The Science and Engineering of Materials, 4 th ed Donald R. Askeland Pradeep P. Phulé. Chapter 3 Ferrous Alloys

Transcription

1 The Science and Engineering of Materials, 4 th ed Donald R. Askeland Pradeep P. Phulé Chapter 3 Ferrous Alloys 1

2 Objectives of Chapter 3 Discuss how to use the eutectoid reaction to control the structure and properties of steels through heat treatment and alloying. Examine two special classes of ferrous alloys: stainless steels and cast irons. 2

3 Chapter Outline 3.1 Designations and Classification of Steels 3.2 Simple Heat Treatments 3.3 Isothermal Heat Treatments 3.4 Quench and Temper Heat Treatments 3.5 Effect of Alloying Elements 3.6 Application of Hardenability 3.7 Specialty Steels 3.8 Surface Treatments 3.9 Weldability of Steel 3.10 Stainless Steels 3.11 Cast Irons 3

4 (a) In a blast furnace, iron ore is reduced using coke (carbon) and air to produce liquid pig iron. The high-carbon content in the pig iron is reduced by introducing oxygen into the basic oxygen furnace to produce liquid steel. An electric arc furnace can be used to produce liquid steel by melting scrap. (b) Schematic of a blast furnace operation. 4

5 Classification of Steels Bsed on composition (Carbon Steels, Low alloy steels, Stainless steels, ) Bsed on steel making method (Electric arc furnace, Blast furnace, ) Bsed on process mehod (Hot rolling, Cold rolling, ) Bsed on product shape (Sheet, Strip, Bar, Plate, ) Bsed on deoxidizing method (Killed steels, Semi-killed steels, Wild steels) Bsed on Microstructure (Ferrite steels, Austenitic steels, Dual phase steels, ) Bsed on properties (Stainless steels, Heat resisting steels, Free cutting steels, ) Bsed on heat treatment (Annealed steels, Quenched and tempered steels, ) Bsed on application (Structural steels, Spring steels, High speed steels, ) Bsed on product quality (Base steels, Quality steels, Special steels, ) 5

6 Systems for designations of steels AISI: American Iron and Steel Institute SAE: Society of Automotive Engineers UNS: Unified Numbering System ASTM: American Society for Testing and Materials ASME: American Society of Mechanical Engineers EN: European Norms ANSI: American National Standards Institute API: American Petroleum Institute AWS: American Welding Society CSA: Canadian Standards Association DIN: Deutsches Institut für Normung (The German Institute for Standardization) JIS: Japanese Institute of Standards 6

7 Designations of Steels AISI & SAE The AISI and SAE provide designation systems for steels that use a four- or five-digit number. XXXX The first number refer to the major alloying elements present. The second number designates the subgroup alloying element OR the relative percent of primary alloying element. The last two or three numbers refer to the percentage of carbon. 1xxx 2xxx 3xxx 4xxx 5xxx 6xxx 7xxx 8xxx 9xxx Carbon steels Nickel steels Nickel-chromium steels Molybdenum steels Chromium steels Chromium-vanadium steels Tungsten steels Nickel-chromium-molybdenum steels Silicon-manganese steels 7

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9 Designations of Steels AISI & SAE Carbon steels 10XX 11XX Plain carbon, Mn 1.00% max Resulfurized free machining 12XX Resulfurized/rephosphorized free machining 15XX Plain carbon, Mn % Manganese steels 13XX Mn 1.75% Nickel steels Nickel-chromium steels Molybdenum steels 23XX Ni 3.50% 25XX Ni 5.00% 31XX Ni 1.25%, Cr % 32XX Ni 1.75%, Cr 1.07% 33XX Ni 3.50%, Cr % 34XX Ni 3.00%, Cr 0.77% 40XX Mo % 44XX Mo % Chromium-molybdenum steels 41XX Cr %, Mo % Nickel-chromiummolybdenum steels Nickel-molybdenum steels 43XX Ni 1.82%, Cr %, Mo 0.25% 47XX Ni 1.05%, Cr 0.45%, Mo % 46XX Ni %, Mo % 48XX Ni 3.50%, Mo 0.25% Chromium steels 50XX Cr % 51XX Cr % 50XXX 51XXX 52XXX Cr 0.50%, C 1.00% min Cr 1.02%, C 1.00% min Cr 1.45%, C 1.00% min Chromium-vanadium steels 61XX Cr %, V % Tungsten-chromium steels 72XX W 1.75%, Cr 0.75% Nickel-chromiummolybdenum steels 81XX Ni 0.30%, Cr 0.40%, Mo 0.12% 86XX Ni 0.55%, Cr 0.50%, Mo 0.20% 87XX Ni 0.55%, Cr 0.50%, Mo 0.25% 88XX Ni 0.55%, Cr 0.50%, Mo 0.35% Silicon-manganese steels 92XX Si %, Mn %, Cr % Nickel-chromiummolybdenum steels 93XX Ni 3.25%, Cr 1.20%, Mo 0.12% 94XX Ni 0.45%, Cr 0.40%, Mo 0.12% 97XX Ni 0.55%, Cr 0.20%, Mo 0.20% 98XX Ni 1.00%, Cr 0.80%, Mo 0.25% 9

