25780297 Het Tretment by Sir Ishaq

Composed by: Aftab Ahmed Laghari (07MT38) Supervised by: Sir M. I. Abro

BY: SIR MUHMMED ISHAQUE ABRO

Classes: 1-2

INTRODUCTION TO SUBJECT:

Strengthening Mechanisms:

•Solid Solution Strengthening•Dispersion Strengthening•Work Hardening/Dislocation Strengthening•Heat Treatment/Thermal Treatment

Strengthening Mechanisms (Solid Solution Strengthening):

•Solid solution strengthening is a phenomenon that occurs when the number of impurity atoms in the lattice of the basic element is so small that they are incapable of forming both stable and metastable precipitation phases under any thermal treatment conditions. •Nevertheless the impurity atoms favor improvement of mechanical properties. This can be accounted for the following. –The presence of impurity atoms in the matrix lattice leads to distortion of the lattice because of the difference in size between the atomic radii of the impurity and the basic component. This in turn leads to the appearance of elastic deformation fields, which retard movement of dislocations in slip planes under the action of applied stresses. –In addition, the impurity atoms can inhibit movement of dislocations by forming impurity atmospheres around them.–Grain size refinement also takes place due to addition of some metallic impurities like AL and Cu •All of the above factors play a leading role in solid solution strengthening.

Strengthening Mechanisms (Dispersion Strengthening):

•In the majority of metal alloys, precipitation of supersaturated solid solutions formed during quenching is followed by precipitation of disperse particles enriched in atoms of the alloying components. •It was found that the strength (hardness) of the alloys increases with the precipitation of these particles.

Tendency for steel to absorb nitrogen as well as carbon, which is sometimes not desired. Requires special salt bath furnace.Not as suitable for volume production as gas carburizing.Disposal of spent salt may be a problem because of poisonous nature.

Relatively easy control of surface carbon content; suitable for high volume production

Molten salts containing NaCN

Liquid Carburizing

Requires special furnaces and atmosphere generators.

Relatively easy control of surface carbon content; suitable for high volume production

Various prepared atmospheres, e.g. H

2+CO+CH

4

Gas Carburizing

Difficult to control surface carbon content, particularly when carbon content must be held below 1%; usually requires individual handling of parts requires special furnaces and atmosphere generators.

Required no special furnace equipment; only restriction is that furnace must be capable of operation at required temperature.

Solid carbon e.g. charcoal – coke mixture

Pack Carburizing

DIS-ADVANTAGESADVANTAGESCARBURIZING MEDIUM

PROCESS


•The increment in the value of these characteristics increases as the dispersion and volume fraction of the particles increase.•This phenomenon has been referred to as dispersion strengthening.

Strengthening Mechanism ( Work Hardening):•An important method used to strengthen steels is deformation strengthening. •Strengthening achieved with crystal deformation can be judged from the shape of stress–strain curves.

What is Heat Treatment? Heating, Soaking and Cooling

Why Heat Treatment is?•To obtained best combination of desired mechanical properties•For example:–High hardness and strength with low toughness value– High ductility and percent elongation with low hardness and strength value–High hardness and strength with high toughness value.

How?•By altering:–Heating Temperature–Soaking Time–Cooling Rate–Heating and Cooling Cycles

0

100

200

300

400

500

600

700

800

900

0 10 20 30

Time

Tem

eper

atu

re


Why?•Metallurgical Characteristics: –Grain size–Microstructure–Precipitation of Secondary Phase–Distortion of Crystal Lattices•are Changing With Heating and Cooling Cycle Grain Size:

•Garin (Demo_4)•Number of grains per unit area•Coarse grain–Small number of grains (4) per unit area.•Fine grain–High number of grains (18) per unit area.

Grain Boundary:•The grain boundary is one of the basic structural elements in polycrystalline materials.•The grain boundary represents an interface between two differently oriented crystals. •This is the region of crystal imperfection. •It is capable of moving and adsorbing impurities. •The boundary has a high diffusive permeability.•Movement of grain boundaries controls the process of recrystallization.•Grain boundaries adsorb impurities. •Embrittlement of metal material is connected with enrichment of grain boundaries in impurities.•Grain boundaries may conventionally be divided into two large groups: low-angle and•large-angle boundaries. •Low-angle boundaries (or subgrain boundaries with an angle of less•than 10o) represent networks or walls of dislocations. •The structure of large-angle boundaries is much more complicated.


Grain Boundary:

Microstructure:•Solid phases present at micro scale within the metal and its alloys •Nomenclature of these phases is based on Physico-chemical attributes–Solid solution, mechanical mixture, chemical compound–Their orientation (equiaxed, preferred) –Morphology or texture (grainular, spherical, plate, needle)

Microstructures of Steels:•Ferrite–Solid solution of carbon in BCC Iron.

Microstructures of Steels:•Austenite–Solid solution of carbon in FCC iron

Pearlite

Ferrite


Microstructures of Steels:•Martensite–Supersaturated solid solution of carbon in BCC iron (BCT)

Microstructures of Steels: Martensite

Microstructures of Steels:•Cementite–Chemical compound of iron and carbon (Fe3C).

OR

Microstructures of Steels:•Pearlite–Mechanical mixture of ferrite and cementite•Leduberite


–Mechnical mixture of austenite and cementite.

Microstructures of Steels:•Bainite–Mechanical mixture of ferrite and cementite–Upper bainite resembles pearlite and lower bainite martensite.

Class: 3-4

TYPES OF HEAT TREATMENT


Types of Heat Treatments:•Ferrous metals and Alloys–Conventional and Non conventional•Non Ferrous metals and Alloys–Age hardening

Heat Treatment of FerrousAlloys(Steel andCast Iron):

Conventional Heat treatment Processes:•What is Normalizing–Heating above the upper critical Temperature and cooling slowly.•What is Annealing –Heating above the upper critical temperature (Hypo) and lower critical (Hyper) and cooling very slow•What is Hardening–Heating above critical temperature and cooling rapidly.–Martensite rather than Pearlite•What is Tempering–Heating Below Critical temperature and cooling slowly.•Note: –Isothermal transformation in both normalizing and annealing–Athermal Transformation in hardening

Heat treatment:Normalising:

• Heat to Upper Critical Temperature, at which point the structure is all Austenite• Cool slowly in air.• Structure will now be fine equi-axed pearlite.• Used to restore the ductility of cold or hot worked materials whilst retaining other properties.

Annealing:• 1.Heat to above Upper Critical Temperature, at which point the structure is all Austenite• 2.Cool very slowly in the furnace.


• 3.Structure will now be large-grained pearlite.• 4.Used to improve the properties of cast and forged steels prior to machining.

Conventional Heat treatment Process:Why Normalizing?

• –To improve mechanical Properties. (i.e. to develop best combination of mech properties.) • –Fine grain microstructure.

When Normalizing?• –After Casting • •To eliminate Dendritic Structure• –After Metal working• •To remove residual stresses.

Why Annealing?• –To improve ductility at the cost of hardness and strength.• –Coarse grain microstructure.• –TO relieve residual stresses.

When Annealing?• –Before and after metal working• –After casting (Hyper steels).

Annealing Types:Diffusion Annealing:

• Annealing is to eliminate inhomogeneities of the• chemical composition which appear during crystallization of alloys. • This annealing is usually carried out in the range of the solid solution at a temperature of 1100–1300OC. • Diffusion annealing can be used primarily to• smooth out a difference in the content of alloying elements • Differences in microhardness are eliminated simultaneously. • The overall hardness of the alloy decreases because liquation regions possessing high hardness are

removed. Some average hardness is obtained. • The success of diffusion annealing largely depends on the steel purity and liquation. This type of

annealing is usually used to improve properties of medium-purity steels.

Softening Annealing:• It is used to produce the structure of globular pearlite. • This structure is very soft and• readily lends itself to deformation during drawing, cold rolling, etc.• Steels with a low-carbon content become too soft after this annealing treatment.• The globular pearlite structure is favorable in steels with a carbon concentration of more than 0.5%. • Another goal of softening is to produce a uniform fine structure with finely dispersed carbon after

quenching.


• The simplest method of softening consists in holding for many hours at a temperature slightly above Ac1. In this case, martensite that is left from the previous treatment is removed and the work hardening caused by deformation (e.g., forging) is eliminated.

• Carbide plates of pearlite fully coagulate only after a long annealing time.• As fine-plate pearlite transforms more easily to globular pearlite.

Annealing Types (Stress Relief Annealing and Recrystallization Annealing):

• Dislocation pile-ups and crystal lattice distortions arising in cold-deformed metals may result in the appearance of macroscopic stresses (stresses of the first kind). Usually these stresses are very high.

• Changes in properties that occur under cold deformation are rectified by employing stress relief annealing.

• The greater the degree of deformation, the lower the heating temperature.• Depending on the temperature and time of annealing, various structural changes take place in a cold-

deformed material. • The changes are divided into recovery and recrystallization processes.• If recovery proceeds without the formation and migration of subgrain boundaries inside the

recrystallized grains, it is called restoring. • If subgrain boundaries are formed and migrate inside the crystallites, recovery is referred to as

polygonization/recrystalization.

Conventional Heat treatment Process:Why Hardening

• To improve the strength and hardness of the material/steel.

