Optimization of mechanical properties of complex, two-stage heat treatment of Cu–Ni (Mn, Mo) austempered ductile iron
Journal of Thermal Analysis and Calorimetry
Optimization of mechanical properties of complex, two-stage heat treatment of Cu-Ni (Mn, Mo) austempered ductile iron
Andrzej Gazda 0 1
Małgorzata Warmuzek 0 1
Adam Bitka 0 1
0 Foundry Research Institute , 73 Zakopianska St., 30-418 Krako ́w , Poland
1 & Andrzej Gazda
The aim of the study was to design and optimize the complex, two-step heat treatment of Cu-Ni (Mn, Mo) ductile iron. A method for the formation and investigation of austempered ductile iron (ADI) by means of complex two-step and as comparative, standard one-step heat treatments has been developed, using quenching dilatometer. Investigations of proceeding phase transformations using differential dilatometric and DSC analysis supported by microstructural observations, hardness and austenite volume measurements have been carried out. An analysis of the temperature sequence of the ausferrite decomposition in the one-step and two-step ADI was performed, which allowed for the separation and identification of the effects responsible for the carbon-enriched austenite decomposition. A quantitative relationship was established between basic dimensional effects revealed on the differential dilatometric curve of ausferrite decomposition, which enables prognosis and optimization of the parameters of complex ADI heat treatment variants. Verification tests were performed on a stand equipped with salt furnaces enabling a quick transfer of samples from one bath to another without changing their initial temperature. Optimization of the two-step ADI heat treatment with the use of quantitative dilatometric analysis of the ausferrite decomposition, allowed to obtain for the temperature step-down heat treatment 390 C/15, 20 min 270 C/130 min the excellent mechanical properties, unattainable by means of standard 1-step ADI heat treatment.
ADI; Ausferrite; Thermal analysis; Dilatometry
Austempered ductile iron (ADI) is a ductile iron subjected
to the heat treatment consisting of austenitizing and
quenching followed by an isothermal transformation—
austempering. ADI has a good machinability, wear
resistance, damping capacity and excellent mechanical
properties—toughness and fatigue strength accompanied by high
The structure and mechanical properties of ADI depend
on the sequence of phase transformations during the
austempering. In the first stage of austenite isothermal
decomposition, ausferrite consisting of fine acicular ferrite
aac and metastable reacted carbon-saturated austenite
cs0 ðCÞ is created. As a result of further step of austempering
within processing window, optimum ausferrite with
maximum amount of fine acicular ferrite asac and reacted stable,
carbon-saturated austenite cs(C) is formed. Austempering
heat treatment parameters should prevent martensite or
carbides formation. Extending the time of isothermal
transformation beyond the processing window leads to
decomposition of carbon-rich austenite to ferrite and
ADI was the subject of many studies dedicated to
optimization of its mechanical and utility properties,
especially for new applications in the special work
conditions. There are many publications, e.g., [
description and identification of the microstructural
components, selection of optimum austenitization and
austempering parameters, role of alloying elements and
other problems related to standard one-step ADI heat
treatment with aim to optimize mechanical and utility
properties. Thermal stability of ausferrite and its
decomposition at elevated temperatures are also of interest
to many works [
]. Retained austenite present in ADI in
the reacted stable and unreacted metastable forms may be
subjected to martensitic transformation proceeding at
different stages of austempering, especially while final
cooling to the room temperature. A description of problems
related to these harmful phenomena decreasing mechanical
properties, mainly ductility, can be found, for example, in
the publications [
1, 12, 15–18
Objective in this study was to optimize a complex
twostep ADI heat treatment, based on the analysis of the
fundamental one-step austempering process. This complex
type of ADI heat treatment has a rich bibliography [
The opinion that complex heat treatment does enable better
mechanical properties, both Rm and A, than those achieved
due to standard one-step austempering is established.
The approach presented in this paper is based on the
weaker assumption that complex heat treatment enables
obtaining better mechanical properties of ADI which are
not possible to obtain in standard, one-step heat treatment.
It means that this complex heat treatment modifies the
strength–elongation relationship, Rm = F (A), in such a
way that new function F0 satisfies the condition that
F0ðAÞ [ FðAÞ at the same strength or F00ðRmÞ [ FðRmÞ at
the same ductility.
