Ferrous alloys upto DP steels from NIT Rourkela.pdf

NIT Rourkela 2025
4th year MM

1. Subject Code: MM4161
Course Instructor: Dr.SrijanAcharya
Assistant Professor
Department of Metallurgical and Materials Engineering,
National Institute of Technology Rourkela
Complex Ferrous and Non-ferrous Alloys

2. Classification of steels
Steels
Low Alloy
Low
Carbon
Plain
carbon
High Strength Low Alloy
Medium
Carbon
High
Carbon
High Alloy
Tool steels
Stainless
steels
Source: “Materials Science and Engineering
An Introduction” by William D. Callister, Jr.

3. Ferrous alloys : Fe-C phase diagram
Source: “Materials Science and Engineering An Introduction” by William D. Callister, Jr.

4. Ferrous alloys Recap : Fe-Fe3C phase diagram
Salient features:
Equillibrium diagram (thermodynamic); does not say about
kinetics (effect of time)
Effect of C on phase stability
Solubility of C in different phases
Invariant reactions
Eutectoid (γ α + Fe3C)
Eutectic (L γ + Fe3C)
Peritectic (L + δ γ)
Steel side (up to ≈ 2.11 wt.% C) and cast iron side (beyond
2.11 wt.% C )
Ac3 (γ γ + α) and Ac1 (γ α + Fe3C) lines
Source: “Physical Metallurgy Principles” by Robert E. Reed-hill
0.02 %

5. Solubility of C in α and γ: role of interstitial void size
Octahedral
FCC
BCC
Tetrahedral
Source: “Steels: Microstructure and Properties ” by Sir HKDH Bhadeshia and Sir R. HoneyKombe

6. Solubility of C in α and γ: role of interstitial void size
Octahedral
FCC
BCC
Tetrahedral
Source: “Steels: Microstructure and Properties ” by Sir HKDH Bhadeshia and Sir R. HoneyKombe
➢C has higher solubility in γ due to larger sizes of octahedral
interstitial sites in FCC-γ
➢In BCC, C occupies octahedral site despite it being smaller than
tetrahedral site
➢For in octahedral interstices movement of 2 nearest-neighbour
Fe atoms is required.
➢For tetrahedral interstices, 4 Fe atoms are of nearest-neighbor,
hence their displacement needs more strain energy.

7. Recap: The Eutectoid Reaction
Source: “Materials Science and Engineering An Introduction” by William D. Callister, Jr.

8. The equilibrium phases in steels at RT
Source: “Materials Science and Engineering An Introduction” by William D. Callister, Jr.
At Eutectoid composition i.e. 0.77 wt.% C:
Pearlite (lamellar mixture of eutectoid ferrite and cementite)
Hypo-eutectoid composition i.e. <0.77 wt.% C:
Proeutectoid ferrite (transformed from γ along Ac
3 )
Pearlite (transformed from γ along Ac
1 )
Hyper-eutectoid composition i.e. >0.77 wt.% C:
Proeutectoid cementite (transformed from γ along Ac
3 )
Pearlite (transformed from γ along Ac
1 )
Ac
3
Ac
1
Hypo- Hyper-

9. Recap: Time-temperature-transformation (T-T-T) Diagram
Source: “Physical Metallurgy Principles” by Robert E. Reed-hill
Isothermal transformation diagram
Kinetics of transformation: effect of time
Important reactions
Pearlitic
Bainitic
Martensitic
Non-equilibrium phases: martensite (ferrite
supersaturated with C ) and bainite (mixture of ferrite
and Fe
3C)
Types of bainite: Upper bainite and Lower bainite
Diffusionless transformation (M
s and M
f)
Hardenability

10. Recap: Continuous cooling transformation (C-C-T) Diagram
Source: “Physical Metallurgy Principles”
by Robert E. Reed-hill
Note the absence of bainitic region in C-C-T diagram

11. Recap: Effect of C on TTT/CCT diagram: Hypo- and Hypereutectoid Steels
Source: “Physical Metallurgy Principles” by Robert E. Reed-hill
❖Note the additional transformati0n line (marked by red
arrow) for proeutectoid ferrite above eutectoid temp.
❖In Hypo- and Hyper-eutectoid compositions, the C-curve
is shifted towards left side
❖Faster transformation to pearlite/bainite due to
heterogeneous nucleation of eutectoid ferrite or
cementite, assisted by pre-existing pro-eutectoid
ferrite or cementite, respectively

