Aluminising to prevent steam oxidation of 9% chrome steels 2

[mnewnhama]

I am loking for new applications for diffusion coatings, especially aluminising and chromising. We have done some work showing that high strength 9%Cr steels such as P91 have poor resistance to steam oxidation, and aluminising is very protective in this regard. Does anybody know of applications where such steels are being used in high temperature steam environment and would benefit from aluminising?
Thanks

 

[metengr]

We have done some work showing that high strength 9%Cr steels such as P91 have poor resistance to steam oxidation, and aluminizing is very protective in this regard.

I disagree; the 9% Cr-Mo materials have good oxidation resistance for their application and strength properties for use in subcritical Power Boilers. Aluminizing will not work for Grade 91 steel because of the interaction between V, Mo and Cr carbides. Aluminum contents are specified to a tight range to achieve formation of the desired carbides for this steel

 

[mnewnhama]

Thanks for the feedback.
I wonder if the reason for our different experiences might be the operating temperature - as temperatures rise towards 650C stem oxidation becomes more problematic. Also, are you saying that aluminising has not worked, or do you suspect that it will not work because of the possible interference with carbide formers. The diffusion zone below the aluminide coating is only several tens of microns below the surface, an should not interfere except in very thin walled components.
A bigger issue may be either the impact on mechanical integrity of a brittle coating, or incomplete coverage of the ooating.

 

[metengr]

mnewnhama;
The 9% Cr alloys do indeed have their service limitation. At 650 deg C this is beyond the recommended service usability (oxidation resistance and creep rupture strength) of this material. At or above 590 deg C you should be evaluating 12 Cr modified alloys (P122, NF12) or Ni-base as in the case with IN 740 or Inconel 617.

Regarding aluminum content of Grade 91, aluminizing would interfere with the optimum ratio of Al (0.04 w%):N (0.03-0.07 w%) to assure optimum creep strength properties in this steel from fine niobium carbonitrides and from vanadium carbides. Too much Al would tie up N and alter the formation of carbonitrides.