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99 Cards in this Set

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forming
indicates changing the shape of an existing solid body
shaping
involves molding and casting of soft or molten materials, finished product is usually at or near the desired shape, forcing molten polymer into a mold and letting it solidify
continuous products
made by rolling, extrusion, drawing processes
discrete products
made by forging, forming, ...
ingot
metal cast into a block shape for convenience, to be further processed
how are cast structures converted to wrought structures?
by plastic-deformation
sinter
heat without melting
forming and shaping processes
rolling (flat, shape), forging, extrusion, drawing, sheet-metal forming, powder metallurgy, processing of plastics and composite materials, forming and shaping of ceramics
flat rolling
high speed foil production
shape rolling
high speed production of i-beams and rails
forging
production of discrete parts using dies
extrusion
production of long, constant shapes; cut into desired lengths
drawing
production of long rod and wire
sheet-metal forming
production of simple or complex shapes with thin walls
powder metallurgy
production of simple or complex shape by compacting and sintering metal powders
processing of plastics and composite materials
production of a wide variety of continuous or discrete products by extrusion, molding, casting, and fabricating processes
forming and shaping of ceramics
production of discrete products by various shaping, dring, firing processes
rolling
process of reducing the thickness or changing the cross section of a long workpiece by compressive forces applied through a set of rolls
which forming and shaping process accounts for 90% of all metals produced by metalworking processes?
rolling
hot rolling
first stage of rolling, rolling carried out at high temps, coarse-grained, brittle, and porous structure of ingot is broken down into a wrought structure with finer grain size and better properties
cold rolling
subsequently, rolling carried out at room temp
-good: improves strength and hardness and surface finish
-bad: causes anisotropy
why does cold rolling require more energy than hot rolling?
because of higher strength of the material at room temp
anisotropic
has different properties when measured in different directions
plates
high thickness (ship hulls, boilers, bridges, ...), more than 6mm
sheets
low thickness (automobile bodies), less than 6mm
foil
used for wrapping candy, thickness of .008mm
flat-rolling process
metal strip with thickness no enters roll gap and is reduced to thickness nf by rotating rolls (surface speed of rolls =Vr), velocity of strip increases from Vo (enters roll gap) to Vf (exits roll gap, highest)
roll gap
space between rolls
neutral point / no-slip point
-point along contact length where the velocity of the strip is the same as the roll
-left of neutral point, roll is faster
-right of "", strip is faster
maximum draft (eq)
different between initial and final thickness, ho-hf, function of roll radius (R) and coeff. of fric (u) s.t. ho-hf=(u^2)R
roll force (eq)
F=Lw(Yavg), F=roll force, L=roll-strip contact length, w=width of strip, Yavg=avg true stress of strip in roll gap
how do you modify roll force eq to get ACTUAL roll force?
add 20%
torque
F*a, a=L/2
Power (kW)
=(2piFLN)/60,000, N=revolutions/min of roll, F=newtons, L=meters
Power (hp)
=(2piFLN)/33,000, F=lbs, L=ft
roll stand
includes housing, chocks, bearings
ways to reduce roll forces
reduce friction at roll-workpiece interface, use smaller diameter rolls to reduce contact area, take smaller reductions per pass to reduce contact area, rolling at elevated temps to lower strength of the material, apply front or back tension to strip
back tension
tension at the entry zone, applied to sheet by applying a braking action to the reel that supplies the sheet into the roll gap
front tension
tension at the exit zone, applied to sheet by increasing rotational speed of the take-up reel
steckel rolling
process carried out by front tension only, with no power supplied to the rolls
roll bending
tendency of rolls to bend during rolling, caused by the difference between forces required to bend strip at center and edges
crown
tendency of rolled strip to be thicker at center than edges, result of roll bending
camber
way to counteract crown by grinding rolls in such a way that their diameter at center is slightly larger than at edges, so that strip has contant uniform thickness
what is the difference between the radius of the max camber point and and the edges of the roll
radius of max camber point is generally .25 mm greater than that of the edges
thermal camber
tendency of rolls to become barrel shaped because of the heat generated by plastic deformation during rolling
what happens to the width of the strip when rolling plates and sheets have high width-to-thickness ratios?
the width of the strip remains effectively constant
spreading
with small width-to-thickness ratios, width increases significantly as it passes through the rolls, like rolling dough with a rolling pin
spreading increases with:
a. decreasing width-to-thickness ratios
b. increasing friction
c. decreasing ratio of the roll radius to the strip thickness
how do you prevent spreading?
using additional rolls in contact with the edges of the rolled product in the roll gap (edger mills)
chatter
-self-excited vibration
-in rolling it leads to periodic variations in the thickness of the rolled sheet and in its surface finish, and consequently, leads to excessive scrap
tandem mills
where chatter in rolling have been found to occur predominantly
how much does chatter slow rolling?
by 50%
cause of chatter?
