Metals Flashcards
What is a ferrous alloy? Give examples.
An alloy with iron as its main constituent.
e.g. cast irons, steels.
Give three advantages of ferrous alloys
- Iron-containing compounds are abundant in the Earth’s crust
- They are very cheap to extract/refine/alloy etc.
- They are incredibly versatile.
What are the major disadvantages of ferrous alloys? (3)
- Poor corrosion resistance (they rust)
- Low conductivity
- High density
What are steels?
They are iron-carbon alloys that can contain a considerable amount of other alloying elements. Steels usually have a carbon content of less than 1 wt%.
What are plain carbon steels?
They are steels which contain very very little impurities (other than carbon and a little manganese).
What are alloy steels?
Steels which have had impurities/other alloying elements added to it in specific concentrations to change the properties.
Give the facts for plain low-carbon steels (6)
- >0.25wt%c
- No impurities added
- average hardenability
- Low tensile strength
- some elasticity
- used in automobiles
What are the facts for High-strength, low alloy steels? (6)
- >0.25 wt%c
- can contain additions of Cr, V, Ni and Mo
- some hardenability
- average tensile strength
- some elasticity
- used in bridges
What are the facts for plain medium-carbon steels? (6)
- Carbon concentrations between 0.25 and 0.6wt%
- no additions
- Some hardenability
- some tensile strength
- average elasticity
- used in hammers
What are the facts for heat treatable medium carbon steels? (6)
- between 0.25-0.6wt%C
- can contain Cr, Ni, and Mo
- Good hardenability
- good tensile strength
- poor elasticity
- used in gears
What are the facts for plain high carbon steels? (6)
- Contains a carbon content between 0.6 and 1.4 wt%
- no additions
- good hardenability
- some tensile strength
- low elasticity
- used in wear applications
What are the facts for Tool (high carbon) steels? (6)
- contains between 0.6-1.4wt%C
- can contain additions of Cr, V, Mo and W
- very hardenable
- good tensile strength
- very low elasticity
- used in drills
What are the facts for Austenitic stainless steels? (5)
- can contain additions of Cr, Ni and Mo
- average hardenability
- average tensile strength
- good elasticity
- used in furnaces
What are cast irons?
A class of ferrous alloys with carbon contents above 2.1 wt%.
What is the carbon wt% of a typical cast iron?
Between 3 and 4.5 wt%
Why is casting often the most convenient fabrication technique for cast irons?
They have a melting point which is significantly lower than steels.
They are very brittle
Why is the carbon in cast irons mostly graphite?
The iron carbide (cementite) decomposes to form alpha-ferrite and graphite(C)
Why do cast irons often need slower cooling rates during solidification?
Slower cooling rates favour graphitization
How many types of cast iron are there?
4
What are the types of cast iron?
Gray iron, Ductile iron, White iron and Malleable iron
What are the facts of Gray iron? (3)
- Has graphite flakes surrounded by an alpha-ferrite or pearlite matrix
- Carbon content between 2.5 and 4 wt%
- Silicon content between 1 and 3 wt%
What are the advantages and disadvantages of Gray iron? (3 advantages, 1 disadvantage)
- Stronger under compression
- Excellent vibrational dampening
- Wear resistant
- Weak and brittle under tension
Why is Gray iron weaker under tension?
The tips of the graphite flakes can act as points of stress concentration
What are the facts for ductile iron?
- Gray iron but has additions of Mg and/or Ce, which significantly changes the microstructure and properties.
- Graphite still forms but as spherical balls
- Matrix is often pearlite, as more ductile, but can be ferrite with excessive heat treatment
What are the advantages of ductile iron? (2)
Stronger and more ductile than Gray iron
What are the facts for white cast iron?
- Less than 1 wt% silicon
- Large amounts of the cementite phase
- The inside has much slower cooling times so Gray iron usually forms
What are the advantages and disadvantages of white cast iron?
- Very rapid cooling rates
- Due to the large amounts of cementite, it is very hard but very brittle so it is virtually unmachinable
What are the facts for Malleable iron?
- It is white iron if it was heat treated at 800-900 degrees for a prolonged time period in a neutral atmosphere.
- Graphite forms in clusters (rosettes)
- surrounded by a ferrite or pearlite matrix, depending on the cooling rate (fast = pearlitic, slow = ferrite)
What are the advantages of malleable iron?
