Assessment of Retained Austenitic Phase in Indefinite Chill Cast Iron Microstructure
Towards Various Techniques
J. Tchoufang Tchuindjang
1, L. Terziev
2, M. Sinnaeve
3, G. Mukadi Kantabilenga
4, J. Lecomte-Beckers
11MMS, Aerospace & Mechanics Dept., University of Liege (BE)
2Chemistry Faculty, University of Sofia (BG)
3R&D – Quality Control Manager, Marichal Ketin Belgium (BE)
4University of Lubumbashi (DRC)
INTRODUCTION
Indefinite Chill Cast Irons also called ICDP when used as rolling mill rolls are high alloyed cast iron belonging to the Fe-Cr-C system. Precipitation at the same time of free graphite and carbides during the casting process leads to “IC” specific designation. ICDP exhibit improved properties regarding hot oxidation and wear resistances as their microstructure is a mixture of a martensitic with bulky Cementite (M3C) and lamellar graphite fully dispersed into the matrix. Mechanical properties involved depend either on the chemical composition of the alloy or the crystallisation
Abstract
Indefinite Chill Cast Irons are high alloyed cast iron alloys belonging to the Fe-Cr-C-X system, with Ni, Cr, Si, Mn and Mo as major alloying elements. The casting process of such an alloy when it is performed in a classic way (air cooling) leads to a mixture of free graphite, Cementite, martensite, and a certain amount of retained austenite (RA) that is
unadvisable in applications involving high stresses and heating-cooling sequences such as Hot Strip Mill. RA yields martensite in ICDP alloys while performing multiple tempering heat treatments in order to complete this phase transformation.
The present study is intend to compare various complex techniques such as XRD, CEMS and VSM used for RA assessment issues, the final goal being the calibration of a classic and rougher method such as Dilatometry to allow a fast and reliable technique for RA measurements.
RESULTS
Co unts 500 1000 Ferrite - red Austenite – blue C-Fe-Si – green Fe-Si - yellow 332672TT 500 1000 32275 BRUT. γ (111) α (100) α(211) Martensite Austenite C-Fe-Si Fe-Si Martensite Austenite C-Fe-Si Fe-Si From XRD Analysis…involved depend either on the chemical composition of the alloy or the crystallisation behaviour and the heat treatments performed subsequently in order to lead to the desired microstructure.
Retained Austenite always present in ICDP microstructure at the end of casting process is of concern regarding applications such as hot rolling mill as this austenitic phase is well-known to be unstable during possible rolling incidents on rolls. Thus subsequent tempering heat treatments are performed at the end of casting procedure in order to allow a fully transformation of RA into an expected and advisable martensite.
From an industrial point of view, it is important to set a fast and reliable method to assess RA presence into ICDP microstructure at the end of every single step of the thermomechanical process for cost issues.
Common techniques such as optical microscopy linked to Image Analysis seem to be unsuitable for such an estimation as austenite and martensite appeared inter mixed in the matrix. The present work is intended to compare different techniques used for RA assessment in ICDP alloys starting from complex, refined and accurate methods such as X-Ray Diffraction (XRD) and even more exotic Conversion Electron Mössbauer Spectroscopy (CEMS) to yield the calibration of a rougher one such as Dilatometry.
