4140 vs 4340
4140 vs 4340
(OP)
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
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





RE: 4140 vs 4340
RE: 4140 vs 4340
Regards,
Cory
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RE: 4140 vs 4340
Additional answers to your question below;
Toughness - advantage 4340 >4140 because of Ni
Fatigue strength - advantage 4340
RE: 4140 vs 4340
RE: 4140 vs 4340
Are there any other grades I should be looking at for this application? (knowing cost, as always, should be minimized)
RE: 4140 vs 4340
*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.
RE: 4140 vs 4340
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.