8620 vs 4320 given case depth and cross section

[zx200driver]

Hi. I'm looking for opinions for a material specification. We have a part that is likely to be manufactured out of 4320H, gas carburized. (8620H is only alternative available). The case depth (50 HRc) requirement is a minimum of .080". Yes, this part must be tough and able to withstand heavy loads and the occasional shock load. Here is my concern: A portion of the loaded section is to be .275" - .320" thick (cross section). If I am interpreting hardenability curves correctly (?), 4320 could have a core hardness well into the 40+ HRc range making the part much less tough and subject to fracture. Are my concerns valid? Would a change to 8260H be a solution? Are there any guidelines or "rules of thumb" for minimum core thickness when using 8620H and 4620H?
Thanks!

 

[dbooker630]

The section sizes you mention are typical of automotive drive gear teeth at the pitchline, with the same core hardness concerns are valid. It is not unlikely to have gear pitchline core hardnesses in the mid 40's HRC, and gearing is certainly a demanding application with regard to tooth microstructure and toughness.

A couple of suggestions to consider if your application cannot tolerate this range but still desire 4320:

Use a medium-temperature quench oil, in the 200-225F range. This should keep the core in the low 40's on 4320 but still give the effective if you use 8620.

If the volume is sufficient, you can order a heat of material with a restricted jominy range. It may be tough to get a good fit from a bar warehouse but it is worth a shot also. For 4320 look for material with a low range hardness at the J4 jominy position, 33-37 HRC.