Influence of Carbon on the Microstructure and Mechanical ... · steels with the use of solid...
Transcript of Influence of Carbon on the Microstructure and Mechanical ... · steels with the use of solid...
1 Institute of Iron and Steel Technology, TU Bergakademie Freiberg, D-09596, Freiberg, Saxony, Germany 2 Institute of Materials Science, TU Bergakademie Freiberg, D-09599, Freiberg, Saxony, Germany
3 Institute of Materials Engineering, TU Bergakademie Freiberg, D-09599, Freiberg, Saxony, Germany
Influence of Carbon on the Microstructure and Mechanical Properties of Cast Austenitic
Fe-19Cr-4Ni-3Mn-0.15N Steels
M. Wendler1 / B. Reichel
2 / A. Weiß
1 / L. Krüger
3 / J. Mola
1
Abstract
The structure and mechanical properties of cast Fe-
19Cr-4Ni-3Mn-0.15N (concentrations in wt%) steels
containing 0.05, 0.15 and 0.25 wt% carbon were
determined under uniaxial tensile loading in the
temperature range of -40 °C to 200 °C. The alloy
development focused on the creation of metastable
austenite capable of exhibiting Transformation- and
Twinning-Induced Plasticity (TRIP/TWIP) effects and
the planar glide of dislocations in the austenite. For the
steel with 0.15%C, these conditions were met
at -40 °C resulting in a tensile strength in excess of
1300 MPa, a uniform elongation of 45%, and a yield
strength of 420 MPa.
Keywords: TRIP/TWIP effect, mechanical properties,
austenitic as-cast steel, CrMnNi steel
1. Introduction
Within the Collaborative Research Center 799 novel
composite materials are being developed which
consist of a highly alloyed TRIP/TWIP CrMnNi cast
steel matrix and partially-stabilized zirconium dioxide
(Mg-PSZ) ceramic particles [1], [2]. To promote the
phase transformation of the ceramic phase from
tetragonal to monoclinic crystal structure, current steel
research activities are focused on the increase of the
strength, especially the yield strength, of cast
austenitic TRIP/TWIP CrMnNi steels by solid solution
strengthening with nitrogen and carbon. To trigger the
deformation-induced martensite or twin formation in
such steels, a metastable austenite is needed. This can
be achieved by adjusting the chemical composition.
Depending on the chemical composition and stacking
fault energy (SFE), such deformation-induced
plasticity mechanisms as γ→α´, γ→ɛ, and γ→ɛ→α´
transformations as well as mechanical twinning of
austenite can occur under external loading [3]. These
plasticity mechanisms are closely linked to the
deformation temperature and can lead to impressive
mechanical properties in comparison with the stable
austenitic steels. To obtain a strain-induced ɛ-
martensite formation or deformation twinning, a low
SFE is required. In high-manganese Fe(Cr)MnC
TWIP/TRIP steels, the γ→ɛ phase transformation is
favoured at SFEs below 15-20 mJ/m². The twinning of
austenite is on the other hand observed at SFEs
between 15-20 mJ/m² and 40 mJ/m² [4]–[7].
Investigations on FeCrMnNi TRIP/TWIP steels have
confirmed the above-mentioned SFE results [8], [9].
In this paper, the effect of carbon on the
microstructure and temperature dependence of stress-
strain behaviour in Fe-19Cr-4Ni-3Mn-0.15N cast
steels containing 0.05-0.25 wt% carbon is
investigated. For the creation of TRIP and TWIP
effects, the chemical compositions are adapted to
achieve low austenite stabilities and SFEs. The aim of
this development is to increase the strength of as-cast
steels with the use of solid solution strengthening by
nitrogen and carbon and yet to increase the strain-
hardening potential by TRIP and TWIP effects.
2. Experimental Procedure
2.1 Fabrication of Cast Steels
The steels used in this study were melted in a vacuum
induction furnace. Initially, a nitrogen partial pressure
of 150 mbar was used to melt down the feedstock.
After complete formation of the molten bath, nitrogen
gas with a partial pressure of 450 mbar was applied for
nitriding. Finally the steels were cast into a water-
cooled copper mould placed in the furnace chamber.
To avoid the pore formation in the cast material, a
nitrogen partial pressure of 1500 mbar was used
during casting. Two ingots of each melt with the
dimensions of 230 mm x 35 mm x 95 mm were
produced and then machined to round tensile
specimens with a gauge diameter of 6 mm. To
eliminate possible formation of strain-induced
martensite in the final manufactured tensile
specimens, heat treatments were performed after
machining of tensile specimens. The chemical
composition of the three manufactured as-cast steels is
given in Table 1.
