chapter 1.pdf

# Ferrous alloys: steels and cast irons Ferrous alloys, characterized by iron as the primary constituent, encompass steels and cast irons, which are produced in large quantities and are crucial engineering materials due to their abundance, economical processing, and versatile properties. Their main drawback is susceptibility to corrosion [3](#page=3). ### 1.1 Classification of ferrous alloys Ferrous alloys are broadly categorized into steels and cast irons [3](#page=3). ### 1.2 Steels Steels are iron-carbon alloys that can incorporate other alloying elements. Their mechanical properties are significantly influenced by carbon content, typically below 1.0 wt% [4](#page=4). #### 1.2.1 Low-carbon steels These steels contain less than approximately 0.25 wt% C. They are not responsive to heat treatments for martensite formation and are strengthened by cold work. Their microstructures consist of ferrite and pearlite, resulting in low strength and softness but excellent ductility and toughness, along with good machinability and weldability. They are also the most economical to produce. Typical applications include automobile body panels, structural shapes, and sheets for pipelines and buildings. Plain low-carbon steels typically exhibit a yield strength of 275 MPa, tensile strengths between 415 and 550 MPa, and ductility of 25% EL [4](#page=4). **High-strength, low-alloy (HSLA) steels** are a subclass of low-carbon alloys containing alloying elements like copper, vanadium, nickel, and molybdenum, with combined concentrations up to 10 wt%. They offer higher strengths than plain low-carbon steels and can be heat-treated for tensile strengths exceeding 480 MPa, while retaining ductility and formability. HSLA steels are more corrosion-resistant than plain carbon steels and have replaced them in structural applications like bridges, towers, and pressure vessels [4](#page=4) [5](#page=5). > **Tip:** Low-carbon steels are ideal for applications where high strength is not the primary requirement but rather formability, weldability, and cost-effectiveness are key. #### 1.2.2 Medium-carbon steels Medium-carbon steels contain between approximately 0.25 and 0.60 wt% C. They can be heat-treated (austenitizing, quenching, and tempering) to enhance mechanical properties, typically resulting in a tempered martensite microstructure. Plain medium-carbon steels have low hardenability, limiting heat treatment effectiveness to thin sections and rapid quenching. Alloying elements like chromium, nickel, and molybdenum improve hardenability, leading to various strength-ductility combinations. These heat-treated alloys are stronger than low-carbon steels but with reduced ductility and toughness. Applications include railway wheels and tracks, gears, crankshafts, and other machine parts requiring high strength, wear resistance, and toughness [5](#page=5). ##### 1.2.2.1 Steel designation systems * **AISI/SAE designation:** A four-digit number system where the first two digits indicate alloy content (e.g., '10' for plain carbon steel, '13', '41', '43' for alloy steels) and the last two digits represent the weight percent carbon multiplied by 100 [5](#page=5). * **UNS (Unified Numbering System):** A single-letter prefix followed by a five-digit number, indexing both ferrous and nonferrous alloys. For steel, the prefix is 'G' followed by the AISI/SAE number, with the fifth digit usually zero [6](#page=6). > **Example:** A 1060 steel is a plain carbon steel with 0.60 wt% C. A G41300 designation indicates an alloy steel with approximately 0.30 wt% C and alloying elements specified by the '41' prefix [6](#page=6) [7](#page=7). #### 1.2.3 High-carbon steels These steels typically contain between 0.60 and 1.4 wt% C. They are the hardest, strongest, and least ductile of the carbon steels. High-carbon steels are usually hardened and tempered, making them highly wear-resistant and capable of holding a sharp cutting edge. Tool and die steels, a subclass of high-carbon alloys, often contain chromium, vanadium, tungsten, and molybdenum, which form hard and wear-resistant carbide compounds (e.g., Cr₂₃C₆, V₄C₃, WC). Applications include cutting tools, dies, knives, razors, hacksaw blades, springs, and high-strength wire [6](#page=6). #### 1.2.4 Stainless steels Stainless steels are characterized by high resistance to corrosion, especially in ambient atmospheres, due to a minimum of 11 wt% chromium. Nickel and molybdenum additions further enhance corrosion resistance. They are classified into three main types based on their predominant microstructural phase: martensitic, ferritic, and austenitic [7](#page=7). * **Austenitic:** Extended austenite phase field to room temperature. They are not heat-treatable and are strengthened by cold work. Austenitic stainless steels are the most corrosion-resistant due to high chromium and nickel content and are produced in the largest quantities. They are non-magnetic [8](#page=8) [9](#page=9). * **Ferritic:** Composed of the α-ferrite (BCC) phase. They are not heat-treatable and are strengthened by cold work. Ferritic stainless steels are magnetic [8](#page=8) [9](#page=9). * **Martensitic:** Capable of heat treatment to achieve a martensite microstructure. Martensitic stainless steels are magnetic [9](#page=9). * **Precipitation-hardenable:** These steels can achieve ultra-high strength through precipitation-hardening heat treatments [8](#page=8) [9](#page=9). Many stainless steels can withstand elevated temperatures and severe environments due to oxidation resistance and maintained mechanical integrity up to approximately 1000°C (1800°F). Applications include gas turbines, boilers, heat-treating furnaces, aircraft, missiles, and nuclear power units. Some austenitic grades, like 316L, are used in biomedical applications such as temporary orthopedic devices [9](#page=9). > **Fact:** The minimum chromium content for a steel to be classified as stainless is 11 wt% [7](#page=7). ### 1.3 Cast irons Cast irons are ferrous alloys with carbon contents exceeding 2.14 wt%, typically ranging from 3.0 to 4.5 wt% C, along with other alloying elements. They melt at significantly lower temperatures (1150°C to 1300°C) compared to steels, making them easy to melt and cast. The brittleness of some cast irons also favors casting as a fabrication method. The equilibrium iron-carbon phase diagram with graphite as the stable carbon phase is relevant for cast irons, differing from the iron-iron carbide diagram primarily at higher carbon concentrations [10](#page=10) [9](#page=9). The tendency to form graphite is influenced by silicon content (greater than 1 wt%) and slower cooling rates during solidification. The microstructure and mechanical behavior depend on composition and heat treatment. The most common types of cast iron are gray, nodular (ductile), white, malleable, and compacted graphite [10](#page=10). #### 1.3.1 Gray iron Gray cast irons typically contain 2.5 to 4.0 wt% C and 1.0 to 3.0 wt% Si. Graphite exists as flakes, usually surrounded by an α-ferrite or pearlite matrix. The fractured surface has a gray appearance due to these flakes [10](#page=10). Mechanically, gray iron is relatively weak and brittle in tension due to stress concentration at the sharp flake tips. However, it