×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

offgassing Ni or Cr from heating stainless steel

offgassing Ni or Cr from heating stainless steel

offgassing Ni or Cr from heating stainless steel

(OP)
Hi all, New to forum.
       Not an exhaustive search but enough to frustrate me.  Cant seem to find any readily available solid information out there regarding this. So I'm hoping there is a metallurgist on this forum that can shed a little light on the subject.  I am constructing a heat exchanger(using air flow to transfer heat, not liquid) where the heat source temperature is somewhat uncontrollable, like in a naturally aspirated wood burning stove, and the system will use natural convection creating a draft as the primary mover of exchanged air and not a fan forced air that would naturally provide additional heat exchanger material cooling.  
      The last unit I built was just carbon steel. but since the heat exchanger is buried in the coal bed I have long term service life concerns where heat scale or corrosion from caustic ash may eat away enough material to cause air leaks similar to what is seen in an automotive tailpipe. in turn contaminating air with CO, etc. that will be circulated into an inhabited room rendering the unit unsuitable for further use.  So I would like to explore stainless as a more corrosion resistant material option for a longer service life.
        My concern is any off-gassing of toxic heavy metals like Ni or Cr while in service, as I am cautioned against while welding these materials, and thermal fatigue from potentially daily heat/cool cycles.  
        So I guess the big question would be are these valid concerns? At what temperature or material state does stainless start off-gassing any toxic/heavy metal fumes or vapors? A little off point, but can a wood fire, naturally or fan aspirated create enough heat to cause this concern? I have read 1600*F is common in the coal bed of a hot wood stove. Would the specific alloy make a difference in this application?  I know 321 is typically used in high heat applications where pressure strength needs to be retained like jet engines, but is also cost prohibitive for this.  Will thermal expansion cracking or heat scaling be an issue with an alloy that is readily available like 303 or 304? What would be the most ideal material from a suitability/cost effective relationship for an application like this?  
       Thanks in advance for any solid light anyone can shed on this issue or point me to other resources/forums discussing this issue, and feel free to make it a discussion thread as I'm sure I'm not the only one pondering this question for one reason or another.  P.S.: I'm ok with logical speculations as they sometimes lead to useful debate or point one in the right direction even if it is inaccurate upfront. However, if you don't actually know and are making a logical speculation that can't be backed up by an industry reference please state it as such so no one takes it as gospel.     
     Thanks again and happy new year everyone.

RE: offgassing Ni or Cr from heating stainless steel

Welding / Cladding: In welding, cladding, the intense heat of the arc or flame vaporizes the base metal and/or the electrode coating. This vaporized metal condenses into tiny particles called fumes. These fume particles can be inhaled. Chromium fume is created by welding or cutting on stainless steel or metals that are coated with a chromium material.

http://www.weldreality.com/Safety%20stainless%20issues.htm

In the electric arc welding, the temperature is very high (of the order of 6,000-8,000 °C for the GTAW, GMAW, FCAW and SAW processes, and up to 10,000 20,000 °C for the PAW process). The adiabatic combustion temperatures for coals are around 2,200°C.

Melting point of steel is about - 1450-1520°C, essentially half of the boiling point, ca. 2900°.

http://www.engineeringtoolbox.com/melting-temperature-metals-d_860.html

Don't think that you should be too concerned about chromium fumes since you will be operating at a lower temperature than the boiling point. This is a problem with welding because of the much higher temperature.

As far as long term durability in this service, you may consider lining the unit with fire bricks.

Consider posting in the materials forums.
  

RE: offgassing Ni or Cr from heating stainless steel

It seems that the nuclear industry must of studied this (using superheated steam) for their heat exchangers. As I remember they send 900 degree steam at 900 psi through PWR heat exchangers.
They have tons of other problems, but I don't think that off-gassing of the stainless components is one of them.

RE: offgassing Ni or Cr from heating stainless steel

(OP)
Thanks for your input guys, re-posted in materials and got some good solid answers.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources