Engineering Materials-istanbul .technical University
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Dr.C.Ergun Mak 214E
MAK214E Summer 2006-2007 Lecture Notes 2
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Dr.C.Ergun Mak 214E
Principals of Heat Treatments
• Heat treatment is a process to apply to a certain alloy compositions to obtain specific properties.
You may want to have followings: –High strength, –High hardness, –Ductility, –Machinability, –Small grain size, –Remove the internal stresses, –Homogenous structure.
You may use heat treatments. Process variables to control the resultant properties: – Processing time, – Processing temperature, – Cooling rate, – The composition of the starting material-alloying elements – The process history of the starting material (any process previously performed). 2
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Dr.C.Ergun Mak 214E
A Basic Heat Treatment Cycle Temperature (C, F, K, etc.)
800
Important Process Parameters •Heating rate •Holding temperature •Holding time •Cooling rate
Heating rate
Treatment Temperature
600
Holding time 400
Cooling rate
200 0 0
1
2
3
Time (day, hr, min, sec, etc.)
Depending on the Cooling rate: • Slow Cooling rate Ö Diffusional phase transformations. • Fast Cooling rate Ö Diffusionless phase transformations. (Quenching)
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Dr.C.Ergun Mak 214E
Types of the phases in steels
4
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Dr.C.Ergun Mak 214E
Steels: Fe-Fe3C Phase Diagram
Ferrous alloys we will involve – Plain Carbon steels – Alloy and tool steels – Stainless steels – Cast irons Phases and Solid solutions – δ Delta iron – γ Austenite – α Ferrite – Fe3C cementite – Martensite
Peritectic reac.
Eutectic reac.
– Bainite. The phase reactiaons: Eutectoid reac. • Peritectic: L 0,53C% + δ 0,009C% Æ γ 0,17C% Dividing point • Eutectic: L 0,53C% Æ γ 0,009C% + Fe3C 6,67C% between cast • Eutectoid: γ 0,77C% Æ α 0,0218C% + Fe3C 6,67C% irons and steels
Dr.C.Ergun Mak 214E
5
Properties of Phases in Steel
6
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Dr.C.Ergun Mak 214E
A1, A2, A3 ve Acm temperatures A3
γ
Acm
γ+Fe3C
α+γ
A1
α+Fe3C
A2: Manyetikliğin kaybolduğu Curie sıcaklıdır: 769oC. 7
Dr.C.Ergun Mak 214E
I II
I
III
γ
y5
III
γ y1 y2
y y2 y3 y4 α+Fe3C
y3
y5
1
Perlite
α
α+γ yy 12 y3 y4
II γ
⇑ Eutectoid Composition
Cementite
α
Perlite
Perlite
8
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Dr.C.Ergun Mak 214E
TTT (Time temperature transformation) Diagrams
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Dr.C.Ergun Mak 214E
TTT diagrams
The first noise shows that the transformation starts.
The second noise shows that the finishes.
T Ostenit Dengesiz ostenit
Tm
Kaba perlit İnce Perlit Üst Beynit Alt Beynit
Reaksiyon Başlamamış
Sürüyor
t (logaritmik skala)
Tamamlanmış 10
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Dr.C.Ergun Mak 214E
TTT diagrams
Phase areas
Fs: Ferrite start temp. Ps: Pearlite start temp. Pf: Pearlite finish temp. Bs: Bainite start temp. Bf: Bainite finish temp. Ms: Martensite start temp. Mf: Martensite finish temp. Coarse Pearlite Fine Pearlite Upper Bainite Lower Bainite
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Dr.C.Ergun Mak 214E
Cooling curves on TTT Diagrams (a) Continues cooling (b) Isothermal cooling
12
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Dr.C.Ergun Mak 214E
Isothermal Heat Treatment: Isothermal Annealing TTT Diagrams
Transformation along Continuous cooling curve
Transformation along isothermal curve
Isothermal annealing for fully pearlitic structure. Ferrite + Perlite for hypoeutectoid steels or Perlite + Cementite for hypereutectoid steels
Dr.C.Ergun Mak 214E
T
13
Soru: Yapılar nedir Ostenit
T Kaba perlit
t (logaritmik skala)
Ostenit Kaba perlit
t (logaritmik skala)
14
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Dr.C.Ergun Mak 214E
Soru: Yapılar nedir T
t (logaritmik skala)
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Dr.C.Ergun Mak 214E
Phase transformation
16
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Dr.C.Ergun Mak 214E
Austenite
Özet Yavaş Soğuma
Austenite
İzotermal Dönüşüm Yayınmalı
Austenite
Çok hızlı Soğuma
Hardness Ü
Yayınmalı
Mechanical Prop vs. Peartlite (α+Fe3C) Microstructure – Ferrite – Coarse Pearlite – Fine Pearlite Bainite (α+Fe3C) – Upper Bainite – Lower Bainite – Martensite Martensite (single)
Yayınmasız
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Dr.C.Ergun Mak 214E
Sıcaklık (oC)
Kritik soğuma hızı
Martenzit
Perlit + Martenzit
İnce perlit
Zaman (s)
Kaba perlit
18
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Dr.C.Ergun Mak 214E
Heat Treatments of Steel •
A Simple Heat Treatments – – – – – –
•
Full Annealing Normalizing Spheroidizing Process Annealing Stress Releif Annealing Homogenizing
Isothermal Heat treatments – Austempering – Isothermal Annealing
•
Diffusionless Transformation Treatments – – – –
•
Surface Hardenning Treatments – – – –
•
Quenching Tempering Martempering Ausforming Carburizing Nitriding Carbonitriding Induction or Flame Hardening
Age Hardening Treatments – Precipitation Hardening Treatment
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Dr.C.Ergun Mak 214E
Simple Heat treatments
20
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Dr.C.Ergun Mak 214E
Full Annealing
Aim: Softest structure (Coarse grains): High ducitlity. • Hypoeutectoid steel: Coarse (grained) pearlite and ferrite • Hypereutectoid steel: Coarse pearlite and sementite • First, austenitize the steel, • A3 + (30 – 50oC) for hypoeutectoid • A1 + (20 – 40oC) for hypereutectoid steels. • Then, slow (furnace) cooling to room temperature.
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Dr.C.Ergun Mak 214E
Normalizing
Aim: homogeneous and fine distribution of pearlite. •Higher strength and slightly lower ductility by refining grains and reducing segregations. •First austenitize the steel • A3 + (50-80oC) for hypoeutectoid • Acm + (50-80oC) for hypereutectoid steels •Air cooling to produce a fine pearlitic structure. •For hypoeutectoid steel; dissolve all the carbides and to response readily to the following treatment (spheroidizing, etc.) or final hardening treatment.
22
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Dr.C.Ergun Mak 214E
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Dr.C.Ergun Mak 214E
Spherodizing Treatment
Aim: Improving Machinability: ¾ Coarse spheroidal cementite particles in ferrite, by decomposition of lamellar cementite into spheres. ¾ Suitable for medium and high C (>0.4%) steels for good machining characteristics. ¾ Heat up to just below A1 temperature (above 690oC) for 15-25 hours, cool in air.
