INTRO materials science is the basic knowledge of internal structure, properties and processing of materials
materials engineering is the fundamental and applied knowledge of materials into useful products
materials science & engineering is bridge between materials science and materials engineering
classification of materials :
1) metallic materials
2) polymeric materials
3) ceramic materials
4) composite materials
5) electronic materials
BRAVAIS LATTICE
The physical structure of solid materials depends primarily on the arrangement of atoms and the bonding force between them. If the atoms are arranged in a repeative pattern in 3D, they form a solid that is said to have crystal structure. Such a material is called a crystalline solid @ material.
Atomic arrangements in crystalline solids can be described by referring
the atoms to points of intersection of a network of lines in 3D.
Such a network is called a space lattice. Each space lattice is formed
by infinite and repeating unit cells.
There are 7 crystal-systems. A.J. Bravais showed 14 standard unit cells to describe all possible lattice networks. Most elemental metals ( ca. 90% ) crystallize upon solidification into these 3 unit cells :
1) body centered cubic (BCC)
2) face centered cubic (FCC)
3) hexagonal close packed (HCP)
HCP
metals with HCP crystal structure at 20oC :
Magnesium(Mg) , Cobalt(Co) , Zirconium(Zr) ,
Titanium(Ti) , Beryllium(Be)
POLYMORPHISM @ ALLOTROPHY
it is the phenomenon where many elements & compounds can exist in more
than one crystalline form under different temperature and pressure
eg.
metal
symbol
at room temp.
at other temp.
Titanium
Ti
HCP
BCC (>883oC)
Iron
Fe
BCC
FCC (910-1394oC)
BCC (1394-1538oC)
PHASE DIAGRAM - PURE WATER
the sketch is as below :
PHASE DIAGRAM - PURE IRON
the sketch is as below :
α = alpha
γ = gamma
δ = delta
PHASE DIAGRAM - BINARY COMPLETELY SOLUBLE ALLOYS
the sketch is as below :
PHASE DIAGRAM - BINARY PARTIALLY SOLUBLE ALLOYS
the sketch is as below :
PHASE DIAGRAM - IRON & IRON CARBIDE
the sketch is as below :
it contains the following solid phases :
α ferrite :
interstitial solid solution ,
BCC ,
maximum solubility of 0.02% C at 723oC
austenite (γ) :
interstitial solid solution ,
FCC ,
maximum solubility of 2.08% C at 1148oC
δ ferrite :
interstitial solid solution ,
BCC ,
maximum solubility of 0.09% C at 1495oC
cementite (Fe3C) :
negligible solubility , 6.67% C & 93.33% Fe
at eutectic point :
L (4.3%C) ---> γ (2.08%C) + Fe3C (6.67%C)
at eutectiod point :
γ (0.8%C) ---> α (0.02%C) + Fe3C (6.67%C)
at peritectic point :
δ (0.09%C) + L (0.53%C) ---> γ (0.17%C)
eutectoid steel =
plain carbon steel with 0.8%C
hypoeutectoid steel =
plain carbon steel with less than 0.8%C
hypereutectoid steel =
plain carbon steel with more than 0.8%C
SLOW COOLING OF PLAIN CARBON STEEL
below is a sketch to explain the slow cooling of eutectoid steel,
hypoeutectoid steel & hypereutectoid steel :
If a sample of 0.8%C plain carbon steel is heated to
ca. 750oC (at point a) and held for a sufficient time,
the structure will become homogeneous austenite (γ).
If it is then cooled slowly to just above 723oC,
its structure will remain as austenite (γ).
Further slow cooling to just below 723oC (at point b)
will cause the entire structure to transform into pearlite.
Pearlite is a mixture of α ferrite and cementite (Fe3C) phases in lamellar structure produced by the eutectoid decomposition of autensite (γ).
If a sample of 0.4%C plain carbon steel is heated to
ca. 900oC (at point c) and held for a sufficient time,
the structure will become homogeneous austenite (γ).
If it is then cooled slowly to temperature d,
proeutectoid ferrite will nucleate and grow mostly
at the grain boundaries of austenite (γ).
If it is cooled slowly from temperature d to e,
the amount of proeutectoid ferrite will increase
and the carbon content of the remaining austenite (γ)
will increase from 0.4 to 0.8%.
