Phyical properties of stainless steel at high temperature 2

[Johnae]

316 or 316L can not work well when temp. is higher than 500 C because low carbon impacts its strength. My question is that in high temperature service, how does carbon affect the phyical properties of stainless steel metallurgically?

 

[Goahead]

May I comment that the question is not clearly expressed?
What do you mean with "can not work well"?
They can work very well if their properties (unspecified) are
suitable for the application at service temperature.
How do you know that it is carbon which "impacts on strength"?

Mechanical properties of metals in general are a function of microstructure, depending on composition and work history.
In this case the structure is austenitic.
It does not seem to be helpful to single out a single element as "affecting physical properties".

http://www.welding-advisers.com/

 

[metengr]

My question is that in high temperature service, how does carbon affect the phyical properties of stainless steel metallurgically?

Carbon content has a direct impact on both ambient and elevated temperature strength. The higher the carbon content, the higher the creep-rupture strength.

 

[EdStainless]

At lower temperatures (up to about 1100F) the carbon contributes to intersticial strengthening. It fills space between the larger metal atoms and make deformation of hte structure more difficult.
At higher temperatures the carbon begins to react with the metal (chromium) and forms carbides in the sructure. These carbides tend to form along grain boundaries and can cause significant reduction in toughness and corrosion resistance.

In a more general sense, various alloy groups are upper temperature limited by two factors, the basic crystal structure of the metal and the metallurgical stability of the specific alloy.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[Johnae]

Thanks for the answers. I got it.

 

[Johnae]

My question came from when I read ASME B16.5 (Pipe flanges and flanged fittings) Table 2-2.2 and Table 2-2.3. Working Pressure decreases a lot as temperature increases for material 316, 316L and 316H. For 316L it shall not be used over 425 C. For 316 and 316H, at temperature over 538 C, use only when the carbon content is 0.04% or higher.

316L has reduced stength at high temperature so isn't generally used for structural applications at elevated temperatures. I believe the reason for that is carbon content and your guy's answers gave me more details.

Thanks

 

[Johnae]

Ed, I have a question for you.You mentioned at another topic about dual certified 316/316L, "...The catch is that neither of these factors helps creap strength or stress rupture. They only increase the low temperature properties."

As I mentioned above, ... For 316L it shall not be used over 425 C. For dual certified 316/316L one of our vendors specifies the maximum temperature is 455 C. is this specification wrong or dual certified 316/316L really has higher creep strength than 316L?

 

[EdStainless]

Good question. In reality no one has the data. We all extrapolate from known test samples. The Code limits are based on the minimum allowed strength and typical creep/stress rupture properties.
I have never seen a 316L sample that didn't meet the 316 strength limits, and exceed them by a lot. What I do know is that if you had samples of 316L (0.015%C), 316 (0.04%C) 316N, and 316H (0.06%C), this is probably the order that the room temp tensile strengths would fall. As far as creep, well the decrease in properties in the creep range is a function fo the carbon level. Even though the 316N starts out strong, it will drop off at the same rate as the 316L. Since it started stronger it may have more useable strength at temp, but this isn't because of better creep resistance.


A personal observation. I have seen a lot more failures in high pressure feedwater heaters made with 304N than with 304 or 304L. I feel that it is due to the higher allowed stress of the 304N (which is in reality 304LN) resulting in a lower effective saftey factor. I can't prove this, but I believe it.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[metengr]

A personal observation. I have seen a lot more failures in high pressure feedwater heaters made with 304N than with 304 or 304L. I feel that it is due to the higher allowed stress of the 304N (which is in reality 304LN) resulting in a lower effective saftey factor. I can't prove this, but I believe it.

No need to prove it, we have experienced it first hand in several of our HP FW heaters. We have switched to T22 tube material.

 

[EdStainless]

Well, what I wasn't going to say is that I have also seen many poorly designed HP FWH over the years. I have seen heaters that were running 150F over the design temp. Things will break.
Being a stainless guy I hate to say it, but if your max steam inlet temp is less than 600F you should use 439, if it is higher then you could use a 304 stainless, but for the upper end I think that the CrMo grades are a better choice.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm