Carbon Steel Interesting Fracture surface

[Hercules28]

Dear all,

I have a very interesting and puzzling fracture surface.
Material is a 1008 Carbon steel. That was bent as shown in one of the pics, then drilled a hole, and then carburized it. Then plated it.

I see cracks going from side to side on the top and the bottom of the core.

So I break the part by hammering it and I see what you see in the pics.

1) You can see the carburized layer that show a brittle fracture all around the piece and along the edges of the hole.

2) Then after that I see another layer that seems to me like a ductile fracture.

3) Then at the center of the piece it seems like a brittle fracture????? There I am confused? How is it possible to have a brittle fracture deeper than a ductile one??

Anyone can shed some light?
Thx

 

[Hercules28]

More pics

 

[redpicker]

The case is very hard and a very low toughness. Small imperfections will initiate a brittle fracture.

These brittle fractures propagate into the core, which is softer ans has a higher toughness, so they proagate in a ductile manner. Whle the toughness of the core is higher than the case, it is low enough that brittle fracture will propagate if the crack is long enough, so once the crack has propagated through the core far enough, it is long enough to propagate in a brittle manner.

Another possibility is that the case cracked when the part was at a higher temperature. At the higher temperature, the core was ductile enough to blunt the advancing crack and stop it from propagating. Later, the temperature dropped low enough so the core was no longer tough enough to blunt the advancing crack and a brittle fracture resulted.

rp

 

[Hercules28]

Redpicker,

I think that explains a lot. How would I see if the crack happened during bending or after the case hardening?

Also is the plating potent of producing cracks?

Thx,

Herc

 

[Guest102023]

You can expect 1008 core material to be fairly ductile but not have terrific low-temperature fracture toughness.

The pictures show a quite brittle fracture, although I think you are correct, there appears to be a more ductile region between the centre and the case. If this was broken at room temperature, the fracture surface tells me the material was embrittled in some manner, perhaps during the case hardening process. Is it a free-machining grade by any chance?

 

[stanweld]

While the photo is non-definitive, I suspect that the center fracture feature is ductile overload.

 

[Hercules28]

A ductile overload would appear as a brittle fracture? ( I did cause that fracture by hammering it!)

 

[swall]

It may just be that the core has excessive grain size if the 1008 was not an aluminum killed steel. Consquently, the large grains just make the fracture in the core look brittle.

 

[Hercules28]

Would you think that the fracture would start at bending? Is there any chance it would happen during quenching?