Machine Design Reviewer Notes

Uses of Alloy Steels

AISI 2330: Bolts, studs, tubing subjected to torsional stresses.
AISI 2340: Quenched and tempered shafting connecting rods, very highly stressed bolts, forgings.
AISI 2350: High capacity gears, shafts, heavy duty machine parts.
AISI 3130: Shafts, bolts, steering knuckles.
AISI 3140: Aircraft and truck engine crankshafts, oil well tool joints, spline shafts, axles, earth moving equipment.
AISI 3150: Wear resisting parts in excavating and farm machinery, gears, forgings.
AISI 3240: Shafts, highly stressed pins and keys, gears.
AISI 3300 series: For heavy parts requiring deep penetration of the heat treatment and high fatigue strength per unit weight.
AISI 4063: Leaf and coil springs.
AISI 4130, 4140: Automotive connecting rods and axles, aircraft parts and tubing.
AISI 4340: Crankshafts, axles, gears, landing gear parts; perhaps the best general purpose AISI steel.
AISI 4640: Gears, splined shafts, hand tools, miscellaneous heavy duty machine parts.
AISI 8630: Connecting rods, bolts, shapes; air hardens after welding.
AISI 8640, 8740: Gears, propeller shafts, knuckles, shapes.

Definitions

Age Hardening: A change in a metal by which its structure recovers from an unstable or metastable condition produced by quenching or cold working.
Alloy: A substance with metallic properties, composed of two or more elements, at least one of which is a metal.
Alloying Elements: In steel, usually metallic elements added to modify properties.
Anisotropy: Exhibiting different properties when tested in different directions (e.g., tensile strength "with the grain" or "across the grain").
Brittleness: The tendency to fracture without appreciable deformation.
Charpy Test: A specimen supported at both ends as a simple beam is broken by the impact of a falling pendulum.
Cold Shortness: Brittleness of metals at ordinary or low temperatures.
Cold Working: Deforming a metal plastically at a temperature below the recrystallization temperature and at a rate to produce strain hardening.
Damping Capacity: The ability of a material to absorb or damp vibrations, absorbing kinetic energy of vibration owing to hysteresis.
Decarburization: Loss of carbon from the surface of steel during hot rolling, forging, and heat treating, when the surrounding medium reacts with the carbon.
Ductility: Property that permits permanent deformation before fracture in tension.
Elasticity: Ability of a material to be deformed and return to its original shape.
Embrittlement: Loss of ductility because of a physical or chemical change of the material.
Free Carbon: Part of the carbon content of steel or iron that is in the form of graphite or temper carbon.
Hard Drawn: A temper produced in a wire, rod, or tube by cold drawing.
Homogenous Materials: Have the same structure at all points.
Isotropic Materials: Have the same properties in all directions.
Izod Test: A specimen, supported at one end as a cantilever beam, is broken by the impact of a falling pendulum.
Killed Steel: Steel deoxidized with a strong deoxidizing agent (e.g., silicon or aluminum) to eliminate a reaction between carbon and oxygen during solidification.
Machinability: A somewhat indefinite property that refers to the relative ease with which a material can be cut.
Malleability: A material's susceptibility to extreme deformation in rolling or hammering.
Mechanical Properties: Properties that have to do with stress and strain.
Percentage Elongation: The extension in the vicinity of the fracture of a tensile specimen, expressed as a percentage of the original gage length.
Percentage Reduction Area: The smallest area at the point of rupture of a tensile specimen divided by the original area.
Physical Properties: Exclude mechanical properties, including density, conductivity, coefficient of thermal expansion.
Plasticity: Ability of a metal to be deformed considerably without rupture.
Poisson's Ratio: The ratio of the lateral strain to the longitudinal strain when the element is loaded with longitudinal tensile force.
$Poisson's\ Ratio = \frac{Lateral\ Strain}{Longitudinal\ Strain}$
Precipitation Heat Treatment: Bringing about the precipitation of a constituent from a supersaturated solid solution by holding the body at an elevated temperature; also called artificial aging.
Proof Stress: Stress that causes a specified permanent deformation of a material, usually 0.01% or less.
Red Shortness: Brittleness in steel when it is red hot.
Relaxation: Associated with creep, is the decreasing stress at a constant strain; important for metals in high temperature service.
Residual Stress: Stresses not due to applied loads or temperature gradients.
Rimmed Steel: Incompletely deoxidized steel.
Solution Heat Treatment: The process of holding an alloy at a suitably high temperature long enough to permit one or more constituents to pass into solid solution and then cooling fast enough to hold the constituents as a supersaturated solution.
Stiffness: The ability to resist deformation.
Strain Hardening: Increasing the hardness and strength by plastic deformation at temperatures lower than the recrystallization range.
Temper: A condition produced in a non-ferrous metal by mechanical or thermal treatment.
Toughness: The capacity of a material to withstand a shock load without breaking.
Transverse Strength: Refers to the results of a transverse bend test, the specimen being mounted as a simple beam; also called rupture modulus.
Work Hardening: Same as strain hardening.
Wrought Steel: Steel that has been hammered, rolled, or drawn in the process of manufacture; it may be plain carbon or alloy steel.