12. Recap: Effect of other alloying elements on TTT/CCT diagram
Material:
AISI 4340 steel (Fe-0.4C-0.4Cr-1.8Ni-0.25Mo)
Figure Source: “The Science and Engineering of Materials” by Askeland et al.
❖Shifting of pearlitic reaction towards right side
due to addition of Ni, Cr and Mo
❖No shift in bainitic reaction rate, as bainitic
transformation rate is determined by
partitioning of C atoms between ferrite and
cementite; not the other alloying elements

13. Effect of C content on mechanical properties
Source: “Materials Science and Engineering An Introduction” by William D. Callister, Jr.

14. Effect of C content on mechanical properties
Source: “Materials Science and Engineering An Introduction” by William D. Callister, Jr.
Note: Izod impact energy tells us about
impact toughness of materials

15. Effect of tempering on martensitic steels
Source: “The Science and Engineering of Materials” by Askeland et al.

16. Contains ≤ 0.3 wt% C
Little or no hardenability
Good weldability
Common Microstructure: pro-eutectoid ferrite and pearlite
Common Processing route: hot-rolling (or, cold drawn/rolled and annealed)
Applications:
Structural beams
Automotive panels
Pipelines
Plain Low C-steels
http://www.phase-trans.msm.cam.ac.uk/
https://www.theworldmaterial.com/astm -a36-steel/
https://www.tatasteeleurope.com/automotive/applications
“Materials Science and Engineering An Introduction” by William D. Callister, Jr.

17. Composition: contains 0.3-0.6 wt. % C
plain medium C-steels have low hardenability (e.g. AISI 1040/EN8 steel)
Ni, Cr, Mo added to improve hardenability (e.g. AISI 4340/EN24 Steel)
Processing route: hot-worked and/or annealed/ normalized (for C-steels) or
Quenching+tempering (for C-steels and alloy steels)
Have higher strength and toughness than low C-steels
Applications:
Railway wheels and tracks
Gears, crankshafts
Medium Carbon steels
https://practicalmaintenance.net
https://www.eatonsteel.com/1040 -hot-rolled-steel-bar.html
1040 steel 4340 steel
Lee et al. (2019), Adv.Engg Mater., 21, 1801116
Mechanical Properties
“Materials Science and Engineering An Introduction” by William D. Callister, Jr.

18. Composition: contains 0.6-1.4 wt. % C
Good hardenability
Difficult to cold work
Poor weldability
Cr or Mo added as carbide formers in high alloy steels (e.g. D2 Steel)
Common Microstructure: pearlitic (for C-steels)
Applications: cutting and milling tools, knives, hammer, blades
High carbon steels
https://practicalmaintenance.net/wp -
content/uploads/High-carbon-AISI-1095-Steel.jpg
https://www.engineersgallery.com/high -speed-steels/

19. High C steels with addition of Cr or Mo carbide formers (e.g. D2 Steel (High-Cr), M1
steel (High Mo))
Precipitation of carbides (M
23C
6 type) at high T gives secondary hardening
Good hardenability
Exceptionally High strength and toughness
Common Microstructure: tempered martensite with carbides
Applications: cutting and milling tools, dies for casting and forming
Tool steels
https://www.engineersgallery.com/high -speed-steels/
https://doi.org/10.3390/ma16051941

20. In conventional structural steels, C provides primary source of strengthening
Increase in C content leads to (1) reduction in Lower ductility and impact toughness, (2) Lower weldability,
(3) Increase in ductile-brittle transition temperature
HSLA/Microalloyed steels - introduction

21. Subset of Low-C steel (C <0.3 wt.); Also called Microalloyed steels
small concentrations (<0.1 wt%) of other alloying elements e.g.
Mo/Ti/Nb/V/Al <0.1 wt.% (carbide forming elements)
Cu/Ni (for SS strengthening)
Stronger and tougher than low C steels
Common Microstructure: fine equiaxed ferrite + carbides/nitrides/carbonitrides + pearlite (depending upon
C %) Table 1: Composition of LC (1020) and HSLA (A656) steels in wt.%
HSLA / Micro-alloyed steels

