TTT Diagrams, Transformations and Heat Treatment in Steels,

Hardenability, CCT Diagrams and Heat Treatment Techniques in steels

Course: Phase Transformations and Heat Treatment of Materials

Course Code: MM1402 NIT Jamshedpur

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Time-Temperature-Transformation (TTT) Diagrams : Isothermal Transformation Diagrams

Progress of Isothermal PhaseTransformation 1. Fraction transformed, f = f(t, T) 2. E.g’s. α β : f is vol. fraction of product phase (β) at any time t α’ α + β : f is volume of β at any time t divided by the final volume of β 3. f = 0 to 1 4. f(t,T) depends

– Nucleation rate – Growth rate – Density and Distribution nucleation sites – Overlap of diffusion field, Impingement of adjacent transformed volumes.

• 5. Possible sequence of events 1. Nuclei form throughout the transformation • So that wide range of particle sizes exists at any time • f depends on nucleation and growth rate 1. All nuclei form right at the beginning • (site saturation) f depends on nucleation sites and growth rate

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Cellular Transformation All of the parent phase is consumed by the transformation process. Transformation terminates by the impingement of adjacent cells growing with a

constant velocity. e.g., 1. Pearlite 2. Cellular Precipitation 3. Massive Transformations 4. Recrystallization Fraction transformed at any time, t, is given by: f = 1-exp(-ktn) n= 1 to 4 (independent of T if there is no change in nucleation mechanism) k: Depends on N & G so sensitive to T For constant N & G rate:

(a). Nucleation at a const. rate during the whole transformation (b) Site saturation –all nucleation occurs at the beginning of transformation (c) Cellular transformation

equation) Avrami-Mehl-(Johnson

)3

exp(143tNv

- f

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Construction of Curves from f

1. Civilian transformation are typified by C –shaped Curves 2. Variation of N & G rates with increasing undercooling 3. At T’s close to Te, driving force is very small. So, N & G rate are slow and long times are required for transformation 4. At large undercoolings, slow diffusion rates limit the rate of Transformation

Percentage transformation vs time for Different Transformation T

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Example of Precipitation Reaction: Ferrite from Austenite 1. Fe-0.14 wt%C 2. Austenized above A3, held at an intermediate T below A3 and quenched to RT in water. 3. At small ΔT below A3 Ferrite nucleates on γ G.Bs ( Grain Boundary Allotriomorphs) and grow in blockey manner 4. α/ γ I/f: curved (incoherent) a.w.a faceted (semi -coherent) are formed. 5. At larger ΔT, Ferrite nucleated at g.b’s grow as plates (Widmanstätten Morphology) 6. Ferrite can also nucleate within γ grains at inclusions

and dislocations. These precipitates are equiaxed at small ΔT and Plate like at large ΔT

Holding T’s for hypo-eutectoid steel (from P& E)

Variation of relative velocity of incoherent to semicoherent I/f w.r.t ΔT (After Aaronson, H.I., Decomposition of Austenite By Diffusional Processes, Inter Science, New York 1962) 13-04-2020 5

Microstructure of Fe-0.5%C alloy

a. 800C 150s B 750C, 40 s

d. 550C, 2 s c. 650C, 9 s

Ferrite is white. Specimens were Austenitized and held at intermediate T and then quenched to RT (Shewmin, P.G., in Transformation in metals, McGraw Hill, Newyork, after H.I. Aaronson 13-04-2020 6

TTT curve for hypoeutectoid steel & T vs composition regions for various morphologies

Typical TTT curve for γ α hypoeutectoid steel. (After Aaronson, H.I., Decomposition of Austenite By Diffusional Processes, Inter Science, New York 1962)

GBA: Grain Boundary Allotropes W: Widmanstatten side plates &/or Intragranular plates M: Maasive FerriteT (After Aaronson, H.I., Decomposition of Austenite By Diffusional Processes, Inter Science, New York 1962)

Practical Heat Treatments : Normalizing, Annealing Transformation occurs continuously during cooling Under these circumstances final microstructure depends on cooling rate

For T < Tw, Ferrite forms predominantly as Widmanstatten plates & T > Tw, GBA

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Eutectoid Transformation γ α + Fe3C : T<A1 γ simultaneously supersaturated w.r.t α & Fe3C. α + Fe3C: Pearlite. Microstructure comprises of lamellae or sheets of Fe3C embedded in α . Pearlite nodules nucleate on G.Bs and grow with a roughly constant radial velocity into the surrounding γ grains

Pearlite colony advancing into Austenite Grain Darken L S and Fisher R M in Decomposition of Austenite & Diffusional Processes, Interscience, NY 1962

Partially transformed has eutectoid steel. Pearlite has Nucleated On G.bs and inclusions. Darken L S and Fisher R M in Decomposition of Austenite & Diffusional Processes, Interscience, NY 1962 13-04-2020 8

Nucleation and Growth of Pearlite 1. Nucleation of α / Fe3C on γ G.B which one? Depends on G.B composition & structure. 2. e.g., Fe3C. To minimize ΔG* , There will be an orientation relationship (k-s) to one of the γ grains. Fe3C: Orthorhombic γ : Fcc (100)c//(1-11)γ1

(010)c//(110) γ1

(001)c//(-112) γ1

γ surrounding this Fe3C nucleus is depleted of C which will drive the pptn. of α . Nucleus is formed next to this nucleus. This process is repeated and the colony spreads sideways along the g.b. After nucleation the colony grows edgewise by the movement of incoherent I/f.