- High strength
- More ductile/malleable
Which cast irons are made with fast cooling times? (2)
White, pearlitic malleable
Which cast irons are made with moderate cooling times? (2)
Pearlitic Gray, Pearlitic ductile
Which cast irons are made with slow cooling times? (3)
Ferritic Gray, Ferritic ductile and Ferritic malleable
What are the three copper alloys?
- Brass
- bronze
- Cu-Be
What are the facts for brass? (2)
- Zinc, as a substitutional impurity, is the major alloying element
- Used as costume jewellery and coins
What is the advantage of brass?
Corrosion resistant
What are the facts for Bronze? (2)
- Alloys of copper and several other elements like tin, Al, Si and Ni
- Used as landing gear
What are the advantages of bronze? (2)
They are stronger than brasses but are still highly corrosion resistant
What are the facts for beryllium coppers? (3)
- Contain between 1 and 2.5 wt% Be
- High strengths are obtained via precipitation-hardening heat treatments
- Used as surgical and dental instruments
What are the advantages and disadvantages of beryllium coppers? (5 advantages, 1 disadvantage)
- Very high tensile strengths
- Excellent electrical properties
- corrosion resistant
- Wear resistant
- May be cast, hot worked or cold worked
- They cost a lot as Be is expensive
What are the facts for aluminium alloys? (5)
Low density (2.7 g/cm3)
Mechanical strength can be improved by alloying/cold working but both diminish resistance to corrosion
Can contain additions of Cu, Mg, Si, Mn, Zn
Depending on the alloy an increase in strength is achieved via solid-solution hardening or precipitation hardening
Used as aircraft parts
What are the advantages/disadvantages of aluminium alloys? (4 ad/ 1 dis)
- Low density of 2.7 g/cm3
- High electrical and thermal conductivities
- Some resistance to corrosion
- High ductility
- Low melting temperature
What are the facts for Mg alloys? (2)
- Very low density of 1.7 g/cm3
- Cannot cold work
- Used in aircrafts
What are the advantages/disadvantages of Mg alloys? (3 dis 2 ad)
- Low melting temp
- Susceptible to corrosion in marine environments
- Fine Mg powder ignites easily
- Low elastic modulus
- Soft
What are the facts for Ti alloys? (3)
- Low density (4.5 g/cm3)
- Hexagonal close-packed crystal structure
- Used in space applications
What are the advantages/disadvantages of Ti-alloys? (5 ad 2 dis)
- High density
- Extremely strong
- Highly ductile
- Easily forged and machined
- Very corrosion resistant
- Is very reactive at high temps
- Very expensive
Facts about refractory metals (2)
- Group consisting of Niobium(Nb), Molybdenum(Mo), Tungsten(W) and Tantalum(Ta)
- Ta and Mo are alloyed with stainless steel to improve corrosion resistance
What are the advantages of the refractory metals (4)
- Very high melting points
- Large elastic moduli
- High strength
- Very hard
Facts for Noble Metals (3)
- Group of 8 metals, but most common are Gold (Ag), Silver (Au) and Platinum (Pt)
- Used in jewellery
- Au and Ag may be strengthened by solid-solution alloying w/ Cu
What is the general description for forming operations
Forming operations are those in which the shape of a piece of metal is changed by plastic deformation
What are the 4 forming operations?
Forging, Extrusion, Rolling and drawing
FERD
What is the difference between hot and cold working?
When the treatment occurs above the temperature at which recrystallization occurs, otherwise it is hot working
What are the benefits and weaknesses of hot-working? (2 ben 1 weak)
- Large deformations are possible and may be repeated because the metal is soft and ductile
- Deformation energy requirements are less
- BUT most metals experience surface oxidation, which leaves a poor finish
What are the benefits and weaknesses of cold-working? (4 ben 2 weak)
- Increases strength but decreases ductility
- Higher-quality surface finish
- Greater variety (and mostly better) mechanical properties
- Closer dimensional control
- Expensive
- Inconvenient
Describe the process of forging
- Usually done on hot metal
- Application of excessive blows or by continuous squeezing
- {Think classic hammering blacksmith and see image}
Describe the process of rolling (2)
- Piece of metal passes between two rollers results in a reduction in thickness
- Can use cold to create high-quality sheets of metal
Describe the process of Extrusion
Metal is pushed through a slit and can make stuff with complicated cross-sectional geometries
See pic
Describe the process of drawing (2)
Pulling a piece of metal through a tapered/slanted die
Multiple dies can be combined to thin the metal gradually
What is the general description for casting?
Fabrication process where 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.
When are the casting techniques used? (3)
- 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 fabrications, this is the most economical.
What are the 3 casting techniques?
