Heat Treatment Process for 8620 Parts

[JenB8]

I work at a naval manufacturing company, and we are investigating the option of bringing heat-treating in-house for one specific part we produce... The part is made from 8620 material and welded into its configuration before heat treating... Below are some of the specs set by our customer and heat treating notes:

- minimum tempering temperature is 975F
- hardness must be 30-33 HRC
- no more than 0.010" total decarburization and approximately 0.03" partial decarburization
- enhanced carbon (carburization) is not permitted

Here's my questions:
1. Does the hardening and tempering processes need to be conducted in an inert atmosphere?
2. How would you determine a cooling rate that is necessary to achieve the above specs?
3. Is there a way to integrate a quenching system into the hardening furnace or would it be better separate?
4. At what temperature does the material need to reach before quenching?

I am just a little clueless when it comes to heat-treating, so any and all suggestions would be appreciated. Thanks!

 

[EdStainless]

1. Not inert but carbon neutral. An atmosphere with the same carbon activity as the alloy.
If you are just running this one furnace the easiest would probably nitrogen + methanol.
You would use pure nitrogen from a liquid bulk tank and methanol from a drum.
Sensors measure the carbon potential in the atmosphere and adjust the methanol injection to keep at the required level.
Your bulk gas supplier (Air Products, Lindie, BOC, and so on) can supply this.
2. You can look up hardening data for the alloy. The attached page is from the Timken Practical Data for Metallurgists.
How you quench partially depends on the size of the part.
In general for a low C alloy like this you would water quench.
Unless this part is small, <3/8" you won't reach RC30. At least it won't be that hard any deeper than about 3/16".
3. Yes, you can buy IQ (integral quench) furnaces. Parts (usually in baskets of racks) mover from austenitizing directly to quench.
Temper is often done in a separate furnace, though for high production it can be added right after quench.
4. The Timken data is based on 1700F austenitize prior to quench.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1642548429/tips/8620_yhdbka.pdf[/url]

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P.E. Metallurgy, consulting work welcomed

 

[bobjustbob]

The total and partial decarb in your question doesn't look right. Probably 0.001" total and 0.003" partial decarb.

30-33HRc is at the upper end of what you will get with 8620. Are there any mechanical property requirements other than surface hardness?

If a customer came to me with a part they required be made from 8620 with a 30-33HRc surface hardness at a 1000°F temper, I don't think I would accept it. I have made 8625 with a 269-302 HB (about 27-32 HRc) spec before and it was pushing it.

You will need an agitated water quench from about 1550°F. I assume you have a weld procedure that has been tested to ensure that your filler is going to achieve the necessary hardness.

Bob

 

[blacksmith37]

Other than agreeing with @ bobjustbob, that I don't think you can do it; I would be concerned about cracks in the welds. This will depend on thickness, preheat , and ? Also I would stress relieve the welds at high temperature so that HAZ are not hard when going into the austenitize furnace.

 

[EdStainless]

While you might not normally pre-heat welds in this alloy for material that sees Q&T you should.
The other part of this how fast you heat up for HT, be gentle.
The required hardness is very borderline.
We used to have a part that originally 8620 Q&T, in reality it was made from 8622H (controlled hardenability).
And we quenched from 1650-1675F.


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P.E. Metallurgy, consulting work welcomed