Query on Reconditioning of manderels.
08-24-2009, 05:10 PM,
#1
Query on Reconditioning of manderels.
Dear Forum members,
I faced this problem recently and I thought somebody here would surely have answers to this.

Task: Reconditioning of Sample Mandrels (WC-Co, 88/12)

Procedure followed:
1) Undercut 1.65mm at the OD
2) Threading-25tpi, 0.2 to 0.3mm deep for better adhesion of SS coating
3) Surface Preparation: #24 grit and later inspected for embedded grit particles
4) Bond Coat: M43NS by APS (40 to 50 microns thickness), Temperature not exceeded 100C while spraying
5) Top Coat: M42C by APS (1.25mm coating thickness), Temperature not exceeded 120C while spraying
6) Machining cut (0.3mm depth/cut)
7) Grinding Cut (0.3mm material from OD, 20-50 microns per pass depending on the finishing/rough cut)
8) Surface Preparation: #24 grit and inspected for embedded grit particles
9) Final coat: Woka3106 by HVOF (0.1mm + .15mm grinding allowance = 0.25mm thickness), Temperature not exceeded 120C While spraying.
10) Final Grinding (Finish:0.2 to 0.3 Ra micron)

As you can see in the pic (Edit by Gordon - see next post), light brown lines (crack like pattern) have appeared on the surface. However, these cracks have not lead to any bubbling, cracking, peeling or any superficial discrepancies. Only problem is that they are VISIBLE !!..

I want suggestions as to what could have lead to occurrence of these lines.

Thanks in advance,
Regards

K09Mad0223
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08-25-2009, 12:49 AM,
#2
RE: Query on Reconditioning of manderels.
   

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The problem attaching image was due to image size (1.8MB). I have reduced image size below the 1MB forum limit. Reason for limit is to help those who have low internet speeds and to keep forum server bandwidth costs at a reasonable level Sad
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08-25-2009, 01:08 AM,
#3
RE: Query on Reconditioning of manderels.
Hi k09

Sign0016 to the Surface Engineering Forum.

Initially, it does look like a cracked coating. But, before I comment further can you tell me:

* does effect disappear with light polishing?
* can you see evidence of cracks when surface is viewed under magnification?
* have you tried dye-penetrant crack detection (may or may not work depending on coating porosity)?
* was grinding done wet or dry (both intermediate and top coat stages)?
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08-25-2009, 10:52 AM, (This post was last modified: 08-25-2009, 10:54 AM by k09.)
#4
RE: Query on Reconditioning of manderels.
(08-25-2009, 01:08 AM)Gordon Wrote: Hi k09

Sign0016 to the Surface Engineering Forum.

Initially, it does look like a cracked coating. But, before I comment further can you tell me:

* does effect disappear with light polishing?
* can you see evidence of cracks when surface is viewed under magnification?
* have you tried dye-penetrant crack detection (may or may not work depending on coating porosity)?
* was grinding done wet or dry (both intermediate and top coat stages)?

1) We tried cleaning it with light polishing but it only made it more prominent.

2) There were no traces of "actual" cracks. the surface was properly leveled and there was no bubbling or cracking of coating either.

3) No, we didnt. somehow, it did not strike me at that time. at that time i felt that porosity of coating itself might give irrelevant indications but now when i think about it i feel, porosity in HVOF is any ways less than 1% so DPT could have helped in some way. We have already removed the coating. Otherwise, would have surely tried that

4) Intermediate stage just water was used and during the final stage a mixture of cutting oil and water was used.

one more important point that i forgot to add was that the lines were not visible immediately after final grinding. It was dipatched from the grinding department and it remained packed for around 2 days. When we opened it after that, poof!! the lines had appeared.
Regards

K09Mad0223
Reply
08-25-2009, 12:52 PM,
#5
RE: Query on Reconditioning of manderels.
Hi k09

I would assume that these indications were cracks in this case. Though I have seen a couple of coatings with crack like indications like these:

1. Crankshaft plasma sprayed with steel coating, ground and then magnetic crack detection employed. Crack like indications found, but laboratory analysis of indications revealed them to be false. Magnetic crack detection is not a reliable method on TS coatings.

