INCO718 thinwall parts heat treatment - maximum hardness 44.5 hrc

[ckiyak]

Hello

I have some thinwall parts made of inco718. Part diameter is about 6 mm and wall thickness is about 1 mm. The problem is hardness value after heat treatment process. Heat treatment process that i did is conventional inco 718 heat treatment (solution heat treatment between 930-1010 C - min 1 hour - air cooling, 2 step aging 720 C/620 C - 8 hours/8 hours - air cooling). I changed solution temperature, cooling rates, cooling media everything that spec allows me. But i have never found a part has hardness lower than 44.5 HRc. This makes me crazy. Please help me.

Thank you.

 

[metengr]

First, how are you conducting the hardness test to verify heat treatment compliance? Do you have access to a metallurgical test lab? What other examinations have you performed on the heat treated material???

 

[EdStainless]

Even though in theory you are above the min thickness for HRC 45, you are too close for my comfort.
I would go to 30N or 45N.
The next question is are you testing ID or OD? what are you using for curvature correction factor?
If you are testing on the ID using HRC your readings will be high by 3-4 points due to the curvature.

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P.E. Metallurgy, Plymouth Tube

 

[ckiyak]

metengr;

Yes ı have access to metallurgical lab. I checked the microstructere and grain size. It wasnt homogenous and grains were really small.(something between ASTM 8-9)


EdStainless;

Actually i always find the hardness between 45-46 HRc. I didnt understand your questıon because i dont know what is ID or OD. But i have never tested heat treated parts in rockwell machine because of the wall thickness. It is about the same size with indenter. Generally, i test it in Microvickers machine then i convert it to HRc ( 0.3 Hv, 40x ).

(Note : Sorry about my english, it not my native language. My grammer could be hard to understand)

 

[EdStainless]

Ahh, in that case I wonder about your conversion. You need to HT a larger sample (a piece of the raw material) of flat material and test it in multiple hardness scales to verify your conversions. The ones that are published are not for these alloys and are very approximate. I would test HV at multiple loads and in various Rockwell scales, build your own reference. I have had to do this for cold worked stainless steels since many are too thin for the standard tests, and in many cases the published conversions were off by 3-5 points.
You said that the part had a diameter, so I presumed that you were testing on a curved surface.

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P.E. Metallurgy, Plymouth Tube

 

[ckiyak]

I had two experiement, first one is oil quenching after solution and the other one is air cooling after solution. I checked the grain size for all the parts. I fıgured out an interesting thing, bigger the grain size higher the hardness. The minimum hardness value that i measured from the parts has minimum grain size (ASTM 10). I couldnt find any proper explanation for this situation. I am really confused.

 

[EdStainless]

This is a precipitation hardened alloy, the size and distribution of the secondary phase is what governs your measured hardness.
The smaller the indenter and lighter the load the more impact the size and distribution will have.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

Contact Special Metals and discuss your issue/concerns with them. I know several of the metallurgists at Special Metals, they are helpful, know their product line and can better assist you.

 

[ckiyak]

Thanks for your help guys. Both of you were really helpful. I am going to contact with them. I will let you know in this post if i can find a solution.

 

[ckiyak]

I did the process at 930 C for 2 hours. Then i cooled it with water (10 C water). Results were awesome. Hardness values are between 42.5-43.0 .

 

[tbuelna]

For a small size part like this, is there any reason you can't machine it from heat treated bar stock? 718 Inconel at Rc 42-43 is not that difficult to machine, unless the part has some complicated features.

 

[ckiyak]

Actually i dont know because i always do heat treatment process to machined parts. But i think it is because of hardness and complexity of the parts.