Carbon Steel Microstructure

[a7992]

I recently posted a thread on pinhole leaks in carbon steel and just sectioned and mounted a sample from one of the tubes in the HVAC unit and found something interesting but confusing...

The microstructure on the OD of the tube (closest to the aluminum fins) looks different than the microstructure on the ID. The ID and into the middle of the tube looks to be austenite and ferrite but the other half of the tube (OD) looks to be just austenite. Does anyone know what could be happening here? I thought it could possibly be spheroidizing on the outside but that doesn't make sense since the outside isn't exposed to the high temps. Has anyone ever seen this in tubing or anywhere before? Help! Thanks!

 

[Flesh]

A comment first; if you are using carbon steel your microstructure should be pearlite and ferrite (for <~1wt%C). I have seen something similar in hydraulic tubing, carbon steel. Generally, the carbon content is quite low to begin with (<0.12wt%C), and the pearlite content is therefore quite low. Moving from the ID to tube OD, the pearlite would essentially vanish. This was an indication of loss of carbon content, or "decarburization", produced at some point during the mfg process (CD, annealed, and formed). Likewise, you may have had your microstructural discrepancy before the part went into service.

 

[strider6]

if your metal has a ferrite-austenite structure seems like a duplex...
are you sure of the type of material? what is the ASTM spec for the material?

Corrosion Prevention & Corrosion Control

 

[0707]

About the thread subject please go to

http://www.msm.cam.ac.uk/phase-trans/2008/Steel_Microstructure/SM.html

http://www.msm.cam.ac.uk/phase-trans/abstracts/M0.html

http://www.msm.cam.ac.uk/phase-trans/abstracts/annealing.twin.html

Hope it helps

luis marques