loss of toughness of casting Ni-alloy 800H after thermal ageing

[Chumpes]

Dear Experts

We faced failures of casting version of Ni-alloy 800H (manaurite 800) after thermal ageing. Operation is at 500°C and excursions at 620°C are anticipated (coke burning). Failure was by crack propagation and no corrosion was experienced. Mechanical test of failed tubes showed total loss of toughness at ambient temperature.

It seems to me this can be some kind of "sigma phase embrittlement" of Ni-alloy 800H but sigma phase formation normally requires a ferrite phase and Ni-alloy 800H is supposed to have very low (zero ?) levels of ferrite. Can there be a ferrite phase or another phase in austenitic structure of casting Ni-alloy 800H that can promote sigma phase formation ?

Can sensitization by chromium carbide precipitation at grain boundary produce significant loss of toughness at high temperature for casting of Ni-alloy 800H ?

Do you see any other metallurgical transformation that can occur at 500-620°C on casting Ni-alloy 800H and that can lead to loss of toughness ?

Which countermeasures (chemical limitations, grain size, ...) would you advise in order to avoid loss of toughness of Ni-Alloy 800H after thermal ageing at 500-620°C ?

Which bad fabrication practice could induce such low ductility of Ni-Alloy 800H (wrong heat treatment / casting conditions) ?

thanks in advance
regards

 

[btrueblood]

"Can sensitization by chromium carbide precipitation at grain boundary produce significant loss of toughness at high temperature for casting of Ni-alloy 800H ?"

Well, yes. From

http://www.specialmetals.com/assets/smc/documents/alloys/incoloy/incoloy-alloys-800h-800ht.pdf

"Chromium carbides precipitate in the alloys at
temperatures between 1000 and 2000°F (540 and 1095°C).
Consequently, alloys 800H and 800HT are similar to other
austenitic alloys in that they can be rendered susceptible to
intergranular corrosion (sensitized) in certain aggressive
environments by exposure to temperatures of 1000 to
1400°F (540-760°C)."

You'd have to judge whether the parts were subjected to aggressive environments.

 

[blacksmith37]

"Coke" implies some level of sulfur which will accelerate inter-granular corrosion. In my experience "coke burning" can reach higher temperatures than anticipated.

 

[Chumpes]

Hello,
Well, thanks for your attempt to provide some guidance... please also note that I carefully selected the sub forum "Material Engineering" and not "Corrosion Engineering"...

@btrueblood :
As it is written in the original post, I was talking about loss of mechanical properties and thermal ageing, not about loss of corrosion resistance... Your statement extracted from material datasheet is only about loss of corrosion resistance by chromium carbide precipitation, not about the loss of thoughness that I was interested in... your answer is off topic but still thank you...

@blacksmith37 :
Yes I agree, the coke burning temperature probably reach higher temperature than expected, and environment may contain some sulfur, but I thougth that 800H was very resistant to sensitization and also that the minimum iron content of 800H would allow avoidance of intergranular corrosion by hot sulfur (affinity of sulfur with nickel, less affinity with iron). Whatever, no corrosion but mechanical damage with zero toughness was experienced...

Does anybody have more information on "Metallurgical Transformations of alloy 800H in the 500-620°C temperature range that can induce loss of mechanical properties (toughness)" ?

Thanks again.

 

[EdStainless]

There are a few reasons that people focus on the loss of corrosion resistance.
1. It happens before you can see significant changes in mechanical properties
2. Corrosion can be measured with high sensitivity
3. At temperatures over 1200F impact toughness isn't usually an issue. People are very willing to give up RT toughness in order to gain creep/SR strength.

Alloy 800 is very metallurgicaly stable, the high carbon versions are somewhat less so.

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P.E. Metallurgy, Plymouth Tube

 

[Chumpes]

Thank you very much Ed
I think that in absence of a corrosive environment, the specific mechanism of sensitization by precipitation of chromium carbide at grain boundary does not impair much mechanical properties such as toughness, am I wrong ?
I do not find much information on metallurgic transformations of 800/800H. Do you have any reference to advise ?
Do you think cast versions may be more prone to metallurgic transformation than other types of product form ?
Thanks again.

 

[Dhurjati Sen]

Dear Chumpes,

Have you checked the Titanium & Aluminium content of the failed tubes?

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[Chumpes]

Hello Dhurjati Sen
I will check if I can have Ti and Al content of failled tubes.
Are these elements the most susceptible to promote temperature embrittlement of Ni-alloy 800H ?
Do you have any guideline for chemical limitations that would help mitigating thermal transformations of Ni-alloy 800H ?
Thanks

 

[Dhurjati Sen]

Dear Chumpes,

Its the opposite. The various inter-metallic compounds formed by each of these with Ni & Cr enhances creep resistance.

Regards.

DHURJATI SEN
Kolkata, India

 

[EdStainless]

The Ti and Al intermetallics (along with carbides) may enhance creep resistance but they will also lower the room temperature toughness.
In these high temp alloys there are often failures related to shutdown and startup practices. As the material cools the toughness can be very low.

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P.E. Metallurgy, Plymouth Tube

 

[Dhurjati Sen]

Dear Chumpes,

Hope you understand the importance of checking Ti & Al content before getting deep into failure analysis and corrosion phenomena discussions.

Dear EdStainless,

Always a pleasure to learn from you.

Best regards.

DHURJATI SEN
Kolkata, India