4340 Heat Treating
4340 Heat Treating
(OP)
I am heat treating 4340 attemting to achieve the following results (similar to ASTM A540-B23):
Tensile 145,000 psi
Yield 130,000 psi
Elongation in 2" of 12%
Reduction in area of 40%
Impact V-notch ft-lbs 25
Brinnell 302-375
I used the following process:
Heated to 1575 F for 3.5 hrs in a protective atmosphere
Quench in 150-180 F agitated oil for 20 min
Snap temper at 400 F immediately after quench for 3 hrs
Wash
Temper at 1100 F for 4.5 hrs
I achived results of:
Tensile 149,270 psi
Yield 135,597 psi
Elongation in 2" of 11.89%
Reduction in area of 30.44%
Impact V-notch ft-lbs 10
Brinnell 302-375
I have changed this process 3 times trying to achieve the results listed, but this is as close as I have come. Previously, the strengths were low as well. In all 3 attempts, the impact hasn't moved.
What am I missing to achieve the desired results?
Tensile 145,000 psi
Yield 130,000 psi
Elongation in 2" of 12%
Reduction in area of 40%
Impact V-notch ft-lbs 25
Brinnell 302-375
I used the following process:
Heated to 1575 F for 3.5 hrs in a protective atmosphere
Quench in 150-180 F agitated oil for 20 min
Snap temper at 400 F immediately after quench for 3 hrs
Wash
Temper at 1100 F for 4.5 hrs
I achived results of:
Tensile 149,270 psi
Yield 135,597 psi
Elongation in 2" of 11.89%
Reduction in area of 30.44%
Impact V-notch ft-lbs 10
Brinnell 302-375
I have changed this process 3 times trying to achieve the results listed, but this is as close as I have come. Previously, the strengths were low as well. In all 3 attempts, the impact hasn't moved.
What am I missing to achieve the desired results?





RE: 4340 Heat Treating
Doesnt prior austenite grain size have an inverse effect on charpy impact strength?
My thought is that your austenization time is too long and it could be reduced to prevent excessive grain growth.
Nick
I love materials science!
RE: 4340 Heat Treating
Yes and Yes. The austenitization time for this material is occurs much more rapidly than you think. If the section size is 1" or greater, I would soak for 1 hour, and check austenitizing temperature every 15 minutes. Once the component has reached the minimum temperature, remove it from the furnace and into the quench tank.
RE: 4340 Heat Treating
RE: 4340 Heat Treating
maintain heat at the austenitizing temperature (1475-1550 deg F) for 15 minutes for every inch of material thickness. Is the net section thickness less than the diameter, if so, this is thickness you need to govern the time for austenitizing (also you should use TC's versus the above to minimize your time at austenitizing temperature). Quench to below 150 deg F.
RE: 4340 Heat Treating
Cutting a sample from the bar stock after heat treatment is completed is not an easy thing to do, but it is the best way to ascertain what your impact toughness values really are. I would also measure the grain size in your current Charpy samples and then compare that with the grain size you obtain from a samples that are sectioned from the heat treated bars at mid-radius. You will very likely see a significant difference in grain size between these two sets of samples.
What is the soak time that you are currently using for each of the bar diameters you listed in your reply?
Maui
RE: 4340 Heat Treating
We are going to run some experiments based upon the information provided specifying 15 minutes per inch of material thickness.
RE: 4340 Heat Treating
RE: 4340 Heat Treating
RE: 4340 Heat Treating
if you run in an atmosphere furnace i would temper at 1025 instead of 1000.
Process as follows
1. Normalize 1650 3@ A/C
2. Harden 1550 3@ oil quench 14-17 minutes in agitated oil
3. Temper 1000 4@ A/C
4. check hardness should be appx. 341 HBW/36 RC
charpies shouldn't be a problem if it is a good heat of steel. also try to start your quench when the oil is at appx. 125-135 degrees. you will get much better properties.
snap tempering does nothing in my experience. your hold times are no where near long enough for the cross section's you are working with.. a good rule of thumb is 1/2 hr/inch + 1 for normalize and hardening cycles and 1 hour/inch for tempering.
RE: 4340 Heat Treating