high temp properties for SAE 4140

[ivymike]

Hello,

Does anyone have a good source of data for the high temp (550degC to 600degC) endurance limit of SAE 4140 steel?

 

[Carburize]

At this temperature the tensile strength of the material will be no greater than 60% of its room temperature value.
If by endurance limit, you mean fatigue endurance limit that also we be expected to fall by at least the same amount since it is usually related to tensile strength however at this temperature other psuedo-creep mechanisms will also start to come into play depending on the time for which the peak loads are applied and a further reduction of life is to be expected from this mechanism.
There is a little bit of data in Atlas of Fatigue Curves edited by H.E. Boyer and published by ASM.

 

[ivymike]

would it surprise you if UTS dropped to 35% of its value at 100degC?

 

[Carburize]

Yes - very much so!
I will take a look at some curves in the office tomorrow but at 100 C I would not expect any change in UTS from its room temperature value.

 

[tbuelna]

ivymike:

At 550degC to 600degC you are well within the tempering range of 4140 alloy steel. Tempering 4140 at 600degC would produce a TS of about 140 ksi. Essentially, you would be "heat-treating" the material at that temp. Chrome-moly steels, like 4140, are generally not used at those types of temperatures.

 

[ivymike]

carb - I was saying that the value at 550 is 35% of the value at 100, not that the value at 100 was 35% of something else.

The question comes up because I'm looking at a piston analysis comparing 4140 vs microalloy steels. The material properties used by the analyst show a sharp drop in strength of 4140 above about 300degC, much more rapid than the drop in strength of the microalloy, such that the 4140 is expected to be weaker than MA above 490degC (20% weaker at 550degC).

I have an SAE paper that shows the contrary - on page 7 of SAE 2000-01-1232, 4140 is shown to be ~25% stronger than MA at 550degC (rotating bending fatigue test).

Any further references would be helpful.

 

[Carburize]

Smithells Metals Reference Book (ISBN 0 7506 3624 6) shows for a BS 1501-620 approx, equivalent to 4140
Temp YS UTS
MPa MPa
RT 286 479
400C 263 595
500C 232 525
600C 216 340

ASM Handbook Vol 1 has the following data for 4340 - (off a graph)
Temp YS UTS
MPa MPa
RT 1400 1400
150C 1250 1350
300C 1000 1250
400C 850 1100
500C 600 700
550C 400 500

 

[Cockroach]

I believe you the correct designation is AISI 4140 and not SAE 4140. At least that is the listing found on pg 76, The Metals Blackbook, Volume 1 (Ferrous Metals).

I have a metallurgical book recommending 250C as maximum "safe" thermal load for this metal. It doesn't surprise me that 4140 shows a sharp drop above 300C.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[NickE]

Cockroach- actually the AISI has handed off all of the steel designations to SAE.

So SAE 4140 is the approved designator for the composition. there is probably also a SAE j-standard (hmmm... J404 IIRC)

 

[ivymike]

I have a metallurgical book recommending 250C as maximum "safe" thermal load for this metal. It doesn't surprise me that 4140 shows a sharp drop above 300C.

Some people sure are conservative... 4140 is a fairly common material for steel pistons, which regularly run over 500degC in hot areas. The microalloy is a cheaper replacement (better machinability), which is used where one can get away with it.

Still scratching my head a bit about whether 4140 is strong enough or not, and whether it's at least stronger than the microalloy @ 550degC.

 

[Cockroach]

Thanks for the update, NickE; I didn't know the switch in AISI to SAE designators. My reference is dated January 1994, time for a new reference maybe! I guess you learn something new everyday, aye?

Yeah, conservative IvyMike, is a safe play when risk outweighs probability of success. I would be more worried of creep at an elevated temperature since material strength drops significantly at 250-ish C. Although a common steel alloy, 4140 has it's limitations...even in the oil field!

Interesting application though. Good luck with it!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[diamondjim]

Sure looks like 4340 is the way to go.
It also has good cold temperature qualities
but lower surface hardness. I have always
wondered if the nickel helps to increase
the endurance limit. I have not seen endurance
limits at the higher temperatures and if it
is still about 56 percent of the UTS at that
temperature.

 

[TVP]

ivymike,

I just reviewed the data in the SAE paper by Cummins. The 4140H (H means it has guaranteed hardenability, see the standard SAE J1268 for more information) was Q&T to a hardness range of 255-285 HB. In order to obtain this hardness, 4140 would be tempered at a temperature in excess of 600 C. ASM HANDBOOK Volume 4 Heat Treating shows data for 4140 tempered at 650 C with hardness of 277 HB and tensile strength of 900 MPa. This shows very good agreement with the Cummins data.

Now, since the 4140 was tempered at such a high temperature, it will show good resistance to softening at temperatures below the tempering temperature. Your analyst probably has data for 4140 that was tempered at a low temperature to a much higher hardness, which means that the properties will drop substantially as the temperature increases. For example, 4140 tempered at 260 C has a TS of 1860 MPa (534 HB), which reduces to 1590 MPa (461 HB) after tempering to 370 C.

If you are tempering the 4140 to a similar range as shown in the SAE paper, then I would have very high confidence in their data. If the 4140 has been tempered to a different hardness range, then things will be somewhat different. Best of luck, and it is nice to see you out of hiding...

 

[ivymike]

TVP, thanks, twice!