Tempering:• Tempering : a process of heating a martensitic steel at a temp. below the eutectoid temp. to make it

softer and more ductile. • Fe3C particles precipitates from the a’ phase → tempered martensite → spheroidite

Spheroidite

CFe' 3+ → αα reheat


Tempering Temperatures:


MARTEMPERING:

• Martempering : a modified quenching procedure used for steels to minimize distortion and cracking upon quenching.

• Austenitizing → quenching in hot oil or molten• salt at a temp. just above the Ms → moderate cooling to Ms

AUSTEMPERING:•Austempering : an isothermal treatment which produce a bainite structure in some carbon steels.


Classes:5-6

HEAT TREATMENT STEPS

MAIN STEPS OF HEAT TREATMENT: • Heating to Austenitizing temperature• Soaking • Cooling

HEATING TO AUSTENITIZING TEMPERATURE:Hypoeutectoid steel:

• Temperature desired is 50oF above the upper critical temperature.• Reason: To dissolve proeutectoid ferrite, otherwise after quenching soft spots in microstructure would result and hardness will decrease.

HEATING TO AUSTENITIZING TEMPERATURE:Hypereutectoid steel:

• Temperature in between Acm and A1 is desired.• Reason: To avoid undesirable coarsening of austenitic grains that would cause cracking on cooling. • Reason: T dissolve the proeutectoid cementite one has to heat above Acm like wise ferrite. But since the temperature rise in this area with increasing small content of carbon is so high (steep) that would cause excess heating of cementite grains and would tend excess coarsening of austenitic grains.


SOAKING TIME (HOMOGENEITY OF AUSTENITE):

• Compositional homogeneity of austenite is essential otherwise surfaces cooled fast would transfer to nonmartenitic and cooled slowly would transfer to martensitic structure.

• Let us think about the composition variation of austenite transformed from hypo eutectoid steel when heated in b/w lower and upper critical temperatures.

• Austenite formed from ferrite would contain very low carbon whereas that from pearlite would contain higher amount.

SOAKING TIME:• Let us apply the lever rule to caculate the compostional difference at particular temperature.• To avoid this compositional difference we have either:• To heat slow • Or Soak at desired temperature• The second is more practical • Soaking time is object- dimensional dependent• One hour per inch thickness is desirable to obtained homogenize austenite.

FACTORS AFFECTING THE COOLING RATE:• Surface condition

• Oxidizing scale thicker than 0.005inch retards the cooling rate (Fig 8.39 page 289 Avener)• Scale formation give birth to non uniform cooling, since scale either is non uniform or some times

removes during transferring from furnace to quenching medium• To protect the scale formation following methods are in practice:• Copper plating• Protective atmosphere (inert gases like hydrogen, dissociated ammonia) • Liquid salts (salts inert for steel)• Cast iron ships are the methods and techniques to avoid formation of scale.

• Size and Mass


• Ratio of surface area to mass is important factor in determining the actual cooling rate.• This ratio is the function of geometrical shape of the object and largest for spherical shapes.• The cooling rate is directly proportional to ratio• Surface area of cylinder = pieDL• Mass= pie/4 D2Ldensity• Ratio=4/D. density• This shows that ratio is inversely proportional to dia.• Hence with increasing the dia the ratio decreases and the cooling rate too.• Experimentally we can demonstrate the effect of dia on the cooling rate via measuring the hardness.• Let us take wires of different dia or plates of different thickness and austenite at same temperature

and quench in same medium. Finally check the hardness

SIZE AND MASS (EXAMPLE OF MEDIUM CARBON STEEL OF 0.4%C):

• Size and Mass• Diameters: 0.5, 1, 2, 3, 4, 5• RC: 59, 58, 41, 35, 30, 20.

• What this data shows?• Steel bar of 0.5 and 1 inch has actual cooling rate grater than critical cooling rate thus martensitic

structure formed.• Surface of 2 and 3 inch sample have intermediate cooling rate thus mixture of martensite and pearlite

would have formed.• Whereas surface of 4 and 5 inch sample received slow cooling thus pearlite and ferrite would be

possible microstructure.

MICROCONSTITUENTS VS. COOLING RATE:• Spheroidite: Spherical “globs” of Fe3C in Ferrite• Pearlite: Layers of a ferrite and Fe3C

• Course Pearlite• Fine Pearlite

• Bainite: 200 – 500 °C Transformation• Martensite: Rapid Cooling

MECHANICAL PROPERTIES VS HEAT TREATMENT:


EFFECT OF HEAT TREATMENT ON TENSILE STRENGTH:

EFFECT OF HEAT TREATMENT ON FAILURE:

EFFECT OF HEAT TREATMENT ON FAILURE MECHANISM:


HEAT TREATMENTS:• A – Furnace Annealed – Slow cooled• N - Normalized - Air cooled.• O - Oil Quenched• WQ – Water quenched.• WT(370) -Water quenched, tempered at 370°C for 1 hour.• WT(705)– Water quenched, tempered at 705°C for 1 hour.

The steels shown in blue on the following slide can be heat treated to harden them by quenching.

Classes: 7-12


FUNDAMENTALS OF HEAT TREATMENT

FUNDAMENTALS OF HEAT TREATMENT:• Phase Diagram• Graphical representation of temperature and composition• There are about eight basic phase diagrams that encompasses the binary alloy system of ferrous and

non ferrous metals.• Most common are:• Completely soluble at liquid and solid stateè Type 1• Completely soluble at liquid and completely insoluble at solid state è Type 2• Completely soluble at liquid and partially soluble at solid state è Type 3• Allotropic transformation• Iron Carbon Diagram• TTT Diagram• Time temperature transformation diagram

TYPE 1 PHASE DIAGRAM: TYPE 2 PHASE DIAGRAM:

TYPE 3 PHASE DIAGRAM:


LEVER RULE:• Composition:

• S=30%B+70%A• L=60%B+30%A

• Amount:• S=mTo/mnx100• S=(60-50)/(60-30)*100• S=33.33%• L=nTo/mn*100• L=(50-30)/(60-30)*100• L=66.66%

ALLOTROPIC TRANSFORMATION:• Allotropy is the phenomenon of an element having different crystal lattices depending on the

particular temperature and pressure.• Iron in the solid state is known in two allotropic states.• Lattice constant of α and δ Iron is 0.286 nm whereas that of γ iron is 0.364 nm.• At low temperatures, α -Fe exhibits a strongly ferromagnetic character. • When it is heated to about 7708C, ferromagnetism vanishes.• In accordance with the latest findings, this is because the lattice loses its ferromagnetic spin

ordering. • The state of iron above 7708C is called β-Fe. The lattice of paramagnetic β crystals is identical to

the lattice of α crystals.

• The transformation stages are symbolized bythe letter A with subscripts indicating theordinal number of the transformation.

• The subscripts 0 and 1 are absent in pure ironbut are observed in carbon alloys of iron.

• The subscript 2 denotes a magnetictransformation of the α-phase, while thesubscripts 3 and 4 stand for transformationof α to γ and γ to δ.


IRON CARBON DIAGRAM:• The structure of iron–carbon alloys

can contain either pure carbon(graphite) ora chemical compound(cementite) as the carbon-enrichedcomponent.

• Cementite is present even in relativelyslowly cooled alloys: a long holding atelevated temperatures is required todecompose cementite to iron andgraphite.

• For this reason the iron–carbondiagram is usually treated as theiron–iron carbide diagram.The former is stable, whereas the latteris metastable.

• The iron–carbon diagram is shown inFigure. Dashed lines stand for the stableFe–C diagram, and solid lines denote themetastable Fe–Fe3C diagram.

Figure: denoting Iron Iron Carbide Diagram

META STABLE IRON IRON CARBIDE DIAGRAM:

Note: Consider above figure of Iron Iron Carbide Diagram• ABCD liquidus line• AHECFD Solidus line• ES, and GS solvious lines• ECF Eutectic line• PSK Eutectoid line• HB Peretectic line• Ao magnetic transformation line• Solid solutions: • Austenite, ferrite• Chemical compound:• Cementite• Mechanical Mixture:• Pearlite, and Leduberite


STABLE IRON CARBON DIAGRAM:

Note: Consider above figure of Iron Iron Carbide Diagram• At very slow cooling rates graphite (i.e. pure carbon ) can crystallize directly during cooling.• ABC’D’ liquidus line• AHE’C’F’D’ Solidus line• E’S’, and GS solvious lines• E’C’F Eutectic line• P’S’K’ Eutectoid line• HB Peretectic line• Ao magnetic transformation line• Solid solutions: • Austenite, ferrite• Pure element:• Graphite• Mechanical Mixture:• Pearlite (F+G), and Leduberite (A+G)

STABLE AND METASTABLE IRON CARBON DIAGRAM:

Note: Consider above figure of Iron Iron Carbide Diagram• Depending on the cooling rates it is possible to obtained partially stable and partially metastable

phase diagram for iron and carbon alloy system.

MICROSTRUCTURES IN IRON CARBON ALLOY SYSTEM:

• Ferrite:• It is ductile. • In the annealed state, it has large elongation (about 40%), • Its Brinell hardness is 65–130 depending on the crystal dimension• It is strongly ferromagnetic up to 7708C (14188F). • At 7238C (1338F), 0.22% C dissolves in ferrite, but at room temperature only thousandths of a

percent of carbon is left in the solution.

