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Lower Furnace Tube Swage

Lower Furnace Tube Swage

Lower Furnace Tube Swage

(OP)
We have recently had cracks in tube swages from 3" to 2.5".  We have identified a significant difference in hardness measurements between the swaged area and the mother tube.  Has anyone seen this in other tube swages?  The metallurgists who have looked at the tubes are saying it is Stress assisted corrosion but two of the cracks have been on the hot side of the tube.  Has anyone seen SAC on the hot side of a tube?

RE: Lower Furnace Tube Swage

I have only seen cracks in swaged superheater tubes that were supplied as Type 347H material. The cause of cracking was creep failure caused by stress-assisted carbide precipitation in elevated temperature service.

RE: Lower Furnace Tube Swage

sort of.

On supercritical waterwalls, there is a common cracking phenomena often called "alligator cracking" . It is "stress assisted" in that  severe longitudinal stresses are generated by the 2 main culprits: hot side to cold side temperature differfence caused by deterioration in inside film heat transfer coefficien ( psuedo dnb) and also tube to tube temperature unbalance ( thermohydraulic sensitivy and/or ledineg static flow instability). These severe stresses may leave gthe hot side of the tube with residual tensile stresses.

The corrosion is related to sulphur compounds in the furnace fluw gases lodging in the microcracks formed by fatigue damage, and the available strain energy due to residual stresses may accelerate the rate of corrosion.

IN those case , tube material is usually T2 . For the swaged case on a drum boiler you had discussed , tube may be CS or T2, and the severe compression imposed during swaging may lead to residual tensile stresses during operation. Even if the swage was stress relieved, the residual strss would equal the yield stress at the stress relieving temp.

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