FAILURE ANALYSIS, PUNCH IN CYCLIC COMPRESSION, AISI 8620, CARBURIZING 2

[PressEquip]

Hello,

For certain field jobs the machine shop makes custom attachments for the pneumatic hammer. They've been doing this for a long time (we've started to formalize the process). Most of the time it works out well but this year the field job was really tough and lots of these attachments broke (see attached photo).

The tool puts the attachment in cyclic impact compression (major load), with some minor shear and moment loads. The skin of the part is carburized to create a hard surface (prevent the surfaces from mushrooming (plastic deformation)) and the core is still ductile.

The attachment is fabricated with gentle radius at transitions in OD to try and minimize any notch effects.

Material AISI 8620 Hot Rolled, Carburized Heat Treatment for ~0.05"+ case depth complete with temper to achieve surface hardness of 50-55 HRC

Preliminary Failure Analysis: During the heat treating process (carburizing, oil quench, temper), the skin of the part transforms into martensite and some micro-cracks occur. During use in the pneumatic hammer the cracks propagated and eventually the part failed in brittle fracture (see photo).

Some Suggestions People Have already Provided

1) Polish the surface in the area where the failure occurred before heat treating (Failed punchs had surface marks left from machining. You can feel slight ridges if you drag your nail along the punch.)
2) Shot Peen the area to put the surface in compressive stress state (helps to prevent the formation and propigation of cracks because the tensile force must over come the residual compressive stress).
3) Mask this area before carburizing to help reduce the hardness
4) Change the heat treatment so attachments skin is not so hard

If you have any helpful thoughts or insight, please reply to this thread

Thanks

A Fan of EngTips, share the knowledge

 

[Tmoose]

Could you post a pic or drawing of the part, indicating where it breaks?

 

[redpicker]

How did you discover these "micro-cracks"? How deep are they? How did you determine they contributed to the failure?

The photo you posted appears to be classic tension-tension fatigue, although it is difficult to make such a determination from such little evidence. Some more information would be helpful, like part size, some other views of the fracture, and, as Tmoose mentioned, a drawing or picture of the part.

rp

 

[tbuelna]

Some Suggestions People Have already Provided -
1) Polish the surface in the area where the failure occurred before heat treating (Failed punchs had surface marks left from machining. You can feel slight ridges if you drag your nail along the punch.)
2) Shot Peen the area to put the surface in compressive stress state (helps to prevent the formation and propigation of cracks because the tensile force must over come the residual compressive stress).
3) Mask this area before carburizing to help reduce the hardness
4) Change the heat treatment so attachments skin is not so hard

The problem you describe with micro-cracks in the carburized case structure of 8620 material is described in this reference pages 57-62. Apparently the case micro-cracks are due to the 8620 alloy composition and carburizing heat treatment. Polishing or shot-peening will not make the micro-cracks go away, nor will changing the heat treat process so the "skin is not so hard". Masking will only prevent the problem on surfaces that are masked, and the problem will still occur on remaining carburized surfaces. The reference above suggests that an alloy with higher nickel content would help reduce the problem.

Since your carburized case is fairly heavy (~.050"), if you can establish the depth of the micro-cracking is not significant, the deficient surface layer can be removed by finish grinding. Lastly, I would recommend switching to a cold rolled vacuum melt quality material rather than the hot rolled material you are using. The cost difference should not be significant for your application.

Good luck.

 

[dbooker630]

Is there any finish grinding or turning done after heat treatment? If not, the micro cracks may be associated with intergranular oxidation, a characteristic of a furnace that is not tight. In some applications 0.001" is tolerable, but perhaps the IGO is much deeper than that.

 

[Maui]

The photo you supplied is very blurry, and it makes diagnosing this fracture surface difficult. But it looks like the failure is from a combination of torsional and compressive loading. I will provide you with some additional options that have apparently not been mentioned previously.

1.) Reduce the maximum loads that you are applying.
2.) Select a different grade of material to manufacture these parts from that possesses a greater level of toughness at the required hardness level. AISI 9310 and 4820 both come to mind. Note that if you select either of these grades you will have to perform a cryogenic or cold treatment as a part of the standard heat treating cycle. After quenching the parts out, temper them using one normal tempering cycle, then cryo treat, and follow that up with one more normal tempering cycle.

Shot peening should also help as suggested previously.

Maui

 

[ornerynorsk]

With the cost of labor and heat treating, would it make more sense to just use S7 or similar to begin with?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[metman]

yes more Nickel => and yes 9310 because it has more Nickel

better yet S7 if cost justifies

sounds like you are machining to shape. You might consider forging also i would expect shot-peening to reduce the extent of micro-cracks upon quenching

Design for RELIABILITY, manufacturability, and maintainability