m et allur gy - marine study · hardness the hardness of a metal is a measure of its ability to...

Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh.

METALLURGY

DUCTILITY • A metal is ductile when it may be drawn out in tension without

rupture.

• Wire drawing depends upon ductility for its successful

operation.

• A ductile metal must be both strong and plastic

• With many materials ductility increase rapidly with heat.

• Is the property of a material which enables it to be drawn

easily into wire form

• The percentage elongation and contraction of area, as

determined from a tensile test are a good practical measures

of ductility

• Ability to undergo permanent change in shape without rupture

or loss of strength if any force applied.

5/27/2014 2 Mohd. Hanif Dewan, Chief Engineer and

Maritime Lecturer & Trainer, Bangladesh.

MALLEABILITY

• The ability to be hammered or rolled out without

cracking.

• Very few metals have good cold malleability, but

most are malleable when heated to a suitable

temperature

• The material that can be shaped by beating or

rolling is said to be malleable.



ELASTICITY

• The elasticity of a metal is its power of returning

to its original shape after deformation by force.

• The ability to return to the original shape or size

after having been deformed or loaded.

• All strain in the stressed material disappears

upon removal of the stress.



PLASTICITY

• The property of flowing to a new shape under

pressure/stress and retaining on the new shape

after removal of pressure/stress.

• This is a rather similar property to malleability, and

involves permanent deformation without rupture.

• It is opposite to elasticity

• The ability to deform permanently when load is

applied.



Modulus of Elasticity E defined as the ratio of

tensile stress to strain and determined in a tensile

test.

Modulus of Rigidity G defined as the ration of

shear stress and strain and determined in a torsion

test.

Bulk Modulus K defined as the ration of pressure

and volumetric strain and found with specialised

equipment for liquids.

Poisson’s ratio ν defined as the ratio of two

mutually perpendicular strains and governs how

the dimensions of a material change such as

reduction in diameter when a bar is stretched.



TOUGHNESS

• Resistance to fracture by blows.

• The materials usually have high tenacity combined with good or fair ductility.

• Toughness decreases with heating.

• A combination of strength and the ability to absorb energy

or deform plastically.

• A condition between brittleness and softness.

• A materials ability to sustain variable load conditions

without failure..

• Materials could be strong and yet brittle but a material is tough has strength



HARDNESS

• The hardness of a metal is a measure of its ability to withstand scratching, wear and abrasion, indentation by harder bodies, etc.

• The machine ability and inability to cut are also hardness property which is important for workshop process.

• Hardness also decreased by heating

• A material’s resistance to erosion or wear will indicate the hardness of the material

• A material’s ability to resist plastic deformation usually by indentation

5/27/2014 8

Mohd. Hanif Dewan, Chief Engineer and


HARDNESS MATERIALS LIST:

Hard materials are diamonds and glass. Soft materials are copper

and lead. Hardness is measured by comparing it to the hardness

of natural minerals and the list is called the Moh scale. The list

runs from 1 to 10 with 1 being the softest ands 10 the hardest.

10 Diamond

9 Corundum

8 Topaz

7 Quartz

6 Feldspar

5 Apatite

4 Fluorite

3 Calcite

2 Gypsum

1 Talc 5/27/2014 9



BRITTLENESS

• Opposite of toughness.

• A brittle material breaks easily under a sharp

blow, although it may resist a steady load quite

well.

• Brittle materials are neither ductile or malleable,

but they often have considerable hardness.

• As a lack of ductility

• Strong materials may also be brittle



STIFFNESS/RIGIDTY

- This is the property of resisting deformation within

the elastic range and for ductile materials is

measured by the Modulus of Elasticity. A high E

value means that there is a small deformation for a

given stress.

- The property of a solid body to resist deformation,

which is sometimes referred to as rigidity.



Strength

• The greater the load which can be carried the

stronger the material and strength of the

material will be higher.



Tensile strength

• This is the main single criterion with reference to

metals.

• This is the ability of a material to withstand

tensile loads without rupture when the material

is in tension

• It is a measure of the material’s ability to withstand the loads upon it in service.



If the material is ductile, we look for the point at which it

starts to stretch like a piece of plasticine. This point is

called the yield point and when it stretches in this manner,

we call it PLASTIC DEFORMATION.

If the material is not ductile, it will snap without becoming

plastic. In this case, we look for the stress at which it snaps

and this is called the ULTIMATE TENSILE STRENGTH.

Most materials behave like a spring up to the yield point

and this is called ELASTIC DEFORMATION and it will

spring back to the same length when the load is removed.



The tensile test is carried out with a standard sized specimen and the force required to stretch it, is plotted against the extension. Typical graphs are shown below.



Ultimate tensile strength (UTS)

(Tensile strength or Ultimate Strength)

- It is the maximum stress that a material can withstand

while being stretched or pulled before failing or breaking.

Tensile strength is not the same as compressive strength

and the values can be quite different.

- UTS is usually found by performing a tensile test and

recording the engineering stress versus strain. The highest

point of the stress-strain curve (see point 1 on the

engineering stress/strain diagrams below) is the UTS.



Stress vs. Strain

curve typical of

aluminum.

1 Ultimate Strength

2 Yield Strength

3 Proportional Limit

Stress

4 Rupture

5 Offset Strain (usually 0.002)



Compressive Strength

• This is the ability of a material to withstand Compressive (squeezing) loads without being crushed when the material is in compression.

Shear Strength

• This is the ability of a material to withstand offset or traverse loads without rupture occurring.

Fatigue Strength • This is the property of a material to withstand continuously varying and alternating loads.

Yeild Strength

The stress a material can withstand without permanent deformation.

Torsional Strength This governs the stress at which a material fails when it is twisted and a test

similar to the tensile test is carried out, only twisting the specimen instead of

stretching it. This is a form of shearing.