If slow cooling condition prevail till point f, the remaining austenite (γ) will transform into pearlite by eutectoid reaction :
austenite (γ) ---> α ferrite + cementite (Fe3C)
The α ferrite in the pearlite is called eutectoid ferrite to distinguish it from proeutectoid ferrite forms earlier above 723oC.
If a sample of 1.2%C plain carbon steel is heated to
ca. 950oC (at point g) and held for a sufficient time,
the structure will become homogeneous austenite (γ).
If it is then cooled slowly to temperature h,
proeutectoid cementite will nucleate and grow mostly
at the grain boundaries of austenite (γ).
If it is cooled slowly from temperature h to i,
the amount of proeutectoid cementile will increase
and the carbon content of the remaining austenite (γ)
will decrease from 1.2 to 0.8%.
If slow cooling condition prevail till point j, the remaining austenite (γ) will transform into pearlite by eutectoid reaction :
austenite (γ) ---> α ferrite + cementite (Fe3C)
The cementite in the pearlite is called eutectoid cementite to distinguish it from proeutectoid cementite forms earlier above 723oC.
RAPID COOLING OF PLAIN CARBON STEEL
If a sample of plain carbon steel in austenitic condition
is rapidly cooled to room temperature by quenching in water,
its structure will be changed austenite (γ) to martensite.
The transformation of austenite (γ) to martensite in plain carbon steels is considered to be diffusionless since the transformation take place so rapidly that the atoms do not have time to intermix. The relative position
of carbon-atoms with respect to iron-atoms are the same in martensite
as they were in the austenite (γ).
If the carbon content is less than 0.2%, the austenite (γ) will
transforms into martensite with BCC α ferrite crystal structure.
As the carbon content increase, BCC is distorted into
BCT (body centered tetragonal) due to the squeezing of
carbon-atom in the interstitial hole of each unit cell.
Martensite is not shown in the equilibrium phase diagram of the iron-carbon system because it is a metastable phase, i.e. it can be easily destroyed
(decomposed) by the application of heat.
The strength and hardness of martensite is directly proportional to the
carbon content; the ductility and toughness is inversely proportional
to the carbon content.
Martensite start, Ms indicates the beginning of transformation. Martensite finish, Mf indicates the ending of transformation. Ms and Mf will be changed to lower temperature with higher carbon content.
ISOTHERMAL DECOMPOSITION OF AUSTENITE (γ)
We have just gone thru the decomposition of austenite (γ)
for slow cooling condition and rapid cooling condition.
Now, we are going to study the decomposition of austenite (γ)
when it is rapidly cooled to temperature below
723oC and then isothermally transformed.
Above is a sketch of TTT (Time Temperature Transformation) diagram
to explain the isothermal decomposition of austenite
(of eutectoid steel).
If a sample of 0.8%C plain carbon steel is heated
to just above 723oC and then rapidly hot-quenched to temperature range of 723oC to 550oC and is isothermally transformed,
coarse pearlite will be produced towards the high end of that temperature range and fine pearlite will be produced towards the low end of that temperature range. Pearite start, Ps indicates the beginning of transformation. Pearite finish, Pf indicates the ending of transformation.
If a sample of 0.8%C plain carbon steel is heated
to just above 723oC and then rapidly hot-quenched to temperature range of 550oC to 250oC and is isothermally transformed,
upper bainite will be produced at the temperature range of 550oC to 350oC
and lower bainite will be produced at temperature range of 350oC to 250oC.
Bainite start, Bs indicates the beginning of transformation. Bainite finish, Bf indicates the ending of transformation.
Bainite is a mixture of α ferrite and very small particles of cementite (Fe3C) phases in nonlamellar structure produced by the eutectoid decomposition of autensite (γ).
Above is a sketch of TTT (Time Temperature Transformation) diagrams
to explain the isothermal decomposition of austenite
(of noneutectoid steel). Noneutectoid steel refers to hypoeutectoid steel and hypereutectoid steel. On the left side is the sketch for hypoeutectoid steel (lower %C higher Ms). On the right side
is the sketch for hypereutectoid steel (higher %C lower Ms).