10 Alloying element Chromium Nickel Manganese Silicon Molybdenum Vanadium Tungsten Effect on the steel Increases hardness without reducing ductility. Refines grain structure and increases toughness. Simplifies heat treatment requirements. Increases strength without reducing ductility. Refines grain structure and increases toughness. Simplifies heat treatment requirements. Added as a deoxidising and desulphurising agent. Considered as alloy when above 1%. Enables oil quenching. Added as a deoxidising agent. Stabilises carbides formed by other alloying elements. Improves oil hardening and air hardening properties. Used with Chromium and Nickel to simplify heat treatment. Widely used in tool steels. Steel retains its hardness at high temperatures. Widely used in tool steels. Tool maintains its hardness even at red heat. 10

11 Alloying element Chromium Nickel Manganese Silicon Molybdenum Vanadium Effect on the steel Cr is commonly added to increases corrosion resistance and oxidation resistance, to increase hardenability, or to improve high-temperature strength. As a hardening element, Chromium is frequently used with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes to increased strength. Chromium is strong carbide former. Ni is a ferrite strengthener. Nickel does not form carbides in steel. It remains in solution in ferrite, strengthening and toughening the ferrite phase. Nickel increases the hardenability and impact strength of steels. Mg is generally beneficial to surface quality especially in resulfurized steels. Manganese contributes to strength and hardness, but less than carbon. Increasing the manganese content decreases ductility and weldability, but less than carbon. Manganese has a significant effect on the hardenability of steel. Si is one of the principal deoxidizers used in steelmaking. Silicon is less effective than manganese in increasing as-rolled strength and hardness. In low-carbon steels, silicon is generally detrimental to surface quality. Mo increases the hardenability of steel. Molybdenum may produce secondary hardening during the tempering ofquenched steels. It enhances the creep strength oflow-alloy steels at elevated temperatures. V increases the yield strength and the tensile strength of carbon steel. The addition of small amounts of Vanadium can significantly increase the strength of steels. Vanadium is one of the primary contributors to precipitation strengthening in micro alloyed steels. When thermo mechanical processing is properly controlled, the ferrite grain size is refined and there is a corresponding increase in toughness. The impact transition temperature also increases when vanadium is added. 11

12 Alloying element Aluminum Niobium Titanium Phosphorus Sulphur Effect on the steel Al is widely used as a deoxidizer. Aluminum can control austenite grain growth in reheated steels and is therefore added to control grain size. Aluminum is the most effective alloy in controlling grain growth prior to quenching. Titanium, zirconium, and vanadium are also valuable grain growth inhibitors, but there carbides are difficult todissolve into solution in austenite. Nb increases the yield strength and, to a lesser degree, the tensile strength of carbon steel. The addition of small amounts of Niobium can significantly increase the yield strength of steels. Niobium can also have a moderate precipitation strengthening effect. Its main contributions are to form precipitates above the transformation temperature, and to retard the recrystallization of austenite, thus promoting a fine-grain microstructure having improved strength and toughness. Ti is used to retard grain growth and thus improve toughness. Titanium is also used to achieve improvements in inclusion characteristics. Titanium causes sulfide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending. P increases strength and hardness, decreases ductility and notch impact toughness of steel. S decreases ductility and notch impact toughness especially in the transverse direction. Weldability decreases with increasing sulfur content. Sulfur is found primarily in the form of sulfide inclusions. Sulfur levels are normally controlled to low levels. The only exception is free-machining steels, where sulfur is added to improve machinability. 12

13 Designations of Steels AISI & SAE Prefix letter (designate the process used to produce the steel): E = Electric furnace. EXXXX M = to designate merchant quality steel. MXXXX If a letter is inserted between the 2 nd and 3 rd number, B = boron has been added to increase hardenability. XXBXX L = lead has been added for improving machinability. XXLXX Suffix letter: H = when hardenability is a major requirement. XXXXH 13

14 Designations of Steels AISI & SAE (Stainless steels) AISI/SAE use three digits sometimes followed by some letters to designate wrought stainless steels. The first digit specifies the alloy classification. The last two digits represent no information about the composition. 2xx 3xx Austenitic stainless steel 4xxx Ferritic stainless steel Martensitic stainless steel Ferritic stainless steel Austenitic stainless steel Martensitic stainless steel

15 Designations of Steels UNS The Unified Numbering System (UNS) for Metals and Alloys is established to correlate several internationally used alloy and metal numbering systems, which are now commonly used by trade associations, societies, producers, and users of alloys and metals. This system does not create the confusion caused due to usage of previous systems that used the same number for different metals or alloys or different identification numbers for the same metal or alloy. 15

16 Designations of Steels UNS The UNS constitutes series of designations for ferrous metals and alloys. Each designation includes a single-letter prefix and five digits. The letter identifies the family of metals. D00001-D99999 F00001-F99999 G00001-G99999 H00001-H99999 J00001-J99999 K00001-K99999 S00001-S99999 T00001-T99999 Steels with specified mechanical properties Cast irons AISI and SAE carbon and alloy steels (except tool steels) AISI and SAE H-steels Cast steels (except tool steels) Miscellaneous steels and ferrous alloys Heat and corrosion resistant steels (stainless), valve steels, iron-base "super alloys" Tool steels, wrought and cast The identification numbers identified from the existing systems are integrated into the UNS designations in order to ensure user convenience. For example, carbon steel identified by the American Iron and Steel Institute is designated as AISI 1020 and covered by the UNS designation as G

