This document discusses the development of bulk nanocrystalline steel. Previous attempts to create ultra-strong materials by decreasing crystal size were limited by strength reductions as size increased due to more defects. A new method is proposed using carbide-free bainite transformation which is displacive and can generate very fine grains down to 10 nanometers. This bainitic transformation meets the design criteria of allowing large components, ultra-fine grain size, and low-cost production method required for bulk nanocrystalline steel.

1. Maraging Steel
Dr. Muhammad Ali Siddiqui
Assistant Professor
Metallurgical Engineering Department, NED
University of Engineering and Technology
Maraging Steels: Very strong
martensitic steels with
strength greater than 3 GPa

2. https://www.phase-
trans.msm.cam.ac.uk/2009/Sagamore/Sagamore.html
Maraging Steels: Very
strong martensitic steels
with strength greater than 3
GPa .

3. https://www.phase-
trans.msm.cam.ac.uk/
2009/Sagamore/2.pdf

4. Maraging Steel
1. Introduction and General Characteristics:
 Maraging steels are characterized with superior strength
combined with excellent toughness properties and
weldability. without loss of malleability. OR
 Maraging steels are carbon free iron-nickel alloys with
additions of cobalt, molybdenum, titanium and aluminium.
 The term maraging is derived from the strengthening
mechanism, which is transforming the alloy to martensite
with subsequent age hardening. (Martensite + Ageing)
• special class of steel contain No-carbon but provides “ultra-
high-strength”
• Their strength comes from precipitation of intermetallic
compounds like (Mo, Ti & Al)Ni3 “NOT FROM CARBON”.
• High strength is maintained up to 350 oC. …? How?

5. cont…
• Since ductile Fe-Ni martensites are formed upon
cooling, cracks are non-existent or negligible.
• These steels can be nitrided to improve case hardness.
• C is considered as an impurity element in these steel
and it should be kept below 0.03%.
2. Application:
• Considered as Military grade … crankshafts, gears, and
the firing pins of automatic weapons that cycle from hot
to cool repeatedly under substantial load.
• Missile Nose. aerospace, e.g. undercarriage parts and
wing fittings, tooling & machinery , e.g. extrusion press
rams and mandrels in tube production, gears
• Ordnance components and fasteners.

6. 3. Types of Maraging Steels
Well known grades 18%, 20% & 25%

7. 4. Production of Maraging Steel
E.A.F/ V.I.M
L.F
V.D/V.C.D
I.C
V.A.R
Shaping (Rolling/Forging )
Solution Treatment + Ageing
Nitriding (If Required)
Most
Important
Step

8. 5. Heat Treatment of Maraging Steel:
Solution
Treatment
+
Aging

9. • steels are solution treated at 820 oC , to absorb all precipitates or
alloying elements & produce uniform austenitic structure.
• upon cooling in air/quenching, an Fe-Ni BCC Martensite is formed
instead of ordinary Tetragonal Martensite (Fe-C).
• This is of lath-like BCC form, softer & tougher than ordinary
martensite but heavily dislocated martensite.
(means high dislocation density = high energy = means favorable site
for precipitation)
• Upon ageing at 480oC for 3 or more hours, coherent precipitates of
intermetallic compound (Mo, Ti & Al)Ni3 are formed.
• The main function of “Co” seems to produce more sites for the
nucleation of (Mo, Ti & Al)Ni3 precipitates. Or Co reduces the
solubility of Ti, Mo & Al in the matrix as a result theses increases the
volume fraction of rich precipitate.

10. 6. Physical Metallurgy of Maraging Steel /
• Effects
a) Ni = Austenitic Stabilizing Element, Increases hardenability, resistant to
fatigue & corrosion
b) Co/Mo = Retain hardness at high temperature, good wear resistant.
c) Ti = Stabilize Ferrite, Refine Grain structure, raises creep strength.
d) Al = stabilize ferrite, form nitride, refine grain size.
Element 200 250 300 350
Substitutional Element Ni 17-19 17-19 18-19 18-19
Substitutional Element Co 8-9 7-8.5 8.5-9.5 11.5-12.5
Substitutional Element Mo 3-3.5 4.5-5.2 4.6-5.2 4.6-5.2
Substitutional Element
(Partially Soluble)
Ti 0.15-0.25 0.3-0.5 0.5-0.8 1.3-1.6
Substitutional Element Al 0.05-0.15 0.3-0.5 0.5-0.8 1.3-1.6
Substitutional Element Fe Balance Balance Balance Balance
• 18%Ni Maraging Steel Grades

11. 6.1 Fe-Ni Phase Diagram

13. • If the precipitation hardening temperaure exceeds 500oC,
over-aging results the strength begin to fall due to reversion of
the austenite.
• similarly increases the time for aging the same effect will be
produce.

