determine the phases present by metal­lographic analysis as well as X-ray diffraction. The last method is sometimes used for quantitative analysis in

* Corresponding author. Present address: EMBL c/o DESY, Notkestrasse 85, Geb. 25A, Notkestrasse 85, 2603, Hamburg, Ger­many. Tel.: + 49-40-89902120; fax: [phone].

spite that the result may be strongly influenced by texture effects. Mo¨ssbauer spectroscopy is a well-known technique used in the study of Fe-containing alloys [4]. The different phases can be distinguished from their different signals, and different magnetic behaviors re­gardless of the state of aggregation of the phases. Martensite and austenite are easily distinguished from their different hyperfine patterns in the Mo¨ssbauer spectra with better accuracy than by other techniques. Due to the low solubility of carbon in oc-Fe in equi­librium, the interstitial solute C can not be detected by Mo¨ssbauer spectroscopy. In the transmission made sig­nals from all the ⁵⁷Fe atoms in the sample are obtained regardless of the state of the aggregation or crystallinity in the material. In the case of conversion electron Mo¨ssbauer spectrocopy (CEMS) the spectrum stems from a region of ~ 10—100 nm below the surface of the sample, and thus becomes an efficient tool to analyse the surface. With Mo¨ssbauer spectroscopy the texture effect does not affect the total area of the subspectra corresponding to the different phases.

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A. Mijovilovich et al. /Materials Science and Engineering A283 (2000) 65—69

Time

Fig. 1. Schematic diagram of two stage annealing. T₁ = intercritical temperature; t₁ = annealing time; T₂ = bainitic temperature (425°C), t₂ = bainitic transformation time.

We used transmission Mo¨ssbauer spectroscopy to determine the amount of austenitic retained phase after the thermal treatments. By CEMS we were able to study the mechanical stability and transformation to martensite during the laminating process. Quantitative X-ray diffraction analyses were employed to determine the relative amounts of retained austenite, and ferritic phases present.

2. Experimental

By a specific thermal treatment of an alloy of compo­sition 0.30%C-1.5%Mn-1.5%Si-0.5%Al-0.5%Mo three initial structures were obtained: ferrite/pearlite/bainite (FPB), spheroidized (ESF) and martensite (MAR). Subsequently they underwent a two stage annealing (Fig. 1) to obtain a final structure of ferrite, bainite, martensite and austenite. We will keep the acronyms of the initial phases when we refer to the samples after the two-stage annealing. For the metallographic analysis a selective etching with Nital 2%, Picral 5% and Na-thio-sulfate [5] was used.

A. Mijovilovich et al. /Materials Science and Engineering A283 (2000) 65—69

67

Table 1

Hyperfine parameters and relative areas (%) for the identified phases

from transmission Mo¨ssbauer spectraa

Sample

45.42 28.02

6.84 13.67

6.08

45.70 26.07

8.34 13.51

6.38

43.80 22.83 29.4 13.72 7.20

48.2

28.4

13.0

7.1

3.2

40.2 028 0.04 13.6 6.0

0.6

0.6

0.6

0.6

0.6

H(T) S (mm s⁻¹) A (mm s⁻¹) Relative area

FPB780
Ferrite 1		33.0	.0-45
Ferrite 2		30.6	30.6-
Ferrite 3		27.9	0.072
Austenite	1	-	−0.023
Austenite	2	-	0.032
MAR780
Ferrite 1		33.0	.0-45
Ferrite 2		30.7	0.064
Ferrite 3		28.4	0.073
Austenite	1	-	−0.027
Austenite	2	-	0.033
Mar810
Ferrite 1		33.0	0.008
Ferrite 2		31.0	0.03
Ferrite 3		29.4	0.033
Austenite	1	-	−0.059-
Austenite	2	-	0.005
Esf750
Ferrite 1		33.0	0.008
Ferrite 2		31.0	0.031
Ferrite 3		29.2	0.025
Austenite	1	-	−0.053-
Austenite	2	-	0.051
Esf840
Ferrite 1		33.0	0.005
Ferrite 2		31.3	0.028
Ferrite 3		29.5	0.041
Austenite	1	-	−0.062
Austenite	2	_	−0.004

' H(T) is the hyperfine magnetic field in Tesla, S (mm s ') is the

mm s ', and A (mm s ') is the

isomer shift refered to a-Fe in quadrupole splitting.

Integrated intensities of X-ray diffraction peaks were used to determine the content of retained austenite [6]. The samples were grounded, embedded in epoxy, pol­ished with 1 |xm diamond paste and measured in a Philips diffractometer (K^-Cu radiation, 0.01°-steps of 20). The reflections used for the quantitative analysis were: (200), (220) and (311) for the y-phase; (200) and (211) for the a-phase.

We measured transmission Mo¨ssbauer spectra of all samples. The measuring temperature was room temper­ature and the source was ⁵⁷Co in Rh matrix. Two samples (FPB780 and MAR780) were also measured at 77 K. Since the steels were 50 nm thick foils the lines were broadened because of thickness effect. It is com­mon to use fits with hyperfine field distributions or Voigtian line profiles [7] to take this effect into account. In fitting these spectra we used the program WOTAN [8] which is based on the integral form of the absorp­tion line calculated by Margulies [9,10]. In this way we

were able to take into account the thickness of the samples. The sample FPB780 was investigated also by CEMS using the same source as above. In this case, as the effective thickness is small, we used a least squared fit with simple Lorentzian lines [11]. The values of x² to measure the quality of the fit ranged from 1.7 to 3.4.

