7. Engg - Review of Histogram Based Image Contrast Enhancement
Review of Engg Mtls
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REVIEW TO ENGINEERING MATERIALS
Materials are usually classified into two main groups. The most convenient way to
Study the properties and uses of engineering materials is to classify them
Into ‘families’ as shown in figure below:
1. Metals
1.1 Ferrous metals These are metals and alloys containing a high proportion of the element iron.
They are the strongest materials available and are used for applications where high
strength is required at relatively low cost and where weight is not of primary importance.
In general metals have high thermal conductivity, a high density, ductility, a relative
stiffness and strength and high electrical conductivity.
As an example of ferrous metals such as: bridge building, the structure of large
buildings, railway lines, locomotives and rolling stock and the bodies and highly stressed
engine parts of road vehicles.
The ferrous metals themselves can also be classified into "families', and these are shown
in figure below:
1.2 Non – ferrous metals These materials refer to the remaining metals known to mankind.
The pure metals are rarely used as structural materials as they lack mechanical strength.
They are used where their special properties such as corrosion resistance, electrical
conductivity and thermal conductivity are required. Copper and aluminum are used as
electrical conductors and, together with sheet zinc and sheet lead, are use as roofing
materials.
They are mainly used with other metals to improve their strength.
Some widely used non-ferrous metals and alloys are classified as shown in figure:
2. Non – metallic materials
2.1 Non – metallic (synthetic materials) These are non – metallic materials that do not exist in nature, although they are
manufactured from natural substances such as oil, coal and clay. Some typical examples
are classified as shown in figure 6.
They combine good corrosion resistance with ease of manufacture by molding to shape
and relatively low cost.
Synthetic adhesives are also being used for the joining of metallic components even in
highly stressed applications.
Plastics: A plastics consists of polymers plus various additives such as dye, fillers,
retardants, etc. Polymers have low electrical conductivity hence they are used for
electrical and thermal insulation. Compared with metals they have low densities, expand
more when there is change in temperature, and are generally more corrosion resistant,
have a lower stiffness, stretch more and are not hard. When loaded they tend to creep i.e.
the extension gradually changes with time. Their properties dependent on temperature.
Ceramic: These are produced by baking naturally occurring clays at high temperatures
after molding to shape. They are used for high – voltage insulators and high –
temperature – resistant cutting tool tips. They are basically Brittle relatively stronger in
compression than in tension, hard, chemically inert, and bad conductors of heat and
electricity.
Composite materials (composites): These are materials made up from, or composed of,
a combination of different materials to take overall advantage of their different
properties. In man-made composites, the advantages of deliberately combining materials
in order to obtain improved or modified properties were understood by ancient
civilizations. An example of this was the reinforcement of air-dried bricks by mixing the
clay with straw. This helped to reduce cracking caused by shrinkage stresses as the clay
dried out. In more recent times, horse hair was used to reinforce the plaster used on the
walls and ceiling of buildings. Again this was to reduce the onset of drying cracks.
Nowadays, especially with the growth of the plastics industry and the development of
high-strength fibers, a vast range combination of materials is available for use in
composites. For example, carbon fiber reinforced frames for tennis rackets and shafts for
golf clubs have revolutionized these sports.
Dual Phase steels
The dual phase is produced by annealing in the (α+γ) region, followed by cooling at a rate
which ensures that the γ-phase transforms to martensite, although some retained austenite
is also usually present, leading to a mixed martensite–austenite (M–A) constituent. To
allow air cooling after annealing, micro alloying elements are added to low-carbon–
manganese–silicon steel, particularly vanadium or molybdenum and chromium.
Vanadium in solid solution in the austenite increases the hardenability but the enhanced
hardenability is due mainly to the presence of fine carbonitride precipitates which are
unlikely to dissolve in either the austenite or the ferrite at the temperatures employed and
thus inhibit the movement of the austenite/ferrite interface during the post-anneal cooling.