2. HVOF WC/Co/Cr on large roll, as-sprayed coating left overnight to cool in spray booth. Next morning showed a similar crack like pattern (perhaps larger spacing). No evidence of cracks when surface viewed at up to X200 magnification. Marks washed off with acetone and failed to reappear. Remembering, magnetic crack detection case above, I tried a little experiment - very lightly dusted surface with very fine iron oxide (magnetite , similar to magnetic particle ink pigment for crack detection) powder. Faint crack indications returned. I assumed this was due to odd magnetic properties of roll material. Perhaps roll cooling overnight collected a little dust in a similar fashion. Anyway, roll went to customer for grinding and inspection (customer aware of crack like indications) and reported no problems.

In your case I think grinding fluids have slowly leach out of cracks to reveal indications as in dye penetrant testing.
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08-25-2009, 01:06 PM,
#6
RE: Query on Reconditioning of manderels.
Hi k09

A few comments on your coating system and procedure:

* Consider a better bond coat - NiAl
* Consider arc spray NiAl followed by 13Cr steel for bond and intermediate coat
* Consider HVOF NiAl in one for both bond and intermediate (maybe 13Cr steel via HVOF)
* Dry grinding of intermediate layer, avoid any cutting fluids and any contamination of coating porosity.
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08-27-2009, 08:23 AM, (This post was last modified: 08-27-2009, 08:25 AM by k09.)
#7
RE: Query on Reconditioning of manderels.
Hi Gordon,

Thanks for your promt reply...

Even i feel that this might be due to oil seeping out after final coating and perhaps using oilfree coolant should solve the problem.

But here is another angle to this problem that was brought to light by a colleague. But this was with reference to another job that was rebuilded (bond coat + intermediate SS coat + Final WC coat) and had similar crack patterns on them. when the component was put in operation, the entire coating peeled off within 120mins!! the coating had come off directly from the threading that was provided before the bond coat.

What my colleague explained me was that the component was already in operation for several hours + when we remove the previous coating by grinding and then machining for thread formation (for SS coating), there COULD be considerable work hardening. This in turn restricts proper surface preparation and bonding of bond coat and SS.

So keeping this perspective, i have the following doubts:

1) Is it true that hardend surface can actually deteriorate the bonding of the coating? and to an extent that it finally peels off..

2) If it is true, then will stress relieving help?

3) Any other suggestions how we can solve this problem

Apart from this, I was also told that if we spray SS in HVOF, a bond coat is usually not required.. how accurate is this information? )

Keeping in mind the above example where the coating peeled off right from the threaded region. does any other reason for faliure come to your mind..

Till now this cracks have appeared only on rebuilded components and there has to be some reason to it...really need to pin-point the problem here..Mad0228 ..!!
Regards

K09Mad0223
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08-27-2009, 12:11 PM,
#8
RE: Query on Reconditioning of manderels.
Hi k09

Quote:1) Is it true that hardend surface can actually deteriorate the bonding of the coating? and to an extent that it finally peels off..
Well, as you said in your post "restricts proper surface preparation" A harder surface will be more resistant to grit blasting - a poorer grit blasted surface texture will lead to poorer bonding of coating. Hardening as such should not significantly alter bonding properties as long as there is a suitable grit blasted profile on the surface.

Quote:2) If it is true, then will stress relieving help?
Softening the surface will make grit blasting easier, but I have my doubts that this is really your problem.

Quote:3) Any other suggestions how we can solve this problem
If you are using Metco 43C (80/20 NiCr) as a bond coat, I would change this to a NiAl type bond material. The 80/20 NiCr materials are primarily for heat and oxidation/corrosion underlay for ceramics at high temperatures and bonding wise is poor compared to the NiAl.

