Steel Flashcards
DMM1: Materials Deck 7
What are carbon steels?
an alloy of iron and carbon which improves hardness and strength
What is Low carbon (mild) steel and what are its properties?
< 0.3% C
cheap, weldable, lower yield stress (250 - 350 MPa), rusts easily
What is medium carbon steel and what are its properties?
0.3 to 0.5% C
Yield Strength: 305-900 Mpa
can be heat treated and welded
low cost and rusts more slowly than mild steel
What is High carbon steel and what are its properties?
0.5 to 0.8% C
Yield Strength: 400-1200 MPa
very hardenable in heat treatment
difficult to weld
rusts slowly
What are the phases of steel carbon alloys?
Interstitial Solid solutions
* Ferrite: BCC. <0.02% carbon
* austenite: FCC. <2% carbon
Metallic compounds
* Fe3C, iron carbide
or cementite: 6.7% carbon
Phase mixtures
* Pearlite. Interlaid plate-like crystals of ferrite and carbide
Non-equilibrium phases
* Martensite. Formed when steel is quenched from austenite
state.
What is normalized steel?
Steel that is cooled slows
can be a eutectoid, hypoeutectoid or
hypereutectoid
generally lower strength
What is the poylmorphic change in pure iron?
under rapid cooling the crystal structure of iron changes from FCC (austenite) to
BCC (ferrite)
What is the polymorphic change in steels?
rapid cooling with intersititial carbons causes the steel to shift from FCC (austenite) to a body centr2ed tetragonal structure called marensite
Describe the structure of Martensite
-Non-equilibrium supersaturated solution
-Microstructure is highly
distorted which revents slip and hence
dislocation
-Very hard, very strong
and very brittle
-Yield strength up to 1200
MPa
How does tempering affect the metal crystal structure?
the product of quencing is very hard and difficult to work with
tempering raises the temperature allowing the structure to progress slighly to an equlibrium state
increases ductility and toughness while maintaining some of the hardness of martensite
How does Quencing affect the metal crystal structure?
rapid cooling of the crystal structure causes it to form a non-equalibrium state which is very hard and strong
How does solutionizing affect the crystal structure?
the alloy is heated and held at a temperature where all the crystal is made of a single phase (austenite)
What is cast iron?
> 2% C, 1-3% Si
*Yield Strength: 250-680 MPa
flakes of carbon sorrounded by ferrite
very easy to cast and cheap to work with
Describe the structure of cast iron
solid flakes of pure carbon (graphite) surrounded by ferrite and pearlite
What is stainless steel?
11% Chromium
prevents corrosion and degration
very expensive
