Engineering Materials-istanbul .technical University
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Dr.C.Ergun Mak 214E
MAK214E Summer 2006-2007 Lecture Notes 3
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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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Strengthening effect of second phase
In order to get a strengthening effect 1. Soft matrix -hard precipitates/particles 2. Homogenuous distribution of precipitates/particles 3. Fine (small) precipitates/particles 4. 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)
Important in precipitation hardening 4
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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. 5
Dr.C.Ergun Mak 214E
Solid-Solution Strengthening
Effect of atomic radii alloying atoms added to Cu on the strengthening
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. 6 (Monel)-german silver
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Dr.C.Ergun Mak 214E
Precipitation (Age) Hardening
Important
Small second phase precipitates behaves as small obstacles to dislocation motion. Starting from a structure having coarse grained precipitates, Steps: 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 dislocation barier 7
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If slowly cooled(not hardening effect) T
%100 β (single phase)
β Slow cooling Equilibrium microstructure: Coarse α Grains in β matrix
α+β
Composition
Time 8
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Steps of Precipitation (Age) hardening treatment
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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 Composition
Time
Forming the coherent small precipitation 10
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Effect of aging time on microstructure and properties of aged material
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Dr.C.Ergun Mak 214E • In the first stage, very small coherent precipitates called -GP zones (Guinier
Hardness
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 the Important dislocation motions. • Then, these zones form larger coherent precipitates. These precipitates stretches the lattice and cause to strengthening the material.
Coherent grain formation
Over Aging
Coarsening the precipitates and loosing their ability to strenghening the material.
GP Zone Lossing of Coherency
Temperature
Coherent Precipitation
β
α
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Loosing of hardening effect during overaging!!
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Overaging
β
α
Important
• 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.
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Effect of aging time and aging temperature on the properties of aged material!!
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Important
taging
Hardness
Temperature
Dr.C.Ergun Mak 214E
Taging(hour) 16
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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.
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Surface Hardening
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Surface Hardening Treatments
•In many machine parts, the ideal properties are; • • • • •
high strength, high toughness, high ductility, high fatigue resistance, high impact resistance.
•This properties require special structural configuration • Hard and strong surface to have excellent wear resistance, fatigue resistance, etc. • Ductile, soft, tough inside for a good ductility, fracture and impact resistance, etc.
•Low C steels:
y Low strength, • Low hardness, • Good Ductility, • Good Toughness,
•High C steels:
y
Opposite properties.
Modify the C content ???
Special treatments are needed to merge both properties in single material:
Surface Hardening 22
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Surface Hardening Treatments of Steel
Methods: 1. Thermo-chemical treatments: modifying the chemical composition at the surface (providing hard carbide and/or nitride layer) • • • •
2.
Introduce C and/or
Carburizing (introducing C) N: use normally inhardenable steel Nitriding (introducing N) Carbonitriding (introducing C and N) Cyaniding (introducing C and N)
Selective heating and quenching treatment: no chemical change in the surface composition. • • •
Induction hardening Flame hardening Laser and electron beam hardenening
Use the existent C and other elements: use hardenable steel 23
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Dr.C.Ergun Mak 214E
Selective heating and quenching
Thermo-chemical surface Hardening
.
The C content of the surface more than that of core after carburizing.
• •
Heating the surface, and the following quenching just transforms austenite to martensite at the surface, since inside never reaches to austenite forming temperatures. 24
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Terminology
• Selectively Heating the Surface - Rapidly heat the surface of a medium-carbon steel above the A3 temperature and then quench the steel. • Case depth - The depth of hardened surface made by surface hardening treatments. • Carburizing - Surface-hardening method of steels by diffusion of carbon diffuses into surface. • Cyaniding - Surface-hardening method of steels by diffusion of both carbon and nitrogen obtained from a bath of liquid cyanide solution. • Carbonitriding - Surface-hardening of steel with carbon and nitrogen obtained from a special gas atmosphere.
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Dr.C.Ergun Surface Mak 214E
(Case) Hardening Treatments: Carburizing
• A thermo-chemical process to harden the normally unhardenable (low carbon) steels by involving diffusion of Carbon (C). • Holding low C steel at 875-925oC in a C rich gas or liquid atmosphere to produce desired case with high carbon content resulting higher hardenability than core. • Then quenching rapidly to room temperature to have case hardened parts with softer core. • Subsequent tempering may be applied. –Depth of hardening (case depth): •No technical limit but generally in excess of 1.5 mm. •Case depth / diameter (or thickness): 5-10 %. –Carburizing time: 2-10 Hrs depending to the desired case thickness. –Carburizing temperature: Depending on the steel, above A3: 875 – 925oC. –Quenching: Carburized parts need to be quenched. Tempering also possible for the desired hardness.
Dr.C.Ergun Surface Mak 214E
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(Case) Hardening Treatments: Carburizing
–Types of Carburizing Process: • Pack Carburizing: The part packed in charcoil activated with BaCO3 inside a steel container, followed by heating at carburizing temperature and then quenching and/or tempering. • Gas Carburizing: Carburizing in methane, ethane or natural gas at 850925oC and free from O2, then quenching and/or tempering. • Liquid Carburizing: In a molten salt such as NaCN for 1 – 4hrs, then followed by the same procedure done in pack carburizing. 27
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Surface (Case) Hardening: Nitriding.
Use nitriding steels with medium carbon and nitride forming alloying elements such as Al, Ti, Cr, etc. – Heating below A1, 500 – 570oC in N2 + H2 atmosphere (gas, liquid or plasma -ammonia, etc). – Holding enough time (longer than carburizing - 10-48 hrs) to obtain desired case depth. –Nitrogen diffusion into the surface –Very hard iron nitride or alloy nitride formation. –No quenching, no scaling or discoloration. –The hardened layer is harder and shallower than tool or carburized steels (1000 VHN). –Also ion nitriding process with some advantageous such as higher hardness, much shorter process time (2-10Hrs). 28
Dr.C.Ergun Mak 214ESurface
(Case) Hardening: Carbonitriding.
–Similar to carburizing but in carbon and nitrogen rich atmosphere with the diffusion of N and C in methane or propane + ammonia.
– Can be applied to plain carbon steels. – At above eutectoid transformation temperatures: 760 – 875oC. – Quenching with blowing gas; much less severe than quenching with water. – 0.1 – 0.75 mm case depth, 60 – 65 Rc case hardness. 29
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Dr.C.Ergun Surface Mak 214E
(Case) Hardening Treatments: Cyanding
– Diffusion of both N and C to the surface in molten cyanide. – Supercritical treatment at 760 – 870oC with case depth 0.250.75mm – Less processing times: less than 1hr. – Water or oil quench under a significant distortion risk.
