Restriction of Aluminium in Chromium Molybdenum Steels 3

[rkg]

A customer has asked for Aluminium to be restricted to 0.01% in Chromium Molybdenum Steels, when used for temp. above 850F. The trouble is that for A335P11 and P22 Aluminium is not required to be reported and for A335 P91, Aluminium restriction is 0.04% as per code. I have following questions:
1. What are the likely reasons for restricting aluminium?
2. Under which service conditions could aluminium be dangerous.
3. What could be the counter-measures(such as some special tests) in case we are unable to meet this requirement.

 

[metengr]

The only explanation that I can offer is that your customer is restricting aluminum to avoid graphitization. Aluminum is used as a de-oxidizer in steel making practice, and it also provides the benefit of refining grain size.

At amounts above 0.06 w%, Al will effect the creep strength of alloy steels, and also promote graphitization above 850 deg F. The tighter aluminum restriction (below 0.06 w%) applies especially to carbon and carbon-moly alloy steels not chromium-moly (Cr-Mo) steels for service at or above 850 deg F, because it is the chromium that prevents graphitization in Cr-Mo alloys (P11 and P22). The Mo enhances elevated temperature strength and creep strength of alloys at amounts 0.5% or greater.

If your customer is concerned about temper or thermal embrittlement, Al does not typically play a role.

 

[rkg]

Dear metengr,

I really appreciate your answer. This actually corroborates my thinking on the matter.

 

[stanweld]

Al in P91 ties up Nitrogen. The high temperature strength of P91 is dependent on formation of Niobium Carbo-Nitride precipitates. Restricting Al will assure their formation.

 

[rkg]

Dear Stanweld,
Thank you for your input. Could you clarify how restricting Al helps formation of these precipitates?

 

[rd400guy]

Not to steal this from stanweld, but niobium (or columbium here in the States) is below aluminum in terms of its "reactivity" with elements like nitrogen in steels. If you have high Al, it will combine with N to form aluminum nitrides. By restricting Al to lower levels, there will be "free" nitrogen to combine with Nb to form the aforementioned niobium carbonitrides, instead of aluminum nitrides / carbonitrides.

 

[EdStainless]

As an example of the Al-N reaction look at steels made for nitriding. They have deliberate Al additions.
The Al reaction is more favorable, so instead of the desired NbCN formation you will get AlN.
Look up the free energies and you will see which is favored.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[rkg]

Stanweld,rd400guy,
Thank you for your answers. I would like to know the importance of small amount of Nb in P91 vis-a-vis Vanadium, in terms of contribution to high temperature strength. In other words,what would happen to high temperature strength of this steel if Nb and N were not present.

 

[metengr]

rkg;
The answer - the long and short term creep strength of this material would be significantly reduced. The T/P91 ferritic steels were developed in the period 1970-1985 as an upgrade over T/P9 material for boiler tubing and high energy piping. The creep strength of T/P91 is derived from the control of C, V, and Nb resulting in precipitation of V and Nb carbides and nitrocarbides, that resist coarsening over time at elevated temperature, which would reduce creep strength.

The nitrogen content is also critical. I have seen some long term creep data on 91 grade materials that shows nitrogen needs to be tied up as carbonitrides, otherwise harmful precipitates can develop over time along grain boundaries that results in a lowering of creep strength (over 100,000 operating hours) by the formation of a “zeta” phase.

Getting back to the aluminum and nitrogen levels, there is a desired ratio of Al/N. This is necessary to prevent formation of AlN that can precipitate and coarsen at grain boundaries resulting in a lowering of creep rupture ductility.

There are numerous papers on short and long term creep rupture studies for Grade 91 material. In some cases there are disagreements in defining what is the optimum Al/N ratio to assure long term creep strength.

 

[rkg]

Could one of you guide me to some literature on the subject?

 

[metengr]

yes.
Please refer to the following publication -
"Materials for Boilers in Ultra Supercritical Power Plants" by R Viswanathan and WT Bakker - from EPRI

This paper provides an excellent overview on the development and evolution of austenitic and ferritic steel material.