17 Designations of Steels UNS AISI/SAE B45 12L H UNS G10400 S31600 G81451 G12144 H10450 Comment The First four digits are the same and the last one is zero. The First three digits are the same and the last two ones are zero. B which represents Boron is equivalent to 1 for the last digit. L which represents Lead is equivalent to 4 for the last digit. H represents hardenability is equivalent to a prefix H before the five digits. 17

18 Designations of Steels UNS (Stainless steels) AISI/SAE Suffix Designator xxxl (304L) xxxs xxxn xxxln (301LN) xxxf (420F) xxxse (303Se) xxxb (302B) xxxh (304H) xxxcu (303Cu) UNS No. xxx03 (S30403) xxx08 xxx51 xxx53 (S30153) xxx20 (S42020) xxx23 (S30323) xxx15 (S30215) xxx09 (S30409) xxx30 (S30330) Description Low carbon (<0.03% as compared to the normal <0.08%) for improved resistance to intergranular corrosion Low carbon (<0.08% as compared to standard <0.2% or higher) Added nitrogen for increased strength Low carbon (<0.03%) plus added nitrogen higher sulfur and phosphorus for improved machinability Added selenium for better machined surfaces Added silicon to increase scaling resistance Wider allowable range of carbon content Added copper 18

19 Designations of Steels ASTM ASTM s designation system for metals consists of a letter followed by an arbitrary sequentially assigned number. Examples 1: ASTM A 582/A 582M-95b (2000), Grade 303Se-Free-Machining Stainless Steel Bars. A describes a ferrous metal, but does not subclassify it as cast iron, carbon steel, alloy steel, tool steel, stainless steel, etc. 582 is a sequential number without any relationship to the metal s properties. M indicates that the standard A582M is written in rationalized SI units (the M comes from the word Metric), hence together 582/A582M includes both inch-pound and SI units. 95 indicates the year of adoption or last revision and a letter b following the year indicates the third revision of the standard in (2000), a number in parentheses, indicates the year of last re-approval. Grade 303Se indicates the grade of the steel, and in this case, it has a Se (selenium) addition. 19

20 Terminology Within the steel industry, the terms Grade, Type, and Class are generally defined as follows: Grade is used to describe chemical composition of steel. Type is used to define the deoxidation practice of steel. Class is used to indicate characteristics such as strength level or surface finish. 20

21 Designations of Steels ASTM Another use of ASTM grade designators is found in pipe, tube, and forging products, where the first letter P refers to pipe, T refers to tube, TP may refer to tube or pipe, and F refers to forging. Example 2: ASTM A 335/A335-03, Grade P22; Seamless Ferritic Alloy-Steel Pipe for High Temperature Service. Example 3: ASTM A 213/A213M-03a, Grade T22; Seamless Ferritic and Austenitic Alloy Steel Boiler, Superheater and Heat-Exchanger Tubes. Example 4: ASTM A 312/A312M-03, Grade TP304; Seamless and Welded Austenitic Stainless Steel Pipe. Example 5: ASTM A 336/A336M-03a, Class F22-Steel Forgings, Alloy, for Pressure and High-Temperature Parts. 21

22 Example 6: ASTM A a Grade A, Grade B, Grade C - Seamless Carbon Steel Pipe for High-Temperature Service. 02 indicates the year of adoption or last revision and a letter a following the year indicates the second revision of the standard in Typically an increase in alphabet (such as letters A, B, C) results in higher tensile or yield strength steels, and if it is an unalloyed carbon steel, an increase in carbon content. Designations of Steels ASTM In this case: Grade A:0.25%C (max), 48 ksi tensile strength (min); Grade B: 0.30%C (max), 60 ksi tensile strength (min); Grade C 0.35%C (max), 70 ksi tensile strength (min). Example 7: ASTM A 48 - Class No. 20A, 25A, 30A - Gray Iron Castings. Class No. 20A: 20 ksi tensile strength (min); Class No. 25A: 25 ksi tensile strength (min); Class No. 30A: 30 ksi tensile strength (min). Example 8: ASTM A , Type 304, 316, 410 Stainless and Heat Resisting Steel Bars and Shapes. Types 304, 316, 410 and others are based on the SAE/AISI designation system for stainless steels. 22

23 Designations of Steels ASME Many of the ASTM specifications have been adopted by the ASME with little or no modification. ASME uses the prefix S and the ASTM designation for these specifications. For example, ASME SA213 and ASTM A 213 are identical. 23

24 Designations of Steels EN Steel Names Steel Names Group 1 refers to steels that are designated according to their application and mechanical or physical properties. These have names that are comprised of one or more letters related to the application, followed by a number related to properties. S P L E B Y R H D T Structural steels Pressure purpose steels Linepipe steels Engineering steels Steels for reinforcing concrete Steels for pre-stressing concrete Rail steels or steels in the form of rails Cold rolled flat products of high strength steels for cold forming Flat products for cold forming Tinmill products (steel products for packaging) M Electric steels Example: EN S 185 (structural steel with min. yield strength of 185 MPa.) (Eq. to UNS K01400). 24