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Selective heating and quenching: Induction Hardening
– High frequency alternating current on the coil induces heat on the part. – Increase the frequency for larger parts to increase the heating depth to austenite T ( even possible to heat to a Temp. higher than melting T. – Lower frequency for smaller parts for a shallower penetration of heat. – Special coil design is possible for complex geometries – Good for medium or high carbon steels as well as low alloy steels. • Heat the surface above A3 by induction. • Then, Quenching with water after heating.
Advantageous: Ease for automation and control, – Safe and clean, – Less scale loses, – Fast start up and quick heating, – Low maintanance and operating costs, – But high investment costs. 31
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Dr.C.Ergun Selective Mak 214E
heating and quenching: Flame Hardening
– Selective hardening with a combustible gas flame to above austenizing temperature of a steel with at least 0.4 C%, – Then, water quenching, – Fuel: oxygen-/acetylene, propane or gas mixture with high heat output. – Good for gear teeth, shear blades, cams, etc, – A good result with 3 – 5mm case depth, – Rc 65 can be obtained with a treatment from 825oC, – Also possible by immersing the work piece into a molten salts, etc. – Types: • Spot Flame Hardening: flame to a spot that needs to be hardened followed by quenching. • Spin Flame Hardening: A rotating work piece in contact with a stationary flame. • Progressive Flame Hardening: The torch and the quenching medium move across the surface of the work piece. 32
Dr.C.Ergun Mak 214E
Selective heating and quenching: Laser and Electron Beam Hardening
– Used for selective hardening, – Steel for these processes should have sufficient C and alloying elements, – A certain portion of the surface can be heated to the desired temperature in very short times, – Vacuum is needed for the operation of electron beam, – Area; 6-10mm2 for electron beam, up to 100mm2 for laser heating , – But high investment and operating costs, – Only good for plain carbon steels. 33
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Advantageous and Characteristics
Advantageous: •Eliminates Distortion and Cracking •Localized hardening makes possible harden certain locations •Heating can be performed with flame, induction coil, laser or electron beam. –Carburizing: • Best for low C steels –Nitriding: Important!!!!! • Lower distortion risk compared to carburizing. • Higher surface hardness, but lower case depth • Good for Cr-Mo alloy steels –Carbonitriding • Good for shallower case depth than carburizing • Yields higher hardness than nitriding.. –Flame hardening: • Suitable for bulky parts and selective hardening for large components. –Induction hardening: • Good for medium carbon low alloy heat treatable steels • Suitable geometry for the induction coils. –Electron beam and laser hardening: • Limited to the plain carbon and low alloy steels.
Dr.C.Ergun Mak 214E
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Effect of Process Variables
Effect of Composition Effect of Processing temperature
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Effect of Process Variables
Effect of Processing temperature
Effect of Processing time
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Types of Steels
•Constructional steels: •Profiles ( Fe37, Fe 42, Fe 50, 1010, 1020, 1040, etc.), •Sheet or plates -deep drawing quality ( low carbon, fine grain), thin plate, galvanize, plates for ship buildings. •Heat treatable steels (for combination of strength and ductility) •Carbon steels •Low Alloy steels (alloyed less than 5%) •Carburizing steels (low carbon steels for case hardening) •Nitriding steels (alloyed with nitride formers such as Al and Cr) •Free cutting steels: (To be easiliy cut by tools: high machinability, high S content) •Spring steels (0.5-0.6 C and good hardenability and elastic properties) •Bolt steels (Good cold formability for thread rolling) •High temperature steels: For boilers and pipes •Sub zero steels (shows no DBTT, generally austenitic steels) •Valve steels (high strength, good toughness and ductility) •Stainless steel (Ferritic, Martensitic, Austenitic, Precipitation Hardening) •Tools steels (Hot work and Cold work Tool steel, High speed steels) •Ball bearing steels •Electrical steels Extra low C with Si up to 3%. •Non-magnetizable steels -austenitic steels •High strength low alloy steels (HSLA) micro alloyed with V or Nb etc. Common in automotive industry. •Dual phase steels (contains martensite in ferrite matrix, obtained with inter-critical range annealing and quenching, widely demanded for transport vehicles) •Maraging steels ( ultra high strength as a result of martensitic transformation and following aging treatment) 38 •Cast steels:
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Types of Steels • Tools steels • Specilaized steels ¾ High strength low alloy steels (HSLA): ¾ Microalloyed Steels: ¾ Maraging steels ¾ Dual phase steels • Stainless steel 9 Ferritic, 9 Martensitic, 9 Austenitic, 9 Precipitation Hardening
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Dr.C.Ergun Mak 214E
Tool steels
Mostly have high C to provide high hardness by quenching and tempering. They have a combination of 9High strength, 9High hardness, 9High toughness, 9High temperature resistance. Applications: • Cutting tools for machining operations. • Dies for die casting, used for casting operations. • Forming dies, used for plastic deformation operation.
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Tool Steels Code M1 T1 H10 H21 H42 A2 D2 O1 S1 L2 P2 W1
Letter: Alloying elements, working cond., heat treatment, etc.
Type of Tool Steel Molybdenum HSS Tungsten HSS Chromium Hot work tool steel Tungsten Hot work tool steel Molibdenum Hot work tool steel Air Hardening Medium Alloy Cold work tool steel High C High Cr Cold work tool steel Oil Hardening Cold work tool steel Shock resistant steel Low alloy special purpose tool steel Low C mold steel Water hardening tool steel
Digits: Designation the Composition
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Secondary Hardening Peak
• Alloying elements may improve the high T stability of these steels: • Water hardenable steels soften rapidly even at relatively low T. • Oil hardenable steels temper more slowly but still soften at high T. • Air hardenable and special tool steels do not soften near A1.
