Flash Gordon
Aluminum
- Joined
- Jan 29, 2004
- Location
- Chicago, IL
I’m machining some complex parts from a forging that gets heat treated after machining. I’m having problems with the parts shrinking after heat treatment. The finished part is roughly 1” x 2” cross section by 14” long. They are shrinking about .017 over the 14” length. I’m sure they are shrinking all over, but it is most apparent over the 14” length. Most dimensions are +/- .020 and are not a problem. However, there are several features that are +/- .002. The parts check good before heat treat, but are under by .017 after heat treat.
I’ve had the forgings annealed and normalized before machining with the hope that it would eliminate any shrinkage during the heat treat process.
Material spec:
Steel forging, ingot or continuous cast, graded D, Spec MIL-S-46172, make from steel, composition 8620, killed, ASTM A304.
Heat treat spec:
Heat treatment per SAE-AMS-2759/7:
A. Austenite in carburizing atmosphere. Case depth .014 to .018 oil or salt quench.
B. Temper 1 hour min to harness RH A80 to 85.
Here are some ideas that were already considered:
1. Leave extra stock on the end of the part and grind to finished length after H.T.
Won’t work since I would be grinding through the case depth and the required
.014 - .018 case depth will not be there anymore. (see H.T. spec)
2. Have the heat treater drive the case depth deeper (i.e. .040 deep) and then grind or machine to finished size after H.T.
Won’t work since I would then need to machine the extra .025 from every
feature of the part. (50 tools and 1 hour cycle time with many slots, holes)
3. Adjust the machining process to account for the shrinkage, leaving some of
the features about .017 longer than the print dimension to account for the shrinkage.
This might work, but it’s not my first choice. I would need to rely on
a consistent amount of shrinkage in every lot of parts from the
heat treater. It would also be a big project to edit the program
to adjust all the dimensions.
Should the parts be shrinking that much during heat treat? Ideally, I'd like do something to the raw forgings that would eliminate any shrinking during H.T. Is there some process like normalizing that would eliminate the shrinkage?
All ideas are appreciated!
I’ve had the forgings annealed and normalized before machining with the hope that it would eliminate any shrinkage during the heat treat process.
Material spec:
Steel forging, ingot or continuous cast, graded D, Spec MIL-S-46172, make from steel, composition 8620, killed, ASTM A304.
Heat treat spec:
Heat treatment per SAE-AMS-2759/7:
A. Austenite in carburizing atmosphere. Case depth .014 to .018 oil or salt quench.
B. Temper 1 hour min to harness RH A80 to 85.
Here are some ideas that were already considered:
1. Leave extra stock on the end of the part and grind to finished length after H.T.
Won’t work since I would be grinding through the case depth and the required
.014 - .018 case depth will not be there anymore. (see H.T. spec)
2. Have the heat treater drive the case depth deeper (i.e. .040 deep) and then grind or machine to finished size after H.T.
Won’t work since I would then need to machine the extra .025 from every
feature of the part. (50 tools and 1 hour cycle time with many slots, holes)
3. Adjust the machining process to account for the shrinkage, leaving some of
the features about .017 longer than the print dimension to account for the shrinkage.
This might work, but it’s not my first choice. I would need to rely on
a consistent amount of shrinkage in every lot of parts from the
heat treater. It would also be a big project to edit the program
to adjust all the dimensions.
Should the parts be shrinking that much during heat treat? Ideally, I'd like do something to the raw forgings that would eliminate any shrinking during H.T. Is there some process like normalizing that would eliminate the shrinkage?
All ideas are appreciated!