askeland phule notes ch12 printable

The Science and Engineering of Materials, 4th edDonald R. Askeland – Pradeep P. Phulé

Chapter 12 – Ferrous Alloys

Objectives of Chapter 12 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.

Chapter Outline

12.1 Designations and Classification of Steels

12.2 Simple Heat Treatments 12.3 Isothermal Heat Treatments 12.4 Quench and Temper Heat

Treatments 12.5 Effect of Alloying Elements 12.6 Application of Hardenability

12.7 Specialty Steels

12.8 Surface Treatments

12.9 Weldability of Steel

12.10 Stainless Steels

12.11 Cast Irons

Chapter Outline (Continued)

Figure 12.1 (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 reduce 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. (Source: www.steel.org. Used with permission of the American Iron and Steel Institute.)

Designations - The AISI (American Iron and Steel Institute) and SAE (Society of Automotive Engineers) provide designation systems for steels that use a four- or five-digit number.

Classifications - Steels can be classified based on their composition or the way they have been processed.

Section 12.1 Designations and Classification

of Steels

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Figure 12.2 (a) The eutectoid portion of the Fe-Fe3C phase diagram. (b) An expanded version of the Fe-C diagram, adapted from several sources.

Figure 12.3 Electron micrographs of (a) pearlite, (b) bainite, and (c) tempered martensite, illustrating the differences in cementite size and shape among these three microconstituents ( 7500). (From The Making, Shaping, and Treating of Steel, 10th Ed. Courtesy of the Association of Iron and Steel Engineers.)

An unalloyed steel tool used for machining aluminum automobile wheels has been found to work well, but the purchase records have been lost and you do not know the steel’s composition. The microstructure of the steel is tempered martensite, and assume that you cannot estimate the composition of the steel from the structure. Design a treatment that may help determine the steel’s carbon content.

Example 12.1 Design of a Method to Determine AISI

Number

Example 12.1 SOLUTION

The first way is to heat the steel to a temperature just below the A1 temperature and hold for a long time. The steel overtempers and large Fe3C spheres form in a ferrite matrix. We then estimate the amount of ferrite and cementite and calculate the carbon content using the lever law. If we measure 16% Fe3C using this method, the carbon content is:

%086.1or 16100)0218.067.6(

)0218.0(CFe % 3

A better approach, however, is to heat the steel above the Acm to produce all austenite. If the steel then cools slowly, it transforms to pearlite and a primary microconstituent. If, when we do this, we estimate that the structure contains 95% pearlite and 5% primary Fe3C, then:

%065.1or 9510077.067.6

-6.67Pearlite %

Process Annealing — Eliminating Cold Work: A low-temperature heat treatment used to eliminate all or part of the effect of cold working in steels.

Annealing and Normalizing — Dispersion Strengthening: Annealing - A heat treatment used to produce a soft, coarse pearlite in steel by austenitizing, then furnace cooling. Normalizing - A simple heat treatment obtained by austenitizing and air cooling to produce a fine pearlitic structure.

Spheroidizing — Improving Machinability: Spheroidite - A microconstituent containing coarse spheroidal cementite particles in a matrix of ferrite, permitting excellent machining characteristics in high-carbon steels.

Section 12.2 Simple Heat Treatments

Figure 12.4 Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels.

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Figure 12.5 The effect of carbon and heat treatment on the properties of plain-carbon steels.

Figure 12.6 The microstructure of spheroidite, with Fe3C particles dispersed in a ferrite matrix ( 850). (From ASM Handbook, Vol. 7, (1972), ASM International, Materials Park, OH 44073.)

Recommend temperatures for the process annealing, annealing, normalizing, and spheroidizing of 1020, 1077, and 10120 steels.

Example 12.2 Determination of Heat Treating

Temperatures

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From Figure 12.2, we find the critical A1, A3, or Acm, temperatures for each steel. We can then specify the heat treatment based on these temperatures.

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 A1 and the nose of the TTT curve, and holding until the austenite transforms to pearlite.

Section 12.3 Isothermal Heat Treatments

Figure 12.7 The austempering and isothermal anneal heat treatments in a 1080 steel.

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Figure 12.8 The TTT diagrams for (a) a 1050 and (b) a 10110 steel.

A heat treatment is needed to produce a uniform microstructure and hardness of HRC 23 in a 1050 steel axle.

Example 12.3 Design of a Heat Treatment for an Axle

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Figure 12.8 The TTT diagrams for (a) a 1050 and (b) a 10110 steel.

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1. Austenitize the steel at 770 + (30 to 55) = 805oC to 825oC, holding for 1 h and obtaining 100% γ.

2. Quench the steel to 600oC and hold for a minimum of 10 s. 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 10 s. After this treatment, the microconstituents present are:

%64100)0218.077.0(

0.0218)(0.5Pearlite

%36100)0218.077.0(

0.5)(0.77αPrimary

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.

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Figure 12.9 Producing complicated structures by interrupting the isothermal heat treatment of a 1050 steel.