• High alloyed steels may pass through a secondary hardening peak near 500oC as the normal cementites dissolve and hard alloy carbides precipitate. • The alloy carbides are particularly stable, resist growth and spheroidization and providing high T resistance of these steels. 42
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SPECIAL STEELS • • • •
High Strength Low Alloy Steels (HSLA) Microalloyed Steels Maraging Steels Dual Phase Steels
High Strength Low Alloy Steels (HSLA) • Low C steels and small amount of carbide former alloying elements, such as Ti, Nb, Ta, Zr, etc. • Carbide forms at grain baundaries and therfore very fine grains. • Therefore high strength and high ductility without heat treatment. • Specifications based on their strength. Commonly used in automotive industry. Microalloyed Steels: • Even less alloying elements than HSLA • Rely partly on a carefully controlled hot-rolling process for precipitation strengthening.- no other heat treatment needed. • Precipitation of carbides of nitrides of Cr, V, Ti or Zr. • Thus dispersion strengthening and fine grain size provides the strength. 43
Dr.C.Ergun Mak 214E
Maraging Steels: Ultra high strength with Martensite + Aging • Steels with low carbon and high alloying element concentrations. • Combination of ¾Solid solution strengthening, ¾Precipitation hardening with a low C martensite + aging. • Austenizing and quenching to produce soft martensite with 0.3% C. • Upon aging at about 500oC, precipitation of intermetallic compounds, such as Ni3Ti, Fe2Mo and Ni3Mo. Dual Phase Steels: • A good combination of strength and ductility in low C steels with an inter-critical annealing of hot or cold rolled sheet steels • A microstructure of martensite and ferrite which is initially ferrite and pearlite. 44
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Stainless Steels
¾ Stainless steels - Excellent resistance to corrosion. Contain at least 12% Cr, a thin protective surface layer of chromium oxide to form under oxygen exposure. ¾ Categories of stainless steels: 9Ferritic SS 9Martensitic SS 9Austenitic SS 9Precipitation-Hardening (PH) SS 9Duplex SS
(a) Ferritic, (b) Martensitic containing large primary carbides and small carbides formed during tempering, (c) Austenitic, (d) PH stainless steels.
Dr.C.Ergun Mak 214E
Phase Diagrams
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Austenite region shrinks and Chromium carbide becomes a stable phase as the Cr content increases in steel
(a) The effect of Cr on the Fe-C phase diagram. Cr content (a) 0 %; (b) 5%; (c) 17%. At low-carbon contents, ferrite is stable at all temperatures. Cr is ferrite stabilizer, Ni and Mn are austenite stabilizer agents 46
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Ferritic Stainless Steels • Up to 30 % Cr with less that 0.12 %C. • Solute Solution Strengthening and strain hardening is possible. • When the C and Cr content high, dispersion strengthening of carbides but embrittles. • General Properties: • Good strength • Excellent corrosion resistance, • Moderate ductility/formability, • Relatively inexpensive.
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Martensitic Stainless Steels
• Less than 17 % Cr to have large enough austenite field to control austenizing temperature and C content. • 0.1 - 1 % C content for martensite formation. • First austenize the steel than quench to get martensitic structure. • Cr content makes possible air or oil quenching. • Combination of hardness, strength and corrosion resistance. • Lower corrosion resistance than the other stainless steels. • Good for high quality knives, ball bearings, valves.
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Austenitic Stainless Steels
¾ Ni content to stabilize γ at the room temperatures. ¾ Due to the FCC structure; 9Excellent ductility, formability. 9No DBTT and high impact resistance even at low temperatures (sub zero applications). 9Non ferromagnetic materials – no ferrite inside. ¾ Strengthening possible with solid solution and cold working. ¾ Expensive due to high Cr and Ni content. ¾ Less than 0.03% C to prevent carbide formation. Otherwise, sensitized to intergranular corrosion with the formation of Cr23C.
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Sensitization
• If C content more than 0.03 % sensitized to intergranular corrosion. • Upon slow cooling between 870 – 425oC, chromium carbide precipitation at the grain boundaries. Austenite region shrinks To prevent sensitization: 1.“Quench anneal treatment”: heat above 870oC to dissolve carbides, then rapidly quench to prevent carbide formation. Or 2.As an alternative solution “Stabilization”: add Ti or Nb to combine C forming TiC or NbC which are more favorable than chromium carbide.
and Chromium carbide becomes a stable phase
18% Cr-8% Ni. At low-carbon contents, austenite is stable at room 51 temperature.
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Precipitation Hardening Stainless Steels
¾A similar composition to austenitic stainless steel with the addition of Al, Nb, or Ta. ¾Strengthening with 9Solid solution strengthening, 9Strain hardening, 9Age hardening, 9Martensitic transformation.
¾High mech. Prop. even at low C contents. 52
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A heat treatment for a typical 17-7 PH with
Starting material in annealed condition: ¾ Conditioning: Austenizing at 760 – 955oC ¾ Quenching and transformation: Cooled to about 15oC or below for martensitic transformation temperature. ¾ Precipitation: Reheated to 500-600oC permitting Ni3Al and other precipitates to form from martensite. ¾ Higher strength obtained with lower aging times. 53
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Duplex Stainless Steels: •A microstructure with a combination of ferrite and austenite. •Small amount of other phase may be present to modify the properties.
Difference
A duplex stainless steel, Ferrite + Austenite Ferrite: Dark, Austenit: Gray.
A dual-phase steel, Ferrite + Martensite Ferrite: Dark, Martensite: Gray.
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Cast Irons
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Cast Irons
Important Cast iron - A ferrous alloys containing 2 < % C < 4 and 0.5 < % Si < 3 Eutectic and Eutectoid reactions controls the microstructure in Cast Irons.
Types of cast irons: ¾Gray cast iron ¾White cast iron ¾Malleable cast iron ¾Nodular (Ductile) cast iron ¾Compacted graphite (vermicular) cast iron
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Fe/C ve Fe/Fe3C Faz diyagramı
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Cast iron: Cooling rate
• Different microstructures form depending on the cooling rate from molten state, • Fast cooling (not quenching) cementite formation (no time for graphite formation). • Very slow cooling rates: graphitization due to decomposition of cementite into ferrite and graphite or pearlite/ferrite and graphite. • Si is a graphite stabilizing agent. • Cr and Bi, cementite stabilizing agents. • Si reduces the amount of C in the eutectic composition. • Therefore, Si content evaluated in Carbon Equivalent concept. 59
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Carbon equivalent and Si
CE% = C% +
1 Si% (CE : Carbon equivalent) 3
The eutectic composition: 4.3 % CE, 9 CE ≤ 4.3 %, hypoeutectic. 9 CE ≥ 4.3 %, hypereutectic. 60
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TTT Diagram of Cast irons
True equilabrium Metastable
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The Matrix: • Slow cooling rates: γ Æα + graphite • Annealing: slow (furnace) cooling leading ferritic matrix. • Normalizing: air (faster) cooling leading pearlitic matrix.
• Quenching and tempering: Tempered martensite. • Austempering: Bainitic matrix or surface hardening. • Matrix structure also effected by the composition of iron: • Si, graphite and ferrite stabilizer, • 0.05% Sn and 0.5% Cu, pearlite stabilizer.