HEAT TREATMENT

• Heat treatment is a general term referring to a cycle of

heating and cooling which alters the internal structure of a

metal and thereby changes its properties

• Metal and alloys are heat treated for a number of

purposes however the primarily to:-

1. Increase their hardness and strength

2. To improved ductility

3. To soften them for subsequent operations (cutting etc)

4. Stress relieving

5. Eliminate the effects of cold work



HEAT TREATMENT OF STEEL

The mechanical properties of materials can be changed by

heat treatment. Let’s first examine how this applies to carbon steels.

CARBON STEELS

In order to understand how carbon steels are heat treated

we need to re-examine the structure. Steels with carbon fall

between the extremes of pure iron and cast iron and are

classified as follows.



All metals form crystals when they cool down and change from liquid into a solid. In carbon steels, the material that forms the crystals is complex. Iron will chemically combine with carbon to form IRON CARBIDE (Fe3C). This is also called CEMENTITE. It is white, very hard and brittle. The more cementite the steel contains, the harder and more brittle it becomes. When it forms in steel, it forms a structure of 13% cementite and 87% iron (ferrite) as shown. This structure is called PEARLITE. Mild steel contains crystals of iron (ferrite) and pearlite as shown. As the % carbon is increased, more pearlite is formed and at 0.9% carbon, the entire structure is pearlite.

NAME

Dead mild

CARBON %

0.1 – 0.15

TYPICAL APPLICATION

pressed steel body panels

Mild steel

Medium carbon steel

High carbon steels

Cast iron

0.15 – 0.3

0.5 – 0.7

0.7 – 1.4

2.3 – 2.4

steel rods and bars

forgings

springs, drills, chisels

engine blocks



1538

1130

2.0

oC

695

910

0.4 0.8 1.2

AUSTENITE

AUSTENITE + FERRITE

FERRITE + PEARLITE

HYPO-EUTECTOID STEELS

PEARLITE

Mixture of Ferrite &

Cementite

EUTECTOID STEELS

AUSTENITE

AUSTENITE + CEMENTITE

AUSTENITE + CEMENTITE

HYPER-EUTECTOID STEELS

IRON – CARBON EQUILIBRIUM DIAGRAM



1538

1130

2.0

oC

695

910

0.4 0.8 1.2

AUSTENITE

FERRITE + CEMENTITE

AUSTENITE + CEMENTITE AUSTENITE + FERRITE

FERRITE

+

PEARLITE

CEMENTITE

+

PEARLITE

AUSTENITE + LIQUID

IRON – CARBON EQUILIBRIUM DIAGRAM



AUSTENITE

• A solid solution of Carbon in face-centred

cubic iron (Allotropic), containing a maximum

0f 1.7 % carbon at 1130oC

• It is soft, ductile and non-magnetic and also

exist in the plain carbon steel above the

upper critical range.

• It may however occur at room requirement,

however, occur at room temperatures in

certain alloy steels



FERRITE

• Ferrite is nearly pure iron.A solid solution of Carbon

in body-centred cubic iron, containing a maximum

of 0.04 % Carbon at 695oC.

• At room temperature, small amounts of manganese,

silicon and other elements may be dissolved in iron

as well as up to 0.007 % Carbon.

• Found only in Hypoeutectoid steel

• It is softest constitute of steel and very ductile and

readily cold-worked



CEMENTITE

• A hard brittle compound of iron and Carbon with

the formula Fe3C

• The hardest constituent of steel

• This may exist in the free state usually as a grain

boundary film, or as a constituent of the

eutectoid pearlite



PEARLITE

• This is the eutectoid structure consisting of alternate lamination of ferrite and cementite.

• It contains 0.83% Carbon and is formed by the breakdown of the austenite solid solution at 695oC

• The properties of pearlite are harder and stronger than ferrite, but softer and more ductile than cementite



If the carbon is increased further, more cementite is

formed and the structure becomes pearlite and cementite as shown.



HEAT TREATMENT of CARBON STEELS

Steels containing carbon can have their properties (hardness,

strength, toughness etc) changed by heat treatment. Basically if

it is heated up to red hot and then cooled very rapidly the steel

becomes harder. Dead mild steel is not much affected by this but

a medium or high carbon steel is.



Principle of heat treatment of steel

• Metals are never heated to the melting point in heat treatment.

• Therefore, all the reactions within the metal during the heating and cooling cycle, take place while the metal is in the solid state

• During ordinary heat treating operations, steel is seldom heated above 983oC.

• In using the iron-iron carbide diagram, we need only to concern ourselves with that part which is always solid steel.

• The area where the Carbon content is 2% or less and the temperature is below 1130oC



COOLING RATE

• Cooling rate is the most important part of heat treatment.

• Different cooling rates are now considered as they have a

significant effect on the properties of the metal.

SLOW COOLING

• Austenite is transformed to course pearlite.

• Slightly more rapid cooling may produce fine pearlite in which

the layers of ferrite and cementite are thinner.

INTERMEDIATE COOLING

• Austenite transforms to a material called Bainite instead of

the usual pearlite.

• When etched, Bainite gives a dark appearance and shows a

circular or needle like form.



FAST COOLING • By quenching in water, the transformation of

austenite is suppressed until about 318oC at which

point a new constituent called Martensite(quite brittle)

begins to form instead of the Bainite or pearlite of

slower cooling rate.

• As the temperature drops lower, the transformation

become complete.

• This temperature vary with the alloy content of the

steel



TIME TEMPERATURE TRANSFORMATION

• In order to obtain steels with the desired properties, we must have some control over the transformation process, and this is indicated in the TTT diagram

• TTT diagram are used to predict the metallurgical structure of a steel sample which is quenched in the austenite region and held to constant elevated temperature below 729oC.

