4140 heat treating. Temper or not 1

[fastline12]

I recently found some documentation regarding brittleness concerns with 4140 if you do not pull back the hardness enough and indication was that if you are desiring a hardness of roughly HRC50 or above, to not introduce any temper because it would actually increase the brittleness of the part. I am curious if anyone has heard of this? I remember seeing a graph and indicated the concerns relative to hardness and the toughness and hardness were not linear, there was a dip in the graph.

We have been doing some small parts without temper for a while now but interested to put this to bed. We flame heat and oil quench and achieving about an HRC50. We really only need a hard case layer but due to the material and cross section, through hardening is almost unavoidable. We have not had any failures yet but would like to avoid that if at all possible.

 

[metengr]

Please review below

https://www.google.com/#hl=en&sclie...e7a5b78bb5c473&bpcl=35466521&biw=1366&bih=539

 

[fastline12]

Excuse my terminology before posting but the "notch toughness" graph seems obvious and concerning for 4140 and many med carbon steels which is why I am asking the question. It seems obvious per the graphs that If trying to achieve the highest and best combination of harness and toughness, temper could be a real concern with much over 200F in which I wonder if it is really worth it? Maybe from a metallurgist standpoint, the choice is obvious.

 

[redpicker]

While you do want to avoid tempering below 1000F if toughness is important, not tempering at all is really beyond foolish. The toughness of the material is very important, but not only will the material have the lowest toughness in the as-quenched condition (even lower than if temperd at 650F, the middle of the embrittling range), but internal residual stresses, combined withthe low toughness, will result in the materal literally tearing itself apart. A temper at 350F would be better than no temper at all.

rp

 

[EdStainless]

I am with Red on this.
At the lower temps you will at least get a little stress relief, which will help toughness.
With 4140 your as quenched hardness should be RC 58-60.
You shold be able to temper at 400F-500F and still be above RC 50 and have a lot better toughness.
This is what they do to springs after winding them.
You need to cool gently in still air, rapid coolign will re-introduce stresses.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[fastline12]

From my notes, it looks like we previously were tempering at 400 for 2hrs and just shut the oven off and allow them to cool slowly. It looks like we have shipped only a hand full without temper. We will certainly put the temper spec back in.

Question, why does the notch toughness graph indicate clearly that there is a zone in which we are reducing toughness though? It looks like anything over 100*F for a temper and you are actually reducing toughness??? I guess I am confused to not looking at this graph correctly

 

[TVP]

Using the graph shown here, tempering at 150 C results in ~ 30 J of impact energy, while tempering at 300 C results in ~ 20 J of impact energy. Not tempering at all is ~ 10 J. There is a metallurgical phenomenon that occurs between 150-300 C that results in a local minimum, but not the absolute minimum. If you want more information on the metallurgical phenomenon, the above link describes temper embrittlement.