why some time austanatic stainless steel grade are specified for stress relieving ???

[Vishal italiya]

i had observed that some clients are provide stress relieving for ss 304 and 316 grades. as we know sensitization would take place at that temp and would damage corrosion resistance. so i would like to know about detailed reason for same and by doing stress relieving what kind of benefit we receive at end of stress relieving ?????

 

[EdStainless]

In cases where other steel components need stress relief and you can live with the risk of IGA on the stainless then a stress relief may be done.
One such case is high pressure feedwater heaters in power plants. The only exposure is high purity water so IGA is not a serious risk.
In most cases though people have selected a stainless steel for corrosion resistance and subjecting it to a stress relief only assure rapid failure in service.

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P.E. Metallurgy, Plymouth Tube

 

[Vishal italiya]

Thank you for your answer.

But I seen that equipment is wholly fabricated with austanetic stainless still and client/ designer specified for stress relieving for same. Before this i had seen case where claded vessel where base metal need PWHT as per code or service requirements and hence we take risk with clad material to meet base material requirements.

I am very clear that stress relieving will not affect much on corrosion properties as to sensitize material it will take to longer time. But my experience says that I had seen so many times that after stress relieving of Sa 240 or SA312 Grades are failing in IGC Test.

 

[puruisen metal]

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[EdStainless]

The issue with sensitization and IGA isn't the formation of grain boundary chrome carbides.
It is the localized depletion of Cr along those grain boundaries.
If you use a higher temp, where Cr will diffuse faster in the alloy then you can form carbides and heal the localized depletion.
When this happens you still have GB carbides, but the material will not show preferential corrosion along the grain boundaries.


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P.E. Metallurgy, Plymouth Tube

 

[Dhurjati Sen]

Dear Vishal italiya ,

Can you specify the stress relief cycle?

Regards.

DHURJATI SEN

 

[cap1a79]

Although it is very rare, there are few cases that stress relieving for stainless steel is performed after cold-forming. As cold-working makes SS be susceptible to stress corrosion cracking, stress relieving is applied to improve SSC resistance.
However, the temperature of SR shall be outside of sensitization range. Refer to the tables of below link. (It is from British Stainless steel association)

https://www.bssa.org.uk/topics.php?article=76

Lee SiHyoung,
WorleyParsons Oman Engineering,

 

[EdStainless]

There are a couple of other issues with SR of SS.
- The risk of distortion, very well designed supports are needed
- The risk that fast cooling (to minimize sensitization) will result in high residual stress. The largest source of residual stresses in differential expansion (or contraction) when cooling.
- The risk of surface oxidation. If this is not removed by pickling the pitting resistance will be significantly decreased.

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P.E. Metallurgy, Plymouth Tube

 

[Vishal italiya]

material SS 316L
loading 300 degree Celsius
Heating rate and cooling rate 150 degree Celsius / hour maximum
Holding temp & time - 620 degree Celsius for 1 hour.

 

[cap1a79]

Vishal - Are you sure about 620 C for 1 hour? Is that considered for avoiding sensitization of SS?

Lee SiHyoung,
WorleyParsons Oman Engineering,

 

[EdStainless]

620C is too low, even for a short period of time.
You should expect some sensitization.
Will the application tolerate it?


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P.E. Metallurgy, Plymouth Tube