Thus, two variants of the austempering course will be
considered. The step-up variant in which the preliminary
stage takes place at a lower temperature guarantees high
strength. The final stage at higher temperature allows the
martensite tempering and additional austenite saturation
with carbon due to the faster carbon diffusion at elevated
The step-down variant in which the initial stage is
carried out at a higher temperature allows austenite to be
saturated with carbon and prevents the harmful formation
of martensite from untransformed austenite. The final stage
at lower temperature inhibits ausferrite growth and reduces
the feathered morphology of acicular ferrite.
Design of austempered ductile iron with specific
properties is based on the direct analysis of diffusionless (shear
mechanism) and the diffusive phase transformation’s
sequence leading to the ausferrite formation and the
indirect analysis, involving the study a sequence of thermal
decomposition of phases produced in austempering
process. From the experimental point of view, in the
laboratory scale, direct analysis requires to perform a specified
heat treatments and at the same time allowing to record the
structure-sensitive physical quantities in device and
determine the correlation between the basic processing
parameters and thermophysical properties. Complementary to
this approach is the indirect (inverse) analysis carried out
by means of non-isothermal thermal analysis methods. In
this approach very useful is the analysis of the sequence of
phase transformations proceeding during heating, reflecting
decomposition of metastable phases produced during ADI
heat treatment. First-order phase transformations, which
are considered, are described by changes of entropy (or
enthalpy) (DH = 0) and volume (DV = 0), so both
calorimetric (DSC) and dilatometric methods are
suitable and very useful here [
Results obtained by these methods will be completed
with the microscopic observations of the microstructure
evolution as affected by controlled heat treatment. The
microstructure effects ascribed to measured thermophysical
properties (enthalpy and volume) will be the base of
analyzing the decomposition path of the initial phase
Thermal analysis methods were used to produce the test
material as well as to analyze the phase transformations
accompanying the decomposition processes of the
ausferrite during controlled temperature changes.
High Speed Quenching Dilatometer Linseis RITA L78,
which allows high heating/cooling rates up to 150 K s-1
(induction heating), has been used to produce relatively
small cylindrical samples 10 mm long and 3 mm in
diameter with a different ausferrite structure and for the
construction of CCT and TTT diagrams. Accurate sample
temperature measurement is obtained by means of K-type
thermocouple welded to the sample.
The dilatometric analyses of the ausferrite were
performed in a Netzsch DIL 402C dilatometer in the range
from ambient temperature to approximately 750 C (below
the Ac1 temperature) with a heating rate q = 5 K min-1 in
a dynamic Ar 5.0 protection atmosphere.
The Netzsch DSC 404C Differential Scanning
Calorimeter was used for calorimetric studies of the
ausferrite decomposition processes. Temperature and
sensitivity calibration of the device was performed using the
melting enthalpies of the pure elements In, Sn, Zn, Al, Ag,
Au. DSC measurements were carried out from ambient to
ca 750 C, using varying heating rates (2–20 K min-1) in
Ar 5.0 protective atmosphere. Each DSC measurement was
repeated under identical conditions, and the resulting
physical baseline curve was subtracted from the first curve
obtained for the as-austempered sample, giving resulting
The HV30/15 Vickers hardness measurements and the
austenite volume fraction evaluated using a Ferrikomp
technical tool, operating on the principle eddy current to
determine the ferrite content, were used as parameters to
characterize the heat treatment effects.
The mass fraction of carbon-enhanced austenite and
carbon content in austenite was determined using the
Bruker D8 Discover X-ray diffractometer.
Examinations of the alloy microstructure and
morphology of graphite were made by means of light
metallographic microscope Axio Observer.Z1m. Graphite was
observed on the polished metallographic cross sections,
while the microstructure morphology on those etched with
4% nital reagent.
More detailed observations of the alloy microstructure
and phase constituents morphology were carried out by
means of scanning electron microscope Scios FEI.
Samples after selected heat treatment variants were
subjected to a static tensile test at room temperature using
EU-20 strength machine (0–100 kN).
