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Super stainless steel 304

Super stainless steel 304

(OP)
Hello everyone,

Anybody know about super stainless steel 304 that is used in super critical temperature boiler in power plant. I was informed that the as supplied material's hardness for this material is low but will increase during heating/operation due to the precipitation of Cu ...which will further increase its creep properties due to this mechanism...The hardness value will then stabilize..any link to this claim/material?

Any comment is highly appreciated...

RE: Super stainless steel 304

There is a SH tube material that was approved as a code case. The material was a Super 304H tube material by Sumitomo.

RE: Super stainless steel 304

(OP)
Thank you very much metengr...can you share link about this material?

RE: Super stainless steel 304

It is called Google, try it.
The UNS is S30432

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P.E. Metallurgy, Plymouth Tube

RE: Super stainless steel 304

(OP)
Thank you metengr and EdStainless..got it..

RE: Super stainless steel 304

(OP)
Thank you XL83NL for a very useful link...

RE: Super stainless steel 304

This is a very odd alloy. It gains strength at temp by the formation of sigma phase. But because of the chemistry balance the sigma is not excessively brittle. Even after long high temp exposure the room temp elongations are still over 25%.
However weld repair on material that has been in service could raise a whole other set of issues.

This is a proprietary grade so if you use it you will be single sourced.

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P.E. Metallurgy, Plymouth Tube

RE: Super stainless steel 304

(OP)
Thank you EdStainless ...I thought the creep property enhancement and strengthening mechanism at high temperature is due to nano precipitation of copper and niobium as claimed ...One thing that confused me regarding the claims is that by having smaller grains the corrosion resistance will increase but what about creep resistance..As far as I know, smaller grain will provide more nucleation site for creep void formation due to bigger surface area....any comment? Or Is the creep mechanism at high temperature not related to surface area..I mean another type of creep mechanism...

RE: Super stainless steel 304

There are many types of creep, but the two main types of creep that can come into play here are diffusional creep and grain boundary sliding. In grain boundary sliding, the grain boundaries can slide relative to one another, so a coarse grained material will not be as affected by this creep mechanism as a fine grained material will. These microscopic displacements along grain boundaries are caused by the glide and climb motion of defects called dislocations. Dislocations will glide until their movement brings them into contact with obstacles that block their glide motion such as an oxide inclusion for example. Dislocation climb may enable dislocations to move past these obstacles through diffusion. The rate of diffusion of point defects to or from moving dislocations becomes the rate-limiting step in the entire creep process under this particular mechanism. Steady-state creep due to grain boundary sliding is therefore a type of diffusion creep and the creep rate is inversely proportional the grain size raised to some exponent. So as the gain size increases, the creep rate decreases.

Maui

RE: Super stainless steel 304

Which is why certain (ASTM) material specifications, especially for alloys which are used for high temperature applications (such as the H-grades in the 300 stainless series), specify a minimum a grain size number, as such alloys are used in the regime where time dependent properties govern.

RE: Super stainless steel 304

In this alloy the secondary phase that forms (calling it sigma just refers to its structure, it is a Cu rich phase) serves as pinning sites to resist creep.
This alloy tries to preserve corrosion resistance by limiting the formation Cr carbides (sensitization) and by having finer grains they have a finer dispersion of secondary phase particles to assist in pinning. In systems like this the behavior is more like age strengthened Ni alloys than traditional stainless.

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P.E. Metallurgy, Plymouth Tube

RE: Super stainless steel 304

Matching weld filler metals were only available from two manufacturers when we were erecting boilers with this material. We purchased the material from both and qualified our welding procedures from both because they were not listed in any SFA specification at the time.

RE: Super stainless steel 304

This alloy is not propeitary because it is in the ASME Code book. ASME cannot offer any advantage to a supplier! This alloy was developed for ultrasupercritical boilers and performs well in service. There are no welding issues on aged material.

RE: Super stainless steel 304

(OP)
Maui...thank you for comment.

EdStainless ...I got it..I thought sigma phase that form from delta ferrite at high temperature that will reduce the toughness of the material.

weldstan,,we are using this material in our newly erected coal fired power plant...

RE: Super stainless steel 304

Thanks Metengr, the last time that I worked with this alloy was about 6 years ago.
At the time I hadn't seen test results on repair of aged material, good to know that it works fine.

While not strictly proprietary I wasn't aware of anyone else making this grade. The Code will list single source alloys (even patented ones) if end users request it, they won't do it for producers.....

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P.E. Metallurgy, Plymouth Tube

RE: Super stainless steel 304

Just be sure you source adequate amounts of filler metals.

RE: Super stainless steel 304

One final thought, we had three different IHI coal fired boilers defining Super 304 H. We required IHI to make all dissimilar metals weld joints so we would only weld Super 304 H to Super 304H when erecting the boiler.

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