4140 or 4340 1

[parsomash1977]

Hi all,

I am mechanical eng and I do not know much about material.I am planning to use 4140 or 4340 as top platen, a square plate with thickness of 1.5". which one would be stronger against large deflection because the platen will be under hydraulic compression on its center by 75000lb?

 

[NickE]

Neither;

4140 and 4340 have the same modulus, they will deflect the same amount under load.

IE: if it's a square plate with 75,000lbs in the exact center, and restrained at the edges, will see the most deflection in the dead center, and the least at the restraints.




Nick

 

[swall]

Nick is right--they will elastically deform the same. Permanent deformation, on the other hand is a different story. You will need to fully harden the plate. You will not do this with 4140; you need 4340. You will be able to achieve a tensile strength of around 250ksi with an oil quench and 400F temper.

 

[MintJulep]

Swall,

From the information available in this thread how were you able to determine that the stress would exceed yield unless hardened?

 

[metengr]

Once again, lack of information in the OP. If you are a mechanical engineer dust off your old strength of materials text book and review the formula for determining deflection in supported plates.

 

[parsomash1977]

Thank you gentlemen,I forgot to mention the hardness which is 32 HRC. I almost got the answer. But, I am worry to go over 32 hrc to get 250 ksi tensile strength because then it would be hard to do machining because I need to make a hole on it for the load cell. and weld a base plate to the top and center of the platen. By the way, this is for a wood bin compression tester.

Thanks

 

[redpicker]

With a thickness of 1.5" and a hardness of 32 HRC, if you are using oil quench and tempered material, I don't think you will see much difference in 4140 and 4340 as far as mechanical properties are concerned. The 4340 may have a bit more ductility and/or toughness, but I am not sure this will make a difference in this application, particularly if all you are concerned about is the amount of flexing (that is, elastic deformation). A little more informaiton on the loads and/or OD size could make a difference, though.

Neither 4140 or 4340 will let you approach 250 KSI tensile with a hardness of 32 HRC.

rpo

 

[parsomash1977]

No, I know to have 250 ksi , I should go for 48-52 HRC. My point was being worry about cost of hardening and then machining. thanks

 

[ornerynorsk]

You're planning on welding something to this at a RC hardness of over 50 ??? 4140 is commenly used in the pre-hard state for bolsters and platens, and is very machineable. Can you make the platen thicker?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.