malleable iron castings 2

[JeffMartinWintle]

We have some malleable iron wire rope clips which have broken. On the inside of the parts can be seen a dark formation. Some formations are larger than others but every broken piece has some measure of this dark formation. Does anyone have any insight into what this dark formation is? Is it rust? How does rust get into the inside (no dark formation appear on the outside of the parts) of a malleable iron casting? Could the dark formation have appeared as a result of the breakage?

 

[KENAT]

Have you looked at it under magnification to see if there are characteristic fatigue wear marks?

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[FACS]

post it in a Metallurgy forum

Charlie

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[JeffMartinWintle]

Kenat, we have, as yet, done no external analysis of these parts. Presently, samples are in Birmingham, AL (sent to my boss) and will be analyzed by a local lab. Would these dark formations be consistent with fatigue failuer?

 

[mfgenggear]

Jeff

It appears parts could have cracks, it then obtained corrosion.
cut up a good & failed parts & then do a failure analysis on both parts.
have all parts 100% magnafluxed.

Mfgenggear

 

[FACS]

Those are Galvanized, you didn't mention that.

From my water well days, we had a problem with pin holes. A tiny, even microscopic, hole in the galvanizing is more of a problem than a large area that isn't galvanized. It's sort of like pressure, the tiny pin hole concentrates the corrosion process, and accelerates the oxidation. That looks like rust to me, and that's why they broke. The pin holes in the galvanizing caused it to rust within. When you look at the parts, the rust probably starts at the surface on at least one section. The pin hole is the tip of the iceberg.




Charlie

http://www.facsco.com

 

[JeffMartinWintle]

Thanks so much for the replies and direction y'all have given. It is most helpful in our overall analysis.

 

[mfgenggear]

it is true plating that is porous will allow contamination or attack the the sub surface of the parts.
but these parts had visual corrosion from the center of the parts, which means possible cracks.
and moisture or contamination was present.
if the parts are cracked it will fail right?

I would think if the plating was porus it it have subsurface oxidation or corrosion.
it would be prudent to strip off the plating & visually verify the surface. then process the part thru NDT.

Mfgenggear

 

[FACS]

I've experienced this many, many times, and the picture demonstrate this. I've had to dig up countless water well casings, and explain this process to people.

Hot dip galvanizing is just not a good process, and that's what these are. A pin hole in the galvanized surface will not show up on the surface, because it is galvanized. It will undermine the galvanized surface, and attack within. You can see clearly on the left hand picture the rust almost looks like a tiny bored hole; and on the surface, there is a discoloration around the hole. This is the rust under the surface of the galvanizing.

These parts did not crack, and then rust. They rusted, and then cracked. The pattern of the rust suggest this. The two parts, the one on the left, and the one in the middle show this fairly clearly.

You can not "strip" off the plating without destroying the evidence. If the parts need to be examined, then a metallurgical laboratory has to take sections, and then make an analysis.


Charlie

http://www.facsco.com

 

[swall]

The photo shows what looks like an internal casting flaw. Malleable iron castings exhibit more solidification shrinkage that gray iron and are more prone to not only shrinkage, but hot tears. I do not see the hot dipped galvanize as a contributor to the failure. But, wait for the lab analysis.

 

[arunmrao]

The casting appears to have an internal shrinkage . The feed metal requirement for malleable iron is compared to that of steel as it is cast white and then heat treated to obtain malleable iron microstructure. Galvanizing appears to be of less significance for the failure.

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[Walterke]

Cutting open a new part. If swall is correct, the black stuff will already be there.

About the pin hole theory:
shouldn't the galvanization (which is done with zinc IIRC) always oxidate faster than the steal because of cathodic protection?
What causes the rust to form in the iron?

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[FACS]

I'd hold back on my theory, and listen to the materials guys... that's what they do. I worked in a foundry long enough to know when to stand back and listen so this might be that time.



Charlie

http://www.facsco.com

 

[Compositepro]

Castings are susceptible to cracking and containing voids and inclusions. The pinhole theory applies to barrier coatings like paint and gold plating. Galvanizing is a sacrificial coating, basically a spread-out sacrificial anode. The fact that it covers the iron surface, thus minimizing exposed electrode area is important to the rate that zinc is lost. But galvanizing does still protect exposed iron in scratches and pinholes by sacrificing itself.

 

[BillPSU]

The galvanizing process include first a cleaning usually with a hot alkaline wash, a rinse, a pickling wash and a rinse, then fluxing, drying and then immersion in liquid zinc. If there are pin holes in the casting both the alkaline rinse and the pickling process would promote the rusting process. There is also a possibility that there is still some water in the casting pin holes and the liquid zinc being 825-850F could be causing the trapped water to expand and actually fracture the part.

Trapped water is a big no-no in galvanizing and can cause steam explosions.

 

[mfgenggear]

Bill

Interesting notation
I also believe that capillary action was taking place into the parts during the clean &/or pickle process prior to plate.
but any thing possible. yes I know wait for the analysis.

HTH
Mfgenggear

 

[dvd]

I wondered about hydrogen embrittlement for galvanized malleable iron and searched on that term with some results. Also found some results on galvanizing embrittlement. Seems like a possibility.

 

[swall]

Malleable iron, being a lower strength material, is not susceptible to hydrogen embrittlement. Zinc embrittlement (aka motlen metal embrittlement) only occurs with parts that are under a tensile load at elevated temperature.

 

[SceneryDriver]

Jeff,
What are you using the malleable iron rope clips for? I hope it's not for anything that must hold a critical load. In entertainment rigging (and rigging more generally), they're specifically prohibited (not really prohibited, but industry best practice is not to use them) because they're not subject to any quality control. Many have casting flaws and metallurgy issues, and because they're imported from overseas, you can't trace them. One popular saying is, "if they fail, you can't sue China."

If you're set on using wire rope clips, use ones from a reputable, traceable, domestic manufacturer, like Crosby. Their clips are forged, and each heat of steel is traceable should there be any issue.

That being said, why use wire rope clips at all? They're bulky, snag stuff, tend to loosen, and are very hard to get torqued to spec. On a good day, wire rope terminated properly, with rated clips, is only good to 75% of the breaking strength of the cable. Usually, it's more like 60% before slippage, because the smaller ones are so damn hard to torque to spec. Not to mention what happens if the clips are put on the cable backwards.

Consider the use of swaged terminations, like products from Nicopress. Copper sleeves are best (no brittle fracture under shock loads) and you can get tin plated copper sleeves for use with galvanized wire rope in outdoor conditions. Stay away from aluminum sleeves, especially in outdoor applications, where corrosion may be present. Stainless sleeves are also available for use with stainless cable. They're rated to 100% of the breaking strength of the cable (or 95% if you're conservative).

All your issues go away if you steer clear of crappy hardware. Based on your picture, it looks like your clips are no larger than 1/4" or so. At that size, a hand swaging tool is perfectly acceptable and usable.


-SceneryDriver