2.2 Heat Treatment
For the complete dissolution of carbides and nitrides
as well as the reduction of segregation, the as-cast
microstructure was homogenized in the austenitic
phase field. Solution annealing treatments were done
under an argon atmosphere for 30 minutes. For the
steels 19NC1505 and 19NC1515, annealing
temperatures of 1050 °C and 1150 °C were used,
respectively. A further increase of the annealing
temperature to 1200 °C was necessary for the
complete dissolution of precipitates in the alloy
19NC1525. Additionally, all the steels were water
quenched to ensure the suppression of precipitation
formation processes during cooling from annealing
temperature.
2.3 Tensile Tests and Magnetic Saturation
Measurements
The heat-treated specimens were tensile tested with a
strain rate of 4 x 10-4
s-1
using a Zwick 1476-type
universal testing machine. With the aid of a thermal
chamber which surrounded the tensile specimen and
the sample holding jaws, different temperatures in the
range of -40 °C to 200 °C could be adjusted.
For the quantification of combined fraction of α׳-
martensite and delta ferrite, a Metis MSAT-type
magnetic saturation device equipped with a Lake
Shore 480 fluxmeter was used. The equipment
measures the magnetic flux density after
magnetization until saturation of steel samples. The
ferromagnetic phase content can be calculated after
corrections for the chemical composition of the steel.
The measurement accuracy is better than 1%. The
room temperature microstructures were studied by
optical microscopy to determine the delta ferrite
fraction.
3. Results and Discussion
3.1 Solidification Mode
The solidification behaviour of austenitic stainless
steels can be classified in five primary solidification
modes: single-phase ferrite (F), primary ferrite with
second-phase austenite (FA), eutectic ferrite and
austenite (E), primary austenite with second-phase
ferrite (AF) and single-phase austenite (A) [10]. For
the processing of high nitrogen steels the knowledge
of the primary solidification mode is essential as it is
closely related to the solid state nitrogen solubility. In
case of a primary austenitic solidification, the nitrogen
solubility is increased in comparison to a primary
ferritic solidification. Therefore, steels are defined as
“high-nitrogen” when there exists a minimum solute
nitrogen content of 0.08 wt% in the ferrite or 0.4 wt%
in the austenite [11]. With the increase of nitrogen
partial pressure, an enhanced nitrogen solubility in the
liquid state and non-porous ingots during (F) or (FA)
solidification mode can be achieved. A further
common way to increase the nitrogen solubility of
stainless steels is alloying with such solubility-raising
elements such as Cr, Mn, V, and Ti [12], [13]. Table 2
lists the solidification modes and solidification
sequences of the investigated steels as determined
with the aid of Thermo-Calc calculations.
3.2 Austenite Stability and As-Cast
Microstructure
The Schaeffler diagram was originally developed for
the prediction of the microstructure at ambient
temperature of high-alloy stainless steels after
welding. Nevertheless, this constitutional diagram
could also be used for the characterization of as-cast
microstructure and for the estimation of austenite
stability against the as-quenched αʼ-martensite
formation [5-7]. The Ni- and Cr-equivalents of the
steels used in this study were calculated with the
following equations [17].
𝑁𝑖𝑒𝑞 = %𝑁𝑖 + 30%𝐶 + 18%𝑁 + 0.5%𝑀𝑛
+ 0.3%𝐶𝑜 + 0.2%𝐶𝑢 − 0.2%𝐴𝑙 (1)
𝐶𝑟𝑒𝑞 = %𝐶𝑟 + %𝑀𝑜 + 4%𝑇𝑖 + 4%𝐴𝑙 + 1.5%𝑆𝑖
+1.5%𝑉 + 0.9%𝑁𝑏 + 0.9%𝑇𝑎 + 0.5%𝑊 (2)
Furthermore, the knowledge of the SFE allows to
evaluate possible occurrence of deformation-induced
twinning. The SFE at room temperature of the alloys
Table 2. Solidification mode and solidification sequence of the as-cast steels.
Alloy Solidification
mode
Solidification sequence
according to Thermo-Calc
19NC1505
FA L → (L+FP) → (L+FP+AE) → (FP+AE)
FP: primary ferrite AE: second-phase austenite
19NC1515
19NC1525
Table 1. Chemical composition of the investigated as-cast steels, in wt%.