The martensite structure found in dual-phase steels is characteristic of plate martensite
having internal micro twins. The retained austenite can transform to martensite during
straining, thereby contributing to the increased strength and work hardening. Interruption
of the cooling, following intercritical annealing, can lead to stabilization of the austenite
with an increased strength on subsequent deformation. The ferrite grains (≈5μm) adjacent
to the martensite islands are generally observed to have a high dislocation density
resulting from the volume and shape change associated with the austenite to martensite
transformation. Dislocations are also usually evident around retained austenitic islands
due to differential contraction of the ferrite and austenite during cooling.
Dual Phase steels offer an outstanding combination of strength and drawability as a result
of their microstructure, in which a hard martensitic or bainitic phase is dispersed in a soft
ferritic phase. These steels have high strain hardening capacity. This gives them good
strain redistribution capacity, and thus drawability. As a result of strain hardening,
finished part mechanical properties, and especially yield strength, are superior to those of
the initial blank.
High finished part mechanical strength lends these steels excellent fatigue strength and
good energy absorption capacity, making them suitable for use in structural parts and
reinforcements. Strain hardening combined with a strong bake hardening effect gives
these steels excellent potential for skin and structural part weight reduction.
However, a number of automotive parts requiring very high strength steels, such as sills
and door reinforcements, have simple shapes. Therefore the steel is only slightly
deformed and the strain-hardening benefits of Dual Phase steels are not achieved. For this
reason, Arcelor Mittal has developed modified (HY - High Yield Strength and HHE -
High Hole Expansion) versions of Dual Phase steels offering high as-delivered yield
strength and good bendability and stretch flangeability. These new versions are
equivalent to the Complex Phase grades being developed on the German market.
Applications
Given their high energy absorption capacity and fatigue strength, cold rolled Dual Phase
Steels are particularly well suited for automotive structural and safety parts such as
longitudinal beams, cross members and reinforcements. Dual Phase 500 can be used to
make visible parts with 20% higher dent resistance than conventional high strength steels,
resulting in a potential weight saving of some 15%. As a result of its mechanical strength,
hot rolled Dual Phase 600 can be used to lightweight structural parts by reducing their
thickness. Relevant automotive applications include:
1. wheel webs
2. light-weighted longitudinal rails
3. shock towers
4. Fasteners.
5. Bumper in Dual Phase 1180 HY (thickness: 1.35 mm)
MICRO ALLOY STEELS
Micro alloyed steel is a type of alloy steel that contains small amounts
of alloying elements (0.05 to 0.15%),
including niobium, vanadium, titanium, molybdenum, zirconium,boron, and rare-earth
metals. They are used to refine the grain microstructure or facilitate precipitation
hardening.
These steels lie, in terms of performance and cost, between carbon steel and low alloy
steel. Yield strength is between 500 and 750 MPa (73,000 and 109,000 psi) without heat
treatment.
Weldability is good, and can even be improved by reducing carbon content while
maintaining strength. Fatigue life and wear resistance are superior to similar heat-treated
steels. The disadvantages are that ductility and toughness are not as good as quenched
and tempered (Q&T) steels. They must also be heated hot enough for all of the alloys to
be in solution; after forming, the material must be quickly cooled to 540 to 600 °C (1,004
to 1,112 °F).
Cold-worked micro alloyed steels do not require as much cold working to achieve the
same strength as other carbon steel; this also leads to greater ductility. Hot-worked micro
alloyed steels can be used from the air-cooled state. If controlled cooling is used, the
material can produce mechanical properties similar to Q&T steels. Machinability is better
than Q&T steels because of their more uniform hardness and their ferrite-
pearlite microstructure.
Because micro alloyed steels are not quenched and tempered, they are not susceptible
to quench cracking, nor do they need to be straightened or stress relieved. However,
because of this, they are through-hardened and do not have a softer and tougher core like
quench and tempered steels.
