Thermal Fatigue of New Tube Materials, 2000 Engineering Conference Proceedings

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Thermal fatigue has been considered as one reason for cracking of recovery boiler composite tubes. The conditions where composite tubes operate are not constant but can vary very much. The solidified layer of smelt against the recovery boiler floor tubes can temporarily melt causing high temperature liquid smelt to touch the floor tubes or steam bubbles to form inside the tube and then collapse. Temperature excursions produce thermal gradients, which create thermal stresses in the composite tubes. When the stresses are high enough and are repeated many times, cracks may finally nucleate in the cladding of a composite tube.

Thermal fatigue tests for composite tubes have been carried out with a testing device which heats and cools the composite tube repeatedly, without dwell times, in order to produce temperature gradients. The stresses/strains in the short composite tube samples produced by the test are different from the stresses in real recovery boiler composite tubes produced by the operation of a boiler. This is evidenced by the fact that the residual stresses in the surfaces of the composite tubes are in compression after thermal fatigue testing, but they are in tension in real recovery boiler composite tubes after operation, when at room temperature.

In this study weld overlay coated, and co-extruded tubes were tested. Some of the claddings were traditional stainless steels and some were nickel-base alloys. Furthermore, chromized carbon steel and uncoated carbon steel tubes were tested. The nickel-base alloy as well as the uncoated carbon steel tubes proved to be more resistant to thermal fatigue cracking compared to the traditional composite stainless steel tubes.