Metallurgy and manufacture of steels Flashcards
What does a eutectoid reaction in the Fe-C system entail?
A eutectoid reaction in the Fe-C system describes the phase transformation of one solid into two different solids, specifically austenite into ferrite and cementite at 0.8 wt% C and 723°C
At what temperature does the bcc ↔ fcc transformation of iron occur?
912°C
What is the maximum carbon content for steel?
The maximum carbon content for steel is less than 2%
What are the three different types of steels based on the iron-carbon phase diagram?
The three different types of steels are hypoeutectoid, eutectoid, and hypereutectoid steels
What is pearlite, and how does it form?
Pearlite is a fine lamellar structure formed directly from austenite in the (α + γ) region of the phase diagram, consisting of alternating layers of ferrite (which is soft and ductile) and cementite
What is the texture of cementite?
hard and brittle
What is the microstructure of low carbon steel?
ferrite formed directly from austenite, with pearlite present as well
How does the cooling rate affect the microstructure of steel?
Affects the fineness of the pearlite in steel; faster cooling rates generally lead to finer pearlite structures
What happens to the carbon during the transformation from austenite to ferrite and cementite?
During the transformation, carbon is expelled from the ferrite and is taken up by the cementite, increasing its concentration there
What is the difference in carbon content between ferrite and cementite?
Ferrite has little carbon and is a body-centered cubic structure, while cementite is a carbide with a higher carbon content and is hard and brittle
What are the two different types of iron?
Allotropic
Polymorphic
What is pearlite like?
Pearlite is analogous to a metal-ceramic nano-composite material
What are the textures of ferrite?
Ferrite is relatively soft and ductile
What are grain boundaries occupied by in eutectoid condition?
Pearlite
What is the microstructure of hypereutectoid steel?
- Fe3C forms at austenite grain boundaries
- Continuous brittle phase
- Cementite surround pearlite
What are steels primarily composed of in general engineering use?
Steels in general engineering use are essentially binary alloys of iron and carbon, often with less than 0.8 wt.% C
What other elements are commonly found in carbon steels?
Carbon steels also typically contain manganese (~0.45-0.9%), phosphorus (0.025-0.060%), and sulphur (0.030-0.050%)
How can the properties of steel be enhanced and controlled?
The properties of steel can be enhanced and controlled through heat treatments
What happens to the phases shown in the phase diagram when steel is cooled rapidly?
If cooling is rapid, the phase diagram becomes invalid as stable phases shown during slow cooling may not form, and instead, metastable phases like martensite may develop
What is martensite, and how is it characterized?
Martensite is a very hard and quite brittle phase that retains the amount of carbon present during its formation. It is a ceramic-like material with many interfaces and high, strongly varying local stresses
For what kind of applications can as-quenched martensite be used?
As-quenched martensite can be used for products like crankshafts, spanners, and high-tension bolts due to its hardness
What does the tempering process involve?
Tempering involves a heat treatment at 200-600˚C, which allows carbon atoms to diffuse out of martensite
How does the hardness of as-quenched martensite compare to air-cooled (pearlitic) steel?
As-quenched martensite is much harder than air-cooled (pearlitic) steel, as indicated by the steeper curve on the hardness versus carbon content graph
What is the typical hardness range of martensitic steel with a carbon content of 0.6 wt.%?
600-800 Vickers (VPN)
What effect does carbon content have on the yield stress of steel?
The yield stress of steel increases with carbon content, as indicated by the yield stress axis on the graph
What influences the microstructure of tempered martensite?
The microstructure of tempered martensite depends on the tempering temperature, which leads to the formation of very fine precipitates of α (alpha) ferrite and Fe3C (cementite)
What is the defining characteristic of stainless steel?
Stainless steel is defined by having greater than 11 wt% chromium (Cr), and may also contain elements like nickel (Ni) and manganese (Mn)
How does chromium protect stainless steel from corrosion and oxidation?
Chromium contributes to the formation of an adherent Cr2O3 film that provides protection against corrosion and oxidation
Why are most stainless steels non-magnetic?
Most stainless steels are austenitic, which is a non-magnetic phase of steel
What happens in a normal oxidising environment to stainless steel?
In a normal oxidising environment, stainless steel automatically forms a tightly bonded chromium-rich oxide film which enhances its corrosion resistance
Compare the corrosion resistance of austenitic stainless steel to other types.
The corrosion resistance of austenitic stainless steel is not as good compared to other types of stainless steel.
What is low alloy steel, and what is its carbon content?
Low alloy steel contains less than 10 wt% alloying elements and typically has low carbon content
What materials are used in the production of molten steel?
The production of molten steel involves limestone and iron ore, which go into a sinter plant, and coal that is transformed into coke for the blast furnace
Why doesn’t a blast furnace produce steel?
A blast furnace doesn’t produce steel because the iron extracted has too much carbon
How is the carbon level reduced in molten steel?
In a basic oxygen vessel, oxygen is blasted into molten steel, which reacts to form CO2 and reduces the carbon level in the molten steel
What might be added during secondary steel making, and what is another method of steel production?
During secondary steel making, elements like chromium might be added, and another method of steel production is using electric arc furnaces which primarily use recycled steel
What is the purpose of a Basic Oxygen Converter (BOC) in steel production?
Used to transfer molten iron into the vessel and blast oxygen into it to further reduce the carbon content to between 0 and 1.5%
Describe the physical characteristics of a Basic Oxygen Converter.
A Basic Oxygen Converter is a cylindrical vessel approximately 8 meters in diameter and 11 meters high, lined with about 1 meter of magnesite bricks
How is energy reclaimed in the Basic Oxygen Converter process?
Energy is reclaimed in the BOC process by burning the exhaust gas to heat a boiler
What is the starting material for an Electric Arc Furnace (EAF), and what is its capacity?
The starting material for an Electric Arc Furnace is typically scrap steel or iron, and an EAF can typically make 150 tonnes of steel in around 90 minutes
How is an electric arc created in an Electric Arc Furnace?
An electric arc is created in an Electric Arc Furnace by passing an electric current through electrodes when the lid containing the electrodes is swung into position
What are the processes involved in steel casting and hot rolling?
Steel casting and hot rolling are forming processes that contribute to microstructure formation in steel production
How much steel was produced in 2018?
1,808 million tonnes of steel were produced
How much carbon dioxide is emitted from making steel each year?
2,730 million tonnes
What makes steel a sustainable material?
- completely recyclable
- possesses good durability
- has low embodied energy
- lightweight steel construction can save energy and resources