Alloy C N Cr Ni Mn Si Fe + others
19NC1505 0.051 0.140 18.90 4.02 2.90 0.48 bal.
19NC1515 0.156 0.140 19.20 4.11 3.20 0.45 bal.
19NC1525 0.264 0.146 19.10 4.17 3.06 0.44 bal.
was therefore calculated using an empirical equation
applicable to high-alloy stainless steels [18].:
The values for the Cr- and Ni-equivalents as well as
the SFEs of the as-cast steels are given in Table 3.
Table 3. Cr-equivalents, Ni-equivalents, and SFEs
calculated based on Equations 1-3.
Alloy Creq
[-]
Nieq
[-]
SFE
[mJm-2]
19NC1505 19.62 9.52 14.00
19NC1515 19.88 12.90 21.02
19NC1525 19.76 16.25 29.81
The calculated SFE value for the steel 19NC1505 is
14 mJm-2
, close to the commonly-reported SFE value
below and above which deformation-induced ε-
martensite formation and deformation-induced
twinning are favored, respectively. The above-
mentioned cast steel exhibits a delta ferrite fraction of
9%. Due to the low solid solution solubility of
nitrogen and carbon in ferrite, these elements are
thought to partition to the austenite phase. This
process raises the SFE of the alloy and leads to an
increased austenite stability. TWIP effect is therefore
expected to be the dominant deformation mechanism
of austenite in the 19NC1505 alloy. This should also
be the case for the steels 19NC1515 and 19NC1525
with SFEs of 21.02 mJm-2
and 29.81 mJm-2
,
respectively. Indicated on the Schaeffler diagram of
Figure 1 are the positions of the three investigated as-
cast steels. There is clearly a good agreement between
the microstructural phases predicted by the diagram
and the delta ferrite fractions determined by light
optical microscopy. Furthermore, the alloy 19NC1525
is predicted by the diagram to have the highest
resistance to as-quenched (similarly deformation-
induced) martensite formation because it is positioned
farthest from the martensite start line, i.e. the line
which separates the austenitic and austenitic-
martensitic phase fields of the Schaeffler diagram.
Figure 1. Positions of the cast steels in the Schaeffler
diagram.
The microstructure of the alloys after water quenching
from the solution annealing temperature is shown in
Figure 2. The alloy 19NC1505 exhibits an austenitic
microstructure containing a continuous network of
approximately 9% delta ferrite. The steel 19NC1515
was found to have a delta ferrite fraction of only 4%
which appeared as isolated regions of more or less
spherical in shape. The difference in the morphology
of delta ferrite in the alloys 19NC1505 and 19NC1515
likely arises from the higher solution annealing
temperature of the latter alloy which accelerates the
Ostwald ripening of ferrite. In the steel 19NC1525, a
fully austenitic microstructure was obtained. The
ferritic regions of the microstructures in Figure 2 are
marked with arrows.
𝛾𝑆𝐹𝐸 = +1.59%𝑁𝑖 − 5.59%𝑆𝑖 + 26.27(%𝐶 + 1.2%𝑁)(%𝐶𝑟 + %𝑀𝑛 + %𝑀𝑜)1
2⁄ + 0.61[%𝑁𝑖(%𝐶𝑟 + %𝑀𝑛)]1
2⁄
+39 − 1.34%𝑀𝑛 + 0.06%𝑀𝑛2 − 1.75%𝐶𝑟 + 0.01%𝐶𝑟2 + 15.21%𝑀𝑜 − 60.69(%𝐶 + 1.2%𝑁)1
2⁄ (3)
c)
a)
δ
Figure 2. Microstructure at RT of solution annealed
cast steels: (a) 19NC1505, austenite with 9% delta
ferrite; (b) 19NC1515, austenite with 4% delta ferrite;
(c) 19NC1525, fully austenitic.
