Ferous Alloys and casting Flashcards
If an alloy is heated but does’nt to reach the lines on a phase diagram then cool what will its microstructure consist off
Martensite
Once a microstructure has formed eg pearlite can it be converted to another microstructure without being heated to ferrite first
No
Give an example of a slow cooling method
Full annealing
What microstructure does full annealing usually produce
Coarse Pearlite
Give an example of moderate cool
Normalising
What microstructure does normalising normally produce
Fine Pearlite
Name a method of fast cooling
Quenching
What microstructures does quenching usually produce
Pearlite or Martensite
How can a Bainitic Microstructure be achieve
Adding other alloying elements to the steel alloy
What sort of process is the process in which austensite becomes Martensite
A diffusion less process
How does the ductility of Tempered Martensite compare to that of Martensite
It is more ductile
What the hardest and strongest microstructure
Martensite
What is the most brittle microstructure
Martensite
What causes the residual stresses in martensite
The differences in densities
Spherodite is very….
Soft and Ductile
What is the most ductile microstructure of steel
Spherodite
Is spherodite formed by heating above or bellow eutectoid temperature and how long is it left for
Bellow and 18-24 hours
Spherodite consists of a ferrite phase containing particles of…
Cementite
Ferrite is a relatively……… &…………….. Microstructure
Soft and ductile
What are some of the characteristics of Cementite
Hard and brittle
What is pearlite made up of
Layers of cementite and ferrite
Which is stronger fine pearlite or coarse pearlite
Fine pearlite is stronger
What is stronger Pearlite or pearlite + cementite
Pearlite + Cementite
Is Bainite is stronger than Pearlite and ferrite
Yes stronger than both
Describe the process of annealing
Heat, Hold, slow cool
What is Annealing used for
To soften and make more ductile
What is process annealing used for
To negate the affects of cold working
How does cold working make a substance harder
Increases the number of dislocations which makes them harder to move
Process Annealing increases the ………….
Electrical Conductivity
What are some of the causes for internal stresses to develop
Plastic deformation during machining and non uniform cooling
What temperature does stress relief annealing cool to
Room Temperature
Does normalizing involve heating above or bellow eutectoid temperature
Above
Does Normalizing involve a phase change
No
Does spherodising take place above or bellow eutectoid temperature
Bellow
Does full annealing take place above or bellow Eutectoid temperature
Above (usually)
In normalising what causes the alloy to become more machinable
Refined grain size and uniform distribution of grain sizes
Normalising takes place 55-85 degrees above…….
Upper critical temperature
What sort of steel (in terms of carbon) is full annealing used to treat
Low or medium
In full annealing the specimen is Austenised ………………. degrees above upper critical temperature
15-40
Full annealing produces ………………. material than normalising
Softer
What does the quenching medium control
The rate of heat extraction and therefor the amount of martensite that forms
What does Tempering usually follow
Quenching
What does tempering involve
Heating martenstitic steel for a specified time between 300-600 degrees bellow the eutectoid temperature
What diffuses from microstructure during tempering
Excess carbon
During tempering of martensite What precipitate forms to reinforce BBC alpha ferrite phase
Fe3C Iron Carbide
What is Precipitation hardening only used for
Non ferrous alloys
Name the 2 stages of precipitation hardening
Solution heat treatment
Precipitation heat treatment
Describe the solution heat treatment stage of precipitation hardening
Heat until all solute is dissolved, just alpha phase present, then rapid cool or quench
Describe the precipitation heat treatment stage of precipitation hardening
Reheat to intermediate temperature to allow precipitation of beta
For precipitation hardening what causes the increase in strength and stiffness
The distortion of crystal lattice, this is caused by precipitation particles, these inhibit the motion of dislocations
What is the shaping process chosen dependent on
Mechanical properties of material
How does the carbon content effect the Yield strength and tensile strength of the steel
Increasing carbon content increases the tensile strength and yield strength
How does increasing carbon content of steel effect the ductility
Increasing carbon content reduces the ductility
Describe the yield strength and the ductility of wrought iron
Low strength and high ductility
Ductile materials can be formed at…….
