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.

.

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.