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4340 Heat Treating

4340 Heat Treating

4340 Heat Treating

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
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

This is mostly just a swag, however what is the section size?

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.

I love materials science!

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.

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

Currently, I am working with 3 different section diameters, 6.5", 8", and 11.5".  However, my latest results were from heat treating the tensile and charpy samples only.  Would that explain the low charpy?  The long austenitize would cause grain size growth due to the small sample size.

RE: 4340 Heat Treating

Yes, fine grain size results in improved notch toughness properties. I checked with my ASM Handbook on line and for 4340 alloy steel to austenitize;

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

You should be cutting samples from the bar stock at mid-radius after heat treatment is completed in order to make your Charpy impact specimens, not loading pre-made Charpy blanks alongside the bar stock inside the heat treat furnace. As Nick and Metengr both pointed out, austenitizing such a small sample for the length of time that you stipulated will produce excessive grain growth, which will adversely affect the Charpy impact toughness. If you choose to heat treat a standard Charpy impact specimen in a controlled atmosphere furnace instead of sectioning it from the heat treated bar stock, then the soak time at the austenitizing temperature for this specimen should not exceed 30 minutes.

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?


RE: 4340 Heat Treating

We used a soak time of 3.5 hours for everything that we did.  We are just getting into optimizing material properties as opposed to hardening material just to get a surface hardness.

We are going to run some experiments based upon the information provided specifying 15 minutes per inch of material thickness.

RE: 4340 Heat Treating

As metengr noted, the austenitizing temperature should be in the range of 1475-1550 F.  3.5 hours at 1575 F will most certainly result in a large austenitic grain size and therefore poor fracture toughness (charpy test).  Use the lowest austenitizing temperature for maximizing toughness.

RE: 4340 Heat Treating

You might preheat at 1200 F and soak to reduce the high temperature soak time.A soak at 1200 F won't have any effect on grain size.

RE: 4340 Heat Treating

I am a commercial heat-treater. depending on the mill and the quality of the heat of steel will depend on your results. Assuming you are just doing this for testing purposes cut the test pieces to 4"x4"x6" run all together in an open furnace if you can, if not run in atmosphere furnace.
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

If hardenability is not your major concern, but it does appear so as you mentioned your real size is 6.5'' ~ 11.5'', I suggest you try 4330. You can very easily achieve the toughness, of course you have to tune the heat treatment.

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