Heat treatment of mild steels

1. MICROSTRUCTURE-PROPERTY
CORRELATION IN LOW CARBON
STEEL AFTER HEAT TREATMENT

2. INDEX
• Introduction
• Iron Carbon Diagram
• Heat Treatment Methods
• Need Of Heat Treatment
• Literature Review
• Effect Of Heat Treatment On Mechanical Property Of Low Carbon Steel
• Effect Of Heat Treatment On Microstructure Of Low Carbon Steel
• New Methods To Improve Mechanical Properties Of Low Carbon Steel
• Summary
• References

3. INTRODUCTION
Steel is a very common material in our life
On the basis of carbon content, the steel is divided as follows:
Low Carbon Steel
 Carbon content = 0.15% to 0.45%
 Most common form of steel as it provides material properties that are acceptable for many
applications.
 Neither externally brittle nor ductile
 Lower tensile strength and malleable
 Large control over the properties.
As the carbon content increases, the metal becomes harder and stronger but less ductile and
more difficult to weld.
Low
carbon
steel
Medium
carbon
steel
High
carbon
steel

4. OVERVIEW OF IRON CARBON SYSTEM
Phases present in the system are:
Ferrite
It is α-iron (B.C.C.);
0.025% carbon.
It can be easily
cold worked.
Cementite
Iron carbide, 6.67%
carbon.
White in colour .
It has an
orthorhombic
crystal structure.
It is a hard, brittle
material
Austenite
2% carbon at 1130°C.
It is tough and non-
magnetic
Known as gamma-
phase iron (γ-Fe)
Metallic, non-magnetic
solid solution of iron,
with an alloying
element.

5. CONTINUED
Pearlite
Microstructure:
Alternate laminations of
ferrite and cementite.
Carbon=0.8%
Mixture of two phases,
ferrite and cementite
(Fe3C).
Forms by the
cooperative growth of
both of these phases at
a single front with the
parent austenite
Martensite
Obtained by quenching
from above upper
critical temperature
Hardest constituent
obtained in given steel
Shows a fine needle-
like microstructure
Diffusion less
transformation
achieved by the
deformation of the
parent lattice into that
of the product.
Bainite
Atomic mechanism of
bainite is similar to that of
martensite.
Plates of bainite form
without any diffusion, but
shortly after
transformation, the carbon
partitions into the residual
austenite and precipitates
as cementite between the
ferrite platelets - this is the
structure of upper bainite.
Lower bainite is obtained
by transformation at a
lower temperature

6. HEAT TREATMENT
It is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties
of a material.
The process starts by heating the material, holding it for some time and then cooling it in the furnace, brine, water and oil.
Types:
Annealing:- Furnace cooling is employed. Annealing is mainly of following types:
 Spherodizing:- Carbon steel is heated to approximately 700oC for over 30 hours to form spheroids. This is the softest and
most ductile form of steel.
 Full annealing:- Carbon steel is heated to approximately above the upper critical temperature. Here all the ferrite
transforms into austenite. Full annealed steel is soft and ductile with no internal stress.
 Diffusion annealing:- The process consists of heating the steel to high temperature (1100- 1200 oC) and then inside the
furnace for a period of about 6 to 8 hours. It is further cooled in the air to room temperature. This process is mainly used
for ingots and large casting.
Normalizing:- Consist of heating the metal to a temperature of 30 to 50 oC above the upper critical temperature for hypo-
eutectoid steels and by the same temperature above the lower critical temperature for hyper-eutectoid steel. The purpose of
normalizing is to refine grain structure, improve machinability and improve tensile strength.

7. CONTINUED
Hardening:- Metal is heated to a temperature of 30-50 oC above the
upper critical point for hypo-eutectoid steels and by the same
temperature above the lower critical temperature for hyper-eutectoid
steels and held, quenched.
Tempering:- This process consists of reheating the hardened steel to
some temperature below the lower critical temperature, followed by
any desired rate of cooling.
The purpose is to relive internal stress, to reduce brittleness and to
make steel tough to resist shock and fatigue.
Other heat treatments include
• Surface hardening
• Austempering
• Martempering

8. WHY HEAT TREATMENT
The purpose of heat treatment is to soften the metal, to change the grain size, to modify the structure of the
material and to relieve the stress set up in the material after hot and cold working.
Heat treatment results into change in following properties in low carbon steels:
 Strength : Ability of a material to resist the externally applied forces.
 Stiffness : Ability of a material to resist deformation under stress.
 Elasticity :Property of materials to regain its original shape after deformation .
 Ductility :It is the property of a material which enables it to draw out into thin wire
 Hardness :Property of a material to resist penetration by another material
 Malleability : It is the ability of materials to be rolled, flattened or hammered into thin sheets
 Plasticity :It is the ability of material to undergo some degree of permanent deformation without rupture or
failure
Heat treatment
Microstructural
changes
Property
changes

