Hi all,
Hoping that someone can provide some insight here as I am having an issue that I cannot seem to get solved. I have been making some parts from 4340 steel bars that are sawn from plate. I then cut those bars down, mill, heat treat, and they go to mag particle inspection. I have made three lots of these parts over the past year with the following results:
Lot #1
Material Heat: #1
Machining Process: #1
Heat Treat Lot: #1
MPI Results: 85 pass/0 fail
Lot #2
Material Heat: #1
Machining Process #1
Heat Treat Lot: #2
MPI Results: 7 pass/16 fail
Lot #3
Material Heat: #1
Machining Process: #1
Heat Treat Lot: #3
MPI Results: 1 pass/25 fail
To summarize, my material supplier is reporting all material as coming from the same heat (note that I did NOT order all of the material at one time. The material came in three different shipments, but they are reporting it all from the same heat). The machining process has not changed since the first lot. The material is being heat treated to 50-53 Rockwell C.
I am not at the shop to take a picture of a failed part, but here is a mock up of the failure cracks that my MPI vendor is reporting:
The top and bottom bold black lines are the where the faces of the plate would be pre-machining. The red lines are the cracks. As you can see, they are nearly perfectly horizontal cracks that run constantly parallel to the surface of the plate, with very few exceptions. Nearly all of the cracks occur at the exact same height in the plate as well. That height is the same on every part. This continues around the entire 2D profile of the part. The parts are about 1/2'' thick.
My material vendor came in and took one of my parts and sent it back to the mill for testing. I just received their test report and they are claiming no issues with the material. They are faulting heat treat or machining (surprise surprise). Despite their results, I am skeptical of that for two main reasons:
1. There is NO WAY I am putting enough stress into this part with a 1/2'' endmill, 1/8'' endmill, and 1/8'' drill to crack it.
2. The location, shape, and consistency of the cracks screams to me that there is a constant layer in the material where the melt is not bonded properly, which is then worsening/splitting during heat treat.
(Why this is happening in lots 2 and 3 but not on 1, I don't know. Separate plates from the same heat or a bad section of the same plate?)
If anyone taking the time to read this has some wisdom to offer, it would be much appreciated. I am not ruling out heat treat, but given the physical evidence that I am seeing I find it unlikely.
Hoping that someone can provide some insight here as I am having an issue that I cannot seem to get solved. I have been making some parts from 4340 steel bars that are sawn from plate. I then cut those bars down, mill, heat treat, and they go to mag particle inspection. I have made three lots of these parts over the past year with the following results:
Lot #1
Material Heat: #1
Machining Process: #1
Heat Treat Lot: #1
MPI Results: 85 pass/0 fail
Lot #2
Material Heat: #1
Machining Process #1
Heat Treat Lot: #2
MPI Results: 7 pass/16 fail
Lot #3
Material Heat: #1
Machining Process: #1
Heat Treat Lot: #3
MPI Results: 1 pass/25 fail
To summarize, my material supplier is reporting all material as coming from the same heat (note that I did NOT order all of the material at one time. The material came in three different shipments, but they are reporting it all from the same heat). The machining process has not changed since the first lot. The material is being heat treated to 50-53 Rockwell C.
I am not at the shop to take a picture of a failed part, but here is a mock up of the failure cracks that my MPI vendor is reporting:
The top and bottom bold black lines are the where the faces of the plate would be pre-machining. The red lines are the cracks. As you can see, they are nearly perfectly horizontal cracks that run constantly parallel to the surface of the plate, with very few exceptions. Nearly all of the cracks occur at the exact same height in the plate as well. That height is the same on every part. This continues around the entire 2D profile of the part. The parts are about 1/2'' thick.
My material vendor came in and took one of my parts and sent it back to the mill for testing. I just received their test report and they are claiming no issues with the material. They are faulting heat treat or machining (surprise surprise). Despite their results, I am skeptical of that for two main reasons:
1. There is NO WAY I am putting enough stress into this part with a 1/2'' endmill, 1/8'' endmill, and 1/8'' drill to crack it.
2. The location, shape, and consistency of the cracks screams to me that there is a constant layer in the material where the melt is not bonded properly, which is then worsening/splitting during heat treat.
(Why this is happening in lots 2 and 3 but not on 1, I don't know. Separate plates from the same heat or a bad section of the same plate?)
If anyone taking the time to read this has some wisdom to offer, it would be much appreciated. I am not ruling out heat treat, but given the physical evidence that I am seeing I find it unlikely.