• This is known as Isothermal transformation



Time (sec)

oC

0

760

725

650

590

540

430

316

260

190

90

TIME TEMPERATURE TRANSFORMATION DIAGRAM

Ferrite

form

Pearlite

starts

Pearlite

forms Pearlite is

complete

Coarse

Pearlite

Fine

Pearlite

Bainite

forming

Upper

Bainite

Lower

Bainite



TIME TEMPERATURE TRANSFORMATION

• However since heat treatment usually

involves continuous cooling, TTT diagrams

are not directly applicable but can be

modified to be useful in at least a qualitative

way for continuous cooling condition



THE AFFECT OF PROCESSING and

MANIPULATION ON METALS

When a metal solidifies grains or crystals are

formed. The grains may be small, large or long

depending on how quickly the material cooled and

what happened to it subsequently. Heat treatment

and other processes carried out on the material

will affect the grain size and orientation and so

dramatically affect the mechanical properties. In

general slow cooling allows large crystals to form

but rapid cooling promotes small crystals. The

grain size affects many mechanical properties

such as hardness, strength and ductility. 5/27/2014 37



MANIPULATIVE PROCESSES

These are processes which shape the solid

material by plastic deformation. If the process is

carried out at temperatures above the

crystallisation temperatures, then re-crystallisation

occurs and the process is called HOT WORKING.

Otherwise the process is called COLD WORKING.

The mechanical properties and surface finish

resulting are very different for the two methods.



HOT ROLLING

This is used to produce sheets, bars and sections. If the

rollers are cylindrical, sheet metal is produced. The hot slab

is forced between rollers and gradually reduced in

thickness until a sheet of metal is obtained. The rollers may

be made to produce rectangular bars, and various shaped

beams such as I sections, U sections, angle sections and T

sections. Steel wire is also produced this way. The steel

starts as a round billet and passes along a line of rollers. At

each stage the reduction speeds up the wire into the next

roller. The wire comes of the last roller at very high speeds

and is deflected into a circular drum so that it coils up. This

product is then used for further drawing into rods or thin

wire to be used for things like springs, screws, fencing and

so on.



COLD ROLLING

The process is similar to hot rolling but the metal is

cold. The result is that the crystals are elongated in

the direction of rolling and the surface is clean and

smooth. The surface is harder and the product is

stronger but less ductile. Cold working is more

difficult that hot working.



FORGING

In this process the metal is forced into shape by

squeezing it between two halves of a die. The dies may

be shaped so that the metal is simply stamped into the

shape required (for example producing coins). The dies

may be a hammer and anvil and the operator must

manipulate the position of the billet to produce the

rough shape for finishing (for example large gun

barrels).



COLD WORKING

Cold working a metal by rolling, coining, cold forging or drawing leaves the surface clean and bright and accurate dimensions can be produced. If the metal is cold worked, the material within the crystal becomes stressed (internal stresses) and the crystals are deformed. For example cold drawing produces long crystals. In order to get rid of these stresses and produce “normal” size crystals, the metal can be heated up to a temperature where it will re-crystallise. That is, new crystals will form and large ones will reduce in size. If the metal is maintained at a substantially higher temperature for a long period of time, the crystals will consume each other and fewer but larger crystals are obtained. This is called “grain growth”. Cold working of metals change the properties quite dramatically. For example, cold rolling or drawing of carbon steels makes the stronger and harder. This is a process called “work hardening”.



HOT WORKING Most metals (but not all) can be shaped more easily when hot. Hot rolling, forging, extrusion and drawing is

easier when done hot than doing it cold. The process produces oxide skin and scale on the material and producing an accurate dimension is not possible.

Hot working, especially rolling, allows the metal to re-crystallise as it is it is produced. This means that expensive heat treatment after may not be needed.

The material produced is tougher and more ductile.



LIQUID CASTING AND MOULDING

When the metal cools it contracts and the final product is

smaller than the mould. This must be taken into account in

the design.

The mould produces rapid cooling at the surface and

slower cooling in the core. This produces different grain

structure and the casting may be very hard on the outside.

Rapid cooling produces fine crystal grains. There are many

different ways of casting.



SAND CASTING

A heavy component such as an engine block would be cast

in a split mould with sand in it. The shape of the component

is made in the sand with a wooden blank. Risers allow the

gasses produced to escape and provide a head of metal to

take up the shrinkage. Without this, the casting would

contain holes and defects.

Sand casting is an expensive method and not ideally suited

for large quantity production. Typical metals

used are cast iron. Cast steel and aluminium alloy.



DIE CASTING Die castings uses a metal mould. The molten metal may be fed in by gravity as with sand casting or forced in under pressure. If the shape is complex, the

pressure injection is the best to ensure all the cavities are filled. Often several moulds are connected to one feed point. The moulds are expensive to produce but

this is offset by the higher rate of production achieved. The rapid cooling produces a good surface finish with a pleasing appearance. Good size tolerance is obtained. The best metals are ones with a high degree

of fluidity such as zinc. Copper, aluminium and magnesium with their alloys are also common.



CENTRIFUGAL CASTING

This is similar to die casting. Several moulds are

connected to one feed point and the whole

assembly is rotated so that the liquid metal is

forced into the moulds. This method is especially

useful for shapes such as rims or tubes. Gear

blanks are often produced this way.



MACHINING

Machining processes involve the removal of

material from a bar, casting, plate or billet to form

the finished shape. This involves turning, milling,

drilling, grinding and so on. Machining processes

are not covered in depth here. The advantage of

machining is that is produces high dimensional

tolerance and surface finish which cannot be

obtained by other methods. It involves material

wastage and high cost of tooling and setting.



Heat treatment Methods

• Annealing

• Normalizing

• Hardening

• Tempering



ANNEALING

Purpose of annealing

1. To soften the steel : improve machinability

2. To relieve internal stress induced by some

previous treatment (rolling, forging, uneven

cooling)

3. To remove coarseness of grain



General term for Annealing

I. Process Annealing

II. Full annealing

III. Spheroidising



i. Process Annealing • Carried out on cold-worked low carbon steel

sheet or wire in order to relieve internal stress

and to soften the material.

