We found thin black areas of polygon shape in some fatigue fracture surfaces of LPBF 18Ni300 maraging steel over-aged at 585C (6 hours). The EDS analysis indicated the higher carbon fraction, but this is the carbon...
The photo and EDS results are enclosed.
Any comments will be appreciated.
18Ni300 maraging steel is typically alloyed with titanium, and with post-process heat treatment, titanium can react with carbon to form titanium carbides (TiC). These carbides can precipitate and become visible under microscopy. The titanium carbides could potentially cause black areas in your material. These carbides could potentially increase the hardness of your material but could also act as stress concentration sites, leading to premature fatigue failure.
To validate this, you could perform more extensive characterization techniques such as transmission electron microscopy (TEM) or X-ray diffraction (XRD), which may provide more clear evidence of the presence of titanium carbides. Another step could be to perform a more detailed EDS or similar analysis to get a clear chemical composition of the black areas.
Please consult with your colleagues or advisors on this, as the actual situation could be influenced by various factors beyond the general information I'm providing here.
With carbon content 0.03% maximum it is not realistic to expect to see any sizable carbides. SEM image of a region is to poor to be of any use, but since you said you see black area, I would suspect that it is some organic contamination (more carbon, darker than surroundings).
thank you for your kind answers. We performed a large experimental program that included cyclic fatigue loading for specimens aged-hardened at different temperatures: 585C, 530C, 490C, 470C, 450C, and no-aged. The only lower fatigue strength than no-aged specimens was exhibited by the specimen aged at 585C but only for the life regime above 8e4 cycles. And for one of the specimens from this regime, we found that black areas. Furthermore, the fatigue crack origin site looked different than all others. See the attached photo taken with an optical microscope with a scanning option. The crack area looks like glass.
You do not have high-mag pictures,but I believe shiny part of your opticla picture is an initial crack - whether preexisting or slowly developed during initial stage of fatigue failure. It became shiny because two sides of crack were rubbing against each other. Also I strongly believe dark areas of your first SE picture are just post fracture contaminations.
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