The principle of conducting minimum two measurement
series for the given treatment and type of analysis and
differentiation of the heat treatment technique
(laboratory - dilatometer/traditional - salt furnaces) was applied
in the work where possible. Each measuring point is the
average of three measurements carried out under the same
conditions. Due to the methodology of thermal analysis
studies, the measurement results are subjected to type B
Material for the study was a ductile cast iron alloyed
with Ni, Cu, Mo and Mn, obtained in a foundry induction
furnace. Table 1 shows the chemical composition of the
examined alloy [
The matrix of the as-cast ductile iron consists of 96%
pearlite and well-shaped graphite (70%VI6 ? 30%V6 acc.
to PN-EN ISO 945-1:2009) characterized by 8.1% volume
fraction and relatively small number of spheroids 60 per
Two types of ADI heat treatments of spheroidal cast iron,
one step and two steps were performed. Selection of the
optimal heat treatment parameters was based on the
analysis of CCT and TTT diagrams, supported by
metallographic studies and hardness measurements, summarized in
the previous paper [
]. The following values of ADI heat
treatment parameters were established, after austenitization
at 910 C for 40 min and cooling down to austempering
temperature at the cooling rate 100 K s-1.
One-step heat treatment—austempering at 270, 310, 350
and 390 C for 150, 120, 90 and 60 min, respectively,
The materials for dilatometric, DSC and structural
investigations were manufactured under laboratory
conditions in the Linseis L78 RITA quenching dilatometer and
in the salt furnace stand. In a first case, dimensional
changes (linear or volumetric) accompanying the phase
transitions induced by the various types or segments of heat
treatment useful for quantitative analysis, were additionally
Characteristics of ADI material
After heat treatment, non-destructive tests were conducted,
and HV30 hardness and carbon-enriched austenite fraction
Va in the alloy were determined, as forecasting and
indicative parameters of the basic mechanical properties of
ADI—tensile strength Rm and ductility (elongation) A.
Figure 1a, b presents dependence of HV30 and Va on
austempering temperature for one-step heat treatment and
calculated relation between hardness and austenite fraction,
Figure 2a, b presents dependence of HV30 and Va on
dwell time of the initial temperature step of a two-step heat
treatment. For the step-up ADI treatment, an increase in the
first-stage duration is accompanied by the increase in
HV30 and a decrease in Va. For the step-down case of this
complex heat treatment, prolonged time of the initial stage
causes an increase in Va and a decrease in HV30, which is
qualitatively in agreement with the results for the one-step
ADI heat treatment.
Morphology of the alloy matrix after different variants
of the heat treatment, observed by means of the scanning
electron microscope, is presented in Fig. 3a–d.
One can see that alloy microstructure formed due to heat
treatment was affected by its parameters, temperature, time
and heat treating mode. In the specimens after one-step
treatment, an increase in the austempering temperature
resulted in a growth of grains (Fig. 3a, b) and in the
thickness of ferrite plates/needles. The two-step treatment
application, of the both variants, resulted in the more
homogeneous microstructure morphology, especially more
uniform ferrite plates/needles dispersion in both examined
specimen (Fig. 3c, d). However, dispersion of ferrite
plates/needles was slightly higher after step-down
treatment (Fig. 3c) in comparison with that observed after
stepup variant (Fig. 3d).
Decomposition of ADI
The obtained ADI material, defined by means of a specific
heat treatment, has been subjected to controlled
destruction, i.e., decomposition studies combined with recording
of dimensional (differential dilatometric analysis) and
thermal (DSC) effects.
Figure 4a shows the recorded sequence of dimensional
changes accompanying the decomposition process of the
ausferrite after one-step treatment carried out at various
constant temperatures. Figure 4b depicts the corresponding
exothermic effects recorded under similar conditions by
DSC curves, less sensitive to structural changes occurring
during heating of ADI.
With increasing austempering temperature, the main
dimensional (volume) effect decreases and exothermal
DSC effect increases, indicating the decomposition of
austenite. In addition, the DSC peaks show a shift toward
the higher temperature which may suggest greater thermal
stability of high-temperature (upper) ausferrite.
The deconvolution of the recorded, complex differential
dilatometric a (Alpha) curves by PeakFit 4.12 has been
performed. Software allows peaks separation by the
selection and fitting of analytical functions, determination
of their temperatures and calculation of the relative
dimensional changes (DL/Lo) of peaks and provides
statistics of fitting.
Generated by software, exemplary differential
dilatometric curves—as measured (upper side) and deconvoluted
Alpha* = (dL/dT)/Lo - baseline(T), elaborated for
decomposition processes after one-step and two-step-down
heat treatments are shown in Figs. 5a, b and 6a, b,
Figure 7a shows an enlarged section of Fig. 4a to
facilitate the interpretation of recorded curves.