• Cementite:• It is brittle and exhibits great hardness (theBrinell hardness is about 800)• It is weakly magnetic up to 2108C (4108F)• It is a poor conductor of electricity and heat, and has a complicated rhombic lattice. • It forms at three stages:• Primary cementite, which crystallizes from the liquid at the line CD; • secondary cementite, which precipitates from the gama solution at the line ES;


• Tertiary cementite, which precipitates from the a solution at the line PQ.

• Graphite:• It is soft. • It is a poor conductor of current but transfers heat well. • Graphite does not melt even at temperatures of 3000–35008C (5430–63308F). • It possesses a hexagonal lattice with the axis relation

• Austenite:• It is soft (but is harder than ferrite) and ductile. • Elongation of austenite is 40–50%.• It has lower conductivity of heat and electricity than ferrite, and is paramagnetic.• Austenite possesses an FCC lattice.

EFFECT OF CARBON:• The structure of the steel containing 0–0.02% C comprises ferrite and tertiary cementite (Figure 3.3).• A further increase in the carbon content leads to the appearance of a new structural component—a

eutectoid of ferrite and cementite (pearlite). • Pearlite appears first as separate inclusions between ferrite grains and then, at 0.8% C, occupies the

entire volume.• Pearlite represents a two-phase mixture, which usually has a lamellar structure (Figure 3.4)

• As the carbon content of steel is raised to over 0.8%, secondary cementite is formed along with pearlite. The secondary cementite is shaped as needles (Figure 3.5).

• The amount of cementite increases as the carbon content is increased.


• At 2% C it occupies 18% of the field of vision of the microscope. A eutectic mixture appears when the carbon content exceeds 2%.

• In rapidly cooled steels, not all the surplus phase (ferrite or cementite) has time to precipitate before a eutectoid is formed.

• Alloys with 3.6% C contain ledeburite (a eutectic mixture of carbon solid solution in g-Fe and iron carbide).

• An electron microscopic image of the carbides is shown in Figure 3.6. • The alloys would be more properly classified with hypoeutectic white cast irons.

LOWER CRITICAL TEMPERATURE:

When steel heated, microstructure changes (new grains forms). The temperature where this growth starts is called Lower Critical Temperature, fix for all % of C, 727oC.

UPPER CRITICAL TEMPERATURE:

• The temperature where new grains formation completes, (All old grains replaced by new grains) is called Upper critical temperature.

• This temperature depends upon % of Carbon in steel. • Can be taken from Iron-Carbon Diagram .

IRON CARBON DIAGRAM (CRITICAL TEMPERATURE LINES):


LIMITATIONS OF IRON-IRON CARBIDE DIAGRAM:• Information about transformation of austenite at varying cooling rates is missing.• Design of heat treatment for desired properties is not possible.• It do not gives the answer of question “How long time is required to start the transformation nor how

long transformation take to complete?”

TTT DIAGRAM:• Time–temperature–transformation diagrams for

isothermal transformation (IT diagrams) andfor continuous cooling transformation (CCTdiagrams) are used to predict the microstructureand hardness after a heat treatment process or tospecify the heat treatment process that will yieldthe desired microstructure and hardness.

• The use of the either type of diagram requiresthat the user be acquainted with its specific features,possibilities, and limitations.

CONSTRUCTION OF TTT DIAGRAM:• Take a steel of 0.8% carbon• Prepare at-least more than 10 specimens• Heat the specimens into furnace set at

800oC.• Take out specimen from muffle furnace

and put it into salt bath set at 700oC. • Leave the specimens in salt bath for

different time intervals and then quenchit into water.

• Polish and etch the specimensfollowing the metallographic steps and


determine the %age of microstructurein each sample.

• Plot the %age of each microstructureagainst the corresponding time.

• Repeat the above steps for another setof 10 specimens at 650oC,500oC,--------200oC.

TTT DIAGRAM OF EUTECTOID STEEL:


TTT DIAGRAM OF HYPEREUTECTOID STEEL(1.13 WT %C )

APPLICATION OF TTT DIAGRAM (INTERMEDIATE COOLING):

APPLICATION OF TTT DIAGRAM (SLOW COOLING):


APPLICATION OF TTT DIAGRAM (FAST COOLING):

APPLICATION OF TTT DIAGRAM:A. Continuous cooling transitionB. diagram for eutectoid steels


• Normalizing : heat the steel into g region → cool it in air → fine pearlite

• Annealing : heat the steel into g region → cool it in furnace

(power off) → coarse pearlite

Classes:13-16

AUSTENITE PHASE TRANSFORMATIO

AUSTENITE-PEARLITE TRANSFORMATION:• Transformation of the FCC lattice of austenite to the BCC lattice of ferrite is hampered due to the

presence of dissolved carbon in austenite. • Because the austenite lattice has enough space to accommodate carbon atoms at the centers of unit

cells. The bcc lattice of ferrite has no such space. • For this reason the solubility of carbon is lowered considerably on transition from austenite to ferrite. • During the β―α a transformation, almost all carbon precipitates from the austenite lattice. • In accordance with the metastable Fe–Fe3C diagram, it precipitates as iron carbide (cementite). This

transformation can be described by three interconnected processes:• Transformation of the g-Fe lattice to the a-Fe lattice • Precipitation of carbon as the carbide Fe3C (cementite)• Coagulation of the carbides• The briefly transformation can be explained by taking following alloys individually.• Hypoeutectoid steel (< 0.8%C)• Eutectoid steel (=0.8%C)• Hyper eutectoid steel (> 0.8% C)

PEARLITE FORMATION IN EUTECTOID STEEL:• The rate at which nuclei of pearlite crystallization

are formed depends on: • Supersaturation of austenite with carbide, which


increases as the temperature is lowered. • The diffusion rate, which decreases

with temperature. • The growth of pearlite islets depends on:

• The diffusion rate of carbon and iron atoms.• Degree of supersaturation • The free energy advantage during the

ferrite formation. • Pearlite islets grow not only through the

formation of new plates but also by way offurther growth of old plates in all directions.

• Carbide plates grow faster than ferrite plates.• The process can start, however, with the• formation of ferrite nuclei. Multiple

alternations of nucleation of ferrite and cementite plates

• and branching of the plates of both phases lead to the formation of plane-parallel and fan-shaped

pearlite plates.

DIFFUSION OF CARBON IN PEARLITE:


MORPHOLOGY OF PEARLITE:

PEARLITE FORMATION IN EUTECTOID STEEL:• A very significant characteristic of pearlite is the plate-to-plate spacing. • Strength properties of steel improve with a decrease in that spacing.• the pearlite transformation takes place on• Pearlite formed at 600–7008C has plate-to-plate spacing equals 0.5–1 mm.• Pearlite formed at 650–6008C has the plate-to-plate distance of 0.4–0.2 mm. In this case, the

eutectoid is finer pearlite.• When austenite precipitates over the temperature interval of 600–5008C an extremely fine eutectoid

mixture is formed, where the plate-to-plate spacing equals ~0.1 mm.• An important characteristic that influences the properties of steel is the dimension of the pearlite

colony. A decrease in the colony dimension is accompanied by a growth of the impact strength and decrease of brittleness.

• For this relatively high strength pearlite is formed by breaking of ferrite and cementite plates, that leads a high density of dislocations inside the ferrite.

• Spheroidization of cementite particles and the formation of a polygonal structure by employing suitable heating cooling cycle is in practice.

PEARLITE FORMATION IN HYPO AND HYPER EUTECTOID STEEL:

• In hypo eutectoid austenite transformation proceeds with birth of pro-eutectoid ferrite, where as in hyper eutectoid pro-eutectoid cementite gets birth first.

(a) coarse pearlite (b) fine pearlite 3000X


• Austenite left for eutectoid reaction is very low in both cases. • During eutectoid reaction eutectoid ferrite at boundries of pro-eutectoid ferrite leaving the eutectoid

cementite free. Vice versa case is true for hyper eutectoid.• This eutectoid transformation is referred as abnormal. • In normal eutectoid transformation, ferrite and cementite grow cooperatively in the form of colonies

with a regular alternation of the two phases. • In the case of abnormal transformation, a coarse mixture of ferrite and cementite does not have a

characteristic eutectoid structure.• Abnormal transformation can be changed to normal with increase in cooling rates.• With a rapid cooling and a correspondingly great undercooling of austenite, the abnormal

transformation can be suppressed altogether.

PEARLITE FORMATION IN HYPO EUTECTOID STEEL (FERRITE PRO AND EUTECTOID):• The ferrite is found in two forms: (Figure 3.7)• Compact equiaxial grains (Eutectoid)• Oriented Widmannstatten plates (Pro Eutectoid)• Compact precipitates of hypoeutectoid ferrite appear predominantly at austenite grain boundaries,

whereas Widmannstatten plates are formed inside grains. • The Widmannstatten ferrite is observed only in coarse grains of austenite of steels with less than

0.4% C. • As the dimensions of austenite grains decrease, the share of ferrite in the form equiaxial grains

grows. • The Widmannstatten ferrite is formed over the

50oc temperature interval from A3 (i.e. 600 to 550oC). • With an increase in the carbon content of steel, the

share of the Widmannstatten ferrite in the structure lowers.

