Volume 6, Issue 1, January 2019 ISSN: 2348-8565 (Online)
International Journal of Modern
Engineering and Research Technology
Website: http://www.ijmert.org Email: editor.ijmert@gmail.com
Anisotropy in Plain Strain Fracture Toughness (Kic) Property of
Maraging Steels
K. Srinivasa Vadayar Kumud Kant Mehta
Associate Professor Principal Scientific Officer
Department of Metallurgical Engineering Office of the Regional Director, Aeronautical Quality
JNTUH College of Engineering Assurance (ORDAQA)
Hyderabad (T.S.) [INDIA] Koraput (O.R.), [INDIA]
Email: ksvadayar@gmail.com Email: kkm2921@gmail.com
Lingampalli Prasad
PG Student
Department of Metallurgical Engineering
JNTUH College of Engineering
Hyderabad (T.S.) [INDIA]
Email: lingampalli_prasad@yahoo.in
ABSTRACT during extrusion (or) prior deformation
processes.
Maraging steels are special class of steels
having excellent strength coupled with good Keywords:— Maraging steels, Double aging,
fracture toughness. These materials are Tensile Strength, Fracture toughness (KIC),
considered as most suitable candidate
materials for aeronautical and missile I. INTRODUCTION
applications for which a good combination of
Maraging steels are low carbon Iron-Nickel
tensile strength and fracture toughness are
alloys with additional alloying elements of
prime requirements. Maraging steel of grade
Cobalt, Molybdenum, Titanium and
C 250 has been chosen for present study. The
Aluminium. These are precipitation
plain strain fracture toughness (KIC) values in
strengthened material used for many
longitudinal and tangential orientation of
applications in wrought forms due to its
crack propagation display anisotropy for
exceptional combination of strength,
single aged starting material. The extent of
toughness, good formability, excellent
anisotropy decreases as the temperature of
machinability and weldability [1]. Another
second aging treatment increases during
important characteristic of Maraging steel is
double aging. The optimum isotropy in
fracture toughness property which is also an
fracture toughness is achieved with second
area of interest for rocket motor casing
aging at a temperature of 485°C. The reason
material [2-4]. The optimum values of
for maximum isotropy in fracture toughness
fracture toughness can be achieved for a
values by re-aging treatments at 485°C / 6 Hrs
particular grade of Maraging steel as a
can be attributed to transformation of higher
function of heat treatment has already been
volume fraction of martensite to precipitates
established [5-14]. Marging steel of grade C
during re-aging, which in turn completely
250 (AMS 6512) is used extensively in
eliminate the impression of flow lines formed
single aged condition and display excellent
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Volume 6 | Issue 1 | January 2019
Anisotropy in Plain Strain Fracture Toughness (Kic) Property of Maraging Steels
Author(s) : K. Srinivasa Vadayar, Kumud Kant Mehta, Lingampalli Prasad | Hyderabad
combination of strength, ductility and isotropic with a suitable heat treatment
toughness. But the problem which is while maintaining the same level of
frequently encountered by designer is strength without deteriorating its ductility.
anisotropy in tensile strength and fracture The specimens for mechanical and
toughness properties. It has been shown that metallographic tests were taken from
the fracture toughness of Maraging steel starting material, flow formed and flow
varies with specimen orientation i.e. the formed plus aged material as per the flow
Maraging steel displays anisotropy in sheet of experiment shown in Figure 1.
fracture toughness. The anisotropy in
fracture toughness misleads the designer to
realize a component having requirement of
isotropic properties. This generally over
estimates (or) underestimates the designer’s
data. The anisotropy in tensile properties
for tube material is generally judged by
their 0.2% proof strength and UTS values
with respect to longitudinal and tangential
orientations.
Several ways have been suggested to arrest
cracks in particular direction to make
fracture toughness an isotropic property in
various alloys, but there are scarcity of
literature in the field of anisotropy of
fracture toughness of Maraging steel.
Therefore, an attempt has been made to
suggest a technique to reduce the extent of
the anisotropy in fracture toughness while
maintaining the same level of typical
strength. In this process, the effect of Figure 1: Flow Chart of Experiment
second aging on fracture toughness and
tensile behaviour of maraging steel has Material obtained from double vacuum
been studied extensively. treatment i.e VIM(Vacuum Induction
Melting) & VAR (Vacuum Arc Remelting)
II. EXPERIMENTAL WORK was extruded to a tube of Maraging steel of
grade C 250 having of size OD (Outside
The present work entitled “Anisotropy in Diameter) -375 mm, ID (Inside Diameter) -
plain strain fracture toughness (K IC) 340 mm with 570 mm length. This
property of maraging steel of grade C 250” extruded tube was double homogenised at
is an attempt to address the above issues. 950°C / 2 Hrs / Water quenching followed
This experiment aims to analyse the tensile by solution annealing at 820°C / 3 Hrs / Air
flow and work hardening behaviour of flow cooling (AC). Chemical composition of
formed and flow formed plus aged high starting material was evaluated using
st rength m araging st eels t hrough optical emission spectroscope (OES, Make:
constitutive relations and correlation of Spectrolab M10) for all important elements
microstructures with mechanical properties. except carbon and sulphur. The carbon and
An attempt has been made to suggest a sulphur contents were analysed by using
technique to make the Maraging steel Leco CS600 analyser.