Sand casting, Die casting, Investment casting and continuous casting
Describe the process of sand casting and give some products it produces. (3 and 1 product)
- Ordinary sand is the mold material
- A two-piece mold is formed by packing sand around a pattern that has the shape of the intended cast
- A gating system is involved to expedite the molten metal into the cavity and to minimise defects.
- Produces large things like fire hydrants
Describe the process of die casting and when is it used? (3 and one)
- Liquid metals are forced into a mold under pressure at a high velocity and allowed to solidify
- The mold is then taken apart to leave the metal structure
- The mold can then be used again
- This technique is used for high volume, low-temperature alloys like Zn, Al and Mg
Describe the process of investment casting and when is it used? (4 steps one use)
- Pattern made from paraffin
- Mold made by enclosing in plaster of Paris
- Melt wax and hollow mold is left
- Pour in metal
- Used for low volume complex shapes like jewellery
Describe the process of continuous casting and give a use (two points and a use)
- The refined and molten metal is cast directly into a continuous strand that can have a rectangular or circular cross-section; solidification occurs in a water-cooled die
- The mechanical and chemical properties are constant throughout
- Used to make big slabs efficiently
What are the two miscellaneous fabrication techniques?
Powder metallurgy and welding
Describe the process of powder metallurgy (4)
- Powdered metals are mixed to the required composition
- The powders are compressed to form the required shape
- The shapes are heated to a point where they are below the melting point, but the particles are hot enough to merge into one another.
- This decreases density as the space between the particles is smaller
When is powder metallurgy used?
When dealing with materials with low ductility
Give a general description of welding
Two or more metal parts are joined to form a single piece when a one-part fabrication is expensive or inconvenient.
What is the process of welding?
{He skipped over it in lectures do who fucking knows}
I think its where the top/bottom of the connecting faces is melted a bit and a filler metal is put in there to seal the edges
What is the general process of annealing?
Heating to the Tanneal (metal specific), then cooling slowly
What are the 5 types of annealing?
Stress relief, Process annealing, Spherodizing (steels), Full anneal (steel), Normalize (steel)
When would you need stress relief (annealing)?
To reduce the stress done by:
- Plastic deformation
- Non-uniform cooling
- Phase transformation
Distortion and warpage and may occur if the residual stresses are not dealt with.
When is process annealing necessary?
When you need to negate the effects of cold-work, so to soften and increase the ductility of a strain-hardened metal
When is spheroidizing a metal necessary?
When steels contain a lot of pearlite, they may be too hard to be machined to be plastically deformed.
The steels then take on a spheroidite structure to have maximum softness and ductility
What is the process of spheroidizing? (2)
Heat at just below the eutectic temperature and hold for 15-25 hrs
Why would we need to fully anneal a metal?
To make a softer steel for good forming
What is the process of a full annealing (3)
- Used on low-medium carbon steels
- Heats to get a coarse pearlite microstructure which is soft and ductile
- This will give a microstructure with small grains and a uniform grain structure
What is normalizing (annealing : steel) used for?
To make the grain size smaller
What do you need to find to determine a hardness profile for a steel?
- Find the distance from the quenched end, using the diameter of the specimen, that the centre, 1/2R, 3/4R and R are situated
- Read off the hardness values for these distances
- Plot this in a radial hardware profile
Determine the radial hardness profile for a 50mm diameter cylindrical specimen of 1040 steel that has been quenched in water.
What is an important fact of steels at the quenched end of a rod?
Alloys have identical hardness at the quenched end.
What is hardenability?
The ability of an alloy to be hardened by the formation of martensite as a result of a given heat treatment
A steel with a high hardenability is one that hardens, or forms martensite, not only at the surface but throughout the structure.
How does precipitation hardening work? (3)
- Heat the metal to a higher temperature to get alpha solid solution
- Quench it to room temp
- Heat to an intermediate temperature to form the precipitate
Why does precipitation hardening greatly increase the hardness and strength of an aluminium alloy?
The precipitate of Cu (and sometimes other metals) get evenly distributed throughout the structure.
This makes it harder for an imperfection to flow/break through the material
What effect does precipitation hardening have on tensile strength and %Elastic Limit?
With time, tensile strength increases dramatically (can double) up to a time, before the precipitate becomes overly enlarged
%Elastic Limit reached minimum with precipitation time
What is kept constant in the Jominy End-Quench test? (2)
Specimen size and shape and quenching treatment
Why does the amount of martensite decrease with slower cooling rate? (2)
More time is allowed for carbon diffusion and the formation of the softer pearlite
