Yield strength of material after it has been exposed to high temperature

[borobam182]

Hello,

I'm not sure if this is the correct place to ask this question, but hopefully you can help me with this issue......

I have some lifting features to be used for lifting an exhaust stack. These features are located inside the 'gas path' and will get heated up to the full exhaust temperature (+600 oC).

However, the lifting features will only ever be used for lifting at an ambient temperature.

The question is - does exposing the lifting features to this high temperature mean that when designing them for ambient use, we should use the 600oC yield strength (does a reduction in yield strength occur because it been exposed to the heat), or should the ambient temperature yield strength be used, as this is the temperature that they will be at when being used?

My assumption was that we should use the yield strength at ambient temperature, basically assume that the yield strength will return the same at ambient before and after its been exposed to high temperature.

This applies as long as no permanent deformation occurs, which I feel would be the case here.

Any thoughts or opinions would be a big help.

Thank you in advance.

Craig.

 

[kingnero]

Depends on the base material.
600°C is rather much, but shouldn't reduce yield by more than 10% for low-alloyed carbon steels when back at room temp.
Depending on your situation, you could use creep resistant steels or even stainless.
Do you have any other specific requirements?

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[metengr]

The YS at 600 deg C has no bearing on lifting at ambient temperature. You should be using the yield strength at ambient temperature for design purposes.

With that said most materials will exhibit degradation at elevated temperature exposure. In these cases, if you are concerned you can perform surface NDT to check for cracks and signs of deformation. However, I suspect this lug will not be exposed to loading in service so there should be no issues. Make sure you have appropriate safety factors for lifting.

 

[borobam182]

Thank you for the feedback guys.

The lug will not be exposed to 'loading' when the equipment is in service, the lug will be 'loaded' when installing the duct at ambient temperatures, the lug will then be exposed to high temperature exhaust gases passing over it, and then the equipment will be turned off, lug will cool back down to ambient temperatures, and then it will be used again.

My feeling was to assume the yield at ambient temperatures, however, I wasn't sure if it should be reduced due to this exposure to high temperatures - possibly changing grain structures etc.

NDT is a good suggestion before the lug is 'reused' to ensure it is still suitable - thank you.

Craig

 

[gtaw]

I would say it depends on the base metal you are using.

While hot rolled low and medium carbon steels may not be adversely affected by the exposure to high operating temperatures, the same may not be true if you are using high performance steels that derive their properties by rolling at lower temperatures or heat treatment. The high performance steels could lose strength, both yield and tensile strength, after exposure to the temperatures you noted.

NDE before using the lugs is always a prudent move.

Best regards - Al

 

[mrfailure]

For piece of mind, you may want to consider taking a replica from the lug surface to see whether the structure has degraded. Even better, you can tensile test if you have a lug that is being retired at end of service to see if it steel meets requirements at ambient temperature.

 

[EdStainless]

If you are using material that has been simply normalized above 600C then you should be fine.
But that is in terms of UTS, long term changes in Yield strength are bound to happen, but it could go either way.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MintJulep]

"Ambient" does NOT mean "about 20C"

Ambient means "existing or present on all sides".

When you say "ambient temperature" you say nothing meaningful.

What happens to an unknown metal when exposed to unknown "gasses" at 600C for a long time is unknown.

 

[metengr]

noun
the temperature of the surrounding environment; technically, the temperature of the air surrounding a power supply or cooling medium; abbreviated ABM

Examples
Ambient room temperature ranges from 68 to 77 degrees Fahrenheit