• The steel is heated to 550 to 650oC below the

critical point

• Prolonged annealing cause the cementite in

the pearlite to ball up or spheroidize

Increase in ductility reduce in TS & hardness 5/27/2014 52 Mohd. Hanif Dewan, Chief Engineer and


ii. Full Annealing

• It carried out on hot-worked and cast steels in

order to obtain grain refinement in combination

with high ductility.

• Compared with normalizing, it produces a softer

steel with better machinability

• For hypoeutectoid steels heating above critical

point (30 - 50oC) holding at this temperature for

a time (thickness), followed by slow cooling

usually in furnace.



iii. Spheroidizing Annealing

• High –carbon steels may be softened by annealing at 650 –

750oC just below the lower critical point, when the cementite of

the pearlite balls up or spheroidizes.

• Resulting structure is one of cementite globules in a ferrite

matrix.

• The steel can be cold drawn and possess good machinability

• Spheroidization readily on a fine pearlite structure when fine

globules of cementite are obtained. Large globules present

difficulties in machining and produce poor surface



Defects uncontrolled temperature

I. Overheating

II. Burning

III. Under annealing



i. Overheating annealing

• Heated above the actual temperature or to long

maintained at annealing temperature, austenite grain

growth will occur

• Upon cooling from this temperature, ferrite is

deposited first at the grain boundaries and then along

certain crystallographic planes

• Known as Wildmanstatten structure – weakness and

brittleness – can be remedies by reannealing

Coarse

pearlite

grains

Ferrite along crystal

planes

Ferrite at grain

boundaries



ii. Burning Annealing

• If heated above the upper critical point to

temperature approaching the solidus, fusion

and subsequent oxidation occur at the grain

boundaries.

• Brittles films of oxide are formed which make

the steel unsuitable for further use and must

be remelted



iii. Under-Annealing

• Structure are not frequently observed in the

heat–affected zones or within the critical point

• The original pearlite will have change to

several small austenite grains

• Upon cooling, ferrite is deposited at the

austenite grain boundaries

Ferrite grains unaltered

Refined ferrite and

pearlite grains



NORMALIZING

• For hypoeutectoid steels heating above critical

point (30 - 50oC) holding at this temperature for

a time (thickness), followed by cooling in still air.

• Produces maximum grain refinement and

consequently the steel slightly harder and

stronger than a fully annealed steel.

• However the properties will vary with section

thickness



HARDENING

• It is done to increase the strength and wear

properties. One of the pre-requisites for

hardening is sufficient carbon and alloy content.

• If there is sufficient carbon content then the steel

can be directly hardened. Otherwise the surface

of the part has to be carbon enriched using

some diffusion treatment hardening techniques.



1. Very slow cooling rate – austenite transforms to lamellar pearlite

2. Increasing cooling rate – depresses the transformation temperature giving a finer, harder pearlite, until second transformation occurs at 150 –130oC when martensite is formed

3. When certain cooling rate known as critical cooling rate – austenite direct to martensite as the hardest structure in a given steel.

4. With certain alloys steels the critical cooling rate may be sufficiently low to enable full hardening to be obtained by oil quenching or even air cooling



MARTENSITE

TROOSTITIC

PEARLITE

SORBITIC PEARLITE

LAMELLAR PEARLITE

Time

Temp

Transformation to martensite

Critical

cooling rate



QUENCHING

• To harden by quenching, a metal (usually steel or cast

iron) must be heated into the austenitic crystal phase

and then quickly cooled. Depending on the alloy and

other considerations (such as concern for maximum

hardness vs. cracking and distortion), cooling may be

done with forced air or other gas (such as nitrogen), oil ,

polymer dissolved in water, or brine. Upon being rapidly

cooled a hard brittle crystalline structure. The quenched

hardness of a metal depends upon its chemical

composition and quenching method.



CASE HARDENING

• Case Hardening is the process of hardening the

surface of a metal, often a low carbon steel, by

infusing elements into the material's surface,

forming a thin layer of a harder alloy.



TEMPERING

Tempering is a process of heat treating, which is used to

increase the toughness of iron-based alloys.

Tempering is usually performed after hardening, to reduce

some of the excess hardness, and is done by heating the

metal to some temperature below the critical temperature for

a certain period of time, then allowed to cool in still air.

The exact temperature determines the amount of hardness

removed, and depends on both the specific composition of

the alloy and on the desired properties in the finished

product. For instance, very hard tools are often tempered at

low temperatures, while springs are tempered to much

higher temperatures. 5/27/2014 65 Mohd. Hanif Dewan, Chief Engineer and


• When temperature region 200 – 450oC the

martensite decomposes into ferrite and the

precipitation of the fine particles of carbide occurs

known. as troostite

• At higher temperatures 450 – 650oC the carbide

particles coalesce thus producing fewer and larges

particles which provide fewer obstacles to

dislocations resulting further increasing toughness

while decrease in strength and hardness and known

as sorbite.

• Sorbite is ideal for components subject to dynamic

stresses such as crankshaft and connecting rod



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SORBITE MARTENSITE TROOSTITE

200 400 600

oC

Hardness

200

800

600

400

1000

EFFECT OF TEMPERING



ALLOYS

Mohd. Hanif Dewan,

Chief Engineer and

Nickel - One of the most widely used alloying elements in

steel. In amounts 0.50% to 5.00% its use in alloy

steels increases the toughness and tensile

strength without detrimental effect on the ductility.

Chromium - Gives resistance to wear and abrasion.

Chromium has an important effect on corrosion

resistance and is present in stainless steels in

amounts of 12% to 20%.

5/27/2014 68

ALLOYS

Mohd. Hanif Dewan,

Chief Engineer and

•Molybdenum - Increases hardenability, toughness to

quenched/tempered steels. It also improves the

strength of steels at high temperatures (red-

hardness).

•Vanadium - Steels containing vanadium have a much finer

grain structure than steels of similar composition

without vanadium.

5/27/2014 69

CREEP

• Creep is strain increase with time under constant load.

