The Chemistry and Structure of Iron and Steel

The Chemistry and Structure of Iron and Steel

اسلاید 1: Ferrous Metallurgy: The Chemistry and Structure of Iron and Steel

اسلاید 2: Pure IronIron from which the residual carbon left over from smelting has been removed.In the pure state it is a very soft grey metalOf no commercial use

اسلاید 3: Wrought IronHas approx 0.05% carbonUsed since about 2000 BCIs stronger than most other pure metals.Made into weapons, armour, cooking pots and vesselsMain limitation to wider uses due to processing (no way of making large items and no welding)

اسلاید 4: Abraham Darby’s Ironbridge

اسلاید 5: Cast IronBetween 2% & 4% carbon contentStandard grey cast iron very brittle due to carbon rosettes in the structure acting as stress-raisersPossible to use heat treatment to improve the structure, this gives materials such as ductile iron and malleable iron (black heart)

اسلاید 6: Ductile iron used in drain grids

اسلاید 7: Grey cast iron showing the graphite flakes in a pearlite matrix

اسلاید 8: Steel0.001% to 1.5% carbonWide range of properties due to:Variation in carbon contentCold workingHeat treatmentAddition of alloying elements

اسلاید 9:

اسلاید 10: Microstructure of SteelFive main constituents:FerriteAusteniteCementitePearliteMartensite

اسلاید 11:

اسلاید 12: FerriteThe structure of pure iron. Has a body-centred cubic (BCC) crystal structure. It is soft and ductile and imparts these properties to the steel. Very little carbon (less than 0.01% carbon will dissolve in ferrite at room temperature). Often known as  iron.

اسلاید 13: A photomicrograph of 0.1% carbon steel (mild steel). The light areas are ferrite.

اسلاید 14: AusteniteThis is the structure of iron at high temperatures (over 912 deg C). Has a face-centre cubic (FCC) crystal structure. This material is important in that it is the structure from which other structures are formed when the material cools from elevated temperatures. Often known as  iron. Not present at room temperatures.

اسلاید 15: CementiteA compound of iron and carbon, iron carbide (Fe3C). It is hard and brittle and its presence in steels causes an increase in hardness and a reduction in ductility and toughness.

اسلاید 16: PearliteA laminated structure formed of alternate layers of ferrite and cementite. It combines the hardness and strength of cementite with the ductility of ferrite and is the key to the wide range of the properties of steels. The laminar structure also acts as a barrier to crack movement as in composites. This gives it toughness.

اسلاید 17: Two-dimensional view of pearlite, consisting of alternating layers of cementite and ferrite.

اسلاید 18: Three-dimensional analogy to the structure of pearlite, i.e. the cabbage represents a single crystal of pearlite, and the water in the bucket the single crystal of ferrite.

اسلاید 19: MartensiteA very hard needle-like structure of iron and carbon.Only formed by very rapid cooling from the austenitic structure (i.e. above upper critical temperature). Needs to be modified by tempering before acceptable properties reached.

اسلاید 20: The needle-like structure of martensite, the white areas are retained austenite.

اسلاید 21: CarbonIn steels none of the carbon is present as free carbon. It is all dissolved in the iron as part of the previously described structures.

اسلاید 22: 0.1% Carbon SteelNote the small amount of pearlite in the structure

اسلاید 23: ApplicationsA typical application of low carbon steel in a car body.

اسلاید 24: Effect of Carbon ContentIncreasing the carbon content decreases the amount of ferrite and increases the proportion of pearlite in the structure.

اسلاید 25: 0.2% Carbon SteelNote the increased amount of pearlite compared with the 0.1% ‘dead mild’ steel

اسلاید 26: Eutectic StructureThis leads to an increase in strength and hardness and a reduction in ductility. This continues until there is 0.8% carbon at which point the structure is 100% pearlite. This is known as a eutectic structure.

اسلاید 27: Over 0.8% CarbonAs carbon content increases beyond 0.8%, no more pearlite can be formed. The excess carbon forms cementite which is deposited in between the pearlite grains. This increases the hardness, but slightly reduces the strength. The ductility of all plain carbon steels over 0.8% carbon is very low.

اسلاید 28:

اسلاید 29: Properties of Carbon Steels

اسلاید 30: Properties of Carbon Steel