exhibits higher strength and ductility under compression. Gray iron is excellent at damping vibrational energy, making it suitable for machine bases and heavy equipment exposed to vibrations. It also offers high wear resistance, good fluidity for intricate casting shapes, low casting shrinkage, and is among the least expensive metallic materials [10](#page=10). > **Example:** Machine tool bases and engine blocks are often made from gray cast iron due to its excellent vibration damping capabilities and low cost [10](#page=10). #### 1.3.2 Ductile (or nodular) iron Adding a small amount of magnesium and/or cerium to gray iron before casting results in graphite forming as nodules or spherelike particles instead of flakes. This alloy is known as ductile or nodular iron. The matrix is typically pearlite in the as-cast state or can be made into ferrite through heat treatment. Ductile iron exhibits higher strength and significantly greater ductility than gray iron, with mechanical properties approaching those of steel. Ferritic ductile irons have tensile strengths between 380 and 480 MPa and ductilities of 10% to 20% EL. Applications include valves, pump bodies, crankshafts, gears, and other automotive and machine components [13](#page=13). #### 1.3.3 White iron and malleable iron * **White cast iron:** Produced in low-silicon cast irons (less than 1.0 wt% Si) with rapid cooling rates, where most carbon exists as cementite (Fe₃C) instead of graphite. It is extremely hard and brittle, making it difficult to machine, but is used for wear-resistant surfaces like rollers in rolling mills. White iron is often an intermediate product in the production of malleable iron [13](#page=13). * **Malleable iron:** Produced by heating white iron at 800°C to 900°C for extended periods in a neutral atmosphere. This process causes cementite decomposition, forming graphite in clusters or rosettes surrounded by a ferrite or pearlite matrix. Malleable iron offers relatively high strength and appreciable ductility, with applications in automotive connecting rods, gears, and fittings for railroad and marine services [13](#page=13). #### 1.3.4 Compacted graphite iron (CGI) CGI is a more recent addition to cast iron types. Graphite in CGI has a wormlike or vermicular shape, intermediate between gray iron flakes and ductile iron nodules. Silicon content ranges from 1.7 to 3.0 wt%, and carbon content is typically 3.1 to 4.0 wt%. CGI offers a balance of properties, making it suitable for applications like diesel engine blocks, exhaust manifolds, and brake discs for high-speed trains [12](#page=12) [13](#page=13) [14](#page=14). --- # Nonferrous alloys: types and properties This section provides a comprehensive overview of nonferrous alloys, categorizing them by their base metals and discussing their compositions, properties, and applications. ### 2.1 Overview of nonferrous alloys Nonferrous alloys are utilized when ferrous alloys present limitations such as high density, low electrical conductivity, or susceptibility to corrosion. They are classified either by their base metal or by shared characteristics. Alloys can be further distinguished as cast (brittle, not amenable to deformation) or wrought (amenable to mechanical deformation). Heat-treatable alloys are those whose mechanical strength can be improved through precipitation hardening or martensitic transformation [15](#page=15). ### 2.2 Copper and its alloys Copper and its alloys have a long history of use due to their desirable physical properties, good corrosion resistance, and amenability to cold working. While unalloyed copper is soft and ductile, alloying enhances its mechanical and corrosion resistance properties. Most copper alloys cannot be strengthened by heat treatment; thus, cold working and solid-solution alloying are primary strengthening methods [16](#page=16). #### 2.2.1 Brasses Brasses are copper alloys where zinc is the main alloying element. The $\alpha$ phase, with an FCC structure, is stable up to approximately 35 wt% Zn and results in ductile, easily cold-worked alloys. Alloys with higher zinc content contain both $\alpha$ and $\beta'$ phases; the $\beta'$ phase (ordered BCC) is harder and stronger, making these alloys suitable for hot working. Common brasses include yellow brass, naval brass, cartridge brass, Muntz metal, and gilding metal. Applications include costume jewelry, cartridge casings, automotive radiators, musical instruments, and coins [16](#page=16). #### 2.2.2 Bronzes Bronzes are copper alloys with tin, aluminum, silicon, or nickel as primary alloying elements. They are generally stronger than brasses while maintaining high corrosion resistance. Bronzes are typically used when good tensile properties are required in addition to corrosion resistance [16](#page=16). #### 2.2.3 Beryllium coppers Beryllium coppers are among the most common heat-treatable copper alloys. They offer exceptional tensile strengths (up to 1400 MPa), excellent electrical and corrosion properties, and wear resistance. High strengths are achieved through precipitation hardening heat treatments. Due to beryllium content, these alloys are costly. Applications include springs, bellows, firing pins, bushings, valves, and diaphragms [16](#page=16) [17](#page=17). > **Tip:** Heat-treatable copper alloys, like beryllium coppers, leverage precipitation hardening to achieve high strengths, making them suitable for demanding applications like springs and high-stress components. ### 2.3 Aluminum and its alloys Aluminum alloys are distinguished by their low density (2.7 g/cm$^3$), high electrical and thermal conductivities, and good corrosion resistance in ambient atmospheres. Their FCC structure provides ductility even at low temperatures. A major limitation is their low melting temperature (660°C), restricting high-temperature applications [18](#page=18). Mechanical strength can be improved by cold work and alloying, though these processes can reduce corrosion resistance. Principal alloying elements include copper, magnesium, silicon, manganese, and zinc. Non-heat-treatable alloys rely on solid-solution strengthening, while others are heat-treatable through precipitation hardening, often involving intermetallic compound formation like MgZn$_2$ [18](#page=18). Aluminum alloys are classified as cast or wrought, with composition and temper designations indicated by a four-digit numbering system and a temper designation (e.g., F, O, H, W, T). Applications span aircraft structural parts, beverage cans, bus bodies, and automotive components. Aluminum alloys are also crucial for reducing fuel consumption in transportation due to their high specific strength (tensile strength-to-specific gravity ratio). Newer aluminum-lithium alloys offer even lower densities, high specific moduli, and excellent fatigue and low-temperature toughness, though at a higher manufacturing cost [18](#page=18). > **Example:** Aluminum alloys are particularly valuable in aerospace and automotive industries for their lightweight properties, contributing significantly to fuel efficiency. ### 2.4 Magnesium and its alloys Magnesium alloys are notable for their extremely low density (1.7 g/cm$^3$), making them ideal for applications where light weight is paramount. Magnesium has an HCP crystal