8.4.3 Merkpunkte zu Kapitel 8.4: Steel

Carbon Steels owe their remarkable properties to the fact that at 996 K there is a phase change of the eutectoid kind:
Phasendiagramm Fe - C
Above 996 K: (Non-magnetic) g - phase, fcc lattice; called austenite, able to dissolve up to 2% carbon and still about 0.8 % at 996 K.
Below 996 K: (Magnetic) a - phase; bcc lattice with hardly any solubility of carbon, called ferrite.  
Even if you would start with a relatively defect free g - phase, the change of lattice type would by necessity introduce many defects and thus lead to some hardening. However, the main hardening effects are due to the need to remove surplus carbon in the a - phase  
Upon slow cooling one obtains pearlite, a mixture of a - Fe and cementite, which is itself an eutectic of a - Fe and Fe3C.  
Upon fast cooling (= quenching) one obtains "lathes" of martensite, a metastable lattice (tetragonal, sort of distorted bcc) with the carbon atoms still dissolved. Martensite is very hard, but brittle  
Tempering below the eutectoid temperature of 996 K will keep part of the hardness, while restoring some ductility: We have "tempered steel", for many years a synonym for the utmost in material strength.
     
Adding more alloying element servews to principially distinct goals:  
Maraging steel hardness
"Repair" certain problems, e.g. add Mn to compensate for unwanted, but unavoidable S in the mix.  
Produce certain wanted properties, e.g. better corrosion resistance by adding Cr.  
However, each addition infringes on all properties; optimizing can be long and hard work.  
Nevertheless, an incredible richness of steel variants with a huge spectrum of properties is known and produced.
What can be done with respect to the yield strength RP (proprotional to hardness) is shown in the diagram for the presently ultimate in strength: maraging steels. Note that the yield strength of pure ferrite is about 50 MPa.
In principle, whatever happens, can be understood by looking at the movement of dislocations.

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