Distortion of Carburized 4340?
Distortion of Carburized 4340?
(OP)
Hi all,
I have a precision needle bearing riding on a shaft inside of a journal. I need to case harden both the shaft and the journal so that the needle bearing can ride on this surface and not mar the surface. The shaft is a hollow steel tube with a wall thickness of 1/8"(OD 1", ID 3/4") made from 4340. The journal is also a hollow tube with a wall thickness of 0.100" (ID 1.125", OD 1.325"). The tolerances on these dimensions are very tight(+/- 0.0005"). My question is, if I batch case harden both of these pieces to 52-58HRc (0.002-0.009"), will there be any distortion of these dimensions? I need the assembly to fit together after it's been heat treated. How much distortion can I expect here?
Thanks,
M
I have a precision needle bearing riding on a shaft inside of a journal. I need to case harden both the shaft and the journal so that the needle bearing can ride on this surface and not mar the surface. The shaft is a hollow steel tube with a wall thickness of 1/8"(OD 1", ID 3/4") made from 4340. The journal is also a hollow tube with a wall thickness of 0.100" (ID 1.125", OD 1.325"). The tolerances on these dimensions are very tight(+/- 0.0005"). My question is, if I batch case harden both of these pieces to 52-58HRc (0.002-0.009"), will there be any distortion of these dimensions? I need the assembly to fit together after it's been heat treated. How much distortion can I expect here?
Thanks,
M





RE: Distortion of Carburized 4340?
RE: Distortion of Carburized 4340?
Since the 4340 is annealed, I'm assuming the core will also harden some during the quench for the case. Any idea what this hardness might come out to given our case spec?
RE: Distortion of Carburized 4340?
RE: Distortion of Carburized 4340?
_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year
RE: Distortion of Carburized 4340?
Maui
www.EngineeringMetallurgy.com
RE: Distortion of Carburized 4340?
Thanks,
M
RE: Distortion of Carburized 4340?
Carburizing is not recommended for 4340. For other processes, you will want to heat treat the 4340 to the desired core hardness prior to case hardening.
If you use ion-nitriding to develop the case hardness, you should experience very little distoriton. I would expect holding +/- 0.0005" to be a problem, since even ion-nitriding may relieve stresses and get the material to move more than that.
Other nitriging processes are likely to be at a higher temperature, so expect more distoriton.
With any of the nitriding processes, the distortion could be either a growth or shirink, but I'd suspect both; that is, your round tubes will become oval. You will have to hone them after processing to be sure you are holding these tolerances.
If you using iduction heating, well, your dimensions are not favorable for this process. I'd expect scrap.
rp
RE: Distortion of Carburized 4340?
After reading the above article, I'd suggest giving these guys a call for some more in-depth answers to the questions that you will have:
http://www.contourhardening.com/services/contractp...
Maui
www.EngineeringMetallurgy.com
RE: Distortion of Carburized 4340?
_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year
RE: Distortion of Carburized 4340?
RE: Distortion of Carburized 4340?
RE: Distortion of Carburized 4340?
RE: Distortion of Carburized 4340?
nitrided
> 800 HV3
NHD HV0.3 = 0.25 +0.05 -0
This defines the surface hardness as 800 HV minimum (using 3 kgf load), and the diffusion zone as 0.25 to 0.30 mm when indented with 0.3 kgf and Vickers indenter. You can further elaborate with gas nitrided, plasma nitrided, etc.
RE: Distortion of Carburized 4340?
Maui
www.EngineeringMetallurgy.com
RE: Distortion of Carburized 4340?
As for the nitrided case depth required, since you are producing a rolling element bearing race surface what matters is the depth/hardness profile of the finish ground journal. During finish grinding, you will need to remove enough material to produce a bearing journal profile that meets tolerances for diameter, runout, surface roughness, cylindricity, etc., as well as any white layer. One characteristic of nitrided cases is the rather rapid reduction in hardness vs depth, which means you need to carefully control the amount of stock removal during finish grinding.
You can produce a case depth of .020" or more with gas nitriding, but the case depth is a function of time in the furnace. A nitrided case of .020" can require 50 hours or more in the furnace, which can be quite expensive. In order to determine the optimum case depth & hardness profile for your bearing journal, you should first perform a Hertzian contact stress analysis of your bearing elements/races. Then estimate how much stock will be removed during finishing operations. This will determine the optimum depth of case you should specify to keep costs down.
Lastly, I don't know how demanding your particular application is, but with rolling element bearing races there can be significant improvement in statistical fatigue life when vacuum melt material is used rather than air melt material. While it would cost a bit more, you might consider using vacuum melt 4340 (AMS 6414).
Good luck to you.
Terry