Not common for Hypoeutectoid steels for cementites spheroidization but good for spheroidizing of oxides, sulfides.
For Hypereutectoid steels, spheroidizing of large carbides for tougher, softer 24 properties.
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Dr.C.Ergun Mak 214E
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Dr.C.Ergun Mak 214E
Process Annealing
Eliminating the effect of Cold Work: • Also called recovery but recrystallization and grain growth possible. • Arrangement of dislocations and formation of new grains and consequently soft structure. • A low-temperature recrystallization heat treatment • Just for hypoetectoid steels. (C < 0.3%). • Heating between 550-650oC for necessary time • Cool in furnace to soften strain hardened- structure high dislocation density. • No further heating to prevent grain growth.
26
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Dr.C.Ergun Mak 214E
Da y
R ec ry s
y er ov c e R
ta lli za ti o n
Process Annealing ra G
in
G
w ro
th
l i k. Sert anım
meler ı geril Kalınt Elek trik Sü iletk nek e nl i ğ lik i
Tane büyüklüğü
0.3
0.4
0.6
Th
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Dr.C.Ergun Mak 214E
¾
Stress relief
Residual stresses are due to thermal and mechanical processes such as casting, inhomogeneous plastic deformation, heat treatment, welding, etc.,
Aim is to reduce internal residual stresses resulted from processes, ¾ Heated up to 500-550oC for necessary time, ¾ Cool slowly in furnace, ¾ Recovery mechanism (Arrangements of the dislocations) ¾ Not major changes on the mechanical properties.
28
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Dr.C.Ergun Mak 214E
Homogenizing
¾ Eliminate the micro-segregation in the cast structure (soaking process of pig casts). ¾ Eliminateing macrosegregations dissolving second phases- oxides, carbides, nitrides, sulfides, etc. ¾ Heat up to high temperatures (1100-1200oC) held 50 hrs, ¾ Then cool in air. ¾ Intermediate heat treatment: Get a suitable microstructure for the subsequent heat treatments. 29
Dr.C.Ergun Mak 214E
Segregation
First solidified solid and the last solidified solids have not the same composition as the last solidified solid. Called as “micro-segregation” 1. To pass slowly the solidification range or 2. Reduced with homogenizing treatment
30
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Dr.C.Ergun Mak 214E
All simple Heat treatments on the same diagram
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Dr.C.Ergun Mak 214E
γ
Yumuşatma Tavı Normalizasyon Su Verme Ms
İnce perlit Martenzit
Perlit + Martenzit
Mf
Kaba perlit 32
16
Figure 12.5 The effect of carbon and heat treatment on the properties of plain-carbon steels.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Dr.C.Ergun Mak 214E
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Dr.C.Ergun Mak 214E
Isothermal Heat treatments
34
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Dr.C.Ergun Mak 214E
T
Austempering
yüzey merkez First austenize. •Quench above Ms •Wait to transform γ to bainite •The final Microstructure: Full Bainite Upper or Lower Bainite depending on the transformation temperature t (logaritmik skala) Bainite can only be obtained by isothermal trasnformation!!!
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Dr.C.Ergun Mak 214E
Diffusionless Heat treatments
36
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Dr.C.Ergun Mak 214E
Quenching
•Aim is to obtain fully martensitic structure (very hard but brittle). •Firs , fully austenizing: •A1 + 30-50oC for hypoeutectoid steels •A3 + 30-50oC for hypereutectoid steels enough time, •Then cool rapidly (quenching) at high cooling rates higher than critical cooling rate to a temperature below Mf (refer to CCT curve for the steel). Quenching –very quick cooling no time for diffusion; a diffusionless transformation forming martensite.
A3
Acm
γ
γ+Fe3C
α+γ
A1
α+Fe3C 37
“Critical Cooling rate”.
Dr.C.Ergun Mak 214E
TTT Curves
The cooling rate just touches the noise is called “Critical Cooling rate”.
¾For martensitic transformation (diffusionless transformation), the cooling rate should be higher than critical cooling rate so that it does not cut the noise and can not start the diffusional mechanisms. ¾Otherwise the diffusional mechanism works and γ austenite may transform other phases depending on the steel composition and the location where the noise is crossed (refer to the next slides for the possible phases that austenite may transform. 38
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Dr.C.Ergun Continuous Mak 214E
Cooling curves (CCC) vs. Isothermal Cooling curves TTT Diagrams
Transformation along Continuous cooling curve
Transformation along isothermal curve
Bainite can only be obtained by isothermal trasnformation!!! 39
Dr.C.Ergun Mak 214E
CCT and IT curves
CCT: diagram
ITT diagram
Temperature
Eutectoid Temperature
A. Slow cooling in furnace (annealing) -Lamellar Coarse pearlite B. Cooling in still air (normalizing) –fine pearlite C. Split transformation (oil quenching) -fine pearlite and martensite D. Rapid cooling (water quenching) -martensite E. Critical cooling rate -Slowest rate to produce no pearlite
Examine the resultant phase in 3 different isothermal cooling conditions and Martensitic transformation Temperatures. 40
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Dr.C.Ergun Mak 214E
What is the difference in the materials properties between the one produced with continues cooling and the one produced by isothermal cooling?
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Dr.C.Ergun Mak 214E
Ötektoit Çelik
γ
T
α+γ
Ostenit
γ+Fe3C
Kaba perlit Dengesiz ostenit
İnce Perlit Üst Beynit
α+Fe3C
Alt Beynit Martenzit Ms Mf
t (logaritmik skala)
42
21
Dr.C.Ergun Mak 214E TTT
diagrams: Isothermal heat treatment curves. Hypoeutoctoid Steel
γÆα Wing for ferrite start temperatures.
Hypoeutectoid steels has a wing for ferrite start temperature whereas hypereutectoid steels, a wing for cementite start temperatures. 43
Dr.C.Ergun Mak 214E TTT
diagrams: Isothermal heat treatment curves. Hypereutoctoid Steel
γÆFe3C Wing for cementite start temperatures.
γ + Fe3C
Hypereutectoid steels, a wing for cementite start temperatures. 44
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Dr.C.Ergun Mak 214E
Interrupting isothermal heat treatment To have different phases in the steel. For example, • Austenize the steel • quench to 650oC, and wait 10s to transform some γ to α and pearlite, • then quench to 350oC and wait for a while 100s to transform a part of the remained γ to bainite, • consequently quench below to Mf to convert the last remained γ to martensite.
Inter. HT
Final microstructure: Ferrite, pearlite, bainite and martensite. 45
Dr.C.Ergun Mak 214E
T
Tempered Martensite
yüzey merkez
Tempering Temperature
To obtain tougher and more ducitle structure. Martensite transforms to very fine ferritic - perlitic structure. • Reheating the martensitic steel below eutectoid temperature. • Temperature level is important for the final hardness.
t (logaritmik skala) 46
23
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Dr.C.Ergun Mak 214E
Fig 12-11 The effect of tempering temperature on the mechanical properties of a 1050 steel.