N&G of pearlite (from P & E)

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Nucleation and Growth of Pearlite (Contd…) • Eutectoid growth analogous to Eutectic

Growth

• Similar equations govern the growth rate

• S0 : observed interlamellar spacing

• S0 α (ΔT)-1

• Growth rate, V α (ΔT)2

• So, V X (S0)1/2 = const.

• In off-eutectoid alloys, Pearlite formation is preceded by precipitation of proeutectoid ferrite or cementite

• For large enough ΔT’s and Small departure from eutectoid composition possible for austenite to transform directly into pearlite(100%) provided the T is low enough to bring the Austenite into hatched region

Effect of transformation T on the volume fraction of Proeutectoid ferrite (from P & E)

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Bainite

• Austenite cooled to large supersaturations below the nose of TTT curve for pearlite transformation

• Mixture of Carbide and Ferrite

• Characterized by its own TTT curve

• Microstructurally different from Pearlite and at sufficiently high T’s competes with Pearlite formation

• Bainite Microstructure depends on T at which it forms.

• Bainite – Upper Bainite (3500C- 5000 C)

– Lower Bainite (Depends on C %)

• Upper Bainite: Needles/Laths of Ferrite with cementite precipitates between the laths

• Lower Bainite: Plates of Ferrite, finer carbide dispersion like in tempered martensite

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Bainite Microstructure

Uppeer Bainite in medium carbon steel (from Metals Society)

Lower Bainite in 0.69 wt % C low-alloy steel (After Heheman R F in Metals Handbook, ASM, 1973

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Effect of Alloying Elements on Hardenability

• Primary aim of adding alloying elements to steel is to increase hardenability (suppress formation of pearlite & /or Bainite )

• Hardenability: Ability of a steel to form martensite on quenching

• Expressed as the distance below the surface at which there is 50% transformation

to martensite after a standard quenching treatment, i.e., depth of hardening

• Alloying elements are classified as

– Ferrite stabilizers (e.g., Cr, Mo, Si)

– Austenite Stabilizers (e.g., Mn, Ni, Cu)

• Austenite stabilizers suppress A1 Temp, i.e., increase the area over which austenite is stable. Ferrite stabilizers increase the area over which ferrite is stable.

• Alloying elements also affect the TTT curve.

• Increasing hardenability = reducing the rate of austenite decomposition

• Achieved in 2 ways:

– Reduce the growth rate of ferrite/pearlite/bainite

– Reduce the nucleation rate of ferrite/pearlite/bainite

• Rate of formation of pearlite/bainite is maximum at the nose of C curve inTTT diagram.

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a) TTT diagrams of low-alloy steels 0.4 % C and 1 % Mn

b) TTT diagrams of low-alloy steels 0.4 % C , 1 % Mn and 0.9% Cr

Effect of Alloying Element on Hardenability (contd…)

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• Alloying element (X) will partition between Ferrite and cementite

• Carbide forming elemen he ts such as Cr, Mo, Mn will concentrate in the carbide

• Si will concentrate in Ferrite

• X in Austenite will be homogeneously distributed

• Pearlite growth will only be as fast as partition occurs by substitutional diffusion

• Most likely diffusion route: α/γ and α/cementite I/f

• Partitioning of alloying elements

(e.g., Fe-0.6 wt% C-0.85%Cr-

0.66%Mn-0.26%Si steel

transformed at 597 C for 2 min.

Effect of Alloying Element on Hardenability (contd…)

Fe3C

From williams, P. R and Miller M.K, Beavan, P.A & Smith G.D.W in Phase Transformations

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Continuous Cooling Diagrams • TTT diagrams (Isothermal Transformation)

: obtained by rapidly quenching to a given T and then measuring f(t, T).

• Practical heat treatments (HTs) : concerned with transformations during continuous cooling.

So, TTT cannot be used to give t, T

of various transformations. Instead CCT diagrams are used.

• CCT diagrams to a 1st approx. are TTT diagrams shifted to the left and to longer times.

• Transformation starts when the product Nt reaches a certain value, α, where N is Nucleation rate, t is time

• At small undercoolings ΔT, N is small and t is also small during cooling.

• So, Transformation does not start when the cooling curve intersects the TTT curve. Transformation is delayed or postponed to occur at lower T’s and longer times.

