16C-NS cracking issue
05-12-2021, 06:14 AM,
#21
RE: 16C-NS cracking issue
Can you explain what is cooling by N2 in this example?  N2 @ 1 bar?  Seems that would be relatively fast cooling.  We usually slow cool from the fusing temperature to room temperature over a period of 24-48 hours, in an insulating environment like vermeculite or glass wool.
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07-16-2021, 07:50 AM,
#22
RE: 16C-NS cracking issue
why porosity accur in NIcrbSi power fusing
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07-16-2021, 12:05 PM,
#23
RE: 16C-NS cracking issue
The melts of most substances have a larger volume than the volume of these substances in the solid state, therefore, when the melt is cooled, shrinkage cavities (closed pores) appear inside it. The greater the difference in the volume of the melt and the solidified material, the greater the porosity. For eutectic alloys NiCrBSi, a very large shrinkage during crystallization is characteristic, therefore, in the fused coatings there is always a characteristic closed porosity.
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07-17-2021, 10:53 AM,
#24
RE: 16C-NS cracking issue
(05-09-2021, 11:20 AM)Lavkesh Wrote: Hi memders ,

what is the solution to minimize the porosity ,please suggest your better opinion .


Regards ,
Lavkesh

(07-16-2021, 12:05 PM)Vadim Verlotski Wrote: The melts of most substances have a larger volume than the volume of these substances in the solid state, therefore, when the melt is cooled, shrinkage cavities (closed pores) appear inside it. The greater the difference in the volume of the melt and the solidified material, the greater the porosity. For eutectic alloys NiCrBSi, a very large shrinkage during crystallization is characteristic, therefore, in the fused coatings there is always a characteristic closed porosity.
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07-17-2021, 11:10 AM,
#25
RE: 16C-NS cracking issue
The best way to reduce the shrinkage porosity of melted NiCrBSi self-fluxing coatings is to reduce the cooling rate of the melt. Unfortunately, this is only possible with reflow in a furnace (retort furnace with protective atmosphere or vacuum furnace). When the coating is melted in air with torches or an inductor, the possibilities for reducing the cooling rate of the melt are very limited.
The second way to reduce shrinkage porosity is to use fillers. The addition of other refractory powders to the NiCrBSi powder, for example, tungsten carbide or molybdenum, makes it possible to reduce the porosity of the coating even upon rapid cooling of the melt.
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07-23-2021, 12:01 PM,
#26
RE: 16C-NS cracking issue
can we control porosity at the time of spraying ?
which is the better gas either Argon or nitrogen for the powder feeder
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07-23-2021, 01:01 PM,
#27
RE: 16C-NS cracking issue
The porosity of the finished, fused NiCrBSi coating has nothing to do with its porosity after spaying. These things are not connected with each other. It is also irrelevant which carrier gas (argon or nitrogen) was used during the deposition of the NiCrBSi powder.
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07-30-2021, 04:12 AM,
#28
RE: 16C-NS cracking issue
(07-17-2021, 11:10 AM)Vadim Verlotski Wrote: The best way to reduce the shrinkage porosity of melted NiCrBSi self-fluxing coatings is to reduce the cooling rate of the melt. Unfortunately, this is only possible with reflow in a furnace (retort furnace with protective atmosphere or vacuum furnace). When the coating is melted in air with torches or an inductor, the possibilities for reducing the cooling rate of the melt are very limited.
The second way to reduce shrinkage porosity is to use fillers. The addition of other refractory powders to the NiCrBSi powder, for example, tungsten carbide or molybdenum, makes it possible to reduce the porosity of the coating even upon rapid cooling of the melt.

Hi Vadim ,  your comment about additing Tungsten Carbide or Moly to NiCrBSi to reduce the porosity, that is interesting.  I will try it some time.  Its a bit counterintuitive.
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07-30-2021, 08:43 AM,
#29
RE: 16C-NS cracking issue
(07-30-2021, 04:12 AM)Stephen Booth Wrote:
(07-17-2021, 11:10 AM)Vadim Verlotski Wrote: The best way to reduce the shrinkage porosity of melted NiCrBSi self-fluxing coatings is to reduce the cooling rate of the melt. Unfortunately, this is only possible with reflow in a furnace (retort furnace with protective atmosphere or vacuum furnace). When the coating is melted in air with torches or an inductor, the possibilities for reducing the cooling rate of the melt are very limited.
The second way to reduce shrinkage porosity is to use fillers. The addition of other refractory powders to the NiCrBSi powder, for example, tungsten carbide or molybdenum, makes it possible to reduce the porosity of the coating even upon rapid cooling of the melt.

Hi Vadim ,  your comment about additing Tungsten Carbide or Moly to NiCrBSi to reduce the porosity, that is interesting.  I will try it some time.  Its a bit counterintuitive.

Hi Stephen, the positive effect of fillers on reducing the porosity of remelted coatings has been known for a long time, as well as the possibility of decreasing porosity by directional solidification (controlled cooling of the melt). Both of these technologies are widely used in casting technology. As an example, I will give a comparison of the structures of fused coatings NiCrBSi and NiCrBSi-WC:


Attached Files Thumbnail(s)
   
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07-30-2021, 09:01 AM,
#30
RE: 16C-NS cracking issue
very interesting. thank you, do the fillers have an effect on cracking during cooling?
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07-30-2021, 11:44 AM,
#31
RE: 16C-NS cracking issue
(07-30-2021, 09:01 AM)Stephen Booth Wrote: very interesting.  thank you, do the fillers have an effect on cracking during cooling?

Yes of course. The coating with fillers becomes harder, but also more brittle than without them. In addition, both tungsten carbide and molybdenum reduce the effective thermal expansion coefficient of the coating, which becomes lower than the CTE of steel (the CTE of self-fluxing nickel alloys without fillers is higher than the CTE of carbon steels). Nevertheless, it is quite possible to obtain coatings from self-fluxing alloys with fillers without cracks, it is only necessary to prevent too rapid cooling of the part after the coating is melted. Reflow is best done in an oven with a protective atmosphere.
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08-04-2021, 02:43 AM,
#32
RE: 16C-NS cracking issue
thank you, I have learned some thing new about an area I thought I had some know-how
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