25 Designations of Steels EN Steel Names Steel Names Group 2 is used for steels that are designated according to their chemical composition and are further divided into four subgroups depending on alloy content. 1. Non alloy steels (except free cutting steels) with Mn < 1% C specified average percent carbon (C) content Example: C45 nominally contains 0.45%C (Eq. to UNS G10450). 2. Non alloy steels with Mn 1 % & Non alloy free cutting steels and alloy steels (except high speed steels) where the content of each alloy element is < 5% by weight 100 specified average percent carbon (C) content + Chemical symbols indicating alloy elements in decreasing order of content Example: 13CrMo4-5 nominally contains 0.13%C, 1%Cr, and 0.5%Mo (Eq. to UNS K11562). Numbers, separated by hyphens, which indicate percent alloy element content multiplied by a factor rounded to the nearest integer. 25 Element Cr, Co, Mn, Ni, Si, W Al, Be, Cu, Mo, Nb, Pb, Ta, Ti, V, Zr Ce, N, P, S B Factor

26 Designations of Steels EN Steel Names Steel Names Group 2 (Continue) 3. Alloy steels (except high speed steels) where the content of at least one alloy element is 5% by weight X specified average percent carbon (C) content + Chemical symbols indicating alloy elements in decreasing order of content Example: X2CrNi18-9 nominally contains 0.02% C, 18%Cr, and 9%Ni. 4. High speed steels HS + Numbers, separated by hyphens, which indicate percent alloy element content (in the order tungsten (W), Molybdenum (Mo), vanadium (V), cobalt (Co)) rounded to the nearest integer Example: HS nominally contains 18%W, 1Mo, 2%V, and 15%Co (Eq. to UNS K11562). 26

27 Alloy steels Non-alloy steels Designations of Steels EN Steel Numbers The structure of steel numbers is set out as follows Steel Group NO. 00 & 90 0x & 9x 1x 2x 3x 4x 5x-8x 08 & & Type Basic steels Quality steels Special steels Tool steels Miscellaneous steels Stainless and heat resistant steels Structural, pressure vessel and engineering steels Special physical properties Other purpose steels

28 Alloy steels Carbon steels Designations of Steels EN EN Steel Number EN Steel Name C15E C15 C45 35S20 11SMn30 11SMnPb30 X10CrNi18-8 X2CrNiN18-10 X2CrNiMo CrNiMo6 25CrMo4 AISI/SAE L L

29 29

30 Steels can be classified based on their composition or the way they have been processed. Carbon steels contain up to ~2% carbon. Decarburized steels contain less than 0.005% C. Classification of Steels Ultra-low carbon steels contain a maximum of 0.03% carbon. They are used for making car bodies and hundreds of other applications. Mild steel contains 0.15 to 0.3% carbon. This steel is used in buildings, bridges, piping, etc. Medium-carbon steels contain 0.3 to 0.6% carbon. These are used in making machinery, tractors, mining equipment, etc. High-carbon steels contain above 0.6% carbon. These are used in making springs, railroad car wheels, and the like. 30

31 Carbon class Carbon range, % Use Low Chain, Nails, Pipe rivets, Sheets for pressing and stamping, wire Mild Bars, Plates, Structural shapes Medium Axles, connecting rods, shafting Crankshafts, scraper blades Automobile springs, Anvils, Band saws, Drop hammer dies Chisels, punches, hand tools Knives, Shear blades, springs High Milling Cutters, Dies, Taps Lathe Tools, Woodworking Tools Files, Reamers Dies for wire drawing Metal cutting saws 31

32 Classification of Steels Cast irons are Fe-C alloys containing 2 to 4% carbon. Alloy steels are compositions that contain more significant levels of alloying elements. The AISI defines alloy steels as steels that exceed one or more of the following composition limits: 1.65% Mn, 0.6% Si, or 0.6% Cu. The total carbon content is up to 1%, and the total alloying element content is below 5%. 32

33 Section 3.2 Simple Heat Treatments Annealing - A heat treatment used to eliminate some or all of the effects of cold working. Annealing at a low temperature may be used to eliminate the residual stresses produced during cold working without affecting the mechanical properties of the finished part. Annealing may be used to completely eliminate the strain hardening achieved during cold working. In this case, the final part is soft and ductile but still has a good surface finish and dimensional accuracy. After annealing, additional cold work can be done since the ductility is restored; by combining repeated cycles of cold working and annealing, large total deformations may be achieved. 33

34 2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. The effect of cold work on the properties of a Cu-35% Zn alloy and the effect of annealing temperature on the properties of a Cu-35% Zn alloy that is cold-worked 75%. 34

35 The Three Stages of Annealing 1. Recovery - A low-temperature annealing heat treatment designed to eliminate residual stresses introduced during deformation without reducing the strength of the cold-worked material. When we first heat the metal, the additional thermal energy permits the dislocations to move and form the boundaries of a polygonized subgrain structure. The dislocation density is virtually unchanged. This low temperature treatment removes the residual stresses due to cold working without causing a change in dislocation density. The mechanical properties of the metal are relatively unchanged because the number of dislocations is not reduced during recovery. Since residual stresses are reduced or even eliminated when the dislocations are rearranged, recovery is often called a stress relief anneal. In addition, recovery restores high electrical conductivity to the metal, permitting us to manufacture copper or aluminum wire for transmission of electrical power that is strong yet still has high conductivity. Recovery often improves the corrosion resistance of the material. 35

36 The Three Stages of Annealing 2. Recrystallization - A medium-temperature annealing heat treatment designed to eliminate all of the effects of the strain hardening produced during cold working. When a cold-worked metal is heated above a certain temperature, rapid recovery eliminates residual stresses and produces the polygonized dislocation structure. New small grains then nucleate at the cell boundaries of the polygonized structure, eliminating most of the dislocations. Because the number of dislocations is greatly reduced, the recrystallized metal has low strength but high ductility. The temperature at which a microstructure of new grains that have very low dislocation density appears is known as the recrystallization temperature. The recrystallization temperature depends on many variables and is not a fixed temperature. 36

