Seeking minimal deformation in a 8620 alloy with a grove...

[jsalomon]

We've been working on a project for a few weeks now and so far it looks like quite a puzzle to get the right heat treatment going. The scenario is the following:

We have shafts madre of alloy 8620, they are about 5" long with 3/4". The special feature they have is a grove which is about 1.5" long and is about .19 with and .09 " deep. The native HT for this piece is carburizing. The problem here is that the "deformation" tolerance has an upper limit of .001" in the grove, which is making our life hard. the casing has to be around .03" to .05" with a 89 to 92 HR15N, we make it with a 7hr cycle in a batch furnace.

At the most recent test we decided to run the process with a carbonitriding cycle (so we could lower the max temperature and try to reduce the deformation) but we got roughly half of the pieces within spec.

Does anyone has a suggestion or solution for this issue???

thank you for your help

 

[redpicker]

A few things you can try. Trying to hold a 0.001" tolerance with a quenching operation is a tall order.

Normalizing the material prior to machining. This won't eliminate the distortion, but it may make it more consistent.

Are you using any stop-off on the parts? If so, see if you can make the stop-off regions symmetric.

What temperature are you quenching from? Try to quench from as low a temperature as you can.

0.03" case is pushing it for carbonitride, but it is achievable.

 

[jsalomon]

Thank you for your response. Here are my answers...

- Normalizing sounds like a good chance to make it as you say, consistent.
- We dont have stop-offs, provider didnt give a go-nogo gage so we are measuring atm. Since its a potential project, we want to make a HT that can achieve what we need. Later on we will analyze the option of fabricating fixtures, etc.
- The hardening process atm is around 845°C
- We didnt have any issues with the case, neither with carburization nor carbonitriding so it's not an issue atm (if we have to reduce the stay time, we will have to re-evaluate.

Thank you

 

[redpicker]

You may want to try dropping the furnace temperature to 820C once you have fully austenitized. That is, drop the furnace to 820C and hold it there for about a half-hour to an hour prior to quenching. You are trying to reduce the temperature gradient by reducing the amount of heat you need to remove during the quench. It's not much, but it may help.

rp

 

[tbuelna]

If the groove surfaces do not require case hardening then mask the area during carburizing and machine the groove after heat treat. The core hardness of carburized 8620 bar is probably less than Rc40, so it is still fairly machinable.

If the groove surfaces require case hardening, then leave more machine stock in the groove area before carburizing and finish grind the groove. Since your carburized case depth can range from .030"-.050" it should pose no problem to add sufficient stock to finish grind the groove surfaces to tolerances well within your .001" requirement.