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Cooling rate and Types of Cast Iron
Important
• Depending on the cooling rate, different cast irons. – Slow cooling: Gray cast iron. • Ferritic, • Ferritic/pearlitic, • Pearlitic. – Fast Cooling: White cast iron. • Malleable (Tempered) cast iron: Annealing Heat treatment of White cast iron. – Nodular cast ion: with the addition of spherodizing agents, such as Mg (and Ce). – Compacted graphite (vermicular) cast iron: Shape of the graphites in between flakes and spheres. 63
Dr.C.Ergun Mak 214E
The eutectic reaction: • Solidification between two eutectics: a graphite containing cast irons: Such as gray, ductile or compacted graphite iron. • Solidification below the lower eutectic temperature: white cast iron. • If solidification starts above lower and finish below it, mottled iron, a mixture of white and gray iron –not desirable. Chilled iron; surface white iron, center gray iron for low cost wear resistant components. • The chill depth: tests for CE measurements. 64
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Gray cast iron
• Slow cooling permits the diffusion of C and forming graphite flakes. – If very slow cooling rate: Ferritic cast iron. – Medium cooling rates Ferritic-perlitic cast iron. – Faster cooling rates Perlitic cast iron. • As the pearlite ratio increase, strength increases but ductility always low due to the notch effect of flake edges. Grafit Lameller α Perlitik DD Ferritik DD
Important
Ferritik-Perlitik DD
Perlit
Artan Soğuma Hızı
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Important
• Most commone one, • Microstructure consists of ferrite and/or pearlite plus graphite flakes – Low almost no ductility – Low strength (higher in compressive loading) – High brittleness – Higher sliding wear resistance – Good thermal conductivity – Self lubrication, – Good machinability – Vibration damping property: engine blocks 66
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• Interconnected graphite flakes connected where the nucleation originated: nucleation sites. • The graphite flakes behaves like small cracks in the cast iron causing stress concentration. Therefore low tensile strength and brittleness with an elongation of 1% or less. • As the flake size (finer graphite) decreases tensile strength increases. The finer the graphite is, the stronger the cast iron is. • At lower CE, the nominal strength is higher.
Notch effect causes stress concentration at the sharp edges of the graphite flakes
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White cast iron
Eutectic reaction: Liquid ⇒ ledeburite microstructure (γ+Fe3C)
γ+S γ
y1 y2 y 3γ+Fe C 3
α+Fe3C
y4
•Fast cooled structure: White cast iron. •Due to cementite: hard and brittle.
S+Fe3C Transformed Ledeburite Ledeburite
S
Important
1
Liquid
2
γ Proeutectic γ
3
Eutectic γ
Eutectic Fe3C 4
Cast iron
Eutectoid Perlite
Eutectic Fe69 3C
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Important
White cast iron
• Microstructure consits of cementite network and pearlite • Extreme brittleness • High hardness • Wear resistance, • Can not machined by tools, only by grinding • An intermediate product for malleable iron
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Important
Malleable cast iron
• Produced by heat treatment of white iron. • During malleablizing, cementite dissolves and graphite clumps or nodules looking like popcorn forms. • Rounded graphite provides a combination of strength and ductility. • Steps for mallebilization: 9Starting material: white cast iron with a CE of 3 %. 9First stage graphitization (FSG): at about 925oC to decompose cementite into austenite and graphite. (Fe3CÆ γ + graphite) 9The austenite decomposes in the subsequent cooling from FSG temperature with two different structures. BETTER DUCTILITY ¾Ferritic Malleable cast iron and TOUGHNESS ¾Pearlitic Malleable cast iron 72
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Important Ferritic Malleable Iron: • The casting is cooled 5-15oC/h through the eutectoid temperature to second stage graphitization (SSG). • The γ transforms to α and excess C diffuses to graphite nodules. • Exceptional toughness.
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Pearlitic Malleable Iron: Important • Cooled in air or oil. • Pearlitic if air cooled, martensitic if oil quenched. • Both hard and brittle. • To improve ductility, the pearlitic malleable iron is drawn below eutectoid T.
Drawing process • Tempers martensite or spheroidies the pearlite, thus reducing the amount of combined C or cementite. • Thus the strength of pearlitic malleable iron decreases and ductility and toughness increase.
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Important
Ductile or Nodular Cast Iron
• Addition of Mg (or Ce) to high CE liquid iron to form spehroidal graphite during solidification. • Best ductility. • Steps:
BEST DUCTILITY and TOUGHNESS
9 Desulfurization: S flake stabilizer. 9 Nodulizing. 9 Inocculation
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Desulfurization: S flake stabilizer. So for low S is needed.
Important
9 High quality starting material; 9 Melting in furnace to remove S; 9 Mix the liquid iron with desulfurizing agents, calcium carbide.
Nodulizing: 9 Add Mg near 1500oC to spherodize the graphite (also remove S and O) in the molten metal, 9 Residual 0.03% Mg is enough for nodulization. 9 Since Mg vaporizes at 1150oC, ferrosilicon used to Mg recoveries. 9 Fading (non violent vaporization or oxidation) of Mg should be controlled with pouring the molten metal within a few minutes, otherwise turns to gray iron. 76
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Inoculation: 9 Mg is a carbide stabilizer in other words white iron forms. 9 Inoculation with ferrosilicon alloys (50 - 85 % Si) and small amounts of Ca, Al, Sr or Ba. 9 So nucleation sites for graphite to grow is provided by inoculation of molten metal. 9 Also this effect fades with time. 77
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Compacted Graphite (Vermecular) Cast Irons • • • •
Graphite shape; intermediate between flakes and spheres. Nodulizing the molten metal with a residual Mg content of 0.015% (<0.03%) May also form with fading of ductile iron. Compared to gray cast iron; better strengths and ductility good thermal conductivity, and vibration damping characteristics. • Similar treatment to that of ductile iron. ¾Low S starting material, ¾Inoculation to nucleate graphite. ¾Pouring shortly after inoculation to prevent fading (loosing the spheoridizing effect of Mg with time due to its evaporation).
Important
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Dr.C.Ergun Mak 214E Uygulama
(a) White cast iron prior to heat treatment. (b) Ferritic malleable iron with graphite nodules and small MnS inclusions. (c) Pearlitic malleable iron drawn to produce a tempered martensite matrix. (d) Annealed ferritic ductile (nodular) iron. (e) As-cast ferritic-pearlitic ductile iron. (f) Normalized pearlitic ductile iron.
