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alanisphoenix (Mechanical) (OP)
30 Sep 07 4:57
hello, i'm new here, and i'm looking for some information. i need to modified a bit the mechanical properties of the P91, because we have a high yield and tensile points, but the elongation is close to the minimum according to ASTM A 335 20%. We have modified the Tempering temperature to the ASTM limit 800ºC, so close to the AC1, and increased the time.. but any recomendation about the chemical composition? I know our carbon content is high around 0.11/0.115, but they dont want to modified it because in the pass with lower carbons they had problems during the rolling process. So what other elements (V, Mo, Cr)can i modified to decrease the hardness in our pipes? any suggestion? thank you so much in advance.
Helpful Member!  metengr (Materials)
30 Sep 07 9:28
Alazne;
The properties of Grade 91 material are based on a very tight chemical composition ranges. I would not recommend any deviation from what is specified in ASTM/ASME A/SA 335. Yes, this material possess higher strength at elevated temperature for forming, so you must have adequate forming capability.

The Al/N ratio and Cr, Mo, V alloy composition ranges were optimized to assure the formation of adequate Al(C:N) precipitates and vanadium carbides for high temperature creep stregnth, and mechanical properties.
Helpful Member!  stanweld (Materials)
1 Oct 07 10:03
Tempering at the maximum permitted temperature will prove risky. The actual temperature may be higher and easily exceed the lower critical temperature, at which point the creep properites will have been compomised and the material will require re-normalizing and tempering to restore them. Similar degradation of properties has been reported when the tempering temperature is very near but still lower than the lower critical temperature and held for a sufficient time.

What are you fabricating from purchased P91 pipe and how are you fabricating it?

alanisphoenix (Mechanical) (OP)
1 Oct 07 14:16
thank you so much both for your help.
We fabricate seamless pipes, so we made it by a rolling process. I know that increasing so much the tempering Temperature, the creep values decrease.. i have not yet the actual creep values, but at tensile properties at elevated temperature (about 400 /500 / 560ºC) are also higher than the minimum ones stated on the EN 10216-2 or DIN, for those temp...so the elongation used to be so close to the minimum.. and thats what i want to change.

So at the laboratory tell us something about the Al, N, C and V combinations too.. but some clients ask for N/Al min. 2 or other kind of requirements so somethimes is not easy mix everything to be according to the standars, to the client  and having a great rolling conditions to avoid problems.
we are thinking about decrease a bit the Vanadium content, to be around 0.22 and will see...
stanweld (Materials)
1 Oct 07 14:47
Normal finishing temperatures during forming should be greater than 1650 F (900 C); after which, normalizing and tempering per A-335 P91 is perfomed. Are you cooling in still air from the normalizing temperature or are you using accelerated cooling methods? Optimum tempering is perfomed between 1380 F (750 C) to 1435 F (780 C) and we prohibit tempering above 1440 F. Elongation properties can be enhanced by increasing time at temperature without adversely affecting Creep properties.

Are you failing to meet the specification's elongation requirements on a significant quantity of material or are they being met but merely on the low side?  

Helpful Member!  Metaljon (Materials)
7 Oct 07 3:51
I would agree with Stanweld that you should not temper this material close to the AC1 temperature.  800 deg C is too close to the limit. Some P91 material we recently tested had a AC1 temperature of 795 deg C.  The effect on properties if the AC1 is exceeded is very dramatic.

Not clear as to what range of properties you are achieving.  Normally I would increase the final tempering time to reduce hardness which should have the affect of increasing elongation.   
alanisphoenix (Mechanical) (OP)
9 Oct 07 14:28
hello,
well, thanks to all of you. we will increase the time for the tempering of course, that was our first action. and now im planning to do some tests at the laboratory. we are going to cut the samples after the normalizing and then we are going to do the tempering at the laboratory at few diferent Tª,to check the mechanical values and the microstructure. we are close to the limit elongation but into the standars, so is not a main problem, but a possible problem... thats why we are just looking for information about how decrease the elongation and hardness.
but i was thinking about "attack" the problem from the first step, the chemical composition... we have decrease a bit the Vanadium on the last heats, so i just have to wait few weeks to seee the results.
thanks again to all of you.

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