Chapter 11 Flashcards
Hot working (pg. 434)
when deformation is achieved at a temperature above temperature where recrystallization occurs
Cold working (pg. 434)
-produces an increase in strength with the attendant decrease in ductility because the metal strain hardens -advantages over hot working include a higher-quality surface finish, better mechanical properties and a greater variety of them, and closer dimensional control of the finished piece
forging (435)
mechanically working or deforming a single piece of a usually hot metal; this may be accomplished by the application of successive blows or by continuous squeezing
rolling (435)
most widely used deformation process, consists of passing a piece of metal between two rolls; a reduction in thickness results from compressive stresses exerted
by the rolls
extrusion (435)
bar of metal is forced through a die orifice by a compressive force that is applied to a ram; the extruded piece that emerges has the desired shape and a reduced cross-sectional area
drawing (436)
pulling of a metal piece through a die having a tapered bore by means of a tensile force that is applied on the exit side
casting (436)
fabrication process in which a completely molten metal is poured into a mold cavity having the desired shape; upon solidification, the metal assumes the shape
of the mold but experiences some shrinkage
sand casting (436)
ordinary sand is used as the mold
material. A two-piece mold is formed by packing sand around a pattern that has the
shape of the intended casting. A gating system is usually incorporated into the mold to
expedite the flow of molten metal into the cavity and to minimize internal casting defects
die casting (436)
liquid metal is forced into a mold under pressure and at a relatively high velocity and allowed to solidify with the pressure maintained
investment casting (436) also called lost wax casting
pattern is made from a wax or plastic that has a low melting temperature. Around the pattern, a fluid slurry is poured that sets up to form a solid mold or investment; plaster of paris is usually used. The mold is then heated, such that the pattern melts and is burned out, leaving behind a mold cavity having the desired shape
lost foam casting (436)
also called expendable pattern casting
- expendable pattern formed by compressing polystyrene beads into the desired shape and then bonding them together by heating
- pattern shapes can be cut from sheets and assembled with glue. Sand is then packed around the pattern to form the mold. As the molten metal is poured into the mold, it replaces the pattern, which vaporizes. The compacted sand remains in place, and, upon solidification, the metal assumes the shape of the mold.
- complex geometries and tight tolerances are possible
continuous casting (437) also called strand casting
- casting and rolling steps are combined,
- using this technique, the refined and molten metal is cast directly into a continuous strand that may have either a rectangular or circular cross section
- solidification occurs in a water-cooled die having the desired cross-sectional geometry
powder metallurgy (437)
compaction of powdered metal followed by a heat treatment to produce a denser piece, makes it possible
to produce a virtually nonporous piece having properties almost equivalent to those of the fully dense parent material
open die (forging) (435)
For open die, two dies having simple geometric shapes (e.g., parallel flat, semicircular) are employed, normally on large work pieces
closed die (forging) (435)
For closed die, a force is brought to bear on two or more die halves having the finished shape such that the metal is deformed in the cavity between them
why are casting techniques applied? (436)
Casting techniques are employed when
- the finished shape is so large or complicated that any other method would be impractical
- a particular alloy is so low in ductility that forming by either hot or cold working would be difficult
- in comparison to other fabrication processes, casting is the most economical
welding (437)
two or more metal parts are joined to form a single piece when one-part fabrication is expensive or inconvenient (laser beam welding is very convenient)
annealing (439)
- heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled
- relieves stresses, increases softness, ductility, and toughness, and produces a specific microstructure
process annealing (439)
- heat treatment that is used to negate the effects of cold work— that is, to soften and increase the ductility of a previously strain-hardened metal
- recovery & recrystallization occurs
stress relief annealing heat treatment (440)
- piece is heated to the recommended temperature, held there long enough to attain a uniform temperature, and finally cooled to room temperature in air
- removes stress
lower critical temperature (440)
- horizontal line at the eutectoid temperature
- below which, under equilibrium conditions, all austenite has transformed into ferrite and cementite phases
upper critical temperature (440)
- phase boundaries denoted as A3 and Acm
- for temperatures and compositions above these boundaries, only the austenite phase prevails
normalizing (440)
- used to refine the grains and produce a more uniform and desirable size distribution because fine-grained pearlitic steels are tougher than coarse-grained ones
- accomplished by heating at least 55!C (100!F) above the upper critical temperature
austenitizing (440)
when alloy transforms completely into austenite