Negative dimensional effect 0 recorded at a temperature
of about 100–170 C is the result of the tempering of
asquenched martensite present in structure, transformation of
270 >> 350 °C
350 >> 270 °C
270 >> 390 °C
390 >> 270 °C
270 >> 350 °C
350 >> 270 °C
270 >> 390 °C
390 >> 270 °C
2-Step heat treating
2-Step heat treating
supersaturated tetragonal ferrite into supersaturated
acicuoccurring in a relatively
wide temperature range of
lar ferrite (asac) ? coherent carbides and gradual stress
]. The positive volumetric effect 1 of the
C and reflects, as indicated by its positive sign,
the decomposition of the unstable austenite with a low
DSC curve corresponds to a broad exothermic effect
carbon content. The magnitude of this effect is clearly
correlated with the magnitude of the effect 0 and increases
with the temperature of the isothermal transformation.
In the case of two-step treatment, no martensite
tempering effect was observed; effect 1 occurs only when the
transformation includes a high austempering temperature
step of 390 C and is lower for step-down heat treatment.
The key effects of ausferrite decomposition are negative
effects 2 and 4 (Fig. 7a), which characterize the
decomposition process after each ADI treatment. A good
correlation was observed between the magnitude of peak 2 and
the hardness HV30 (Fig. 7b), indicating that this effect is
related to the loss of strength properties of the acicular
ferrite. In temperature range of peak 2, other processes also
take place, such as cementite precipitation from
supersaturated austenite or coagulation of previously precipitated
carbides; therefore, good fitting requires the assumption of
a double peak 2a, 2b for calculations. It is obvious that the
quality of fitting increases with the number of peaks added
to the calculations so the assumption of pre-conditions is
subjected to the rule—do not multiply peaks over
necessity, defined by their physical meaning.
The source of negative value of the main effect of
ausferrite tempering is dimensional (volumetric) changes
accompanying the mentioned processes as well as high
expansion coefficient of the vanishing austenite phase.
Fitting of dilatometric curves after these one- and
twostep heat treatment variants when an upper ausferrite is
produced is only possible by assuming the presence of
additional peak 3, reflecting the decomposition of austenite
saturated with carbon, as evidenced by the positive sign of
the effect at high temperature. This is the only case where
the decomposition effect of high carbon austenite can be
explicitly observed in the decomposition curve.
The experiments of the isothermal decomposition of
ausferrite at 400, 450, 500 and 550 C for about 120 min
followed by decomposition during heating showed that
even after the complete disappearance of effect 2 (450 C/
120 min), peak 4 remains unchanged. Therefore, it is not
the result of the decomposition of the products that
preceded it, but that originated in the earlier phase of the
decomposition path. Coagulation of the cementite is the
only process proceeding in this high temperature range
which is mainly the product of the austenite decomposition
and indirectly related to its content in the alloy.
Due to the relatively small values of effect 4, which in
case of peak 3 appearance is split into double effects 4a,
4b, no satisfactory correlation can be found with
carbonenriched austenite ratio.
Effects 5 and 6 were not analyzed in detail, although
they could have an informative significance. Peak 5
appearing only for very high temperature of austempering
reflects the ferrite recrystallization and cementite
graphitization processes imposed on sharp specific heat (baseline)
change in the vicinity of the Curie point. Such an effect
proves that processing window is exceeded and eliminates
these heat treatment variants. Effect 6 depicts an eutectoid
Figures 8–10a, b present dependence of values of
deconvoluted peaks 2 and 4 found for realized step-up and
step-down variants of complex heat treatment.
It is worth noting that highest values and sensitivities of
effect 2 and especially effect 4 were obtained for the case
step-down heat treatment 390 270 C (Fig. 9b).
Figure 11a shows dependence of dilatometric effects 4
versus 2 calculated for all one-step heat treatment
temperature variants with outlined confidence bands at 95%
The compatibility of relations 4 versus 2 (Fig. 11a) and Va
versus HV30 (Fig. 1b) is a prerequisite for this type of
relationships to be used to design optimal heat treatment.