structure and a low elastic modulus (45 GPa). At room temperature, they are difficult to deform, necessitating casting or hot working between 200°C and 350°C. Magnesium alloys have moderately low melting points (651°C). They are susceptible to corrosion in marine environments but exhibit reasonable resistance in normal atmospheres, possibly due to impurities. Fine magnesium powder is pyrophoric [19](#page=19). Alloying elements include aluminum, zinc, manganese, and rare earths. Magnesium alloys are classified as cast or wrought, with a designation system similar to aluminum. Applications include aircraft and missile components, luggage, handheld devices, automotive parts (steering wheels, seat frames), and electronic equipment. Magnesium alloys are increasingly replacing engineering plastics due to their superior stiffness, recyclability, and lower cost for comparable densities [19](#page=19) [21](#page=21). ### 2.5 Titanium and its alloys Titanium and its alloys are relatively new engineering materials with an exceptional property combination. They possess a low density (4.5 g/cm$^3$), a high melting point (1668°C), and high tensile strengths (up to 1400 MPa), resulting in remarkable specific strengths. Titanium alloys are also highly ductile and forgeable [22](#page=22). Titanium exists as an $\alpha$ phase (HCP) at room temperature, transforming to a $\beta$ phase (BCC) at 883°C. Alloying elements influence this transformation temperature; $\beta$-phase stabilizers like V, Nb, and Mo promote the $\beta$ phase at room temperature. Titanium alloys are categorized into four types based on phase composition: $\alpha$, $\beta$, $\alpha + \beta$, and near $\alpha$ [22](#page=22). * **$\alpha$-titanium alloys:** Often alloyed with Al and Sn, they excel in high-temperature applications due to superior creep resistance. They cannot be strengthened by heat treatment [22](#page=22). * **$\beta$ titanium alloys:** Contain sufficient $\beta$-stabilizing elements to retain the metastable $\beta$ phase at room temperature after rapid cooling. They are highly forgeable and have high fracture toughness [22](#page=22). * **$\alpha + \beta$ materials:** Alloyed with stabilizers for both phases, their strength can be controlled by heat treatment, leading to diverse microstructures [22](#page=22). * **Near $\alpha$ alloys:** Contain a small proportion of $\beta$ phase, offering a greater diversity of microstructures and properties than pure $\alpha$ materials [22](#page=22). The primary limitation of titanium is its chemical reactivity at elevated temperatures, leading to expensive refining and melting processes. Despite this, titanium alloys exhibit outstanding corrosion resistance in various environments, including air, seawater, and industrial settings. They are also highly biocompatible, making them suitable for dental and orthopedic implants. Common applications include airplane structures, space vehicles, and the petroleum and chemical industries [22](#page=22) [23](#page=23). ### 2.6 Refractory metals Refractory metals are characterized by their extremely high melting temperatures, ranging from 2468°C (niobium) to 3410°C (tungsten). This high melting point is attributed to strong interatomic bonding, which also contributes to their large elastic moduli, high strengths, and hardness at both ambient and elevated temperatures. Examples include niobium (Nb), molybdenum (Mo), tungsten (W), and tantalum (Ta). Tantalum and molybdenum are used to enhance the corrosion resistance of stainless steel. Molybdenum alloys are used in extrusion dies and aerospace structures, while tungsten alloys are found in light filaments and X-ray tubes. Tantalum's immunity to chemical attack below 150°C makes it ideal for highly corrosive applications [23](#page=23) [24](#page=24). ### 2.7 Superalloys Superalloys possess superior combinations of properties, primarily used in aircraft turbine components that endure high temperatures and oxidizing environments. Density is a critical factor, as lower density reduces centrifugal stresses in rotating parts. Superalloys are classified by their predominant base metal: iron-nickel, nickel, or cobalt. Other alloying elements include refractory metals, chromium, and titanium. They can be wrought or cast. Applications extend to nuclear reactors and petrochemical equipment [24](#page=24). ### 2.8 Noble metals The noble or precious metals are a group of eight elements known for their expense and superior properties, including softness, ductility, and oxidation resistance. These include silver, gold, platinum, palladium, rhodium, ruthenium, iridium, and osmium. Silver and gold can be strengthened by solid-solution alloying with copper (e.g., sterling silver is 7.5 wt% Cu). They are used in jewelry, dental restorations, and electrical contacts. Platinum finds use in chemical laboratory equipment, as a catalyst, and in thermocouples [25](#page=25). ### 2.9 Miscellaneous nonferrous alloys #### 2.9.1 Nickel and its alloys Nickel alloys offer high resistance to corrosion, particularly in basic environments. Nickel plating is used as a protective measure on corrosion-susceptible metals. Monel, a nickel-copper alloy (approximately 65 wt% Ni, 28 wt% Cu), exhibits high strength and excellent corrosion resistance in acidic and petroleum solutions. Nickel is also a key component in stainless steels and superalloys [25](#page=25). #### 2.9.2 Lead, tin, and their alloys Lead and tin alloys are mechanically soft and weak, with low melting temperatures and recrystallization temperatures below room temperature. They offer good resistance to many corrosive environments. Lead-tin alloys are used as solders due to their low melting points. Lead alloys are found in X-ray shields and storage batteries. Tin is primarily used as a coating for steel cans, preventing chemical reactions between the steel and food products [25](#page=25). #### 2.9.3 Zinc and its alloys Unalloyed zinc is soft, has a low melting temperature, and a subambient recrystallization temperature. It is susceptible to corrosion in many common environments. Galvanized steel, coated with zinc, utilizes the sacrificial corrosion of zinc to protect the steel. Zinc alloys are used in padlocks, plumbing fixtures, automotive parts, and office equipment [25](#page=25). #### 2.9.4 Zirconium and its alloys Zirconium and its alloys are ductile and possess mechanical properties comparable to titanium alloys and austenitic stainless steels. Their primary advantage is exceptional corrosion resistance, even in superheated water. Zirconium's transparency to thermal neutrons makes its alloys suitable for cladding uranium fuel in nuclear reactors. They are also cost-effective materials for heat exchangers, reactor vessels, and piping in the chemical-processing and nuclear industries [25](#page=25). --- # Metal fabrication techniques: forming, casting, and welding This section explores fundamental metal fabrication techniques, focusing on shaping metal through forming, liquid state processes like casting, and joining methods such as welding, alongside advancements like 3D printing [27](#page=27). ### 3.1 Overview of metal fabrication Metal fabrication encompasses processes that transform refined, alloyed, and heat-treated metals into finished