Corrosion

Corrosion: The natural deterioration of a metal in which metallic atoms leave the metal or form compounds in the presence of water or gases.
General Corrosion: May be minimized by the use of corrosion-resistant materials and the addition of protective coatings and liners.
Galvanic Corrosion: Occurs when dissimilar metals exist at different electrical potentials in the presence of an electrolyte. It may be reduced by careful design and selection of materials regarding dissimilar metals and the use of sacrificial anodes.
Localized Corrosion: Can be especially damaging in the presence of other destructive forces such as stress, fatigue, and other forms of chemical attack.
Stress-Corrosion Cracking (SCC): Occurs at grain boundaries under tensile stress. It propagates as stress opens cracks that are subject to corrosion, ultimately weakening the metal until failure.
- Effective means of reducing SCC:
  1. Proper design
  2. Reducing stress
  3. Removing corrosive agents
  4. Avoiding areas of chloride and hydroxide ion concentration.
Chloride Stress Corrosion: Occurs in austenitic stainless steels under tensile stress in the presence of oxygen, chloride ions, and high temperature. It is controlled by the removal of oxygen and chloride ions in the environment and the use of low carbon steels.

Important Properties

Brittleness: Tendency to fracture without appreciable deformation.
Ductility: Property that permits permanent deformation before fracture in tension.
Elasticity: Ability of a material to be deformed and return to its original shape.
Hardness: Resistance to indentation.
Machinability: Relative ease with which a material can be cut.
Malleability: Susceptibility to extreme deformation in rolling and hammering.
Plasticity: Ability of a metal to be deformed considerably without rupture.
Stiffness: Ability to resist deformation.
Toughness: Ability to withstand shock load without breaking.

Important Facts about Engineering Materials

Steel is the most prevalent engineering metal due to the abundance of iron ore, low cost, simplicity of production, and high performance.
Metals are the most commonly used materials in engineering design.
Metallurgy: The study that encompasses the procurement and production of metals.
Extractive Metallurgy: The study that covers the refinement of pure metals from their ores.
Iron (Fe) is obtained from oxides ( $Fe<em>2O</em>3$ ).
Gangue: The earth and stone mixed with the iron oxides.
Blast Furnace: The place in which the process to reduce iron oxides to pure iron is carried out.
Coke: Coal that has been previously burned.
Bessemer and oxygen processes are used to reduce the carbon content and purify the iron.

Classifications of Plain Carbon Steels:

Low Carbon Steel: (less than 0.3% carbon) are used for wire, structural shapes, and screw machine parts.
Medium-Carbon Steel: (0.30-0.70% carbon) are used for axles, gears and similar parts requiring medium to high hardness and high strength.
High-Carbon Steel: (0.70-1.4% carbon) are used for drills, cutting tools, and knives.
Low-alloy steels: Steels containing less than 8% total alloying elements and have higher strength than plain carbon steels. Types of low-alloy steels are structural steel, high-strength steel and ultra high strength steel.
High-alloy steels: Contain more than 8% total alloying elements.
Stainless Steel: A steel formed by the addition of chromium.

Classifications of Stainless Steels:

Ferritic Stainless Steels: Contain more than 12 - 27 percent chromium and contain no nickel (part of the AISI 400 series). Turbine blades are manufactured from ferritic stainless steels.
Martensitic Stainless Steels: Are heat-treatable and contain no nickel and primarily in higher carbon contents (part of the AISI 400 series). Surgical instruments are manufactured from martensitic stainless steels.
Austenitic Stainless Steels: Contains 4 - 22% nickel as an alloying element and commonly used for general corrosive purposes. Basic composition is 8% nickel and 28% chromium or 18\8 type. Austenitic stainless steels are weldable, non-magnetic, hardenable and can be polished to a mirror finish.

Heat Treatment Practice

Annealing: Heating above the transformation range, usually 1300 to 1350°F, and cooling slowly to soften the metal and increase ease in machining.
Hardening: Heating above the transformation temperature and quenching, usually for the purpose of increasing the hardness.
Normalizing: Heating to some 100°F above the transformation range with subsequent cooling to below that range in still air at room temperature to produce uniform structure of the metal.
Stress Relieving: Heating to a subcritical temperature, about 1100 to 1300°F and holding at that temperature for a suitable time for the purpose of reducing internal residual stresses.
Tempering: Reheating to a temperature below the transformation range, followed by any desired rate of cooling to attain the desired properties of the metal.
Case Hardening: Process of hardening the surface or case of a metal to provide a hard, wear resistant surface while retaining toughness in the core.

Metal Forming Processes

Rolling: Process of forming metal parts by the use of dies after the metal is heated to its plastic range.
Forging: Process of forming metal parts by the use of powerful pressure from a hammer or press to obtain the desired shape, after the metal has been heated to its plastic range.

AISI and SAE Designation of Steel

AISI Y XXXX
SAE XXXX
- Y is a letter, used in AISI only, to indicate the method of manufacturing
- First number (or first two numbers) represents class of steel
- Second number indicates the approximate percentage of the principal alloying element
- Last two numbers indicate 100 times the approximate percentage of carbon present in the metal.