3.3 Strain-Induced Martensite Formation
The initial microstructure at room temperature (prior
to deformation) of the alloys shows no as-quenched
martensite. To check the possible formation of as-
quenched martensite at lower temperatures, the heat-
treated cast steels were cooled down to -196 °C in
liquid nitrogen. No αʼ-martensite formation could be
induced by the preceding treatment indicating the high
stability of the alloys with respect to the athermal αʼ-
martensite formation. During tensile testing at low
temperatures, however, strain-induced αʼ-martensite
formation was observed in all of the cast steels. As the
carbon concentration increased, the alloys showed a
higher resistance to the deformation-induced
martensite formation and thus a lower Mdγ→αʼ
temperature. The steel 19NC1505 exhibited the lowest
austenite stability and thus the highest Mdγ→αʼ
temperature (106 °C) of all alloys. Mdγ→αʼ
temperatures of 66 °C and 42 °C were found for the
steels 19NC1515 and 19NC1525, respectively. With
decreasing the tensile test temperature, the chemical
driving force for the γ→αʼ martensite transformation
increases. Therefore, less mechanical work is required
at lower temperatures to supply the critical driving
force for the martensite formation. As a consequence,
the TRIP effect is facilitated at lower temperatures and
a higher amount of martensite can be obtained by
deformation at lower deformation temperatures. This
is true as long as the chemical driving force for the
martensitic transformation increases at lower
temperatures. Shown in Figure 3 is the evolution of
strain-induced martensite fraction after tensile test up
to fracture at various temperatures. Some retained
austenite persists in the microstructure of all alloys
even in the case of the lowest deformation
temperature.
Figure 3. Strain-induced αʼ-martensite fractions
formed at various tensile testing temperatures. The
values were obtained by magnetic saturation
measurements.
The microstructure of the steel 19NC1505 after
deformation until fracture at 100 °C is shown in
Figure 4. It can be seen that the intersections of the
straight slip bands in the austenite serve as nucleation
sites for the strain-induced αʼ-martensite. Due to their
low SFE values, all of the alloys studied in this
research exhibited a similar martensite formation
behaviour.
δ
b)
c)
Figure 4. SEM image in BSE contrast of the steel
19NC1505 strained until fracture at 100 °C (austenite
with slip bands of a brighter contrast and the dark
αʼ-martensite regions formed within the bands most
often at the intersections; load axis aligned
horizontally).
3.4 Mechanical Properties
Figure 5 shows the yield strength as a function of
temperature. At the temperature of -40 °C, the steel
19NC1525 exhibits the highest 0.2% proof stress of all
alloys amounting to 413 MPa. This could be explained
by the solid solution strengthening effect of the
interstitial elements (C+N=0.41%) assisted by the
absence or negligibility of stress-induced martensite
formation. The latter process is known to reduce the
proof stress and appears to be responsible for the low
proof stress of the 19NC1505 alloy at -40 °C [19],
[20]. In spite of the lower interstitial content of the
19NC1505 alloy compared to the 19NC1525 alloy, it
shows a proof stress close to that of the latter in the
temperature range of 60 °C to 100 °C. At 200 °C, the
proof stress of the 19NC1505 alloy reaches 236 MPa,
clearly higher than that of the 19NC1525 alloy. These
observations might be justified by the phase
strengthening effect of delta ferrite and the superior
glide planarity in the 19NC1505 alloy caused by its
lower stacking fault energy and hindered cross slip.
Figure 5. Temperature dependence of the 0.2% proof
stress.
The temperature dependence of ultimate tensile
strength (UTS) for the studied as-cast steels is shown
in Figure 6. At temperatures above approximately
100 °C, where the strain-induced martensite formation
is absent or very limited in all alloys, the tensile
strength increases with increasing carbon
concentration. This is despite the lower delta ferrite
fraction of alloys with a higher carbon content. A
possible explanation can be sought by referring to the
elongation values in Figure 7. The uniform elongation
of alloys increases with increasing carbon
concentration. The larger plastic strains
accommodated by the 19NC1525 alloy are therefore
thought to be responsible for its higher UTS values at
temperatures of 100 °C and 200 °C. At lower
temperatures, UTS values are also influenced by the
strain-induced martensite formation. The martensite
which tends to form in the slip bands of austenite acts
as an obstacle to the dislocation glide in the bands. As
a consequence, the strain-hardening increases and
additional slip bands have to be activated to sustain
the plastic deformation. At tensile deformation
temperatures associated with the strain-induced αʼ-
martensite formation, the tensile strength should be, as
a rule, proportional to the fraction of strain-induced
martensite. The higher tensile strength of the steel
19NC1505 at low tensile test temperatures should thus
be related to the higher martensite fractions formed
during deformation. A deviation from this material
behaviour occurs at -40 °C where the tensile strength
of the 19NC1515 steel exceeds that of the 19NC1505
steel. At this temperature, an equal strain-induced αʼ-
martensite fraction of approximately 81% formed in
both alloys. In this case, the higher carbon content of
the 19NC1515 alloy leads to an enhanced tetragonal
distortion of the strain-induced martensite and results
in an increased phase strengthening effect.
