6.1 Aircraft Materials - Ferrous Flashcards
What is the base material for ferrous metals?
Iron
What is Iron in it’s pure form?
Very soft, malleable and ductile metal which is easy to form and shape
What does Iron and Oxygen combined cause?
Iron oxide (rust)
What is Iron usually alloyed with?
Carbon
At what percentage of Carbon alloying with Iron causes cast Iron?
2% +
Why is cast iron limited in aviation?
Low strength to weight ratio and brittleness.
How is Iron extracted from Iron Ore?
By mixing it with coke and limestone and heating it in a furnace
How is steel made?
The pure iron is remelted in a special furnace where carbon is introduced along with other alloying elements
What is the main restriction of steel in a/c?
Its high density (approximately three times the density of aluminium)
What percentage of aircraft is usually steel?
9-16%
What considerations are used for alloying elements in steel?
Service temperature, strength, stiffness fatigue properties and fabricability.
What is the carbon content in low, medium and high steels
Low - 0.1-0.3%
Medium - 0.3-0.5%
High - 0.5 - 1.05%
What system do Alloy Steels use for designation?
AISI (American Iron and Steel Institute)
What does 4340 mean in the AISI system?
nickel-chromium-molybdenum alloy containing 0.4% carbon.
What is Tempering?
Tempering is a method used to decrease the hardness, thereby increasing the ductility and toughness of the quenched steel
What is Hardening?
To harden steel, it is heated above its critical temperature so carbon can disperse uniformly in the iron matrix. Once this occurs, the alloy is cooled rapidly by quenching it in water, oil, or brine (salt water).
What is Annealing?
Annealing softens steels and relieves internal stresses. The annealing process entails heating a steel to 50 ̊Fahrenheit above its upper critical limit and soaking it at this temperature.
What is Normalising?
The normalizing process involves heating steel to 100 ̊F above its upper critical limit and soaking it for a prescribed time. After soaking, the steel is allowed to cool at room temperature.
What is Quenching?
The rate that steel will cool down when quenched is governed by the medium in which the metal is quenched water is the fastest quenching medium, followed by oil, still air and a warm oven.
What is the definition of Ferrite?
Pure iron. It has a body centred cubic lattice that will absorb up to .02% carbon.
What is the definition of Austenite?
Face centred cubic form of iron, which can absorb a lot more carbon into solid solution (up to approximately 2%) than ferrite.
What is Cementite also called?
Iron Carbide
What is the definition of Pearlite?
Two-phased, lamellar (or layered) structure composed of alternating layers of alpha-ferrite (88%) and cementite (12%) that occurs in some steels and cast irons
What percentage of Carbon will the steel cool to Austenite?
0.8%
What are the important factors of the Heat Treatment process?
Carbon content, temperature and time taken to cool.
What is the other name for CRES
Stainless Steel
What does CRES stand for?
Corrosion-Resistant Steel
What is the main properties of CRES?
Corrosion resistance, strength, toughness and resistance to high temperatures.
What series are the 200 & 300 CRES classed as?
Austenitic
What series is the 400 CRES classed as?
Martensitic
What is the most widely used CRES for general use?
300 series, called 18-8
What three groups can CRES be divided into?
Austenitic, Ferritic, Martensitic
What alloys are 200 & 300 series CRES
Chromium- Nickel
Chromium-Nickel-Magnesium
How much chromium to austenitic CRES contain?
18%
Is CRES heat treatable and if so by what method?
CRES is not heat treatable!
What are Austenitic CRES good for in the aviation sector?
Only really Sea-Water applications
How much carbon does Ferritic CRES contain?
Less than 0.1%
What types of treatment can Ferritic CRES not receive?
Cannot be Heat treated or Quenched
Is Ferritic CRES magnetic or not?
It is magnetic
What is Ferritic CRES usually used for?
Piping and Tubing
What CRES can be heat treated and by what methods?
Martensitic
Quench and Temper
What is Martensitic steel used for on a/c?
Turbine Blades
What CRES has the lowest corrosion resistance?
Martensitic
What is the main disadvantages of stainless steel?
They are more di cult to cut and form than many materials
• They have a much greater expansion coe cient than other steels, and they conduct heat at a lower rate, making welding more di cult. In welding CRES, inert-gas arc welding is preferred, because this process causes less deformation due to heat expansion of the metal and it prevents oxidisation.
• Many of the stainless steels lose their corrosion resistance under high temperatures.
• Because of its toughness, stainless steel is more di cult to cut, form, shear, machine or drill than ordinary steel. For this reason, the technician, who is to work with this material successfully, must be experienced in the necessary processes or must be directed by an experienced technician.
When Precipitation-Hardening Stainless Steels, what must they have little of?
Carbon
What are Precipitation-Hardened Stainless Steels
used for on a/c?
Where high strength and excellent corrosion resistance are required. Also used in elevated temperature locations.
