4140 vs 4340

[rmetzger]

we are looking for alternatives for a 4340 shaft material and are exploring using 4140. The final shaft will be inductioned hardened to Rc52-55 with a core hardness of 33-38 if we keep the same specs. Any thoughts?

 

[metengr]

What is the shaft diameter for your application?

 

[rmetzger]

13mm OD for most applications but also 15 and 17mm on a few

 

[metengr]

From the hardenability curves (Rockwell C versus distance from quenched end) that I have on hand, the 4140 alloy should hit about 47 HRc at the core (center) of your 13mm shaft diameters (0.5") with a water quench. For the 15 to 17 mm (0.6-0.67") shaft diameters, it will be closer to 42 HRc.

Keep in mind that 4340 alloy steel provides excellent toughness and increased fatigue strength in addition to increased hardenability in comparison to 4140 alloy steel. These also need to be factored into your material substitution.

 

[Carburize]

There is a danger with trying to use 4340 for small diameter parts. It can be so hardeneable that quench cracking can occur unless a very slow quench rate is used. At times specific high chemistry heats of 4340 can even crack when quenched into hot oils.
4140 should be perfectly adequate for this application.

 

[rmetzger]

thanks for the info - I'll make sure proper testing is done before we let 4140 out on the market (if we do) to check for failures due to the loss of properties.