Room Temperature
How does increasing the temperature effect the strength the ductility of a material
Makes it weaker but more ductile
How does the quality of casting compare to forging
Lower quality
What are the 2 main categories of metal alloys
Ferrous and non-ferrous
What has higher carbon content, steels or cast iron and what are these contents
Steel has less, <1.4 and cast iron has 3-4.5
What is the oldest way to extract iron from ore
Bloomary process
What does the bloomary process involve
Heating the iron with charcoal until soft but not molten
Describe the output of a blast furnace
A very high carbon (4-5 wt%) iron called big iron. which contains lots of impurities. Pig iron is very brittle
What are the inputs and outputs of the blast furnace
Limestone, iron ore, coke and hot air in. Pig iron, Slag and exhaust gas out.
What are the chemical reactions involved in the blast furnace
Heat generation C+02=C02 Reduction of iron ore CO2+C=2CO 3CO+Fe2O3=2Fe+3CO2 Purification CaCO3=CaO+CO2 CaO+SiO2+AL2O3=Slag
What is coke formed from
Coal
What is a Cupola furnace used for
Making pig iron ready for casting
What are puddling furnace’s used for
Remove excess carbon from pig iron
What does the Bessemer process involve
Blowing air through molten iron to remove C as CO2
What does a basic oxygen furnace involve
Blowing pure oxygen through molten iron to remove C as CO2
Basic Oxygen furnace accounts for 70% of steel production in USA what is the other 30%
Electric arc Furnace- better quality but more expensive
What types of iron contain flakes of Graphite and what cooling methods to produces each
White iron (fast cool) , pearlitic grey iron (moderate cool) and Ferritic grey iron (slow cool)
What types of iron produce nodules of graphite and what are the cooling methods to produce each
Ductile pearlitic iron (moderate)
Ductile Ferritic iron (slow)
What elements should be added to produce ductile iron
Mg/Ce
How would you produce pearlitic and ferritic malleable iron which have graphite in rosettes
Reheat white iron at 700 degrees C for about 30 hours
What are the carbon and silicon contents of grey iron
3-4wt% carbon and 1-3wt% silicon
What are some of the properties of grey iron
Weak and brittle under tension strong under compression good vibration dampening cheap cant be deformed
What are some properties of white iron
hard but brittle
<1wt% Si
more cementite that grey iron
can be found on the surface of grey iron
Which type of iron is ductile
Malleable iron
What are some advantages of casting
Versatility
can be used for any molten metal
can be used to create complex shapes
What are some disadvantages of casting
Porosity, poor tolerances and surface finishes
What are some problems that can occur during casting
Turbulence while pouring and Gas entrapment
What is equation for the velocity down the downsprue
v= (2gh)^1/2
What is the equation of the flow rate from the continuity law
Q=Av
What is the equation for the filling time of the mould
t=V/Q
What is the equation of Reynolds number
(velocityDiameterDensity)/Viscosity
What is the Reynolds number defined as
Inertial versus viscous forces of fluid flow
if the Reynolds number is greater than………. it can cause damage
20000
What happens to metals during solidification
They shrink
What can the shrinkage of metals cause
A cavity to form in the inside the cast
What part of a closed mould ensures that the cavity is filled
The riser
What is the equation for solidification time
t= mould constant * (volume/area)^n
n=2
In a dendritic what does the width of the mushy zone depend on
Length of freezing zone
In dendritic structure what causes micro voids to form
Pockets of liquid trapped inside growing dendritic structure
What are the limitations of quality in cast parts and what are the causes
Voids caused by air entrapment
Damaged mould caused by turbulence
Cavities due to shrinkage
Micro cavities when solidifying mushy zone
Rapid cooling can produce concentration gradients