products. The selection of fabrication methods is governed by the metal's properties, the desired final shape and size, and cost-effectiveness. These techniques are broadly categorized into forming operations, casting, powder metallurgy, welding, machining, and 3D printing, often requiring a combination of methods for completion [27](#page=27). ### 3.2 Forming operations Forming operations reshape metal through plastic deformation, requiring external forces that exceed the material's yield strength. Most metals are ductile enough for these processes, allowing permanent deformation without fracture [27](#page=27). #### 3.2.1 Hot working vs. cold working * **Hot working** involves deformation above the recrystallization temperature [27](#page=27). * **Advantages:** Allows for large deformations, successive repetitions due to sustained softness and ductility, and requires less energy [27](#page=27). * **Disadvantages:** Prone to surface oxidation, leading to material loss and a poor surface finish [27](#page=27). * **Cold working** is deformation below the recrystallization temperature [27](#page=27). * **Advantages:** Results in a higher quality surface finish, superior and a wider range of mechanical properties due to strain hardening, and closer dimensional control [27](#page=27). * **Disadvantages:** Increases strength at the expense of ductility [27](#page=27). * A combination of cold working and process annealing (heating to soften) can be used for extensive deformation, though it is costly and inconvenient [27](#page=27). #### 3.2.2 Specific forming techniques * **Forging**: Involves mechanically deforming a metal piece, usually hot, through successive blows or continuous squeezing [28](#page=28). * **Closed-die forging**: Metal is deformed within the cavity of two or more die halves that possess the final shape [28](#page=28). * **Open-die forging**: Uses dies with simple geometric shapes on large workpieces [28](#page=28). * **Characteristics**: Forged articles exhibit excellent grain structures and superior mechanical properties, exemplified by wrenches, crankshafts, and connecting rods [28](#page=28). * **Rolling**: The most common deformation process, where metal passes between two rolls, reducing thickness via compressive stress [28](#page=28). * **Applications**: Cold rolling produces sheet, strip, and foil with high-quality finishes. Grooved rolls are used for circular shapes, I-beams, and railroad rails [28](#page=28). * **Extrusion**: A metal bar is forced through a die orifice by a ram, creating a shape with a reduced cross-sectional area [28](#page=28). * **Products**: Used for rods and tubing with complex cross-sections, including seamless tubing [28](#page=28). * **Drawing**: A metal piece is pulled through a tapered die by a tensile force, reducing its cross-section and increasing its length [28](#page=28). * **Applications**: Commonly used for producing rod, wire, and tubing, potentially involving a series of dies [28](#page=28). ### 3.3 Casting Casting involves pouring molten metal into a mold cavity of a desired shape, which the metal assumes upon solidification, though some shrinkage occurs. Casting is employed when [29](#page=29): 1. The final shape is too large or complex for other methods [29](#page=29). 2. The alloy has low ductility, making forming difficult [29](#page=29). 3. Casting is the most economical option compared to other processes [29](#page=29). #### 3.3.1 Common casting techniques * **Sand casting**: Uses ordinary sand as the mold material, formed around a pattern [29](#page=29). * **Characteristics**: Features a gating system for molten metal flow and defect minimization. Common for automotive cylinder blocks, fire hydrants, and large pipe fittings [29](#page=29). * **Die casting**: Liquid metal is injected into a permanent steel die under pressure and at high velocity [29](#page=29). * **Characteristics**: Enables rapid casting rates and is cost-effective for thousands of castings with a single die set. Best suited for smaller parts and low-melting-point alloys like zinc, aluminum, and magnesium [29](#page=29). * **Investment casting (lost-wax casting)**: Uses a wax or plastic pattern that is melted and burned out after a ceramic slurry mold is formed around it [29](#page=29). * **Applications**: Ideal for high dimensional accuracy, fine detail reproduction, and excellent finishes, such as in jewelry, dental crowns, gas turbine blades, and jet engine impellers [29](#page=29). * **Lost-foam casting (expendable pattern casting)**: Employs a foam pattern that vaporizes when molten metal is poured into the sand-packed mold [30](#page=30). * **Advantages**: Allows for complex geometries and tight tolerances, is simpler, quicker, and less expensive than sand casting, and generates less waste [30](#page=30). * **Common materials and applications**: Used for cast irons and aluminum alloys in automotive engine blocks, cylinder heads, and electric motor frames [30](#page=30). * **Continuous casting (strand casting)**: Molten metal is cast directly into a continuous strand with a rectangular or circular cross-section in a water-cooled die [30](#page=30). * **Advantages**: Produces more uniform chemical composition and mechanical properties compared to ingot casting. It is highly automated and efficient [30](#page=30). ### 3.4 Powder metallurgy (P/M) Powder metallurgy involves compacting metal powder followed by heat treatment to create a dense piece [30](#page=30). * **Advantages**: Produces virtually nonporous parts with properties approaching those of fully dense materials. Expedites fabrication of metals with high melting temperatures and allows for economical production of parts with close dimensional tolerances, such as bushings and gears [30](#page=30). * **Suitability**: Particularly useful for metals with low ductility, requiring minimal plastic deformation of powder particles [30](#page=30). ### 3.5 Welding Welding joins two or more metal parts to form a single piece when separate fabrication is impractical or expensive. The bond is metallurgical, involving diffusion, rather than purely mechanical [31](#page=31). #### 3.5.1 Arc and gas welding These methods involve heating the workpieces and filler material (welding rod) to melting temperatures, forming a fusion joint upon solidification [31](#page=31). * **Heat-affected zone (HAZ)**: The region adjacent to the weld may undergo microstructural and property alterations, including: 1. Recrystallization and grain growth in cold-worked materials, leading to reduced strength and toughness [31](#page=31). 2. Formation of residual stresses that can weaken the joint [31](#page=31). 3. Formation of undesirable brittle phases like martensite in steels upon rapid cooling [31](#page=31). 