How much hardness change can I expect from a low-level stress relief on a 17-4 SS part?

[rollingcloud]

It's a casted part that is required to undergo the following heat treatment:
1. Homogenization Heat Treatment
Temperature: Heat to 2100°F ± 25°F (1149°C ± 14°C).
Hold at heat for not less than 90 minutes.
Cooling: Cool as required to room temperature.
2. Solution Heat Treatment
Temperature: Heat to 1900°F ± 25°F (1038°C ± 14°C).
Hold at heat for 1 hour per inch (25 mm) of section thickness
Cooling: Cool as required to below 70°F (21°C).
3. Precipitation Heat Treatment
Temperature: Heat to a temperature within the range of 985°F to 1015°F (529°C to 546°C).
Hold at the selected temperature for not less than 90 minutes.
Cooling: Cool in air or inert gas to room temperature.

The required hardness is 34-42 HRC. The actual hardness was 31-36 HRC from various locations, a bit of uneven hardness. A few samples were reworked due to dimensional issues. The supplier used hot straightening. The straightening process was as follows:
Heated to 300°C (572°F) for 1 hour, straightened, and cooled to room temperature. After straightening, the hardness increased to 37-38 HRC, which is now within the required range. My guess is that due to the low temperature, it effectively acted as a low-level stress relief. I want to know if this level of stress relief alone can increase the hardness by 2-6 HRC, or if the hardness increase was mainly due to the strain hardening of the straightening process. Is there anything I should watch out for? The AMS spec controls the re-heat process but does not mention anything about additional stress relief as performed by the supplier.

Edit: The supplier claimed they performed a reheat treatment (solution and aging) after the straightening.

 

[mfgenggear]

None
I believe AMS 2759/3 specifies a stress relief.
Reason : when proper stress relief it does not change the hardness.
But precipitation heat treat us a little different. But follow the specification to do it correctly.

 

[rollingcloud]

Thanks. It looks like 2759/11 is for stress relief specifically. I just realized that the supplier claimed they performed a reheat treatment (solution and aging) after the straightening. Is reheat treatment actually necessary?

 

[mfgenggear]

Op
Was this a finished part.
Not really. But the supplied is allowed to rework parts by reheat treating. On Simi finished parts.
Close tolerance attributes are finished after heat treat. Straightening has to follow the spec.

 

[rollingcloud]

Yes, it's a finished part.
Per the AMS material spec, if they proceed with reheat treatment, they are required to do additional testing on one specimen. Since it’s a rework, it will destroy one part. I think my concern is the effects of straightening; maybe it created a lot of internal stresses, which is why the supplier wanted to reheat treat? The physical straightening step has no controlling procedures/spec.

 

[MFAMET]

17-4 PH SS is a tempered martensite. Normally, the tempered martensite hardness will drop with tempering temperature over 650F. !7-4 PH SS hardness increasing is due to the formation of carbide precipitates However, there is "tempered martensite embrittlement" occurring at 480-570F, particularly coupled with high phosphorus content. This will cause an increasing hardness with a lower toughness. High carbon hard drawn steel wires also show similar characters while heated for residual stress relief. I doubt "straightening" alone could make significant harness increase.

 

[rollingcloud]

MFAMET,
Thank you for the great info. I made an edit to the original post, supplier claimed they performed a reheat treatment (at least aging was done but could be both solution and aging) after the straightening. To your point, a reheat treatment in this case would be ideal to regain the toughness and ductility?

 

[mfgenggear]

Op
it was not cool they re heat treated a finished part. A dimensional verification on close tolerance is required.

I am on the road right now . I will pull and read the spec as soon as I get a chance.

Ps why are they straightening a finished part.
For next add an inspect before and after heat treat. Straightening should be done right after heat treat. But this is a low distortion process. Makes no sense.
I would have to pull all records and review for irregularities.

 

[rollingcloud]

mfgenggear,
oops, my bad, I was thinking in terms of the casting drawing, it's not 100% finished part, its only finished per the casting drawing, it will still need to get machined, however, the final machining drawing only includes drilling, so the thickness and the overall shape of the part are finished.
Yes, I agree, a reheat would require us to re-inspect the dimensions.
Initially, they attempted straightening at room temperature, but it did not work out due to the high hardness.

 

[mfgenggear]

OP
What was straighten?
Casting generally have liberal stock for final machining. With castings its critical to indicate prior to machining 5o ensure it will clean up. Generally
If it is out to much it is scrap.
It is critical the manufacture correct the issue by adding stock or correcting the the master. I use to ask for first articles from the supplier.
Before running production. Then using sample inspection during receiving
This is a little out of topic, but is important. The casting supplier are notorious for shipping bad parts.
And controls have to be in place.

 

[rollingcloud]

A couple areas were too thick and one flat surface was slightly warped. I suppose they should add a machining operation to reduce the thickness issue since the straightening would only correct the warped surface, right?
We typically ask for first article when it's the supplier's first lot.

 

[mfgenggear]

Op yes that's how to correct it. But there has to be enough stock to clean up. And it's crucial to indicate in.

 

[mfgenggear]

Op
If it needs to be flat to it self. Indicate it in.
Then machine to clean up. For flange face it's a must for sealing.

 

[EdStainless]

So they re-heat treated, do they have certification AMS5-2750?
If you do the identical HT twice the properties will drop slightly.
Usually by 1-2 points.
In your first HT either the solution temp was not uniform, or the cooling was very non-uniform, or the aging temp was not within +/-15F.
Your variation in hardness needs to investigated, it is excessive.
I could harken back to variations in the composition of the castings.

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P.E. Metallurgy, consulting work welcomed