Reason for difference in machinability between two identical sockets? 4

[coreman73]

I have received two sockets for evaluation. One is related to supplier "A" and one to supplier "B". Allegedly, the socket from "A" represents parts that are showing excessive tooling issues during forming/heading. In addition, machining of this socket introduced surface burrs. All issues are found ONLY at the drive end of "A" sockets. The "go" gage would not fit the drive end as well.

In comparison, the socket related to supplier "B" does not have any complaints and has been manufactured without issue (nothing wrong with formation of drive end).

All sockets were formed from the same steel grade, which was provided by the same material vendor. I have tested the chemistry and they are virutally identical. Surface hardness is also comparable with drive ends at ~85 HRB. I'll go ahead and just mention that serration ends show ~97 HRB. Values are very consistent.

The only further manufacturing information I received was that supplier "A" and "B" used an identical process flow as follows: initial heading operation, annealing operation, coating operation and final heading operation. The annealing step was intended to make slugs more workable prior to secondary heading.

I have included three photomicrographs of each socket from "A" (Figure 1) and "B" (Figure 2). Based on the photos, is there something with the structure of "A" that might have led to the increased difficulty of machining versus "B"?

The only difference is that "B" has slightly larger grain size and noticeably larger carbide particles, which are found mostly along the grain boundaries.

Sorry for the long post but I'm stumped. There are no other differences between the sockets. I'd greatly appreciate any input.

 

[redpicker]

I would look into the annealing operation used after the initial heading operation. Perhaps one vendor is performing a sub-critical anneal while the other is utilizing a full anneal.

rp

 

[CoryPad]

Nothing really obvious from the microstructures. Can you do a compression test of the starting material. Perhaps there is a difference in the strain hardening behavior, though I don't see why there would be if the composition and microstructures are similar.

What are the tooling issues? Wear? Perhaps you have differences in the surface roughness and/or coatings and lubrication conditions.

 

[TVP]

I agree with redpicker, the annealing operation is likely the source of the difference. Before getting into that detail, I wanted to ask a question about the chemical composition analysis: Did you test both samples for N in addition to the usual suspects C, Mn, Si, etc.? Nitrogen has a significant negative impact on formability/workability, and is sometimes overlooked in terms of importance. It doesn't appear that there is much sulfur in either of these samples, based on the absence of MnS inclusions.

With regards to the microstructures of your two samples, there is definitely a difference, as you already noted. Larger ferrite grain size and larger, more spheroidized cementite particles will reduce the flow stress and increase formability/workability. Sample A is not as homogeneous as B, with less globularization of the cementite and some remnant directionality (lower left to upper right in the 100x image). I would expect Sample A to have a higher flow stress, which is likely related to the tooling issues (higher forming forces, increased tool breakage and/or wear). The less homogeneous microstructure will be more likely to have areas of flow localization, which can impact subsequent machining (burr formation). See if you can get details about the annealing process (sub-critical vs. intercritical, time vs. temperature history, etc.).

 

[EdStainless]

Is "A" having machining issues because it is softer?
The actual time and temp of anneal sound like the place to start. Check on how they are measuring temp also.

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