International Journal of Modern Engineering and Research Technology
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Volume 6 | Issue 1 | January 2019
Anisotropy in Plain Strain Fracture Toughness (Kic) Property of Maraging Steels
Author(s) : K. Srinivasa Vadayar, Kumud Kant Mehta, Lingampalli Prasad | Hyderabad
As shown in figure 2 samples were selected round specimen as shown in figure 3. In
from extruded tube for study the effect of present study Instron 8500 universal testing
double aging on fracture toughness and machine was used for tensile test. An
tensile properties. Initially, the starting extensometer of Instron make having gauge
material was aged at temperature 485°C / 6 length of 25 mm and calibration class of 0.5
Hrs / Air cooling for evaluation of tensile was used to measure average linear strain
and fracture toughness properties and with accuracy of 1x10 -4 . The crosshead
designated as numeral 1 throughout the speed maintained during test was 0.5 mm /
thesis. Subsequently, these aged samples min so as to maintain the strain rate of
were pre-cracked and re-aged at three 0.0088 mm/mm/min which is within the
different aging cycles. These three different range specified in ASTM standard.
aging cycles were denoted using numerals 2
(150°C / 6 Hrs / Air cooling), 3 (250°C / 6 The starting material in single aged
Hrs / Air cooling) and 4 (485°C / 6 Hrs / condition was designated as TL1 and TT1,
Air cooling) along with some prefix where the second alphabetical letters L and
indicating the type of test referred in that T denotes directionality of the specimens
section of the thesis, respectively. that is longitudinal and tangential,
respectively. The suffix 1 represents single
Fracture toughness sample aging of starting material. Similarly, the
double aged specimens were designated as
TL2, TT2; TL3, TT3 and TL4, TT4
corresponding to tensile specimens in 2, 3
and 4 aged conditions, respectively.
Gauge Length(G)25.0 mm Diameter (D)
6.25 mm Radius(R) 5 mm Reduced section
length (A)32 mm
Figure 3: Round tensile specimen drawing
[ASTM E 8 M]
Fracture Toughness Test
Plain strain fracture toughness test was
Figure 2: Schematic representation of sample carried out with a CT (compact tension)
extraction plan from extruded tube. specimen as shown in the figure 4. The
specimens were extracted from starting
Tensile Test material and aged with a heat treatment
Room temperature tensile test was carried cycle of 485°C / 6 Hrs / Air cooling.
out on round specimens selected from Subsequently, all these aged specimens
extruded tube material. Tensile test was were fatigue pre-cracked as per the cycles
carried out as per standard ASTM E 8 M on given in ASTM standard. The pre-cracked
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Volume 6 | Issue 1 | January 2019
Anisotropy in Plain Strain Fracture Toughness (Kic) Property of Maraging Steels
Author(s) : K. Srinivasa Vadayar, Kumud Kant Mehta, Lingampalli Prasad | Hyderabad
specimens were then subjected to different Scanning Electron Microscopy
aging treatments. The specimens with single Fracture toughness tested specimens were
aging treatment denoted as FTCL1 and characterized by using scanning electron
FTLC1 were directly subjected to fracture microscope of make Carl Zeiss of model
toughness testing using machine of make EVO 18 attached with EDX facility.
Waltar Bai W-B as per the standard ASTM Fracture study was carried out in secondary
E 399. The letters FT denotes fracture electron mode. Energy dispersive X-ray
toughness, whereas letters CL and LC are spectroscopy was carried out during SEM
the crack plane orientations defined in analysis to find out chemical composition
ASTM E 399 standard as shown in Fig. 5.
of some particles in the matrix.
Similarly, the designation for double aged
specimens was given as FTCL2, FTLC2; III. RESULTS AND DISCUSSIONS
FTCL3, FTLC3 and FTCL4, FTLC4 for
samples corresponding to second aging The analyzed chemical composition of
cycle of 2, 3 and 4. starting material is given in Table.1. The
chemical composition of each element is
given in weight percent and lie well within
the range of specification of Maraging steel
grade C 250 .