22. Common Microstructure: ferritic equiaxed + with/out pearlite + with/out carbides/carbonitrides
Processing route involves hot rolling above austenitization temperature to induce (1) precipitation of
carbides/carbonitrides and (2) retard growth of dynamically recrystallized γ-grains by pinning
Primary Strengthening mechanisms:
grain refinement
precipitation strengthening
Table 1: Composition of LC (1020) and HSLA (A656)
steels in wt.%
Table 2: Comparison of Mechanical properties
“Materials Science and Engineering An Introduction” by William D. Callister, Jr.
HSLA / Micro-alloyed steels

23. Interstitial Free (IF) Steels
Steels with C, N < 30 ppm
Ti and/or Nb added to stabilize (form carbides/nitrides with) remaining
interstitial C and N atoms
Almost no C, N available as interstitials
predominantly ferritic (BCC) structure;
no pearlite
Majumdar et al., Met Trans A, 2006, 37A, 3541-53
S. Hoile (2000), Materials Science
and Technology, 16:10, 1079-1093

24. Interstitial Free (IF) Steels (contd.)
First commercial development in Japan during 1970s following the
introduction of vacuum degassing technology
Extremely high ductility with low strength; ideal for car body panels
produced by sheet metal forming
Also called Extra Deep Drawing Steels (EDDS)
Tensile curve does not show yield point elongation due to insufficient
interstitial atoms
Acharya et al., Mater. Sci. Eng. A, 2013, 565, 405–413
IF
IF-HS
IF
IF-HS

25. Advanced High strength Steels (AHSS) for Automotive Applications
Lightweight and high strength cuts energy use & GHG; challenge: keep safety & cost.
UltraLight Steel Auto Body (ULSAB) Consortium: thirty-five steel manufacturers,
representing twenty-two countries; reduced weight by 25% using lighter and stronger
steels vs. conventional steels.
https://ahssinsights.org/
ULSAB was replaced by Future Steel Vehicle (FSV)
program aimed for battery Electric vehicles: 35–39% mass
saving;
AHSS Steels have been key to achieve energy efficiency.
Examples: Dual Phase (DP) steels , TRIP, TWIP, Complex
Phase (CP), HF (Hot formed)
2021 Global Formability Diagram comparing strength and elongation of current
and emerging steel grades.

26. Dual Phase (DP) Steels
Immensely popular in automotive applications
Dual phase microstructure:
Primary phase: ferrite
Secondary phase: martensite (+ bainite + retained austenite)
https://www.thyssenkrupp -steel.com/en/products/sheet-coated-products/multiphase-steel/dual-phase-steel/

27. Dual Phase (DP) Steels: composition and processing
Composition:
C: 0.06–0.15 wt.% (for strengthening martensite)
Mn: 1.5–3% wt.% (for stabilizing austenite)
Cr (<0.5 wt.%) and Mo (0.2-0.4 wt.%) (to improve
hardenability)
Si: 0.15 wt.% (to promote ferrite transformation)
V and Nb (for precipitation strengthening and
microstructure refinement; similar to HSLA)
General Processing Route : cold rolling → intercritical annealing (ICA) → Quenching
Tasan et al., Annu. Rev. Mater. Res. 2015. 45:391–431
ICA gives ferrite + austenite; Austenite is richer in C
Quenching results in formation of high-C hard martensite

28. Dual Phase (DP) Steels: mechanical properties
Lower YS (due to soft ferrite phase) and continuous yielding (due to 2-phase microstructure )
High strain hardening response rate and high UTS/YS ratio
Considerable ductility and toughness (crashworthiness)
https://ahssinsights.org/metallurgy/steel-grades/ahss/dual-phase/

29. Dual Phase (DP) Steels: Deformation mechanism
Strain partitioning: Ferrite deforms first, martensite resists →
strain gradients form.
Mismatch at interfaces produces geometrically necessary
dislocations (GNDs) which maintain the compatibility between
ferrite and martensite grains at their interface.
Pile-ups of the dislocations at ferrite-martensite interfaces cause
work hardening.
Progressive activation: At higher strain, martensite start to
plastically deform, adding more dislocations.
Continuous hardening: Interfaces keep generating obstacles,
delaying strain hardening saturation/necking
https://ahssinsights.org/metallurgy/steel-grades/ahss/dual-phase/