37 The Three Stages of Annealing 3. Grain growth - Movement of grain boundaries by diffusion in order to reduce the amount of grain boundary area. At still higher annealing temperatures, both recovery and recrystallization occur rapidly, producing a fine recrystallized grain structure. If the temperature is high enough, the grains begin to grow, with favored grains consuming the smaller grains. Grain growth is almost always undesirable. Grain growth will occur in most materials if they are subjected to a high enough temperature and is not related to cold working. Recrystallization orrecovery are notneeded for grain growth tooccur. 37

38 The effect of annealing temperature on the microstructure of cold-worked metals. (a) cold-worked, (b) after recovery, (c) after recrystallization, and (d) after grain growth. 38

39 Photomicrographs showing the effect of annealing temperature on grain size in brass. Twin boundaries can also be observed in the structures. (a) Annealed at 400 o C, (b) annealed at 650 o C, and (c) annealed at 800 o C. 39

40 Process Annealing - Eliminating Cold Work: A low-temperature (80 C to 170 C below the A 1 ) heat treatment used to eliminate all or part of the effect of cold working in steels with less than about 0.25% C. Annealing and Normalizing - Dispersion Strengthening: Annealing (full anneal) - A heat treatment used to produce a soft, coarse pearlite in steel by austenitizing (hypoeutectoid: about 30 C above the A 3 -hypereutectoid: about 30 C above the A 1 ), then furnace cooling. Normalizing - A simple heat treatment obtained by austenitizing (about 55 C above the A 3 or A cm ) and air cooling to produce a fine pearlitic structure. The faster cooling gives fine pearlite and provides higher strength. Spheroidizing - Improving Machinability: Spheroidite - A microconstituent containing coarse spheroidal cementite particles in a matrix of ferrite, permitting excellent machining characteristics in high-carbon steels. It requires several hours at about 30 C below the A 1. 40

41 Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels. 41

42 The effect of carbon and heat treatment on the properties of plain-carbon steels. 42

43 The microstructure of spheroidite, with Fe 3 C particles dispersed in a ferrite matrix ( 850). 43

44 Example 3.1 Determination of Heat Treating Temperatures Recommend temperatures for the process annealing, annealing, normalizing, and spheroidizing of 1020, 1077, and steels. An expanded version of the Fe-C diagram, adapted from several sources. 44

45 Example 3.1 SOLUTION We can specify the heat treatment based on critical A 1, A 3, or A cm temperatures. 45

46 Section 3.3 Isothermal Heat Treatments Austempering - The isothermal heat treatment by which austenite transforms to bainite. Isothermal annealing - Heat treatment of a steel by austenitizing, cooling rapidly to a temperature between the A 1 and the nose of the TTT curve, and holding until the austenite transforms to pearlite. 46

47 1050 steel steel 47

48 Example 3.2 Design of a Heat Treatment for an Axle A heat treatment is needed to produce a uniform microstructure and hardness of HRC 23 in a 1050 steel axle steel 48

49 (a) The eutectoid portion of the Fe-Fe 3 C phase diagram. (b) An expanded version of the Fe-C diagram, adapted from several sources. 49

50 Example 3.2 SOLUTION We might attempt this task in several ways. We could austenitize the steel, then cool at an appropriate rate by annealing or normalizing to obtain the correct hardness. By doing this, however, we find that the structure and hardness vary from the surface to the center of the axle. A better approach is to use an isothermal heat treatment. 1. Austenitize the steel at (30 to 55) = 805 o C to 825 o C, holding for 1 h and obtaining 100% γ. 2. Quench the steel to 600 o C and hold for a minimum of 10s. Primary ferrite begins to precipitate from the unstable austenite after about 1.0 s. After 1.5 s, pearlite begins to grow, and the austenite is completely transformed to ferrite and pearlite after about 10s. After this treatment, the microconstituents present are: Primary α Pearlite ( ) ( ) (0.5 ( ) ) % 64% 3. Cool in air-to-room temperature, preserving the equilibrium amounts of primary ferrite and pearlite. The microstructure and hardness are uniform because of the isothermal anneal.

51 Producing complicated structures by interrupting the isothermal heat treatment of a 1050 steel. 51

52 Dark feathers of bainite surrounded by light martensite, obtained by interrupting the isothermal transformation process ( 1500). 52

53 Section 3.4 Quench and Temper Heat Treatments Retained austenite - Austenite that is unable to transform into martensite during quenching because of the volume expansion associated with the reaction. Marquenching (Martempering) - Quenching austenite to a temperature just above the M s and holding until the temperature is equalized throughout the steel before further cooling to produce martensite. Tempering (In metallic materials) - A heat treatment used to soften the material and to increase its toughness. Tempered martensite - The microconstituent of ferrite and cementite formed when martensite is tempered. Quench cracks - Cracks that form at the surface of a steel during quenching due to tensile residual stresses that are produced because of the volume change that accompanies the austenite-to-martensite transformation. 53

