Carbon steel vs Stainless steel at high temperatures - I'm confused...

[USAeng]

We make a nozzle that shoots oil into a furnace that is currently made of 1144 stressproof steel. The furnace that the nozzle assembly sits in is about 1500 degrees F. Carbon steel like this is not intended to be used at 1000+ degrees F (according to what I have read in the past)... normally I would think you would want to use stainless steel in these temps.

The carbon steel pieces seem to hold up though... some issues with threads siezing but other than that things seem to hold up...

I dont understand why the carbon steel holds up in those temps... is the recommendation on carbon steels being used in lower service temps more for structural purposes?

The only load on this part is the forces from the pressures of the fluid that is inside of about 50-60psi

can anyone help explain how this works? Thanks

 

[mcguire]

I would think that the part doesn't reach anything near 1500F, probably because of the oil stream cooling it from the interior.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[USAeng]

oh I should have mentioned that this nozzle is only in use to light the fire... normally no air or fuel is being passed through it.... so it definately sees those temperatures

 

[arunmrao]

Try measuring the temperature of the nozzle and the mystery will be solved. It is not reaching those high temperatures to be affected. The blower air and the relatively cool oil,must be removing the heat. You may independently place an old nozzle in the furnace and expose it to 1500F , and then watch!

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[USAeng]

supposedly the boiler operator measured the temp there via thermacouple at 1500F... from what I understand 1500F is the temp that the nozzle sees... are you saying this is not possible if it is made of CS? and note that the fuel/air is not flowing under normal circumstances

 

[TVP]

USAeng,

We all believe that the temperature must be considerably lower than 1500 F because carbon steels will rapidly oxidize at that temperature. That is practically the austenitizing temperature for 1144.

 

[racookpe1978]

Where is the 1500 degrees?

If it is the "air temp" of the furnace, then each further step outward away from the burner is cooler.

Temperature on a bare steel bar will drop 700 degrees in less than 4-6 inches in still air - how long is your injection tube, and where is it fastened to the furnace wall? Did you actually measure the whole tube with a IR thermometer?

More, you've got to realize that a "limit" of 1000 degrees for a class of steel doesn't mean that the steel is useless if it reached 1050, 1100, or even 1250 degrees. It will be less strong than rated, or may change crystal structure over time as the number of cycles add up, but it doesn't melt like a candy bar at 1001 degrees.

 

[USAeng]

yeah I realize that it wont melt like a candy bar lol... I guess I am wondering if in fact a stainless steel is better to use in this application then? There is a force I wasnt thinking of that is from a torque that is applied to a cap nut that holds the nozzle at the end of the igniter... I figured out roughly that it has an axial force on the thread area of about 2ksi... I wonder if that force is enough to deform the steel from creep over a few weeks/months in those temps if made from the steels mentioned above?

 

[EdStainless]

The other issue is the local oxidation/reduction conditions.
If this is in a place with little or no excess oxygen then heating to 1500F is no big deal.

Actually a lot of parts like this are made from various grades of 2.5%Cr, 5%Cr, 9%Cr, and 13%Cr alloys. These are not really stainless, but Cr is very powerful at helping resist high temp oxidation. So depending on conditions a little Cr may do everything that you need.


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