Colmonoy Coating Explosive Failure

[jackboot]

If anyone has any ideas about this puzzling situation - please respond.

We coating pump plungers with colmonoy. We have been doing this for 30 years.

Recently, we have had failures -usually during grinding- where the colmonoy will explode and shower the work area with colmonoy shards. However, this can occur anytime during the process- but always when the part has cooled. The explosive force is substancial enough that have taken safety precautions to protect those in the work area.

The plunger is 316L stainless and the part is ground to a 3.00 in diameter. Colmonoy is applied to a groove in the plunger to a depth of approximately 0.030 in. The run-out angles on the end of the groove are 5 deg (groove transition to part OD). The plunger is ground and the finished depth of the colmonoy is approximately 0.018 in.

We have been tearing our hair out and can not find the culprit.

jackboot

 

[Metalguy]

Is the colmonoy a weld overlay? If so, sounds like it could involve severe underbead cracking-I'd look for hydrogen.

 

[jackboot]

COLMONOY IS A HARD SURFACING THAT IS APPLIED BY SPRAYING (POWDER METAL) ONTO A WORK PIECE. THE POWER IS THEN FUSED BY HEATING THE WORK PIECE UNTIL THE COLMONOY IS BONDED. COLMONOY IS A NICKEL CHROMIUM BORIDE/CARBIDE MIXTURE WITH A SURFACE HARDNESS.

JACKBOOT

 

[Metalguy]

I would advise you to take some known good samples and some of the new ones to a good met. lab. and study the bonding- line area. I have only limited experience with plasma and flame-sprayed coatings, but did have a real problem once when plasma-spraying (with a very powerful plasma setup) so large pump impeller wear-ring areas.

After many $$$$$ of testing, experimenting, etc. the problem was finally traced to excessive dust in the spray area. But this dust was from the spraying itself. We couldn't spray in the open because the parts were contaminated (radioactive particles imbedded all over the surface). We measured the bond strength by using a special exoxy glue to fasten a round metal piece right to the top of the sprayed surface, and then pulling it off in a tensile testing machine. IIRC, we wanted ~7,000 psi strength. We got that sometimes, but unfused overspray dust particles were to prevalent-they are easily seen on the bond line after the pull test.

The lab. I used is in Phoenix, METL (Materials Eng. and Testing Labs.). BTW, I have no financial connection with them, but they did extremely good work for me a few years ago.

 

[Metalguy]

Oops, that's Metals Eng. etc.

 

[kenvlach]

jackboot,
I checked the Comoloy site (

http://www.wallcolmonoy.com/)

for details of the material and application. From your description, you may be using Colmonoy 5, which can be applied by Oxyacetylene, DC Electric Arc, GTAW, or Spraywelder and fused at 1880°F (1025°C). General description and usage: “Contains wear-resistant chromium borides and carbides. Has greater impact resistance and workability than Colmonoy 6. For wear rings, plungers, dies. Finished with carbide tools and grinding.” Also, a B:Cr ratio of 1:5 by weight indicates a high volume fraction of boride.

It seems to me that your problem is due to 2 effects: Lack of diffusional bonding to the substrate 316 SS and extremely high residual stress (perhaps thermally induced hoop stress) within the Colmoloy. Your grinding opens many metal-ceramic interfaces which act as crack initiators for brittle fracture.

Since you did not experience the problem before, there must be some change in the surface preparation, Colmonoy powder, application (e.g., fuel/air ratio if oxyacetylene), or diffusion bonding heat treatment & subsequent cooling. An oxidizing flame would obviously lead to oxidation of the 316 and hence lack of bonding; a reducing flame could create excessive Cr carbide. If the Colmonoy powder had too much initial oxide, this would prevent sintering.
My guess is that there is excessive ceramic phase, perhaps as surface oxidization which limited sintering of metal grains or else an interlinking ceramic network (carbide, boride, oxide) which reduced the CTE of the Colmoloy below that of the 316 SS, thereby creating the residual stress during cooling. This hoop stress was great enough that when flaws were introduced during grinding (& w/o good bonding to the SS), the Colmoloy ring exploded.

Like Metalguy suggested, get some labwork done. Look at the SS/Colmoloy interface, look at the Colmoloy fracture surface, and check the Colmoloy matrix for a fully dense metal phase and whether the ceramic particles are discrete (preferable) or networked (bad). You might also try to determine the CTE of the Colmoloy for comparison with 316 SS.

 

[jackboot]

The Colmonoy alloy is #62.

I have called the head-honcho at

http://www.wallcolmonoy.com

and explained the problem.

Here were his suggestions:

Our process to bond the colmonoy to the plungers was:
1. Preheat to 200F
2. Dust coat with colmonoy
3. Heat to 500F
4. Spray
5. Fuse

Now;
316L and other stainless steels can have some trace amounts of Aluminum. The percentage is so small that it will only show up in mass spectrometer - however, this Aluminum will form an oxide layer 1 um thick and ruin the bond between the two materials. Also, the coolant in the machines can have an anti-foaming agent that will form Silicone Oxide and have the same effect. Titanium will also form oxides that will cause the same problems.

So do this instead,

1. Shot blast and degrease
2. Immediately dust coat to cover surface to prevent oxidation
3. Preheat 400F
4. Spray and fuse

I will report back to this thread if we have success doing this.

jackboot

 

[gkim]

jackboot:

I think your problem may be related to excessive heating of the part during deposition. There may be a CTE mismatch between your coating and substrate; your coating probably has lower CTE than the substrate. When the part cools, the substrate will shrink more than the coating; thereby, leaving you with a compressive residual stress within the coating. The higher the process temperature, the higher the compressive stress. Excessive compressive stress can lead to the coating exploding off the surface. Try to reduce the heat input into the part by external cooling or parameter changes.

Good luck.

gkim

 

[jackboot]

gkim,

That may be a factor as well. We had been heating the part to bright red for the fusing of the colmonoy (950 F). We had discussed this as being part of the problem - but we have done this type of process for approximately 30 years. Never, Never, have we seen the likes of failures experienced recently.

Notions of reducing vs. oxidizing flame have been beating to death - but the welder claims this how he has done it for ever. But the number of varibles are infinite - so we don't really know.

However, as a follow up - the suggestions (which include a fast fuse; that is not heating the part to 950F) have so far worked.

So, the problems may have been contributed to many factors in varing degress.

I appreciate the help from everyone.

jackboot