microstructure difference P9 vs P91 2

[QC1971]

What differences in microstructure can be expected between P9 (9Cr-1Mo) and P91 (9Cr-1Mo-V). I am familiar with the P91 as we use it in the power industry however I have heard very little on the P9 material as we do not use it.

 

[stanweld]

P9 material may be in the fully annealed condition, in which case the microstructure will be predominantly coarse ferrite. It may also be in the normalized (P9 is air hardenable) and tempered condition, in which case the microstructure could be coarse, tempered martensite. You will also not see the fine carbide, carbonitride precipitates in either of the above matrices as is exihbited in P91.

 

[QC1971]

Thank you Stanweld. It sounds like it is criitical to specify what condition the P9 material is supplied. I would expect that in the martensitic form that you would experience many of the same fabrication issues as P91.

 

[stanweld]

Not really. The high temperatures creep properties of P9 are not nearly so dependent on the correct microstructure. The best high temperature properites of P9 will be exhibited in the annealed condition.

P9 is also used in hydrogen and sulfur service in the petrochem industry at temperatures near 750 F, in which service the material will often times be specified in the normalized and tempered condition.

In terms of hydrogen cracking prevention, the required preheat is essentially the same as that for P91. PWHT is normally performed at 1350 F to 1400 F, which is only slightly lower than that normally prescribed for P91. The weld metal and substantial part of the HAZ will transform to martensite regardless of the initial heat treat condition of P9.

 

[metengr]

Not much more to add as Stanweld notes. We have a fair amount of SA 213 T9 superheater and reheat tubing in our aged Power Boilers. The T9 material does not behave like T91 material. T9 material is more forgiving in terms of forming and post weld heat treatment. I have had very little problem with T9 boiler tubing because it was originally designed for corrosion (high temperature oxidation) resistance versus creep strength. As a matter of fact, the creep resistance of T9 is actually lower in comparison to Grade 22 tube material, as reflected in slightly lower allowable stress values (so higher alloy does not necessarily equate to higher creep strength) in ASME Section II, Part D stress tables. The reason is that for a constant molybdenum content (of 1% mass percent) for the family of Cr-Mo alloys, as the chromium content increases no additional precipitates as mentioned above form, so you gain no benefit in creep strength.