Failure analysis of ratchet 3

[coreman73]

I've received two Craftsman 1/2" drive teardrop ratchets in finished/plated condition that experienced premature failure during routine torque test. Both failed at the same site located at the fork.

Results so far:
1. Bulk hardness of 46-48 HRC meets spec (40-48 HRC)
2. Chemical composition meets spec of 6140
3. Fractures originated at a clearly oxidized corner likely due to pre-existing process step. Failure mode is most likely mixed brittle/ductile due to application of unidirectional torque overload.

I've included some photos focused on the fracture face specific to the initiation site. There is heavy scale related only to this initiation area. Higher magnification through this area also appears to show a very thin subsurface layer, which I can only guess is internal oxidation. There is no decarburization found along any part of the fracture surface or outside surfaces. The fracture surface shows no trace of plating.

I can clearly conclude this part failed due a pre-existing crack due to the heavy scale on fracture surface.

I have a couple of questions:
1. Even though it's tough to pinpoint, I was looking for some advice on which process step the crack might have originated from. It's pretty obvious that it was there before plating but I'm a little surprised that I'm not able to see the presence of any plating on the fracture surface.
2. What might the thin sub-fracture surface layer be?

As usual, I really appreciate any and all help that you all can provide me.

 

[Guest102023]

1) Forging. Can't see what else. Possibly worn tool/die or lubrication issues? You first need to rule out material quality issues, which you have partly done.
2) Oxide. Test that out by EDS analysis.

Also, look for surface decarburization, a clue to when it happened.

 

[coreman73]

Thanks Brimstoner.

1) I considered forging at first due to the corner being so heavily oxided and the appearance on the fracture surface. I thought that if it was a forging crack though that I would have been able to see a decarburized layer. My etching didn't show any sign of decarb at all.

2) If it was oxide then what should that tell us?

As an extra point, why would I not have been able to see plating penetration into this crack? I was expecting to see this. Could it be that so much oxide was inside the crack that the plating couldn't get in?

 

[redpicker]

It sure looks like a forging lap to me. The large oxides shown in the red circle in Photo 5 suggest that, but the smaller oxides in the decarb layer that you can see in photos 6-9 are the strongest indication. Oxides in a decarb layer like that pretty much rule out a quench crack.

BTW, I think the magnification you have listed on Photo 9 (500x)is inconsistent with Photo 6 (1000x). They both look to be the same magnification, to me, just one is un-etched and one is etched.

It would be unlikely that you would get any plating on a lap like this because it was fairly small (and probably very tight) at the time of plating and very little plating solution would circulate into the crack.

On the tool; it's a Craftsman, you can take it back to Sears and they'll replace it for free ;^)

rp

 

[coreman73]

Thanks Redpicker. I always look forward to your responses.

That does make a solid case for forging lap. As decarb showing up during etching, maybe I don't understand something. I was under the impression that decarb layers would show up as lightly colored in contrast to surrounding areas. My photo does show a very thin dark layer just under the fracture surface. Are you saying this is really a decarb layer even though it etches dark instead of light?

Thanks for the catch on magnifications! Yes, that should read 1000x instead of 500x.

One more question I think I need clarification on. Are the large, gray particles on the fracture surface technically referred to as oxides or scale and why?

 

[metengr]

Excerpt from ASM Handbook Volume 11 -

Laps and Folds. Laps are surface irregularities that appear as linear defects and are caused by the folding over of hot metal at the surface. These folds are forged into the surface but are not metallurgically bonded (welded) because of the oxide present between the surfaces (Fig. 20). Thus, a discontinuity with a sharp notch is created.

 

[Guest102023]

Coreman73,

Well, the scale can't really be anything other than iron oxide.

I played around with two of the micrographs to highlight the subsurface condition - there is a layer of scattered inclusions less than 0.0002" deep lining the defect. At first I thought the dark edge of the etched micrograph was just staining from the etchant, but it is due to the scattered inclusions. See attached. There does not appear to be significant decarburization though.

 

[coreman73]

A big thanks to all that posted.

Brimstoner,
Ok so the thin, dark etched layer beneath the surface is just related to the oxide (?) inclusions and not anything to do with decarb?

That brings up another point of confusion for me. I have seen and been advised that forge cracks or forge defects (such as the forge lap we have here) should be outlined by a decarburized layer. Is that true and if so then why am I not able to reveal one for this ratchet?

 

[Guest102023]

Decarburization will occur when a surface is exposed to temperatures above about 1600F, but practically it indicates something more like 1700-1800F.
I did not see decarb here, but that may just mean the lap was closed tight and not exposed to the atmosphere. Or more likely, that such a high temperature is not reached in your mfg. process (which I don't know in detail - that would be helpful).