High-strength low-alloy (HSLA) steels
The requirement for structural steels to be welded satisfactorily has led to steels with
lower C (<0.1%) content. Unfortunately, lowering the C content reduces the strength and
this has to be compensated for by refining the grain size. This is difficult to achieve with
plain C-steels rolled in the austenite range, but the addition of small amounts of strong
carbide-forming elements (e.g. <0.1% Nb) causes the austenite boundaries to be pinned
by second-phase particles and fine grain sizes (<10μm) to be produced by controlled
rolling.
Nitrides and carbonitrides as well as carbides, predominantly fcc and mutually soluble in
each other, may feature as suitable grain refiners in HSLA steels; examples include AlN,
Nb(CN), V(CN), (NbV)CN, TiC and Ti(CN). The solubility of these particles in the
austenite decreases in the order VC, TiC, NbC while the nitrides, with generally lower
solubility, decrease in solubility in the order VN, AlN, TiN and NbN. Because of the low
solubility of NbC, Nb is perhaps the most effective grain size controller. However, Al,V
andTi are effective in high-nitrogen steels, Al because it forms only a nitride, V and Ti by
forming V(CN) and Ti(CN), which are less soluble in austenite than either VC or TiC.
The major strengthening mechanism in HSLA steels is grain refinement, but the required
strength level is usually obtained by additional precipitation strengthening in the ferrite.
VC, for example, is more soluble in austenite than NbC, so if V and Nb are used in
combination, then on transformation of the austenite to ferrite, NbC provides the grain
refinement and VC precipitation strengthening.
Solid-solution strengthening of the ferrite is also possible. Phosphorus is normally
regarded as deleterious due to grain boundary segregation, but it is a powerful
strengthener, second only to carbon. In car construction, where the design pressure is for
lighter bodies and energy saving, HSLA steels, rephosphorized and bake-hardened to
increase the strength further, have allowed sheet gauges to be reduced by 10–15% while
maintaining dent resistance.
TRIP steels (TRansformation Induced Plasticity)
TRIP steels offer an outstanding combination of strength and ductility as a result of their
microstructure. They are thus suitable for structural and reinforcement parts of complex
shape. The microstructure of these steels is composed of islands of hard residual austenite
and carbide free bainite dispersed in a soft ferritic matrix. Austenite is transformed into
martensite during plastic deformation (TRIP: Transformation Induced Plasticity effect),
making it possible to achieve greater elongations and lending these steels their excellent
combination of strength and ductility.
These steels have high strain hardening capacity. They exhibit good strain redistribution
and thus good drawability. As a result of strain hardening, the mechanical properties, and
especially the yield strength, of the finished part are far superior to those of the initial
blank.
High strain hardening capacity and high mechanical strength lend these steels excellent
energy absorption capacity. TRIP steels also exhibit a strong bake hardening (BH) effect
following deformation, which further improves their crash performance.
The TRIP range of steels comprises three cold rolled grades in both uncoated and coated
formats (TRIP 590, TRIP 690 and TRIP 780) and one hot rolled grade (TRIP 780),
identified by their minimum tensile strength expressed in MPa.
Applications
As a result of their high energy absorption capacity and fatigue strength, TRIP steels are
particularly well suited for automotive structural and safety parts such as cross members,
longitudinal beams, B- pillar reinforcements, sills and bumper reinforcements.
Maraging steels
A serious limitation in producing high-strength steels is the associated reduction in
fracture toughness. Carbon is one of the elements which mostly affect the toughness and
hence in alloy steels it is reduced to as low a level as possible, consistent with good
strength. Developments in the technology of high-alloy steels have produced high
strengths in steels with very low carbon contents (<0.03%) by a combination of
martensite and age hardening, called maraging.
