Failure analysis of cracked pliers 3

[coreman73]

About a week ago, I got word that some of our plier products were showing up cracked after heat treating. At that time, I was told there was reason to believe the cracking might have been related to a raw material defect or possibly introduced during forging. I was sent some raw material samples for evaluation. I found multiple sites around the perimeter of the 1070 bar stock showing rolled in scale defects. These were as deep as 0.017" and were fully decarburized. If needed, I can provide photos.

Rather than assume these defects were the root cause, I had some actual cracked pliers sent to me in heat treated condition. The attached file "cracked pliers" illustrates the results.

Cracking is only associated with parting lines. For some reason, these samples were treated by an acid bath after heat treating in order to reveal the cracks. I have no idea why this was done. I've never heard of this. Unfortunately as a result, as shown by some of the photos in the second attachment, there is both general surface pitting as well as pitting along the crack paths.

Hopefully the attachment provides enough information. For me, it seems that the cracks originated during forging, since they're found exclusively along the parting lines and the cracks show extensive scale. I don't see how the raw material defects could be responsible since cracking is not seen anywhere else except for these parting lines. What's a little confusing is that there are some deep internal voids adjacent to the crack paths that are filled with scale. If it's raw material related then I didn't find any such defects on the raw material I originally examined.

Any opinions on the root cause of cracking would be appreciated.

 

[coreman73]

Just out of curiosity, I have a few questions about the decarb layer. What are some possible places it could have come from? Could it have only been formed during austenitizing? Did an earlier heat treatment step (i.e. forging, annealing) introduce it? Could it have come from the raw material?

 

[metengr]

Could it have only been formed during austenitizing? Did an earlier heat treatment step (i.e. forging, annealing) introduce it? Could it have come from the raw material?

Yes to all three statements above.

 

[coreman73]

Thanks metengr. I guess there's no way to really determine where exactly it originated from since this part has been heat treated.

 

[metengr]

coreman73'
There is a way to possibly track down the source of decarburization. It requires detective work where you conduct a detailed review of all elevated temperature thermal process treatments from acceptance of raw material to finish product.
The cost of doing this may not be worth it because you need to layout a diagram and methodically begin eliminating sources of decarburization based on known process conditions. If process controls are poor, material when fully austenitized at elevated temperature and exposed to atmosphere can result in decarburization.