54 The marquenching or martempering heat treatment designed to reduce residual stresses and quench cracking. 54

55 The effect of tempering temperature on the mechanical properties of a 1050 steel. 55

56 Example 3.3 Design of a Quench and Temper Treatment A rotating shaft that delivers power from an electric motor is made from a 1050 steel. Its yield strength should be at least 145,000 psi, yet it should also have at least 15% elongation in order to provide toughness. Design a heat treatment to produce this part. Example 3.3 SOLUTION 1. Austenitize above the A 3 temperature of 770 o C for 1 h. An appropriate temperature may be = 825 o C. 2. Quench rapidly to room temperature. Since the M f is about 250 o C, martensite will form. 3. Temper by heating the steel to 440 o C. Normally, 1 h will be sufficient if the steel is not too thick. 4. Cool to room temperature. 56

57 Retained austenite (white) trapped between martensite needles (black) ( 1000). As the martensite plates form during quenching, they surround and isolate small pools of austenite, which deform to accommodate the lower density martensite. As the transformation progresses, however, for the remaining pools of austenite to transform, the surrounding martensite must deform. Because the strong martensite resists the transformation, either the existing martensite cracks or the austenite remains trapped in the structure as retained austenite. Martensite softens and becomes more ductile during tempering. After tempering, the retained austenite cools below the M s and M f temperatures and transforms to martensite, since the surrounding tempered martensite can deform. But now the steel contains more of the hard, brittle martensite! A second tempering step may be needed to eliminate the martensite formed from the retained austenite. 57

58 Increasing carbon reduces the M s and M f temperatures in plain-carbon steels. Retained austenite is also more of a problem for high-carbon steels. The martensite start and finish temperatures are reduced when the carbon content increases. High-carbon steels must be refrigerated to produce all martensite. 58

59 Formation of quench cracks caused by residual stresses produced during quenching. When steels are quenched, the surface of the quenched steel cools rapidly and transforms to martensite. When the austenite in the center later transforms, the hard surface is placed in tension, while the center is compressed. If the residual stresses exceed the yield strength, quench cracks form at the surface. One solution is marquenching (martempering) process. 59

60 The surface always cools faster than the center of the part. As the size of the part increases, the cooling rate at any location is slower. The cooling rate depends on the temperature and heat transfer characteristics of the quenching medium. The H coefficient is equivalent to the heat transfer coefficient. Agitation helps break the vapor blanket and improves the overall heat transfer rate by bringing cooler liquid into contact with the parts being quenched. 60

61 Normalized Annealed The CCT diagram (solid lines) for a 1080 steel compared with the TTT diagram (dashed lines). The CCT diagrams describe how austenite transforms during continuous cooling. These diagrams give the cooling rates needed to obtain martensite in quench and temper treatments. 61

62 The CCT diagram for a low-alloy, 0.2% C Steel. 62

63 Hardenability In plain carbon steels, the nose of the TTT and CCT curves occurs at very short times. Plain carbon steels have low hardenability only very high cooling rates produce all martensite. All common alloying elements in steel shift the TTT and CCT diagrams to longer times. Section 3.5 Effect of Alloying Elements Alloy steels have high hardenability even cooling in air may produce martensite. A low-carbon, high alloy steel may easily form martensite but, because of the low-carbon content, the martensite is not hard. In thin sections of steel, the rapid quench produces distortion and cracking. In thick steels, we are unable to produce martensite. 63

64 (a) TTT and (b) CCT curves for a 4340 steel. 64

65 Effect on the Phase Stability - When alloying elements are added to steel, the binary Fe-Fe 3 C stability is affected and the phase diagram is altered. Alloying elements reduce the carbon content at which the eutectoid reaction occurs and change the A 1, A 3, and A cm temperatures. The effect of 6% manganese on the stability ranges of the phases in the eutectoid portion of the Fe-Fe 3 C phase diagram. A steel containing only 0.6% C is hypoeutectoid and would operate at 700 C without forming austenite; the otherwise same steel containing 6% Mn is hypereutectoid and austenite forms at 700 C. carbon equivalent (CE) 65

66 Shape of the TTT Diagram - Alloying elements may introduce a bay region into the TTT diagram. Ausforming is a thermomechanical heat treatment in which austenite is plastically deformed below the A 1 temperature, then permitted to transform to bainite or martensite. Ausformed steels - A steel can be austenitized, quenched to the bay region, plastically deformed, and finally quenched to produce martensite. When alloying elements introduce a bay region into the TTT diagram, the steel can beausformed. 66

67 Tempering - Alloying elements reduce the rate of tempering compared with that of a plain-carbon steel. This effect may permit the alloy steels to operate more successfully at higher temperatures than plain carbon steels since overaging will not occur during service. The effect of alloying elements on the phases formed during the tempering of steels. The airhardenable steel shows a secondary hardening peak. Secondary hardening peak - Unusually high hardness in a steel tempered at a high temperature caused by the precipitation of alloy carbides. 67

68 Section 3.6 Application of Hardenability 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. The set-up for the Jominy test used for determining the hardenability of a steel. 68

69 Jominy distance - The distance from the quenched end of a Jominy bar. The Jominy distance is related to the cooling rate. 69

70 Hardenability curves - Graphs showing the effect of the cooling rate on the hardness of as-quenched steel. The hardenability curves for several steels. 70

71 The Grossman chart used to determine the hardenability at the center of a steel bar for different quenchants. 71

72 Design a quenching process to produce a minimum hardness of HRC 40 at the center of a 1.5-in. diameter 4320 steel bar. Example 3.4 SOLUTION Example 3.4 Design of a Quenching Process We can estimate the Jominy distance for HRC 40 of the 4320 steel. 72