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(a) Annealed ferritic ductile iron (b) As-cast ferritic-pearlitic ductile iron (c) Normalized perlitic ductile iron with. • Compared to gray iron, excellent strength, ductility and toughness. • Compared to malleable iron, higher ductility and strength, but slightly lower toughness due to higher Si content. 80
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Coding System
Yokes: Bağlantı Gear: Dişli Drum: arka fren dış kabı Hausing: gövde
Knuckle: oynak nokta Cap:kapak Hub: merkez kısım 81
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Gray Cast Iron
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Malleable Cast Iron
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Nodular Cast Iron
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MAK214E Summer 2006-2007 Lecture Notes 3
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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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Strengthening effect of second phase
In order to get a strengthening effect 1. Soft matrix -hard precipitates/particles 2. Homogenuous distribution of precipitates/particles 3. Fine (small) precipitates/particles 4. 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)
Important in precipitation hardening 4
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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. 5
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Solid-Solution Strengthening
Effect of atomic radii alloying atoms added to Cu on the strengthening
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. 6 (Monel)-german silver
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Precipitation (Age) Hardening
Important
Small second phase precipitates behaves as small obstacles to dislocation motion. Starting from a structure having coarse grained precipitates, Steps: 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 dislocation barier 7
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If slowly cooled(not hardening effect) T
%100 β (single phase)
β Slow cooling Equilibrium microstructure: Coarse α Grains in β matrix
α+β
Composition
Time 8
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Steps of Precipitation (Age) hardening treatment
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Precipitation (Age) hardening
Important T
Solution treatment
Same mic ro
Sıcaklık
g Quenchin
α+β
structure
β
taging
α-Grains in β matrix Composition
Time
Forming the coherent small precipitation 10
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Effect of aging time on microstructure and properties of aged material
11
Dr.C.Ergun Mak 214E • In the first stage, very small coherent precipitates called -GP zones (Guinier
Hardness
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 the Important dislocation motions. • Then, these zones form larger coherent precipitates. These precipitates stretches the lattice and cause to strengthening the material.
Coherent grain formation
Over Aging
Coarsening the precipitates and loosing their ability to strenghening the material.
GP Zone Lossing of Coherency
Temperature
Coherent Precipitation
β
α
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Loosing of hardening effect during overaging!!
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Overaging
β
α
Important
• 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.
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Effect of aging time and aging temperature on the properties of aged material!!
15
Important
taging
Hardness
Temperature
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Taging(hour) 16
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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.
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Surface Hardening
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Surface Hardening Treatments
•In many machine parts, the ideal properties are; • • • • •
high strength, high toughness, high ductility, high fatigue resistance, high impact resistance.
•This properties require special structural configuration • Hard and strong surface to have excellent wear resistance, fatigue resistance, etc. • Ductile, soft, tough inside for a good ductility, fracture and impact resistance, etc.
•Low C steels:
y Low strength, • Low hardness, • Good Ductility, • Good Toughness,
•High C steels:
y
Opposite properties.
Modify the C content ???
Special treatments are needed to merge both properties in single material:
Surface Hardening 22
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Surface Hardening Treatments of Steel
Methods: 1. Thermo-chemical treatments: modifying the chemical composition at the surface (providing hard carbide and/or nitride layer) • • • •
2.
Introduce C and/or
Carburizing (introducing C) N: use normally inhardenable steel Nitriding (introducing N) Carbonitriding (introducing C and N) Cyaniding (introducing C and N)
Selective heating and quenching treatment: no chemical change in the surface composition. • • •
Induction hardening Flame hardening Laser and electron beam hardenening
Use the existent C and other elements: use hardenable steel 23
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Selective heating and quenching
Thermo-chemical surface Hardening
.
The C content of the surface more than that of core after carburizing.
• •
Heating the surface, and the following quenching just transforms austenite to martensite at the surface, since inside never reaches to austenite forming temperatures. 24
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Terminology
• Selectively Heating the Surface - Rapidly heat the surface of a medium-carbon steel above the A3 temperature and then quench the steel. • Case depth - The depth of hardened surface made by surface hardening treatments. • Carburizing - Surface-hardening method of steels by diffusion of carbon diffuses into surface. • Cyaniding - Surface-hardening method of steels by diffusion of both carbon and nitrogen obtained from a bath of liquid cyanide solution. • Carbonitriding - Surface-hardening of steel with carbon and nitrogen obtained from a special gas atmosphere.
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(Case) Hardening Treatments: Carburizing
• A thermo-chemical process to harden the normally unhardenable (low carbon) steels by involving diffusion of Carbon (C). • Holding low C steel at 875-925oC in a C rich gas or liquid atmosphere to produce desired case with high carbon content resulting higher hardenability than core. • Then quenching rapidly to room temperature to have case hardened parts with softer core. • Subsequent tempering may be applied. –Depth of hardening (case depth): •No technical limit but generally in excess of 1.5 mm. •Case depth / diameter (or thickness): 5-10 %. –Carburizing time: 2-10 Hrs depending to the desired case thickness. –Carburizing temperature: Depending on the steel, above A3: 875 – 925oC. –Quenching: Carburized parts need to be quenched. Tempering also possible for the desired hardness.
Dr.C.Ergun Surface Mak 214E
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(Case) Hardening Treatments: Carburizing
–Types of Carburizing Process: • Pack Carburizing: The part packed in charcoil activated with BaCO3 inside a steel container, followed by heating at carburizing temperature and then quenching and/or tempering. • Gas Carburizing: Carburizing in methane, ethane or natural gas at 850925oC and free from O2, then quenching and/or tempering. • Liquid Carburizing: In a molten salt such as NaCN for 1 – 4hrs, then followed by the same procedure done in pack carburizing. 27
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Surface (Case) Hardening: Nitriding.
Use nitriding steels with medium carbon and nitride forming alloying elements such as Al, Ti, Cr, etc. – Heating below A1, 500 – 570oC in N2 + H2 atmosphere (gas, liquid or plasma -ammonia, etc). – Holding enough time (longer than carburizing - 10-48 hrs) to obtain desired case depth. –Nitrogen diffusion into the surface –Very hard iron nitride or alloy nitride formation. –No quenching, no scaling or discoloration. –The hardened layer is harder and shallower than tool or carburized steels (1000 VHN). –Also ion nitriding process with some advantageous such as higher hardness, much shorter process time (2-10Hrs). 28
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(Case) Hardening: Carbonitriding.
–Similar to carburizing but in carbon and nitrogen rich atmosphere with the diffusion of N and C in methane or propane + ammonia.
– Can be applied to plain carbon steels. – At above eutectoid transformation temperatures: 760 – 875oC. – Quenching with blowing gas; much less severe than quenching with water. – 0.1 – 0.75 mm case depth, 60 – 65 Rc case hardness. 29
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(Case) Hardening Treatments: Cyanding
– Diffusion of both N and C to the surface in molten cyanide. – Supercritical treatment at 760 – 870oC with case depth 0.250.75mm – Less processing times: less than 1hr. – Water or oil quench under a significant distortion risk.