This is confirmed in Fig. 11b where the data computed for
complex, two-step heat treatment are added. Only
stepdown heat treatment 390 270 C provides hope for
better mechanical properties than one-step heat ADI
Based on the analysis of the dilatometric test results
(Fig. 11b), one- and two-stage heat treatments were
performed in the work stand equipped with austenitization
2-step heat treating 270 >> 350 °C
2-step heat treating 350 >> 270 °C
Peak 2 Peak 4
The samples after selected heat treatment variants were
subjected to a static tensile test at room temperature using
an EU-20 strength machine. Figure 12 presents
dependence of ductility (elongation) on tensile strength Rm for
standard one-step and complex two-step ADI heat
The forecast rightness based on dilatometric analysis has
been confirmed; for step-down heat treatments 390 C/
15 min 270 C and 390 C/20 min 270 C,
mechanical properties have been obtained which cannot be
achieved by means of standard one-step ADI heat
Observations of the fracture surface topography carried
out by means of the scanning electron microscopy showed
that decohesion mechanism was modified by the heat
The transcrystalline cleavage fracture was observed in
material in the as-cast state (Fig. 13a). In the heat-treated
material, more of the microregions of the plastic
deformation were revealed, while temperature of the isothermal
austempering increased from 310 to 390 C (Fig. 13b, c).
10 15 20 25
1st step dwell time/min
furnace and two salt furnaces to carry out complex
austempering, with the possibility of rapid transfer of
samples from one bath to another without changing the
sample initial temperature.
Fig. 13 Morphology of the fracture
surface in the examined specimens,
SEM; as-cast state, transcrystalline,
cleavage fracture in the matrix, with
visible decohesion at graphite/matrix
interface (a), one-step at 310 C,
transcrystalline, mixed cleavage/ductile
fracture in the matrix, with visible
decohesion at graphite/matrix interface
(b), one-step at 390 C, transcrystalline,
ductile fracture in the matrix [with
small microregion of brittle cracks,
with visible decohesion at
graphite/matrix interface] (c), 2-step
(down) 390 270 C, transcrystalline,
mixed cleavage/ductile fracture in the
matrix, with visible decohesion at
graphite/matrix interface (d)
On the fracture surface in specimens after heat treatment
(310, 390 C), the dimples characteristic for ductile
fracture was visible. Similar morphology to that observed in
the specimen austempered at 390 C, characteristic for
ductile fracture, was observed in specimen after two-step
heat treatment (Fig. 13d).
The specific features of the fracture surface morphology
observed in the examined specimens reflect changes in
their fracture mechanism due to alloy microstructure
evolution. It was caused, first of all, by effect of heat treatment
parameters on change of the microstructure phase
composition: rise of products of the isothermal annealing,
especially ausferrite, in place of those primary, as pearlite and
ferrite. This resulted in the transition of fracture
mechanism from that transcrystalline cleavage in as-cast state to
transcrystalline ductile or mixed after austempering. The
ductile dimples in alloy matrix observed on a fracture
surface after heat treatment indicate its local plastic
deformation. As it is related to energy absorption, in a
macroscope scale, material plasticity increases.
A method for the precise production and investigation
of austempered ductile iron (ADI) has been developed
using standard one-step and complex, two-step heat
treatments by means of quenching dilatometer.
Investigations of proceeding phase transformations
using differential dilatometric analysis supported by
microstructural observations, hardness and austenite
volume measurements have been carried out.
Analysis of the temperature sequence of the ausferrite
decomposition in the one-step and two-step ADI was
carried out, which allowed for the separation and
identification of the effects responsible for acicular
ferrite and carbon-enriched austenite decomposition.
A quantitative relationship was established between
basic dimensional effects on the dilatometric
differential curve of ausferrite decomposition, which enables
prognosis and optimization of the parameters of
complex ADI heat treatment variants.
Evolution of alloy phase composition observed as an
effect of the austempering heat treatment led to change
in decohesion mechanism from cleavage to ductile
which gave a macroscope effect in the form of increase
in alloy ductility.
Microstructure constituents morphology observed as a
result of the used heat treatment parameters can have
some impact on final macroscope material properties,
especially on strength/ductility ratio. However, noticed
effect of differences in the retained austenite volume
fraction should not be omitted in the procedure of the
optimization of the austempering parameters.
Quantitative dilatometric analysis of the ausferrite
decomposition allowed to obtain for the optimal
temperature step-down heat treatment 390 C/15, 20
min 270 C/130 min the excellent mechanical
properties, unattainable by means of standard 1-step
ADI heat treatment.
Acknowledgements This work was financially supported by Polish
National Science Centre (NCN) in the Project No. UMO-2013/09/B/
Open Access This article is distributed under the terms of the
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