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Steel
Properties of Materials- Chapter 3
Question | Answer |
---|---|
Steel's origins | Melting of steel using charcoal (1500 BC) |
Types of Steel | • Structural Steel (Beams, columns, pipes, bars, plates, etc) • Fastening products (Connections like bolts, nuts, washers, etc) • Reinforcing steel “rebar's” (for concrete reinforcing) • Others (structuralforms, pans, etc) |
Why civil engineers should study steel as a material? | Civil engineers are not responsible for the production of steel, but it is highly important for them to understand the properties of steel that can affect it’s behavior as a structural element. |
Basic material required in production: | • Iron Ore: Pig iron. • Coal (Coke)(Carbon): To produce iron oxides in the ore. • Limestone: Removes impurities |
Cast Iron characteristics | High carbon Brittle behavior Products are shaped by casting Relatively low tensile strength |
Steel characteristics | Low carbon content Ductile behavior Products are shaped by rolling, hammering, stretching Higher tensile strength |
Steel Heat Treatment purpose | Altering steel properties (hardening, softening, surface treatment, etc.) |
Types of Heat treatment | Annealing, normalizing, hardening, tempering |
Annealing heat treatment | Objectives: • Refine grains • Soften steel • Remove internal stress • Remove gases • Increase durability & toughness • Change electrical & magnetic properties |
Normalizing heat treatment | Same as annealing except a different rate of cooling is used to produce a uniform fine grained microstructure. Example: Structural plates. |
Hardening heat treatment | Cooling at a faster rate to produce a very hard and brittle material (ex: camshaft for carrying torque to rear car wheels) |
Tempering heat treatment | Transforming steel with undesirable brittleness to more ductile and tough steel through heating and cooling to about 40 °C followed by reheating through immersion in oil or nitrate salts. Example: Structural beam in a building |
Steel Alloys | Definition: Additives (materials, agents) added to steel to improve its characteristics. Properties improved: • Hardibility. • Corrosionresistance. • Mechanicability. • Ductility. • Strength. |
Types of steel alloys | • Low alloy: ex. High strength alloy steel • High alloy: ex. Stainless steel Note: Steel in construction are predominantly low (& medium) carbon plain steel. |
Structural steel | Hot-rolled structural elements (plates, bars) |
How is Steel identified? | – Grade: Chemical composition or yield strength or mechanical properties. – Type – Class |
What is special steel (high performance steel)? | Combination of materials that possess the optimum combination of material properties required to build a cost effective structure that will be safe and durable throughout their service life. |
Fastening Product | Bolts • Nuts • Washers • Rods |
Reinforcing steel | Used in cement concrete for the purpose of carrying tensile stresses since concrete has negligible tensile strength |
Types of reinforcing steel | • Conventional reinforcing steel • Pre‐stressed reinforcing steel |
Forms of reinforcing steel | • • • Plain bars Deformed bars: Good bond preventing slippage in beams & columns Wire Fabric: Economical for concrete slabs & pavements |
Tension Test | Determines: • Yieldpoint • Ultimate strength • Elongation • Reduction in area |
Impact Test | Determines: • Toughness of material |
Hardness Test | Determines: • Resistance of material to localized plastic deformation (dents or scratches) |
Steel Tests | Tension, torsion, impact, bend, hardness |
Corrosion | Destruction of material by electrochemical reaction to the environment (Ex: formation of rust) |
What is required for corrosion to occur? | • Two side electrode • Medium to move • Liquid to support the movement of electrons |
How to prevent corrosion? | • Barrier coating • Inhibitive primer coating • Sacrificial primers (Zinc coating) |