Austenite-ferrite boundary

A STEM method for investigation of alloying element accumulation at austenite-ferrite boundaries in C-Fe-Mo alloys has been used as a method capable of yielding results eombining high accuracy with sufficient rate to permit the analysis of many such boundaries in a given speeimen within a reasonable period of time [2001 Fie]. 

Fle] Fleteher, H.A., Garratt-Reed, A.J., Aaronson, H.I., Purdy, G.R., Reynolds, W.T., Smith, G.D.W., A STEM Method for Investigating Alloying Element Accmnulation at Austenite-Ferrite Boundaries in an Fe-C-Mo Alloy , Scr. Mater., 45(5), 561-567 (2001) (Experimental, Interface Phenomena, Morphology, 25)... 

Guh] Guha, A., Aaronson, H.L, Rigsbee, J.M., Observations on Epsilon Carbide Precipitation at Austenite - Ferrite Boundaries in an Fe-C-Si Alloy , J. Metals, 28(12), A54-A54 (1976) (Abstract)... 

Figure 9. Microstructure of 2 V4 Cr-1 Mo steel. Key 0, ferrite grain peppered with M C , pearlite (platelets of alternating M,C and ferrite) and K, ghost boundary of prior austenite grain boundary.

Similar comments are applicable to weld metal hydrogen cracks, with the further observations that cracking in weld metals is frequently transverse to the weld and that in weld metals of the acicular ferrite type, cracking is most common in the primary ferrite (if it is present) at the prior austenite grain boundaries, as in Fig. 1.3(a). In relatively hard, alloyed weld metals (e.g. in the Cr-Mo steels) any transverse weld metal cracking is usually perpendicular to the weld surface, but in C-Mn weld metals the chevron type of cracking (Fig. 1.3(c)) at 45° to the surface is common. 

Ferritic stainless steel has the reputation of being less sensitive to intergranular corrosion than austenitic stainless steel. This type of corrosion can nevertheless take place under certain conditions of thermal treatment [20]. The diffusion coefficients of both carbon and chromium in ferrite are larger than in austenite. Grain boundary precipitation of carbides and nitrides of chromium can therefore occur at temperatures of 540-600 °C already. The behavior differs from that of austenitic stainless steel, which becomes sensitized at higher temperatures only. Because of the larger diffusion... 

Concept Check 9.9 Briefly explain why a proeutectoid phase (ferrite or cementite) forms along austenite grain boundaries. Hint Consult Section 4.6. 

Eno] Calculation (central atoms model) Ferrite and austenite phase boundaries... 

Enol] Enomoto, M., Aaronson H.I., Nucleation Kinetics of Proeutectoid Ferrite at Austenite Grain Boundaries in Fe-C-X Alloys , Metall Trans. A, 17A(8), 1385-1397, (1986) (Phase Relations, Thermodyn., Calculation, Experimental, Kinetics, 53)... 

On slow cooling the reverse changes occur. Ferrite precipitates, generally at the grain boundaries of the austenite, which becomes progressively richer in carbon. Just above A, the austenite is substantially of eutectoid composition, 0.76% carbon. 

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