Model 6 Metallic Structures, Processing, and Properties

Metals are generally divided into 2 groups:

• ferrous (iron based)

• nonferrous

Ferrous metal alloys

Steels (low alloy) and cast irons (high alloy)

• cast iron

• stainless steel

• wrought iron

• mild steel

• steel

Nomenclature for steels

Provides a structured system of rules and numerical codes (like AISI or SAE) to classify + identify their chemical composition and mechanical properties.

• 10xx Plain Carbon Steels

• 11xx Plain Carbon Steels (resulfurized for machinability)

• 15xx Mn (1.00 - 1.65%)

• 40xx Mo (0.20 - 0.30%)

• 43xx Ni (1.65 - 2.00%), Cr (0.40 - 0.90%), Mo (0.20 - 0.30%)

• 44xx Mo (0.5%)

* “xx” is the weight percent of C (x 100); e.g., “1060 steel” is plain carbon steel with 0.60 wt% C *

Bessemer Process

A steel ore refinement process that used a receptacle to blow oxygen down on molten iron.

Gray Iron

• Graphite flakes

• Weak and brittle tension

• stronger in compression

• excellent vibration dampening

• wear resistant

White Iron

• <wt%Si

• pearlite + cementite

• very hard and brittle

Ductile Iron

• Add Mg or Ce

• graphite as nodules not flakes

• matrix often pearlite - stronger but less ductile

Malleable Iron

• Heat treat white iron at 800 - 900^oC

• Graphite in rosettes

• Reasonably stronger and more ductile

Limitations of Ferrous Alloys

• Relatively high densities

• Relatively low electrical conductivity

However, they are still more durable and stronger than nonferrous alloys

Nonferrous Alloys

• Cu Alloys

• Al Alloys

• Mg Alloys

• Ti Alloys

• Noble Metals

• Refractory metals

Plain carbon steel

• A ferrous alloy where carbon is the prime element

Stainless steel

• A steel alloy, highly resistant to corrosion in a variety of environments. Predominant alloying element is chromium

White cast iron

• Low silicon very brittle cast iron

• carbon is combines to form cementite

Gray cast iron

• A cast iron alloyed with silicon in which the graphite exists in the form of flakes

Bronze

• A copper-rich copper tin alloy

• Aluminum, silicon, and nickel bronzes are also possible

Brass

• A copper-rich zinc-rich alloy

Alloy steel

• A ferrous alloy containing appreciable concentrations of alloying elements

Cast Iron

• Generically, a ferrous alloy, the carbon content of which is greater than the maximum solubility in austenite at the eutectic temperature

Hot Working

Fabrication of metals process

• Deformation temperature high enough for recrystallization

• Large deformations

Cold Working

• Deformation below recrystallization temp

• Strain hardening occurs

• Small deformations

Metal forming Methods

• Forging (hammering; stamping) - wrenches, crank shafts

• Rolling (hot or cold rolling) - i-beams, rails, plate

• drawing - rods, wire, tubing

• extrusion - rods, tubing

Casting

• metal melted in furnace, perhaps alloying elements added, then cast in a mold

• common and inexpensive.

• gives good production of shapes.

• weaker products, internal defects.

• good option for brittle materials

Sand casting

• Sand can with stand T >1600^oC, is inexpensive and easy to mold

• To create mold pack sand around mold (pattern) of desired shape

• Poor in metal, let it cool (solidify) and remove part

Investment (lost wax) casting

1. Mold formed by pouring plaster of paris around wax pattern. Plaster allowed to harden.

2. Wax is melted and then poured from mold—hollow mold cavity remains.

3. Molten metal is poured into mold and allowed to solidify.

Die casting

• high volume

• for alloys having low melting temperatures

Continuous Casting

• simple shapes (rectangle slabs, cylinders, etc.)

Metals can be …

glasses

First 3 metals discovered

• copper

• gold

• silver

How can the processing method affect the final properties of a metal?

• change in mechanical properties

• change in physical properties

• change in thermal properties

Why would one choose to dye cast a metal over sand casting?

• die casting has higher accuracy and gives a better finish