Chapter 3 Materials
Materials are classified as metallic and nonmetallic, metallic is divided into ferrous and nonferrous. Here are some examples.
FERROUS
1. IRON
2. STEEL
3. VARIOUS ALLOYS
NONFERROUS
1. COPPER
2. ALUMINUM
3. TITANIUM
Nonmetallic materials such ceramics, glass, and graphite are inorganic materials; wood, rubber, and plastics are organic materials.
PROPERTIES
A material has certain properties that define behavior and characteristics under various conditions
Desirable Properties
Static strength and dynamic strength are desirable in any material, along with low cost. Some materials are used even if they have some poor characteristics, an example would be in casting materials the following is desirable.
o Low melting point
o Good fluidity when melted
o A minimum of porosity
o Low shrinkage
Definition of Properties
o Brittle, Breaks with a clean break, the harder a metal is the more brittle it is.
o Cold short is the name given to a metal that can not be worked under a hammer or rolling and can not be bent cold without the edges cracking. It must not be worked under a pressure of dull red.
o Ductile: Easily drawn out, pliable, material is ductile when it can be extended by pulling,
o Elastic limit: The greatest strain that a substance can endure and still completely spring back to the original shape when the strain is released.
o Fusible: Capable of being melted of liquefied by the action of heat.
o Hardness: The ability to resist damage.
o Homogeneous: Of the same kind or nature
o Hot short: more or less brittle when heated.
o Melting point of a solid: The temperature which a solid becomes a liquid or gas. Melting points range from -39 degrees F to 3000 degrees F.
o Resilience: The act or quality of elasticity, the property of springing back , or recoiling upon removal of a pressure (as with a spring).
o Specific gravity: The weight of a given substance relative to an equal bulk of some other substance, which is taken as a standard of comparison.
o Strength: The ability of a substance to resist force without breaking or yielding, solidity or toughness.
o Tensile strength: The greatest longitudinal stress that can be applied to a substance without tearing it apart.
o Toughness: A substance having the ability to resist great strain, and absorb energy without failure.
Metals
Is any element (silver, iron, etc) that carries a positive charge when dissolve in acid solution and seeks the negative pole in an electric cell. Metals are good conductors of heat and electricity, and can be melted, formed or machined.
Ferrous Metals
Ferrous metals are those metals that contain iron, usually they are magnetic, and can be pure iron or an alloy.
IRON
Pure iron is crystalline in structure, and is a relativity soft metal. There are three temperatures that affect iron, they are 2782 degrees F, the temperature iron solidifies, 1648 degrees F, and 1416 degrees F. Here are the four solid phases of iron.
- Alpha iron- is soft, magnetic and incapable of dissolving carbon from atmospheric temperature to 1416 degrees F.
- Beta iron -is feebly magnetic, hard, and brittle, it occurs from 1416 to 1648 degrees F.
- Gamma iron- takes up carbon, and occurs between 1648 and 2554 degrees F. There are elements that effect gamma iron, carbon; nickel and manganese help prevent it passing into alpha iron. Chromium, tungsten, aluminum, silicon, phosphorus, arsenic, and sulfur helps in the passage from hard beta iron to soft alpha iron.
Delta iron- This type of iron has very little use, reaching 2554 to 2782 degrees F indicates that the internal structure of the metal has changed from gamma to delta iron.
Pig Iron
Pig iron is combined with carbon, silicon, sulfur, phosphorus, and manganese, with carbon making up 2 to 4.5 percent of the content.
Cast Iron
Is really remelted pig iron, it is not malleable, the carbon content is over 2 percent, and cannot be formed, shaped, rolled or drawn into shapes of any use. In industry cast iron is used for castings, and there are four types.
o Gray cast iron- mostly used in castings, has a high percentage of graphite, and is tough with low tensile strength. The carbon content is from 2.5 to 3.5 percent.
o White cast iron- is a casting of extreme hardness and brittleness, with a lower carbon content of 2.0 to 2.5 percent.
o Malleable cast iron- castings made of hard, brittle white cast iron and then annealed, which transforms it to a form of steel.
o Wrought iron- is actually a form of low carbon steel which contains a large amount of slag, up to 1 to 2 percent. What is called the puddling process removes this slag.
STEEL
This general term describes an alloy in which iron is the base metal, and carbon, is the most important added element anywhere from 0 to 2.0 percent. This is a form of what is called pure steel, and has never been made in large quantity. Manganese is the third alloying element of plain carbon steels along with small quantities of silicon, phosphorus, sulfur and other elements. Plain carbon steels contain a few hundredths to 1.4 percent carbon; properties depend on carbon content and heat treatment.
Low carbon steels are used in the rolled or the annealed condition; contrast this with high carbon steels are used where hardness is needed. Increasing the carbon content of steel to a certain percent increases its strength. Mild steel with a content of 0.1 percent carbon has a tensile strength of approximately 50,000 psi, increasing the carbon content to 1.2 percent then the tensile strength is 140,000 psi. A carbon content of 2.0 percent is the theoretical upper limit for steel.
Other elements are added to steel as well, they are as follows, and their effects.
o Phosphorus- enhances the hardness of steel and is able to better resist abrasion; however, it makes steel weak against shocks and vibrations.
o Sulfur- increases brittleness, content should not exceed 0.02 to 0.05 percent
o Manganese- increases strength, hardness and soundness of steel
o Nickel- increases strength and toughness of steel
o Aluminum- improves soundness of ingots and castings
o Vanadium- makes steel nonfatigable, ductile, high tensile strength, and high elastic limit; also resistant to shocks
o Molybdenum- is used in crankshafts, propeller shafts, gun, and rifle barrels also boilerplates
HIGH SPEED STEELS
Can be used as cutting tools for they retain hardness at high temperatures, even if the edges becomes red hot. High speed steels contain 12 to 20 percent tungsten, 2 to 3 percent chromium, and 1 to 2 percent vanadium along with cobalt which is sometimes added. Carbon content is low from 0.65 to 0.75 percent. The two most commonly used high speed steels are 18-4-1 and 14-4-2; the numbers refer to tungsten, chromium and vanadium.
STAINLESS STEELS
When properly heat treated and finished stainless steel resist oxidation and corrosion although they are not absolutely corrosion resistant. Medical and dental instruments contain 12 to 14 percent chromium.
CAST STEEL
Is stronger than cast iron when used in castings, stainless steel casting resist oxidation at temperatures up to 1800 degrees F. Cast steel does not pour as sharply as iron and shrinkage is greater.
