So that figure gives you about 4-5 min.
But if the material was not fully annealed in the first place the time would be more like 10sec.
The material does not need residual carbides, just to still have non-uniform composition. Even small local composition gradients greatly accelerate the reactions.
And of course if the temperature actually hit 650C then the time would be more like 2 min.
Many people are confused by this issue. If you're using the material at high temperature continuously, there isn't an issue at all related to sensitization- the corrosion mechanisms you're worried about at high temperature are totally different. If the material is brought down to lower temperatures periodically and exposed to aqueous corrosion or polythionic acid attack conditions etc., then you're going to have problems- and the solution is the stabilized grades or higher alloys. In many cases, the low temperature corrosion cases can be mitigated by proper shutdown and inerting procedures. In others, this is impossible or impractical and the stabilized grades are required.
Note too that sensitization often occurs as the result of temperature exposure from welding. Carbide migration to grain boundaries happens during cooling from welding, leaving grain boundaries susceptible to intergranular attack as molten metal describes.
in fact, in place of 304H, stainless steel concerned is 18 to 25 Cr, 8 to 14 Ni and C around 0,6 content.
there is a Ferrite content required from 5 to 20FN. will this ferrite content prevent a sensitization of this steel during a PWHT of 2 hours ?
