The Fe-Fe carbide metastable phase diagram is based on equilibrium cooling or very slow cooling rates. This is seldom encountered in welding for the exception of very high heat input processes such as electroslag welding. However, the diagram is useful in understanding the phase transformations that occur during welding in the weld and HAZ. The example illustrated in the phase diagram is for the transformation of a 0.2% C steel from the austenite region (the vertical red dashed line). Note that a carbon steel and weld filler metal will have alloys in them, such as manganese (Mn) which will alter the transformation temperatures. For example, the addition of 1% Mn will drop the eutectoid (100% pearlite) carbon content from 0.8% to 0.75% and slightly expand the austenite field downward (lower the A3 temperature), since Mn is an austenite stabilizing element.

Slow cooling rate:
In the composite region of the weld, ferrite will nucleate at high angle grain corners and boundaries when cooled just below the A3 temperature. Growth will be along a planar front into the austenite. Since the solubility of carbon in ferrite is quite low, the carbon will be expulsed from the ferrite, along its planar front, into the austenite. The redistribution of the carbon along the planar front occurs efficiently because of the high temperatures involved.The resulting ferrite is blocky or polygonal ferrite (PF). In the austenite and ferrite region of the phase diagram, slow cooling and the high temperature combine to permit rapid carbon diffusion from the ferrite into the austenite. As the temperature drops below the A1, the carbon enriched austenite regions transform into pearlite, so that a mixed structure of ferrite and pearlite (P) is obtained.

Medium cooling rate:
Again, ferrite will nucleate, grow along a planar front and carbon will be expulsed from the ferrite, along its planar front, into the austenite as in the slow cooling rate case. However, the redistribution of the carbon along the planar front is not as efficient. The result is a "build-up" of carbon along the advancing ferrite front (a higher concentration of carbon at the front and a steeper carbon gradient into the austenite). The ferrite formed is a proeutectoid ferrite or grain boundary ferrite (GF). The GF growth occurs at incoherent (large-angle) phase boundaries; whereas, the PF grows at both semi-coherent and incoherent boundaries. Growth at incoherent phase boundaries occurs with faster cooling rates (higher undercooling). As the temperature cools below the Ws line (Widmanstatten start temperature), the GF breaks through the carbon "build-up" along its front with austenite in the form of fingers or needles. Carbon expulsion is again efficient and along the sides of the ferrite needles. This results in the microstructure known as Widmanstatten side plates or intragranular Widmanstatten ferrite (WF).
Another reaction which can occur at cooling just below the A1 temperature is the formation of acicular ferrite (AF). AF is similar in shape to the WF, except that its aspect ratio (=length over width) is smaller than WF's. Also, it is believed that the AF needs a high density of nucleations sites to form. Suitable nucleation sites would be slag inclusions. The formation of these inclusions is favoured by the presence of oxygen (300 ppm is believed to be the optimum amount).

Fast cooling rate:
At this cooling rate, the A1 is depressed to below 690° C and the carbon has little time to diffuse into the austenite as GF forms. Instead, the carbon concentrates along the advancing GF fingers or needles and precipitates between them as Fe carbide. This type of microstructure is known as upper bainite (B) or feathery bainite. In accordance with the weld microstructure identification proposed in "A Scheme for the Quantitative Description of Ferritic Weld Metal Microstructures" written by D.J. Abson and R.E. Dolby, upper bainite has been grouped under the category of ferrite with aligned M-A-C or AC.

Very fast cooling rate:
One of two microstructures will occur with very fast cooling rates: lower bainite and/or martensite. If the cooling rate permits some carbon diffusion, laths of bainite will nucleate and grow. Lower bainite is also grouped under the category of AC. Since the carbon diffusion is very slow Martensite (M) forms if the cooling rate permits no carbon diffusion. The type of martensite formed is a lath or low carbon type. Martensite is rare in weldments with 0.1 to 0.25% C and 1.0 to 2.0% Mn carbon steel welded with suitable fillers such as E6010 and E7018. Generally, martensite formation would have to be artificially enhanced by the use of a very low heat input and environmental conditions which enhance rapid cooling (i.e. weld exposure to subzero air temperatures or underwater welding).