TRANSFORMATIONS OF AUSTENITE : G → A + FE3C: • A. Diffusional transformations 1) At slightly lower T below 727 : ℃ DT <<

• Coarse pearlite: nucleation rate is very low.: diffusion rate is very high.

2) As the Tt (trans. temp.) decreases to 500 ℃• Fine pearlite

: nucleation rate increases.: diffusion rate decreases.

Strength : s (MPa) = 139 + 46.4 S-1 S : intermetallic spacing


MECHANICAL PROPERTIES OF PEARLITE:• Mechanical properties of pearlite are dependent on cementite content , since it is much harder and

more brittle than ferrite. • Therefore increasing the fraction of Fe3C will make the resulting material harder and stronger and

less ductile and tough.

• The layer thickness is also important for the mechanical behavior of the pearlite. • Fine pearlite is harder and stronger than coarse pearlite. • Coarse pearlite is more ductile than fine pearlite because of greater restriction to plastic deformation

of the fine pearlite.


MARTENSITE:• Supersaturated solid solution of carbon in BCC iron (BCT)• Very hard & very brittle• Diffusionless transformation • of FCC to BCT (more volume!)• Therefore no compositional change during transformation.

CHARACTERISTICS OF MARTENSITE:• The martensite transformation is realized on rapid cooling of steel from a temperature above A1 in,

for example, water. • In this case, diffusive precipitation of austenite• to a mixture of two phases (ferrite and carbide) is suppressed. • The concentration of carbon in martensite corresponds• to that in austenite. • The main difference between the martensite transformation• and the pearlite transformation is that the former is• diffusionless.• Transformation of austenite to martensite starts from the• martensite start temperature (Ms).

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.


• The Ms temperature is dependent on carbon content of the steel but is independent on cooling rate. Normally rapid cooling rate decrease/suppress the transformation of austenite to pearlite, but in case of martensite cooling rate is ineffective.

• With the martensite transformation, a certain amount of retained austenite is left.• On cooling below Ms, the amount of martensite increases rapidly owing to the quick formation of

new plates. The initially formed plates do not grow with time. This feature distinguishes the martensite transformation from its pearlite counterpart; in the latter case new colonies nucleate and old colonies continue growing.

• The martensite lattice is regularly oriented relative to the austenite lattice. A certain orientation relationship exists between the lattices. With the pearlite transformation, lattices of the phases comprising the eutectoid mixture exhibit a random orientation with respect to the starting austenite grain.

MARTENSITE MARPHOLOGY:• By morphology, martensite can be divided into two basic types:

• Plate and massive martensite. • They are different in shape, mutual arrangement of crystals,

substructure, and habit plane. • Plate (needle) martensite:

• It is found most frequently in high-carbon steels and carbon-free iron alloys.

• Its texture is shaped as thin lenticular plates (Figure).• Its neighboring plates are not parallel to one another.

• Massive (lath) martensite:• It can be observed in low- and medium-carbon steels. • Crystals of this type of martensite are shaped as

interconnected plates having approximately the sameorientation. The habit plane of laths is close to the{1 1 1}A plane.

• Plates of massive martensite are separated withlow-angle boundaries.

MECHANICAL PROPERTIES OF MARTENSITE:


MARTENSITE FORMATION:• Time temperature transformation [TTT] diagrams) for isothermal martensite formation show a

typical C-curve behavior, indicating that both thermal and athermal characters are present. • Athermal or burst martensite formation is observed in steels; whereas isothermal martensite

formation is observed only in iron alloys that do not contain carbon.

BAINITE TRANSFORMATION:• The bainite transformation is intermediate between pearlite and martensite transformations.• The kinetics of this transformation and the structures formed exhibit features of both diffusive

pearlite transformation and diffusionless martensite transformation.• The bainite transformation mechanism involves γ―α rearrangement of the lattice, redistribution of

carbon, and precipitation of carbide.• Most researchers are of the opinion that ferrite precipitates from austenite by the martensitic

mechanism. This is attested to by the presence of retained austenite in alloyed steels, a similarity in the structure of lower bainite and martensite, and the resemblance of upper bainite to low-carbon martensite.

• Closeness of the bainite transformation to its pearlite and martensite counterparts can be explained as follows.

• The diffusive movement of atoms of the basic component, iron, is almost completely suppressed over the bainite transformation range.

• Therefore the γ―α formation of ferrite is difficult because pearlite precipitation is suppressed. However, carbon diffusion is rather active and causes precipitation of carbides.

BAINITE MORPHOLOGY:• Based on morphology bainite is classed into:

• Upper bainite• Lower bainite

• Upper bainite has a feathery structure, whereas lower bainite exhibits an acicular morphology, which is close to that of martensite

• The difference in• the structures of upper and lower bainites is attributed to a different mobility of carbon in the upper

and lower parts of the bainite temperature range.


• An electron microscopic analysis showed that the a-phase substructure of upper bainite resembles the substructure of massive martensite in low-carbon steels, while the a-phase structure of lower bainite approximates the structure of martensite in high-carbon steels.

• In upper bainite, carbide particles can precipitate both at lath boundaries and inside laths. This fact suggests that here carbides precipitate directly from austenite.

• In lower bainite, carbide is found inside the a-phase. This means that carbide is formed during precipitation of a supersaturated solid solution of carbon in the a-phase. Both upper and lower bainites exhibit a high density of dislocations inside the a-phase.

• Cementite is the carbide phase in upper bainite, and e-carbide in lower bainite. As the• holding time is increased, e-carbide turns into cementite.

BAINITE:• Temperature range of bainite formation 250 < Tt < 500 , below the nose in TTT diagram℃ ℃

• Upper (550-350°C)• Lower (350-250°C)

• Diffusion rate is very low.• Nucleation rate is very high

MECHANICAL PROPERTIES OF BAINITE:

• Bainitic steels have a finer structure, therefore

• They are stronger and harderthan pearlitic ones.

• They have a good combination ofstrength and ductility

Class: 17

AUSTENITE GRAIN SIZE CONTROL

AUSTENITE GRAIN SIZE AND GRAIN SIZE CONTROL:• The austenite grain size produced on heating above the critical points is of significant importance.

Because:• A coarse austenite grain determines a coarse plate structure of martensite during quenching or a coarse cellular network of ferrite (cementite) precipitates at the boundary of the


initial austenite grains during annealing or normalization. The pearlite structure is also the coarser, the larger the pearlite grain.• In result of a coarse-grain structure of steel lower mechanical properties are obtained. For this reason a fine-grain structure of steel is preferable in practice.

• Since austenite appears during heating of a ferrite–carbide mixture, growth centers of the austenite phase are very numerous. At initial stages of heating austenite grains are extremely small, on the order of 10–20 mm. But with an increase in the heating temperature or holding time in the austenite range, the grains begin to grow intensively.

• Based on growth rate of austenite grain during heating, steels are classed into:• Hereditarily coarse-grained steels. • Hereditarily fine-grained steels.• The difference is the grain growth kinetics with an increase in temperature.• Hereditarily coarse-grained steels. The steel in which grain gradually and rather uniformly becomes

larger as the temperature is raised above Ac3• In hereditarily fine-grained steels, fine grains are preserved up to about 950oC. On transition through

the coarsening temperature, separate grains start growing intensively and variations in grain size arise. Near 1100–1200oC, grains of hereditarily fine-grained steels may be even larger than those of hereditarily coarse-grained steels.

• Such differences in the growth of grains in steels are explained by the differences in number and state of disperse nonmetal inclusions such as, , aluminum nitrides, certain carbides, and oxides.

• These articles retard movement of grain boundaries until temperatures are reached at which the particles dissolve in austenite.

• The barrier effect of the particles diminishes nonuniformly, which leads to variations in grain size.• A standard test can be used to distinguish between the steel classes.

• If a noticeable growth of austenite grains is not observed for 8 h after carburization at 925oC, the steel is assumed to be a hereditarily fine-grained one. • Extrapure steels, those produced with a minimum amount of foreign impurities, nitrogen and oxygen, are distinguished by a rapid growth of grains above the critical point Ac3.

• In the case of the usual commercial steels, a grain 20–25 mm in size corresponds to standard heating for quenching, normalization, or annealing.

• As the temperature is elevated to 1200–1250oC the grain size reaches 0.1 mm.• In large forgings and welds, grains of several millimeters in size occur. • In ingots and castings, grains can be as large as several centimeters.• On cooling:

• Several pearlite colonies are formed in every grain, So an austenite grain is broken into several grains.• This is also true of the ferrite–pearlite structure of a hypoeutectoid steel. But in the latter case a network of excess ferrite is formed at grain boundaries.

• When steel undergoes quenching:• A large number of martensite crystals appear in every austenite grain.

Classes: 18-21


EFFECT OF ALLOYING ELEMENTS

EFFECT OF ALLOYING ELEMENTS ON HEAT TREATMENT PROCESSING OF STEEL:• The position of the critical points A3 and A4 and the location of the eutectoid temperature A1 are of

great significance in heat treatment because they determine the lowest temperature to which a steel should be heated for quenching, annealing, or normalization.

• Different alloying elements have different effects on the position of the critical points A3 and A4. • Therefore alloying elements are accordingly divided into two large groups, each in turn broken down

into two subgroups.

• Elements of A group of 1st group (Ni, Co, Mn, Pt, Pd, Rh, and Ir) decrease A3 and increase A4 points, thus broadening of γ phase results.