• Creep is temperature dependent – the

higher the temperature the greater the

effect



FRETTING

A type of wear that occurs between tight-fitting

surfaces subjected to cyclic relative motion of

extremely small amplitude. Usually, fretting is

accompanied by corrosion, especially of the very

fine wear debris.

FRETTING CORROSION

The accelerated deterioration at the interface

between contacting surfaces as the result of

corrosion and slight oscillatory movement between

the two surfaces. 5/27/2014 71



IMPURITIES

Mohd. Hanif Dewan,

Chief Engineer and

Sulphur

– The presence of free sulphur in a steel product is detrimental to its properties, especially toughness.

Phosphorous

– Its presence in steel is usually regarded as an undesirable impurity due to its embrittling effect, for this reason its content in most steels is limited to a maximum of 0.050%.

5/27/2014 72

Welding Metallurgy



Heat Affected Zone Welding Concerns



Heat Affected Zone Welding Concerns

Changes in Structure Resulting in Changes in Properties

Cold Cracking Due to Hydrogen

Two major concerns occur in the heat affected zone which

effect weldability these are,

a.) changes in structure as a result of the thermal cycle

experienced by the passage of the weld and the resulting

changes in mechanical properties coincident with these

structural changes, and

b.) the occurrence of cold or delayed cracking due to the

absorption of hydrogen during welding.



First let’s review the thermal cycles experienced in the heat affected zone as a result of the passage of the weld. The

figure illustrated here shows the temperature vs time curve at

various distances from the weld metal. Note that almost

every thermal cycle imaginable occurs over this short

distance of the heat affected zone. Thus a variety of

structural and property variations are expected.



Look At Two Types of Alloy Systems



There are two types of alloy systems which we will

consider, those which do not have an allotropic

phase change during heating like copper, and

those which have an allotropic phase change on

heating like steel. We will first consider those

materials which do not have an allotropic phase

change. The top schematic illustrates this type of

material. We will however consider that this

material has been cold worked (not the elongated

cold worked grains present in the base material

(region A). The weld metal is represented by

region C, and the heat affected zone is region B.



Note that the heat of welding has effected the structure of this material even though there are no allotropic transformations. Recall that cold worked structures undergo recover, recrystalization and grain growth when heated to ever increasing temperatures. So it is in this material. As we traverse from the cold worked elongated grains in the unaffected base metal, we come to a region where the cold worked grains undergo recovery and then shortly there after they recrystalize into fine equaled new grains. Traversing still closer to the weld region we note grain growth where the more favorably oriented grains consume neighboring grains and grain growth occurs. The grains within the weld epitaxially nucleate from

the grains in the heat affected zone at the fusion boundary, and grain growth continues into the solidifying weld metal making very large grains. 5/27/2014 79



Introductory Welding Metallurgy,

AWS, 1979

Cold Worked Alloy Without Allotropic Transformation



One of the factors that occur when cold worked grains

recrystalize and grain grow occurs we have already

discussed, and that is the material softens. Thus the heat

affected zone and weld metal will not hold the same

strength level as the cold worked base metal. Another

consequence of increased grain size is perhaps equally

important and that is that the larger grains are more brittle.

A “Charpy” impact test is used to determine how much impact energy a structure will absorb over various

temperature ranges. Note that the larger grain size

material will become brittle and not absorb much of an

impact load even at temperatures around room

temperature and above.



Welding Precipitation

Hardened Alloys Without

Allotropic Phase Changes

Welded In:

• Full Hard Condition

• Solution Annealed

Condition



A second way of strengthening materials without allotropic

phase changes is by precipitation strengthening. (The first

we just discussed was cold working). Recall that in

precipitation strengthening, the base metal is solutionized,

rapidly cooled and then aged at some moderately elevated

temperature to promote precipitate formation. There are

two ways that precipitation hardened material can be

welded. One is to weld on the full hard, that is the already

aged base metal. The second is to weld on material which

has been solution annealed and rapidly cooled, but not yet

given the ageing heat treatment. In either case, when

welding, the heat affected zone will see some additional

time at temperature (varied temperature over the distance

of the HAZ) as illustrated above, and this will effect the

aged or overaged condition of the precipitates.

5/27/2014 83



Annealed upon

Cooling



When welding on the already aged (full hard) material,

the unaffected base metal will have aged precipitates that are just the right size for strengthening. The heat affected zone, on the other hand, will experience some

additional heating. In the region farthest from the weld the heat will be sufficient to overage the precipitates with the resulting loss in strength. In regions closer to the weld, the heat will be so excessive that the

temperature will exceed the two phase region and the single phase solutionizing region on the phase diagram will be entered. Again, a loss in strength will

occur, but this region at least might be able to be re-aged to recover some strength.



Let us now turn our attention to the materials which do

have an allotropic phase change during heating. A typical

material like steel is ferrite at low temperatures and

transforms to austenite when heated. Each time the

material goes through one of these phase changes, new

finer equaled grains grow starting from the grain

boundaries of the previous grains present. So in the case

of cold worked steels in the base metal, the elongated cold

worked grains will undergo recovery, recrystalization and

grain growth just as discussed above. But now the

recrystallized grains at higher temperature will undergo the

allotropic phase change, reducing the grain size again

which then is followed by grain growth at still higher

temperature (nearer the weld). This variation in grain

structure is schematically shown in the lower figure above.




AWS, 1979

Steel Alloys With Allotropic Transformation



This illustration shows the various regions in the heat

effected zone and what microstructure would be predicted

as related to the iron-carbon phase diagram. Note that at

the far extent of the element in the base metal, ferrite and

commentate arte expected. Closer to the weld some dual

phase ferrite austenite will occur at temperature of welding.

Closer yet we would expect single phase austenite, and

then maybe some austenite of delta ferrite and liquid

mixtures until at the maximum temperature the liquid phase

would be present as the welding arc traverses. These are

the structures at temperature, but we now must consider

what happens during cooling.