Fig. 3. Mo¨ssbauer spectra for: (a), MAR780 and (b), FP780 samples.

Table 2

Hyperfine parameters and percentages of phases from CEMS spectra

(surfaces)^a

48.12

39.36

3.37

0.18

0.6 3.79

H(T) S (mm s − (mm ¹) Relative area

FPB780
Ferrite 1		33.0	0.00
Ferrite 2		31.0	0.04
Ferrite 3		27.0	−0.08
Austenite	1	-	−0.18
Austenite	2		0.00

¹ Symbols as in Table 1.

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A. Mijovilovich et al. /Materials Science and Engineering A283 (2000) 65—69

-8-6-4-2 0 2 4 6 8

Velocity (mm/s) Fig. 4. CEMS spectra for sample FP780.

Table 3

XRD results for retained austenite y phase (volume%) for different

two-stage thermal treatments

Volume (%) of 7
phase
13.2 +	.5
7.1 +	.5
16.1 +	.5
17.8 +	.5
17.2 +	.5

Sample Thermal treatment

FPB780 780°C 20 min−425°C 10 min

ESF750 750°C 20 min−425°C 10 min

ESF840 840°C 20 min−425°C 10 min

MAR780 780°C 20 min−425°C 10 min

MAR810 810°C 20 min−425°C 10 min

40 45 50 55 60 65 70 75 80 85 90 95 Angle-28 (°)

Fig. 5. X-ray diffraction pattern of a multiphased structure obtained from martensitic initial structure-MAR-780 (intercritically annealed at 780°C for 20 min and at 425°C for 10 min).

3. Results and discussion

bainite shows dark gray, and both, the martensite and the austenite become light gray. The microstructure of the FPB sample is heterogeneous with fine and coarser regions (Fig. 2a). The final structure for the sample obtained from an initial martensite is finer (Fig. 2b). For the samples obtained from the spheroidized initial structure an increase of the amount of bainite and martensite-austenite with increasing intercritical tem­perature is found (Fig. 2c, d).

The main contributions to the Mo¨ssbauer spectra arise from the ferrite and martensite phases of the steel, which lead to magnetically split spectra. We used the designations Ferrite 1, 2 and 3 to denote subspectra corresponding to Fe atoms in the ferritic/bainitic/ martensitic matrix with three different environments, following the nomenclature of Uwakweh et al. [12]. But due to the different nomenclatures used in the literature it is not possible to give unique assignments for the different Fe-C configurations [13]. The austenitic para­magnetic phase is clearly distinguishable with two con­tributions: austenite 1 and 2 for the singlet and the doublet, respectively. It amounts < 21% in all samples. Since there is insignificant contribution of cementite or other carbide precipitates seen in the spectra, they have been neglected in the fitting. The hyperfine parameters and the percentages of each phase as determined from the Mo¨ssbauer spectra are listed in Table 1. Typical Mo¨ssbauer spectra are shown in Fig. 3.

The Mo¨ssbauer results indicate similar austenite con­tents for the MAR and FPB samples. For the ESF samples it is clearly observed that by the treatment at higher temperature more austenite is retained. By com­paring the room temperature and liquid nitrogen spec­tra of FPB780 (ferrite-perlite system) and MAR780 (martensite rich sample), we conclude that both exhibit similar austenite contents.

We measured the FPB780 sample also with CEMS and deduced a decrease of the austenitic phase, indicat­ing a transformation from austenite to martensite in the surface during the polishing process (see Table 2 and the corresponding spectrum in Fig. 4). This was simi­larly observed by [14].

The quantitative determination of phases by XRD is given in Table 3 and a characteristic pattern is shown in Fig. 5. The Mo¨ssbauer results for the austenite content differs significantly from the XRD results, except for the ESF samples where both techniques indicate the same trend. This is ascribed to a cristallographic texture effect that strongly influences the XRD measurements. Mo¨ssbauer results concerning the spin texture in these steels will be published elsewhere [15].

Photographs of optical microscopy are shown in Fig. 1. Three phases can be distinguished in the photo­graphs, namely martensite plus austenite, bainite and ferrite. With the used etching, ferrite becomes gray,

4. Conclusions

Metallographic analysis showed the presence of bainite, ferrite and martensite-austenite in all samples.

A. Mijovilovich et al. /Materials Science and Engineering A283 (2000) 65—69

69

The finer micro structure is present in the samples ob­tained from the martensitic phase due to its acicular morphology.

From the micrographs it is seen that the amount of carbide precipitates is negligible which is in concordance with the Mo¨ssbauer results, which do not indicate any significant contribution from carbides. For the evalua­tion of the retained austenite the Mo¨ssbauer spectra indicate that the annealing at higher temperature is effective in stabilizing the austenite phase in the spheroidized sample. Results for other samples under different thermal treatments are similar, in disagreement with XRD measurements. The difference was attributed to a texture effect.

From CEMS results a significant decrease of the austenite content in the surface due to the mechanical polishing is deduced.

Acknowledgements

The support of the Brazilian research agencies Fapemig, CAPES and CNPq is greatfully acknowl­edged.

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