There are a few differences in isothermal transformation diagram
between eutectiod one and noneutectoid one. First major difference
is that the S-curves has been shifted to the left. Thus, it is impossible
to quench this steel from austenite region to produce a entirely
mastensitic structure. Second major difference is that another transformation line has been added which indicates
the start of formation of proeutectoid ferrite
(for hypoeutectoid steel)
and
the start of formation of proeutectoid cementite
(for hypereutectoid steel).
CONTINUOUS COOLING TRANSFORMATION
We have gone thru the decomposition austenite (γ)
by slowing cooling, rapid cooling and isothermal transformation.
Now, we are going to study the decomposition of austenite (γ)
by Continuous Cooling Transformation (CCT). The motivation for us to study
CCT is that CCT is widely used in industrial heat treatment operation
instead of isothermal transformation. However, we will be focusing on
eutectoid steel only cos' CCT for noneutectoid steel
is too advanced for me (I'm not a metallurgist).
Above is a sketch of CCT diagrams of eutectoid steel. There are a few differences between TTT diagram and CCT diagram. First major difference is that there is no bainite transformation in CCT diagram because it is impossible for bainite to from on continuous cooling unless there is an interuption to quench. Second major difference is that there is a
split transformation in CCT diagram. Please view the cooling curve
(with oil quench) which passes thru the truncated S-curve.
That particular cooling curve starts with the formation
of pearlite, but there is insufficient time to complete the
austenite-pearlite transformation. The remaining austenite will transformed into mastensite at ca. 220oC. The microstructure of this steel consists of a mixture of pearlite and martensite. Third major difference is
that there is a curve which represents the critical cooling rate
in CCT diagram. Cooling at a rate faster than this rate will produce a fully hardened martensitic structure. The ability of a steel to fully transform into martensite is known as its hardenability.
QUENCHING OF PLAIN CARBON STEEL
The term quenching indicates heating of steel into the austenitic region and followed by rapid cooling (by immersion in liquids or gases or by contact with metal). The rapid cooling produces a supersaturated mixture
solution of iron and carbon called martensite, which is extremely hard (cf the original material). Unfortunately, it is very brittle.
Thus, martensite is a undesirable product without a heat-treatment called
tempering.
ANNEALING & NORMALIZING OF PLAIN CARBON STEEL
below is a sketch to explain annealing and normalizing :
The term annealing indicates heating of steel and followed by slow cooling. There are two major applications of annealing.
First, it can restore the original properties of hardened steel
(in other words, it makes the quenching & tempering process reversible).
Second, it can relieve residual stresses of cold-worked steel.
In full annealing, hypoeutectoid steel and eutectoid steel
are heated into the austenitic region
(ca. 40oC above the austenite-ferrite boundary) and
hypereutectoid steel are heated into the austenitic-cementite region
(ca. 40oC above the 723oC), it is then held
at these respective temperatures for a sufficient time in order for
the carbon atoms to diffuse in the materials,
finally it is cooled slowly (usually in the furnace where it is treated).
The word full implies complete transformation.
Process annealing aka stress relief softens cold-worked low carbon steel by relieving residual stresses. This treatment is usually
applied to hypoeutectoid steel with less than 0.3% carbon, is performed
at below 723oC (550oC to 650oC).
The term normalizing indicates heating (higher temperature
cf full annealing) of steel into the austenitic region and followed by slow cooling (in still air). It is often used as final treatment for steel.
Its objectives could be :
1) to produce coarser grain structure
(easier to machine if the material is a low carbon steel)
2) to increase the hardness (faster cooling rate cf full annealing)
3) to reduce compositional segregation in castings / forgings
TEMPERING OF PLAIN CARBON STEEL
The tempering of quenched steel (martensitic steel)
is intended to improve its ductility by relieving
the internal stresses and correcting the distorted unit cells (BCT).
It can be done by heating to a temperature range
(Ms to 723oC), held for sufficient time
and followed by slow cooling (in still air).
The usual heating temperature range is 400-600oC alhough
200oC is enough to relieve the internal stresses.
Martempering is a quenching process whereby a steel in austenitic condition is hot-quenched above Ms, held shortly
to prevent austenite-bainite transformation, then cooled slowly to room
temperature. The end product is in martensitic condition.
Austempering is a quenching process whereby a steel in austenitic condition is hot-quenched above Ms, held isothermally
to allow austenite-bainite transformation, then cooled slowly to room
temperature. The end product is in bainitic condition.