Figure 12.10 Dark feathers of bainite surrounded by light martensite, obtained by interrupting the isothermal transformation process ( 1500). (ASM Handbook, Vol. 9 Metallography and Microstructure (1985), ASM International, Materials Park, OH 44073.)

Retained austenite - Austenite that is unable to transform into martensite during quenching because of the volume expansion associated with the reaction.

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.

Marquenching - Quenching austenite to a temperature just above the MS and holding until the temperature is equalized throughout the steel before further cooling to produce martensite.

Section 12.4 Quench and Temper Heat Treatments

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Figure 12.11 The effect of tempering temperature on the mechanical properties of a 1050 steel.

Example 12.4 Design of a Quench and

Temper TreatmentA 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.

1. Austenitize above the A3 temperature of 770oC for 1 h. An appropriate temperature may be 770 + 55 = 825oC.

2. Quench rapidly to room temperature. Since the Mf is about 250oC, martensite will form.

3. Temper by heating the steel to 440oC. Normally, 1 h will be sufficient if the steel is not too thick.

4. Cool to room temperature.

Figure 12.12 Retained austenite (white) trapped between martensite needles (black) ( 1000). (From ASM Handbook, Vol. 8, (1973), ASM International, Materials Park, OH 44073.)

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Figure 12.13 Increasing carbon reduces the Ms and Mf temperatures in plain-carbon steels.

Figure 12.14 Formation of quench cracks caused by residual stresses produced during quenching. The figure illustrates the development of stresses as the austenite transforms to martensite during cooling.

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Figure 12.15 The marquenching heat treatment designed to reduce residual stresses ands quench cracking.

Figure 12.16 The CCT diagram (solid lines) for a 1080 steel compared with the TTT diagram (dashed lines).

Figure 12.17 The CCT diagram for a low-alloy, 0.2% C Steel.

Hardenability - Alloy steels have high hardenability. Effect on the Phase Stability - When alloying elements

are added to steel, the binary Fe-Fe3C stability is affected and the phase diagram is altered.

Shape of the TTT Diagram - Ausforming is a thermomechanical heat treatment in which austenite is plastically deformed below the A1 temperature, then permitted to transform to bainite or martensite.

Tempering - Alloying elements reduce the rate of tempering compared with that of a plain-carbon steel.

Section 12.5 Effect of Alloying Elements

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Figure 12.18 (a) TTT and (b) CCT curves for a 4340 steel.

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Figure 12.19 The effect of 6% manganese on the stability ranges of the phases in the eutectoid portion of the Fe-Fe3C phase diagram.

Figure 12.20 When alloying elements introduce a bay region into the TTT diagram, the steel can be ausformed.

Figure 12.21 The effect of alloying elements on the phases formed during the tempering of steels. The air-hardenable steel shows a secondary hardening peak.

Jominy test - The test used to evaluate hardenability. An austenitized steel bar is quenched at one end only, thus producing a range of cooling rates along the bar.

Hardenability curves - Graphs showing the effect of the cooling rate on the hardness of as-quenched steel.

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

Section 12.6 Application of Hardenability

Figure 12.22 The set-up for the Jominy test used for determining the hardenability of a steel.

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Figure 12.23 The hardenability curves for several steels.

A gear made from 9310 steel, which has an as-quenched hardness at a critical location of HRC 40, wears at an excessive rate. Tests have shown that an as-quenched hardness of at least HRC 50 is required at that critical location. Design a steel that would be appropriate.

Example 12.5 Design of a Wear-Resistant Gear

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From Figure 12.23, a hardness of HRC 40 in a 9310 steel corresponds to a Jominy distance of 10/16 in. (10oC/s). If we assume the same Jominy distance, the other steels shown in Figure 12.23 have the following hardnesses at the critical location:

1050 HRC 28 1080 HRC 36 4320 HRC 31

8640 HRC 52 4340 HRC 60

In Table 12-1, we find that the 86xx steels contain less alloying elements than the 43xx steels; thus the 8640 steel is probably less expensive than the 4340 steel and might be our best choice. We must also consider other factors such as durability.

Figure 12.24 The Grossman chart used to determine the hardenability at the center of a steel bar for different quenchants.

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 12.6 Design of a Quenching Process

Figure 12.24 The Grossman chart used to determine the hardenability at the center of a steel bar for different quenchants.

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Several quenching media are listed in Table 12-2. We can find an approximate H coefficient for each of the quenching media, then use Figure 12.24 to estimate the Jominy distance in a 1.5-in. diameter bar for each media. Finally, we can use the hardenability curve (Figure 12.23) to find the hardness in the 4320 steel. The results are listed below.

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, H2O is less corrosive than the brine quenchant.

Tool steels - A group of high-carbon steels that provide combinations of high hardness, toughness, or resistance to elevated temperatures.

Secondary hardening peak - Unusually high hardness in a steel tempered at a high temperature caused by the precipitation of alloy carbides.

Dual-phase steels - Special steels treated to produce martensite dispersed in a ferrite matrix.

Maraging steels - A special class of alloy steels that obtain high strengths by a combination of the martensitic and age-hardening reactions.

