3.2,
First of all mixed with what?
Probably ferrite.
However, there are number of things.
Low carbon, low nitrogen, to slow cooling. Two of them, all three. But probably too slow cooling from the normalize, allowing the carbon to diffuse and form ferrite instead of martensite. Ferrite is a diffusion mechanism. Martensite a shear mechanism. Slow down the cooling allow the C to diffuse and you get ferrite even in higher alloys like Grade 91.
This can be a problem especially with thicker sections like flanges. The heat dissipating from the inside slows down the cooling of the outside. Some alloys (self tempering) use this. Its not good for Grade 91.
Its also possible the normalizing was too low in temp or too short, or both, and didn't allow the carbon enough time or enough diffusion to go into solution completely. Carbon has to be in solution to force more of the FCC of autensite to become the HCP of martensite instead of the BCC of ferrite.
It's possible the lower critical temp was exceeded during the PWHT or the tempering step, or quenching step was too slow. This would allow some tranformation of martensite to ferrite during PWHT or tempering, or incomplete martensite formation during quenching. The PWHT chart should tell the story. May need to check recorder calibration also. The only way to correct is completely re-normalize the part and repeat Q & T step correctly.
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