0.20C-0.28Si-1.25Mn-0.15Cr Steel

Steel name: 0.20C-0.28Si-1.25Mn-0.15Cr
Diagram No.: 5204
Type of diagram: CCT
Chemical composition in weight %: 0.20% C, 0.28% Si, 1.25% Mn, 0.015% P, 0.008% S, 0.15% Cr, 0.044% Al, 25 ppm B
Steel group: Low alloyed steels
Note: Fig. show a typical continuous cooling transformation (CCT) diagram of low alloyed boron added steel. Against each cooling rate curve Ar3, Ps, and Pf corresponding to proeutectoid ferrite start, pearlite start and pearlite finish temperatures respectively have been marked.
It can be inferred from Fig. that at all the curves associated with different transformation temperatures have diminishing tendencies as the cooling rates are increased. However, decrease is gradual upto cooling rate of 5C/s. The proclivity towards decline becomes quite sharp at and above cooling rates of 10C/s, which continues till it again levels off at cooling rates of 20C/s and beyond. The CCT diagram can be interpreted in terms of first formation of proeutectoid ferrite from austenite labeled Ar3 line in the figure. There is not much variations in this temperature with cooling rates. The region between Ar3 line and the Ps line gets widened upon increase in cooling rates. The lines representing Ps and Pf remain somewhat parallel upto cooling rate of 5 C/s. Marked change in width of separation occurs after this cooling rate and area get widened progressively as it approaches towards higher cooling rates. Therefore, cooling rate of 5 C/s is critical and seems to signal departure from formation of mere pearlite and ferrite phase mixture. This may also be the cooling rate above which bainite may form.
Reference: Not shown in this demo version.

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