4. Sensitization in some stainless steels, leading to intergranular corrosion [31](#page=31). #### 3.5.2 Laser beam welding A highly focused laser beam serves as the heat source, melting the parent metal to create a fusion joint, often without filler material [31](#page=31). * **Advantages**: Non-contact process, minimal distortion, rapid and automated, low energy input resulting in a minimal HAZ, precise and small welds, versatility in joining various metals, and porosity-free welds with high strength [31](#page=31). * **Applications**: Widely used in automotive and electronics industries for high quality and speed [31](#page=31). ### 3.6 3D printing (Additive Manufacturing) Additive manufacturing (AM), or 3D printing, creates functional objects by incrementally adding raw material, often layer by layer, from computer-aided design (CAD) data. This contrasts with subtractive manufacturing (e.g., machining) [32](#page=32). * **Advantages**: Enables creation of complex shapes, cost-effective fabrication of customized, one-of-a-kind products with short lead times, minimal waste, and easy design modifications without retooling [32](#page=32). * **Disadvantages**: Higher costs for large production runs, limited material availability, fewer color and finish choices (though increasing), and often inferior mechanical properties compared to traditional methods. Property and dimensional reproducibility can also be issues [32](#page=32). #### 3.6.1 3D printing process flow 1. Generate a digital 3D model using CAD software, 3D scanner, or photogrammetry [33](#page=33). 2. Convert the model to a file format (e.g., STL) defining surface geometry [33](#page=33). 3. Use "slicer" software to divide the model into horizontal layers and generate tool paths [33](#page=33). 4. The printer creates the physical object layer by layer [33](#page=33). 5. End-part finishing (e.g., sanding, painting) may be necessary [33](#page=33). #### 3.6.2 3D printing of metallic materials Metallic materials typically use powder or wire feedstock with a laser or electron beam as the energy source [33](#page=33). * **Direct Energy Deposition (DED)**: A focused laser or electron beam melts metal powder or wire as it's deposited by a nozzle onto the workpiece surface, solidifying in layers [33](#page=33). * **Characteristics**: Similar to multipass welding, requiring control of homogeneity, microstructure, and porosity. Post-processing may be needed if control is insufficient [34](#page=34). * **Powder Bed Fusion (PBF)**: Utilizes a powder feedstock spread in thin layers onto a build platform [34](#page=34). * **Process**: A laser or electron beam selectively melts or sinters powder particles according to the CAD model's tool path, creating solid layers. Unmelted powder is recovered and reused [34](#page=34). * **Selective Laser Sintering (SLS)**: A PBF process using a laser where powder particles coalesce without melting [34](#page=34). * **Materials**: Pure gold, copper, titanium, tantalum, niobium, and alloys of aluminum, copper, cobalt, nickel, iron, and titanium [34](#page=34). * **Applications**: Concentrated in biomedical and aerospace industries, but expanding into automotive, architecture, medical, and dental sectors (#page=34, 35). Potential applications include fully 3D-printed automobiles, complex aircraft engine parts, architectural models, personalized medical devices, and even tissue engineering (bioprinting). Dental and shoe manufacturing also leverage this technology [34](#page=34) [35](#page=35) [36](#page=36). --- # Heat treatment processes Heat treatment processes are crucial thermal treatments applied to metals and alloys to modify their microstructure and, consequently, their mechanical properties. These processes typically involve three stages: heating to a specific temperature, holding at that temperature (soaking), and cooling [36](#page=36). ### 4.1 Annealing processes Annealing is a heat treatment involving heating a material to an elevated temperature for a prolonged period, followed by slow cooling. The primary goals of annealing are to relieve internal stresses, increase softness, ductility, and toughness, and to achieve a specific microstructure [36](#page=36). #### 4.1.1 Process annealing Process annealing is employed to counteract the effects of cold work, thereby softening and increasing the ductility of a strain-hardened metal. This is particularly useful during fabrication steps requiring significant plastic deformation, allowing for continued deformation without fracture or excessive energy expenditure. This process facilitates recovery and recrystallization, and it is typically performed at a temperature above the recrystallization temperature but low enough to avoid significant grain growth or oxidation [36](#page=36). #### 4.1.2 Stress relief Internal residual stresses can arise from plastic deformation (e.g., machining, grinding), non-uniform cooling after high-temperature processing (e.g., welding, casting), or phase transformations with differing densities between parent and product phases. These stresses can lead to distortion or cracking. Stress relief annealing involves heating the component to a relatively low temperature (to avoid affecting prior cold work or heat treatments), holding it to equalize temperature, and then cooling it in air [37](#page=37). #### 4.1.3 Annealing of ferrous alloys Specific annealing procedures are used for steel alloys, often referencing critical temperatures on the iron-iron carbide phase diagram [37](#page=37). * **Normalizing:** This process refines grains and promotes a more uniform grain size distribution in steels that have undergone plastic deformation. It involves heating the steel to at least 55°C above the upper critical temperature (A3 for hypoeutectectoid steels, Acm for hypereutectectoid steels) to form austenite (austenitizing), followed by cooling in air (#page=37, page=38). This results in tougher, fine-grained pearlitic steels compared to coarse-grained ones [37](#page=37) [38](#page=38). * **Full annealing:** This treatment is applied to low- and medium-carbon steels intended for machining or extensive plastic deformation. The steel is heated to approximately 50°C above the A3 or A1 line (depending on carbon content) to form austenite. Subsequently, it is cooled very slowly in the furnace, resulting in coarse pearlite and proeutectoid phases, yielding a relatively soft and ductile microstructure [38](#page=38). * **Spheroidizing:** Medium- and high-carbon steels that are still too hard for machining or deformation, even with coarse pearlite, can be spheroidized. This process converts cementite (Fe3C) into spherical particles, maximizing softness and ductility. Methods include heating just below the eutectoid temperature (around 700°C or 1300°F) for extended periods, heating above the eutectoid and cooling slowly, or alternating heating and cooling around the A1 line. Prior cold work can accelerate spheroidite formation [38](#page=38). ### 4.2 Heat treatment of steels Specialized heat treatments are applied to steels to achieve desired properties, primarily through controlling the formation of martensite and subsequent tempering. #### 4.2.1 Hardenability Hardenability describes a steel alloy's capacity to form martensite upon quenching for a given heat treatment. It is not the same as hardness (resistance to indentation) but rather indicates how quickly hardness decreases with depth into a specimen due to reduced martensite content. High hardenability means martensite forms extensively throughout the interior, not just at the surface [39](#page=39). ##### 4.2.1.1 The Jominy end-quench test The Jominy end-quench test is a standardized method for determining hardenability. A cylindrical specimen is austenitized and then quenched