The microstructure in this case was pearlitic with some steadite and free carbide but no free ferrite.
The effect observed was opposite to the previously discussed test of cast iron with pearlite matrix and steadite phase, in that aging of ferri-tized/resolutionized gray iron improved machinability.
These irons had some free ferrite and no free cementite or steadite. Optimal aging time depends upon particular "free manganese" content and could be evaluated.
Hard Spots/Machinability--Carb ides and steadite
are typical examples of hard spots.
These two constituents are iron carbides and steadite (iron phosphides).
Iron carbide and steadite are eutectic phases between iron (Fe) and carbon (C), and Fe and phosphorus (P), respectively.
These voids have a shape that is similar to the carbide and steadite constituents that would be found in the structure.
In addition, tools such as these are also helpful in identifying sand grains, determining the elements responsible for carbide stabilization, distinguishing between iron carbide and steadite, and identifying graphite or iron oxide linings in gas holes.
Chemical analyses should always be included when microstructural problems such as carbides, steadite and poor graphite shape are suspected.
Phosphorus, in excess of the 0.02% that is soluble in austenite, exists in gray iron as a binary eutectic of iron and iron phosphide called steadite
. The chemical composition of this eutectic is 10.2% phosphorus and 89.8% iron.