email@example.com METAL FABRICATION METHODS-I CASTING FORMING • Forging (wrenches, crankshafts) force die Ao blank JOINING • Rolling (I-beams, rails) Ad often at elev. T • Drawing force • Extrusion (rods, wire, tubing) die Ao die Ad Adapted from Fig. 11.7, Callister 6e. (rods, tubing) tensile force 6 FORMING TEMPERATURE • Hot working --recrystallization • Cold working --recrystallization --less energy to deform --oxidation: poor finish --lower strength --less energy to deform --oxidation: poor finish --lower strength • Cold worked microstructures --generally are very anisotropic! --Forged (a) --Swaged (b) --Fracture resistant! (c) 7 METAL FABRICATION METHODS-II FORMING CASTING • Sand Casting (large parts, e.g., auto engine blocks) • Investment Casting JOINING • Die Casting (high volume, low T alloys) • Continuous Casting (simple slab shapes) (low volume, complex shapes e.g., jewelry, turbine blades) plaster die formed around wax prototype 8 METAL FABRICATION METHODS-III FORMING • Powder Processing (materials w/low ductility) CASTING JOINING • Welding (when one large part is impractical) filler metal (melted) base metal (melted) fused base metal unaffected piece 1 heat affected zone unaffected Adapted from Fig. piece 2 11.8, Callister 6e. (Fig. 11.8 from • Heat affected zone: Iron Castings Handbook, C.F. Walton and T.J. (region in which the Opar (Ed.), 1981.) microstructure has been changed). 9 Thermal processing of metals Annealing: Heat to Tanneal, then cool slowly. Outline • Heat Treatment of Steels – Hardenability – Influence of quenching medium, specimen size, and geometry • Annealing Processes – Annealing of ferrous alloys Full annealing Normalizing – Process annealing – Stress relief • Precipitation Hardening 160 The complete isothermal transformation diagram for an iron-carbon alloy of eutectoid composition. 140 120 100 80 A: austenite B: bainite M: martensite P: pearlite 60 40 20 0 1 10 100 Heat Treatment of Steels • Conventional heat treatment procedures for producing martensitic steels involves – continuous and rapid cooling of an austenitized specimen in some type of quenching medium, such as water, oil, or air • The optimum properties of a steel that has been quenched and then tempered can be realized only if, – during the quenching heat treatment, the specimen has been converted to a high content of martensite Non-uniform Cooling Rate during Quenching • During the quenching treatment, – it is impossible to cool the specimen at a uniform rate throughout – the surface will always cool more rapidly than interior regions. • The austenite will transform over a range of temperatures, yielding a possible variation of microstructure & properties with position within a specimen. Heat Treatments of Steels (Cont’d) • The successful heat treating of steels to produce a predominantly martensite microstructure throughout the cross section depends mainly on three factors: 1. the composition of the alloy 2. the type and character of the quenching medium 3. the size and shape of the specimen Hardenability of Steels • Hardenability: A measure of the ability a specific alloy to be hardened by forming martensite as a result of given heat treatment • The Jominy end-quench test: – to measure hardenability Jominy End Quench Test • Ability to form martensite • Jominy end quench test to measure hardenability 1” specimen (heated to phase field) 24°C water flat ground 4” Adapted from Fig. 11.10, Callister 6e. Hardenability Curves • Hardness vs. distance from the quenched end 1” specimen (heated to phase field) 24°C water flat ground 4” Adapted from Fig. 11.11, Callister 6e. • A steel that is highly hardenable will retain large hardness values for relatively long distances; a low hardenable one will not. Why hardness changes with position? • The cooling rate varies with position Adapted from Fig. 11.12, Callister 6e. Effect of Alloying Elements on Hardenability (1.85 Ni, 0.80 Cr, & 0.25Mo) (1.0 Cr & 0.20 Mo) (0.55 Ni, 0.50 Cr, & 0.20 Mo) (0.85 Cr) (plain carbon steel) Distance from quench end Hardenability Curves for Five Steel Alloys (Each Containing 0.4 wt% C) Hardenability One plain carbon steel (1040) and four alloy steels • All five alloys have identical hardness at the quenched end (57 HRC); this hardness is a function of carbon content. • The shape of the curves relates to hardenability. The hardenability from low to high: 1040 steel < 5140 steel < 8640 steel < 4140 steel < 4340 steel • A water-quenched specimen of the 1040 plain carbon steel would harden only to a shallow depth below the surface, whereas for the other four alloy steels the high quenched hardness would persist to a much greater depth. Effect of Carbon Content on Hardenability The hardenability increases with the carbon content. FIG. 11.14 Hardenability curves for four 8600 series alloys of indicated carbon content. Alloying Elements and Hardenability • The alloying element content and carbon content • change the shapes of the hardenability curve The principal reason for using alloying elements in the standard grades of steels is to increase hardenability. Quenching Mediums (1): Water • The most commonly used quenching medium • Inexpensive and convenient to use • Provide very rapid cooling • Especially used for low-carbon steel, which requires a very rapid change in temperature in order to obtain good hardness and strength • Can cause internal stresses, distortion, or cracking Quenching Mediums (2): Oil • More gentle than water • Used for more critical parts, such as parts that have thin sections or sharp edges • Razor blades, springs, and knife blades • Does not produce steel that is as hard or strong as steel