3 Flashcards
ferrous metals are among the most useful of all the metals.
Wide range of
* Mechanical
* Physical
* Chemical properties
Iron Making
3 raw materials (iron ore + coal + limestone) are dumped to the top of a blast furnace.
➢ Hot air is used to initiate the reaction of carbon and oxygen to remove the oxygen from iron- oxide.
Steelmaking
This process is essentially refining the pig iron by reduction of the percentage of carbon, manganese, silicon, and adding various elements and limestone.
➢The most common method is called Basic Oxygen Furnace (BOF).
➢Steel scrap is added to BOF.
Casting of Ingots
Traditionally, the next step in steelmaking is the shaping of molten steel into a solid form (ingot).
➢ During solidification, the solubility of gases decreases sharply as the temperature decreases.
➢ Rejected gases like oxygen causes porosity in ingots.
➢ Then the ingots are further re-shaped by one of these processes: rolling, forging or
casting.
Continuous Casting
stead of the traditional way, “continuous casting” process is used which improves efficiency by eliminating the need for ingots!!!
➢ Molten metal is poured into a tundish. After the molten metal is cleaned and equalized in temperature, it is poured through a water-cooled mold and begins to solidify.
➢ The cooling rate is such that the metal develops a solidified skin (shell).
➢ The center of the metal is still liquid at the top section.
Most important alloying elements are:
Carbon:
* Increases strength, hardness and wear resistance. * Decreases ductility, weldability, toughness.
Chromium:
* Increases corrosion and wear resistance, toughness, hardenability, high-temperature strength.
Nickel:
* Increases strength, toughness, corrosion resistance, and hardenability.
Sulfur:
* Increases machinability when combined with manganese. * Decreases impact strength, ductility, surface quality and weldability.
Stainless Steels
Have a high content of chromium (>10%).
* Have a high corrosion resistance, strength and ductility.
* In the presence of oxygen (which is in air), these steels develop a thin and hard film of chromium oxide that protects the metal from the corrosion (passivation).
* The higher the carbon content is, the lower is the corrosion resistance.
* Applications: cutlery, kitchen equipment, health care and surgical equipment, chemical and food processing.
Unwanted Impurity Elements
Hydrogen: Severely embrittles steels which is known as Hydrogen Embrittlement. (If steel is heated during the processing, most of the hydrogen is taken out.)
Nitrogen: Increases strength, hardness; however it decreases ductility and toughness.
Oxygen: Slightly increases the strength; however it severely decreases toughness.
polymer
Advantages of polymers compared to metals:
➢ low density
➢ corrosion resistance
➢ resistance to chemicals
➢ low electrical and thermal conductivity
➢ noise reduction
➢ wide choice of colors and transparencies ➢ low cost
Disadvantages of polymers compared to metals:
➢ low strength and stiffness
➢ high coefficient of thermal expansion ➢ low useful service temperature range
Raw material for polymers:
Natural organic materials from animal and vegetable products (such as cellulose used in paper making). Examples include: DNA, wool, silk.
➢ Synthetic (man-made) organic polymer.
Polymerization
By adding some catalyst, and/or applying heat and pressure, polymerization process takes place: Monomers are linked into polymers in repeating units.
➢ The degree of polymerization (DP) denotes the average number of monomer units joined together in a polymer.
➢ The higher the DP is, the higher is the polymer’s tensile and impact strength, and viscosity (hence the resistance to flow).
➢ High viscosity adversely affects the ease of shaping and thus raises the overall cost of processing.
4 types of polymers
Linear (not necessarily straight line!) ▪ Branched
o Entanglement
o Difficult to move a branched polymer as in the analogy of pile of tree branches ▪ Cross-linked
o These polymers are called thermosets or thermoset plastics.
o Hardness, strength, stiffness and brittleness increase as cross-linking increases. ▪ Network
o Highly cross-linked
o Thermoplastic polymers that have already been formed or shaped can be cross-linked to obtain greater strength, by subjecting them to high-energy radiation such as ultraviolet light, x-rays, or electron beams. But, excessive radiation can cause degradation of polymer!
thermoplastics
▪ For certain polymers, as the temperature is raised above glass temperature or melting temperature, we find that it becomes easier to form or mold them into desired shapes.
▪ The increased temperature weakens the secondary bonds, and the adjacent chains can thus move more easily under the shaping forces. If we then cool the polymer, it returnsThe electrical conductivity of some polymers can be increased by doping to its original hardness and strength.
▪ That means, the process is reversible!!!.
▪ These polymers are known as thermoplastics.
With increasing temperature, strength and E decrease, but toughness increases.
classification of polymers
Thermoplastics Thermosets Elastomers
Thermosets
When the long chain molecules in a polymer are cross-linked in 3D arrangement, the structures becomes one giant molecule with strong covalent bonds.
These polymers are called thermosets because during polymerization the network is completed and the shape is set.
Properties and Applications of Thermoset Plastics Epoxies:
Polyesters:
➢ High mechanical and electrical properties
➢ Strong adhesive properties
➢ Fiber-reinforced epoxies have excellent mechanical properties, and used in structural components.
➢ Good chemical and electrical properties
➢ Applications: boats, seats, chairs, automotive bodies, swimming pools, playground components, luggage.
