8620 surface perlite 1

[Turda]

Hi,
I realy have hard time dealing with surface perlite for carburized 8620. I'm using endo and the case depth I'm aiming for is .030". I'm looking for any tips to reduce surface perlite/bainite.
Thank you,
T

 

[metengr]

Turda;
Could you please describe your carburizing procedure in some detail? This way proper help can be provided. In the meantime, even if you are familiar with heat treatment of steels, the information below might be of help to you;

http://www.key-to-steel.com/default.aspx?ID=CheckArticle&NM=125

 

[Turda]

Thank you for reply,carburizing procedure is a result of many trials in order to obtain less distortion on the part. I ended up with 1700 1.15 C 1H then 1500 .95 15 min and quench(hot oil). I tried longer cycles at 1660 .9 with almost the same results for surface perlite.I'm tring at this moment to completely remove any left over cuuting fluids.

 

[redpicker]

Your carburize cycle doesn't sound right to me. Years ago, when I was carburizing 8620 regularly, a 0.030" case would take 6-8 hours at 1700 with a carbon potential of 0.90/0.95%. Even with such a high carbon potential, I doubt you are getting high enough carbon on the surface to form 100% martensite.

Check your carbon content in the case. Running a round coupon and collecting turnings at 0.005" depth works well. With 8620, I'm guessing you'd need over 0.60%C to eliminate the pearlite. I'd be suprised if your getting over 0.40% at 0.005" deep. I'm guessing you'll need longer time at a lower carbon potential to acheive your goals.

rp

 

[Yallapragada1022]

Your carburizing recipe does not sound right to me either. You can still use the Boost & diffuse method you have used and my recommendation would be the following:
Boost:
Temp - 1700 deg. F, Carbon potential - 1.20 to 1.25
Time - 3:15 hours
Diffuse:
Drop potential to 0.90 to 0.95%.
Time - 2:00 hours.
Equalize:
Drop temperature to 1625 deg.C or less for distortion control.
Soak for 30 minutes and qench.

You can check the carbon content as recommended in the previous post.

You can also go to higher carburizing temperature (1750 deg. F) and reduce the cycle time. You have few options to look at and I hope this helps.

 

[Turda]

To hold the carbon set point to a certain value the furnace is making gas/air additions. I'm wondering if the air addition for drroping the carbon from 1.20 to .90 could be a source for my surface perlite/bainite (10 microns layer)Thx

 

[TVP]

Turda,

Yes, non-martensitic transformation products (NMPT) like bainite and pearlite occur in the surface layer when the atmosphere contains oxygen. Intergranular oxidation and decarburization are other potential problems. It sounds like you need to better control the atmosphere. Do you have a good reference on heat treating, such as Carburizing: Microstructures and Properties by Geofrrey Parrish (published by ASM International)? There is even specific information on lean-alloys like SAE 8620 and the problem of NMTPs.