engg materials Flashcards
abrasives and abrasion
abrasive is a substance used to wear down the surface of other material
abrasion is the ability to wear or tear down the surface of other substance
what should a abrasive posses?
hardness: ability to resist scratching
toughness: ability to scratch other substance
refractoriness: ability to withstand temp
mineral hardeness and mohs hardness order
it is the measure of hardness and resistance to scratching between minerals
talc gypsum calcite fluorite apatite orthoclase quartz topaz corundum diamond
classification of abrasives
natural abrasives - non siliceous and siliceous
non siliceous : pure crystalline carbon,hardest,chemically inert nd not affected by acid or alkalis
diamond, corundum, emery
siliceous: garnet quartz
synthetic-carborundum,alundum
diamond, emery and corundum
diamond: crystalline carbon,chemically inert,hardest,ranks first in mohrs scale
types: bolt-off color diamond used in drill points
carbando-black color diamond used to cut glass and stone
corundum:pure crystalline alumina,transparent,have different colors based on impurities present used in grinding glasses,gems
emery:fine grained opaque black material of 55-75 alumina,20-40 magnetite,12 other not sharp, used for manual polishing
classification of refractories
based on chemical composition
acidic:attacked by alkalis sio2,zro2
neutral:stable to both graphite,chromites
basic:attacked by acids magnesia,dolomite
based on physical form
shaped:fixed shape when delivered
unshaped:without definite form nd are given shpe on application
based on temperature
low:1600-1800 fireclay
high:1800-2000chrmoite
super:>2000 zro2
properties of refractories
rul:ability to withstabd very high temp
dimensional stability:
thermal spaling: fracture of refractory due to compression in structure,variation in coefficient of thermal expansion,temp gradient
porosity:avg % of open pore space in overall refractory volume
thermal expansion: measure of refractory about its linear stability when it i exposed to different ranges of temp
portland cement and manufacture
manufactured from chalk and clay, hardens under water and resembles Portland stone in color
composition: tricalcium silicate,dicalcium silicate,tricalcium aluminate,tetracalciumaluminoferrite, gypsum
raw material:lime,silica,alumina
lime=limestone or chalk silica and alumina=clay or bauxite
wet process
raw materials-homogenised by crushing,grinding and blending such that they pass thrugh no.200 sieve
turned into slurry by adding 30-40% water
head at 2750F in horizontal revolving kilns
natural gas,coal,petroleum are used for burning
uneconomical due to high fuel requirement
dry process
raw material……
fed into kiln and burned in dry state
saves fuel and water consumption but dustier
dry wet
in kiln, water from raw is driven off and limestone is decomposed to lime and carbondioxide
burning zone,portion of kiln,silica and alumina from clay reacts with lime to produce calcium aluminate
rotation and shape of kiln allow miv=mixture to flow down and increases temp
in hotter regions of kiln,calcium silicates are formed
black or greenish in color in the form of small pellets are cement clinkers-hard,irregular,ball shaped
cooled and stored in clinker silos
when needed,clinker is mixed with 2-5 gypsum to retard the setting time of cement when it is mixed with water
grounded to fine powder and stored in storage bins or cement silos
placed on pallets in dry place
garnet and quartz
garnet-trisilicates of alumina,magnesia and ferrous oxide, soft in nature, used along with glue to make paper or cloth used in watches,glass g4rinding
quartz-pure crystalline silica, used as grinding pigments in paint industry,granules in grinding machines and hydrated form is used in sand paper
carborundum and alundum
carborundum-composition sic prepared by heating the mix of sand,coke and saw dust at high temp in an electric furnance used as refractory material
alundum-resistance to acids,stble at high temp not as hard as carborundum,less brittle and tougher used in grinding hard steel and manufacturing of abrasive wheels
wheel bond nd types
it determines how wheel holds abrasives
silicate:mix graded abrasive with sodium silicate jelly,moulded,dried and baked
shellac:mix abrasive with shellac resin,moulded,dried and baked
rubber:embedding abrasive grains on rubber sheets,cutting the sheets into circles of required diameter and finally vulcanising by adding sulphur and heating
synthetic resin:abrasive grains mix with resins and plasticizer,moulded into desired shape by applying heat and pressure
ceramic:mix abrasive with feldspar,clay and water,moulded,dried and fired
types of grit
glasspaper aluminium oxide silicon carbide garnet paper steel wool
characteristics of refractories
used for construction of furnances,kilns,crucibles,etc
act as thermal barriers b/w hot barrier and wall of containing vessel
insure strong physical protection,preventing erosion of walls by circulating hot medium
reperesesnt chemical protective barrier against corrosion
act as thermal insulation
glass and composition
amorphous, rigid, brittle, hard and transparent super cooled liquid resitant to heat transparent mixture of silicates of potassium or sodium composition: soda lime glass potash lime glass potash lead glass
manufacture of glass
raw materials-sand,sodium carbonate,calcium carbonate
mixing-raw powdered nd mixed homogenously with this broken glass pow is added to get batch
fusion:batch is transferred into pot furnance and heated at 1400-1500c at this temp-substances react with batch and mix of silicates called glass
fining:molten glass kept at high temp to remove gas bubbles
fabrication:coloring material is added to molten glass and it is fabricated to desired shape by blowing,moulding or pressing
coloring material: cobalt oxides-blue,cuprous salt-red,chromium oxides-green,ferric iron-yellow,potassium dichromate-green
annealing of glass:slow cooling if hot are cooled suddenly it will break,so cooled by passing through several hot chambers