As shown in Figure 6 the microstructure of
annealed sample displays homogeneously
distributed lath martensite. The average
grain size of annealed samples calculated
using linear intercept method of ASTM E
112 is ~ 65 m. The starting material in
Figure 4: Fracture toughness specimen drawing aged condition consists of colony of lath
[ASTM E 399] martensite.
Figure 6: Optical micrograph of (a) Solution Annealed
Figure 5: Schematic of crack plane orientation for and (b) Aged
hollow cylinder [ASTM E 399]
TABLE 1: CHEMICAL COMPOSITION OF EXTRUDED AND SOLUTION ANNEALED MARAGING STEEL
OF GRADE C 250
Elements Wt % C Si Mn Ti S P Ni Al Cr Co Mo Fe
Specified val- 0.03 0.1 0.1 0.3 0.01 0.01 17.0 0.05 0.5 7.0 4.6 Bal.
ues in Max Max Max 0.5 Max Max 19.0 0.15 Max 8.5 5.2
Obtained val-
ues 0.005 0.005 0.01 0.44 0.002 0.005 18.09 0.11 0.01 8.04 4.89 Bal
International Journal of Modern Engineering and Research Technology
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Volume 6 | Issue 1 | January 2019
Anisotropy in Plain Strain Fracture Toughness (Kic) Property of Maraging Steels
Author(s) : K. Srinivasa Vadayar, Kumud Kant Mehta, Lingampalli Prasad | Hyderabad
Tensile Properties compared to L. The overall proof stress
values remain nearly same for single and
Solution Annealed and Aged extruded tube double aged conditions except for
material specimens TL4 and TT4. The values of
The tensile parameters of extruded tube in 0.2% proof stress are moderately high for
solution annealed and aged material are TL4 and TT4 specimens. Similar variation
given in Table 2. It can be seen that after can be observed for values of UTS.
aging the strength values are significantly However, the values of percent elongation
improved while the elongation is reduced. remain almost same for all the specimens
The 0.2% proof stress values are almost subjected to either single or double ageing.
same for both longitudinal and tangential The engineering stress - strain curves for
aged specimens. The UTS and elongation single and double aged specimens in L and
values moderately decreases and slightly T orientations have been shown in Figures.
increases, respectively in tangential 7 and 8. Both the L and T specimens
direction as compared to axial orientation. display continuous and uniform decrease in
stress with increase in strain till fracture. It
Single and double aged material is evident that the double aged specimens
Tensile test parameters of starting materials TL4 and TT4 have higher values of stresses
evaluated on single and double aged as compared to other specimens after yield
specimens in both Longitudinal (L) and stress. This difference is very clear for TT4
Tangential (T) directions are given in Table as compared to TL4 specimens as can be
-3. It is seen that the 0.2 % proof stress seen in the inset figures provided within the
values are always lower in T direction as figures 7 and 8.
Table 2: Tensile Test Parameters of Extruded Tube in Solution Annealed and Aged
Conditions.
Tensile test parameters
Condition of material 0.2% Proof Ultimate ten- % Elongation at
stress in sile stress gauge length
MPa (UTS) in MPa 4*Diameter
Extruded and Solution annealed
865 1045 19
(longitudinal direction)
Extruded and Solution annealed
+
1725 1786 13
aged
(longitudinal direction)
Extruded and Solution annealed
+
1726 1778 14
aged
(tangential direction)
International Journal of Modern Engineering and Research Technology
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Volume 6 | Issue 1 | January 2019
Anisotropy in Plain Strain Fracture Toughness (Kic) Property of Maraging Steels
Author(s) : K. Srinivasa Vadayar, Kumud Kant Mehta, Lingampalli Prasad | Hyderabad
Table 3: Tensile Test Parameters of Single and Double aged Materials in Longitudinal and
Tangential Direction
Tensile test parameters
Sample
Condition of material identifica- 0.2% Proof Ultimate ten- % Elongation
tion stress in sile stress at gauge
MPa (UTS) in MPa length 50 mm
Solution Annealed and TL1 1727 1786 13
aged at 485 °C / 6 Hrs /AC
TT1 1726 1778 14
Solution Annealed and
aged at 485 °C / 6 Hrs /AC TL2 1744 1805 14
+ Fatigue cracking + Re-
aging at 150°C X 6 Hrs
TT2 1728 1782 14
AC
Solution Annealed and
aged at 485 °C / 6 Hrs /AC TL3 1733 1788 13
+ Fatigue cracking + Re-
aging at 250°C / 6 Hrs /
TT3 1726 1782 14
AC
Solution Annealed and TL4 1785 1844 13
aged at 485 °C / 6 Hrs /AC
+ Fatigue cracking + Re-
aging at 485°C / 6 Hrs / TT4 1780 1841 13
AC
Table 4: Voce’s Fitted Flow Curve Parameters of Single and Double aged Specimens in L and
T orientations.