14. Why Ni is selected for Maraging steels ?
How it can with stand high strength at high temperature?
• High-nickel steels introduces considerable thermal
hysteresis in the polymorphic transformation.
• steel containing 15% nickel will not begin to transform on
cooling until a temperature of about 250oC has been
reached, when martensite begins to form.
• On re-heating the structure, however, martensite does not
begin to change to austenite until a temperature
approaching 600oC.
• This hysteresis, or lag, in transformation is fundamental to
the use of nickel in maraging steels.

16. Bulk
Nanocrystalline
Steel
Dr. Muhammad Ali
Siddiqui
Assistant Professor
Metallurgical Engineering Department,
NED University of Engineering and
Technology
IMR-CAS, China 25-04-2019,
H. K. D. H. Bhadeshia (Right)

18. World first Bulk Nanostructured steel ever created
https://www.phase-
trans.msm.cam.ac.uk/2005/SWpaper/index.html
https://www.phase-
trans.msm.cam.ac.uk/2005/nanocrystalline.Bhadeshia.IS.2
005.pdf
https://www.tandfonline.com/doi/pdf/10.1179/174328405X639
99

19. • Imagine, a steel
1.Exceptionally strong, = GPa
2.Be made in large chunks = bulk crystalline
3.Easy to manufacture
4.Low cost which is affordable = cheap
How ?

20. Problem: to design a bulk
nanocrystalline steel which is
very strong, tough, cheap ….

21. Before describing this novel
material, it is important to
review the meaning of
strength,

22. • Put an apple on 1 m2 = 1 pa
• 100 MPa = I00 million apples on 1 m2
• 1GPa = billion apples on 1 m2
• 1TPa = 1000 billion apples on 1 m2
Understanding unit

23. • Brenner achieved
tensile strength =
greater than 13 GPa
in an iron whisker
about 1.5 mm (1500
µm) in length.
• Theoretically =
possible to achieve a
tensile strength of 21
or 22 GPa in ideal
crystals of iron.
Brenner, 1956
Theoretical Strength

24. • strength of a crystal
increases sharply as it is
made smaller because the
probability of avoiding
defects increases.
• Note these are the crystals
only.
• Strength collapses as we
make bigger in size
because of defects
increases.
Brenner, 1956
Now remember Aim ~ 22 Gpa, if we eliminating
the defects in the materials.
whisker

25. 1. Strengthening by Deformation
• It has been possible for some time to obtain
commercially, steel wire which has an ultimate tensile
strength of 5.5 GPa and yet is very ductile in fracture.
• Made by Kobe Steel Japan.
Scifer, Scientific Iron
Scifer, 5.5
GPa with
ductility!

26. • Scifer, as the wire is known.
• Made by drawing a dual-phase
microstructure of martensite
and ferrite in Fe-0.2C-0.8Si-
1Mn wt% steel in the form of
10 mm diameter rods, into
strands which individually have
a diameter of about 8 µm.

27. Figure: Field-ion microscope image of Scifer,
showing the very fine dislocation cell structure.
Bhadeshia and Harada, 1993
The dislocation cell size
in the material becomes
about 10–15 nm, from
which much of the
strength of Scifer is
derived.
Figure 4b:
Comparison of size-sensitivity of
single-crystals whiskers of iron and
Scifer
The fact that the properties are
here achieved by introducing
defects, also means that the
strength of Scifer is insensitive
to its size as shown in Fig. 4b.

28. • See strength 5.5 GPa .
• We can not make a knot with Carbon fiber which has 3.3GPa
strength & virtually zero ductility.
• Scifer, as the wire is known is made by drawing a dual-phase
microstructure of martensite and ferrite in Fe–0.2C–0.8Si–1Mn
(wt-%) steel.
So can we make a cable bridge from this = ?

29. 1 Denier: weight in grams, of 9 km of
fibre or yarn.
50-10 Denier
Scifer is 9 Denier

30. • Scifer is just 9 denier in this classification; this highlights one
of the difficulties in using deformation to increase strength.
• The deformation necessary to accumulate a large number
density of defects limits the size and form of the product, in
the case of Scifer to that of a textile thread.
• Deformation processes such as equichannel angular processing
and accumulative roll-bonding maintain the overall dimensions
but the range of shapes that can be achieved is limited.
So we can use it for cutting semi conductors
not for making bridge cables
So can we make a cable bridge from this = ?

