Case hardening S2 Tool Steel? 2

[metalman8357]

Hi all,

I have a lot of mandrels that we are planning on redesigning. They are subjected to high torsional impact loads and fractographic analysis shows a brittle fatigue failure. We are going to select a new material for this application, and we are aiming to have a softer core and just case harden the shaft for wear resistance. The shaft is 0.25", and ideally I would like to achieve a core hardness of 35-38 HRC with a 0.008 - 0.012" case hardened to 52-56HRC. My question is, without having to make these mandrels from a new material, would I be able to carburize S2 Tool steel to meet this hardness profile? If not what about nitriding? I don't want to have to get a large quantity of these made just for testing purposes, and I want to see if we can get away with using our existing inventory. Also, I am looking for suggestions for a future material to use for this application. I want something that will be cheaper than S2, but will also have the ability to form the hardness profile that I am looking for.

Thanks,
M

 

[redpicker]

You haven't mentioned your operation temperature, but you may consider either a 4815/4820 carburized or, for larger section sizes, 9310/9315 carburized. Not knowing the application and/or operation temperatures, these may not be appropiate, but it is an option. 0.008-0.012 is a pretty light case for carburized product, you may be better off just going with S2 tempered to you desired hardness range and haveing it nitrided.

rp

 

[metalman8357]

Operating temperature is ambient temperature. The shaft has a hexagonal recess on one end that a hexagonal bit fits into. The shaft is loaded for short pulses at a time, with torque on the bit around 300 in/lbs. Again, the shaft is 0.25" in diameter, and the amount of material above each flat on the hex recess is 0.053".

 

[mfgenggear]

Metal

I am with RP, heat treat the S2 to 35-38 Rc then finish machine, then Nitride,
it will work great except the nitride case depth will be approximately .010 max total case depth.
the outer surface should be 60 min HRc.
this will be the least amount of distortion.
the problem with nitride it is very hard & fragile. it will chip easily.
carbonitriding may be another option.

HTH
Mfgenggear

 

[TVP]

I do not recommend case carburizing for this. Nitriding is a better option. If you want optimal performance, do the following:

1. Induce residual compressive stresses by shot peening or roller burnishing after nitriding.
2. Polish the surface after SP or RB. This can be hand polishing, vibratory finishing with appropriate media, etc.

 

[Screwman1]

This sounds like a fastener drive bit holder. S2 is the traditional material for these parts, but the heat treat that is used most often is to thru harden and temper back to the Rc 55 - 58 range to get maximum strength. Since you are using this in an impulse or impact tool, that is not going to work real well because of the propensity for just the failures that you are experiencing. What we have done in similar situations is to switch to S7 and thru harden to the Rc 42 -45 range. with the hex recess that you are broaching into that piece, I'm not sure if putting a case on it is the best strategy if you are already having fatigue issues. Under the torsional loads, with all the sharp corners you will have a lot of crack initiators.

 

[mfgenggear]

Maybe core harden & then Induction harden the the tips?

Mfgenggear