Section 12.7 Specialty Steels

Figure 12.25 Microstructure of a dual-phase steel, showing islands of light martensite in a ferrite matrix ( 2500). (From G. Speich, ‘‘Physical Metallurgy of Dual-Phase Steels,’’ Fundamentals of Dual-Phase Steels, The Metallurgical Society of AIME, 1981.)

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 below the surface of a steel at which hardening occurs by surface hardening and carburizing processes.

Carburizing - A group of surface-hardening techniques by which carbon diffuses into steel.

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.

Section 12.8 Surface Treatments

Figure 12.26 (a) Surface hardening by localized heating. (b) Only the surface heats above the A1 temperature and is quenched to martensite.

Figure 12.27 Carburizing of a low-carbon steel to produce a high-carbon, wear-resistant surface.

Design the materials and heat treatments for an automobile axle and drive gear (Figure 12.28).

Example 12.7 Design of Surface-Hardening Treatments

for a Drive Train

Figure 12.28 Sketch of axle and gear assembly (for example 12.7).

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 A3 temperature (about 770oC). 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 A3 temperature (about 860oC), 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. This high-carbon case, which transforms to martensite during quenching, is tempered to control the hardness.

g ™ is

Figure 12.29 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, and (c) the structure after cooling in a steel of high hardenability.

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 5oC/s.

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.

Example 12.8 Structures of Heat-Affected Zones

Stainless steels - A group of ferrous alloys that contain at least 11% Cr, providing extraordinary corrosion resistance.

Categories of stainless steels:• Ferritic Stainless Steels• Martensitic Stainless Steels• Austenitic Stainless Steels• Precipitation-Hardening (PH) Stainless Steels• Duplex Stainless Steels

Section 12.10 Stainless Steels

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Figure 12.30 (a) The effect of 17% chromium on the iron-carbon phase diagram. At low-carbon contents, ferrite is stable at all temperatures. (b) A section of the iron-chromium-nickel-carbon phase diagram at a constant 18% Cr-8% Ni. At low-carbon contents, austenite is stable at room temperatures.

Figure 12.31 (a) Martensitic stainless steel containing large primary carbides and small carbides formed during tempering ( 350). (b) Austenitic stainless steel ( 500). (From ASM Handbook, Vols. 7 and 8, (1972, 1973), ASM International, Materials Park, OH 44073.)

In order to efficiently recycle stainless steel scrap, we wish to separate the high-nickel stainless steel from the low-nickel stainless steel. Design a method for doing this.

Performing a chemical analysis on each piece of scrap is tedious and expensive. Sorting based on hardness might be less expensive; however, because of the different types of treatments—such as annealing, cold working, or quench and tempering—the hardness may not be related to the steel composition.

The high-nickel stainless steels are ordinarily austenitic, whereas the low-nickel alloys are ferritic or martensitic. An ordinary magnet will be attracted to the low-nickel ferritic and martensitic steels, but will not be attracted to the high-nickel austenitic steel. We might specify this simple and inexpensive magnetic test for our separation process.

Example 12.9 Design of a Test to Separate

Stainless Steels

Cast iron - Ferrous alloys containing sufficient carbon so that the eutectic reaction occurs during solidification.

Eutectic and Eutectoid reaction in Cast Irons Types of cast irons:

• Gray cast iron• White cast iron• Malleable cast iron• Ductile or nodular, cast iron• Compacted graphite cast iron

Section 12.11 Cast Irons

Figure 12.32 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.

Figure 12.33 The iron-carbon phase diagram showing the relationship between the stable iron-graphite equilibria (solid lines) and the metastable iron-cementite reactions (dashed lines).

Figure 12.34 The transformation diagram for austenite in a cast iron.

Figure 12.35 (a) Sketch and (b) photomicrograph of the flake graphite in gray cast iron (x 100).

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Figure 12.36 The effect of the cooling rate or casting size on the tensile properties of two gray cast irons.

Figure 12.37 The heat treatments for ferritic and pearlitic malleable irons.

Figure 12.38 (a) White cast iron prior to heat treatment ( 100). (b) Ferritic malleable iron with graphite nodules and small MnS inclusions in a ferrite matrix ( 200). (c) Pearlitic malleable iron drawn to produce a tempered martensite matrix ( 500). (Images (b) and (c) are from Metals Handbook, Vols. 7 and 8, (1972, 1973), ASM International, Materials Park, OH 44073.) (d) Annealed ductile iron with a ferrite matrix ( 250). (e) As-cast ductile iron with a matrix of ferrite (white) and pearlite ( 250). (f) Normalized ductile iron with a pearlite matrix ( 250).

Figure 12.17 (Repeated for Problem 12.20) The CCT diagram for a low-alloy, 0.2% C steel.

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Figure 12.23 (Repeated for Problem 12.54) The hardenability curves for several steels.

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Figure 12.30b (Repeated for Problem 12.48) (b) A section of the iron-chromium-nickel-carbon phase diagram at a constant 18% Cr-8% Ni. At low-carbon contents, austenite is stable at room temperature.

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