Page345
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Dr.C.Ergun Mak 214E
Martempering (Marquenching) During quenching; •Surface cools faster and transforms first to martensite. •Center transforms later. •If residual stresses are greater than yield strength, quench cracks may occur. •Martempering reduces the risk of residual stresses and their results. •Quench the steel from austenite region to above Ms •Wait to equalize the temperatures of surface and center, then quench to room temperature produce martensite.
48
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Dr.C.Ergun Mak 214E
T
Austempering
yüzey merkez
Bs
Bf
Bainite
Ms
First austenize. • Quench above Ms • Wait to transform γ to bainite • The final Microstructure: Full Bainite
Mf t (logaritmik skala) 49
Dr.C.Ergun Mak 214E
Effect of Alloying Elements Effect of C on Ms and Mf
As C content increases in the steel, •The martensite start temperature, Ms •The finish temperatures, Mf decrease. So, amount of retained austenite (not demanded), the residual stresses due to the increase in the temperature difference betwen austenite and Ms 50 increase, thus the quench cracking risk increases.
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Dr.C.Ergun Mak 214E
Ausforming
A thermomechanical heat treatment in which austenite is plastically deformed below the A1 temperature, then permitted to transform to bainite or martensite.
The bay area obtained by alloying • First, quench the steel austenite region to Bay area, • Then apply forming processes avoiding to enter pearlite and/or bainite region, Then; • If quench to below Mf: martensite forms. • If cooled slowly: bainite forms 51
Dr.C.Ergun Mak 214E
Effect of Alloying Elements
Important
1. Increase the hardenability: Alloying elements increase the hardenability of steel. Martensite can form through the large thickness of the parts even very slow cooling rates. 2. Change the shape of Fe-Fe3C phase diagram: (Mn and Ni, austenite stabilizer agent (γ at room T), Cr; ferrite stabilizer) 3. Introduce a bay area in the TTT Diagram; (Ausforming (austenite + forming) becomes possible); 4. Improve the respone toTempering treatment: Alloying elements reduce the rate of tempering compared with that of a plain-carbon steel. Secondary hardening becomes possible. 5. Other: solid solution strengthening, alloy carbides, corrosion resistance, etc. can be obtained. 52
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Dr.C.Ergun Mak 214E
Effect of Alloying Elements Hardenability
•Certain alloying elements in the steel moves the noise of the TTT curve to the right direction. •The practical significance; Very low cooling rates, (cooling in air), can produce martensite. •Whole volume of the fabricated massy body can be transformed to martensite even cooling in air.
Dr.C.Ergun Mak 214E
Hardenability
Certain alloying elements, increase the hardenability. In the plain carbon steels, 1050, the surface is hard, but not in deep. The alloyed steel, 4340 hardened deeper. So the hardenability of 4340 is much better. Even slow cooling rates may produce the martensite in all cross-section. But hardness is not high since lower C content. 53
Effect of Alloying elements Secondary hardening: Carbide precipitation
A bay area may appear. Special processes possible such as “ausforming”.
Alloying elements can also reduce the effect of tempering compared to the plain carbon steels. The alloy steel can be used at high temperatures. 54
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Dr.C.Ergun Mak 214E
Hardenability Curves and Jominy Tests •
•
•
Jominy test - The test used to evaluate hardenability. An austenitized steel bar is quenched at one end only, thus producing a range of cooling rates along the bar. Hardenability curves - Graphs showing the effect of the cooling rate on the hardness of asquenched steel. Jominy distance - The distance from the quenched end of a Jominy bar. The Jominy distance is related to the cooling rate. Jomminy distance for various steels can be seen in the figure. Plain carbon steels have shallow jomminy distance while alloyed steels may have very deep. However, C provides higher surface hardness compared to the other alloying elements. 55
Dr.C.Ergun Mak 214E The
cooling rates provided by various quenchants (quenching media) The cooling rate provided by the quenchants are represented by a constant value “H”.
the relation between the diameter of the work piece and jomminy distance in the Figure for a given “H” values.
56
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Dr.C.Ergun Mak 214E
A machine part of 1050 steel was quenched in a medium (H=0.2) and hardness at a certain location is 28 HRC. Predict the hardness change at the same point if the oil is agitated during quenching.
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Dr.C.Ergun Mak 214E
From Figure 12-23, Page 353
4 16
10 16
4 16
From Figure 12-23, Page 353
4 (inch) ⇒ 39 HRC 16 58
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Dr.C.Ergun Mak 214E
An AISI 9310 steel bar with a diameter of 40mm have a hardness of 42HRC at the center after quenching. What is the minimum severity of quenching medium in terms of “H coefficient”. Which quenchant would you recommend to produce the aimed hardness in the steel with the minimum risk of quench cracks?
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Dr.C.Ergun Mak 214E
From Figure 12-23, Page 353
From Figure 12-24, Page 355 40 mm = 1.6 inch
6.5 16
6.5 16
H value should be between 0.5 and 1. But the correct H to provide sufficient cooling rate is “1”. The quenchant should be still water (Table 12-2, page 348). 60
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Dr.C.Ergun Mak 214E
The fallowing heat treatments were applied to a shaft of 25mm diameter and made of 1050 steel. a) Heat at 820oC, quench to 25oC in water, temper for one hour at 400oC. Cool to room temperature in air. b) Heat at 820oC, quench to 400oC in a salt bath, hold for two min. Cool to room temperature in air.
Describe the resultant microstructure and estimate the hardnesses at the end of each treatment. Make comments about the mechanical behaviour of shafts at the end of each treatment. 61
Dr.C.Ergun Mak 214E Figure 12-8 (a) page 342
a) Tempering of Martensite: Micorstructure:Tempered Martensite b) Austempering: (Isothermal heat treatment) Microstructure: Lower Bainite 62
31
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Dr.C.Ergun Mak 214E
•Strength / Hardness, (Wear resistance) • Ductility, • Machinability, • Small grain size, • Residual stresses stresses, • Homogenous structure. (Quenching ? Marquenching may be better) (Retained austenite) Figure 12-5 page 340 63
Dr.C.Ergun Mak 214E
Strengthening of materials
• Strain hardening: due to the increase in dislocation density and their interaction with each other, obstacles, grain boundaries, etc. • Martensite strengthening: • Solid Solution hardening: Addition of different atoms provide additional strength to the material caused by the lattice distortion due to the mismatch of the atoms. • Dispersion strengthening: The strengthening of a metal or an alloy by incorporating chemically stable submicron size particles of a nonmetallic or intermetallic phases that impede dislocation movement at elevated temperatures (hard particles in matrix). • Precipitation hardening: hardening in metals caused by the precipitation of a constituent from a supersaturated solid solution.