Commonly Used Metals

Steel
- Plain Carbon (10XX)
- Free Cutting (11XX)
- Manganese (13XX)
- Boron (14XX)
- Nickel (2XXX)
- Nickel-Chromium (3XXX)
- Heat and corrosion resistant (303XX)
- Molybdenum (4XXX)
- Molybdenum-Chromium (41XX)
- Molybdenum-chromium-nickel (43XX)
- Molybdenum-nickel (46XX)
- Molybdenum-chromium-nickel (47XX)
- Molybdenum-nickel (48XX)
- Chromium (5XXX)
- Heat and corrosion resistant (514XX, 515XX)
- Chromium-vanadium (6XXX)
- Nickel-chromium-molybdenum (8XXX)
- Silicon-manganese (92XX)
- Nickel-chromium-molybdenum (except 92XX) (9XXX)
Wrought Iron
- Description: Iron formed by hammering and rolling operations
- Uses: Rivets, welded steam and water pipes
Cast Iron
- Description: Iron formed by casting
- Uses: Cylinder blocks, brake drums, gears, machine tool ways
Malleable Iron
- Description: Heat treated cast iron which is strong, ductile and easily machined
- Uses: Gears
Nodular Cast Iron
- Description: Cast iron added with magnesium and cerium to become stronger and more ductile
- Uses: Casings Crankshafts, hubs, rolls, forming die
Cast Steel
- Description: Steel formed by casting
- Uses: Gears, crankshafts, cylinder barrels
Wrought Steel
- Description: Steel formed by hammering, rolling or drawing
- Uses: Bars, tubes
Stainless Steel
- Description: Steel obtained by addition of chromium
- Uses: Steam turbine blades, valves
Brass
- Description: Alloy of copper and zinc
- Uses: Propeller shafts, piston rods, screws, etc.
Bronze
- Description: Alloy of copper, tin and phosphorous
- Uses: Clutch disks, pump rods, shafts, valve stems, etc.

Tabulated Properties of Materials

Tables of different materials show the following important properties: ultimate strength, yield stress, endurance limit, BHN, modulus of elasticity, elongation, density.
Tables in Faires: Tables AT 4-AT 11, pp. 568-582 (Appendix).
Tables in Vallance: Table 2-4, p. 25, Table 2-5, p.27, Table 2-6, p. 30

Classification of Alloy Steels

Low-alloy structural steels were developed for structural uses where light weight is important.
Low-carbon alloy steels (0.10 -0.25% C) used chiefly for carburizing.
Medium-carbon alloy steels (0.25 -0.50% C) usually quenched and tempered to hardness between 250 and 400 Brinell.
High-carbon alloy steels (0.50 -0.70% C or more) ordinarily heat treated to hardnesses between 375 and 500 Brinell, for use as springs, wear resisting parts, etc.
High-alloy steels such as stainless steels.

Functions of Alloying Elements

Aluminum (Al): Efficient deoxidizer, an alloy in nitriding steels (nitralloys), and it promotes fine grain size.
Boron (B): In very small amounts (0.001% or less) is an economical hardenability agent in low or medium carbon deoxidized steels.
Chromium (Cr): Improves hardenability economically, resistance to corrosion (with other alloys), strength at high temperatures, and wearing properties (high carbon).
Cobalt (Co): Improves red hardness.
Columbium (Cb): Often used to stabilize stainless steel.
Copper (Cu): Improves steel's resistance to atmospheric corrosion.
Lead (Pb): Improves machinability, but affects different alloys differently.
Manganese (Mn): Improves strength and increases hardenability moderately, counteracts brittleness from sulfur.
Molybdenum (Mo): Increases hardenability markedly and economically, tends to counteract temper brittleness, improving creep strength and red hardness; it improves wear by forming abrasion-resistant particles.
Nickel (Ni): Strengthens unquenched and annealed steels, toughens steel (especially at low temperatures), and simplifies heat treatment by lessening distortion. It is the most effective element for reducing the brittleness of steel at very low temperature.
Phosphorous (P): Increases hardenability, strengthens low carbon steels, improves machinability of free cutting steels, and improves resistance to corrosion.
Selenium (Se): Improves machinability of stainless steel; also added to leaded resulfurized carbon steels for the same purpose.
Silicon (Si): Strengthens low alloy steels and improves resistance to high temperature oxidation; it is a good general purpose deoxidizer and promotes fine grain.
Tantalum (Ta): Is a stabilizer.
Titanium (Ti): Is used for deoxidation and for stabilizing austenitic stainless steels; it increases the hardness and strength of low carbon steel and improves creep strength.
Tungsten (W): Increases hardenability markedly in small amounts and improves hardness and strength at high temperature.
Vanadium (V): Promotes fine grain structure, improves the ratio of endurance strength to ultimate strength of medium carbon steels, increases hardenability strongly when dissolved, and results in retention of strength and hardness at high temperature; it is the most effective element in retarding softening during tempering.