Cyclic Thermal Fatigue vs Creep Fatigue in Steel

[DCA0077]

Hi All,

Could someone help me with the following question. Which of the following steel structure scenarios will fail first:

1. The steel structure operating at a constant temperature of 300 °C (creep fatigue)
2. The steel structure operating at temperatures cycling continuously between 100 °C and 300 °C, cycle period 1 hour. (cyclic thermal fatigue)

Thanks

 

[cloa]

Too vague. What is failure for the structure e.g. deformation or strength loss? What's the steel alloy? And what's the dimensions? Thermal cycling won't produce any fatigue if its thin. 300 C is just near low end for steel creep.

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[davefitz]

The question is not correctly posed.

Few materials exhibit creep at 300C; 425 C + is a better choice. And fatigue is a phenomena of alternating stresses; a constant tress state does not generate fatigue damage.

The rate of life consumption for both creep and for fatigue are well known for properly formed materials, but the combined failrue modes of creep + fatigue simultaneoulsy operating on the workpiece is yet a work i progress and the object of many pHd theses.

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[DCA0077]

Thanks for the reply.

We have a carbon steel gas duct transporting furnace off gas to a bag house. The shell temperature of the gas duct cycles between 100 °C and 300°C every hour on a daily basis due to a change in temperature of the gas inside the duct. We need to replace some of the duct section every 2 years because of deformation. My question is the deformation because of creep failure at high temperature or cyclic thermal fatigue? Will we reduce the duct deformation over time if keep the shell temperature at 300 °C.

 

[stanweld]

Temperature cycling and failure to account for it in the design is more likely the reason for your problem. One assumes the designer did take into account the thermal expansion from ambient to 300 C.

 

[metengr]

The original post is impossible to answer with the given information. You need to know the service stresses and geometry of the object.

 

[davefitz]

The failure is due to forgetting to include an expansion joint in the design.

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