64
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Dr.C.Ergun Mak 214E
• • • •
Dispersion Hardening
Soft matrix-hard precipitates/particles Homogenuous distribution of precipitates/particles Fine precipitates/particles Spherical precipitates/particles
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Dr.C.Ergun Mak 214E
Phase diagrams with respect to solubility
a) Unlimited solubility: One material can completely dissolve in a second material without creating a second phase. b) Insolubility: One element can not dissolve in another in any amount. c) Limited solubility: One element can dissolve in another only in certain amount. a)
b)
c)
66
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Dr.C.Ergun Solubility Mak 214E
and Solid Solutions
Complete solute solution of Cu and Ni atoms
Precipitation of a new phase: a Cu- Zn compound
(a) Liquid Cu and Ni: complete solubility. Phases and solubility: (Solid Cu-Ni alloys: complete solid The three phases of water. solubility in random lattice sites). • Water and alcohol - unlimited solubility. (a) In Cu-Zn alloys containing more than • Salt and water - limited solubility. 30% Zn, a second phase forms • Oil and water - no solubility. limited solubility of Zn in Cu. 67
Dr.C.Ergun Mak 214E
•
•
For Unlimited Solid Solubility
Hume-Rothery rules - The conditions for unlimited solid solubility. Hume-Rothery’s rules are necessary but are not sufficient for materials to show unlimited solid solubility. Hume-Rothery rules: • Size factor • Crystal structure • Valence • Electronegativity
68
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Dr.C.Ergun Mak 214E
Solid-Solution Strengthening
Effect of atomic radii alloying atoms added to Cu on the strengthening
Dr.C.Ergun Mak 214E
Important
• •
Effect of Zn content in Cu on the properties of solid solution.
The mechanical properties of Cu-Ni alloys. Pay attention to 60% Ni -40% Cu. 69
Precipitation (Age) Hardening
Small second phase precipitates behaves as small obstacles to dislocation motion. Starting from a structure having coarse grained precipitates, 1. Solution treatment: heating the material to the single phase ragion. 2. Queching the material to room temperature having a supersaturated solid solution with a metastable single phase microstructure. 3. Aging the material at (reheating to) an intermediate temperature to activate solid state diffusion to form fine grained precititates.
•
Overaging- aging the material too long causes coarser precipitates loosing the effectiveness to behave as an 70 dislocation barier
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Dr.C.Ergun Mak 214E
Çökelme sertleşmesi
• İç yapıda, dislokasyon hareketlerini engelleyerek dayanımın artmasına sebep olan çok küçük ikinci fazların çökeltilmesi işlemidir. Yaşlandırma sertleşmesi: • Önce Çözündürme işlemi (solution treatment) yapılarak çökelen sert olan 2. faz, tek faz içerisinde tamamen çözülür. • Daha sonra yapı, hızlı soğutma (su vererek-suda soğutmak) ile ikinci fazın çökelmesi engellenir ve aşırı doymuş katı çözelti elde edilir. • Daha sonra yaşlandırma işleminde; aşırı doymuş katı çözelti, çözündürme sıcaklığından daha düşük olana yaşlandırma sıcaklığına tekrar ısıtılarak çok küçük bağdaşık (koherent) ikinci faz tanecikleri çökeltilir. (Bu çökeltiler dislokasyonlara engel teşkil ederek malzemenin dayanımını arttırır). • Aşırı yaşlanma: çökelmelerin çok büyüyerek bağdaşıklığın (koherentliğin ) kaybolmasi 71
Dr.C.Ergun Mak 214E
If slowly cooled-(not hardening) T
%100 β (single phase)
β Slow cooling Equilibrium microstructure: Coarse α Grains in β matrix
α+β
Composition
Time 72
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Dr.C.Ergun Mak 214E
Precipitation (Age) hardening
Important
T Solution treatment
Same mic ro
Sıcaklık
g Quenchin
α+β
structure
β
taging
α-Grains in β matrix Time
Composition
Forming the coherent small precipitation 73
Dr.C.Ergun Mak 214E
Hardness
• In the first stage, very small coherent precipitates called -GP zones (Guinier preston zones) forms, • The empty spaces below the dislocation are good location for nucleating of these GP zones (decreasing the energy of the system), thus prevents Important the dislocation motions. • Then, these zones form larger coherent precipitates. These precipitates stretches the lattice and cause to strengthening the material. Coarsening the precipitates and loosing Over their ability to Coherent grain Aging strenghening the material. formation GP Zone Lossing of Coherency
Temperature
Coherent Precipitation
β
α
74
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Dr.C.Ergun Mak 214E
Overaging
β
α
• Overaging: As the precipitates coarsen, the misfit stresses become too large to sustain. • Then the coherency would be lost the the precipitates becomes uncoherent. • Thus the effectiveness of the hardening decreases. • If the material aged long enough, the starting coarse microstructure will be formed.
75
taging
Hardness
Temperature
Dr.C.Ergun Mak 214E
Taging(hour)
76
38
Dr.C.Ergun Mak 214E an Design
age hardening treatment giving the temperature for each step for the alloy having 2 wt.% Cu.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
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Dr.C.Ergun Mak 214E
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
78
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Quizz: What is the streghthening mechanism of age hardening? Explain briefly the steps for a typical age hardening treatment.
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Dr.C.Ergun Mak 214E
( ) Austempering is an isothermal heat treatment which transforms austenite to pearlite ( ) Process annealing is used to soften steels after quenching ( ) Cooling rate in oil quenching is always higher than in water quenching …….. steels cointain ferrite and martensite in their room temperature microstructure. The first manufacturing step to obtain pearlitic melable cast iron from whit cast iron is ………. Dimond brale indenter and 150kgf major load are used to conduct ......... Test Hardness of hard metals can be measure by using ............... Tests. Hardness of ceramics can be measure by using ............... Tests.
Aşırı yaşlanmış Al alaşımlarında dayanımın düşmesinin sebepleri; çökelti matris
81
Dr.C.Ergun Mak 214E Bolt class 6.8 should satisfy the ultimate tensile strength of ….MPa and yield strength of ……..MPa. a) 800/600 b) 480/600 c) 600/480 d) none of them ……….can be seen in macroscopic examinations of metals which resulted from ……. a) flow lines/plastic deformation b) welded section / low hardenability c) dislocations/casting process d) none of them Spherodizing of high C steels is done at temperatureres between .... And .... (a) 690oC-A3 (b) Acm-800oC (c) 690oC-A1 (d) none of them ..........occurs at the temperature higher than 60% of melting point in ..... (a)Grain growth/process annealing (b)Full annealing age hardening (c)Overtempering/stress relief (d)none of them Upeer bainite is .......... (a)Harder than martensite (c)Softer than ferrite
(b)harder than coarse pearlite (d) none of them
Fromation of .................. İs the sequence of age hardening (a) Supersaturated solid solution / GP zones / non-coherent precipitates/ coherent precipitates ( b) Supersaturated solid solution / GP zones / coherent precipitates / non-coherent precipitates (c) GP zones / coherent precipitates / non-coherent precipitates/ Supersaturated solid solution (d) None of them 82
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Jomminy tests are used to evaluate ..... Of steels a) Ductile/brittle transition b) microstructure c) hardness d) none of them The risk of quench cracking can be resuced by using.......treatment a) Tempering b) annealing c) Martempering d) austempering Secondary hardening can be seen in ......steels (a) High alloy (b) Acm-800oC (c) carburizing (d) high carbon Galvanized steels is produced by coating......... On the surfaces of sheets (a)Pb (b)Sn (c)Zn (d)none of them Deep drawing quality steels must exhibit high...... (a)Hardenability (b)strength (c)ductility (d)density
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MAK214E Summer 2006-2007 Lecture Notes 2
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Dr.C.Ergun Mak 214E
Principals of Heat Treatments
• Heat treatment is a process to apply to a certain alloy compositions to obtain specific properties.
