Personal and Public Transport- materials Flashcards
Why do we heat treat ferrous metals
Generally alters the properties of Metals
- For stress relief
- Achieve final strength and harness
- To release the effects of strain hardening and to prepare the material for further deformation
What do all the following terms from the image mean?
Cementite- consisting of 6.67% carbon dissolved in BCC iron – it is an interstitial compound, Fe3C, that is extremely hard and brittle
- *Ferrite- carbon dissolved (BCC) iron, up to a maximum of 0.025% at 723C; it is an interstitial solid solution that is very soft, ductile and malleable**
- *Pearlite**- full pearlite at ~ .8% of carbon – mixture of ferrite and Cementite phases – depends on cooling process of strength – very tough, max toughness at eutectoid range (0.8% carbon)
- *Austenite** - > 2% C (FCC), very soft and ductile. Forms Ferrite and Cementite when cooled
How do Martensite and Dendrites form?
- *Martensite** – (BCT) Formed when austenite is cooled very rapidly (quenched) so that the C atoms in austenite have no time to diffuse to form Cementite - Results in many dislocations
- *Dendrites** the skeleton shaped grains formed during the solidification of many metals; also a microstructure feature, formed only when cooling an alloy from a liquid, and consist of skeleton shaped grains which are drawn using curved lines
What is a eutectoid steel?
Eutectoid steel is a steel having a composition of 0.8% carbon
What is a mild/low carbon steel?
< 0.3% C tensile, moderate strength, low costs, e.g. bolts, steel plates, wheel rims
What uis a medium carbon steel?
0.3 - 0.6% C - reduced weldability e.g. axles wires, brake springs (hot drawn/rolled, formed into helical shape, hardened then tempered)
What is High Carbon -
0.6 – 0.9%C - strong and durable, toughness and high tensile strength, high wear resistance
Grain structure of ferrous metals as carbon contnet increases?
As carbon increases characteristics chnage from
soft to tough to hard+brittle
What is carbon content of cast iron?
2 – 4 %C inexpensive, strong, brittle, compressive e.g. tools
- Has carbon in the form of graphite instead of Cementite like steels
- Easily cast
- Good is machinability
What is grey cast iron
a cast iron when molten iron, containing 2.8% to 4.0% carbon, is slowly or moderately cooled in a mould; the resultant structure has graphite flakes in a pearlite or ferrite matrix – it is very strong in compression, but weak in tension
- excellent dampening properties
- silicon (>1.5%) is added to form graphite
What is malleable cast iron?
malleable cast iron a cast iron produced when white cast iron is reheated to 800∞C and soaked for 30 to 50 hours; a moderate cooling rate produces graphite rosettes in a pearlite matrix while a slow cooling rate produces graphite rosettes in a ferrite matrix\
- – it has higher tensile properties than gray cast iron, and is also tougher
- Spherical shape = fewer stress concentrations
What is white cast iron?
white cast iron a cast iron produced
when molten iron, containing 2.8% to
4.0% carbon, is rapidly cooled in a
mould, the resultant structure has
dendrites of pearlite in a cementite
matrix; it is extremely hard and
brittle
- used in wrecking balls
What is Annealing?
Heating to red heat then cooled slowly in furnace.
- Relieves any stress from distorted grains. Large uniform grains.
- Not really wanted due to poor toughness.
- Increases ductility
- Lowers strength + harness
What is process Annealing?
Usually used during cold working processes, when material has to be made more ductile and stresses have to be relived for the process to continue (e.g. sheet and wires)
- For low carbon steels (<0.3% C)
- Heated to below red heat then cooled in air. Ferrite is recrystallised (form equiaxed grains) but Pearlite remains unchanged.
What is sperodising Annealing (subcritical annealing)
For anything not low carbon (>0.3% C)
- Piece is “soaked” in 650-700 C to allow Cementite to form in spheres due to surface tension. Then allowed to cool slowly in a furnace
- Improves Machinability
What is Nomalising?
- Heated to above red heat (austenitising) then cooled in air. Smaller equiaxed grains.
- Relieves stress
- Improves Machinability
What is tempering?
Tempering is a softening process that is applied to materials that have been previously hardened by quenching, for example, martensite. These materials are usually extremely hard and brittle and will fail under minor impacts. The steel is heated to between 200oC and 450oC, soaked and allowed to air cool. This process relieves the internal stresses, and results
in a slight decrease in the hardness but a significant increase in the toughness of the material.