interaction between structural dynamics of the mill stand and the dynamics of the rolling operation
ways to reduce chatter
increasing dist between the stands of the rolling mill, increasing the strip width, decreasing the reduction per pass (draft), increasing roll radius, increasing strip-roll friction, incorporating dampers in the roll supports
cast structure
dendritic with coarse and nonuniform grains, brittle and porous
dendritic
having a branching structure similar to a tree
flat rolling process
1. break down material by hot rolling, converts cast structure to wrought structure-->resulting wrought structure is bloom, slab, or billet
2. cold rolling
wrought structure
finer grains and better ductility by breaking up brittle grain boundaries and closing up internal defects (porosity)
bloom
square cross section, becomes i-beams and railroad rails
slab
rectangular cross section, becomes plates and sheets
billet
square with smaller cross section than bloom, rods and bars
conditioned
prepared for subsequent operation; surface of bloom, slab, and billet is usually conditioned before rolling
scarfing
conditioning by torch
pickling
acid etching, by blasting with water or grinding
pack rolling
flat-rolling operation in which 2 or more layers of metal are rolled together to increase productivity
ex: foil is pack rolled in 2 layers, only the top and bottom outer layers have been in contact with rolls, foil-to-foil side is matte, foil-to-roll side is shiny
yield-point elongation
phenomenon that causes surface irregularities called stretcher strains or luder's bands, happens to rolled mild steel when subsequently stretched during sheet-forming operations
how do you correct yield-point elongation?
sheet metal undergoes a final light pass of 1% reduction known as temper rolling or skin pass
leveling rolls
to improve flatness, rolled strip goes through series of "", the workpiece is flexed in opposite directions as it passes through the sets of rollers
(T/F) Each roller is usually powered individually by a separate motor.
T
surface defects
scale, rust, scratches, gouges, pits, cracks
wavy edges
result of roll bending, strip is thinner along edges than at its enter causing the edges to elongate more than the center, the edges buckle because they are constrained by the central region from expanding freely in the longitudinal (rolling) direction
alligatoring
complex phenomenon and typically is caused by nonuniform bulk deformation of the billet during rolling or by the presence of defects in the original cast material
residual stresses in rolled metals
small diameter rolls: compressive "" on surfaces and tensile "" in bulk
large diameter rolls: opposite
gage number
number identifying the thickness of a sheet, smaller=thicker
two-high rolling mills
used for hot rolling in initial breakdown passes on cast ingots or in continuous casting with roll diameters from .6m to 1.4m
three-high mill
direction of material movement is reversed after each pass, using elevator mechanism and various manipulators
four-high mills and cluster mills (sendzimir or Z mill)
based on the principle that small-diameter rolls lower roll forces (why? because of small roll-strip contact area) and power requirements and reduce spreading, also smaller rolls are cheaper to replace
tandem rolling
strip is rolled continuously through a number of stands to thinner gages with each pass, speed and thickness of strip is critical
stand
consists of a set of rolls with its own housing and controls
train
group of stands
heat checking
cracking from thermal cycling, results from using rolls made for cold rolling for hot rolling
spalling
cracking or flaking of surface layers
why does the bottom surface of an aluminum beverage can have longitudinal scratches on it?
because the surface of the can is a replica of the surface finish of the roll, which is produced by grinding, makes it easy to see the rolling direction of the original aluminum sheet
are lubricants used in hot rolling?
hot rolling of FERROUS alloys is carried out without lubricants although graphite may be used, NONFERROUS alloys are hot rolled with various compounded oils, emulsions, and fatty acids
are lubricants used in cold rolling?
cold rolling is carried out with water-soluble oils or low-viscosity lubricants, such as mineral oil, emulsions, paraffin, fatty oils
shape rolling
forms straight and long structural shapes (i-beams, rails), the stock goes through a set of specially designed rolls
cold shape rolling
can be done with the starting materials in the shape of wire with various cross sections, roll-pass design requires considerable experience
roll-pass design
the design of a series of rolls, requires considerable experience to avoid external and internal defects, hold dimensional tolerances, and reduce roll wear
roll forging (also cross rolling)
the cross section of a round bar is shaped by passing it through a pair of rolls with profiled grooves, used to produce tapered shafts and leaf springs, table knives
skew rolling
similar to roll forging, used for making ball bearings, round wire is fed into the roll gap and spherical blanks are formed by the action of rotating rolls
ring rolling
a thick ring is expanded into a large-diameter thinner one by placing it between 2 rolls and driving 1 while the other is idle, thickness is reduced by bringing the rolls closer together as they rotate
thread rolling
cold-forming process by which straight or tapered threads are formed on round rods or wire, threads are formed with each stroke of a pair of flat reciprocating dies, makes screws and bolts
thread rolling advantages
threads with good strength and without loss of material, smooth surface finish, induces compressive residual stresses on surfaces (thus improving fatigue life)
internal thread rolling
carried out with a fluteless forming tap, similar to external thread rolling, produces internal threads with good strength
lubrication in thread rolling
important to obtaining good surface finish and surface integrity
rotary tube piercing
also mannesmann process, hot-working operation for making long, thick-walled seamless pipe and tubing
tube rolling
used to reduce diameter and thickness of pipes and tubing
integrated mills
large facilities that involve complete integration of activities, from production of hot metal in a blast furnace to casting and rolling of finished products
minimills
scrap metal is:
a. melted in electric-arc furnaces
b. cast continuously
c. rolled directly into specific lines of products
each minimill makes 1 type of product (rod, bar), scrap metal is obtained locally from old machinery (cars)