The maraging steels are based on a Fe–Ni containing between 18% and 25% Ni to
produce massive martensite on air cooling to room temperature. Additional hardening of
the martensite is achieved by precipitation of various intermetallic compounds,
principally Ni3Mo or Ni3(Mo, Ti), brought about by the addition of roughly 5% Mo and
8% Co as well as small amounts of Ti and Al; the alloys are solution heat-treated at
815◦C and aged at about 485◦C. Many substitutional elements can produce age hardening
in Fe–Ni martensites, some strong (Ti, Be), some moderate (Al, Nb, Mn, Mo, Si, Ta, V)
and other weak (Co, Cu, Zr) hardeners.
There can, however, be rather strong interactions between elements such as Co and Mo,
in that the hardening produced when these two elements are present together is much
greater than if added individually. It is found that A3B-type compounds are favored at
high Ni or (Ni+Co) contents and A2B Laves phases at lower contents.
In the unaged condition maraging steels have yield strength of about 0.7GNm−2. On
ageing this increases up to 2.0GNm−2 and the precipitation strengthening is due to an
Orowan mechanism according to the relation σ =σ0 + (αμb/L), where σ0 is the matrix
strength, α a constant and L the interprecipitate spacing.
The primary precipitation-strengthening effect arises from the (Co+Mo) combination, but
Ti plays a double role as a supplementary hardener and a refining agent to tie up residual
carbon.
The alloys generally have good weldability, resistance to hydrogen embrittlement and
stress corrosion, but are used mainly (particularly the 18% Ni alloy) for their excellent
combination of high strength and toughness.
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HEAT TREATMENT PROCESS
The heat treatment includes heating and cooling operations or the sequence of two or
more such operations applied to any material in order to modify its metallurgical
structure and alter its physical, mechanical and chemical properties.
Usually it consists of heating the material to some specific temperature, holding at this
temperature for a definite period and cooling to room temperature or below with a
definite rate.
Annealing, Normalizing, Hardening and Tempering are the four widely used heat
treatment processes that affect the structure and properties, and are assigned to meet the
specific requirements from the semi-fabricated and finished components. Steels being the
most widely used materials in major engineering fabrications undergo various heat
treatment cycles depending on the requirements.
Also aluminum and nickel alloys are exposed to heat treatment for enhancement of
properties. A brief discussion on the principles of various heat treatment processes of
steels are presented in the text to follow.
Annealing
Annealing refers to a wide group of heat treatment processes and is performed primarily
for homogenization, recrystallization or relief of residual stress in typical cold worked or
welded components.
Depending upon the temperature conditions under which it is performed, annealing
eliminates chemical or physical non-homogeneity produced of phase transformations.
Few important variants of annealing are full annealing, isothermal annealing, spheroidise
annealing, recrystallization annealing, and stress relief annealing.
Full annealing (conventional annealing)
Full annealing process consists of three steps. First step is heating the steel
component to above A3 (upper critical temperature for ferrite) temperature for
hypoeutectoid steels and above A1 (lower critical temperature) temperature for
hypereutectoid steels by 30-500C. In Figure, the terms α, γ and Fe3C refer to
ferrite, austenite and cementite phases.
The second step is holding the steel component at this temperature for a definite
holding (soaking) period of at least 20 minutes per cm of the thick section to
assure equalization of temperature throughout the cross-section of the component
and complete austenization. Final step is to cool the hot steel component to room
temperature slowly in the furnace, which is also called as furnace cooling. The
full annealing is used to relieve the internal stresses induced due to cold working,
welding, etc, to reduce hardness and increase ductility, to refine the grain
structure, to make the material homogenous in respect of chemical composition,
to increase uniformity of phase distribution, and to increase machinability.
Isothermal annealing
Isothermal annealing consists of four steps. The first step is heating the steel components
similar as in the case of full annealing.
The second step is slightly fast cooling from the usual austenitizing temperature to a
constant temperature just below A1.
The third step is to hold at this reduced temperature for sufficient soaking period for the
completion of transformation and the final step involves cooling the steel component to
room temperature in air.