We have already seen that the cooling rate from

welding can vary depending upon a number of

weld variables. The two most important are

preheat and heat input. The cooling rate is fastest

when no preheat and low heat input are used to

make the weld. On the other hand, the cooling

rate is slowest when high preheat and high heat

input are employed.




AWS, 1979



As we have learned before, the cooing rate from austenite can

effect the room temperature structure as defined by the

continuous cooling transformation diagram. Rapid cooling results

in non-equilibrium hard brittle martensite. Slow cooling results in

some higher temperature transformation products such as

bainite, ferrite and pearlite which tend to be softer. Examining

two welding procedures here, one with no preheat (number 1)

and the other with preheat (number 2) we find some differences

in structure. The no preheat weld has a narrower HAZ and rapid

cooling and the austenite transforms to martensite on cooling

giving a hard martensite peak near the fusion line. The weld with

preheat has a wider HAZ, a slower cooling rate producing ferrite

pearlite and bainite and the fusion line peak is softer. There is

also more outer HAZ region grain growth and overaging so that

the softening in the HAZ is greater. Thus, once again, welding

procedures have to be carefully tailored for the material being

welded.

5/27/2014 93



How does the hydrogen get into the heat effected

zone where the cold cracking is often observed?

Liquid metal can absorb more hydrogen than solid

austenite, and austenite more than ferrite. When

welds are made on wet material or with wet

electrodes, the hydrogen is absorbed into the

liquid. As the liquid solidifies, if forces some of the

hydrogen which it is trying to get rid of into the

surrounding hot austenite. If there is still too much

to be absorbed even in a supersaturated solid,

some hydrogen porosity may form in the weld

metal, a sure sign that poor procedures were

followed.



During cooling, the cooler material tries to push

hydrogen out while at the same time the solidifying

weld metal tries to push hydrogen out. Note that

the large grained region of the HAZ which just may

have the hardest most susceptible martensitic

microstructure thus acquired hydrogen from both

directions and a supersaturated condition exists

there.



The hydrogen then slowly diffuses to any location

where is can relieve the stress of being stuck in the lattice in the supersaturated condition. The hydrogen atoms are often carried by dislocation and the preferred site for collection is often inclusions. At this

point, they can either weaken the surrounding structure or the hydrogen atoms can recombine and form molecular hydrogen gas and exert an internal

pressure. As this pressure grows, the crack slowly expands until a critical size is reached and catastrophic failure occurs. This takes time at low

temperature , thus the common name of cold cracking or delayed cracking applies.



The time after welding has an effect. As time

proceeds, the hydrogen diffuses away from the

high concentration in the most critical portion of the

heat affected zone. If hydrogen diffuses away

before the critical crack length is reach, the weld

has occurrence of some micro cracks but

catastrophic failure does not occur. On the other

hand, if hydrogen diffusion is slower than that

failure may occur. Elevated temperature post weld

treatment will allow fast hydrogen diffusion and

may help in the reduction of cold cracking.



Dickinson 5/27/2014 102



The above diagram summarizes the discussions

about delayed cracking. The red regions are crack

sensitive regions while the blue represents the

safe region. Materials with high hardenabilty will

promote the formation of martensite, and materials

with high carbon content will produce a harder

martensite. Increases in heat input and preheat

and stress reliving practices increases the safety

against hydrogen delayed cracking. And the

decrease in hydrogen in the welding process

likewise increases the safety region.



Why Preheat?

• Preheat reduces the temperature

differential between the weld region and the base metal

– Reduces the cooling rate, which reduces the

chance of forming martensite in steels

– Reduces distortion and shrinkage stress

– Reduces the danger of weld cracking

– Allows hydrogen to escape

0.1.1.5.1.T9.95.12 5/27/2014 104 Mohd. Hanif Dewan, Chief Engineer and


Using Preheat to Avoid Hydrogen

Cracking • If the base material is preheated, heat flows more

slowly out of the weld region

– Slower cooling rates avoid martensite formation

• Preheat allows hydrogen to diffuse from the metal

Cooling rate T - Tbase)2

Steel

Cooling rate T - Tbase)3

T base

T base



Why Post-Weld Heat Treat?

• The fast cooling rates associated with welding often produce martensite

• During postweld heat treatment, martensite is tempered (transforms to ferrite and carbides)

– Reduces hardness

– Reduces strength

– Increases ductility

– Increases toughness

• Residual stress is also reduced by the postweld heat treatment

Carbon and Low-Alloy Steels



Postweld Heat Treatment and

Hydrogen Cracking

• Postweld heat treatment (~ 1200°F) tempers any martensite that may have formed

– Increase in ductility and toughness

– Reduction in strength and hardness

• Residual stress is decreased by postweld heat treatment

• Rule of thumb: hold at temperature for 1 hour per inch of plate thickness; minimum hold of 30 minutes

Steel



Base Metal Welding Concerns



Lamellar Tearing

• Occurs in thick plate subjected to high transverse welding stress

• Related to elongated non-metallic inclusions, sulfides and silicates, lying parallel to plate surface and producing regions of reduced ductility

• Prevention by

– Low sulfur steel

– Specify minimum ductility levels in transverse direction

– Avoid designs with heavy through-thickness direction stress

Cracking in Welds



Improve Cleanliness Improve through thickness properties

Buttering



This illustrates how the rolled out inclusions (mainly MnS)

can de-bond from the base metal matrix and under the

action of short transverse (through thickness) stresses they

can actually link to form a stepped like fracture. Improving

cleanliness of the steel during steel processing, and

improving through thickness properties by steel making

processed line calcium or rare earth treatment which

produces inclusions which to not roll out a long stringer

during plate processing can help. Also laying a weld bead

on top of the plate which has lower strength and improved

ductility before welding the attachment can help by letting

the weld bead take the shrinkage stresses rather than

transmitting them into the base plate.