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Selective heating and quenching: Induction Hardening
– High frequency alternating current on the coil induces heat on the part. – Increase the frequency for larger parts to increase the heating depth to austenite T ( even possible to heat to a Temp. higher than melting T. – Lower frequency for smaller parts for a shallower penetration of heat. – Special coil design is possible for complex geometries – Good for medium or high carbon steels as well as low alloy steels. • Heat the surface above A3 by induction. • Then, Quenching with water after heating.
Advantageous: Ease for automation and control, – Safe and clean, – Less scale loses, – Fast start up and quick heating, – Low maintanance and operating costs, – But high investment costs. 31
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heating and quenching: Flame Hardening
– Selective hardening with a combustible gas flame to above austenizing temperature of a steel with at least 0.4 C%, – Then, water quenching, – Fuel: oxygen-/acetylene, propane or gas mixture with high heat output. – Good for gear teeth, shear blades, cams, etc, – A good result with 3 – 5mm case depth, – Rc 65 can be obtained with a treatment from 825oC, – Also possible by immersing the work piece into a molten salts, etc. – Types: • Spot Flame Hardening: flame to a spot that needs to be hardened followed by quenching. • Spin Flame Hardening: A rotating work piece in contact with a stationary flame. • Progressive Flame Hardening: The torch and the quenching medium move across the surface of the work piece. 32
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Selective heating and quenching: Laser and Electron Beam Hardening
– Used for selective hardening, – Steel for these processes should have sufficient C and alloying elements, – A certain portion of the surface can be heated to the desired temperature in very short times, – Vacuum is needed for the operation of electron beam, – Area; 6-10mm2 for electron beam, up to 100mm2 for laser heating , – But high investment and operating costs, – Only good for plain carbon steels. 33
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Advantageous and Characteristics
Advantageous: •Eliminates Distortion and Cracking •Localized hardening makes possible harden certain locations •Heating can be performed with flame, induction coil, laser or electron beam. –Carburizing: • Best for low C steels –Nitriding: Important!!!!! • Lower distortion risk compared to carburizing. • Higher surface hardness, but lower case depth • Good for Cr-Mo alloy steels –Carbonitriding • Good for shallower case depth than carburizing • Yields higher hardness than nitriding.. –Flame hardening: • Suitable for bulky parts and selective hardening for large components. –Induction hardening: • Good for medium carbon low alloy heat treatable steels • Suitable geometry for the induction coils. –Electron beam and laser hardening: • Limited to the plain carbon and low alloy steels.
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Effect of Process Variables
Effect of Composition Effect of Processing temperature
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Effect of Process Variables
Effect of Processing temperature
Effect of Processing time
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Types of Steels
•Constructional steels: •Profiles ( Fe37, Fe 42, Fe 50, 1010, 1020, 1040, etc.), •Sheet or plates -deep drawing quality ( low carbon, fine grain), thin plate, galvanize, plates for ship buildings. •Heat treatable steels (for combination of strength and ductility) •Carbon steels •Low Alloy steels (alloyed less than 5%) •Carburizing steels (low carbon steels for case hardening) •Nitriding steels (alloyed with nitride formers such as Al and Cr) •Free cutting steels: (To be easiliy cut by tools: high machinability, high S content) •Spring steels (0.5-0.6 C and good hardenability and elastic properties) •Bolt steels (Good cold formability for thread rolling) •High temperature steels: For boilers and pipes •Sub zero steels (shows no DBTT, generally austenitic steels) •Valve steels (high strength, good toughness and ductility) •Stainless steel (Ferritic, Martensitic, Austenitic, Precipitation Hardening) •Tools steels (Hot work and Cold work Tool steel, High speed steels) •Ball bearing steels •Electrical steels Extra low C with Si up to 3%. •Non-magnetizable steels -austenitic steels •High strength low alloy steels (HSLA) micro alloyed with V or Nb etc. Common in automotive industry. •Dual phase steels (contains martensite in ferrite matrix, obtained with inter-critical range annealing and quenching, widely demanded for transport vehicles) •Maraging steels ( ultra high strength as a result of martensitic transformation and following aging treatment) 38 •Cast steels:
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Types of Steels • Tools steels • Specilaized steels ¾ High strength low alloy steels (HSLA): ¾ Microalloyed Steels: ¾ Maraging steels ¾ Dual phase steels • Stainless steel 9 Ferritic, 9 Martensitic, 9 Austenitic, 9 Precipitation Hardening
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Tool steels
Mostly have high C to provide high hardness by quenching and tempering. They have a combination of 9High strength, 9High hardness, 9High toughness, 9High temperature resistance. Applications: • Cutting tools for machining operations. • Dies for die casting, used for casting operations. • Forming dies, used for plastic deformation operation.
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Tool Steels Code M1 T1 H10 H21 H42 A2 D2 O1 S1 L2 P2 W1
Letter: Alloying elements, working cond., heat treatment, etc.
Type of Tool Steel Molybdenum HSS Tungsten HSS Chromium Hot work tool steel Tungsten Hot work tool steel Molibdenum Hot work tool steel Air Hardening Medium Alloy Cold work tool steel High C High Cr Cold work tool steel Oil Hardening Cold work tool steel Shock resistant steel Low alloy special purpose tool steel Low C mold steel Water hardening tool steel
Digits: Designation the Composition
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Secondary Hardening Peak
• Alloying elements may improve the high T stability of these steels: • Water hardenable steels soften rapidly even at relatively low T. • Oil hardenable steels temper more slowly but still soften at high T. • Air hardenable and special tool steels do not soften near A1.
• High alloyed steels may pass through a secondary hardening peak near 500oC as the normal cementites dissolve and hard alloy carbides precipitate. • The alloy carbides are particularly stable, resist growth and spheroidization and providing high T resistance of these steels. 42
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SPECIAL STEELS • • • •
High Strength Low Alloy Steels (HSLA) Microalloyed Steels Maraging Steels Dual Phase Steels
High Strength Low Alloy Steels (HSLA) • Low C steels and small amount of carbide former alloying elements, such as Ti, Nb, Ta, Zr, etc. • Carbide forms at grain baundaries and therfore very fine grains. • Therefore high strength and high ductility without heat treatment. • Specifications based on their strength. Commonly used in automotive industry. Microalloyed Steels: • Even less alloying elements than HSLA • Rely partly on a carefully controlled hot-rolling process for precipitation strengthening.- no other heat treatment needed. • Precipitation of carbides of nitrides of Cr, V, Ti or Zr. • Thus dispersion strengthening and fine grain size provides the strength. 43
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Maraging Steels: Ultra high strength with Martensite + Aging • Steels with low carbon and high alloying element concentrations. • Combination of ¾Solid solution strengthening, ¾Precipitation hardening with a low C martensite + aging. • Austenizing and quenching to produce soft martensite with 0.3% C. • Upon aging at about 500oC, precipitation of intermetallic compounds, such as Ni3Ti, Fe2Mo and Ni3Mo. Dual Phase Steels: • A good combination of strength and ductility in low C steels with an inter-critical annealing of hot or cold rolled sheet steels • A microstructure of martensite and ferrite which is initially ferrite and pearlite. 44
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Stainless Steels
¾ Stainless steels - Excellent resistance to corrosion. Contain at least 12% Cr, a thin protective surface layer of chromium oxide to form under oxygen exposure. ¾ Categories of stainless steels: 9Ferritic SS 9Martensitic SS 9Austenitic SS 9Precipitation-Hardening (PH) SS 9Duplex SS
(a) Ferritic, (b) Martensitic containing large primary carbides and small carbides formed during tempering, (c) Austenitic, (d) PH stainless steels.
