Quench vs. Hydrogen Assisted Crack 2

[evan_z]

I am seeing some strange cracking patterns in castings when performing MPI after the quench and temper operation. In some cases they appear similar to HAC. Can anyone suggest a way to differentiate between the two?

 

[gtaw]

Castings rarely have a HAZ.

Best regards - Al

 

[evan_z]

I am referring to Hydrogen Assisted Cracking.

 

[blacksmith37]

The only time I ever saw HAC of recently heat-treated parts was when someone mistakenly acid pickled as quenched steel ( about HRc 65).

 

[WKTaylor]

What alloy/spec and heat treatment process... to what temper/condition, etc...?

Regards, Wil Taylor

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[metengr]

Most large industrial castings I have seen have major weld repairs especially the Gr 91 castings. This is typical and may not be reported to the purchaser for certain grades.

So you can have HAC or delayed cracking in weld repaired areas. The trick is to know the weld repaired locations and evaluate the crack morphology. In areas where no weld repairs have been confirmed the likelihood of HAC is remote.

 

[arunmrao]

metengr,

Good comment.

However, I would guess that the foundry would carry out weld repair before quench and temper operation. Also, in the event of major weld repairs the castings are normalized before quench and temper operation.

This good foundry practice would minimize the risk of cracks.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[Coctyle]

Is it correct that you do not know if the cracks existed before HT or not? Heating the part to the austenitizing temperature should allow hydrogen to diffuse out, therefore you would be unlikely to see HAC after HT unless, as someone alluded to already, the part was exposed to hydrogen after HT, such as pickling or weld repair after HT. I agree that either of those situations are unlikely, but not impossible.

If the crack existed before HT, the inside of the crack would have been exposed to the heat treat atmosphere. Maybe if there was decarburization that could not be completely recovered during HT, you would see the effect of the crack surface being exposed to the atmosphere because the decarb just inside the opening of the crack would be more fully eliminated than at a similar depth away from the crack.

 

[evan_z]

we perform MPI before HT in the areas that we typically see hot tears. The parts go through Normalize and then an AQT. These cracks occur before any weld repair is done.

 

[Coctyle]

So, you saw the cracks before HT, or you did not? Or are these cracks not in an area where hot tears typically occur and therefore not checked before HT?

 

[JoeyV87]

It would have to be Austenitic to want to repair before Q&T, doing that in Carbon or low alloy steel would be silly.

It is really important to know the grade, it might be delayed cracking on weld repairs, it's not common on base material unless it's an HSLA (HY80 etc). If its a recurring problem, you could put a degas op in after your first normalise. The most effective is 300C for 24 hours per inch section thickness, but that might not be practical.

 

[evan_z]

We are not seeing the cracks before HT in this case. There are no weld repairs. It is an area we fought hot tears in the past, so we MPI prior to HT. It is a proprietary grade.

The reason I brought this question up is the similarity of a "Fish eye" to a quench crack. This part is having a high percentage of cracks after Q&T, and has had some failures in the field. Origin appearance is very similar, just in some cases it's internal, and others is closer to a surface. I'm just trying to confirm I am not fighting the wrong issue.

 

[metengr]

From your description this sounds like quench cracks OR casting defects subsurface that are opening during HT.

 

[arunmrao]

Maybe you need to normalize the castings before hardening.Alternately have a milder quenching media to avoid cracks.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[tbuelna]

This might be of help:

https://www.sfsa.org/misc/duncan/duncan.ppt