6.1- Aircraft Materials- Ferrous Flashcards
Where does the word ferrous come from?
Latin for ferrum, referring to metals and alloys.
What is important about any alloy containing iron?
It is ferrous and magnetic.
What is iron in the periodic table?
Fe, atomic number 26.
What are pure irons properties?
Ductile, soft and easily formed.
What is the density of pure iron?
7.9g/cm³
How is oxidisation of pure iron reduced?
By alloying with carbon and other elements.
How is iron produced?
Mixing iron ore with coke and limestone and submitting it too hot air.
What is the slag?
The reaction of limestone with impurities in the iron and coke, it sits on the top.
What percentage of cast iron is carbon?
3.8-4.7%
What is the issue with cast iron?
Due to its high carbon content it is brittle.
Cast iron isn’t often used on aircraft, but where can it be found?
Valve guides due to its porosity allowing it too hold lubricant and piston rings.
How many tonnes of steel is made annually?
1.6 billion tonnes.
How is steel made?
Cast iron is re-melted, pure oxygen is then forced in, combining with carbon and burning. A controlled amount of carbon is then put back in.
What happens to the ingots of molten steel after they solidify?
They are heated to 1200C, and rolled.
What are the considerations for alloy selection?
Service Temp
Strength
Stiffness
Fatigue
Fabricability
How much carbon is in steel?
Up to 2%. Most common is 0.15% to 1%.
What happens when the carbon content of iron reaches 6%?
The metal is too brittle, it cannot be formed, machined, processed or used.
What is tensile strength?
Withstand stress in tension.
What is compressive stress?
Withstand pressing or squeezing.
What is shear strength?
Withstand stress acting coplanar with a material cross section.
What is torsional strength?
Resist rotational shear.
What is flexural strength?
Bending strength.
What is fatigue strength (endurance)?
Resist repeated loading.
What is impact strength?
Ability to resist shock.
What is hardness?
Resist cutting, penetration and abrasion.
What is malleability?
To be bent, formed or shaped without cracking or breaking.
What is ductility?
To be drawn into wire stock, extrusions or rods.
What is brittleness?
Tendency to break or shatter.
What is conductivity?
Enables it too carry heat or electricity?
What is thermal expansion?
To expand when heated and shrink when cooled.
What is elasticity?
Return to its original shape after normal stretching or bending.
What is toughness?
Resist tearing or breaking when it is bent or stretched.
What is fusibility?
The ability of a metal to be joined by heating and melting.
Name some of the more common alloying ingredients.
Carbon, sulphur, silicon, phosphorus, nickel and chromium.
What happens when carbon is alloyed with iron?
Iron carbites in cementite form are made.
What decreases when carbon content increases?
Malleability and weldability.
What percentage is a low carbon steel?
0.1-0.3
What percentage is a medium carbon steel?
0.3-0.5
What percentage is a high carbon steel?
0.5-1.05
What does sulphur do and how can it be counteracted?
Makes it brittle and manganese.
What does silicon do as a alloying agent?
Hardener, and in small quantities ductility.
In low carbon steels what can silicon cause?
Bad surface finish.
What does phosphorus do as a alloying agent?
Raises the yield strength and resistance to atmospheric corrosion.
What is the most amount of phosphorus used to avoid brittleness?
0.05%
What does nickel do as a alloying agent?
Strength, hardness and yield strength.
What does chromium do as a alloying agent?
Wear and corrosion resistance.
What does molybdenum do as a alloying agent?
Reduces grain size, better impact strength and elastic limit.
Is Chrome Molybdenum alloy the most commonly used?
Yes
What is the SAE designation for chrome-moly?
4130 (1% moly 0.3% carbon)
Heat treated 4130 has how many times more tensile strength than 1025?
4 times.
What does vanadium do as a alloying agent?
When combined with chromium, produces strong, tough, ductile steel alloy.
What does tungsten do as a alloying agent?
High melting point.
Where is tungsten used?
Breaker contacts in magnetos and high speed cutting tools.
Where is vanadium used?
Tooling
Where is molybdenum used?
Engine parts
Where is chromium used?
Balls and rollers or bearings.
What does CRES contain?
Large amounts of chromium and nickel.
What is cres well suited for?
High temp areas such as firewalls and exhausts.
What is the only time that cres needs a protective coating?
When in contact with light weight alloys.
What three categories of stainless steel are there?
Austenitic
Ferritic
Martensitic
What are 200/300 series stainless steels?
Austenitic
How does an austenite structure form?
When steels are heated to above their critical range (800 to 925f) and held there.
How can austenite steels be hardened?
Cold working, heat treatment only anneals them.
What series are ferritic stainless steels?
400, chromium is the main alloying element.
What are some of the characteristics of ferritic stainless steels?
Magnetic, good ductility and welded easily.
How are ferritic stainless steels hardened?
Cold rolling.
What is martensitic stainless steels?
Essentially a high carbon version of ferritic, with higher chromium. The combination offers heat treatment options and corrosion resistance.
How are steels classified?
Society of Automotive Engineers (SAE) system, 4 digit system.
What do the 4 SAE digits represent?
First digit- principle alloying element
Last two- carbon percentage
What SAE numbers are carbon steels?
10xx
11xx
12xx
15xx
What SAE numbers are Manganese steels?
13xx
What SAE numbers molybdenum steels?
40xx
44xx
What is a eutectoid?
A metal that can exist in more than one different form at the same temp.
What cubic form is ferrite?
Body centre cubic
What cubic form is Austenite?
Face centred cubic, it can absorb more carbon than ferrite.
What is cementite also known as?
Iron carbide
What is pearlite?
Two phased lamellar structure composed of alternating layers of alpha-ferrite (88%) and cementite.
How is pearlite formed?
By eutectoid reaction when austenite is slowly cooled below 727c.
In simple terms how is steel hardened?
Raised above critical temp, steel becomes austenite and absorbs carbon in solid state. Then quenched and carbon is trapped.
What are the important factors in heat treatment?
Carbon content, temp and time taken to cool.
What is a BCT crystal?
Body centred tetragonal structure- martensite, stronger harder and more brittle than BCC.
What happens above the critical temp to the structure?
Goes from BCC to FCC.
What is quenching?
Rapidly cool a material in medium such as water, oil or air.
What is annealing?
It softens and releases stress in the metal.
How does annealing work?
Heat to 10c above critical and heat soak, then cooled slowly.
What is the normalising process?
Heating steel to 38c above critical limit and soaking for prescribed time, then cooled at room temp.
What steels are best suited to case hardening?
Low carbon low alloy steels.
What two methods are used to case harden?
Carburising and nitriding.
What is carburising?
Heat treatment process that diffuses carbon into the surface of a metal?
What is nitriding?
Diffuses nitrogen into it to increase hardness.
What are the three methods of carburising?
Pack- fire clay container
Gas- carbon monoxide
Liquid- sodium cyanide bath
How is nitriding carried out?
Hardening and tempering, heated to 540c and surrounded with ammonia gas.
What parts are commonly nitrided?
Engine crankshafts and cylinder walls for wear resistance.
What carbon content can flame hardening be carried out on?
At least 0.4% after normalisation.
What is induction hardening?
The same has flame hardening but using a coil surrounding the metal.