31. 2. Strengthening of Carbon Nanotubes
Morinobu Endo, 2004
Single-walled carbon tubes can be imagined to be constructed from
sheets of graphene consisting of sp2 carbon arranged in a two-
dimensional hexagonal lattice.

32. Claimed strength of carbon
nanotube is 130 GPa
Edwards, Acta Astronautica, 2000
Claimed modulus is 1.2 TPa
(1000 GPa) 6X greater than
Steel
Terrones et al., Phil. Trans. Roy. Soc., 2004

33. Fig: Space-elevator concept (originally due to Arthur
C. Clark), requiring a cable 120 000 km in length.
The Cable would be launched in both directions from
geosynchronous orbit at a height of 36 000 km
People starting research to built an Space elevator
(Russian Concept)
What is wrong with this ?

34. •Table: The measured strength
of carbon nanotube based
ropes as a function of their
length.
•The calculation of strength is
based on the cross-sectional
area of the-nanotube shell
•In the latter case the strength
of short nanotubes decreases
by about an order of magnitude.
https://www.phase-
trans.msm.cam.ac.uk/2005/SWpaper/index.html

35. as soon as make it big the strength
collapses due to increase in the
defects as we scale up
[as we know that about Fe in 1956.
(22 GPa) ]
Equilibrium number of defects (1020)
Strength of a nanotube rope 2 mm
long is less than 2000 MPa.

36. Limit of Nanotube

37. •Strength produced by deformation
limits shape: wires, sheets...
•Strength in small particles relies on
perfection. Doomed as size increases.
Summary
So far; we are unsuccessful to produce Bulk Nanocrystalline Steel

38. 3. Stored Energy :
Fracture of Gigatubes
Suppose that gigatubes of carbon could be made capable
of supporting a stress of 130 GPa.
Would this allow for safe engineering design?
One aspect of safe
design is that fast
fracture should be
avoided; most metals
absorb energy in the
form of plastic
deformation before
ultimate fracture.
Energy absorption in
an accident is a key
aspect of automobile
safety.
Carbon nanotubes are not in
this sense defect tolerant; their
deformation prior to fracture is
elastic.

39. • The stored energy density in a tube
stressed to 130 GPa, given an elastic
modulus along its length of 1.2 TPa is
in excess of that associated with
dynamite,
Comment: Structures in tension, which reversibly store energy
far in excess of their ability to do work during fracture must be
regarded as unsafe.

40. 4. Thermomechanical
processing
• Smallest size possible in polycrystalline substance?
• Back in 1960 (Micro alloying = dramatic change in
grainsize improves the quality of steel)
• 10 billion tons of steel are in service today by micro
alloying only. (HSLA steels)

41. • During the processing, fine austenite (γ)
grains are generated by a combination of
deformation and recrystallisation; the
austenite finally transforms into fine grains of
ferrite (α), with a size typically of 10 µm.
• The recent search has been for processes
which reduce the grain size dramatically to
less than 1 µm .
• Fine grains represent one of the few
mechanisms available to increase both
strength and toughness.

42. Yokota & Bhadeshia, 2004
Fig. A

43. What is recalescence?
• In simpler terms, when something transforms from
one state to another (like from a solid to a liquid), it
releases heat. This process is called recalescence.
Now, if this heat is released really quickly, it can be
challenging for it to spread out or dissipate.
• Example: It's like when you microwave something for
too long, and it becomes very hot in one spot but
stays cool in another. The quick release of heat
during recalescence makes it hard for the heat to
spread out smoothly.

44. • The reason for this is recalescence, which is the heating
of the sample caused by release of the latent heat of
transformation at a rate so high that it cannot easily be
dissipated by diffusion.
• This recalescence reduces the effective undercooling and
hence the driving force for transformation.
• It is seen from Fig. A that the recalescence corrected
curves show better agreement with the experimental data,
indicating that at large undercoolings, the achievement of
fine grain size is limited by the need to dissipate enthalpy
during rapid transformation.

45. Limit of Thermomechanical Treatment

46. Thermomechanical processing
limited by recalescence
Summary
Need to store the heat
Reduce rate
Transform at low temperature
Heating up the steel by itself

47. Courtesy of Tsuji,
Ito, Saito,
Minamino, Scripta
Mater. 47 (2002)
893.
Howe, Materials
Science and
Technology 16
(2000) 1264.
Another
problem = ?

48. Fine crystals by transformation
1. Introduce work-hardening
capacity
2. Need to store the heat
3. Reduce rate
4. Transform at low temperature
Requirement for Scale up:

49. Design criteria for Bulk
Nanocrystalline Steel
1. It should ideally be possible to
manufacture components which are large
in all dimensions, not simply in the form of
wires or thin sheets.

