I have performed differential thermal analysis (DTA) to measure the A1 temperature of grade 1.4003 ferritic stainless steels. My temperature difference curves show two peaks in the temperature range from 600°C to 850°C.
The second peak located between 780°C and 830°C in the attached figure is the α→γ phase transformation.
I wonder what is the first peak centered at around 710°C in my DTA curve?
As far as I know, the Curie temperature is in this region. But according to literature, the latent heat associated with Curie point should is very small. The peak is rather high. => possible but unlikely.
The tested materials are cold rolled sheet metals in annealed condition. I do not believe that they contain any sigma phase in the initial condition. The DTA curve shown was measured in heating carried out using the heating rate of 6K / min. According to Thermocalc calculations, sigma phase is stable in the temperature range T< 550°C.
I think it is unlikely that sigma phase possibly formed during the DTA measurement would cause such a high peak. And why would the peak show at around 700°C?
How did you determine the second peak for the α→γ phase transformation?
Just a thought: You DSC curve looks through a big exothermic peak with several small endothermic peaks on it. The DSC baseline might be drifted so much that the first peak had artifact. It still can be magnetic transformation or dissolutions.
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