Exploitation
of X-ray diffraction in characterisation of C45 ferritic-pearlitic
steel properties
D. Šimek1, A. Oswald2, R. Schmidtchen2,
M. Motylenko1, D. Rafaja1, G. Lehmann2
1Institute of Materials Science, TU-Freiberg, Gustav-Zeuner-Str. 5,
D-09599
2Institute of Metal Forming, TU-Freiberg, Bernhard von Cotta Str.
4, D-09599 Freiberg,
e-mail: simek@fzu.cz
Ferritic-pearlitic (F-P) steel C45 containing 0.45 wt.% of
carbon exhibits a variable microstructure according to its thermo-mechanical
treatment. After forming in austenite state, if the cooling below approx.
850 °C is slow enough, the ferritic-pearlitic
microstructure develops. The primary ferrite crystallizes at the austenitic
grain boundaries above the eutectoid temperature of 727 °C, below which
the rest austenite is transformed into lamellar pearlite.
The relative amount of primary ferrite (Xf) depends on the cooling rate, the prior
austenite grain size (the smaller the grains the larger the grain boundary
crystallisation area) and the upper limit is approx. 60%. The faster the
cooling, the less primary ferrite is developed in favour of pearlite.
The
correlation of mechanical properties of F-P steels wit their microstructure
were thoroughly studied [1,2,3] as the key factors
affecting the strength of pearlite, its interlamellar distance (ILD, S) was identified [1,2]. In F-P steels, the volume fraction of pearlite (Xp = 1 – Xf) play also an
important role, but the correlation of yield strength with these two parameters
is not clear [3].
The
intention of study was to investigate the exposure of the microstructure in the
X-ray diffraction (XRD) and the correlation of XRD features with mechanical
properties of C45 F-P steel directly. The aim of the survey was to develop a
fast analytical method for an on-line control of manufacturing process
optimisation. In order to cope with the problem, a series of F-P
microstructures was produced by hot rolling at various temperatures and by
different regimes of cooling (ambient air, lead bath of 550 °C, dipping in
water). Subsequent heating was optionally also applied. Further, selected F-P
microstructures representing the broad range of hot rolled F-P steels were
subjected to gradual cold drawing with or without intermediate reheating. This
process represented the industrial treatment from ingots till cold-drawn hard
wires.
Figure 1. Correlation of dislocation-induced microstrain and over-all lamellar density in hot rolled ferritic-pearlitic C45 samples and in fully pearlitic (C80) samples. The cold drawn and
subsequently annealed C45 samples are also included
The
investigation of the hot rolled samples revealed that the ferrite phase is
under an apparent compressive residual stress regardless of the cut of the
specimen surface (cross-sectional or longitudinal). This behaviour excludes the
macroscopic residual stress. It is a consequence of two-component
microstructure (ferrite/pearlite). The TEM
investigation revealed the misfit dislocations at the ferrite cementite interfaces in the pearlite,
while the primary ferrite grains were almost defect-free. The 3rd
kind dislocation induced mean squared microstrain (e2disl) obtained from XRD was found to correlate with
the over-all density of cementite lamellas calculated
as Xp/S (Figure 1). The same tendency is held
also for fully pearlitic samples of steel C80D (0.80
wt.% carbon). The dependence is rather quadratic
instead of linear as supposed in e.g. [4]. It is the nature of the misfit
dislocations that are not randomly distributed but organized in an equidistant
grid, so that their displacement fields are more similar to dislocation
dipoles. The ultimate tensile strength was found to be directly proportional to
the mean squared microstrain in hot-rolled samples
(Figure 2).
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|
|
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Figure
2. Correlation of
UTS with mean squared dislocation induced microstrain |
Figure
3. Comparison of
UTS predicted from the X-ray diffraction experiment and true UTS evaluated in
the tensile test |
With cold
drawing, for cross-sectional reduction till about 50%, the dislocation density increases
and the correlation of UTS with the mean squared microstrain
can still be observed, however, with a certain offset in the microstrain compared to hot rolled samples. Afterwards, for
higher deformation or after annealing, the proportionality is lost. On the
other hand, macroscopic residual stress is now observed in the ferrite phase,
which is compressive along drawing direction. It is the result of the easier
plastic deformation of the ferrite compared to harder cementite,
which, after relief of the drawing force, compresses the ferrite.
A new
empirical correlation was found between the UTS and the (extrapolated) lattice
parameter observed on crystallographic direction á111ñ in the drawing direction (a111). The smaller is the a111 (the stronger the
compressive force), the higher is the UTS of the steel in the tensile test. The
XRD here allows to determine which mechanism is
driving the tensile properties. Thus, a single measurement in rolling/drawing
direction is suitable for estimation of the UTS. If the a111 is larger than the stress-free lattice parameter,
the sample is hot rolled or annealed and UTS depends on the dislocation density
(analysed from the line broadening). If a111
is smaller, the UTS is dependent upon this lattice
parameter. The XRD-predicted UTS than gives a good agreement with the true
experimental UTS evaluated from the tensile test (Figure 3).
The XRD is
able to predict the ultimate tensile strength of ferritic-pearlitic
steel produced by the hot rolling or cold drawing in a broad range of its
experimental values (600 – 2000 MPa) with
the error band of approx. ± 100 MPa (cf.
Figure 3). It appears therefore as a promising analytical method for a fast
on-line control of the steel production. The correlation with the
microstructure moreover allows to estimate the mesoscopic
microstructure parameter (pearlite volume fraction or
its interlamellar distance) if additional information
about the thermo-mechanical history is known.
1. O. P.
Modi, N. Deshmukh, D. P. Mondal, A. K. Jha, A. H. Yegneswaran, H. K. Khaira, Materials Characterization 46, (2001), pp. 347-352
2. A. M. Elwazri, P.
Wanjara,
3. K. K. Ray, D. Mondal,
Acta Metall. Mater.
39, (1991), pp. 2201-2208
4. T.
Ungár,