73 Example 3.4 SOLUTION (Continue) The last three methods, based on brine or agitated water, are satisfactory. Using an unagitated brine quenchant might be least expensive, since no extra equipment is needed to agitate the quenching bath. However, H 2 O is less corrosive than the brine quenchant. 73

74 Section 3.7 Specialty Steels Tool steels - A group of high-carbon steels that provide combinations of high hardness, toughness, or resistance to elevated temperatures. Their applications include cutting tools in machining operations, dies for die casting, forming dies, etc. High-strength-low-alloy (HSLA) steels - A group of low-carbon steels containing small amounts of alloying elements. Careful processing permits precipitation of carbides and nitrides of Nb, V, Ti, or Zr, which provide dispersion strengthening and a fine grain size. Dual-phase (DP) steels - Special steels treated to produce martensite dispersed in a ferrite matrix. These low-carbon steels do not contain enough alloying elements to have good hardenability using the normal quenching processes. 74

75 Transformation induced plasticity (TRIP) steels - A group of steels with a microstructure that consists of a continuous ferrite matrix, a harder second phase (martensite and/or bainite), and retained austenite. TRIP steels exhibit better ductility and formability at a given strength level because of the transformation of retained austenite to martensite during plastic deformation. Interstitial-free steels - These are steels containing Nb and Ti. They react with C and S to form precipitates of carbides and sulfides, leaving the ferrite nearly free of interstitial elements. Maraging steels - A special class of low-carbon high alloyed steels that obtain high strengths by a combination of the martensitic and agehardening reactions. Galvanized steel is coated with a thin layer of zinc and terne steel is coated with lead. 75

76 a) Microstructure of a dual-phase steel, showing islands of white martensite in a light gray ferrite matrix. b) Microstructure of a TRIP steel, showing ferrite (light gray) + bainite (black along grain boundaries) + retained austenite (white). 76

77 Section 3.8 Surface Treatments Selectively Heating the Surface - Rapidly heat the surface of a mediumcarbon steel above the A 3 temperature (the center remains below the A 1 ). After the steel is quenched, the center is still a soft mixture of ferrite and pearlite, while the surface is martensite. Case depth - The depth below the surface of a steel at which hardening occurs by surface hardening and carburizing processes. (a) Surface hardening by localized heating. (b) Only the surface heats above the A 3 temperature and is quenched to martensite. 77

78 Carburizing - A group of surface-hardening techniques by which carbon diffuses into steel at a temperature above the A 3. A high carbon content is produced at the surface due to rapid diffusion and the high solubility of carbon in austenite. When the steel is then quenched and tempered, the surface becomes a high-carbon tempered martensite, while the ferritic center remains soft and ductile. The thickness of the hardened surface, again called the case depth, is much smaller in carburized steels than in flame- or induction hardened steels. Carburizing of a low-carbon steel to produce a high-carbon, wearresistant surface. 78

79 Cyaniding - Hardening the surface of steel with carbon and nitrogen obtained from a bath of liquid cyanide solution. Carbonitriding - Hardening the surface of steel with carbon and nitrogen obtained from a special gas atmosphere (carbon monoxide and ammonia). Nitriding - Hardening the surface of steel with nitrogen obtained from a special gas atmosphere below the A 1 temperature. In each of these processes, compressive residual stresses are introduced at the surface, providing excellent fatigue resistance in addition to the good combination of hardness, strength, and toughness. 79

80 Example 3.5 Design of Surface-Hardening Treatments for a Drive Train Design the materials and heat treatments for an automobile axle and drive gear. Sketch of axle and gear assembly. 80

81 Example 3.5 SOLUTION The axle might be made from a forged 1050 steel containing a matrix of ferrite and pearlite. The axle could be surface-hardened, perhaps by moving the axle through an induction coil to selectively heat the surface of the steel above the A 3 temperature (about 770 o C). After the coil passes any particular location of the axle, the cold interior quenches the surface to martensite. Tempering then softens the martensite to improve ductility. Carburize a 1010 steel for the gear. By performing a gas carburizing process above the A 3 temperature (about 860 o C), we introduce about 1.0% C in a very thin case at the surface of the gear teeth. This high-carbon case, which transforms to martensite during quenching, is tempered to control the hardness. 81

82 Section 3.9 Weldability of Steel Low-carbon steels Many low-carbon steels weld easily. The strength of the welded regions in these materials is higher than the base material because of the finer pearlite microstructure that forms during cooling of the heat-affected zone. Medium- and high-carbon steels Welding of medium- and high-carbon steels is comparatively more difficult since martensite can form in the heat-affected zone rather easily, thereby causing a weldment with poor toughness. Several strategies such as preheating the material or minimizing incorporation of hydrogen have been developed to counter these problems. The incorporation of hydrogen causes the steel to become brittle. 82

83 During welding, the metal nearest the weld heats above the A 1 temperature and austenite forms. During cooling, the austenite in this heat-affected zone transforms to a new structure, depending on the cooling rate and the CCT diagram for the steel. Plain low-carbon steels have such a low hardenability that normal cooling rates seldom produce martensite. An alloy steel may have to be preheated to slow down the cooling rate or post-heated to temper any martensite that forms. 83

84 84 The development of the heat-affected zone in a weld: (a) the structure at the maximum temperature (b) the structure after cooling in a steel of low hardenability (c) the structure after cooling in a steel of high hardenability.

