6.1 Ferrous Flashcards
How is iron extracted from iron ore
By mixing coke and limestone and heating in a furnace
What are the main restrictions of using steel on aircraft
High density (approx 3 times than aluminium) and susceptibility to corrosion
What are the advantages of high strength steels
High strength and high modulus of elasticity
How much carbon does low, medium and high steels contain
Low - 0.1 - 0.3%
Medium - 0.3 - 0.5%
High - 0.5 - 1.05%
When would carbon steels with 0.3 - 0.5% be used
For surface finish purposes
When would steels with 0.5 - 1.05% be used
Extreme hardness required, typical applications are springs, files and cutting tools
What is the purpose of using sulphur as an alloying agent
Decreases ductility and weld ability with increasing content.
Increases machinability
What is the benefit of using manganese as an alloying agent
Contributes to strength and hardness but less than carbon
Increasing amounts of manganese will result in a decrease of ductility and weld ability
What is the purpose of silicon as an alloying agent
One of the principle de-oxidisers in steel making
Bad for surface quality
What is the purpose of phosphorus as an alloying agent
Increase of strength and hardness and corrosion resistance but decreases ductility
What is the purpose of Nickel as an alloying agent
Increases harden-ability and impact strength of steels
What is the purpose of chromium as an alloying agent
Increase corrosion resistance, hardenability and high temperature strength
What is the benefit from using molybdenum as an alloying agent
Increases hardenability, increases creep strength at high temps
What are the benefits of using Vanadium as an alloying agent
Increases yield strength and tensile strength of carbon steels
What is the purpose of using titanium as an alloying agent
Increases toughness
What is the AISI
The American Institute of Steel and Iron
What is SAE
Society of Automotive Engineers
What is the purpose of tempering
Decrease hardness and increase ductility and toughness in steel after that have been quenched. Also relieves internal stresses caused during quenching
Why would steels need to be treated after they have been quenched
In their quenched state they are far to hard and brittle for any use on aircraft
What are the main ways of temperature control during the tempering process
Pyrometers and thermocouple controlled ovens. A less accurate way is by observing the colour
Why cannot pure iron, wrought iron or extremely low carbon steels be hardened by heat treatment
As they have no hardening agent (Carbon)
What provides the slowest and quickest quench
Oil for slowest quench and brine for the fastest
What happens to an alloy if it is quenched too quickly
The structure will be known as martensite, a super hard material which would be too hard and brittle for most uses
What is the purpose of annealing
Softens steels and relieves internal stresses
How is the annealing process carried out
Heating a steel to 50°F above its upper critical limit and soaking it at this temperature. When the required soak time has been met it is cooled very slowly in a furnace or bath of hot sand
What will the main characteristics of steel be when it has been annealed
Very soft and ductile
What is the purpose of normalising
When a steel is forged, welded or machined, it will have residual stresses contained within the material. These stresses can lead to structural failure. The process of relieving these stresses is known as normalising
What is the normalising process
Heating steel to 100°F above upper critical limit and soaking it for a prescribed amount of time. After soaking the steel is allowed to cool at room temperature. This allows the particles of carbon to precipitate out of the Austenitic structure and relives internal stresses
What is a Eutectoid
A metal can exist in more than one form at the same temperature
I.e - Ferrite
- Austenite
- Cementite
- Pearlite
What is a Ferrite structure
The name given to pure iron. A BCC lattice that will absorb up to 0.2% carbon
What is Austenite
An FCC form of iron which can absorb a lot of carbon into solid solution (up to approx 2%) than Ferrite
What is cementite
A chemical compound made up from iron and carbon
What is cementite also known as
Iron carbide
What is pearlite
A two phased layered structure composed of alternating layers of alpha-Ferrite(88%) and cementite (12%)
It forms by a eutectoid reaction
What are the most important characteristics of CRES
Corrosion resistance, strength, toughness and resistance to high temperatures
How are stainless steels identified
By a 3 digit system (I.e 200 and 300 series)
What are 200 and 300 stainless steels known as
Austenitic
What is 400 series stainless steel known as
Martensitic
What series of stainless steels are most commonly used
300 series called the 18-8 as they contain 18% chromium and 8% Nickel
How are stainless steels divided into 3 general groups
Based on their structures. Austenitic, Ferritic, Martensitic
What are Austenitic stainless steels (200 & 300 series)
Chromium Nikel and chromium Nikel manganese alloys
Hardened only by heat treatment
Heat treatment only anneals them
Non magnetic in annealed condition but may be slightly magnetic after cold working
What uses to stainless steels have in the aircraft industry