You may want to have followings: –High strength, –High hardness, –Ductility, –Machinability, –Small grain size, –Remove the internal stresses, –Homogenous structure.
You may use heat treatments. Process variables to control the resultant properties: – Processing time, – Processing temperature, – Cooling rate, – The composition of the starting material-alloying elements – The process history of the starting material (any process previously performed). 2
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Dr.C.Ergun Mak 214E
A Basic Heat Treatment Cycle Temperature (C, F, K, etc.)
800
Important Process Parameters •Heating rate •Holding temperature •Holding time •Cooling rate
Heating rate
Treatment Temperature
600
Holding time 400
Cooling rate
200 0 0
1
2
3
Time (day, hr, min, sec, etc.)
Depending on the Cooling rate: • Slow Cooling rate Ö Diffusional phase transformations. • Fast Cooling rate Ö Diffusionless phase transformations. (Quenching)
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Dr.C.Ergun Mak 214E
Types of the phases in steels
4
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Steels: Fe-Fe3C Phase Diagram
Ferrous alloys we will involve – Plain Carbon steels – Alloy and tool steels – Stainless steels – Cast irons Phases and Solid solutions – δ Delta iron – γ Austenite – α Ferrite – Fe3C cementite – Martensite
Peritectic reac.
Eutectic reac.
– Bainite. The phase reactiaons: Eutectoid reac. • Peritectic: L 0,53C% + δ 0,009C% Æ γ 0,17C% Dividing point • Eutectic: L 0,53C% Æ γ 0,009C% + Fe3C 6,67C% between cast • Eutectoid: γ 0,77C% Æ α 0,0218C% + Fe3C 6,67C% irons and steels
Dr.C.Ergun Mak 214E
5
Properties of Phases in Steel
6
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Dr.C.Ergun Mak 214E
A1, A2, A3 ve Acm temperatures A3
γ
Acm
γ+Fe3C
α+γ
A1
α+Fe3C
A2: Manyetikliğin kaybolduğu Curie sıcaklıdır: 769oC. 7
Dr.C.Ergun Mak 214E
I II
I
III
γ
y5
III
γ y1 y2
y y2 y3 y4 α+Fe3C
y3
y5
1
Perlite
α
α+γ yy 12 y3 y4
II γ
⇑ Eutectoid Composition
Cementite
α
Perlite
Perlite
8
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Dr.C.Ergun Mak 214E
TTT (Time temperature transformation) Diagrams
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Dr.C.Ergun Mak 214E
TTT diagrams
The first noise shows that the transformation starts.
The second noise shows that the finishes.
T Ostenit Dengesiz ostenit
Tm
Kaba perlit İnce Perlit Üst Beynit Alt Beynit
Reaksiyon Başlamamış
Sürüyor
t (logaritmik skala)
Tamamlanmış 10
5
Dr.C.Ergun Mak 214E
TTT diagrams
Phase areas
Fs: Ferrite start temp. Ps: Pearlite start temp. Pf: Pearlite finish temp. Bs: Bainite start temp. Bf: Bainite finish temp. Ms: Martensite start temp. Mf: Martensite finish temp. Coarse Pearlite Fine Pearlite Upper Bainite Lower Bainite
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Cooling curves on TTT Diagrams (a) Continues cooling (b) Isothermal cooling
12
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Isothermal Heat Treatment: Isothermal Annealing TTT Diagrams
Transformation along Continuous cooling curve
Transformation along isothermal curve
Isothermal annealing for fully pearlitic structure. Ferrite + Perlite for hypoeutectoid steels or Perlite + Cementite for hypereutectoid steels
Dr.C.Ergun Mak 214E
T
13
Soru: Yapılar nedir Ostenit
T Kaba perlit
t (logaritmik skala)
Ostenit Kaba perlit
t (logaritmik skala)
14
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Dr.C.Ergun Mak 214E
Soru: Yapılar nedir T
t (logaritmik skala)
15
Dr.C.Ergun Mak 214E
Phase transformation
16
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Austenite
Özet Yavaş Soğuma
Austenite
İzotermal Dönüşüm Yayınmalı
Austenite
Çok hızlı Soğuma
Hardness Ü
Yayınmalı
Mechanical Prop vs. Peartlite (α+Fe3C) Microstructure – Ferrite – Coarse Pearlite – Fine Pearlite Bainite (α+Fe3C) – Upper Bainite – Lower Bainite – Martensite Martensite (single)
Yayınmasız
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Dr.C.Ergun Mak 214E
Sıcaklık (oC)
Kritik soğuma hızı
Martenzit
Perlit + Martenzit
İnce perlit
Zaman (s)
Kaba perlit
18
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Dr.C.Ergun Mak 214E
Heat Treatments of Steel •
A Simple Heat Treatments – – – – – –
•
Full Annealing Normalizing Spheroidizing Process Annealing Stress Releif Annealing Homogenizing
Isothermal Heat treatments – Austempering – Isothermal Annealing
•
Diffusionless Transformation Treatments – – – –
•
Surface Hardenning Treatments – – – –
•
Quenching Tempering Martempering Ausforming Carburizing Nitriding Carbonitriding Induction or Flame Hardening
Age Hardening Treatments – Precipitation Hardening Treatment
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Simple Heat treatments
20
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Full Annealing
Aim: Softest structure (Coarse grains): High ducitlity. • Hypoeutectoid steel: Coarse (grained) pearlite and ferrite • Hypereutectoid steel: Coarse pearlite and sementite • First, austenitize the steel, • A3 + (30 – 50oC) for hypoeutectoid • A1 + (20 – 40oC) for hypereutectoid steels. • Then, slow (furnace) cooling to room temperature.
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Normalizing
Aim: homogeneous and fine distribution of pearlite. •Higher strength and slightly lower ductility by refining grains and reducing segregations. •First austenitize the steel • A3 + (50-80oC) for hypoeutectoid • Acm + (50-80oC) for hypereutectoid steels •Air cooling to produce a fine pearlitic structure. •For hypoeutectoid steel; dissolve all the carbides and to response readily to the following treatment (spheroidizing, etc.) or final hardening treatment.
22
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Dr.C.Ergun Mak 214E
Spherodizing Treatment
Aim: Improving Machinability: ¾ Coarse spheroidal cementite particles in ferrite, by decomposition of lamellar cementite into spheres. ¾ Suitable for medium and high C (>0.4%) steels for good machining characteristics. ¾ Heat up to just below A1 temperature (above 690oC) for 15-25 hours, cool in air.