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Phase Diagrams
45
Austenite region shrinks and Chromium carbide becomes a stable phase as the Cr content increases in steel
(a) The effect of Cr on the Fe-C phase diagram. Cr content (a) 0 %; (b) 5%; (c) 17%. At low-carbon contents, ferrite is stable at all temperatures. Cr is ferrite stabilizer, Ni and Mn are austenite stabilizer agents 46
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Ferritic Stainless Steels • Up to 30 % Cr with less that 0.12 %C. • Solute Solution Strengthening and strain hardening is possible. • When the C and Cr content high, dispersion strengthening of carbides but embrittles. • General Properties: • Good strength • Excellent corrosion resistance, • Moderate ductility/formability, • Relatively inexpensive.
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Martensitic Stainless Steels
• Less than 17 % Cr to have large enough austenite field to control austenizing temperature and C content. • 0.1 - 1 % C content for martensite formation. • First austenize the steel than quench to get martensitic structure. • Cr content makes possible air or oil quenching. • Combination of hardness, strength and corrosion resistance. • Lower corrosion resistance than the other stainless steels. • Good for high quality knives, ball bearings, valves.
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Austenitic Stainless Steels
¾ Ni content to stabilize γ at the room temperatures. ¾ Due to the FCC structure; 9Excellent ductility, formability. 9No DBTT and high impact resistance even at low temperatures (sub zero applications). 9Non ferromagnetic materials – no ferrite inside. ¾ Strengthening possible with solid solution and cold working. ¾ Expensive due to high Cr and Ni content. ¾ Less than 0.03% C to prevent carbide formation. Otherwise, sensitized to intergranular corrosion with the formation of Cr23C.
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Sensitization
• If C content more than 0.03 % sensitized to intergranular corrosion. • Upon slow cooling between 870 – 425oC, chromium carbide precipitation at the grain boundaries. Austenite region shrinks To prevent sensitization: 1.“Quench anneal treatment”: heat above 870oC to dissolve carbides, then rapidly quench to prevent carbide formation. Or 2.As an alternative solution “Stabilization”: add Ti or Nb to combine C forming TiC or NbC which are more favorable than chromium carbide.
and Chromium carbide becomes a stable phase
18% Cr-8% Ni. At low-carbon contents, austenite is stable at room 51 temperature.
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Precipitation Hardening Stainless Steels
¾A similar composition to austenitic stainless steel with the addition of Al, Nb, or Ta. ¾Strengthening with 9Solid solution strengthening, 9Strain hardening, 9Age hardening, 9Martensitic transformation.
¾High mech. Prop. even at low C contents. 52
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A heat treatment for a typical 17-7 PH with
Starting material in annealed condition: ¾ Conditioning: Austenizing at 760 – 955oC ¾ Quenching and transformation: Cooled to about 15oC or below for martensitic transformation temperature. ¾ Precipitation: Reheated to 500-600oC permitting Ni3Al and other precipitates to form from martensite. ¾ Higher strength obtained with lower aging times. 53
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Duplex Stainless Steels: •A microstructure with a combination of ferrite and austenite. •Small amount of other phase may be present to modify the properties.
Difference
A duplex stainless steel, Ferrite + Austenite Ferrite: Dark, Austenit: Gray.
A dual-phase steel, Ferrite + Martensite Ferrite: Dark, Martensite: Gray.
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Cast Irons
55
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Cast Irons
Important Cast iron - A ferrous alloys containing 2 < % C < 4 and 0.5 < % Si < 3 Eutectic and Eutectoid reactions controls the microstructure in Cast Irons.
Types of cast irons: ¾Gray cast iron ¾White cast iron ¾Malleable cast iron ¾Nodular (Ductile) cast iron ¾Compacted graphite (vermicular) cast iron
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Fe/C ve Fe/Fe3C Faz diyagramı
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Cast iron: Cooling rate
• Different microstructures form depending on the cooling rate from molten state, • Fast cooling (not quenching) cementite formation (no time for graphite formation). • Very slow cooling rates: graphitization due to decomposition of cementite into ferrite and graphite or pearlite/ferrite and graphite. • Si is a graphite stabilizing agent. • Cr and Bi, cementite stabilizing agents. • Si reduces the amount of C in the eutectic composition. • Therefore, Si content evaluated in Carbon Equivalent concept. 59
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Carbon equivalent and Si
CE% = C% +
1 Si% (CE : Carbon equivalent) 3
The eutectic composition: 4.3 % CE, 9 CE ≤ 4.3 %, hypoeutectic. 9 CE ≥ 4.3 %, hypereutectic. 60
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TTT Diagram of Cast irons
True equilabrium Metastable
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The Matrix: • Slow cooling rates: γ Æα + graphite • Annealing: slow (furnace) cooling leading ferritic matrix. • Normalizing: air (faster) cooling leading pearlitic matrix.
• Quenching and tempering: Tempered martensite. • Austempering: Bainitic matrix or surface hardening. • Matrix structure also effected by the composition of iron: • Si, graphite and ferrite stabilizer, • 0.05% Sn and 0.5% Cu, pearlite stabilizer.
