Grain Structure of Forged 440C

We have tested two different 440C shafts in a simulated failure mode to analyze the strength or torque of when the shaft will break. Upon completion of the test we have found some very odd results.

Shaft #1 had the highest strength and the most refined grain structure. This part is forged from 440C and then heat treat to a hardness of 56 HRC.

Shaft #2 had roughly 65% of the strength of shaft#1. It is also forged and heat treated to a hardness of 56 HRC. However, the grain structure is completely different. I am unsure of what to make of it.

Any ideas? I am thinking that it is a heat treating process but I am unsure. I know there was mention that the pieces were heated multiple times due to the pieces shifting while being heat treated. Both shafts are HRC 56.


Structure and its coarse grains indicates a overheating. Verify parameters of forging process and hardening too. Look at temperature, strain rate, and soaking.
The reason why material failed depends on forging temperature, strain rate and soaking. In other words, higher temperature!
Coarse grain is the result of something wrong in this process. For this reason, look at Hrc after forging process, before slow cooling after this one. This value could be a good info about exaggerated forging temperature or exaggerated strain rate because you could have an idea about the influence of retained austenite whose amount depends on above parameters.
The higher retained austenite and the higher forging temperature, the lower Hrc after cooling.
In case of only one tempering after forging (just to avoid cooling cracks) HRc cannot reach Hrc = 56 if the structure had been overheating or “burned”.
For this reason I agree about "incipient melting "but an easy optical microscope testing of grain coarsened piece would point up some grain boundaries with a evidence of melting (dentrides together decoesion between the grain boundaries)
However, the main problem of a hot forging /rolling process of this high carbon stainless steel grades is the habit to consider these ones as an austenic or low C martensitic grade. A lot of people forget that higher Carbon means lower liquidus/solidus borderline.
Finally, I think that if you collect the correct information about the process, they wouldn’t be asking us in this forum about the “bad” structure as they would have figured this out on their own.

Above is a snippet.

Can you upload images of the grain structures from both samples? This may help us to provide some appropriate guidance.

Sorry Maui, I found no images in the cited thread.