Ferrous Metals Flashcards
How is steel made?
Pure iron is smelted and mixed with carbon and other additives based on the type of steel desired.
Restrictions of steel
High density
Susceptible to corrosion
What percentage of an aircrafts structure is steel?
9-16
What are the main advantages of steel?
High modulus of elasticity
High strength
What are the alloy selection considerations?
Service temperature Strength Stiffness Fatigue properties Fabricallity
Effects of carbon in steel
Primary hardening element
Allows steel to be heat treated
Hardness increases with carbon percentage up to around 1.05%
Low carbon steel percentage and uses
0.1-0.3%
Manufacture of safety wire and secondary structures
Medium carbon steel percentage and applications
0.3-0.5%
Machining or surface hardness is desired
High carbon steel percentage and uses
0.5-1.05%
For extreme hardness as in springs, filed, and cutting tools
Effects of sulphur in steel
Increases machinability in free machining steel
Reduces ductility and weld ability with increasing content
Effects of manganese on steel
Increases strength and hardness, but not as much as carbon
Decreases ductility and weldability but not as much as carbon
Has a significant effect on hardenability
Effects of silicon on steel
One of the principal de-oxidisers
Detrimental to surface quality in low carbon steels
Less effective than manganese at increasing rolled strength and toughness
Effects of phosphorus on steel
Increases strength hardness and corrosion resistance
Reduces ductility
Effects of nickel on steel
Increases hardenability and impact strength
Effects of chromium on steel
Increases corrosion and oxidisation resistance
Improve high temperature strength
Increases hardenability
What is chromium used with to achieve superior mechanical properties?
Nickel as a toughening element
Effects of vanadium on steel
Increases yield and tensile strengths
Small amounts can significantly increase the strength
Effects of molybdenum on steel
Increases hardenability
Secondary hardening of quenched steels
Increases creep strength in low allow steel at elevated temperatures
Effect of titanium on steel
Improves toughness
What is the AISI?
American iron and steel institute
First two digits in a steel designation represent what?
Primary alloying element
Second 2/3 digits in steel designation represent what?
Carbon percentage in the alloy
What type of steel is 10XX?
Pure carbon steel
41XX steel type
Chromium and molybdenum alloys
43XX type steels
Nickel, chromium, and molybdenum alloys
52100 steel
Chromium alloy with 1% carbon
93XX steel type
Same as 43XX but with a different ratio.
What are the 2 heat treatment processes used to soften metal?
Tempering
Annealing
How is steel hardened?
It gets heated above its critical temperature for a uniform dispersal of carbon in the iron matrix before it gets quenched in water, oil, or brine depending on the quench speed required
What has the fastest and slowest quenching speeds for oil, water and brine?
Oil- slowest
Water- medium
Brine- fastest
How is annealing done?
The steel is heated 50degrees above its critical temperature and left to cool slowly.
What can’t be hardened?
Pure iron
Low carbon steels
Wrought iron
What is normalising?
Relieving stresses from metals by heating it to 100F above upper critical temperature, soaking it for a period, and leaving it to cool at room temperature
What is a ferrite state?
Pure iron. has a body centred cubic lattice that absorbs .02% carbon
What is a austenite state?
Face centred cubic that can absorb up to 2% carbon
What is cementite?
Chemical compound of iron and carbon, the more carbon the higher the cementite level and hardness, but leaving the metal more brittle
What is pearlite?
2 phased laminar structure made of alternating alpha ferrite (88%) and cementite (12%)
What is CRES?
Corrosion-resistant steel
What does the 3 digit system refer to?
200 and 300 series- austenitic
400 series- martensitic or ferritic
What are the 200 and 300 series alloys?
Chromium nickel
Chromium nickel manganese
How much carbon is in the 400 series CRES?
Below 0.1%
How are austenitic CRES hardened and heat treated?
Hardened through cold working
Can only be annealed
Can ferritic CRES be heat treated?
No, they can only be work or strain hardened due to the low carbon levels.
What are the martensitic 400 series alloys?
Straight chromium alloys
How are martensitic CRES hardened?
By cooling them from high temperatures
What could the 200 and 300 series CRES be used for?
Under water applications
What could the ferritic CRES be used for?
Piping and tubing
What could the martensitic CRES be used for?
Turbine blades
Some disadvantages of CRES
Hard to cut or form, so technician needs to be experienced.
Can lose corrosion resistance to high temperatures
What makes up precipitation hardened steel?
Very little carbon
15-17% chromium
4-7% nickel
Other alloying elements
What can precipitation hardened steels be used for?
Airframes that need high strength and corrosion resistance.
Marine environments
Elevated temperature locations
What are high strength, low alloy steels and what to applications?
Iron based alloys that are hardened to very high strengths
Application is based on strength range required but higher ranges can be used for landing gear.
What is hydrogen embrittlement?
A phenomenon that causes the hydrogen in some metals to mitigate to areas such as grain boundaries when under load.
The hydrogen can also combine with the carbon to make methane which creates pressure that can cause cracking.
What metals are mainly affected by hydrogen embrittlement?
Ferrous and titanium
How can hydrogen embrittlement be avoided?
By a baking operation at 385F if the cracking has not initiated and the part is bare and porous
What should you be aware of when reworking steel alloys?
Hydrogen embrittlement
Stress concentration
Untempered martensite
When does stainless steel need protection and how is it protected?
Only when in contact with light weight alloys.
Cadmium plating, zinc spray, chromium passivating
How are other steels protected?
Cadmium plating and layer of paint Phosphating and paint Aluminium spray Hard chromium plating Chemical nickel plating Silver plating, not in contact with aluminium
What is galvanic corrosion?
Corossion caused by 2 dissimilar metals coming into contact with each other, based on how far apart in the nobility scale they are.
Typically an electrolyte is also needed to start the corrosion.
What can be case hardened?
Ferrous metals
What are the 2 methods of case hardening?
Carburising
Nitriding
What are the 3 types of carburising and how are they done?
Gas similar to pack but with a carbon rich atmosphere instead.
Pack- heating component while packed with carbon rich material
Liquid heating the material in a bath of sodium or barium carbide
How is flame hardening done?
By heating the material above its upper transitional level and quenching it immediately afterwards.
What is induction hardening?
Heating up the material with a magnetic coil, then shutting off the coil and quenching the material in water.
What is nitriding?
By heating up the metal in an ammonia rich environment, the nitrogen can penetrate the surface and harden the material, while leaving it with a better surface finish compared to carburising.