Figure depicts the heat treatment cycles of full annealing and isothermal annealing. The
terms α, γ, P, PS and PFrefer to ferrite, austenite, pearlite, pearlite starting and pearlite
finish, respectively.
Isothermal annealing has distinct advantages over full annealing which are given below.
1. Reduced annealing time, especially for alloy steels which need very slow cooling to obtain
the required reduction in hardness by the full annealing.
2. More homogeneity in structure is obtained as the transformation occurs at the same time
throughout the cross section.
3. Improved machinability and surface finish is obtained after machining as compared to that
of the full annealed components.
Isothermal annealing is primarily used for medium carbon, high carbon and some of the
alloy steels to improve their machinability.
Spheroidise annealing
Spheroidise annealing is one of the variant of the annealing process that produces typical
microstructure consisting of the globules (spheroid) of cementite or carbides in the matrix
of ferrite. The following methods are used for spheroidise annealing
Holding at just below A1 Holding the steel component at just below the lower critical
temperature (A1) transforms the pearlite to globular cementite particles. But this process
is very slow and requires more time for obtaining spheroidised structure.
Thermal cycling around A1 In this method, the thermal cycling in the narrow temperature
range around A1 transforms cementite lamellae from pearlite to spheroidal. Figure
depicts a typical heat treatment cycle to produce spheroidised structure.
During heating above A1, cementite or carbides try to dissolve and during cooling they
try to re-form. This repeated action spheroidises the carbide particles.
Spheroidised structures are softer than the fully annealed structures and have excellent
machinability.
This heat treatment is utilized to high carbon and air hardened alloy steels to soften them
and to increase machinability, and to reduce the decarburization while hardening of thin
sections such as safety razor blades and needles.
Figure depicts a typical heat treatment cycle to produce spheroidised structure.
Recrystallization annealing
Recrystallization annealing process consists of heating a steel component below A1
temperature i.e. at temperature between 6250C and 6750C (recrystallization temperature
range of steel), holding at this temperature and subsequent cooling.
This type of annealing is applied either before cold working or as an intermediate
operation to remove strain hardening between multi-step cold working operations. In
certain case,
recrystallization annealing may also be applied as final heat treatment.
The cold worked ferrite recrystallizes and cementite tries to spheroidise during this
annealing process.
Recrystallization annealing relieves the internal stresses in the cold worked steels and
weldments, and improves the ductility and softness of the steel. Refinement in grain size
is also possible by the control of degree of cold work prior to annealing or by control of
annealing temperature and time.
Stress relief annealing
Stress relief annealing process consists of three steps. The first step is heating the cold
worked steel to a temperature between 5000C and 5500C i.e. below its recrystallization
temperature. The second step involves holding the steel component at this temperature
for 1-2 hours. The final step is to cool the steel component to room temperature in air.
The stress relief annealing partly relieves the internal stress in cold worked steels without
loss of strength and hardness i.e. without change in the microstructure. It reduces the risk
of distortion while machining, and increases corrosion resistance. Since only low carbon
steels can be cold worked, the process is applicable to hypoeutectoid steels containing
less than 0.4% carbon. This annealing process is also used on components to relieve
internal stresses developed from rapid cooling and phase changes.
Normalizing
Normalizing process consists of three steps. The first step involves heating the
steel component above the A3 cm temperature for hypoeutectoid steels and above
A(upper critical temperature for cementite) temperature for hypereutectoid steels
by 300C to 500C (Figure 4.7.5).
The second step involves holding the steel component long enough at this
temperature for homogeneous austenization. The final step involves cooling the
hot steel component to room temperature in still air.
Due to air cooling, normalized components show slightly different structure and
properties than annealed components.
The properties of normalized components are not much different from those of
annealed components. However, normalizing takes less time and is more
convenient and economical than annealing and hence is a more common heat
treatment in industries.