MATERIALS
Po sition [°2Theta] 4 0 50 60 7 0 80 90 0 -10 -8 -6 -4 -2 0 2 4 6 8 10 94 96 98 100 4 3 2 1 6 5 32275 brut R el at iv e tr an sm is si on , % Velocity, mm/s -10 -8 -6 -4 -2 0 2 4 6 8 10 94 96 98 100 32814h 1TT R el at iv e tr a n sm is si o n , % Velocity, mm/s 1 2 3 4 5 6 -10 -8 -6 -4 -2 0 2 4 6 8 10 94 96 98 100 6 5 4 3 2 1 32814h 2TT R el at iv e tr a sn m is si on , % Velocity, mm/sCEMS Spectra at Room T°- Changes in Martensite (sextets 1, 2,3and doublet 4 contributions) and RA (singlet 5and doublet 6 contributions) Volume Fractions with progressive tempering
Sample number % Martensite % Retained Austenite -○- 1 (Reference) 100 0 --○-- 2 (3 x Temper.) 99.3 0.7 -*- 3 (3 x Temper.) 98.6 1.4 -■- 4 (2 x Temper.) 95.2 4.8 -x- 5 (2 x Temper.) 95.2 4.8 -∆- 6 (1 x Temper.) 95.0 5.0 -□- 7 (1 x Temper.) 95.0 5.0 -▼- 8 (As-cast) 68.2 31.8 -◊- 9 (As-cast) 67.5 32.5 After 3 times tempering,
microstructure is fully martensitic with around 35 to 45% of Cementite, and 1 to 3% of free graphite. Average Hardness is around 600 HV, with a more or less wide scattering depending on the material itself or the
XRD Spectrum for a fully martensitic sample (n°2), after 3 times tempering
Position [°2Theta] 40 50 60 70 80 90 0 γ(200) α(200) γ(220) γ (311)
XRD Spectrum for sample n°8 in the as-cast conditions
As-cast conditions (Sample 8) After 1 Tempering (Sample 7) After 2 Tempering (Sample 3)
Martensite volume fractions as obtained from magnetic measurements starting from
a fully martensitic reference Magnetisation measured (M) as a function of
coercive force applied (He) gives an idea of RA
volume fraction in the defined sample
From VSM Measurements…
From CEMS Technique…
C Si Mn Ni Cr Mo Fe
3.2/3.5 0.9/1.1 0.9/1.0 4.2/4.5 1.3/1.9 0.3/0.4 Bal.
Average chemical composition of ICDP (-%weight )
ICDP microstructure (as-cast conditions) with free graphite (dark) and martensite-austenite mixture
(Nital etching –Bright Field) ICDP
TECHNIQUES USED FOR R.A. ASSESSMENT
REFERENCES
[1] Mössbauer study of retained austenitic phase in multiphase steels. A. MIJOVILOVICH & al – Materials Science and Engineering A283, 2000 [2] Carbon content of retained austenite in quenched steels – H. K. D. H. BHADESHIA – Metal Science Vol 17, 1983
[3] Effet des inclusions et des carbures dans des aciers et fontes fortement alliés, soumis à des sollicitations mécaniques sévères – J. LECOMTE-BECKERS & J. TCHOUFANG TCHUINDJANG - Convention RW 114877, Rapport Final, 2004
Acknowledgements
The authors would like to thank Marichal Ketin Belgium for providing materials and both the “Communauté Française de Belgique”and the “Région Wallonne” for their support.
0 10 20 30 40 50 60 70 80 90 100 % R e ta in e d A u s te n it e - -1 2 3 4 5 6 7 8 9 Sample Number XRD CEMS (Reference) VSM Dilatometry
on the material itself or the heat treatments performed.
CONCLUSIONS
Dilatometry seems to overrate retained austenite volume fraction in the as-cast conditions while good agreements are made on the overall batch of techniques when considering heat treated material where retained austenite transformation occurred. CEMS seems to be the more accurate technique for RA assessment.
ICDP microstructure
(as-cast conditions) with free graphite (dark), Cementite and Matrix (As-polished; Crossed
Nichols)
Comparisons results with various techniques for retained austenite volume fraction content in ICDP in as-cast conditions and after multiple tempering 5 m m
ø 2m m Sa m p le
Pic k-u p c o ils
M a g n et M a g n e tic fie ld M a g n et
Vibrating Sample Magnetometer (VSM) (Max. Field of 6.5 kOe)
Assuming every single sample to be made of a magnetic phase (α-Fe) with a volume fraction of x and a saturation at room T°of σs(RT) = 217 emu/g, the remaining part of the sample being non
magnetic (volume fraction of [1-x]).
Moduleur de Doppler Compteur multicanaux Détecteur Accélérateur Compteur Monocanal Pré-accélérateur Vibrateur Source (57Co) Absorbeur Ordinateur