• 1st group elements show considerably high solubility in Fe crystals.


• Elements of B subgroup of 1st group (N, C,Cu, Zn, Au, Re) have considerably limitedsolubility, therefore at certain concentrationof these elements in iron alloys, chemicalcompounds are formed and eutectic oreutectoid transformations are observed.Thus heterogeneous region results which inturn limit the γ phase region.

• Elements of A group of 2nd group [Cr, Mo,W,Si, T, Al, and Be (Cr group)] play distinct rolefrom the elements of the first group.

• Elements entering the second group elevate thepoint A3 and lower the point A4 as their contentin the alloy is raised.

• This leads initially to narrowing and then to acomplete closing of the region of the γ-solidsolution as shown schematically in Figure

• Elements of B subgroup (Zr, Ta, Nb, Ce, etc.) of 2nd group causes the appearance of other phasesin the equilibrium diagrams before the g-phase rangeis closed.

• In this case the narrow range of the γ-phase is limitedby adjacent heterogeneous regions

• The aforementioned division of alloying elements into groups allows one to predict to some extent the effect of the elements on the critical points of carbon steel.

• For example, considering the diagram lines that correspond tothe transition of Fe from one allotropic form to another, it canbe expected that:

• The elements extending the g-phase range (Ni group) willlower the α―γ iron transition point Ac3.

• While the elements narrowing the γ-phase range (Cr group)will elevate that point.

• A similar effect of the elements is observed, to a certainextent, in the pearlite transformation Ac1.

• Figure illustrates the influence of the most importantalloying elements on the position of the critical point Ac1. As is seen:

• The elements narrowing the γ-phase range do raise the critical point Ac1• While the elements broadening the g-phase range lower it.


• The effect of alloying elements manifests itself in a shift of the critical points with respect not only to temperature but also concentration.

• Figure illustrates how the content of alloying elements in steel affects the carbon concentration at the eutectoid point.

• As can be seen from the figure, all the alloying elements shift the eutectoid point to the left, i.e., toward lowering of the carbon concentration, and consequently decrease the carbon content of alloy.

• In analogy to the shift of the eutectoid point to the left, the addition of most alloying elements in steel is followed by a leftward displacement of the point E in the Fe–C equilibrium diagram, which determines the solubility limit of carbon in austenite.

• The point E is shifted most by Cr, Si, W, Mo, V, and Ti,• All these elements narrow the g-phase range in alloys of the iron-alloying element system.


• Therefore the introduction of alloying elements into a carbon steel is accompanied by:• Shift of the equilibrium critical points with respect to both temperature and carbon concentration.

• The greater the shift, the larger the amount of the elements introduced.• In commercial alloy steels, which are multicomponent systems, alloying elements can be found

• (1) in the free state; • (2) as intermetallic compounds with iron or with each other;• (3) as oxides, sulfides, and other nonmetal inclusions; • (4) in the carbide phase as a solution in cementite or in the form of independent compounds with carbon (special carbides); or• (5) as a solution in iron.

• As to the character of their distribution in steel, alloying elements may be divided into two groups:• Elements that do not form carbides in steel, such as Ni, Si, Co, Al, Cu, and N• Elements that form stable carbides in steel, such as Cr, Mn, Mo, W, V, Ti, Zr, and Nb

ASSIGNMENT/PRESENTATION:• Explain in detail the effect of alloying elements on austenite transformation into pearlite, bainite, and

martensite phases. Support your answer with suitable diagrams. • Reference “Page # 174 to 186 Steel Heat Treatment Hand Book, 2nd Addition by George E Totten,

Ph.D, FASM

Classes:23-26HARDENABILITY

HARDENABILITY:• The ability of a steel to increase hardness during quenching is called its hardenability or hardening

capacity.• The hardening capacity is characterized by the maximum hardness that can be obtained on the

surface of a given steel product by quenching, whereas hardenability refers the depth to which maximum hardness is achieved.

• To achieve maximum hardness it is necessary to observe basic conditions:


• The rate of cooling should be equal to or higher than the critical rate at which quenching gives martensite alone (inevitability with some retained austenite, of course, but without bainite);• All carbon at the quenching temperature should be in the solid solution in austenite (the quenching temperature should be above the critical points Ac1 and Ac3 by 30–50oC for hypereutectoid and hypoeutectoid steels, respectively).

• Although hardening capacity and hardenability are used inplace of each other but have different meaning and different factors are used to control them.

• The hardening capacity of a steel is determined by the factors affecting the hardness of martensite, while hardenability is determined by those affecting the quantity of the martensite obtained and the hardness penetration depth.

• Upon quenching, steel can feature high hardening capacity and low hardenability at the same time. • Such a steel would correspond to the schematic curve 1 in Figure. • If for a work piece of the same diameter D cooled under the same conditions, the distribution of

hardness over the cross section is characterized by curve 2; such a steel possesses medium or poor hardening capacity but good hardenability.

• Finally, steel that corresponds to curve 3 wouldpossess high hardening capacity and high hardenability.

• Hardenability is an inherent property of the material itself,whereas hardness capacity is a state that depends onother factors as well.

• Generally hardenability is determined by the distance belowthe surface at which 50HRC is obtained.

• The hardenability depends on:• Carbon content.• The amount of alloying elements dissolved in the austenite during

the austenitizing treatment. • The austenite grain size. • Figure shows the hardness distributions within the cross sections of bars of 100 mm diameter after

quenching three different kinds of steel [2].• In spite of quenching the W1 steel in water (i.e., the more severe quenching) and the other two

grades in oil, the W1 steel has the lowest hardenability because it does not contain alloying elements. • The highest hardenability in this case is that of the D2 steel, which has the greatest amount of

alloying elements.• It is interesting to note that :

• when a steel has high hardenability it achieves a high hardness throughout the entire heavy section (as D2) even when it is quenched in a milder quenchant (oil).• When a steel has low hardenability its hardness decreases rapidly below the surface (as W1), even when it is quenched in the more severe quenchant (water).


HARDENABILITY (GOVERNING FACTOR):• The austenite condition (chemical composition, grain size, homogeneity of austenite) prior to

quenching has the decisive effect on the hardening capacity and, especially, hardenability of steel. • Other factors are secondary or derive from the basic three. These factors are:

• The carbon content• Type, and amount of alloying elements at the time of quenching• The quenching temperature (austenitizing temperature).• The holding time at a given temperature.

• Mainly grain size and composition of steel determines the depth to which martensite zone can penetrate.

HARDENABILITY (AUSTENITIZING CONDITION):• To improve the hardening capacity and hardenability of steel, the austenitizing conditions should be

such as to ensure that a maximum amount of carbon passes from the ferrite–carbide mixture to the solution and, at the same time, no marked growth of grains occurs as a result of overheating, as this would lead to a high brittleness and the formation of quenching cracks.

• The quenching temperature should be maintained as constant as possible, and the holding time should be just enough to ensure uniform heating of the workpiece and dissolution of carbides.

• For their complete dissolution in austenite, coarse-plate and coarse-grain carbides need more time than thin-plate and fine-grain ones.

• Steels alloyed with carbide forming elements should be heated to a temperature considerably exceeding Ac3. Because if small amount of carbides available in the structure impede enlargement of grains and the nuclei of the new phase facilitate transformation of austenite in the pearlite range and increase the critical rate of quenching, thus decreasing the hardenability of the steel.

FACTORS INFLUENCING HARDENABILITY:• Shape and size of the cross section• Hardenability of the material• Quenching conditions

• Quenching conditions include:i. The specific quenchant ( with its inherent chemical and physical properties), Bath

temperature and Agitation rate.

FACTORS INFLUENCING HARDENABILITY (SHAPE AND SIZE OF THE CROSS SECTION):• The cross-sectional shape has a remarkable influence on heat extraction during quenching and

consequently on the resulting hardening depth.• Bars of rectangular cross sections always


achieve less depth of hardening than roundbars of the same cross-sectional size.

• Chart shown in Figure is used to convertsquare and rectangular cross sections toequivalent

• circular cross sections. • For example,

• A 38-mm square and a 25 100-mmrectangular cross section are eachequivalent to a 40-mm diameter circularcross section; • A 60 100-mm rectangular cross sectionis equivalent to an 80-mm diametercircle [2].

• The influence of cross-sectional size whenquenching the same grade of steel under thesame quenching conditions is shown in Figure 5.4A.

• Steeper hardness decreases from surfaceto core and substantially lower core hardnessvalues result from quenching a larger cross section.

• The influence of hardenability and quenching conditions are shown in Figure • Figure shows that an unalloyed (shallow-hardening) steel and an alloyed steel of high hardenability

is quenched in (a) water and (b) oil. • Figure conceives that:

• The critical cooling rate (Vcrit) of the unalloyedsteel is higher than the critical cooling rate of thealloyed steel. • Only the cross section that have been cooled at ahigher cooling rate than Vcrit could transformedto martensite and attained high hardness.• In unalloyed steel water quench give birth tomartensite to limited crosection whereas oil quenchcould not formed martensite. • In case of alloyed steel oil quenching provided

cooling rates higher than critical within a quite largedepth, whereas water quench produced martensite


upto core.

