ME 437 Mechanical System Design

ME 437 Mechanical System Design

Answer to material science questions

1. Ductility is a measure of the material’s ability to deform without fracture. Measure: % elongation before fracture. Design: important in forming. Brittle behavior occurs when % elongation drops roughly below 1%.

1. Toughness is a measure of the material’s ability to absorb energy without fracture. High strength and ductility determine toughness.

1. Strength is the material’s resistance to plastic deformation (yielding). Strength also refers to the material’s resistance to fracture. When strength is used without any other qualifiers, it refers to yield strength.

1. Endurance limit is the stress level that does not lead to fatigue failure in steels and some other metals.

1. Fatigue strength is the stress level for which fatigue failure would not happen for a specified number of loading cycles.

1. Hardness is another measure of resistance to deformation. Measures: Brinell, Rockwell, Vickers. Design: Important criteria for resistance to wear and surface deformation. Also it is an indicator of the material’s strength.

1. Hardenability is a measure of the material’s ability to be deep hardened in heat treatment. Design: Important when very high strengths are required through heat treatment of thick sections.

1. Stiffness is a measure of the material’s resistance to elastic deformation. Measure: Modulus of elasticity. Design: It is important in design of high accuracy machines. Also impacts vibration characteristics of the machines.

1. Porosityrefers to the degree of material solidity. Itis characterized by voids and pores within a material such as P/M parts containing tiny voids within their structure.

1. Machinability is a measure of a material’s ease of machining (chip forming) and quality of surface finish.

1. Factors related to machinability are strength, hardness, chip breakup, heat resistance, work-hardening, stickiness, and the like. Design: machinability has great impact on the cost of manufacturing.

1. Formability is the materials ability to be easily formed without rupture or failure. A formable material is ductile and has low yield strength.

1. Weldability is the material’s ability to be easily welded. Weldability usually depends on the welding process. There are many side effects to welding such as formation of brittle compounds, distortion, change of properties, and cracking. Alternatives to welding are brazing, soldering, and adhesive bonding. Brazing uses a filler material with lower melting temperature than the materials to be welded. Soldering is a form of brazing but with a filler that melts below 450 degrees C.

1. A polycrystalline material is a material with a microstructure composed of many tiny grains of crystals joined along grain boundaries.

1. In work hardening (strain hardening) atomic planes in crystals slide along weak planes leading to grain elongation in the direction of stress.

1. Work hardened materials become stronger and harder as the slip planes deform and grains elongate. They develop considerable residual stresses. If a work hardened material is heated above the recrystalization temperature, the grains reshape and the normal stress-free crystal structure is restored. The material looses its strength but the ductility is also restored. The lowest recrystallization temperature for aluminum is 150 degrees C.

1. A solid solution is an alloy in which atoms of one material (say carbon) fill the space within the atomic structure of another material (say iron).

1. An intermetallic compound is a substance formed through covalent or ionic bonding of a metal and a non-metal. These materials are often hard and brittle such as iron carbide in the iron-carbon solutions.

1. Phase diagram is a graph indicating the substances forming in an alloy at certain temperatures and certain compositions of alloying elements under equilibrium (held constant) conditions.

1. -ferrite is the room temperature equilibrium structure of iron. -ferrite has a BCC structure. Carbon does not dissolve very well in -ferrite and this is fortunate because all the heat treatment capability of steels depend on this fact. -ferrite is relatively soft and deformable.

1. Austenite is the high temperature equilibrium structure of iron (at about 1700 F). Austenite has a FCC structure. Unlike -ferrite, austenite can dissolve a lot of carbon in solid solution form.

1. Cementite is iron carbide or simply carbide when the context is steel composition. Cementite is a strong and brittle intermetallic compound and if it forms along the grain boundaries, it makes the alloy quite brittle.

1. Pearlite is a micro structure of steel that is composed of layers of -ferrite and cementite. In low carbon steels the alloy structure is mostly ferrite and some pearlite. In high carbon steels, the structure is mostly pearlite.

1. Cast iron is an alloy of iron and carbon in which the carbon content is more than 2.11% of the alloy by weight. Other common elements such as silicon are present at their normal levels.

1. The main feature in the micro-structure of the gray cast iron is the existence of carbon in the form of graphite flakes. The matrix can be primarily ferrite (low strength – class 20), or pearlite with lower carbon content (medium strength – class 40). Alloying and heat treatment can create higher strengths (up to class –80).

1. Gray cast irons have high compressive and low tensile strengths, they have excellent machinability and good wear resistance. Gray cast iron has very good damping and vibration characteristics. Due to high silicon content gray cast iron is more corrosion resistant than plain carbon steels, and has more fluidity for casting processes. Gray cast iron is also quite inexpensive.

1. By adding certain alloying elements to cast iron and controlling the cooling process, the graphite shape and size can be controlled to be spherical leading to a cast iron called the ductile iron.

1. Properties of ductile iron: Changing the graphite form from flakes to nodules considerably improves the cast iron’s ductility. Also by controlling the matrix structure, higher strengths (up to 90 ksi) can be attained. Due to the required processing, ductile iron is more expensive that gray cast iron.

1. Process heat treatment is used to prepare a material for processing such as softening for machining or forming, or restoring ductility for further deformation processing.

1. Full annealing involves holding the material (steel) at elevated temperatures followed by slow cooling. The micro structure of steel would become a coarse pearlite which is quite soft and ductile.

