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
full annealing (441)
for ferrous alloys, austenitizing, followed by cooling slowly to room temperature
spheroidizing (441)
for steels, a heat treatment normally carried out at a temperature just below the eutectoid in which the spheroidite microstructure is produced
hardenability (442)
ability of an alloy to be hardened by the formation of martensite as a result of a given heat treatment
Jominy end-quench test (442)
- a standardized laboratory test used to assess the hardenability of ferrous alloys
- all factors that may influence the depth to which a piece hardens (i.e., specimen size and shape and quenching treatment) are maintained constant
stainless steels
-highly resistant to corrosion (rusting) in a variety of environments
-predominant alloying element is
chromium
severity of quench (446)
indicate the rate of cooling; the more rapid the quench, the more severe is the quench (water produces most severe)
precipitation hardening (451)
- the strength and hardness of some metal alloys may be enhanced by the formation of extremely small, uniformly dispersed particles of a second phase within the original phase matrix
- small particles are called precipitates
age hardening (451)
strength develops with time, or as the alloy ages
solution heat treatment (453)
- part of precipitation hardening
- all solute atoms are dissolved to form a single phase solid solution
precipitation heat treatment (454)
- part of precipitation hardening
- supersaturated alpha solid solution is ordinarily heated to an intermediate temperature T2 within the alpha-beta two-phase region, at which temperature diffusion rates become appreciable, the beta precipitate phase begins to form as finely dispersed particles of composition
overaging (454)
reduction in strength and hardness that occurs after long time periods
zones (455)
clusters of atoms that are too small to be considered precipitate particles
natural aging (457)
age harden at the normal ambient temperature
artificial aging (457)
carried out at elevated temperatures
plain carbon steels
contain only residual concentrations of impurities other than carbon and a little manganese
alloy steels
more alloying elements are intentionally added
in specific concentrations
high-strength, low-alloy (HSLA) steels
-contain other alloying elements such as copper, vanadium, nickel, and molybdenum
-they possess higher strengths than
the plain low-carbon steels
-ductile, formable, and machinable
medium-carbon steels
-carbon concentrations between about 0.25 and 0.60
wt%
-may be heat-treated by austenitizing, quenching, and then tempering to improve their mechanical properties. -often utilized in the tempered condition, having microstructures of tempered martensite
high-carbon steels
- carbon contents between 0.60 and 1.4 wt%
- hardest, strongest, and yet least ductile of the carbon steels
cast irons
- ferrous alloys with carbon contents above 2.14 wt%
- easily melted and amenable to casting
gray cast irons
- graphite exists in the form of flakes, which are normally surrounded by an a-ferrite or pearlite matrix
- because of these graphite flakes, a fractured surface takes on a gray appearance—hence its name
- comparatively weak and brittle in tension as a consequence of its microstructure
ductile or nodule iron
magnesium and/or cerium added to the gray iron before casting produces graphite that resembles as nodules or spherelike particles instead of flakes
white cast iron
- for low-silicon cast irons and rapid cooling rates, most of the carbon exists as cementite instead of graphite
- a fracture surface of this alloy has a white appearance
malleable cast iron
-white cast iron that has been heat-treated
to convert the cementite into graphite clusters; a relatively ductile cast iron
compacted graphite iron
a cast iron alloyed with silicon and a small amount of magnesium, cerium, or other additives, in which the graphite exists as wormlike particles
cast alloys
alloys that are so brittle that forming or shaping by appreciable deformation is not possible
wrought alloys
amenable to mechanical deformation
unalloyed copper
- so soft and ductile that it is difficult to machine
- has an almost unlimited capacity to be cold worked
- highly resistant to corrosion
brasses
- most common copper alloys
- zinc is the predominant alloying element
bronzes
alloys are somewhat stronger than the brasses,
yet they still have a high degree of corrosion resistance
aluminum and its alloys
relatively low density, high electrical and thermal conductivities, and a resistance to corrosion
temper designation
a letter and possibly a one- to three-digit number, which indicates the mechanical and/or heat treatment to which the alloy has been subjected
specific strength
tensile strength–specific gravity ratio
refractory metals
metals that have extremely high melting temperatures
superalloys
superlative combinations of properties
noble metals
expensive and are superior or notable (noble) in properties—characteristically soft, ductile, and oxidation resistant
noble metals: silver, gold, platinum, palladium, rhodium, ruthenium, iridium, and osmium
forming operations
those in which a metal piece is shaped by plastic deformation
hardenability curve
plots hardness versus distance from the quenched end of a Jominy specimen
