Cooling Rate During Austenite to Ferrite Transformation and Grain Size
During cooling of austenite, the new bcc ferrite crystals start to grow from many points. The number of starting points determines the number of ferrite grains and consequently the grain size. This grain size is important because the engineering properties are dependent on it. Small grains are favourable.
Slide 7 : Microstructure of pearlite. (x 1000)
By adding elements like aluminium and niobium, the number of starting points can be increased. Another important factor is the cooling rate. When cooling is slow, the new ferrite grains develop from only a few most favourable sites. At high cooling rates, the number of starting points will be much higher and the grain size smaller. Slides 7 - 9 shows steels with various grain sizes, produced at different finish rolling temperatures.
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