PWHT heating and cooling rates 2

[sakthi06.dec]

Why is heating and cooling rates not applied from ambient temperature? (per Sec-I, why is it above 425°C and per B31.1, it is 315°C)? I am in need of a technical reason for it.
Also, does it have any hidden meaning that the temperature of furnace and the item that is to be stress relieved in furnace shall be below 425°C?

 

[Dhurjati Sen]

Dear sakthi06.dec,

Refer to the Fe-C phase diagram. Its an equilibrium / quasi equilibrium diagram. To get the desired phase changes, you need to homogenize the temperature along the cross-section of the specimen. Thus, heating and cooling rates are required.

Above 300 / 400⁰C, the phenomenon of recovery, recrystallisation and grain growth come into play. Hence, the rates are applied from that temperature.

B31.1 seems to be more stringent in this regard, but do check the latest revisions.

Yes, the specimen can be heated up to 315 / 425⁰ C or cooled below that temperature at any rate.

Regards.

DHURJATI SEN
Kolkata, India