DETERMINATION OF HARDENABILITY:• Grossmann’s Hardenability Test• Jominy End-quench Hardenability Test• Hardenability Bands

DETERMINATION OF HARDENABILITY (GROSSMANN’S HARDENABILITY TEST):• Grossmann’s method of testing hardenability uses a number of cylindrical steel bars of different

diameters hardened in a given quenching medium. • After sectioning each bar at midlength and examining it metallographically, the bar that has 50%

martensite at its center is selected, and the diameter of this bar is designated as the critical diameter (Dcrit).

• The hardness value corresponding to 50% martensite will be determined exactly at the center of the bar of Dcrit.

• Other bars with diameters smaller than Dcrit have more than 50% martensite in the center of the cross section and correspondingly higher hardness,

• While bars having diameters larger than Dcrit attain 50% martensite only up to a certain depth as shown in Figure.

• The critical diameter Dcrit is valid for the quenching medium in which the bars have been quenched. If one varies the quenching medium, a different critical diameter will be obtained for the same steel.


DETERMINATION OF HARDENABILITY (JOMINY HARDENABILITY TEST):• The end-quench hardenability test developed by Jominy and Boegehold is commonly referred to as

the Jominy test. • It is used worldwide, described in many national standards, and available as an international

standard. • This test has the following significant advantages:• It characterizes the hardenability of a steel from a single specimen, allowing a wide range of cooling

rates during a single test.• It is reasonably reproducible.

• Procedure: • Normally the steel test specimen (25 mm diameter 100 mm) is heated to the appropriate

austenitizing temperature and soaked for 30 min. • It is then quickly transferred to the supporting fixture (Jominy apparatus) and quenched from the

lower end by spraying with a jet of water under specified conditions as illustrated in Figure A. • The cooling rate is the highest at the end where the water jet impinges on the specimen and

decreases from the quenched end, producing a variety of microstructures and hardnesses as a function of distance from the quenched end.

• After quenching, two parallel flats, approximately 0.45 mm below surface, are ground on opposite sides of the specimen and hardness values (usually HRC) are measured at 1/16 in. intervals from the quenched end and plotted as the Jominy hardenability curve (see Figure B).

• When the distance is measured in millimeters, the hardness values are taken at every 2 mm from the quenched end for at least a total distance of 20 or 40 mm, depending on the steepness of the hardenability curve, and then every 10 mm.

• On the upper margin of the Jominy hardenability diagram, approximate cooling rates at 700oC may be plotted at several distances from the quenched end.

••


• 4340: Very hardenable, More expensive

• l1040: Less hardenable, Less expensive

• The Jominy end-quench test is used mostly for:• Low-alloy steels for carburizing (core hardenability) • Structural steels, which are typically through-hardened in oils and tempered.

• The Jominy end-quench test is suitable for all steels except those of very low or very high hardenability, i.e., D1< 1.0 in. or D1>6.0 in.


• The standard Jominy end-quench test cannot be used for highly alloyed air-hardened steels. These steels harden not only by heat extraction through the quenched end but also by heat extraction by the surrounding air. Thiseffect increases with increasing distance from the quenched end.

ASSIGNMENT:• Jominy Test procedure used for hardenability determination of shallow steel is little bit different than

alloyed steel. Describe it according to ASTM A255 standard. • Reference Page# 231 to 233. Steel Heat Treatment Hand Book, 2nd Addition by George

E Totten, Ph.D, FASM

• What is hardenability bond. How it is used for determination of hardenability.• Reference Page# 237 to 240. Steel Heat Treatment Hand Book, 2nd Addition by George

E Totten, Ph.D, FASM

Classes:27-32CASE HARDENING

CASE HARDENING:• Case hardening is a simple method of hardening steel. • It is less complex than hardening and tempering. • This techniques is used for steels with a low carbon content. • Carbon is added to the outer surface of the steel, to a depth of approximately 0.03mm. • One advantage of this method of hardening steel is that the inner core is left untouched and so still

processes properties such as flexibility and is still relatively soft.

APPLICATION:• This is a very common process that is used to harden the outer surface of parts such as:

• Gear teeth, Cams, Shafts, Bearings• Fasteners, Pins, Tools, Molds, Dies etc

• Reason• Above types of components normally works under the

dynamic forces conditions wherei. High fatigue

ii. High toughnessiii. And high friction resistance

• Is required• This is only achieved best by case hardening• Most of these processes are used to case harden steel and other

iron alloys, including low carbon steels, alloy steels, tool steels etc.

TYPES:• There are several types of case hardening:


• Carburizing• Nitriding• carbonitriding• Chromizing• Bronizing• Cyaniding or carbonitriding• Induction hardening• Flame hardening

• In first five cases chemical structure of the metal surface is changed by diffusing atoms of an alternate element (doping agent) which results in alterations to the micro-structure on the crystals on the surface.

• In last two methods doping agent is not required therefore the steel must contain at least 0.3% C

PROCESS:• The basic method is fairly simple:

• Heating • Dipping in the atmosphere containing “doping” substance either in gasseous or liquid

state. • Heating to diffuse dopant into the surface.

• The duration and temperature control the concentration and depth of the doping.

CASE HARDENING STAGES:• STAGE ONE: • The steel is heated to red heat. It may only be necessary to harden one part of the steel and so heat

can be concentrated in this area.• STAGE TWO: • The steel is removed from the brazing hearth with blacksmiths tongs and plunged into case

hardening compound and allowed to cool a little. The case hardening compound is high in carbon.• STAGE THREE: • The steel is heated again to a red colour, removed from the brazing hearth and plunged into cold,

clean water.


PROCESS VARIABLES:• The process variables normally fixed in specifications for a carburized part are:

• Case depth. • Hardness of core, and case. • Case carbon content. • Post Heat treatment. • Material

CASE DEPTH:• For design purposes, the variables of greatest concern are:

• case depth • case hardness.

• Case depth is described in two ways:• Total case depth• Effective case depth.

• Total case depth refers to the total depth of carbon penetration and is ordinary determined by metallographic examination or by chemical analysis of successive layer of the case.

• Effective case depth is the depth below the surface to which a specified hardness is exceeded, commonly the depth of 50 HRC is considered as effective case depth

• Total case depths of greater than 0.075 inch (1.8mm) are rarely used in production because of the excessive time required to develop deep cases. Approximately 9 hr at 1700oF (912oC) are required for producing a 0.075-inch (1.8mm) case.

CASE CARBON CONTENT:• It depends on:

• Mechanical properties and relative microstructure desired.• High carbon content would give birth to retain austenite.• High carbon content at surface is required when high wear properties are required.

POST HEAT TREATMENTS:


CARBURIZING:• It is oldest and cheapest methods of case hardening• It is applicable of steel containing 0.1% to 0.2%C steel.• Heating to 1700oF in carburizing (CO) atmosphere.

• Fe + 2CO ====è FeC + CO2• FeC represents dissolved carbon in Fe.

CARBURIZING TYPES:• Pack carburizing

• Heating of charcoal, coke, and barium carbonate (energizing compound to enhance carbon pick up character of steel) with sample in a closed packed chamber.

• It is efficient and economical for small lots of parts or of large massive parts. • Main disadvantage is that it is not suitable for thin cases.• Minimum case depth 0.03inch with 0.01 inch tolerance.

• Gas carburizing• Uses propane (C3H8), methane (CH4), natural gas (87% methane), and carbon

monoxide. in a sealed furnace.• Gas carburizing allows quicker handling, closer quality, and greater flexibility of

operation in comparison with pack carburizing.• Liquid carburizing


• Used sodium cyanide (NaCN) and other salts (BaCl2)• Thickness 0.005 in. to 0.030 in.

PROCESS STEPS OF GAS CARBURIZING:• Formation of CO. • Absorption of C. • Diffusion of C.• CH4 ==è C + 2H2. • CO2 + H2.==è CO + H2O.• CH4 + H2O ==è CO + 3H2 • Formation of H2O is causing decarburization. Some times CO2 may also takes place

ADSORPTION OF CARBON:• Fe + 2CO. ==è Fe(C) + CO2.• Fe + CH4 ==è Fe(C) + 2H2 • Fe + CO + H2 ==è Fe(C) + H2O• Formation of CO2 and Water can be minimized by further addition of CH4, other wise carburization

process will not proceed, since CO2 and H2O are decarburizing agents.• CH4 + CO2 ==è2CO + 2H2• CH4 + H2O ==è CO + 3H2• Fick’s first law


• J = - D ¶c/¶x. • Where:

• J is flux, or amount of diffusing substance that passes through unit cross sectional area per unit time.

• D is the diffusion coefficient.• ¶c/¶x is the concentration gradient of the diffusing substance.

• The transport of carbon in austenite depends on: • Diffusion coefficient values. • The carbon concentration gradient

CASE DEPTH:• d = j ét • Where d is case depth and j is constant proportionality

coefficient, t is time

DECARBURIZATION:• It is reversal of carburization and is the problem in high carbon steels and tool steels• Depletion of carbon left the soft skin and case do not transform to martensite on subsequent

hardening• Endothermic environment is the key to avoid decarburization.

