stress relief at 400 F

[Tmoose]

A big fan shaft (fairly mundane European spec structural steel) developed a major bow after sitting for 6 weeks, when the in-housing temp might have reached 400F. The gravity induced stress is just a few thousand psi. During original manufacture some flanges were welded to the shaft, and some PWHT (details unknown, certs not yet provided) was done before finish machining. After running several months prior to the 6 month standstill the runout toward the ends of the long bearing journal was virtually NIL.

Circumstances suggest sitting caused the bow.

So far all generic metallurgical info suggests almost no stress relief takes place at 400F.

Any good references to help understand what could or could not happen?

 

[metengr]

It is called residual stress, and this is common for shafts of large rotating equipment that are not supported midsection during shipment or come to a sudden stop above ambient temperature. Periodic rotation is required and/or slow speed rotation to work the deflection out of the shaft if this occurs. Typically, this is no big deal UNLESS the service temperature is high enough to cause a permanent set (actual plastic deformation) or excessive bending loads caused a permanent set.