Precipitation of Ferrite in the SH-CCT Diagram for welding

Reconsideration of SH-CCT Diagram for welding (I)

Abstract

The nucleation and growth processes of ferrite precipitation from austenite in low-carbon low-alloy high strength steels were investigated to make use of SH-CCT diagrams for welding. There were ferrite allotriomorphs and ferrite sawteeth which precipitated from austenite grainboundary, rodlike or needlelike ferrite which precipitated in austenite grain. Growth morphologies of these ferrites were independent of the strength level of testing steels and had the following growth processes : 1) In the case of slow cooling; grainboundary ferrite grew to massive ferrite, ferrite sawteeth and broad ferrite sideplate without carbide precipitation; intragranular ferrite grew to massive ferrite and rodlike ferrite without carbide precipitation; 2) in the case of rapid cooling, grainboundary ferrite grew to narrow ferrite sideplate, intragranular ferrite grew to needlelike ferrite without carbide precipitation.
We defined the temperature region where ferrite grew to the configurations described above as the F transformation region in the SH-CCT diagrams for welding.
As Zwischenstufengefuge (Zw) transformation proceeded continuously after ferrite transformation, it was difficult to accurately find the begining of transformation of Zw and the difference of Zw and ferrite by dilatation curve and microstructure. The starting line of Zw transformation following ferrite transformation, for these reasons, turns out as shown by broken line. The structual percentage of ferrite and Zw is exhibited as sum'of them under the Zw transformation area.
Starting line of pearlite transformation does not correspond to the finishing line of ferrite transformation nor the starting line of Zw transformation corresponds to the finishing lines of ferrite and pearlite transformation.