Heat treated 4340 and 4140 and strength values 5

[canmecheng]

Hello,

I have read quite a few posts regarding 4340 and 4140 so I apologize if this question has been asked before.

Do the strength properties in a large shafts (4" & 13" in diameters) vary through the thickness after Quench and temper heat treatment of 4140 and 4340. Would I see differences of hardness depending on the distance from the surface?

If so, would any strength specification be indicating average strength?

What is the best sequence to ensure I have proper strength in large shafts that require significant machining. For example, I have a 13" round, which half gets machined to 12.5" and the other half gets machined to 9.5" diameter. Would I rough machine, then heat treat such that the hardness suggests acceptable strength, in this caase 40HRC, then send a coupon sample for strength testing per A370. What diameter would the coupon be taken from? Then perform the final machining.

Are there any standards that specify the amount of machining that would be acceptable and still retain the strength values of the original raw material? For example if I have 43" rounds of A434-BD.

Thanks

 

[weldstan]

Greater depth of hardness is obtained with the 4340 alloy. And yes the hardness will vary through thickness with large diameter bars.
Rough machine, heat treat and final machine.

 

[TVP]

I agree with weldstan. My only comment is to your specific question about coupon sample location-- this is commonly done by the customer explicitly stating where the specimen should obtained from, preferably on an engineering drawing. If you want the thickest section to have at least 40 HRC, then make sure that the heat treater knows this, and that the hardenability of the alloy, whether 4140 or 4340, can achieve the requirement without excessive distortion, cracking, etc.

 

[canmecheng]

Hi,

Thank you for the feedback.

I agree with you TVP. The most simple solution would be to specify on the drawing that the sample for testing physicals (per A370) needs to be at a distance from the center which is midway to the center from the smallest diameter section after final machining. Then I can be assured that the average strength in that section is represented by the physicals.

Best

 

[FennLane]

The first question you really need to answer is what tensile strength are you trying to obtain.

For example I would think that for a tensile strength of around 850/1000 MPa the Limited Ruling Section would be around 10" diameter.

For lower strength levels you would be able to use a larger section.

Clearly the hardenability of 4140 would mean that for this strength level the Ruling Section would be smaller.

If you are going to start by using bars of larger diameter than the ruling section then you should expect properties to fall off towards the centre of the bar and you may be better off to use material in a normalised condition and harden and temper after you have removed some of the stock.

If you want a level of strength that is in the 850/1000 MPa range in 13" diameter you will need to consider a steel which is much more costly like a 4340 VAR or 300M but I am not sure if these materials are available in a sufficiently large diameter.

 

[redpicker]

Without knowing what strength levels you are needing, it is difficult to offer advice.

If 80 KSI (550 MPa) yield is accaptable, it isn't going to make a lot of difference. If you need 160 KSI yield (1100 MPa) yield, you are going to have difficulty even with 4340 in the 13" diameter rounds.

Could you make 10.5" diameter work? If so, you should be able to find 4145M heat treated to 321 HBW with the oil field suppliers (Timken or Ascometal). Considering the recent fall of oil prices, you might find heat treated material at an attractive price. In this section size, I would not consider 4140.

rp

 

[tbuelna]

Attached is a table that might elaborate on what FennLane was saying.