Curves A, B , C, D : cooling curves. Solid curve: TTT diagram Bold curve: CCT diagram From P & E

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Jominy End-Quench Test

End quench test specimen (From P & E, from Atlas of Isothermal Transformation & Cooling Transformation Diagrams, ASM, Metals Park)

End quench test specimen (from Atlas of Isothermal Transformation & Cooling Transformation Diagrams, ASM, Metals Park) 13-04-2020 17

Jominy-End Quench Test

1. Jominy End Quench test: widely used 2. Standard dimensions: round bar (25.4mm diameter, 102mm long) 3. heated to T> A3 then placed on a rig 4. One end of the rod is quenched by a standard jet of water 5. Cooling rate decreases along the bar from the quenched end, 6. Hardness along the length of the bar will be different

From Wilson, Metallurgy and Heat Treatment of Tool Steels

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Heat Treatment Techniques

Normalization • Performed to refine grain size of hypoeutectoid steels and to obtain a

carbide size and distribution that provides a favorable starting point for subsequent HT

• Austenitization T > 100 to 150 °F above the A3 line followed by an air cool. • Austenitizing Time : ~ 1 hour / in. of thickness

Full Anneal • To make the steel relatively soft and to endow it with good machinability. • Microstructure is large-grained ferrite and coarse pearlite. • Austenitization at HT to produce a large grained austenite and to dissolve

all the carbides. • The steel is then cooled slowly enough to allow the desired microstructure

to form by the transformation of the austenite at T near to the A1 line.

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Heat Treatment Techniques (contd..)

Isothermal Annealing.

• Aim as the full anneal. More efficient for small parts

• furnace cooling replaced by transferring the parts between const. T baths containing molten bath (salts).

Salts : mixtures of compounds such as alkali metal hydroxides and nitrates that are chosen for particular temperature ranges.

Spheroidization

• Spheroidized microstructure: carbides are discrete globules embedded within the ferritic matrix and also resident on G.B’s.

• After spheroidization, steels are softest and are most easily mechanically formed.

• Often part of overall treatments given to high carbon tool steels that are mechanically formed into particular shapes before being hardened.

• Produced by cooling very slowly from 100°F above the eutectoid isotherm or by a long IT just below it.

• Spheroidization can also be obtained by cyclic Thermal processing above and below the A1 line.

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Heat Treatment Techniques (contd..)

Quench and Temper.

• HTs : produce parts with desired hardness either throughout or in the vicinity of its surface.

• Involves austenitization, & quenching to RT or below and reheating to and holding at a tempering temperature below the A1 line.

• Quenching is to form martensite. The tempering T & t are selected to produce the required hardness.

• Excellent properties can be obtained by this procedure.

• Limitations for plain carbon steels. why? Diffusional transformation processes are not easily avoided during cooling.

• Consequentially, very severe quenching rates are necessary.

• The rapid quench rates are undesirable- Thermal stress and distortion are more likely.

• These stresses can help cause quench-cracking.

• Led to the development of low alloy steels- alloying elements slow down diffusional transformations to enable martensite to be produced during slower cooling rates.

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Martempering

• A modification of the quench process – relieves thermally induced stresses.

• The part is first quenched rapidly until its T has fallen below that of the nose of the TTT diagram but is still > the Ms point.

• Isothermally held to relieve thermal stress and then finally cooled through the Ms point.

• Is possible when the bainitic transformation takes much longer to begin compared to changes at the nose of TTT diagram.

Maraging

• Similar to Age Hardening.

• Low C % and contain Ni = 18 and 25wt% . Quenched from the austenitic condition, to form a soft but heavily dislocated martensite.

• High Ni% lowers Ms to ~ 150◦C, on reheating martensite austenite is not reformed until the steel is held between 500◦C and 600◦C. At 400–500◦C, precipitation of intermetallic phases takes place.

• Mo & Ti are necessary additions, results in the precipitation of Ni3Mo, Ni3Ti and the Laves phase,Fe2Mo. Co is a useful alloying element, Co decreases solubility of Mo.

• Maraging steels can be heat treated to give a yield stress around 2000MPa accompanied by good ductility and toughness.

Heat Treatment Techniques (contd..)

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Austempering

• Bainite can have useful hardness and toughness values .

• When they are adequate for the application, the formation of bainite can be chosen instead of martensite.

• The steel is quenched to a T below the nose and held there until the bainitic transformation is completed. Avoids problems associated with the rapid volume changes during the martensitic transformation.

Ausforming

• A steel is said to have been ausformed when martensite is produced from plastically deformed austenite.

• Ausforming provided some of the strongest, toughest steels + very good fatigue resistance

• Limitations: high concentrations of expensive alloying elements and must be subjected to large deformations which impose heavy work loads on rolling mills.

• these steels are particularly useful where a high strength to weight ratio is required and where cost is a secondary factor.

• Typical applications have included parts for undercarriages of aircraft, special springs and bolts.

Heat Treatment Techniques (contd..)

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