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Localized Softness after Induction Hardening (440c)

Localized Softness after Induction Hardening (440c)

Localized Softness after Induction Hardening (440c)

(OP)
Dear Engineering Hive Mind of the Internets,

I currently work for a facility that through-hardens 440c small Bearing Races via Induction Hardening.

Recently, a part that was "only soft on one side" was brought to me for analysis. After polishing/microhardness/etch, there is a visibly "shiny" area about .030" in from the "softest" surface. Under a microscope, this area does not show any of the "carbon etch pits" that are exhibited throughout the rest of the sample, and in fact shows very little microstructural details at all. We have been finding one or two parts that exhibit this behavior every few days.

I saw the exact same sort of structure from a part that was heat treated with some of the rusty scale (from when the forgings were shipped over here) still on it. Softness and a lack of carbon near the rusty edge, etc.

Disclosure: We buy forgings and tubings in the spherodized, annealed condition. We have seen wild variation in the material before.

HT Procedure: 2030 F for ~2 mimuntes, submerge on a roundness holding die in room temperature quenchant until cooled.


My questions are many:

1) What are the chances of this being a bit of surface de-carb? IIRC this would be caused by a localized over-heating, but the ol' brain fluid is pretty viscous this morning, corrections are invited and encouraged.

2) These tend to be roto-rocked/cold-rocked/indie-rocked forgings. Would there be something in the processes there that would encourage this behavior?

3) How do I keep my sanity while using material that makes these parts that undergo the same process yet turn out wildly different?

4) Could this be a "centered in the coil" IH problem? (I don't believe this to be the case myself having observed a few, but you never know.)

 

RE: Localized Softness after Induction Hardening (440c)

Just a guess--it is possible that your soft area is retained austenite. This is consistent with the way that it does not etch to show normal martensitic structure with carbides. To rule this in or out, try a subzero treatment to see if it develops normal hardness and martensitic structure.

RE: Localized Softness after Induction Hardening (440c)

I agree with swall, check to see if it is retained austenite.  Otherwise, it is likely decarburization.  Keep in mind that the parts may be decarburized as you receive them, and may not have anything to do with your induction hardening process.  Spheroidize annealing Type 440C is difficult, as decarburization is such a problem.

RE: Localized Softness after Induction Hardening (440c)

(OP)
I have applied a sub-zero treatment and the hardness only came up about 2 points, so I'm going to guess it's not a retained austenite.

If others have had decarb issues with forgings like this it would make more sense with some of the other things I've found out today.

 

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