with a water jet at one end (#page=39, page=40). This creates a gradient of cooling rates along the specimen's length, with the fastest cooling at the quenched end. After cooling, hardness measurements are taken at intervals along the specimen, and the results are plotted as a hardenability curve showing hardness versus distance from the quenched end [39](#page=39) [40](#page=40). * **Hardenability curves:** These plots illustrate how hardness diminishes with decreasing cooling rate (increasing distance from the quenched end). A steel with high hardenability maintains high hardness for longer distances. Cooling rate at 700°C (1300°F) is often used as a parameter, correlating to distance from the quenched end. Alloy elements like nickel, chromium, and molybdenum increase hardenability by delaying the austenite-to-pearlite and bainite transformations, allowing more martensite to form. Carbon content primarily affects the maximum achievable hardness at the quenched end, while alloying elements influence the depth of hardening (#page=41, page=42). Hardenability can also be represented as a band to account for variations in composition and grain size [40](#page=40) [41](#page=41) [42](#page=42) [43](#page=43). #### 4.2.2 Influence of quenching medium, specimen size, and geometry The cooling rate achieved during quenching is influenced by: * **Quenching medium:** Water provides the most severe quench, followed by oil, and then air. Agitation of the medium also enhances heat removal. Aqueous polymer quenchants offer intermediate cooling rates [43](#page=43). * **Specimen size and geometry:** Heat must transfer from the interior to the surface before dissipation into the quenching medium. Larger diameters lead to slower cooling rates, especially at the center. The surface area-to-mass ratio is critical; shapes with higher ratios (e.g., irregular shapes with edges) cool faster and harden more deeply [44](#page=44) [45](#page=45). Hardenability curves, combined with information on cooling rates for various quenching conditions and specimen geometries, help predict hardness distribution and select appropriate steels and heat treatments for specific applications (#page=44, page=45). A minimum of 80% martensite is often required for high-stress applications, while 50% is sufficient for moderately stressed parts [44](#page=44) [45](#page=45). #### 4.2.3 Quenching and tempering Many steels are heat-treated by austenitizing, rapidly quenching to form martensite, and then tempering. Tempering involves reheating the quenched steel to an intermediate temperature to reduce brittleness and improve toughness, while retaining much of the hardness gained from quenching. The resulting properties are dependent on alloy composition, quenching medium, and specimen diameter (#page=47, page=48) [47](#page=47) [48](#page=48). ### 4.3 Precipitation hardening Precipitation hardening, also known as age hardening, is a process that enhances the strength and hardness of certain metal alloys by forming extremely small, uniformly dispersed particles of a second phase within the original phase matrix (#page=48, page=49). This is achieved through specific phase transformations induced by heat treatments. It is distinct from tempered martensite strengthening in steels, differing in the underlying hardening mechanisms [48](#page=48) [49](#page=49). #### 4.3.1 Heat treatments for precipitation hardening Precipitation hardening requires alloys that exhibit specific phase diagram characteristics: a significant maximum solubility of one component in another, and a solubility limit that drastically decreases with decreasing temperature (#page=50, page=51) [50](#page=50) [51](#page=51). * **Solution heat treating:** The alloy is heated into the single-phase solid solution region (e.g., the $\alpha$ phase field) to dissolve all solute atoms. This is followed by rapid cooling (quenching) to a low temperature to suppress diffusion and prevent the formation of the second phase, resulting in a supersaturated solid solution. The alloy in this state is typically relatively soft and weak [51](#page=51). * **Precipitation heat treating (aging):** The supersaturated solid solution is then heated to an intermediate temperature within the two-phase region. At this temperature, diffusion becomes appreciable, allowing the second phase ($\beta$ precipitate) to form as finely dispersed particles. The size, distribution, and character of these precipitate particles, which depend on aging temperature and time, determine the alloy's final strength and hardness (#page=51, page=52). Overaging occurs after prolonged aging times, leading to a reduction in strength and hardness as precipitate particles coarsen [51](#page=51) [52](#page=52). #### 4.3.2 Mechanism of hardening In precipitation hardening, the strengthening arises from the interaction of dislocations with the numerous fine precipitate particles (#page=52, page=53). These precipitates impede dislocation movement, thus increasing the alloy's yield strength and hardness. For example, in aluminum-copper alloys, initial hardening involves the formation of small clusters (zones), followed by transition phases ($\theta''$ and $\theta'$) before the equilibrium $\theta$ phase forms. Maximum strength is often achieved with the $\theta''$ phase. As precipitation temperature increases, the time required to reach peak strength decreases, but ductility tends to decrease with increasing strength [52](#page=52) [53](#page=53) [54](#page=54). --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Ferrous alloy | An alloy where iron is the primary constituent, including steels and cast irons. These alloys are known for their abundance, economic production, and versatility in mechanical and physical properties, though they can be susceptible to corrosion. | | Nonferrous alloy | An alloy that does not have iron as its principal constituent. This broad category includes alloys based on metals like copper, aluminum, magnesium, titanium, and others, each offering distinct property combinations for various applications. | | Steel | An iron-carbon alloy that typically contains less than 1.0 wt% carbon, often with other alloying elements. Steels are highly versatile and can be heat-treated to achieve a wide range of mechanical properties, making them crucial engineering materials. | | Cast iron | A class of ferrous alloys with carbon content generally above 2.14 wt%. Cast irons are easily melted and cast, and their properties depend heavily on the form of carbon present (graphite or cementite) and cooling rates during solidification. | | Gray cast iron | A type of cast iron where graphite exists in the form of flakes, giving a gray appearance to fractured surfaces. It is known for its good damping capacity and wear resistance but is relatively weak and brittle in tension due to stress concentration at flake tips. | | Ductile (nodular) iron | A type of cast iron where graphite is present in nodular or spherical shapes, achieved by adding magnesium and/or cerium. This form of graphite significantly improves strength and ductility compared to gray cast iron, with mechanical properties approaching those of steel. | | Malleable cast iron | A type of cast iron produced by heat-treating white cast iron. The process