is it possible to get "yield stress at ambient temperature" AS the "von misses stress at very high temperature"?

 I didn't do any thermal analysis but only doing structural analysis but i am using measured temperature values to my structural model. I created temperature loading with time (Using amplitude method as you mentioned). Plastic perfect material model created with respect with temperature.

I used Nonlinear Static Analysis with Linear Perfect plastic materialmodel. (fy=612 at Temperature=20°C)

The yield stress and elastic modulus values wereinput with respect to temperature (Using FIELD)

The temperature was input as a boundary conditionwith respect to time (Using Amplitude)

Then I use Nonlinear Static Analysis to run thefile in a time frame (say up to Total time=107).

However, after some analysis time (say Total time=75min) we had some numerical problem.

Therefore we used STABILZE and the analysis wascompleted successfully.

Near Failure, (In the attached diagram, atTotal time = 107), I get the stress distribution shown in figure.

In the joist due to high temperature(T=320°C), the maximum possible fy is 560 at coldflange near failure (Total time=107).

However, I'm getting  von misses of 612.That's the problem.

Specially, I'm getting these values after theSTABILIZE is effective. (I mean after Total time=75)

 If i go through the resultsfile von-misses values increases with step time up to ambient yieldstress (Fy=612) there after it keeps same

Could you please comment on this? Or please adviseme how von misses stress created while we use temperature loading?

I  mean, is it possible to get von missesstress of 612 at very high temperatures?