85 Example 3.6 Structures of Heat-Affected Zones Compare the structures in the heat-affected zones of welds in 1080 and 4340 steels if the cooling rate in the heat-affected zone is 5 o C/s. 85

86 Example 3.6 SOLUTION The cooling rate in the weld produces the following structures: 1080: 100% pearlite 4340: Bainite and martensite The high hardenability of the alloy steel reduces the weldability, permitting martensite to form and embrittle the weld. 86

87 Section 3.10 Stainless Steels Stainless steels - A group of ferrous alloys that contain at least 11% Cr, providing extraordinary corrosion resistance by permitting a thin, protective surface layer of chromium oxide to form when the steel is exposed to oxygen. Categories of stainless steels: Ferritic Stainless Steels Austenitic Stainless Steels Martensitic Stainless Steels Duplex Stainless Steels Precipitation-Hardening (PH) Stainless Steels 87

88 Ferritic Stainless Steels contain up to 30% Cr and less than 0.12% C. Because of the BCC structure, the ferritic stainless steels have good strengths and moderate ductilities derived from solid-solution strengthening and strain hardening. They have excellent corrosion resistance, moderate formability, and are ferromagnetic, not heat treatable, and relatively inexpensive. Austenitic Stainless Steels - Nickel, which is an austenite stabilizing element, increases the size of the austenite field, while nearly eliminating ferrite from the iron-chromium-carbon alloys. If the carbon content is below about 0.03%, the carbides do not form and the steel is virtually all austenite at room temperature. The FCC austenitic stainless steels have excellent ductility, formability, and corrosion resistance. Strength is obtained by extensive solid-solution strengthening, and the austenitic stainless steels may be cold worked to higher strengths than the ferritic stainless steels. They are expensive and not ferromagnetic. 88

89 Martensitic Stainless Steels - The combination of hardness, strength, and corrosion resistance makes the alloys attractive for applications such as high-quality knives, ball bearings, and valves. Duplex Stainless Steels - By appropriate control of the composition and heat treatment, a duplex stainless steel containing approximately 50% ferrite and 50% austenite can be produced. This combination provides a set of mechanical properties, corrosion resistance, formability, and weldability not obtained in any one of the usual stainless steels. Precipitation-Hardening (PH) Stainless Steels contain Al, Nb, or Ta and derive their properties from solid-solution strengthening, strain hardening, age hardening, and the martensitic reaction. The steel is first heated and quenched to permit the austenite to transform to martensite. Reheating permits precipitates such as Ni 3 Al to form from the martensite. High strengths are obtained even with low carbon contents. 89

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91 (a) Martensitic stainless steel containing large primary carbides and small carbides formed during tempering ( 350). (b) Austenitic stainless steel ( 500). 91

92 Section 3.11 Cast Irons Cast iron - iron-carbon-silicon alloys, typically containing 2 4% C and 0.5 3% Si, that pass through the eutectic reaction during solidification. Types of cast irons: Gray cast iron White cast iron Malleable cast iron Ductile or nodular, cast iron Compacted graphite cast iron 92

93 Schematic drawings of the five types of cast iron: (a) gray iron, (b) white iron, (c) malleable iron, (d) ductile iron, and (e) compacted graphite iron. 93

94 Gray cast iron - Cast iron which, during solidification, contains graphite flakes, causing low strength and poor ductility. This is the most widely used type of cast iron. It has high compressive strength, good machinability, good resistance to sliding wear, good resistance to thermal fatigue, good thermal conductivity, and good vibration damping. The graphite flakes concentrate stresses and cause low strength and ductility. (a) Sketch and (b) photomicrograph of the flake graphite in gray cast iron (x 100). 94

95 White cast iron - Cast iron that produces cementite rather than graphite during solidification. The white irons are hard and brittle. A fractured surface of this material appears white. Malleable cast iron - Cast iron obtained by a lengthy heat treatment, during which cementite decomposes to produce rounded clumps of graphite. Good strength, ductility, and toughness are obtained as a result of this structure. It is formed by the heat treatment of white cast iron, produces rounded clumps of graphite. It exhibits better ductility than gray or white cast irons. It is also very machinable. 95

96 Ductile or nodular cast iron - Cast iron treated with magnesium to cause graphite to precipitate during solidification as spheres, permitting excellent strength and ductility. The ductile irons are stronger but not as tough as malleable irons. Compared with gray iron, ductile cast iron has excellent strength and ductility. Compacted graphite cast iron - A cast iron treated with small amounts of magnesium and titanium to cause graphite to grow during solidification as an interconnected, coral-shaped precipitate, giving properties midway between gray and ductile iron. The compacted graphite permits strengths and ductilities that exceed those of gray cast iron, but allows the iron to retain good thermal conductivity and vibration damping properties. 96

97 The gray irons are specified by a class number of 20 to 80. A class 20 gray iron has a nominal tensile strength of 20,000 psi. In thick castings, coarse graphite flakes and a ferrite matrix produce tensile strengths as low as 12,000 psi. In thin castings, fine graphite and pearlite form and give tensile strengths near 40,000 psi. Higher strengths are obtained by reducing the carbon equivalent, by alloying, or by heat treatment. The effect of the cooling rate or casting size on the tensile properties of two gray cast irons. 97

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