Quote:Apart from this, I was also told that if we spray SS in HVOF, a bond coat is usually not required.. how accurate is this information? )
Well to some extent. Care should be taken with term SS stainless steel - thick intermediate coatings applied purely as a means of dimensional restoration require to be as stress free as possible. 13Cr steel and high carbon steel are known to be good due to low shrink, but austenitic stainless steels like 316 are the opposite with high shrink, high stress and low thickness limitation.

I would try where possible to reduce total coating system thickness. Do you always need to "Undercut 1.65mm at the OD"? I assume this is to clean up worn areas so only clean up as much as needed. Ideal situation would be where only top coat is needed (bearing in mind thickness limitation). I would be inclined to stay with a NiAl bond coat when using an intermediate coating or consider NiAl as bond and intermediate in one. The need for thread cut preparation could also be debated.
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09-01-2009, 10:17 AM,
#9
RE: Query on Reconditioning of manderels.
UPDATE:

This is what happened recently:

Sprayed a cylindrical job with NiAl...Sprayed SS 42C...grinding just with water....sprayed TC 88/12.

after 6-8 passes and cooling for subsequent spray pass we observed similar lines..this time on the as-sprayed surface..

disheartend, that it would again lead to prominent crack-patterend lines after grinding (as in the previous case)...we decide to try a few tests.. first thing in my mind was to heat the job.

I preheated the job with HVOF main flame (around 4-6 passes at coating speed)...and then let it cool down to room temperature...

POOFFF!!! the lines had disappeared... i sprayed the job to the final thickness required and even got it ground....the lines did not appear again.

the only conclusion i cud make here is that the lines that appeared were just traces of oil seeping out.

im goin to try this preheating technique on a cylindrical job that has visible lines even after grinding... if it does work we can conform that it is actually oil.

will report as soon as im done with it..
Regards

K09Mad0223
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09-07-2009, 09:25 AM,
#10
RE: Query on Reconditioning of manderels.
Carried out a few more "Pre-heat" Experiments on a reconditioned cylindrical rod...

pic 1) Rod cleaned with solvent. Only a few lines visible on the surface. mostly at the end of the rods.
   

pic 2&3) Rod Preheated to around 100C with HVOF main flame. Cracks appear with oil seeping out. You can also see the oil droplets. Cracks emerged all over the 1.2mt Long rod.
   
   

pic 4) Rod is cooled to ambient temperature and cleaned with solvent. oil marks go away but it still has fine fissures.
   

-
Regards

K09Mad0223
Reply
09-07-2009, 04:59 PM,
#11
RE: Query on Reconditioning of manderels.
Hi k09

Thanks for updates Smile

Well it would certainly appear that you have a genuine cracked coating.

I think you will have to look carefully at your coating procedures, changing/removing intermediate coating, dry grinding intermediate coating or spraying straight on without finishing.
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09-08-2009, 11:26 AM, (This post was last modified: 09-08-2009, 11:34 AM by k09.)
#12
RE: Query on Reconditioning of manderels.
Hi Gordon,

thanks for ur inputs).. What i have observed is that these cracks only appear on parts tht come for Re-conditioning. And especially those which had been in operation for not less than 2-5years..

Well..we have never bypassed any procedure and ensure that everything happens as per the correct procedure chart..but one thing that we overlooked was the diffrence in preheating passes required for a new component and an already used component..

Well after your valuable inputs and my exhaustive (and not soo pocket friendly) experimentsBig Grin, i feel i have pin pointed the problem here.. And offcourse this wud prove uselful for other members of forum.. Eventually..all's well that ends wellBig Grin
Regards

K09Mad0223
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11-11-2009, 10:43 PM,
#13
RE: Query on Reconditioning of manderels.
I would be really concerned about the difference in CTE's between the SS and WC. That combination is very difficult to spray without cracking, even without the effect of oil seeping out.
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