What environment is Precipitation-Hardened Stainless Steels good in?
Marine Environment
What type of stainless steel is used on a/c landing gear?
High-Strength Low-Alloy Steels - 300 M
What is High-Strength Low-Alloy Steels usually a base of?
Iron Alloys
What is the most widely used high strength steels ?
4330M (heat-treated to 220- 240 ksi)
4340M (heat-treated to 275-300 ksi)
What are 4330M and 4340M steels also known as?
Vacuum Remelt Steels
Because of the manufacturing process used to obtain their superior properties.
What are the properties of Vacuum remelt steels?
increased ductility, fatigue and fracture toughness properties.
For temperature applications up to 900F what steel can be used?
9Ni-4Co-0.30C steel heat treated to 220-240 ksi is used
What strength is used for bar and forging applications?
200ksi or lower
What is the advantage of 15-5 PH steel?
Manufacturing costs are reduced and improved corrosion resistance
What size thickness is 15-5 PH practical over?
0.063”
For repair applications what stainless steel type and thickness is acceptable?
7-7 PH CRES sheet up to 0.090 inch thick when heat treated to 150-170 ksi
What is 300 series austenitic stainless steel sheets used for?
lower strength applications where corrosion or hygiene concerns dictate material needs
What material is hydrogen embrittlement usually found?
Ferrous and titanium alloys
What materials are not impacted by hydrogen embrittlement?
Aluminium, 300 series stainless steels and precipitation hardenable steels
How early can a hydrogen embrittlement fracture occur?
30% of the yield strength after only a few thousand service hours.
When can the hydrogen be introduced in Hydrogen Embrittlement?
During processes such as pickling and welding
What should be introduced during the welding process to stop hydrogen embrittlement
Hydrogen Free Electrodes
What temperature can the material be placed in to remove hydrogen embrittlement?
375F (190C) for 3 hours
The susceptibility of hydrogen embrittlement to increase with what?
Hardness and Strength
At what ksi are steel heat treated parts highly susceptible to hydrogen embrittlement?
200 ksi
What must be done to stress concentrations in metal?
Should always be removed
What happens if caution is not taken whilst reworking low alloy carbon steels?
Formation of untempered martensite, or the generation of undesirable residual tensile stresses.
What caution should be taken whilst reworking low alloy carbon steels?
Should be done by hand or with slow-speed hand-held power tools.
When do stainless steels require protective treatment? - and what with?
When in contact with Iight weight alloys
Cadmium plating, zinc spraying or chromate passivating, to avoid galvanic corrosion of the light alloys.
What protections do non-stainless steels have?
Cadmium plating plus a paint finish • Phosphating plus a paint finish • Aluminium spraying • Hard chromium plating • Chemical nickel plating • Silver plating (not in contact with aluminium)
What is the most noble metal on the galvanic scale?
Platinum
What is the least noble on the galvanic scale?
Magnesium
Is a more noble metal anodic or cathodic?
Cathodic
What is Case Hardening?
A hard durable surface that is suitable for load bearing, whilst retaining a tough core
What is Case Hardening best suited to?
Low Carbon Steels
What happens to High Carbon steels that are case hardened?
Become Brittle
What two methods are used for case hardening?
Carburising and Nitriding
Is case hardening suitable for ferrous or non-ferrous metals
Ferrous
NOT SUITABLE FOR NON-FERROUS!!!!
What is carburising?
Produces a thin layer of high carbon steel on the surface of a low carbon steel component
What are the three methods of carburising?
Pack
Gas
Liquid
What is Pack Carburising?
Heating the component up to temperatures of 1700 degrees Fahrenheit whilst it is packed into a carbon rich material such as charcoal. The carbon penetrates the surface of metal causing it to harden.
What is Gas Carburising`?
Similar to pack carburising, but utilises a carbon dioxide rich atmosphere within the oven instead of a carbon rich solid.
What is liquid carburising?
Heating the metal up in a bath of either sodium cyanide or barium cyanide. Either liquid will provide the necessary carbon to surface harden the metal.
What is flame-hardening?
Achieved by heating the surface to above the upper phase transition temperature by means of a travelling oxy-acetylene torch and immediately quenching it with a jet of water supplied from a lance built into the torch assembly.
What carbon percentage has the material got to have before it can be flame-hardened?
At least 0.4% and only after the steel has been normalised.
What is induction heating?
A coil carries high frequency current, which produces eddy currents in the surface of the steel, raising its temperature. The lower the current frequency used the deeper the heating. As soon as the surface of the steel has reached the necessary temperature, the current is switched o and it is immediately quenched by jets of water.
What is Nitriding?
Involves heating the material to 1000 ̊F in an ammonia rich environment. The ammonia breaks down allowing the nitrogen to penetrate the surface.
What Heat Treatment produces the best finish
- Nitriding
- Carburising
Nitriding
