Types of cast iron – Cementite formation

The role of carbon in Cementite formation.

What is cementite?

Cementite, also known as iron carbide, is a particular compound (interstitial compound) given by a specific combination of iron and carbon. More precisely, by weight, Cementite is given by 93.3% carbon and 6.67% iron with a molecular formula Fe3C.

The cementite is then identifiable inside the Fe-C Diagram which we have talked about in the previous article “Types of Cast iron”. The Fe-C alloy is the basic alloy of all types of cast iron.

How to identify cementite in the Fe-C phase diagrams?

Cementite appearsDIAGRAMMA DI STATO FE-C_CEMENTITE at different times inside the Fe-C alloy composition and thus it is possible to distinguish:

  • Primary cementite when Cementite crystallizes directly from the liquid cast iron (identified by the CD curve in the Fe-C phase diagram);
  • Secondary cementite when Cementite is separated from Primary austenite (identified by the ES curve in the Fe-C phase diagram);
  • Tertiary cementite when Cementite is originated from Ferrite (identified by the PQ curve in the Fe-C phase diagram);
  • Eutectic cementite when the Cementite is the result of Ledeburite transformation;
  • Eutectoidic cementite when Cementite is equivalent to one of the two pearlitic phases.

How can Cementite be identified on the casting?

Different ways in which cementite forms itself determine also different morphologies:

When analyzing such morphology on the microscope, Cementite can presents itself as follows:

Primary cementiteCEMENTITE PRIMARIA_FASSMET – this is how Primary cementite appears on the microscope with coarsed sharp crystals.

 

 

 

 

 

 

 

Ledeburite cementiteCEMENTITE LEDEBURITICA_FASSMET – this is how Ledeburite cementite appears on the microscope. Cementite appears really similar to Leopard skin with a rounded shape.

 

 

 

 

 

 

Secondary cementiteCEMENTITE SECONDARIA_FASSMET – this is how Secondary cementite appears on the microscope with white layers visible on the matrix.

 

 

 

 

Pearlitic cementiteCEMENTITE PERLITICA_FASSMET – this is how Pearlitic cementite appears on the microscope with ferrite layers similar to a footprint.

 

 

 

Not being visible to the eye, when chemical composition is given, the presence and the type of cementite can be evaluated by analyzing the following representation. Such representation allows to verify the quantity of the individual elements inside the microstructure at room temperature and thus once solidification is completed.

RAPPORTI QUANTITATIVI FRA COSTITUENTI MICROSTRUTTURALI_FASSMET

Alternatively, both to the analysis through metallographic microscope and to the study of the quantity in which the elements are present in the microstructure, the cementite can be analyzed through the use of thermal analysis.

Thermal analysis is based on the study of the cooling curve. Such study can be conducted directly in foundry on cast iron samples that are left to solidify. Thanks to the solidification of such samples, a valid estimation of the quantity of the cementite inside the cast iron that will be produced can be obtained.

Please see the following video to see how thermal analysis occurs and the dedicated website page to read further information related to thermal analysis.