5/27/2014 Mohd. Hanif Dewan, Chief Engineer and

Maritime Lecturer & Trainer, Bangladesh. 111

Multipass Welds • Heat from subsequent passes affects the structure and

properties of previous passes

– Tempering

– Reheating to form austenite

– Transformation from austenite upon cooling

• Complex Microstructure.

• In a multi-pass weld, the heating and cooling cycles of one

pass are superimposed upon those of previous passes.

Portions of previous passes are heated high enough to form

austenite again, and upon cooling this austenite once again

can transform to ferrite and pearlite or to martensite. Some

portions of previous weld passes will not transform to

austenite but will be tempered by the heat from subsequent

passes. All in all, this leads to a rather complicated structure

in multi-pass welds.



Multipass Welds

• Exhibit a range of microstructures

• Variation of mechanical properties across joint

• Postweld heat treatment tempers the structure – Reduces property

variations across the joint

Steel



Reheat Cracking • Mo-V and Mo-B steels susceptible

• Due to high temperature embrittlement of the heat-affected zone and the presence of residual stress

• Coarse-grained region near fusion line most susceptible

• Prevention by

– Low heat input welding

– Intermediate stress relief of partially completed welds

– Design to avoid high restraint

– Restrict vanadium additions to 0.1% in steels

– Dress the weld toe region to remove possible areas of stress concentration

Cracking in Welds



Steels containing molybdenum or vanadium resist creep at

elevated-temperature. These steels, along with thick

sections of high-strength, low-alloy steels, are subject to

reheat cracking in combination with residual stress and low

creep-ductility in the HAZ.

During postweld heat treatment, cracks form along the

grain boundaries in the HAZ, particularly in the coarse-

grained region near the fusion line.

Defects at the weld toe can promote reheat cracking;

therefore, grinding or peening the weld toe can help

prevent this cracking.

The cracked area must be heat treated to restore ductility

prior to repair. Then it can be cut out beyond the ends of

the cracks and rewelded.



Knife-Line Attack in the HAZ

• Cr23C6 precipitate in HAZ

– Band where peak temperature is 800-1600°F

• Can occur even in stabilized grades

– Peak temperature dissolves titanium carbides

– Cooling rate doesn’t allow them to form again

Weld

HAZ

Knife-line attack

Stainless Steel



A discrete band in the heat affected zone of the austenitic

stainless steel welds experiences peak temperatures in the

800°-1600°F temperature range associated with

sensitization.

Chromium carbide precipitation in this region can lower the

chromium content near the grain boundaries to less than

12%, thereby causing sensitization.

Stabilized grades can also suffer from knife-line attack.

Elevated temperatures in the heat-affected zone can

dissolve titanium and niobium carbides. The fast cooling

rates in the welded joint do not allow these carbides to

reform. This leaves excess free carbon, which can then

form chromium carbides.



WELDING FAULTS

Root Faults

For deep vee multi run welds the first run or root weld is critical to the quality of the welds laying on top. Typical faults may be caused by too high or low a current of too large a rod.



Fusions Faults

The three main causes of this is too low current for rod, too high a

travel rate or when too small a rod is used on a cold surface.

Bead Edge Defects

normally in the form of under cutting or edge craters. The main

cause for this is incorrect current setting. Too high will lead to

undercutting, too low to edge craters. Similar efects may occur at

the correct current due to incorrect arc length. Edge faults are

particularly common in vertical welding or 'weave' welding. The

general cause for the latter being a failure to pause at the

extremes of the weave. Edge defects are stress raisers and lead

to premature weld failure.

Porosity

May have many causes the most common being moisture in the

rod coating or in the weld joint. Poor rod material selection is also

a factor 5/27/2014 119



Heat Cracks

this is a destructive fault caused generally due to incompatiablity of

the Weld material and weld Rod. Indeed in some cases the

material may be deemed unweldable. Heat cracks occur during or

just after the cooling off period and are caused by impurites in the

base metal segrateing to form layers in the middle of the weld. The

layers prevent fusion of the crystals. The two main substances

causing this are Carbon and Sulphur. A switch to 'basic'

electrodes may help.

Anouther cause is temsion acroos the weld which , even without

segregation in the weld, cause a crack. This occurs during a

narrow critical temerpature range as the bead coagulates. During

this period the deformation property is small, if the shrinkage of the

base material is greater than the allowed stretch of the weld then a

crack will result. One method of preventing this is to clamp the

piece inducing a compressive force on the weld during the cooling

period

5/27/2014 120



Shrinkage Cracks Thes form due to similar effect of allowed weld deformation being less than base metal shrnkage although it is not associated with the critical temerpature rang above and therefore cannot be elleviated by

compression. The use of 'basic' electrodes can help

Hydrogen cracks This is generally associated what either hardened material or material hardened during the welding process. The hydrogen source can be moisture, oil, grease etc. Ensuring that the rod is dry is essential and preheating the weld joint to 50'C will help. The cracking occurs adjacent to the weld pool and allied to the tension created during the welding porcess will generate a through weld crack.

Slag Inclusion This common fault is caused by insufficient cleaning of the weld between runs. If necessary as well as using a chipping hammer and brush grind

back each weld run with an angle grinder. Once the slag is in the weld it is near impossible to removed it by welding only

5/27/2014 121



Metallurgical Testing



Non-Destructive Testing

- This is carried out on components rather than on test

pieces, they are designed to indicate flaws occurring due or after manufacture. They give no indication of the mechanical properties of the material.

- Surface flaws may be detected by visual means aided by dye penetrant or magnetic crack detection. - Internal flaws may be detected by X-ray or ultrasonic testing.

- In addition to this there are special equipment able to exam machine finish.



Liquid Penetrant Methods - The surface is first cleaned using an volatile cleaner and

degreaser.

- A fluorescent dye is then applied and a certain time

allowed for it to enter any flaws under capillary action.

Using the cleaning spray, the surface is then wiped clean. -

- An ultra violet light is shone on the surface, any flaws

showing up as the dye fluoresce.

