Fatigue damage and heat treatment 1

[Sparky4598]

We are using some field used parts as an adapter for a test part. Some parts were sent out to be heat treated to a higher hardness and some are being used as is.

My question is does the heat treatment process in any way reset or undo any fatigue damage that may have been done to the part? Say the part has 100,000 cycles on it at some stress above the endurance limit, does the process of bringing the part up to temperature during heat treatment sort of reverse the cycle count? Maybe not to zero, but bring the effective cycle count back down to like 1000 or something?

The material is very similar to AISI 4130 steel. As is part is 23-28HRc and hardened part is 40-45HRc.

Any other info needed just let me know, I'm not super familiar with materials science.

Thanks in advance for any responses!

 

[geesaman.d]

Fatigue is considered to be a couple of modes: first, dislocations in the crystal structure shift around and move to grain boundaries.

Second, these become cracks and the cracks grow at a rate proportional to the load and cycle count.

Third, these cracks from microscopic to macroscopic.

I could see some improvement in the crystal structure happening. I do not expect the cracking to reset unless you truly melt the metal. My gut says you're well into that second stage.

 

[EdStainless]

100k cycles at high load and you will have internal voids and cracks that formed.
These are unaffected by re-heat treatment.
As Geesaman said it would only help you if you were in the initial stage of fatigue.

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P.E. Metallurgy, consulting work welcomed

 

[MFAMET]

Fatigue failure usually starts from the surface where it receives the max tensile stress during application. This implies a hardened surface and/or a defect free surface could improve the fatigue life. A tensile residual stress could reduce the fatigue life.
The fatigue life is limited under a load. So, a used material will have a shorter lifetime than the new one. The re-heat treat of the used one could reset its fatigue property.

 

[MagBen]

Macro cracks/voids are unlikely to be eliminated. micro fatigue cracks (unlike corrosion cracks) can be potentially healed by heat treatment.

1. stress relief, can help to reduce the likelihood of further cracking.
2. Fatigue cracking can create grain boundaries that are not aligned properly. Heat treatment can realign the grain structure, which can help to reduce the stress concentrations that contribute to fatigue cracking.
3. Grain growth could fill in small gaps and potentially close cracks/voids
4. Could improve toughness, making it more resistant to further fatigue cracking.

 

[EdStainless]

I have re-heat treated parts that had seen some service but still looked good.
The result was parts that cracked in quench because of unseen fatigue cracks in their initial stages.
We actually prohibited any re-heat treatment of used parts for fear of making cracks worse but still not being able to catch it with standard NDT.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[MagBen]

yes, re-heat could make it worse. practically i agree with what Ed suggests to prohibit re-heat to re-use. My previous opinion was more of theoretical

 

[mfgenggear]

If there is any close tolerance attributes kiss it good bye after heat treat.
Scrap it out

 

[MFAMET]

as I mentioned previously, the fatigue crack starts from material surface due to the formation of "slip bands" by a greater tensile stress on surface. The failure caused by the interior defect, such as voids or microcracks is very rare. The used parts could have minute cracks on surface generated by fatigue process that is the areas need to be cared. Otherwise, re-heat treatment is
practical.