Normalizing is used for high-carbon (hypereutectoid) steels to eliminate the
cementite network that may develop upon slow cooling in the temperature range
from point Acm to point A1. Normalizing is also used to relieve internal stresses
induced by heat treating, welding, casting, forging, forming, or machining.
Normalizing also improves the ductility without reducing the hardness and
strength.
Hardening
Different techniques to improve the
hardness of the steels are conventional
hardening, martempering and
austempering.
Conventional hardening
Conventional hardening process consists of four steps. The first step involves heating the
steel to above A3 temperature for hypoeutectoid steels and above A1 temperature for
hypereutectoid steels by 500C.
The second step involves holding the steel components for sufficient socking time for
homogeneous austenization. The third step involves cooling of hot steel components at a
rate just exceeding the critical cooling rate of the steel to room temperature or below
room temperature.
The final step involves the tempering of the martensite to achieve the desired hardness.
Detailed explanation about tempering is given in the subsequent sections. In this
conventional hardening process, the austenite transforms to martensite. This martensite
structure improves the hardness.
Following are a few salient features in conventional hardening of steel.
1. Proper quenching medium should be used such that the component gets cooled at a rate
just exceeding the critical cooling rate of that steel.
2. Alloy steels have less critical cooling rate and hence some of the alloy steels can be
hardened by simple air cooling.
3. High carbon steels have slightly more critical cooling rate and has to be hardened by oil
quenching.
4. Medium carbon steels have still higher critical cooling rates and hence water or brine
quenching is necessary.
Figure depicts the conventional hardening process which involves quenching and
tempering. During quenching outer surface is cooled quicker than the center. Thinner
parts are cooled faster than the parts with greater cross-sectional areas. In other words the
transformation of the austenite is proceeding at different rates. Hence there is a limit to
the overall size of the part in this hardening process.
Martempering (marquenching)
Martempering process overcomes the limitation of the conventional hardening
process. Figure depicts the martempering process. This process follows
interrupted quenching operation. In other words, the cooling is stopped at a point
above the martensite transformation region to allow sufficient time for the center
to cool to the temperature as the surface.
Further cooling is continued through the martensite region, followed by the usual
tempering. In this process, the transformation of austenite to martensite takes
place at the same time throughout the structure of the metal part.
Austempering
This process is also used to overcome the limitation of the conventional hardening
process. Figure depicts the austempering process. Here the quench is interrupted at a
higher temperature than for martempering to allow the metal at the center of the part to
reach the same temperature as the surface.
By maintaining that temperature, both the center and surface are allowed to transform to
bainite and are then cooled to room temperature.
Austempering causes less distortion and cracking than that in the case of martempering
and avoids the tempering operation. Austempering also improves the impact toughness
and the ductility of the metal than that in the case of martempering and conventional
hardening.
Tempering
The hardened steel is not readily suitable for engineering applications. It possesses
following three drawbacks.
• Martensite obtained after hardening is extremely brittle and will result in failure
of engineering components by cracking.
• Formation of martensite from austenite by quenching produces high internal
stresses in the hardened steel.
• Structures obtained after hardening consists of martensite and retained austenite.
Both these phases are metastable and will change to stable phases with time
which subsequently results in change in dimensions and properties of the steel in
service.
Tempering helps in reduce these problems. Tempering is achieved by heating hardened
steel to a temperature below A1, which is in the range of 1000C to 6800C, hold the
component at this temperature for a soaking period of 1 to 2 hours (can be increases up to
4 hours for large sections and alloy steels), and subsequently cooling back to room
temperature.
The tempering temperature is decided based on the type of steel. Highly alloyed tool
steels are tempered in the range of 5000C - 6000C. Low alloy construction steels are
tempered above 4000C to get a good combination of strength and ductility. Spring steels
are tempered between 3000C - 4000C to get the desired properties. Figure depicts the
influence of tempering temperature on the properties of steel. It is observed that the
increase in the tempering temperature decreases the hardness and internal stresses while
increases the toughness.