FORMATION OF H2O AND CO2 DURING CARBURIZING AT DIFFERENT TEMPERATURES:

MICROSTRUCTURES:• At surface microstructure would be pearlite with cementite network and at core pearlite with ferrite

network (figure 8.7 avener).• The amount of ferrite will increase towards core.• The carbon gradient from case to core is shown in figure 8.71.• The relation between time and temperature to case depth is shown in figure 8.72


CYANIDING AND CARBURIZING:• Cyaniding: Surface hardening using liquid salt baths • Carbonitriding: Surface hardening using C and N gas atmosphere (carbonitriding).• The temperature used is lower than carburizing (b/w 1400 to 1600oF)• This technique is used for producing thinner cases.• Up to 0.01in for cyaniding• Up to 0.03 in for carbonitriding

CARBONITRIDING:• The process in which the carbon and nitrogen gases are used directly the source of hardening agent.• It is the modified carburizing. The most descriptive term would be nitrocarburizing • The process is also know as dry cyaniding, gas cyaniding and nicarbing.• The gases used in the process is the mixture of:

• Carrier gas (mixture of N, H, CO). It is used at +ve pressure to prevent air infiltration and act as diluent for active gases (hydrocarbon and ammonia) thus making the process easy to control.• Enriching gas (usually propane and natural gas) the main carburizing agent source.• Ammonia NH3. At operating temperature the NH3 dissociates to provide N to surface of steel.

• The endothermic generator is used for production of CO.

NITRIDING:• Nitrogen is diffused in the surface of special alloy steels

at temperatures around ~510 to 600°C.• Suitable for low carbon alloy steels containing Al, Cr,

Mo, V, Ni that can form nitride compound • Mechanism: NH3è N +3H• Surface hardness achieved: up to 1000 VHN • Case Depth: 0.1 to 0. 6 mm• Applications: Gears, valves, cutters, sprockets, pump


boring tools, fuel-injection pump parts.• Depending on case depth, nitriding time normally range

upto 120h• 0.038% is the maximum solubility of nitrogen at standard

pressure and austenitizing temperature (810oC)


• Assignment• Define and differentiate the types of nitriding process shown in table

SURFACE HEAT TREATMENTS:• Carburizing -> HRC 60, Thickness: 0.025 – 4 mm• Nitriding -> HRC 70, Thickness: 0.025 – 0.05 mm• Carbonitriding -> HRC 70, Thickness: 0.07-0.5 mm• Chromizing and Boronizing -> HRC 70

HEAT TREATMENT WITH GASEOUS ATMOSPHERE:• The absorption of material from a gaseous atmosphere occurs in several steps

• Processes in the gaseous atmosphere: i. Formation of transportable gas molecules and transfer of these molecules through

the gas phase onto the surface of the metal with subsequent physical adsorption of the gas molecules

• Processes in the interface:


i. Dissociation of the gas molecules and chemisorption of the gas atoms, penetration of the atoms through the surface of the metal with transition of the atoms from the state of chemisorption to the interstitially solute state in the solid solution

• Diffusion of the atoms from the surface into the core of the material• Independent of the composition of the initial gases, the gaseous atmospheres used in heattreating at

processing temperatures consist of the elementary molecules:• carbon monoxide CO, carbon dioxide CO2, hydrogen H2, water vapor H2O, oxygen O2,

ammonia NH3, and sometimes also methane CH4, • All of which are able to react with one another and with the catalyzing surface of the component

(and the furnace wall), thus releasing or absorbing• carbon, oxygen, nitrogen, and hydrogen.

• Reactions among the constituents of the gas are described as homogeneous reactions; reactions between elements of the gas and elements of the component surface are described as heterogeneous reactions.

• The heterogeneous reactions that take place in the interface between gaseous atmosphere and component surface are chemophysical processes and responsible for the mass transfer.

FLAME HARDENING:Demo-3Demo-3

INDUCTION HARDENING:Demo_4Demo_4

CHROMIZING:• Diffuse chromium into the surface 0.001 – 0.002 in.• Pack the parts in Cr rich powders or dip in a molten salt bath containing Cr salts.

BROMIZING:• Performed on tool steels, nickel and cobalt based alloy steels.• When used on low carbon steels, corrosion resistance is improved.

Classes:34-39DIFFUSION

DIFFUSION:• Diffusion is process of substance (solid, liquid or gas ) transport to an other substance.

to

t1

t2

t3

xo x1 x2 x3


• It is temperature and time dependent.• The rate of diffusion is dependent on:• Type of substance to diffuse.

• Temperature• Time• Concentration gradient.

• Examples of gas diffusion: diffusion of perfume in air, diffusion of smoke in air, etc.• Examples of liquid diffusion: diffusion of syrup in water• Example of Solid diffusion: Solid solution of carbon in BCC or FCC iron, porosity movement,

inclusion movement.• Path of diffusing substance is concentration dependent.

• At low concentration diffusing path is random, zigzag and unpredictable• At high concentration there could be systemic flow.

IMPORTANCE OF DIFFUSION:• Diffusion plays a crucial role in…

• Alloying metals => bronze, silver, gold• Strengthening and heat treatment processes

i. Hardening the surfaces of steel• High temperature mechanical behavior• Phase transformations

i. Mass transport during FCC to BCC • Environmental degradation

i. Corrosion, etc.

DIFFUSION DEMO:• Glass tube filled with water.• At time t = 0, add some drops of ink to one end of the tube.• Measure the diffusion distance, x, over some time.• Compare the results with theory.

HOW DO ATOMS MOVE IN SOLIDS ?WHY DO ATOMS MOVE IN SOLIDS ?• Diffusion, simply, is atoms moving from one lattice site to another in a stepwise manner

• Transport of material by moving atoms• Two conditions are to be met:

• An empty adjacent site• Enough energy to break bonds and cause lattice distortions during displacement

• What is the energy source ?• HEAT !

• What else ?• Concentration gradient !


TYPES OF DIFFUSION:• Self Diffusion

• In pure and perfect metals the atoms do not exist at any defined position. Their location is constantly changed by intersinck property/character of metals.

• Inter diffusion• Self diffusion in an alloy.• Interdiffusion means the diffusion of one component through lattice of other.• Ferrite. Diffusion of carbon in BCC lattice of Fe.

• Volume diffusion• Diffusion in bulk quantity.

• Grain boundary diffusion• Atomic migration along grain boundaries.

• Surface diffusion• Diffusion of vapor/gas in solid

DIFFUSION:• Self-diffusion: In an elemental solid, atoms also migrate.

Interdiffusion: In an alloy, atoms tend to migrate from regions of high conc. to regions of low conc

• Interdiffusion: In an alloy, atoms tend to migrate from regions of large concentration.


DIFFUSION MECHANISMS:• Gases & Liquids – random (Brownian) motion• Solids – vacancy diffusion or interstitial diffusion

DIFFUSION MECHANISMS IN SOLIDS (I):

ACTIVATION ENERGY:• Energy required to diffuse; energy required to pull the atom away from its present neighbors• Activation energy is dependent on:

• Size of atom• Type of atom ( eg tungsten atom is more closely bounded than Fe atom)

DIFFUSION MECHANISMS IN SOLIDS (II):• Interstitial Diffusion

increasing elapsed time


DIFFUSION MECHANISMS IN SOLIDS (III):Substitutional Diffusion:

• applies to substitutional impurities• atoms exchange with vacancies• rate depends on:

• number of vacancies• activation energy to exchange.

PROCESSING USING DIFFUSION (1):• Case Hardening:

• Diffuse carbon atoms into the host iron atoms

at the surface.• Example of interstitial diffusion is a case hardened gear.

• Result: The "Case" is hard to deform: C atoms"lock" planes from shearing.hard to crack: C atoms put

• the surface in compression.

Much faster than vacancy diffusion, why ? Smaller atoms like B, C, H, O. Weaker interaction with the larger atoms. More vacant sites, no need to create a vacancy !


PROCESSING USING DIFFUSION• Doping silicon with phosphorus for n-type semiconductors:

o Process:• 1. Deposit P rich layers on surface.

2. Heat it. 3. Result: Doped semiconductor regions

DIFFUSION LAW:• How do we quantify the amount or rate of diffusion?

o Ficks Law (Steady state and Non steady State)

• Measured empiricallyo Make thin film (membrane) of known surface areao Impose concentration gradiento Measure how fast atoms or molecules diffuse through the membrane

MODELING DIFFUSION: FLUX

magnified image of a computer chip

0.5 mm

light regions: Si atoms

light regions: Al atoms


CONCENTRATION GRADIENT:

CONCENTRATION PROFILES & FLUX


STEADY STATE DIFFUSION:

DIFFUSION AND TEMPERATURE:


SIZE IMPACT ON DIFFUSION:


Smaller atoms diffuse faster

FICK’S SECOND LAW ; NON-STEADY STATE DIFFUSION:

• In most practical cases, J (flux) and dC/dx (concentration gradient) change with time (t).


o Net accumulation or depletion of species diffusing• How do we express a time dependent concentration?

NON-STEADY STATE DIFFUSION:

SUMMARY:• STRUCTURE & DIFFUSION:

Classes:40-43

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used

herein under license.