50. 2. There are commercially available steels
in which the distance between interfaces is of the order
of 250–100 nm. The novelty is in approaching a
structural scale in polycrystalline metals that is an order
of magnitude smaller.
3. The material concerned must be cheap to produce. A
good standard for an affordable material is that its cost
must be similar to that of bottled water when
considering weight or volume.

51. • The following conditions are required to
achieve this:
1.

52. 2.
3.

53. 4.

54. • All of these conditions can in principle be met by
the phase transformation of austenite into
bainite, partly because the reaction is
particularly amenable to control by either
isothermal or continuous cooling heat treatment.
• Furthermore, the transformation is displacive,
i.e., it leads to a shape deformation which is
macroscopically an invariant plane strain with a
large shear component, as illustrated in figure.

56. Hard bainite
• Steel transformed into carbide-free bainite can
satisfy these criteria.
• Bainite and martensite are generated from austenite
without diffusion by a displacive mechanism.
• This leads to solute-trapping and also a huge strain
energy term, both of which reduce the heat of
transformation.

57. • The growth of individual plates in both these
transformations is fast, but unlike martensite,
the overall rate of reaction is much smaller for
bainite.
• This is because the transformation
propagates by a sub-unit mechanism in
which the rate is controlled by nucleation
rather than growth. This mitigates
recalescence.

58. • Suppose we now attempt to calculate
the lowest temperature at which bainite
can be induced to grow. We have the
theory to address this proposition.

59. 0
200
400
600
800
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Carbon / wt%
Temperature
/
K
Fe-2Si-3Mn-C wt%
BS
MS
There is in principle no lower limit to the
temperature at which bainite can be generated.

60. Fig: Calculated time required
to initiate bainite at BS
temperature.
• On the other hand, the rate at
which bainite forms slows
down drastically as the
transformation temperature is
reduced, as shown by the
calculations in the right plot of
Fig.
• It may take hundreds or
thousands of years to
generate bainite at room
temperature.

61. C Si Mn Mo Cr V P
0.98 1.46 1.89 0.26 1.26 0.09 < 0.002
wt%
Low transformation temperature
Bainitic hardenability
Reasonable transformation time
Elimination of cementite
Austenite grain size control
Avoidance of temper embrittlement
• For practical purposes, a transformation time
of tens of days is reasonable.

62. Time
1200 oC
2 days
1000 o
C
15 min
Isothermal
transformation
125 oC-
325 oC
hours-
months
slow
cooling
Air
cooling
Quench
Austenitisation
Homogenisation

63. 0
100
200
300
400
500
600
700
1.E+00 1.E+02 1.E+04 1.E+06 1.E+08
Time / s
Temperature/
o
C
BS ~ 350o
C
MS = 120o
C

65. X-ray diffraction results
0
20
40
60
80
100
200 250 300 325
Temperature/o
C
Percentage
of
phase
bainitic ferrite
retained austenite
Bainite plates which are only 20–40 nm thick
carbon-enriched austenite

67. 50 nm
a
g
g
g a
Bainite plates
which are only 20–
40 nm thick
carbon-
enriched
austenite

68. 200 Å
g
g
a
a
a
Caballero, Mateo,
Bhadeshia

69. 200 Å
g
g
a
a
a
Very strong
Huge uniform ductility
No deformation
No rapid cooling
No residual stresses
Cheap
Uniform in very large sections

70. Low temperature transformation: 0.25 T/Tm
Fine microstructure: 20-40 nm thick plates
Harder than most martensites (710 HV)
Carbide-free
Designed using theory alone

71. “more serious battlefield threats”

74. Chatterjee & Bhadeshia, 2004
Fe-1.75C-Si-Mn wt%
2104

75. Moreover

77. ballistic mass efficiency
consider unit area of armour

78. Cobalt (1.5 wt%) and aluminium (1 wt%)
increase the stability of ferrite relative
to austenite
Refine austenite grain size
Faster Transformation
C Si Mn Mo Cr V P
0.98 1.46 1.89 0.26 1.26 0.09 < 0.002

79. Original 5h 3/4d 63 550
Co 4h 11h 77 640
Co + Al 1h 8h 76 640
200oC
250oC
300oC
Steel Beginning End % Bainite HV
Original 4d 9d 69 618
Co 2d 5d 79 690
Co+ Al 16h 3d 78 690
Original 2.5h 1/2d 55 420
Co 1h 5h 66 490
Co + Al 0.5h 4h 66 490

80. original

81. Co

82. Co+Al

88. Need to improve mechanical
stability of austenite

89. Thanks