Not common for Hypoeutectoid steels for cementites spheroidization but good for spheroidizing of oxides, sulfides.
For Hypereutectoid steels, spheroidizing of large carbides for tougher, softer 24 properties.
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Process Annealing
Eliminating the effect of Cold Work: • Also called recovery but recrystallization and grain growth possible. • Arrangement of dislocations and formation of new grains and consequently soft structure. • A low-temperature recrystallization heat treatment • Just for hypoetectoid steels. (C < 0.3%). • Heating between 550-650oC for necessary time • Cool in furnace to soften strain hardened- structure high dislocation density. • No further heating to prevent grain growth.
26
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Dr.C.Ergun Mak 214E
Da y
R ec ry s
y er ov c e R
ta lli za ti o n
Process Annealing ra G
in
G
w ro
th
l i k. Sert anım
meler ı geril Kalınt Elek trik Sü iletk nek e nl i ğ lik i
Tane büyüklüğü
0.3
0.4
0.6
Th
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Dr.C.Ergun Mak 214E
¾
Stress relief
Residual stresses are due to thermal and mechanical processes such as casting, inhomogeneous plastic deformation, heat treatment, welding, etc.,
Aim is to reduce internal residual stresses resulted from processes, ¾ Heated up to 500-550oC for necessary time, ¾ Cool slowly in furnace, ¾ Recovery mechanism (Arrangements of the dislocations) ¾ Not major changes on the mechanical properties.
28
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Dr.C.Ergun Mak 214E
Homogenizing
¾ Eliminate the micro-segregation in the cast structure (soaking process of pig casts). ¾ Eliminateing macrosegregations dissolving second phases- oxides, carbides, nitrides, sulfides, etc. ¾ Heat up to high temperatures (1100-1200oC) held 50 hrs, ¾ Then cool in air. ¾ Intermediate heat treatment: Get a suitable microstructure for the subsequent heat treatments. 29
Dr.C.Ergun Mak 214E
Segregation
First solidified solid and the last solidified solids have not the same composition as the last solidified solid. Called as “micro-segregation” 1. To pass slowly the solidification range or 2. Reduced with homogenizing treatment
30
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All simple Heat treatments on the same diagram
31
Dr.C.Ergun Mak 214E
γ
Yumuşatma Tavı Normalizasyon Su Verme Ms
İnce perlit Martenzit
Perlit + Martenzit
Mf
Kaba perlit 32
16
Figure 12.5 The effect of carbon and heat treatment on the properties of plain-carbon steels.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Dr.C.Ergun Mak 214E
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Isothermal Heat treatments
34
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T
Austempering
yüzey merkez First austenize. •Quench above Ms •Wait to transform γ to bainite •The final Microstructure: Full Bainite Upper or Lower Bainite depending on the transformation temperature t (logaritmik skala) Bainite can only be obtained by isothermal trasnformation!!!
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Dr.C.Ergun Mak 214E
Diffusionless Heat treatments
36
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Quenching
•Aim is to obtain fully martensitic structure (very hard but brittle). •Firs , fully austenizing: •A1 + 30-50oC for hypoeutectoid steels •A3 + 30-50oC for hypereutectoid steels enough time, •Then cool rapidly (quenching) at high cooling rates higher than critical cooling rate to a temperature below Mf (refer to CCT curve for the steel). Quenching –very quick cooling no time for diffusion; a diffusionless transformation forming martensite.
A3
Acm
γ
γ+Fe3C
α+γ
A1
α+Fe3C 37
“Critical Cooling rate”.
Dr.C.Ergun Mak 214E
TTT Curves
The cooling rate just touches the noise is called “Critical Cooling rate”.
¾For martensitic transformation (diffusionless transformation), the cooling rate should be higher than critical cooling rate so that it does not cut the noise and can not start the diffusional mechanisms. ¾Otherwise the diffusional mechanism works and γ austenite may transform other phases depending on the steel composition and the location where the noise is crossed (refer to the next slides for the possible phases that austenite may transform. 38
19
Dr.C.Ergun Continuous Mak 214E
Cooling curves (CCC) vs. Isothermal Cooling curves TTT Diagrams
Transformation along Continuous cooling curve
Transformation along isothermal curve
Bainite can only be obtained by isothermal trasnformation!!! 39
Dr.C.Ergun Mak 214E
CCT and IT curves
CCT: diagram
ITT diagram
Temperature
Eutectoid Temperature
A. Slow cooling in furnace (annealing) -Lamellar Coarse pearlite B. Cooling in still air (normalizing) –fine pearlite C. Split transformation (oil quenching) -fine pearlite and martensite D. Rapid cooling (water quenching) -martensite E. Critical cooling rate -Slowest rate to produce no pearlite
Examine the resultant phase in 3 different isothermal cooling conditions and Martensitic transformation Temperatures. 40
20
Dr.C.Ergun Mak 214E
What is the difference in the materials properties between the one produced with continues cooling and the one produced by isothermal cooling?
41
Dr.C.Ergun Mak 214E
Ötektoit Çelik
γ
T
α+γ
Ostenit
γ+Fe3C
Kaba perlit Dengesiz ostenit
İnce Perlit Üst Beynit
α+Fe3C
Alt Beynit Martenzit Ms Mf
t (logaritmik skala)
42
21
Dr.C.Ergun Mak 214E TTT
diagrams: Isothermal heat treatment curves. Hypoeutoctoid Steel
γÆα Wing for ferrite start temperatures.
Hypoeutectoid steels has a wing for ferrite start temperature whereas hypereutectoid steels, a wing for cementite start temperatures. 43
Dr.C.Ergun Mak 214E TTT
diagrams: Isothermal heat treatment curves. Hypereutoctoid Steel
γÆFe3C Wing for cementite start temperatures.
γ + Fe3C
Hypereutectoid steels, a wing for cementite start temperatures. 44
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Dr.C.Ergun Mak 214E
Interrupting isothermal heat treatment To have different phases in the steel. For example, • Austenize the steel • quench to 650oC, and wait 10s to transform some γ to α and pearlite, • then quench to 350oC and wait for a while 100s to transform a part of the remained γ to bainite, • consequently quench below to Mf to convert the last remained γ to martensite.
Inter. HT
Final microstructure: Ferrite, pearlite, bainite and martensite. 45
Dr.C.Ergun Mak 214E
T
Tempered Martensite
yüzey merkez
Tempering Temperature
To obtain tougher and more ducitle structure. Martensite transforms to very fine ferritic - perlitic structure. • Reheating the martensitic steel below eutectoid temperature. • Temperature level is important for the final hardness.
t (logaritmik skala) 46
23
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Dr.C.Ergun Mak 214E
Fig 12-11 The effect of tempering temperature on the mechanical properties of a 1050 steel.
