Presentation on theme: "Chapter 12 – Ferrous Alloys"— Presentation transcript:
1 Chapter 12 – Ferrous Alloys The Science and Engineering of Materials, 4th ed Donald R. Askeland – Pradeep P. PhuléChapter 12 – Ferrous Alloys
2 Objectives of Chapter 12Discuss 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.
3 Chapter Outline 12.1 Designations and Classification of Steels 12.2 Simple Heat Treatments12.3 Isothermal Heat Treatments12.4 Quench and Temper Heat Treatments12.5 Effect of Alloying Elements12.6 Application of Hardenability
5 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: Used with permission of the American Iron and Steel Institute.)
6 Section 12.1 Designations and Classification of Steels 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.
8 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.)
10 Example 12.1 Design of a Method to Determine AISI Number 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.
11 Example 12.1 SOLUTIONThe 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: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:
12 Section 12.2 Simple Heat Treatments 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.
18 Example 12.2 SOLUTIONFrom 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.
19 Section 12.3 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 A1 and the nose of the TTT curve, and holding until the austenite transforms to pearlite.
24 Example 12.3 SOLUTIONAustenitize the steel at (30 to 55) = 805oC to 825oC, holding for 1 h and obtaining 100% γ.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: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.
26 Figure 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 )
27 Section 12.4 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.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.
29 Example 12.4 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 12.4 SOLUTIONAustenitize above the A3 temperature of 770oC for 1 h. An appropriate temperature may be = 825oC.Quench rapidly to room temperature. Since the Mf is about 250oC, martensite will form.Temper by heating the steel to 440oC. Normally, 1 h will be sufficient if the steel is not too thick.Cool to room temperature.
37 Section 12.5 Effect of Alloying Elements 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.
42 Section 12.6 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.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.
48 Example 12.5 SOLUTIONFrom 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 have the following hardnesses at the critical location:1050 HRC HRC HRC 318640 HRC HRC 60In 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.
53 Example 12.6 SOLUTIONSeveral quenching media are listed in Table We can find an approximate H coefficient for each of the quenching media, then use Figure 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.
54 Section 12.7 Specialty Steels 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.
55 Figure 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.)
56 Section 12.8 Surface Treatments 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.
60 Example 12.7 SOLUTIONThe 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.
62 Example 12.8 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 5oC/s.Example 12.8 SOLUTIONThe cooling rate in the weld produces the following structures:1080: 100% pearlite4340: Bainite and martensiteThe high hardenability of the alloy steel reduces the weldability, permitting martensite to form and embrittle the weld.
63 Section 12.10 Stainless Steels Stainless steels - A group of ferrous alloys that contain at least 11% Cr, providing extraordinary corrosion resistance.Categories of stainless steels:Ferritic Stainless SteelsMartensitic Stainless SteelsAustenitic Stainless SteelsPrecipitation-Hardening (PH) Stainless SteelsDuplex Stainless Steels
66 Figure (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 )
67 Example 12.9 Design of a Test to Separate Stainless Steels 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.Example 12.9 SOLUTIONPerforming 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.
68 Section Cast IronsCast iron - Ferrous alloys containing sufficient carbon so that the eutectic reaction occurs during solidification.Eutectic and Eutectoid reaction in Cast IronsTypes of cast irons:Gray cast ironWhite cast ironMalleable cast ironDuctile or nodular, cast ironCompacted graphite cast iron
74 Figure 12.37 The heat treatments for ferritic and pearlitic malleable irons.
75 Figure 12. 38 (a) White cast iron prior to heat treatment ( 100) Figure (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 ) (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).