FOR INTERNAL USE ONLY
Figure 7 The austenite region of the iron-iron carbide diagram showing maximum solubility of up to 2%C
FERRITE (α)
3.2
The region shown in Figure 9 where carbon is dissolved in bcc iron is very narrow, extending to only 0.02% carbon at 723°C. This phase is termed ferrite or alpha phase. Although the carbon content of ferrite is very low other elements may dissolve appreciably in it so ferrite cannot be considered as “pure iron”.
Figure 8
3.3
The ferrite region of the iron-iron carbide diagram
PERITECTIC
The region at the top left portion of the phase diagram enlarged in Figure 10 is where the iron reverts back to the bcc structure known as delta ferrite. Here again the solubility for carbon is low, only 0.1 wt.% at 1493°C. The part of the diagram at 0.16% carbon having the appearance of an inverted eutectoid is called a peritectic. At this point a two phase mixture of liquid and solid (austenite) transforms on cooling to a single phase solid of austenite. This portion of the phase diagram will not be discussed in detail, but it should be recognized since it has been invoked to explain various hot cracking phenomena in welding.
WELDING AND COATING METALLURGY2
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