9. LITERATURE REVIEW
1. Effect Of Heat Treatment On Mechanical Property
Of Low Carbon Steel
In one such study, the specimen was subjected to :
 Full annealing at 900 degrees for 2 hours
 Normalizing at 900 degrees for 2 hours in furnace followed by air
cooling
 Hardening from 900 degrees followed by oil bath quenching
 Tempering at different temperature ranges in order to determine the
property change.
 Austempering also performed to develop all round properties
Table 1 : Comparison of UTS ,YS and %elongation of
specimen tempered
Table 2 : Hardness comparison after tempering

10. In another study, different heat treatment processes were applied on low carbon steel to study the
behaviour
Results
Effect Of Heat Treatment
2. On Microstructure Of Low Carbon Steel
1. On Mechanical property Of Low Carbon Steel
Results

11. NEW METHODS TO IMPROVE MECHANICAL PROPERTIES
OF LOW CARBON STEEL
1. Rapid Heat Treatment (RHT): Annealing at a temperature above AC3 to be transformed to austenite in a very short time
(0.001 – 0.5 sec.).
2. Intercritical Heat Treatment (IHT): Original ferrite-pearlite structure, transforms to dual-phase structure of ferrite and
martensite
• The tensile strength of the used steel in its original
state is about 370 MPa, and after RHT it can reach
800 MPa.
• Considerable increase in tensile strength could be
achieved by subjecting it to IHT, with the proper
selection of the annealing temperature (1170 MPa
after quenching from 840 oC).
The mechanical properties obtained after IHT

12. CONTINUED
Process 3:
Special heat treatment cycle -step quenching, used to produce a dual-phase (DP) microstructure in low carbon
steel.
Dual-phase (DP) steels: advanced high-strength (AHS) steels, primarily comprised martensite and ferrite
By producing this DP microstructure, the mechanical properties of the investigated steel such
as yield stress, tensile strength, and Vickers hardness have increased 14, 55, and 38%,
respectively.
The figure shows the ferrite-
martensite (DP)
microstructure of steel after
Step Quenching (SQ).
Three different quenching paths, namely intercritical quenching,
intermediate quenching, and step quenching (SQ) can be used to
produce DP steels with different microstructures and mechanical
properties

13. SUMMARY
Out of the steel grades present, low carbon steels are the most common because of the diversity in properties.
Ferrite is the softest phase having BCC structure while martensite being the hardest and brittle phase having BCT structure.
Annealing results in soft and ductile material with decreased internal stresses which is a result of microstructure containing
ferrite and pearlite
Normalizing results in refined grain structure with a good toughness level and improved tensile strength as compared to
annealing
Hardening or quenching treatment results in high hardness having the microstructure containing martensite which is
known for its high hardness
Tempering temperature results in decrease in strength i.e. UTS and Yield strength values while an increase in ductility is
observed in terms of percentage elongation.
Other treatments like intercritical treatment, Rapid heat treatment etc. improve the mechanical properties of such steels
considerably.
There exist a strong structure property co-relation in the materials like steel due to which we can obtain a plenty of grades of steel
as per as the desired applications.

14. REFERENCES
• A Project Report On Heat Treatment Of Low Carbon Steel Bachelor Of Technology (Mechanical Engineering)
• Heat Treatment Of Low Carbon Steels, Ya. S. Finkel Shtein, V. A. Gladkovskii, And G. S. Batist
• Mechanical Properties Improvement Of Low Carbon Steel By Combined Heat Treatments J. Abou-Jahjah And J. Dobránszky.
• Effect Of Heat Treatment On Mechanical Properties And Microstructure Of Nst 37-2 Steel D. A. Fadare , T. G. Fadara And
O. Y. Akanbi, Department Of Mechanical Engineering, University Of Ibadan, P.M.B 1, Ibadan, Nigeria
• Enhancing The Mechanical Properties And Formability Of Low Carbon Steel With Dual-Phase Microstructures M. Habibi, R.
Hashemi, E. Sadeghi, A. Fazaeli, A. Ghazanfari, And H. Lashini
• Effect Of Heat Treatment On Microstructure And Property Of Cr13 Super Martensitic Stainless Steel Liu Yu-Rong', Ye Dong',
Yong Qi-Long' , Su Jie' , Zhao Kun-Yu' , Jiang Wen' (1. College Of Materials Science And Engineering, Kunming University Of
Science And Technology, Kunming 650093, Yunnan, China; 2. Institute Of Structural Materials, Central Iron And Steel
Research Institute, Beijing 100081, China
• Heat Treatment: Principles and Techniques-By T.V Rajan, C.P Sharma, Ashok Sharma
• Physical Metallurgy- Vijendar Singh.
• Putatunda Sushil K Material science and Engineering Vol 315, sept 2001
• Principles and application of heat treatment of CI, Isfahan University Iran, 1987

15. THANK YOU