4340 Temper Embrittlement and Application 2

[mewhg]

I have some small .150" dia pins 1" long that are in a critical area and are under bending fatigue, light impact and wear. Pins are machined with some small radiused grooves, heat treated then centerless ground. I am thinking of using 4340 at 50 HRC.

Is temper embrittlement something I should be concerned about with this alloy when heat treated to 50 HRC?


Are there special heat treating practices with this alloy considering how small the part is?

TIA

 

[israelkk]

You should be concerned with grind induces micro cracks that you should check using NDT rest. See MIL-STD-866B GRINDING OF CHROME PLATED STEEL AND STEEL PARTS HEAT TREATED TO 180,000 PSI OR OVER. You can get it freely at

http://assist.daps.dla.mil/quicksearch/

 

[metengr]

The 4340 low alloy steel should be immune to temper embrittlement. It has excellent hardening capability and toughness (because of the nickel content).

SAE-AISI 4340 steel is considered the standard by which other ultrahigh-strength steels are compared. It combines deep hardenability with high ductility, toughness, and strength. It has high fatigue and creep resistance. It is often used where severe service conditions exist and where high strength in heavy sections is required. In thin sections, this steel is air hardening; in practice, it is usually oil quenched. It is especially immune to temper embrittlement. It does not soften readily at elevated temperatures; that is, it exhibits good retention of strength. Hydrogen embrittlement is a problem for 4340 heat treated to tensile strengths greater than about 1400 MPa (200 ksi). Parts exposed to hydrogen, such as during pickling and plating, should be baked subsequently. This steel exhibits extremely poor resistance to stress-corrosion cracking when tempered to tensile strengths of 1500 to 1950 MPa (220 to 280 ksi). It can be readily nitrided, which often improves fatigue life.

Source; ASM Desk Edition

 

[israelkk]

Accoring to MIL-H-6875H HEAT TREATMENT OF STEEL, PROCESS FOR, the heat treatment between 220 ksi and 260 ksi is not allowed and no tempering data is given (probably because of temper embrittlement) see at

http://assist.daps.dla.mil/quicksearch/

 

[metengr]

israelkk;
I would suspect that the reason for not allowing strength levels of 220 to 260 Ksi for this material has nothing to do with TE. Presuming that tramp elements are kept to an absolute minimum. As I mentioned above, susceptibility to TE is very low in 4340 steel. The restriction to the aforementioned strength levels has more to do with increased susceptibility to hydrogen embrittlement or SCC.

 

[israelkk]

metengr

Please look and see that MIL-H-6875H allowS the 260-280 ksi treatment. It just requires to Cool to-lOO°F for 1 hour minimum within 2 hours after quenching and
before tempering. In previous versions they even gave the recommended tempering temperature for this treatment.

 

[metengr]

israelkk;
This subzero treatment is to assure complete martensitic transformation before tempering. I have 4340 quench and temper data starting from 400 deg F up to 1200 deg F tempers. There is no dip or loss of toughness from temper embrittlement.
Table 7 Typical mechanical properties of 4340 steel
Oil quenched from 845 °C (1550 °F) and tempered at various temperatures

Tempering
temperature Tensile strength Yield strength Elongation
in 50 mm
(2 in.), % Reduction
in area, % Hardness Izod
impact energy
°C °F MPa ksi MPa ksi %E RA HB HRc Jft · lbf
205 400 1980 287 1860 270 11 39 520 53 20 15
315 600 1760 255 1620 235 12 44 490 49.5 14 10
425 800 1500 217 1365 198 14 48 440 46 16 12
540 1000 1240 180 1160 168 17 53 360 39 47 35
650 1200 1020 148 860 125 20 60 290 31 100 74
705 1300 860 125 740 108 23 63 250 24 102 75

 

[israelkk]

If so why the MIL allow the 260-280 ksi treatment which is far more susceptibile to hydrogen embrittlement or SCC

 

[metengr]

israelkk;
That is an excellent question. I did not post the fracture toughness data for 4340 material, which in my opinion, is more useful and beneficial to understanding the implications of temper embrittlement.

In looking at more published CVN impact data and testing, perhaps the military is just being more conservative with this material. If you really wanted to focus on the ASM International published data above, one could make an argument that the CVN impact data "slightly bottoms out" at 255 Ksi (which falls in the range you mentioned). However,to me this is a stretch. There are times that metallurgists can't come to agreement on a specific issue. I certainly would not use this material with strength levels at or above 200 Ksi in an environment that would induce SCC, period.

Again, I would prefer to hang my hat on more meaningful test data, like fracture toughness. The fracture toughness data below presents a darn good reason why 4340 can be used in various tempers;

Hardness UTS* CVN Ksi(Sqrt) in
HB
550 296 Ksi 14 ft-lb 48
430 220 22 68
380 187 31 100

*equivalent strength from hardness

 

[mewhg]

Gentlemen,

Thanks for all the good info. I am going to proceed with a Quench, then Cryo, then temper to 50HRC. I have had good luck with the direct Cryo immediately after quench in some 4370 parts that needed to be 60HRC.