quenched by water • Less chance of producing internal stresses, distortion, or cracking • More effective when oil is heated slightly above room temperature to 100°F or 150°F (40°C or 65°C): reduced viscosity Quenching Mediums (3): Air • More gentle than oil • Does not produce steel that is as hard or strong as steel quenched by water or oil • Less chance of producing internal stresses, distortion, or cracking • Generally used only on steels that have a very high alloy content Special alloys (such as Cr and Mo) are selected because they are known to cause materials to harden even though a slower quenching method is used The heated sample is placed on a screen. Cool air is blown at high speed from below it. Effect of Quenching Medium Medium Severity of Quench Hardness air small small oil moderate moderate water large large The severity of quench: water > oil > air Quenching Operations in Heat Treatment Removing Samples from a Heat-Treating Furnace Parts Quenched in a Group (Figs. 11-3 and 11-4 in Metallurgy Fundamentals, by D. A. Brandt and J. C. Warner) Effect of Part Size FIG. 11.17 Radial hardness profiles for (a) 50 mm (2 in.) diameter cylindrical 1040 and 4140 steel specimens quenched in mildly agitated water, and (b) 50 and 100 mm (2 and 4 in.) diameter cylindrical specimens of 4140 steel quenched Effect of Part Geometry • When surface-to-volume ratio increases cooling rate increases hardness increases Position Cooling rate center small surface large Hardness small large Annealing Processes • Annealing: a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. • Three stages of annealing 1. Heating to the desired temperature 2. Holding or “soaking” at that temperature 3. Cooling, usually to room temperature Purposes for Annealing 1. Relieve Internal Stresses • Internal stresses can build up in metal as a result of processing. – Stresses may be caused by previous processing operations such as welding, cold working, casting, forging, or machining. • If internal stresses are allowed to remain in a metal, the part may eventually distort or crack. • Annealing helps relieve internal stresses and reduce the chances for distortion and cracking. Purposes for Annealing (Cont’d) 2. Increasing Softness, Machinability, and Formability • A softer and more ductile material is easier to machine in the machine shop. • An annealed part will respond better to forming operations. 3. Refinement of Grain Structures • After some types of metalworking (particularly cold working), the crystal structures are elongated. • Annealing can change the shape of the grains back to the desired form. The Iron–Iron Carbide Phase Diagram P 2.14 E 4.30 L + Fe3C F G M O N H 0.76 0.022 Cementite Fe3C C 6.70 Temperature Regime of Steel Heat Treatment FIG. 11.9 The iron-iron carbide phase diagram in the vicinity of the eutectoid, indicating heat treating temperature ranges for the plain carbon steels. • Most heat treating operations begin with heating the alloy into the austenitic phase field to dissolve the carbide in the iron • Steel heat treating practice rarely involves the use of temperatures above 1040°C (1900°F) Precipitation hardening of metals (Heat treatment) • Strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix. • This strengthening is accomplished by phase transformations induced by heat treatment. • Age hardness is also used to designate the process since strength develops with time, or as the alloy ages. Precipitation hardening (Cont.) • Requisite features of phase diagrams of alloy systems for precipitation hardening: – Appreciable maximum solubility of one component in the other (on the order of several percent). – Solubility limit that rapidly decreases in concentration of the major component with temperature reduction. – Composition of precipitation-hardenable alloy must be less than the maximum solubility. Precipitation hardening • Particles impede dislocations. • Ex: Al-Cu system • Procedure: --Pt A: solution heat treat (get a solid solution) --Pt B: quench to room temp. --Pt C: reheat to nucleate small q crystals within a crystals. • Other precipitation systems: • Cu-Be • Cu-Sn • Mg-Al Precipitation hardening (Cont.) • Types of precipitation hardening process: – Solution heat treatment – Precipitation heat treatment. Temperature vs. time plot showing both solution and precipitation heat treatment for precipitation hardening. Precipitation hardening (Cont.) Solution heat treatment: • Quenching (rapid cooling) prevent diffusion and the formation of a new phase. Resulting phase will be supersaturated in B atoms (alloy relatively soft and weak). a Phase is retained at room temperature for relatively long periods (long diffusion rate at room temperature). Precipitation heat treatment: • The characteristics of the phase and the strength and hardness of the alloy depends on the precipitation temperature T2 and the aging time at this temperature. Precipitation effect on TS, %EL • 2014 Al Alloy: • TS peaks with precipitation time. • Increasing T accelerates process. • %EL reaches minimum with precipitation time. Effect of aging on dislocation motion • Peak-aged --avg. particle size = 64b --closer spaced particles efficiently stop dislocations. • Over-aged --avg. particle size = 361b --more widely spaced particles not as effective. SUMMARY • Steels: increase TS, Hardness (and cost) by adding --C (low alloy steels) --Cr, V, Ni, Mo, W (high alloy steels) --ductility usually decreases w/additions. • Non-ferrous: --Cu, Al, Ti, Mg, Refractory, and noble metals. • Hardenability --increases with alloy content. • Precipitation hardening --effective means to increase strength in Al, Cu, and Mg alloys.