Strange cracks or laminations along beam of wrenches 1

[coreman73]

I have received a couple of wrenches that are showing the as-titled surface defects. These wrenches are in finished condition and were made from steel grade 4047. Steel and hardness results meet spec.

After microscopic examination, I found some defects that are outlined by a thin decarb layer. I've attached some photos showing what I've found so far.

I am trying to determine if these prominent defects originated during formation of the steel billet/raw material or from hot forging (forging lap). If I had to guess then I'd say they're rolled-in scale defects from the billet based on similar looking defects I've seen before in some billets from this particular supplier. Also, I wouldn't think hot forging could produce defects of such depth. Regardless, please let me know what you guys think. I'd appreciate it.

 

[metengr]

coreman73;
These defects appear to be related to typical laps or folds from forging based on the micrographs you supplied.

The problem with rolled-in scale is are you assuming the billets were rolled by working prior to forging? Keep in mind that rounds can be supplied from continuous casting ready for forging, in lieu of traditional ingot to billet shaping.

 

[coreman73]

Thanks metengr,

I will try and find out what condition the billets were received in from the material supplier and if they were rolled by working prior to forging.

Just for the sake of comparison, I've attached some unrelated photos of a billet I was asked to examine recently and found what appeared to be rolled-in scale defects. These current wrench defects look suspiciously similar to them.

 

[coreman73]

I was just informed that the billet is received as hot rolled bar stock. Bars are then cut down to billet lengths using cold shearing. No other work is done to the billets prior to hot forging.

From this info, would it be much more possible that these defects are most likely rolled-in scale?

 

[metengr]

coreman;
Yes, rolling can produce very similar surface defects as forging. In my mind, the important take away from all of this is preventing these types of surface defects from continuing on in your process chain and reaching final product form. You need to create some type of back stop(s) with your supply vendors.
1. Evaluate what manufacturing processes (es) were used from supply to end product.
2. What is required (QC hold points, sampling, inspections, or NDT) to ensure defect free product forms throughout the manufacturing process to avoid having these defects become a problem in the finished product. Obviously, there was a breakdown somewhere in this manufacturing chain and you need to understand where and how to prevent this from occurring.

 

[coreman73]

Thanks again metengr,
Yes, I do agree the most important thing is to take preventative measures. Of course, it's always nice to be able to pinpoint the origin of such defects.

I've got a raw material auditing program established to try and catch these defects before they have a chance to reach production. Unfortunately, the plant from which this material comes from appears too busy to keep up with sending me regular samples from their incoming coils. I hope this evidence will help change that.

 

[stanweld]

Hot rolled bar, reforging billets need to have scabs and laps/seams of specified depth removed prior to placing in the oven for forging. Recommend changing reforging billet supplier if the frequency of these defects is too high.

 

[Guest102023]

Any link in the supply chain to the pliers cracking problem you posted recently?

 

[coreman73]

Stanweld,
I will make that suggestion to the plant that they need to consider removing all scale remnants as well as any laps and seams. I think though that if this problem continues then the material supplier will have to be changed for sure.

brimstoner,
The plant informed me that they have stopped the cracking problem for the time being at least. They somehow established that their forging dies along with multiple blows were causing a weak/sheer plane in the material which then in line with the report I sent them (including valuable info I got from you guys) was being separated at cold trim. They said they redesigned the dies for better material flow and reduced the blows to a minimum.

 

[BMKR]

Do these get quenched and tempered in the as forged state? laps can be machined off(if you have enough material) before H/T, if not during Q&T they open up more and become cracks.

 

[coreman73]

JConnBMKR,
These go through several machining steps including soft vibe after forging. There is even an additional hard vibe step administered after heat treating to remove even more surface material.

Unfortunately though, I don't believe these processes really remove enough material to get rid of such laps/rolled-in scale defects. The last time I audited our vibe processes there was no more than maybe 0.006" total material being removed.