1. When maximum softness is not required, the material can be air cooled. The process is called normalizing and the material would have a non-uniform micro-structure.

1. Process annealing heats the material above the recrystalization temperature to restore material’s ductility. The material is then cooled in the air. A stress relief anneal is similar to process anneal except that the cooling is slower. Stress relief anneal is often used on large castings or welded structures.

1. Spherodization is the softening process used on high carbon steels. By proper heating, the cementite shape and distribution can be altered into finely dispersed spheroids leading to better ductility for forming processes.

1. Martensite is a non-equilibrium micro-structure. Martensite happens when steel attempts to switch from high temperature austenite lattice (FCC) to low temperature ferrite lattice (BBC). Without sufficient time for the carbon atoms to defuse, the structure becomesa deformed austenite (FCC) lattice. The amount of deformation depends on the speed of quenching.

1. Martensite is extremely hard and brittle like ceramics.

1. In steels, Martensite is formed by rapidly quenching steel from austenite temperature to a low temperature.

1. When martensite is heated (tempered) it regains some of its ductility but loses some of its strength – a good tradeoff. Depending on the tempering temperature a wide range of properties and micro-structures can be obtained.

1. The formation of martensite depends on the quenching temperature. If the quenching temperature is not low enough, some of the austenite would not transform into martensite. This is called the retained austenite.

Rapid quenching can cause cracking due to distortions and rapid contractions. In austempering, the material is held above martensite transformation until all the austenite transforms into bainite, a strong microstructure of dispersed carbide and ferrite.

Marquenching rapidly cools the material just above the martensite transformation and then gradually cools the material before the bainite transformation starts. The final structure is martensite that needs to be tempered.

1. If a material with retained austenite is subjected to colder temperatures than the quenching temperature, the retained austenite transforms into untempered martensite. This could become a potentially dangerous situation. For example, retained austenite can turn into brittle martensite when an aircraft component is subjected to very low temperatures at high altitudes.

1. Heat treatment is expensive and adds some degree of uncertainty to the design. Heat treatment can create non-uniform temperatures leading to stress concentration, possible cracking, and distortion. Heat treated materials are difficult to process.

1. Surface hardening methods: Flame hardening, carburizing, nitriding, induction hardening, laser beam hardening.

1. Plain carbon steels are alloys obtained through oxidation of pig iron to reduce carbon and other elements in iron. No other special process is performed to balance the alloying elements and for that reason these are the least expensive grades of steel.

1. Low carbon steel contains less than 0.3% carbon. Low carbon steel has low strength, high toughness, good fatigue strength, and high ductility. It has excellent formability and weldability. Its hardenability is quite low – it cannot be heat-treated.

1. Medium carbon steels contain between 0.3% and 0.8% carbon. They have higher strength, and lower ductility compared to low carbon steels. They can be quenched and tempered to increase their strength and hardness but their hardenability is still poor.

1. High carbon steels contain more than 0.8% carbon. They have higher strength than other carbon steels but their ductility and formability is quite limited. Their surface can be hardened by quenching but their hardenability is still very limited.

1. Objectives of adding alloying elements to carbon steels are: increasing hardenability, increasing strength, improving ductility, improving machinability and weldability, improving corrosion resistance, attaining more desired high temperature or low temperature properties.

1. Primary alloying elements in AISI 4340 steel are Mo, Cr, and Ni. It contains 0.40% carbon. This steel has high hardenability and can be heat treated to obtain high strength with good toughness. This alloy has good weldability.

1. In HSLA steels, small amounts of alloying elements are carefully balanced to create a low cost steel for structural applications. These steels are twice as strong as plain carbon steels and yet they have good weldability, and some corrosion resistance. These steels are not developed for heat treatment as their hardenability is quite low.

1. Quench-and-tempered steels are developed to have through hardening properties for heat treatment. Their yield strength can exceed 150 ksi, and because of rather low carbon content, they are weldable. These steels are also tough and have good corrosion resistance.

1. Free machining steels have added alloying elements (usually sulfur) to improve the machinability of the steel. In these steels chips break up easily.

1. The main alloying element of stainless steel is chromium (Cr).

1. Four classes of stainless steel: Ferritic stainless steel (good corrosion resistance, poor ductility, good weldability, non-heat treatable, least expensive stainless steel). Martensitic stainless steel (heat treatable, more expensive than ferritic). Austenetic stainless steel (highest corrosion resistance, high formability, non-heat treatable but can be work hardened to high strengths, twice as expensive as ferritic). Precipitation hardening stainless (can attain yield strengths of up to 250 ksi, very expensive).

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1. Alloys of copper and tin are called bronzes. Bronzes are high strength, wear resistant, corrosion resistant, and are commonly used for sliding bearing applications especially phosphor bronze. Other applications include jewelry, screws, pump rods and valve stems, and high strength parts subjected to corrosive media.

1. Brass is an alloy of copper and zinc. Brasses are corrosion resistant and very ductile. They have high thermal and electrical conductivity. Applications include musical instruments, springs, screw machine parts, ornamental pieces, heat exchangers.

1. High temperature alloys are alloys that retain their strength and rigidity at high temperatures. Example: Many nickel alloys such as Inconel.

1. Engineering plastics: Polycarbonate, polypropylene, polyethylene

1. Are there any engineering plastics that have metal elements in their molecular structure? No