Page345
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Martempering (Marquenching) During quenching; •Surface cools faster and transforms first to martensite. •Center transforms later. •If residual stresses are greater than yield strength, quench cracks may occur. •Martempering reduces the risk of residual stresses and their results. •Quench the steel from austenite region to above Ms •Wait to equalize the temperatures of surface and center, then quench to room temperature produce martensite.
48
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Dr.C.Ergun Mak 214E
T
Austempering
yüzey merkez
Bs
Bf
Bainite
Ms
First austenize. • Quench above Ms • Wait to transform γ to bainite • The final Microstructure: Full Bainite
Mf t (logaritmik skala) 49
Dr.C.Ergun Mak 214E
Effect of Alloying Elements Effect of C on Ms and Mf
As C content increases in the steel, •The martensite start temperature, Ms •The finish temperatures, Mf decrease. So, amount of retained austenite (not demanded), the residual stresses due to the increase in the temperature difference betwen austenite and Ms 50 increase, thus the quench cracking risk increases.
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Dr.C.Ergun Mak 214E
Ausforming
A thermomechanical heat treatment in which austenite is plastically deformed below the A1 temperature, then permitted to transform to bainite or martensite.
The bay area obtained by alloying • First, quench the steel austenite region to Bay area, • Then apply forming processes avoiding to enter pearlite and/or bainite region, Then; • If quench to below Mf: martensite forms. • If cooled slowly: bainite forms 51
Dr.C.Ergun Mak 214E
Effect of Alloying Elements
Important
1. Increase the hardenability: Alloying elements increase the hardenability of steel. Martensite can form through the large thickness of the parts even very slow cooling rates. 2. Change the shape of Fe-Fe3C phase diagram: (Mn and Ni, austenite stabilizer agent (γ at room T), Cr; ferrite stabilizer) 3. Introduce a bay area in the TTT Diagram; (Ausforming (austenite + forming) becomes possible); 4. Improve the respone toTempering treatment: Alloying elements reduce the rate of tempering compared with that of a plain-carbon steel. Secondary hardening becomes possible. 5. Other: solid solution strengthening, alloy carbides, corrosion resistance, etc. can be obtained. 52
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Dr.C.Ergun Mak 214E
Effect of Alloying Elements Hardenability
•Certain alloying elements in the steel moves the noise of the TTT curve to the right direction. •The practical significance; Very low cooling rates, (cooling in air), can produce martensite. •Whole volume of the fabricated massy body can be transformed to martensite even cooling in air.
Dr.C.Ergun Mak 214E
Hardenability
Certain alloying elements, increase the hardenability. In the plain carbon steels, 1050, the surface is hard, but not in deep. The alloyed steel, 4340 hardened deeper. So the hardenability of 4340 is much better. Even slow cooling rates may produce the martensite in all cross-section. But hardness is not high since lower C content. 53
Effect of Alloying elements Secondary hardening: Carbide precipitation
A bay area may appear. Special processes possible such as “ausforming”.
Alloying elements can also reduce the effect of tempering compared to the plain carbon steels. The alloy steel can be used at high temperatures. 54
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Dr.C.Ergun Mak 214E
Hardenability Curves and Jominy Tests •
•
•
Jominy test - The test used to evaluate hardenability. An austenitized steel bar is quenched at one end only, thus producing a range of cooling rates along the bar. Hardenability curves - Graphs showing the effect of the cooling rate on the hardness of asquenched steel. Jominy distance - The distance from the quenched end of a Jominy bar. The Jominy distance is related to the cooling rate. Jomminy distance for various steels can be seen in the figure. Plain carbon steels have shallow jomminy distance while alloyed steels may have very deep. However, C provides higher surface hardness compared to the other alloying elements. 55
Dr.C.Ergun Mak 214E The
cooling rates provided by various quenchants (quenching media) The cooling rate provided by the quenchants are represented by a constant value “H”.
the relation between the diameter of the work piece and jomminy distance in the Figure for a given “H” values.
56
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Dr.C.Ergun Mak 214E
A machine part of 1050 steel was quenched in a medium (H=0.2) and hardness at a certain location is 28 HRC. Predict the hardness change at the same point if the oil is agitated during quenching.
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Dr.C.Ergun Mak 214E
From Figure 12-23, Page 353
4 16
10 16
4 16
From Figure 12-23, Page 353
4 (inch) ⇒ 39 HRC 16 58
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An AISI 9310 steel bar with a diameter of 40mm have a hardness of 42HRC at the center after quenching. What is the minimum severity of quenching medium in terms of “H coefficient”. Which quenchant would you recommend to produce the aimed hardness in the steel with the minimum risk of quench cracks?
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From Figure 12-23, Page 353
From Figure 12-24, Page 355 40 mm = 1.6 inch
6.5 16
6.5 16
H value should be between 0.5 and 1. But the correct H to provide sufficient cooling rate is “1”. The quenchant should be still water (Table 12-2, page 348). 60
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Dr.C.Ergun Mak 214E
The fallowing heat treatments were applied to a shaft of 25mm diameter and made of 1050 steel. a) Heat at 820oC, quench to 25oC in water, temper for one hour at 400oC. Cool to room temperature in air. b) Heat at 820oC, quench to 400oC in a salt bath, hold for two min. Cool to room temperature in air.
Describe the resultant microstructure and estimate the hardnesses at the end of each treatment. Make comments about the mechanical behaviour of shafts at the end of each treatment. 61
Dr.C.Ergun Mak 214E Figure 12-8 (a) page 342
a) Tempering of Martensite: Micorstructure:Tempered Martensite b) Austempering: (Isothermal heat treatment) Microstructure: Lower Bainite 62
31
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Dr.C.Ergun Mak 214E
•Strength / Hardness, (Wear resistance) • Ductility, • Machinability, • Small grain size, • Residual stresses stresses, • Homogenous structure. (Quenching ? Marquenching may be better) (Retained austenite) Figure 12-5 page 340 63
Dr.C.Ergun Mak 214E
Strengthening of materials
• Strain hardening: due to the increase in dislocation density and their interaction with each other, obstacles, grain boundaries, etc. • Martensite strengthening: • Solid Solution hardening: Addition of different atoms provide additional strength to the material caused by the lattice distortion due to the mismatch of the atoms. • Dispersion strengthening: The strengthening of a metal or an alloy by incorporating chemically stable submicron size particles of a nonmetallic or intermetallic phases that impede dislocation movement at elevated temperatures (hard particles in matrix). • Precipitation hardening: hardening in metals caused by the precipitation of a constituent from a supersaturated solid solution.