Limited uses however can be used for sea water applications as they are resistant to most corrosives
What are ferritic stainless steels (400 series)
Contains very low carbon
Cannot be hardened by heat treatment and quenching
Can be work hardened/strain hardened
Always magnetic
15-30% chromium
What use on aircraft would ferritic stainless steels have
Limited to piping and tubing
Excellent corrosion resistance at high temperatures and under sulphide corrosion
What are martensitic steels (400 series)
Straight chromium alloys that harden intensely if they are allowed to cool rapidly from high temperatures
12-18% chromium, no Nikel
Heat treatable by quench and temper
Used in turbine blades
What kind of conditions/uses would martensitic 400 series stainless steels be used for
Mild conditions. They will rust in moderate to severe corrosion conditions due to having low chromium content and no Nikel
What are the disadvantages of stainless steels
- More difficult to cut
- Welding more difficult
- Loose corrosion resistance under high temperatures
- As it is hard to weld and cut, the technician must be well trained to work with it successfully
What are precipitation hardened stainless steels
- Very little carbon
- 15-17% chromium, 4-7% Nikel
- Solution heat treated and can be hardened to very high strengths
- Good corrosion resistance in marine environment
What typical uses would precipitation hardened stainless steels have in the aircraft industry
- Airframe applications where high strength and excellent corrosion resistance is required
- Also used in elevated temperature conditions
- Very good corrosion resistance in marine environment
What are high strength low alloy steels
Iron based alloys that can be hardened to very high strengths
180-200ksi usually 4130
What is 300m stainless steel used for
Landing gear
What are the most widely used high strength steels in the aircraft industry
4330m heat treated to 220-240ksi
4340m heat treated to 275-300ksi
What are 4330m and 4340m steels also known as and why
Vacuum remelt steels due to the manufacturing process to obtain superior properties
This process results in increased ductility, fatigue and fracture toughness properties
What is hydrogen embrittlement
Occurs in particularly ferrous and titanium alloys under sustained loads at stresses far below the actual ultimate tensile stress
What percentage of the ultimate tensile strength could steel fail if it has suffered from hydrogen embrittlement
As low as 30% it’s ultimate tensile strength
Other than sustained loads how would metals suffer from hydrogen embrittlement
Can be introduced during processes such as welding and pickling
What stainless steels are not affected by hydrogen embrittlement
300m and precipitation hardened steels
What precautions must be observed when doing reword operations on steel parts
- Stress concentrations
- Hydrogen embrittlement
- Untempered martensite
When would a stainless steel need a protective surface treatment
Only when in contact with light weight alloys
They are coated with
- Cadmium plating
- Zinc spraying or chromate passivating
This is to avoid galvanic corrosion of the light weight alloys
What types of surface treatment would non stainless steels have
- Cadmium plating with a paint finish
- Phosphating with a paint finish
- Aluminium spraying
- Hard chromium plating
- Chemical Nikel plating
- Silver plating (as long as not in contact with aluminium)
What is galvanic corrosion
A measure of how dissimilar metals will corrode when placed against each other in an assembly.
What is case hardening of steel
Some steels require a hard durable surface that is suitable for load baring, whilst retaining a tough core
Best suited to low carbon steels
What types of metals is case hardening not suitable for
Non ferrous metals
What is the purpose of carburising
Produces a thin layer of high carbon steel on the surface of a low carbon component
What are the 3 methods of carburising
- Pack carburising
- Gas carburising
- Liquid carburising
What is pack carburising
Heating the component to temperatures of 1700°f whilst it is packed in to a carbon rich material such as charcoal. The carbon penetrates the surface of the metal causing it to harden
What is gas carburising
Similar to pack carburising, but utilises the carbon dioxide rich atmosphere within the oven instead of a carbon rich solid
What is liquid carburising
Works by heating up the metal 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
Heated above upper critical temperature with oxy acetylene torch and immediately quenched with a jet of water supplied by a lance attached to the assembly
When can steels only be flame hardened
Only on steels with a carbon content of at least 0.4% and only after the steel has been normalised
What is induction hardening
Similar process to flame hardening except that the whole circumference of the steel is heated simultaneously by an induction coil
The coil heats the steels temperature, after it has reached its required temperature it is quenched
What is nitriding
Produces an improved surface finish compared to carburising as the component is hardened, tempered and ground to its final dimensions prior to surface hardening
Heating material to 1000°f in an amonia rich environment. The amonia brakes down allowing the nitrogen to penetrate the surface