Dye penetrant method - The surface is cleaned and the low viscosity penetrant

sprayed on.

- After a set time the surface is again cleaned. A developer

is then used which coats the surface in a fine white chalky

dust he dye seeps out and stains the developer typically a

red colour. 5/27/2014 124



Both these methods are based loosely on the old

paraffin and chalk method.



Magnetic crack detection

A component is place between two poles of a magnet The

lines of magnetism concentrate around flaws. Magnetic

particles are then applied, in a light oil or dry sprayed, onto

the surface where they indicate the lines of magnetism and

any anomalies. This method of testing a has a few

limitations. Firstly it cannot be used on materials which

cannot be magnetised such as austenitic steel and non-

ferrous metals. Secondly it would not detect a crack which

ran parallel to the lines of magnetism.



- The test pieces are machined to a standard size

depending on the thickness of the metal in

question.

- When a material is tested under a tensile load, it

changes shape by elongating. Initially the

extension is in proportion to the increasing tensile

load. If a graph is plotted showing extension for

various loads, then a straight line is obtained at

first. If the loading is continued the graph deviates

as shown.

Tensile Testing



When the test piece reaches the Yield point there is a failure of the crystalline structure of the metal, not

along the grain boundaries as has been the case, but through the grains themselves. This is known as slip. A partial recovery is made at the lower yield point, then the extension starts to increase. If the load is removed at any stage along the curve Y-U the material will have a corresponding permanent

deformation. This termed permanent set. Maximum loading occurs at the ultimate Load U and at this stage local wasting or extension will start. Normally this starts at about the centre of the specimen and will rapidly be followed by failure. 5/27/2014 128



Within the limit of the straight line, if the load is removed

the material will return to its original length. The graph can

be plotted as load and extension or as stress and strain.

Stress is load per unit area. Strain is extension divided by

original length.

Hookes law states that within the elastic limit, stress is

proportional to strain.

Stress ∞ Strain Stress = Strain x Constant

Constant = Strain / Stress This constant is called Young's Modulus of elasticity.



For a material which does not

have a marked yield point such

as aluminium, there is a

substitute stress specified. This

is termed the proof stress.

Proof stress is determined from

a load/extension or stress/strain

graph. It is obtained by drawing

a line parallel to the straight

portion and distant from it on a

horizontal scale, by an amount

representing a particular non-

proportional elongation. e.g.

0.1% proof stress is found

through 0.1% non-proportional

elongation



Creep Testing

- Creep tests are carried out

under controlled temperature

over an extended period of

time in the order of

10,000hrs.

- The test piece is similar to

the type used for tensile

tests and creep is usually

thought of as being

responsible for extensions

of metal only. In fact creep

can cause compression or other forms of deformation

5/27/2014 132



- Temperature of the test is around that of recrystallization

which for steels starts around 400oC. For other metals the

recrystallization temperature is different being about 200oC

for copper and room temperature for tin and lead.

- At the start of the test the initial load must be applied

without shock. This load, normally well below the limit

strength limit of the material, will extend the test piece slowly.

- The load is kept steady through the test and the

temperature is maintained accurately.

- Extension is plotted and is seen to proceed in three distinct

stages.



HARDNESS TESTING

The basis of the Brinell hardness

testing is the resistance to

deformation of a surface by a

loaded steel ball.

Oil is pumped into the chamber

between the pistons until there is

sufficient pressure to raise the

Weight so that it is floating. The ball

is now forced into the specimen

material at the same force. The

loading for steel and metals of

similar hardness is 3,000Kg. The

load is allowed to act for 15 sec to

ensure that plastic flow occurs.



The surface diameter of the indentation is measured with the aid of a microscope which is traversed over

the test piece on a graduated slide with a vernier. Cross wires in he microscope enable the operator to accurately align the instrument. Both the loading and

ball diameter (10mm) are known, by measuring the indentation diameter the hardness can be calculated. For softer materials the loading is reduced, copper being 1000Kg and aluminium 500Kg. The diameter of

the indentation must be less than half the ball diameter.



The thickness of the specimen must be not less than 10x the depth of the impression. The edge of the

impression will tend to sink with the ball if the surface has been work hardened; otherwise the local deformation will tend to cause piling up of the metal

around the indent If the hardness test is used on very hard materials, the steel ball will flatten. This method is not reliable for reading over 600. It is used in preference to other

methods where the material has large crystals, e.g. Cast iron. Mild Steel 130, Cast Iron 200, white cast iron 400,

nitrided surface 750.



BRITTLE FRACTURE TESTING

Under low temperature conditions , impact or shock

loading on a material can cause cracking in a material

which is normally ductile at room temperature

Critical stressing in a material

Griffith equation sc = Kic / ж Pc

where sc is the critical stress in a material

Kic is the fracture toughness of a material

Pc is the micro-crack length within the materials

The presence of these micro cracks (porous materials or

defects) can act to cause transcrystalline type failures with

a bright crystalline appearance.

Testing is carried out via the Charpy notched piece test at various temperatures between -200o to +200oC



To reduce the effects of brittle fracture the carbon content in

carbon steels is kept as low as practical. Grains within the

materials are kept as small as possible by heat treatment

and normalizing.

Alloying elements may also be added.

Factors which affect the transition temperature are

1. Elements; Carbon, silicon, phosphorus and sulphur raise

the temperature.

Nickel and manganese lower the temperature.

2. Grain size; the smaller the grain size the lower the

transition temperature, hence grain refinement is beneficial.

3. Work hardening; this appears to increase transition

temperature.

4. Notches; possibly occurring during assembly e.g. weld

defects or machine marks. Notches can increase tendency to

brittle fracture.

5/27/2014 139



18/80 stainless steel

It is this property of stainless steel that makes it so suitable

for use in LPG carriers. Hardness Testing 5/27/2014 140 Mohd. Hanif Dewan, Chief Engineer and


ANY QUESTION?

THANK YOU! 5/27/2014



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