Furthermore, the larger plastic strains accommodated
by the NC1515 alloy cause an additional strengthening
by work hardening.
α αʼ
δ
Figure 6. Temperature dependence of the tensile
strength.
Shown in Figure 7 are the uniform and total
elongations of the three investigated steels. The high
ductilities are caused by the superimposition of such
plasticity mechanisms as dislocation glide in the
austenite as well as TRIP and TWIP effects. The steels
exhibit a maximum in both uniform and total
elongations at intermediate temperatures. As the
carbon concentration increases, the maximum
elongation occurs at a lower temperature. This is the
case for the alloys 19NC1505 and 19NC1515. For
these alloys, the temperatures associated with the
maximum elongation are just below their respective
temperatures. This does not hold for the 19NC1525
alloy in which the maximum uniform and total
elongations occur at 80 °C, higher than its Mdγ→αʼ
temperature of 42 °C. Therefore, the highest total
elongation of 74% at 80 °C cannot be associated with
the strain-induced αʼ-martensite formation. To identify
occurring deformation processes at 80 °C of the steel
19NC1525, Electron Backscattering Diffraction
(EBSD) measurements with a step size of 0.1 μm were
carried out on a tensile specimen strained until
fracture. According to the EBSD phase analysis in
Figure 8a, the microstructure remains fully austenitic
and free of ε-martensite. To evaluate a possible
deformation-induced twin formation, austenite
boundaries satisfying the twin orientation relationship
were sought. According to Figure 8b, in addition to
low- and high-angle grain boundaries, a high density
of twin boundaries were observed in the
microstructure of the 19NC1525 alloy, confirming the
occurrence of the TWIP effect.
The fracture elongation of all alloys decreases
significantly with increasing strain-induced αʼ-
martensite fractions at temperatures below the Mdγ→αʼ
temperature. Because of the high interstitials content
of the alloys, the transformed martensite is unable to
deform sufficiently so that the strain is mainly
accommodated by the softer austenite phase. Reduced
fraction of austenite which remains available for
plastic deformation leads to deteriorated elongation
properties below the Mdγ→αʼ
temperature. Formation of
strain-induced martensite also results in a continuous
decrease in the post-uniform elongation of alloys. This
is represented for different alloys with the aid of
hatched areas in Figure 7.
Figure 7. Temperature dependence of the uniform and
total elongation.
4. Conclusions
The influence, on the microstructure formation
processes and mechanical properties, of carbon
concentration and temperature was studied in three
high-alloy Fe-19Cr-4Ni-3Mn-0.15N cast steels. The
alloys containing 0.05%C and 0.15%C showed an
initial microstructure consisting of an austenitic matrix
and delta ferrite fractions of 9% and 4%, respectively.
The alloy containing 0.25%C was fully austenitic
before tensile deformation. No as-quenched martensite
formed in the investigated cast steels by liquid
nitrogen treatment at -196 °C. Strain-induced γ→αʼ
martensite formation was, however, triggered in all
alloys below their respective Mdγ→αʼ
temperature. The
increase of the carbon content was found to increase
the austenite stability and depress the Mdγ→αʼ
temperature. The strain-induced αʼ-martensite
formation preferentially occurred at the intersections
of deformation bands in the austenite. The austenitic-
ferritic steel grade with 0.05%C exhibited lower
elongations in comparison to the other steels. This
could be explained on the one hand by the existence of
9% delta ferrite and thus the reduced fraction of
metastable austenite which is essential for the TRIP or
TWIP effects. In the steel with 0.15%C, the
occurrence of the TRIP effect at -40 °C resulted in a
tensile strength of 1326 MPa combined with moderate
elongations. The steel containing 0.25%C exhibited a
fracture elongation of 74% at 80 °C. This high
plasticity was microstructurally associated with
intensive deformation-induced twinning. The results
demonstrate that with increasing carbon content, the
austenite is stabilized and the αʼ-martensite fraction is
reduced, causing an attenuated strengthening by TRIP
effect.
Acknowledgements
The authors gratefully acknowledge the German
Research Foundation (DFG) for the financial support
of this research which is part of the Collaborative
Research Center 799 (CRC 799). Special thanks
should be given to Dr. mult. rer. nat. O. Fabrichnaya
for the Thermo-Calc calculations.
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