Heating 1.25Cr-0.5Mo to 900 deg C for 6.6 hours 3

[MadDog88]

Hello all,

I am getting a refractory lined vessel fabricated for hydrogen service (reformed gas outlet header) and I am concerned that the refractory dry-out temperature and duration will cause material degradation to the 1.25Cr-0.5Mo shell (1100mm OD, 20mm thick).

The dry-out schedule states:

24 hour minimum curing
Heat up to 500 deg C at 25 deg/hour
Heat up to operatimg temperature (900 deg C) at 50 deg/hour.
Hold at 900 deg C for 6.6 hours minimum
Cool down at a maximum of 100 deg C/hour

Will this temperature and hold time at this temperature cause any material degradation? eg. spheroidisation, distortion, oxidation etc..

Thanks in advance for your help.

 

[unclesyd]

Are you going to heat the header in the furnace or is it going to be heated with portable blowers?

I see no problem with the bake out and cure as you state. The time isn't long enough to do damage.

The only problem I've ever seen with a furnace cure was a slight collapse of the shell, but it was later determined that there could have been flame impingment on the shell. The shell was ok at the ends. If it is a furnace cure make sure everything is in order and operating properly, the same goes with portable heaters.

 

[metengr]

My main question is what is the actual metal temperature during curing? Are you planning to have T/C's installed to monitor the refractory temperature and tube metal temperature during curing?

For our Fossil boilers that have the lower furnace lined with refractory up to several inches in thickness, the gas temperature to dry and cure the refractory is similar to your stated temperatures except the refractory protects the tube metal. We have the contractor install T/C’s and have never seen metal temperatures above 350 deg C for carbon steel boiler tubing. So in reality you should not see actual metal temperatures reach gas temperatures.

So to answer your question, I would say that the Grade 11 vessel material will not see any significant exposure to temperature that would seriously oxidize or spheroidize the metal provided the refractory has been correctly applied and cures under heat. The maximum metal temperature that I would allow for the vessel is 700 deg C to assure remaining safely below the lower critical transformation temperature.

I would suggest you have the contractor that is curing the refractory install T/C's to monitor curing temperature and metal temperature.

 

[PAN]

- Should we consider about maximum holding time at 900 C?
- Should we inspect hardness of shell material after dry-out?

 

[metengr]

The 900 deg C does not represent vessel metal temperature unless the T/C's would indicate as such. Refractory provides insulation and would not conduct heat directly to the shell surface. My guess is the temperature of the vessel surface would be considerably lower.

Regarding hardness testing. Unless you exceed the lower critical transformation temperature of the vessel material - I would not bother with hardness testing.

 

[MadDog88]

Thanks for your comments. I now see the advantage of using portable blowers to dry the refractory in order to keep the shell temperature down. T/C's on the shell is another good idea.

What is the exact lower critical transformation temperature for 1.25Cr 0.5Mo? I have done a quick search on the internet but haven't been able to find it. Are you aware of any on-line refernces to this information for this and other materials?

Thanks again.

Cheers

 

[CoryPad]

MadDog88,

The best on-line reference for materials information is ASM International. They have the entire ASM Handbook series available via Internet subscription for only $279 per year to members. Membership is only another $107 per year. You can obtain more information at:

http://www.asminternational.org

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[metengr]

CoryPad’s recommendation is a good one.


If you have access to ASME B31.1 Power Piping Code, the stated lower critical transformation temperature for !.25% Cr- 1/2% Mo alloy steel is 1430 deg F (775 deg C).

 

[HeatTreater]

I have a couple of comments regarding your Refractory Heatup Schedule. I am presently on a job in northwest Alberta on a refractory cure on a waste heat boiler that has just had a "patch" repair on the refractory about 7 feet by 7 feet in size. The thicknes is 8" with T-Tech Insulation of 2" in thickness, between the shell of the vessel and the "Blue Ram" refractory.
Not knowing what the make up is of the refractory that you are using, I would like to mention that a ramp up of 50C p/hr to a temp of 500C from ambient may cause major spalling and cracking and refractory falling off the walls of your work piece. I would recommend a "hold" temperature in your procedure of whatever amount of time it takes to remove any moisture. Somewhere around the 250F (farenheit) until steaming becomes lazy and not under any sort of pressure at the vent stack of your portable heatup.

Further,, have your contractor install the TC's between the shell of your vessel and the refractory, I personally install them at the lower portion of the refractory as well as in the middle and near the top. As well, on the surface of your refractory. Control is the Key word in dryouts, the more TC's you have, the more control you have with the portable burners and fans.

Just my 2 cents

 

[MadDog88]

Thankyou all for your help.

Cheers

 

[NickelMet]

One more thing to remember about the metal temperature. For 1¼Cr-½Mo material, oxidation resistance (or oxidation rates) may become an issue. From my NACE book, a maximum temperature for long-term exposure to air for this material is 1100°F (593°C).

This may not be an issue in the refractory-lined areas, but may come into play where the metal is exposed (if it is at all).

Just some food for thought...

~NiM