decomposes cementite into graphite in the form of rosettes or clusters, resulting in improved strength and ductility, suitable for applications like connecting rods and pipe fittings. | | Compacted graphite iron (CGI) | A cast iron with graphite in a wormlike or vermicular shape, intermediate between the flakes of gray iron and the nodules of ductile iron. CGIs offer a balance of properties, including higher thermal conductivity and better resistance to thermal shock than other cast irons. | | Brass | An alloy of copper with zinc as the primary alloying element. Brasses vary in properties based on zinc content, with alpha (𝛼) brasses being soft and ductile, and alpha-beta (𝛼 + 𝛽) brasses being harder and stronger, suitable for applications like cartridge casings and automotive radiators. | | Bronze | An alloy of copper with other elements such as tin, aluminum, silicon, or nickel. Bronzes are generally stronger than brasses while maintaining good corrosion resistance, making them suitable for bearings, gears, and steam fittings. | | Aluminum alloy | Alloys where aluminum is the base metal, characterized by low density, good electrical and thermal conductivity, and corrosion resistance. They can be strengthened by cold work or heat treatment (precipitation hardening) and are widely used in aerospace and automotive industries. | | Magnesium alloy | Alloys based on magnesium, known for their exceptionally low density, making them ideal for lightweight applications like aircraft components. They are typically processed by casting or hot working due to their limited cold deformability and susceptibility to corrosion in certain environments. | | Titanium alloy | Alloys with titanium as the base metal, offering a combination of high strength, low density, high melting point, and excellent corrosion resistance. Their properties are influenced by the presence of alpha (𝛼) and beta (𝛽) phases, leading to classifications like 𝛼, 𝛽, and 𝛼 + 𝛽 alloys. | | Refractory metals | Metals characterized by extremely high melting temperatures, including niobium (Nb), molybdenum (Mo), tungsten (W), and tantalum (Ta). Their strong interatomic bonding results in high strength, hardness, and elastic moduli, making them suitable for high-temperature applications. | | Superalloys | Alloys designed to withstand extreme conditions, particularly high temperatures and oxidizing environments, commonly used in aircraft turbine components. They are based on iron, nickel, or cobalt and possess excellent mechanical integrity under severe operating conditions. | | Noble metals | A group of precious metals known for their softness, ductility, and superior oxidation resistance. This includes silver, gold, platinum, palladium, and others, frequently used in jewelry, dental restorations, and catalytic applications. | | Hot working | A metal forming process where deformation occurs at a temperature above the recrystallization temperature. This allows for large deformations, requires less energy, and results in a fine-grained, ductile product, though surface oxidation can be a concern. | | Cold working | A metal forming process where deformation occurs at temperatures below the recrystallization temperature. It leads to increased strength and hardness due to strain hardening but reduces ductility. Advantages include better surface finish and dimensional control. | | Forging | A forming operation that shapes metal by applying localized compressive forces, typically through repeated blows or continuous squeezing. It results in excellent grain structure and mechanical properties, used for components like wrenches and crankshafts. | | Rolling | A deformation process where a metal piece is passed between two rotating rolls to reduce its thickness. It is widely used for producing sheet, strip, foil, and various structural shapes like I-beams and rails. | | Extrusion | A forming process where a metal billet is forced through a die orifice by a ram. This creates a continuous profile with a reduced cross-sectional area, suitable for producing rods, tubing, and complex shapes. | | Drawing | A forming process where a metal piece is pulled through a tapered die by a tensile force. This reduces the cross-sectional area and increases the length, commonly used for producing wire, rods, and tubing. | | Casting | A fabrication process where molten metal is poured into a mold cavity of a desired shape and allowed to solidify. It is used for complex shapes, alloys with low ductility, or when casting is more economical than other methods. | | Sand casting | A casting technique using ordinary sand as the mold material. It is a versatile and widely used method for producing large and complex parts such as engine blocks and pipe fittings. | | Die casting | A casting process where molten metal is injected into a permanent metal mold (die) under high pressure. This method allows for rapid production and intricate details, typically used for low-melting-point alloys like aluminum and zinc. | | Investment casting (Lost-wax casting) | A casting process where a wax or plastic pattern is coated with a ceramic slurry to form a mold. After the pattern is melted out, molten metal is poured in, producing castings with high dimensional accuracy and fine detail, often used for jewelry and turbine blades. | | Lost-foam casting | A variation of investment casting that uses a foam pattern which vaporizes when molten metal is poured into the mold. This process is simpler, quicker, and less expensive than traditional investment casting, suitable for complex geometries. | | Continuous casting (Strand casting) | A process where molten metal is continuously solidified into a strand with a specific cross-section. This method is efficient and produces metal with more uniform composition and properties compared to ingot casting. | | Powder metallurgy (P/M) | A fabrication technique involving the compaction of metal powder into a desired shape, followed by a heat treatment to achieve densification. It is effective for producing parts from metals with low ductility or high melting points and for achieving tight dimensional tolerances. | | Welding | A joining process where two or more metal parts are fused together to form a single piece, typically by melting the base metals and often using a filler material. The heat-affected zone (HAZ) adjacent to the weld can experience microstructural and property changes. | | 3D printing (Additive Manufacturing) | A fabrication technology where functional objects are created by incrementally adding material, often in a layer-like fashion, from digital design data. It allows for complex geometries, customization, and reduced material waste. | | Direct energy deposition (DED) | A 3D printing technique where a focused energy source (laser or electron beam) melts metal powder or wire as it is deposited onto a surface, building up a part layer by layer. It is analogous to multipass welding. | | Powder bed fusion (PBF) | A 3D printing technique where a layer of powder is selectively melted or sintered by an energy source (laser or electron beam). The process is repeated layer by layer to build the final object, with unmelted powder being recovered. | | Annealing | A heat treatment involving heating a material to an elevated temperature