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CON 251 Metals Processing. “Manufacturing” From Refined Steel Material to Shaped Product. Shaped Products. The product may be intermediate, as in a rolled steel beam prior to “fabrication” The product may be final, as in a steel casting for a bridge saddle or a bollard .
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CON 251 Metals Processing “Manufacturing”From Refined Steel Material to Shaped Product
Shaped Products • The product may be intermediate, as in a rolled steel beam prior to “fabrication” • The product may be final, as in a steel casting for a bridge saddle or a bollard
Manufactured Steel Products Two basic categories of manufactured steel products: • Cast Steel Products made in one basic way in foundries. (Molten metal cast into finished shape) • Wrought Steel Products made one of several methods in mills. (Shaped through deformation)
Casting • Casting is generally performed by pouring molten steel into sand molds. • Casting is used instead of wrought methods when the steel product to be made is of such size and/or complexity that it is uneconomical to produce by other methods. • Example (big): Bridge saddles, ship anchors, mooring cleats, bollards
Wrought Steel Products(Formed Through Deformation) • Forging - ( Hammering into shape) • Extruding - ( Squeezing through a shaped die) • Hot Rolling - Progressive forming with various rolls. (with or without cold finishing)
Forging • Forging: Method of forming hot metal by squeezing between heat-resistant dies. • Open-Die Forging: A large press squeezes (not strikes) steel between two heat-resisting surfaces…can be used to shape very large steel ingots (the ingot may weigh several hundred tons, the forge can squeeze with a force of several thousand tons) • Closed-Die Forging: A large hammer pounds the steel between two heat-resisting shaped dies until the product is in the desired shape
Extrusion • Extruding: Method of converting semi-finished shapes into lengths of uniform cross-section by forcing preheated, plastic steel through a very tough, heat-resistant die (analogy: toothpaste) • Bars, tubing, pipes, and many unusual cross-sections can be extruded. More complex shapes can be extruded than can be rolled. • More economical for small quantities than other forming methods. • A limitation is that cross-section must be uniform. Therefore, Plain round bars could be extruded or rolled, but Ribbed round bars (like rebar) could not be extruded, must be rolled
Hot Rolling • Hot Rolling: Used to make semi finished shapes as well as some finished products. • May be started with reheating of large steel ingots from steel producer, or may be sequenced directly after the continuous strand casting process. • Hot steel passes through a system of heat-resistant rolls which gradually, roll by roll, change the ingot or strand into one of three basic intermediate shapes: • Slabs: Flat, rectangular shapes with width > 2x thickness; Will later become plates, sheets, strips, or products like pipe and tubing (made from plates, sheets, strips) • Blooms: Rectangular cross-sections, generally larger than 36 sq. in.; Will later become structural shapes, rails, seamless pipe • Billets: Rectangular cross-sections, less than 36 sq. in. Will later become bars (including rebar), rods, wire
Hot & Cold Finishing • Hot Finishing: (Hot Rolling) This is basically a continuation of hot rolling, where the semi-finished product continues while still hot through more rolls to become the finished structural shape, rail, plate, bar, sheet, etc. • Cold Finishing: (Cold Rolling) Transition from semi-finished to finished product via room-temperature finishing processes such as rolling, reduction, drawing:
Cold Finishing Processes • Prior to the cold finishing processes, the semi-finished products from the initial hot-rolling steps must be “cleaned up” by: Descaling: This is removal of the surface oxide scale, usually by a process involving dipping in sulfuric or hydrochloric acid (this process is known as pickling) • Rinsing: with both hot and cold water • Drying: usually using steam • Oiling: as a temporary sealant and lubricant, in preparation for cold finishing
Cold Rolling • Cold Rolling: passing the semi finished metal through another series of rolls to impart desired final shapes and/or mechanical properties and surface finishes • Cold Reduction: actually another type of cold rolling, but specifically to drastically reduce the thickness of an already-flat hot-rolled product • (such as sheets or strips) in order to improve strength, finish, flatness Example: Half-dollar thick, ¾-mile long steel strip + 20 minutes of cold reduction = playing-card thick, 2-mile long strip • Cold Drawing: making smaller cross-sections (small bars or wire) from hot- rolled bars or rods by pulling the latter through a hard, abrasion-resistant die…cold drawn wire can achieve tensile strength of 500ksi.
Protective Finishes • Protective Finishes: • Metallic Coatings • Vitreous Coatings • Laminated Coatings • Painted Coatings • Needed to protect from corrosion
Metallic Coatings • Hot dip processes (submerging in molten bath): • Galvanizing – heavy coating with zinc • Aluminizing – coating with aluminum/silicon • Tin, others • Electroplating (electrolytic metal transfer) • Metallizing (spraying molten metal onto surface to be coated) • Cladding (direct application of thin sheet of coating metal)
Vitreous Coatings • Glass-on-steel linings for process piping, tanks • Porcelain enamel coatings on building panels, plumbing fixtures
Laminated Coatings • Thin, tough plastic films applied with thermosetting adhesives
Painted Coatings • Petroleum Based • Water Based • Epoxy Based
Corrosion Two basic types: Oxidation (Ferrous metals): Iron reacts with oxygen, forming iron oxide (rust) Galvanic (Dissimilar metals): Creation of an electrolytic cell (water & dissolved minerals are typically the electrolyte) One metal becomes the positive electrode (anode)… oxidation & loss of material One metal becomes the negative electrode (cathode)… reduction & gaining of material
Galvanic series Magnesium, Aluminum Zinc, Iron Steel, Cast Iron Lead, Brass Copper, Bronze Nickel, Stainless steel Silver, Graphite Example: sacrificial zinc anodes on marine steel hulls Corrosion can be impeded by: coatings, alloying, and other methods