4140 vs 4340

[GTFiji]

I am looking at these two materials (4140 vs 4340) for my application, as suggested by users of this forum. I have a 62" octagonal bar (1" flat to flat) heat treated to a minimum of 150k UTS. I need to machine a square bar to my shape and then heat treat. I need the bar to hold tolerences and not bend (warp) more than 1/16" overall flatness.

What are the tradeoffs/pros & cons of these two materials as compared to one another?
-cost
-properties (after heat treat)
-toughness
-fatigue strength
-dimensional stability

 

[israelkk]

You will be better off with final machining the bar after heat treatment. At 150 ksi the bar is nicely machinable. This way the bar will not warp or will warp much less and with no decarborization.

 

[CoryPad]

This is a simple question. These two alloys are essentially the same, so the properties essentially are the same. The big difference: a 1" diameter is at/beyond the limit of full hardening for 4140, therefore you should use 4340.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[metengr]

What are the tradeoffs/pros & cons of these two materials as compared to one another?
-cost
-properties (after heat treat)
-toughness
-fatigue strength
-dimensional stability

Additional answers to your question below;

Toughness - advantage 4340 >4140 because of Ni
Fatigue strength - advantage 4340

 

[dbooker630]

4340 may cost a bit more because of the nickel cost but that is probably the only drawback. The price of nickel has been coming back down slightly.

 

[GTFiji]

Great feedback.

Are there any other grades I should be looking at for this application? (knowing cost, as always, should be minimized)

 

[Metalguy]

Most of your alloy selection depends on how much fatigue strength and impact resistance you REALLY need. Yes, 4340 is a little better than 4340 but 300M is a little better than 4340 (all in general).

*If* you can specify or select heats of 4140, it should work fine unless your requirements are severe. You can spec. it for hardenability--some good heats will thru harden beyond 2". Just ask for a heat that is on the high side of the limits for C, Mn, Cr and Mo-at least 3 of them should do it. Make sure not to end up with a large grain size from heat treating at too high a temp.--1550 F is fine for 4140, and 1475F works for 4340.

 

[MOB1]

According to my information there should not be anything between the two steels.
Assuming you need a 2 inch square, the equivalent round bar is 2 1/2 inch and this is the limiting ruling section (diameter which will transform to martensite on quenching) for 4140 while 4340 is good for 4 inches. The impact requiremnets for the two grades at this strength level are the same and I would not have considered there to be a difference in fatigue properties at the same strength level.