AGE HARDENINGAGE HARDENING:

• Principally there are two methods of increasing the strength and hardness of the alloy.o Work hardeningo Heat treatment

• The most important heat treatment process for non ferrous alloys are age hardening/ Precipitation hardening

• The alloys that could be age hardened should have partial solubility at solid state.• The alloy composition that could be hardened should cross the solvus line.• There are two stages in age hardening

o Solution treatmento Aging Process

SOLUTION AND AGING STEPS:


AGE HARDENING STEPS:

• Solution treatmento -heating to a single phase regiono Quench to TR à precipitation is slow, insignificant precipitates

• Aging• Heat the supersaturated solid solution to an intermediate T• Diffusion of Cu in Al à fine dispersion• The alloy is cooled to room temperature

AGING PROCESS:• Aging at elevated temperature to facilitate/speed up the precipitation of trapped element• Precipitation speed is temperature dependent.


• Figure indicates the optimal temperature• Natural Aging alloys

o Precipitation at room temperature b/w 4 to 5 days.• Artificial aging alloys

o Which require reheating to elevated temperature.• Refrigeration retards the rate of aging

o At 32oF beginning of the aging can be delayed for several hours in case of Al Alloys.o Dry ice (-50 to -100oF) retards aging for extended period.

• Al alloy rivets which age at room temperature are kept in deep-refrigerator to retard their aging process until they are driven

MECHANISM OF AGE HARDENING:• Precipitation of excess element as fine submicroscopic particles at slip plane.• Distortion of lattice structure that interfere with movement of dislocation.• Distortion of lattice is the key factor behind the hardening than precipitation of submicroscopic

particles.• Mag can dissolve 46% Lead at eutectic temperature and 2% at room temperature. Although

precipitation is high but with less distortion so no significant hardening takes place• On the other hand copper beryllium alloy shows 2.2% solubility at eutectic temperature and 0.2%

at room temperature. Since excess distortion in lattice is involved so significant increase in hardness and strength is reported.

AGE HARDENING CURVES:• The most quoted age hardening curve is that for

Al-Cu alloys performed in the late 40s. Keep inmind that age hardening was known empirically(Alfred Wilm) as a technologically useful treatmentfrom the early days of aluminum alloys.

• Higher Cu contents result in higher maximumhardnesses because larger volume fractions ofprecipitate are possible

AL-CU PRECIPITATION SEQUENCE:• The sequence is:

a0 ® a1 + GP-zones ® a2 + q“® a3 + q’® a4 + q• The phase are:

an - fcc aluminum; nth subscript denotes each equilibrium


GP zones - mono-atomic layers of Cu on (001)Al q“ - thin discs, fully coherent with matrix q’ - disc-shaped, semi-coherent on (001)q’ bct. q - incoherent interface, ~spherical, complex

• body-centered tetragonal (bct).

AL-CU PPT STRUCTURES:

AL-CU MICROSTRUCTURES:• This tableau shows which of the different ppt types are associated with which part of the hardening

curve.

AL-CU DRIVING FORCES:• Each precipitate has a different free energy curve w.r.t composition. Exception is the GP zone,

which may be regarded as continuous with the alloy (leading to the possibility of spinodal decomposition, discussed later).


• P&E fig. 5.27 illustrates the sequence of successively greater free energy decreases and also successively greater ∆G*.

• P&E fig. 5.28 illustrates the point that the nucleation barriers are much smaller for each individual nucleation step when the next precipitate nucleates heterogeneously on the previous structure.

AL-CU PHASE RELATIONSHIPS:• The explanation of age hardening depends on

understanding the metastable phases that can appear.

NUCLEATION SITES, REVERSION:• The nucleation sites vary depending on

circumstances.• q“ most likely nucleates on GP zones by adding

additional layers of Cu atoms.• Similarly, q’ nucleates on q“ by in-situ

transformation.• However, q’ can also nucleate on dislocations,

see P&E fig. 5.31a.• The full sequence is only observable for annealing temperatures below the GP solvus. Any of the

intermediate precipitates can be dissolved, reverted, by increasing the temperature above the relevant solvus, fig. 5.32

AL-AG: EXAMPLE 2:• The age hardening curve has the same double peak as for the Al-Cu series, but the separation is

more pronounced.

TYPICAL P-H ALLOYS:• Al

o 2014 Forged Aircraft Fittings, Al Structureso 2024 High strength forgings, Rivets


o 6061 Furniture, Vacuum Cleaners, Canoeso 7075 Aircraft Structures, Olympic Bikes

• Cuo Beryllium Bronze Surgical Instruments ,

Non sparking tools, Springs, Nuts, Gears• Mg

o AM 100A Sand Castingso AZ80A Extruded products

• Nio Rene' 41 High Temperatureo Inconel 700 up to 1800Fo Udimet 500

• Feo A-286 High Strength Stainlesso 17-10P

AGE HARDENING IN STEEL:• Ferrite has a very low solubility for carbon and

therefore age hardening (also called quenchhardening) occurs here too. To avoid it, thesoluble carbon levels must be reduced, whichis a common objective of the IF or interstitial-freesteel grades. These have additions of carbideformers such as Ti or Nb to sequester the C.

Classes:44HEAT TREATMENT DEFECT

DISTORTION_ HEAT TREATMENT DEFECT:• Distortion is a term that is very familiar to all that are involved with thermal processing techniques

especially in the field of heat treatment. • No matter how careful one is, distortion cannot be avoided. It is important to at least understand

the basic causes of the distortion problem. • Distortion describes the movement of a metal during its heat treatment process. • The distortion will manifest itself in one of two forms, or a combination of both:

o Shape distortiono Size distortion

• Shape distortion can occur as a direct result of one or any combinations of the following:o Forgingo Rolling


o Castingo Machining stresses induced due to manufacturing operationso Grain sizeo Variations in homogeneity of the materialo Incomplete phase changes

• Size distortion occurs as a direct result of changing the surface chemistry of steel, whereas change in surface chemistry only takes place in surface heat treatment.

• The outstanding importance of steels in engineering is based on their ability to change mechanical properties over a wide range when subjected to controlled heat treatment.

• For unalloyed carbon steels, for example, the hardness can be increased by up to 500% just by changing the cooling rate from the austenitizing temperature from extremely slow to extremely fast.

• However quenching at a rate faster than in still air does not only determine the desired mechanical properties but also induce formation of thermal and transformational stresses that lead to changes in size and shape and thus may result in quenching cracks that damage the workpiece

• Transformational stresses are caused due to change in microphases, whereas change in microphases cause the change in volume.

• For example:o Austenite phase has the smallest volume,

• Untempered martensite phase has the largest phase.• If there are mixed phases, any residual austenite will transform to martensite over time or with the

application of heat. This will cause a dimensional change in the steel.

COUPLING EFFECTS AMONG THE THREE DIFFERENT CHARACTERISTICS:

• Figure schematically represents the coupling effects among the three different characteristics of quenching:

o cooling rate, metallic structure, and internal stresses. • The cooling rate influences the phase transformation of the metallic structure, whereas the latent

heat due to structural changes affects the cooling rate. • All phase transformations of austenite during quenching are accompanied by volume expansion.

As a consequence, locally and temporally different changes of structure and temperature cause nonuniform volumetric changes in the quenched part that can result in transformational and thermal stresses. These stresses accelerate or hinder the phase transformation and influence the volume expansion.

• While the phase transformation brings out a defined metallic structure, the volumetric dilatation and thermal and transformational stresses result in deformation and residual stresses.

• At room temperature, both characteristics influence the material properties.


Classes:45-46QUENCHING AND QUENCHING MEDIA

COMMON QUENCHANTS:• The most common quenchants in ardening practice are liquids including:

o watero water that contains salt, aqueous polymer solutionso hardening oils.

• Inert gases, molten salt, molten metal, and fluidized beds are also used.• Quenching techniques used for liquid media are:

o Immersion quenching. o Spray quenching.

• Immersion quenchingo The part is submerged into an unagitated or agitated quenchant, o It is the most widely used technique.

• Spray quenchingo It refers to spraying the liquid through nozzles onto those areas of the hot workpiece where

higher cooling rates are desired.

QUENCHING MEDIA:Four commonly used quenching media:

• Brine – the fastest cooling rate• Water – moderate cooling rate• Oil – slowest cooling rate• Gas – used in automatic furnaces, usually liquid nitrogen, can be very fast cooling.

Too rapid cooling can cause cracking in complex and heavy sections.

HEAT REMOVAL STAGES:• During quenching in liquid media with boiling temperatures far below the initial temperature of

the body, three stages of heat removal occur. These are referred as • The film boiling or vapor blanket stage.• The nucleate boiling stage.• The convection stage.• In the film-boiling stage the surface temperature of the workpiece is sufficiently high to vaporize

the quenching liquid and form a stable film around the part.


• The vapor film has an insulating effect; therefore the cooling rate during film boiling is relatively slow.

• The temperature above vapor blanket occurs is called the Leidenfrost temperature after Johann Gottlieb Leidenfrost.

• When surface temperature is less than the Leidenfrost temperature, the vapor film collapses and the nucleate boiling begins [11].

• In this stage, the liquid in contact with the hot surface evaporates, and immediately the vapor bubbles leave the surface.

• This causes strong convection, which results in a high rate of heat transfer from the metal to the fluid.

• Upon further cooling, the surface temperature becomes less than the boiling point of the liquid, and the surface is permanently wetted by the fluid.

• The cooling rate is low and determined mainly by the rate of convection and the viscosity of the liquid quenchant.

ASSIGNMENT:

Documents

25780297 Het Tretment by Sir Ishaq