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Dr.C.Ergun Mak 214E
• • • •
Dispersion Hardening
Soft matrix-hard precipitates/particles Homogenuous distribution of precipitates/particles Fine precipitates/particles Spherical precipitates/particles
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Phase diagrams with respect to solubility
a) Unlimited solubility: One material can completely dissolve in a second material without creating a second phase. b) Insolubility: One element can not dissolve in another in any amount. c) Limited solubility: One element can dissolve in another only in certain amount. a)
b)
c)
66
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Dr.C.Ergun Solubility Mak 214E
and Solid Solutions
Complete solute solution of Cu and Ni atoms
Precipitation of a new phase: a Cu- Zn compound
(a) Liquid Cu and Ni: complete solubility. Phases and solubility: (Solid Cu-Ni alloys: complete solid The three phases of water. solubility in random lattice sites). • Water and alcohol - unlimited solubility. (a) In Cu-Zn alloys containing more than • Salt and water - limited solubility. 30% Zn, a second phase forms • Oil and water - no solubility. limited solubility of Zn in Cu. 67
Dr.C.Ergun Mak 214E
•
•
For Unlimited Solid Solubility
Hume-Rothery rules - The conditions for unlimited solid solubility. Hume-Rothery’s rules are necessary but are not sufficient for materials to show unlimited solid solubility. Hume-Rothery rules: • Size factor • Crystal structure • Valence • Electronegativity
68
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Dr.C.Ergun Mak 214E
Solid-Solution Strengthening
Effect of atomic radii alloying atoms added to Cu on the strengthening
Dr.C.Ergun Mak 214E
Important
• •
Effect of Zn content in Cu on the properties of solid solution.
The mechanical properties of Cu-Ni alloys. Pay attention to 60% Ni -40% Cu. 69
Precipitation (Age) Hardening
Small second phase precipitates behaves as small obstacles to dislocation motion. Starting from a structure having coarse grained precipitates, 1. Solution treatment: heating the material to the single phase ragion. 2. Queching the material to room temperature having a supersaturated solid solution with a metastable single phase microstructure. 3. Aging the material at (reheating to) an intermediate temperature to activate solid state diffusion to form fine grained precititates.
•
Overaging- aging the material too long causes coarser precipitates loosing the effectiveness to behave as an 70 dislocation barier
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Dr.C.Ergun Mak 214E
Çökelme sertleşmesi
• İç yapıda, dislokasyon hareketlerini engelleyerek dayanımın artmasına sebep olan çok küçük ikinci fazların çökeltilmesi işlemidir. Yaşlandırma sertleşmesi: • Önce Çözündürme işlemi (solution treatment) yapılarak çökelen sert olan 2. faz, tek faz içerisinde tamamen çözülür. • Daha sonra yapı, hızlı soğutma (su vererek-suda soğutmak) ile ikinci fazın çökelmesi engellenir ve aşırı doymuş katı çözelti elde edilir. • Daha sonra yaşlandırma işleminde; aşırı doymuş katı çözelti, çözündürme sıcaklığından daha düşük olana yaşlandırma sıcaklığına tekrar ısıtılarak çok küçük bağdaşık (koherent) ikinci faz tanecikleri çökeltilir. (Bu çökeltiler dislokasyonlara engel teşkil ederek malzemenin dayanımını arttırır). • Aşırı yaşlanma: çökelmelerin çok büyüyerek bağdaşıklığın (koherentliğin ) kaybolmasi 71
Dr.C.Ergun Mak 214E
If slowly cooled-(not hardening) T
%100 β (single phase)
β Slow cooling Equilibrium microstructure: Coarse α Grains in β matrix
α+β
Composition
Time 72
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Dr.C.Ergun Mak 214E
Precipitation (Age) hardening
Important
T Solution treatment
Same mic ro
Sıcaklık
g Quenchin
α+β
structure
β
taging
α-Grains in β matrix Time
Composition
Forming the coherent small precipitation 73
Dr.C.Ergun Mak 214E
Hardness
• In the first stage, very small coherent precipitates called -GP zones (Guinier preston zones) forms, • The empty spaces below the dislocation are good location for nucleating of these GP zones (decreasing the energy of the system), thus prevents Important the dislocation motions. • Then, these zones form larger coherent precipitates. These precipitates stretches the lattice and cause to strengthening the material. Coarsening the precipitates and loosing Over their ability to Coherent grain Aging strenghening the material. formation GP Zone Lossing of Coherency
Temperature
Coherent Precipitation
β
α
74
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Overaging
β
α
• Overaging: As the precipitates coarsen, the misfit stresses become too large to sustain. • Then the coherency would be lost the the precipitates becomes uncoherent. • Thus the effectiveness of the hardening decreases. • If the material aged long enough, the starting coarse microstructure will be formed.
75
taging
Hardness
Temperature
Dr.C.Ergun Mak 214E
Taging(hour)
76
38
Dr.C.Ergun Mak 214E an Design
age hardening treatment giving the temperature for each step for the alloy having 2 wt.% Cu.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
78
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Quizz: What is the streghthening mechanism of age hardening? Explain briefly the steps for a typical age hardening treatment.
80
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Dr.C.Ergun Mak 214E
( ) Austempering is an isothermal heat treatment which transforms austenite to pearlite ( ) Process annealing is used to soften steels after quenching ( ) Cooling rate in oil quenching is always higher than in water quenching …….. steels cointain ferrite and martensite in their room temperature microstructure. The first manufacturing step to obtain pearlitic melable cast iron from whit cast iron is ………. Dimond brale indenter and 150kgf major load are used to conduct ......... Test Hardness of hard metals can be measure by using ............... Tests. Hardness of ceramics can be measure by using ............... Tests.
Aşırı yaşlanmış Al alaşımlarında dayanımın düşmesinin sebepleri; çökelti matris
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Dr.C.Ergun Mak 214E Bolt class 6.8 should satisfy the ultimate tensile strength of ….MPa and yield strength of ……..MPa. a) 800/600 b) 480/600 c) 600/480 d) none of them ……….can be seen in macroscopic examinations of metals which resulted from ……. a) flow lines/plastic deformation b) welded section / low hardenability c) dislocations/casting process d) none of them Spherodizing of high C steels is done at temperatureres between .... And .... (a) 690oC-A3 (b) Acm-800oC (c) 690oC-A1 (d) none of them ..........occurs at the temperature higher than 60% of melting point in ..... (a)Grain growth/process annealing (b)Full annealing age hardening (c)Overtempering/stress relief (d)none of them Upeer bainite is .......... (a)Harder than martensite (c)Softer than ferrite
(b)harder than coarse pearlite (d) none of them
Fromation of .................. İs the sequence of age hardening (a) Supersaturated solid solution / GP zones / non-coherent precipitates/ coherent precipitates ( b) Supersaturated solid solution / GP zones / coherent precipitates / non-coherent precipitates (c) GP zones / coherent precipitates / non-coherent precipitates/ Supersaturated solid solution (d) None of them 82
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Dr.C.Ergun Mak 214E
Jomminy tests are used to evaluate ..... Of steels a) Ductile/brittle transition b) microstructure c) hardness d) none of them The risk of quench cracking can be resuced by using.......treatment a) Tempering b) annealing c) Martempering d) austempering Secondary hardening can be seen in ......steels (a) High alloy (b) Acm-800oC (c) carburizing (d) high carbon Galvanized steels is produced by coating......... On the surfaces of sheets (a)Pb (b)Sn (c)Zn (d)none of them Deep drawing quality steels must exhibit high...... (a)Hardenability (b)strength (c)ductility (d)density
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