for a specific duration, followed by slow cooling. It is used to relieve stresses, increase softness, ductility, and toughness, or to produce a specific microstructure. | | Process annealing | An annealing heat treatment used to soften and increase the ductility of a previously strain-hardened metal. It allows for further plastic deformation during fabrication processes by promoting recovery and recrystallization. | | Stress relief | A heat treatment performed to eliminate internal residual stresses within a metal piece. It involves heating to a moderate temperature and slow cooling, preventing distortion or warpage caused by stored stresses from processes like machining or welding. | | Normalizing | A heat treatment for steels that involves heating above the upper critical temperature (austenitizing) and then cooling in air. It refines the grain structure, producing a more uniform and desirable distribution of pearlite, enhancing toughness. | | Full annealing | A heat treatment for low- and medium-carbon steels where the material is heated above the A3 or A1 line, held to form austenite, and then furnace cooled slowly. This results in a coarse pearlite microstructure, yielding maximum softness and ductility for machining or deformation. | | Spheroidizing | A heat treatment that converts the carbide phase in medium- and high-carbon steels into spherical particles (spheroids) within a ferrite matrix. This process maximizes softness and ductility, making the steel easier to machine or deform. | | Hardenability | A measure of the depth to which a steel alloy can be hardened by the formation of martensite upon quenching. It is influenced by alloy composition and dictates how effectively the material can achieve a desired hardness throughout its cross-section. | | Jominy end-quench test | A standardized test used to determine the hardenability of steel. A specimen is quenched from one end, creating a gradient of cooling rates, and hardness measurements are taken along its length to generate a hardenability curve. | | Precipitation hardening | A heat treatment process used to strengthen certain alloys by forming extremely small, uniformly dispersed particles of a second phase (precipitates) within the matrix. This impedes dislocation movement, increasing strength and hardness. | | Solution heat treatment | The first step in precipitation hardening, where the alloy is heated to dissolve all solute atoms into a single-phase solid solution. Rapid cooling (quenching) then preserves this supersaturated state at a lower temperature. | | Precipitation heat treatment | The second step in precipitation hardening, where the supersaturated solid solution is heated to an intermediate temperature to allow fine precipitate particles to form and grow over time. This controlled aging process leads to increased strength and hardness. | | Overaging | The stage in precipitation hardening where prolonged aging at an elevated temperature causes the precipitate particles to grow too large, leading to a decrease in strength and hardness. | | Temper designation | A code used for aluminum alloys to indicate the condition of the material, which is achieved through specific heat treatments or strain hardening. Examples include F (as-fabricated), O (annealed), H (strain-hardened), W (solution heat-treated), and T (solution heat-treated and stabilized). | | Specific strength | A material property defined as the ratio of tensile strength to specific gravity. It is an important consideration for lightweight applications, such as in the transportation industry, where high strength-to-weight ratios are desirable. | | Weld metal | The region within a fusion weld that consists of melted and re-solidified base metal and any filler metal. Its composition and microstructure can differ from the base metal. | | Heat-affected zone (HAZ) | The region adjacent to the weld metal in a welded joint that has not been melted but has undergone microstructural or property changes due to the heat of welding. | | Austenitizing | The process of heating a ferrous alloy to a temperature at which it transforms into austenite. This is a critical step in many heat treatments for steels, such as hardening. | | Martensite | A very hard and brittle phase formed in steels by rapid cooling (quenching) from the austenite phase. Its formation is dependent on the cooling rate and alloy composition. | | Pearlite | A lamellar microstructure in steels consisting of alternating layers of ferrite and cementite. It is typically formed upon slow cooling of austenite and is relatively soft and ductile. | | Bainite | A microstructure in steels that forms at cooling rates intermediate between those that produce pearlite and martensite. It consists of fine carbide precipitates dispersed within a ferrite matrix and offers a good combination of strength and toughness. | | Cementite | A hard, brittle intermetallic compound of iron and carbon ($Fe_3C$), with a fixed composition of 6.7 wt% C. It is a key constituent in many steels and cast irons. | | Ferrite | A solid solution of carbon in alpha-iron (BCC iron structure). It is relatively soft and ductile and is a primary constituent in many steels and cast irons. | | Austenite | A solid solution of carbon in gamma-iron (FCC iron structure). It exists at elevated temperatures in iron-carbon alloys and is the phase from which martensite, pearlite, or bainite can form upon cooling. | | Alloy steel | Steels to which specific amounts of alloying elements (other than carbon) are added to improve properties such as hardenability, strength, toughness, or corrosion resistance. Examples include chromium, nickel, molybdenum, and vanadium steels. | | Plain carbon steel | Steels containing carbon as the primary alloying element, with only small residual amounts of other impurities like manganese, silicon, phosphorus, and sulfur. Their properties are largely determined by carbon content and heat treatment. | | High-strength, low-alloy (HSLA) steel | Low-carbon steels with small additions of alloying elements (e.g., copper, vanadium, niobium) to enhance strength and toughness without significantly sacrificing ductility or weldability. | | Tool steel | High-carbon steels, often alloyed with elements like chromium, tungsten, and vanadium, designed for hardness, wear resistance, and ability to hold a sharp cutting edge. Used for cutting tools, dies, and molds. | | Stainless steel | Steels containing a minimum of 11 wt% chromium, providing excellent resistance to corrosion and oxidation. They are classified by their microstructure: ferritic, austenitic, or martensitic. | | Lower critical temperature ($A_1$) | The temperature below which austenite transforms into ferrite and cementite under equilibrium conditions in the iron-carbon phase diagram. | | Upper critical temperature ($A_3$ or $A_{cm}$) | The temperature above which austenite is the sole phase present in hypoeutectoid ($A_3$) or hypereutectoid ($A_{cm}$) steels, respectively. | | Natural aging | Precipitation hardening that occurs spontaneously at room temperature over time, often observed in certain aluminum alloys. | | Artificial aging | Precipitation hardening performed at elevated temperatures to accelerate the formation and growth of precipitate particles. |