S pe ci m en
s S -KV
Condition Identifica- s0 (MPa) (R2)*
(MPa) (MPa)
tion
TL1 1823 1605 552 0.995
Single aged starting material
TT1 1810 1564 652 0.997
TL2 1843 1625 570 0.992
TT2 1817 1620 576 0.999
TL3 1824 1591 639 0.995
Double aged starting material
TT3 1818 1570 601 0.993
TL4 1871 1661 587 0.999
TT4 1873 1684 605 0.995
International Journal of Modern Engineering and Research Technology
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Volume 6 | Issue 1 | January 2019
Anisotropy in Plain Strain Fracture Toughness (Kic) Property of Maraging Steels
Author(s) : K. Srinivasa Vadayar, Kumud Kant Mehta, Lingampalli Prasad | Hyderabad
The true stress - strain curves for single and
double aged materials have been shown in
Figure 9. The flow curves in L orientations
are overlapping each other in elastic regime
in L orientations for both single and double
aged specimens. However, in the complete
plastic strain regime TL4 is observed to
have higher stresses up to UTS as compared
to TL1, TL2 and TL3 specimens (Fig. 9a).
On the other hand, the elastic and plastic
regimes are not overlapped in T orientation
for both single and double aged specimens.
The flow stress curve for TT4 specimen is
Figure 7: Engineering stress - strain curves in L
observed above the flow stress curves of
orientation for single and double aged specimens of
starting material. other specimens such as TT1, TT2 and TT3.
True stress and true plastic strain curves for
single and double aged specimens in L and
T orientations are given in Fig. 10(a-b). An
attempt has been made to best fit the
experimental flow curves data. For this, a
number of models, such as Hollomon,
Ludwik, Ludwigson etc., have been tried
but finally it was observed that the Voce’s
relation fits best with R 2 values between
0.992-0.999 and 0.993-0.999 for L and T
specimens, respectively which reflect an
adequate fit of flow curves data.
Figure 8: Engineering stress - strain curves in T E
orientation for single and double aged specimens of
starting material. q-1
Figure 9: True stress - strain curves of single and double aged specimens of starting material in (a) L and (b) T
orientations.
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Anisotropy in Plain Strain Fracture Toughness (Kic) Property of Maraging Steels
Author(s) : K. Srinivasa Vadayar, Kumud Kant Mehta, Lingampalli Prasad | Hyderabad
Figure 10: True stress - plastic strain curves in (a) L and (b) T orientations for single and double aged specimens of
starting material.
Flow curve parameters of all the single and aged followed by fatigue pre-cracking.
double aged specimens in L and T direction After pre-cracking the specimens CL1 and
is fitted with (Eq.1) and corresponding LC1 were directly subjected to fracture
values of flow parameters (S, 0 and K v) toughness testing whereas the specimens
are given in Table-6. The numerical value CL2, LC2, CL3, LC3, CL4 and LC4 were re
of K v decreases for L specimens from TL1 -aged as per cycle given in Table-5 and then
to TL3 and then slightly increases for subjected to fracture toughness testing. The
specimen TL4. In contrast, the values of K v variation in K IC values can be observed in
increases and then decreases from TT1 to Table-5. The K IC values of CL orientation
TT2 and TT2 to TT4, respectively. On the specimens increases remain constant and
other hand, the value of S increases, then decreases again to initial values from
decreases and then sharply increases from CL1 to CL2, CL2 to CL3 and CL3 to CL4,
specimens TL1 to TL2, TL2 to TL3 and respectively. On the other hand, the values
TL3 to TL4, respectively. In contrast, the of K IC continuously decrease from
value of S continuously increases from specimens LC1 to LC4. It is to be noticed
TT1 to TT4. The values of 0 increases that the specimens CL3 and LC3 have same
decreases and then sharply increases from KIC values of ~118 MPa√m. Similarly, the
specimens TL1 to TL2, TL2 to TL3 and specimens CL4 and LC4 have same K IC
TL3 to TL4, respectively. It is observed that value that is 113 MPa√m. The anisotropy in
the variation for 0 is also same for T fracture toughness is highest between CL1
orientation samples. It can be noticed that and LC1 for single aged specimens. This
the values of S and 0 are always higher anisotropy in fracture toughness decreases
for fourth double aging cycle as compared slightly after second aging of specimens to
to single and other two double aging cycles. 150º C / 6Hrs / AC (CL2 and LC2). When
second aging is performed at 250º C /
IV. FRACTURE TOUGHNESS 6Hrs / AC and 485º C / 6Hrs / AC it is
observed that the CL and LC specimens
The plain strain fracture toughness (K IC) display isotropic fracture toughness values.
