Transcript MC 251 Metals Lecture 2 “Manufacturing” From Refined Steel

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
Cast Bars & Engine Block
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
Forged Steel Parts
Extrusion
• Extruding: Method of converting semi-finished shapes
into lengths of uniform cross-section by forcing
preheated, plastic steel through a very tough, heatresistant 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
Aluminum Extrusions
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 Rolling I & H Beams
Rolled Sheet & Rail
Coiled & Straight Re-bar
Hot & Cold Finishing
• Hot Finishing: (Hot Rolling) This is basically a
continuation of hot rolling, where the semifinished 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 roomtemperature finishing processes such as rolling,
reduction, drawing:
Cold Finishing Processes
• Prior to the cold finishing processes, the semifinished 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
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Protective Finishes:
Metallic Coatings
Vitreous Coatings
Laminated Coatings
Painted Coatings
Needed to protect from corrosion
Metallic Coatings
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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
Sacrificial Anodes