crack in NiCr

[Panf]

Hi all,
I need help. We need to make dublelayer coating - NiCr+Stainless steel by HVOF GTV K2. The thikness of first layer must be 0,7mm, the thikness of Stainless steel - 0,5mm. After NiCr spraying we havn't any problems, but during stainless steel spraying we had strange problem - the coating cracked when the thikness of SS was 0,1mm. The crack was unusual - it was not axial and not radial. The lenght of crack is 40...50mm. Who did anybody have similar problem? What you may suggest me?

Thank's

PS substrate material is Steel 316L

[Stephen Booth]

Hi Panf:

A couple quick points, assuming you equipment is in good working order, and the coating parameters are accurate for the base material and the powder. I would suggest the following to be considered:

1. Increase the rotational and traverse speed of the process, so you make thinner passes of the ss and the NiCr
2. Then the other suggestion is to a) increase the preheat of the substrate prior to spraying, to about 200C, but keep the interpass below about 300C during the build up or B) control the interpass below 150C or even better lower during the entire process.

Remember SS has a high thermal expansion, so if you start spraying relatively cool, and finish relatively hot, the expansion would crack the coating.

Of course there are many other factors, but this is my 2 cents

[landemarre]

Hi all,
I need help. We need to make dublelayer coating - NiCr+Stainless steel by HVOF GTV K2. The thikness of first layer must be 0,7mm, the thikness of Stainless steel - 0,5mm. After NiCr spraying we havn't any problems, but during stainless steel spraying we had strange problem - the coating cracked when the thikness of SS was 0,1mm. The crack was unusual - it was not axial and not radial. The lenght of crack is 40...50mm. Who did anybody have similar problem? What you may suggest me?

Thank's

PS substrate material is Steel 316L

Hello

This a problem of thermal stress. You need work on substrate temperature during process (try to have the most constant temperature on it), and this is not easy with hvof gun. Your thickness is too high for me. You have to work on NiCr and stainless coating because residual stress is already in your NiCr and probably if you do an higher NiCr thickness you will have the same trouble.

OL

[Panf]

Hi Stephen Booth
Hi landemarre
I was sure, to decrease residual stresses during spraying of coatings with high thickness I need to use NiCr for example. I was sure this material has low level of residual stresses and the application of this material is build-up. If I wrong, please, tell me what can I use for build-up.
P.S. the coating must be corrosion resistance in ammonia.

[Stephen Booth]

well how about a NiCrBSi fused coating?

[Panf]

Unfortunately it's not possible because we can't to heat up this detail to 1000 degree.

I attached the photo of crack.
   

[Gordon]

Hi Panf

Sorry for taking so long to approve your attachment

If after taking previous posters (Stephen Booth and landemarre) advice on controlling temperature and coating thickness per pass, you still have problems, you will need to look at either: reducing thickness of coating or changing coating materials to higher thickness tolerant materials. Stephen's suggestion of NiCrBSi may not be a bad coating in the HVOF as-sprayed condition (not fused). NiCr and many austenitic stainless steels tend to be high shrink and form stressful coatings and can be problematical on thick coatings. A NiAl or 13Cr steel would be much better for building-up the majority of thickness if you need to go this thick or thicker. There is probably a wide choice of top coat materials resistant to ammonia (gas or ammonium hydroxide?) you could consider, depending on what other properties you want from the coating.

[Panf]

Thank's a lot.
Dear Gordon, could I ask you about 13Cr steel which is sprayed by HVOF. Does it need to spray the bond coating before spraying one? If it needs, what material do you advise as the bond coating?

[plasmajet1990]

Hi Panf

Sorry for taking so long to approve your attachment

If after taking previous posters (Stephen Booth and landemarre) advice on controlling temperature and coating thickness per pass, you still have problems, you will need to look at either: reducing thickness of coating or changing coating materials to higher thickness tolerant materials. Stephen's suggestion of NiCrBSi may not be a bad coating in the HVOF as-sprayed condition (not fused). NiCr and many austenitic stainless steels tend to be high shrink and form stressful coatings and can be problematical on thick coatings. A NiAl or 13Cr steel would be much better for building-up the majority of thickness if you need to go this thick or thicker. There is probably a wide choice of top coat materials resistant to ammonia (gas or ammonium hydroxide?) you could consider, depending on what other properties you want from the coating.

Hello,

Your picture shows a plunger type PERONI or URACA used in an ammonium (NH3) or ammonium carbamat (NH2-CO2-NH4) factory.
If the plunger is used for ammonium it is recommanded a NiAl + 13 Cr Arc Spray recondition.
For ammonium, usualy, the plunger is not made of SS.
If the plunger is made of SS, I think it is used for pumping ammonium carbamat.
The ammonium carbamat is a verry corrosive product.
That is why I do not recommand you to use 13 Cr.
Are you shure that the plunger works with ammonium?

Ion

[Panf]

Yes, you are right. This is plunger URACA. And it works in ammonium carbamat (NH2-CO2-NH4). I said the plunger is made from SS 316L, because we wanted to spray stainless steel with NiCr bondcoat. We had no problems on first and second plungers. But... the coating broke on third plunger.

[plasmajet1990]

Yes, you are right. This is plunger URACA. And it works in ammonium carbamat (NH2-CO2-NH4). I said the plunger is made from SS 316L, because we wanted to spray stainless steel with NiCr bondcoat. We had no problems on first and second plungers. But... the coating broke on third plunger.

Hi,

OK.
For ammonium carbamat, SS 4435.2 (2 mins a SS without ferrite) is very good.

Regards,
Ion

[Panf]

Hi plasamjet1990,
thanks for your answer. We had some experience in repairing Uraca's plungers. Early we sprayed FeCrNiMoSiC coating by HVOF to substrate which was made from 316L. However, sometimes the unlayer corrosion were observed. After change the coating material on FeCrNiBSiC this problem was finished. We have low porosity, not bad efficiency and high adhesion that's why we wouldn't like to change a method of spraying.
Now we need to build-up big thikness. But I afraid to make 3 layers coating (Ni-Al + SS 4435 + FeCrNiBSiC). We use this material (FeCrNiBSiC) because it has high hardness ~800HV, so the life time of plunger more then 3...4 times.