values of single and double aged starting
materials are given in Table-5. Initially, all
the specimens of starting materials were
International Journal of Modern Engineering and Research Technology
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Volume 6 | Issue 1 | January 2019
Anisotropy in Plain Strain Fracture Toughness (Kic) Property of Maraging Steels
Author(s) : K. Srinivasa Vadayar, Kumud Kant Mehta, Lingampalli Prasad | Hyderabad
Table-5. Fracture Toughness Values of V. FRACTOGRAPHY
Single and Double aged Starting Materials.
Condition of Material Scanning electron microscopic (SEM)
images of fractured tensile test specimens
Condition of Sample Fracture which were subjected to single and double
material identifi- toughness aging treatments have been shown in figure
[Initially all sam- cation values (KIC) 11. All the SEM fractographs show
ples solutionized (MPa√m ) concentric equiaxed ductile dimples of
at 820º C / 3Hrs / different sizes.
AC]
CL1 111.315 (a) Single Aging at 485°C
485º C / 6Hrs / AC
LC1 127.895
485º C / 6Hrs / AC + CL2 117.96 (b) Double Aging 485°C & 150°C
Pre-cracking + Aging
at 150º C / 6Hrs / AC
LC2 124.446
(c) Double Aging 485°C & 250°C
485º C / 6Hrs / AC + CL3 117.661
Pre-cracking +Aging
LC3 117.92
at 250º C / 6Hrs / AC (d) Double Aging at 485°C
485º C / 6Hrs / AC + CL4 112.69
Pre-cracking+Aging
at 485º C / 6Hrs / AC LC4 112.90
Longitudinal Circumferential
Figure 11: SEM images of tensile fractured samples
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Volume 6 | Issue 1 | January 2019
Anisotropy in Plain Strain Fracture Toughness (Kic) Property of Maraging Steels
Author(s) : K. Srinivasa Vadayar, Kumud Kant Mehta, Lingampalli Prasad | Hyderabad
VI. CONCLUSIONS Approach to Selection of Material and
Manufacturing Processes for Rocket
Following conclusions are made based on Motor Cases” using Weighted
the results obtained from the present work: Performance Index, 2002, Vol.11, pp.
The tensile flow curves for single and 444–449.
double aged specimens show best fit with [5] NASA, National Space Transportation
Voce’s model and the maximum strength is System, Vol. 1 and 2, U.S.
realized only after re-aging at 485°C / 6Hrs/ Government Printing Office,
Air cooling. Washington, DC, June 1988.
The plain strain fracture toughness (K IC)
[6] P . Ludwi k, “El em ent e der
values in longitudinal and tangential
Technologischen Mechanic Verlag”,
orientation of crack propagation display
Von Julius Springer, Leipzig, 1909,
anisotropy for single aged starting material.
The extent of anisotropy decreases as the pp. 32.
temperature of second aging treatment [7] J.H. Hollomon, “Tensile
increases during double aging. The Deformation”, Trans. AIME,
optimum isotropy in fracture toughness is
1945, Vol. 162, pp. 268-290.
achieved at second aging temperature of
485°C. The reason for maximum isotropy in [8] D.C. Ludwigson, “Modified Stress-
fracture toughness values by re-aging Strain Relation for FCC Metals and
treatments at 485°C / 6 Hrs can be Alloys”, Metall. Trans., 1971, Vol. 2,
attributed to transformation of higher pp. 2825–2828.
volume fraction of martensite precipitates
during re-aging, which in turn completely [9] E. Voce, The Relationship Between
eliminate the impression of flow lines Stress and Strain from Homogeneous
formed during extrusion (or) prior Deformation, J. Inst. Met., 1948, Vol.
deformation processes. 48, pp. 537-562.
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[11] C.H. Crussard and B. Jaoul,
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International Journal of Modern Engineering and Research Technology
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Volume 6 | Issue 1 | January 2019
Anisotropy in Plain Strain Fracture Toughness (Kic) Property of Maraging Steels
Author(s) : K. Srinivasa Vadayar, Kumud Kant Mehta, Lingampalli Prasad | Hyderabad
–3599.
[13] R. F. Decker, J. T. Eash, and A. J.
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[14] W. Sha and Z. Guo, Maraging Steels:
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*****
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