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 .)
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 A 1 temperature and hold for a long time. The steel overtempers and large Fe 3 C 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% Fe 3 C using this method, the carbon content is: A better approach, however, is to heat the steel above the A cm 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 Fe 3 C, then:
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.
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.
Austenitize the steel at 770 + (30 to 55) = 805 o C to 825 o C, holding for 1 h and obtaining 100% γ .
Quench the steel to 600 o C 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.
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 M S and holding until the temperature is equalized throughout the steel before further cooling to produce martensite.
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 SOLUTION
Austenitize above the A 3 temperature of 770 o C for 1 h. An appropriate temperature may be 770 + 55 = 825 o C.
Quench rapidly to room temperature. Since the M f is about 250 o C, martensite will form.
Temper by heating the steel to 440 o C. Normally, 1 h will be sufficient if the steel is not too thick.
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 .)
Example 12.5 SOLUTION From Figure 12.23, a hardness of HRC 40 in a 9310 steel corresponds to a Jominy distance of 10/16 in. (10 o C/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.
Example 12.6 SOLUTION 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, H 2 O 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 .)
Example 12.7 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. This high-carbon case, which transforms to martensite during quenching, is tempered to control the hardness.
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. Example 12.8 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. Example 12.8 Structures of Heat-Affected Zones
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. Example 12.9 SOLUTION 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
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).