Monday, April 27, 2009

Iron and Steel

Iron is abundant in earth crust (4-5%) in the form of ores (hematite (Fe)2 O3 [70% iron SG 4.5-5.3] & magnetite) as oxides, carbonates, silicates and sulfides.

Blast furnace reduces ore to metal where iron ore, coke (carbonaceous material) and limestone (hold silica and alumina impurities) were mixed to form pig iron and slag (non metallic product) under 1650°C that burn oxygen away.

[Carbon: Manganese: Silicon] Pig irons (3.5-4.5: 0.25-1: 0.01% 1670°C) are to be steel in three forms which are wrought iron (-0.01 %- 1535°C, Open hearth furnace), steel (0.5%-1.7%: <1.6%: <0.6%, Basic oxygen furnace) and cast iron (5%, Electric arc furnace). Wrought iron resists corrosion [pipe] is made of molten metal and hot slag through rolling and pressing. Cast iron [Beam] with high compressive strength but low tensile strength is made of pig iron and other material plus cupola to purify iron.

Alloy materials are to give hardness, tensile strength, corrosion resistance, ductility and toughness. [-Note alloy components and effects-]

Steel comes in three types for semi finished product which are blooms, slabs and billets and three types of reinforcing steel which are plain bar, deformed bar and plain-deformed wire fabrics. Blooms are casting for subsequent rolling or forging. They are rectangular in cross section. Slabs are flat and wide. Billets are square lengths of steel which are shipped to rod mills for rolling. Plain bar is round surface which is not good in bonding with concrete thus it is not used in sections subjected to tension and bending. Deformed bar is protrusions surface where got in bonding and used in section subjected to tension and bending. P-D wire fabrics (wires pass each others) are used in concrete slabs and pavements to resist temperature and shrinkage stresses.

Properties of steel are 1% carbon by weight, 0.25% for structural steel by weight. It’s affected by chemical, heat treatment, and mechanical work (hot/cold: increase strength, decrease ductility).
Steel corrosion is defined as the destruction of steel by electrochemical reaction detected by rust formation. Elements that necessary for its occurrence are Fe, Oxygen, Hydroxide, and water. [Draw]

Yield point is a stress point where strain increase without increase in stress. Yield strength is an offset of stress corresponding to known strain. Tension test is to determine metal strength by first determine the length, area before machine will stretch the specimen until it breaks. Data of load applied will be recorded and elongation count. Average stress & strain obtained by σ=P/A and ε=Lf/L

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