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Cooling rate and Types of Cast Iron
Important
• Depending on the cooling rate, different cast irons. – Slow cooling: Gray cast iron. • Ferritic, • Ferritic/pearlitic, • Pearlitic. – Fast Cooling: White cast iron. • Malleable (Tempered) cast iron: Annealing Heat treatment of White cast iron. – Nodular cast ion: with the addition of spherodizing agents, such as Mg (and Ce). – Compacted graphite (vermicular) cast iron: Shape of the graphites in between flakes and spheres. 63
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The eutectic reaction: • Solidification between two eutectics: a graphite containing cast irons: Such as gray, ductile or compacted graphite iron. • Solidification below the lower eutectic temperature: white cast iron. • If solidification starts above lower and finish below it, mottled iron, a mixture of white and gray iron –not desirable. Chilled iron; surface white iron, center gray iron for low cost wear resistant components. • The chill depth: tests for CE measurements. 64
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Gray cast iron
• Slow cooling permits the diffusion of C and forming graphite flakes. – If very slow cooling rate: Ferritic cast iron. – Medium cooling rates Ferritic-perlitic cast iron. – Faster cooling rates Perlitic cast iron. • As the pearlite ratio increase, strength increases but ductility always low due to the notch effect of flake edges. Grafit Lameller α Perlitik DD Ferritik DD
Important
Ferritik-Perlitik DD
Perlit
Artan Soğuma Hızı
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Important
• Most commone one, • Microstructure consists of ferrite and/or pearlite plus graphite flakes – Low almost no ductility – Low strength (higher in compressive loading) – High brittleness – Higher sliding wear resistance – Good thermal conductivity – Self lubrication, – Good machinability – Vibration damping property: engine blocks 66
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• Interconnected graphite flakes connected where the nucleation originated: nucleation sites. • The graphite flakes behaves like small cracks in the cast iron causing stress concentration. Therefore low tensile strength and brittleness with an elongation of 1% or less. • As the flake size (finer graphite) decreases tensile strength increases. The finer the graphite is, the stronger the cast iron is. • At lower CE, the nominal strength is higher.
Notch effect causes stress concentration at the sharp edges of the graphite flakes
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White cast iron
Eutectic reaction: Liquid ⇒ ledeburite microstructure (γ+Fe3C)
γ+S γ
y1 y2 y 3γ+Fe C 3
α+Fe3C
y4
•Fast cooled structure: White cast iron. •Due to cementite: hard and brittle.
S+Fe3C Transformed Ledeburite Ledeburite
S
Important
1
Liquid
2
γ Proeutectic γ
3
Eutectic γ
Eutectic Fe3C 4
Cast iron
Eutectoid Perlite
Eutectic Fe69 3C
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Important
White cast iron
• Microstructure consits of cementite network and pearlite • Extreme brittleness • High hardness • Wear resistance, • Can not machined by tools, only by grinding • An intermediate product for malleable iron
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Important
Malleable cast iron
• Produced by heat treatment of white iron. • During malleablizing, cementite dissolves and graphite clumps or nodules looking like popcorn forms. • Rounded graphite provides a combination of strength and ductility. • Steps for mallebilization: 9Starting material: white cast iron with a CE of 3 %. 9First stage graphitization (FSG): at about 925oC to decompose cementite into austenite and graphite. (Fe3CÆ γ + graphite) 9The austenite decomposes in the subsequent cooling from FSG temperature with two different structures. BETTER DUCTILITY ¾Ferritic Malleable cast iron and TOUGHNESS ¾Pearlitic Malleable cast iron 72
Dr.C.Ergun Mak 214E
Important Ferritic Malleable Iron: • The casting is cooled 5-15oC/h through the eutectoid temperature to second stage graphitization (SSG). • The γ transforms to α and excess C diffuses to graphite nodules. • Exceptional toughness.
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Dr.C.Ergun Mak 214E
Pearlitic Malleable Iron: Important • Cooled in air or oil. • Pearlitic if air cooled, martensitic if oil quenched. • Both hard and brittle. • To improve ductility, the pearlitic malleable iron is drawn below eutectoid T.
Drawing process • Tempers martensite or spheroidies the pearlite, thus reducing the amount of combined C or cementite. • Thus the strength of pearlitic malleable iron decreases and ductility and toughness increase.
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Dr.C.Ergun Mak 214E
Important
Ductile or Nodular Cast Iron
• Addition of Mg (or Ce) to high CE liquid iron to form spehroidal graphite during solidification. • Best ductility. • Steps:
BEST DUCTILITY and TOUGHNESS
9 Desulfurization: S flake stabilizer. 9 Nodulizing. 9 Inocculation
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Dr.C.Ergun Mak 214E
Desulfurization: S flake stabilizer. So for low S is needed.
Important
9 High quality starting material; 9 Melting in furnace to remove S; 9 Mix the liquid iron with desulfurizing agents, calcium carbide.
Nodulizing: 9 Add Mg near 1500oC to spherodize the graphite (also remove S and O) in the molten metal, 9 Residual 0.03% Mg is enough for nodulization. 9 Since Mg vaporizes at 1150oC, ferrosilicon used to Mg recoveries. 9 Fading (non violent vaporization or oxidation) of Mg should be controlled with pouring the molten metal within a few minutes, otherwise turns to gray iron. 76
Dr.C.Ergun Mak 214E
Inoculation: 9 Mg is a carbide stabilizer in other words white iron forms. 9 Inoculation with ferrosilicon alloys (50 - 85 % Si) and small amounts of Ca, Al, Sr or Ba. 9 So nucleation sites for graphite to grow is provided by inoculation of molten metal. 9 Also this effect fades with time. 77
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Dr.C.Ergun Mak 214E
Compacted Graphite (Vermecular) Cast Irons • • • •
Graphite shape; intermediate between flakes and spheres. Nodulizing the molten metal with a residual Mg content of 0.015% (<0.03%) May also form with fading of ductile iron. Compared to gray cast iron; better strengths and ductility good thermal conductivity, and vibration damping characteristics. • Similar treatment to that of ductile iron. ¾Low S starting material, ¾Inoculation to nucleate graphite. ¾Pouring shortly after inoculation to prevent fading (loosing the spheoridizing effect of Mg with time due to its evaporation).
Important
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Dr.C.Ergun Mak 214E Uygulama
(a) White cast iron prior to heat treatment. (b) Ferritic malleable iron with graphite nodules and small MnS inclusions. (c) Pearlitic malleable iron drawn to produce a tempered martensite matrix. (d) Annealed ferritic ductile (nodular) iron. (e) As-cast ferritic-pearlitic ductile iron. (f) Normalized pearlitic ductile iron.
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Dr.C.Ergun Mak 214E Uygulama
(a) Annealed ferritic ductile iron (b) As-cast ferritic-pearlitic ductile iron (c) Normalized perlitic ductile iron with. • Compared to gray iron, excellent strength, ductility and toughness. • Compared to malleable iron, higher ductility and strength, but slightly lower toughness due to higher Si content. 80
Dr.C.Ergun Mak 214E
Coding System
Yokes: Bağlantı Gear: Dişli Drum: arka fren dış kabı Hausing: gövde
Knuckle: oynak nokta Cap:kapak Hub: merkez kısım 81
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Dr.C.Ergun Mak 214E
Gray Cast Iron
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Dr.C.Ergun Mak 214E
Malleable Cast Iron
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Dr.C.Ergun Mak 214E
Nodular Cast Iron
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