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)
   
Reply
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?
Reply
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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07-27-2022, 11:53 AM,
#33
RE: 16C-NS cracking issue
(05-10-2021, 04:10 PM)Vadim Verlotski Wrote:
(05-10-2021, 12:55 PM)Lavkesh Wrote: Hello Sir ,
We do cooling very slow,but still Longitudinal cracks are genrate in the coating .
please tell me What to do?

Regards,
Lavkesh

If the cooling is slow (it was not clear from the first post), then the matter can be much more serious. This is because the coating material has a higher coefficient of thermal expansion than the substrate, which leads to tensile stresses in the coating upon cooling after reflow. Usually this difference in thermal expansion does not cause cracking on slow cooling, so I recommended slow cooling. However, if, even with slow cooling, tensile stresses destroy the coating, then the problem may not be solved at all with this powder material, which is very hard and does not tolerate tensile stresses well. If I were you, I would try to repeat the manufacture of a part with a less brittle coating of the NiCrBSi system, for example, Höganäs 76-W-50 or Höganäs 72-W-40 (the hardness of 74-W-60 is 58HRC, the hardness of 76-W-50 is 50HRC,  and 72-W-40 is only 37HRC hard).

Dear team,

we doing the thermal spraying induction fusing of the NiCrBSi powder for the plunger .

we facing the porosity issue. and some cracking issue . If i go more than 1020°C then small pin hole was created after machining.

1) Please suggest me how the control the porosity.
2) what is the fusing temp for the induction fusing .
3)how to eliminate the sagging issue.
4) what is the prefect temp range in the induction fusing of the NiCrBsi powder / Hogan's 74-w-60.
5) we doing the steel grit blasting #14 and surface roughness before spraying is the 11micro -mm
6) coating thickness is 1.00mm .
7)induction preheating temp is 150°C
8)spraying torch is metco 6P-2
9) Induction fusing Rpm -80 and travel speed is 6mm/sec
10) total length of the shaft is 500mm
11) induction fuse 1st pass temp is 400°C 2nd pass 600°C 3rd pass 750°C 4th pass 875°C 5th pass 975°C.
12) slow cool in the air .

please suggest us the reason for the pinhole and porosity .

Regards,
Lavkesh


Find

[img=1000x1000]PHOTO-2022-07-26-11-06-26[/img]
Reply
07-27-2022, 11:54 AM,
#34
RE: 16C-NS cracking issue
(07-27-2022, 11:53 AM)Lavkesh Wrote:
(05-10-2021, 04:10 PM)Vadim Verlotski Wrote:
(05-10-2021, 12:55 PM)Lavkesh Wrote: Hello Sir ,
We do cooling very slow,but still Longitudinal cracks are genrate in the coating .
please tell me What to do?

Regards,
Lavkesh

If the cooling is slow (it was not clear from the first post), then the matter can be much more serious. This is because the coating material has a higher coefficient of thermal expansion than the substrate, which leads to tensile stresses in the coating upon cooling after reflow. Usually this difference in thermal expansion does not cause cracking on slow cooling, so I recommended slow cooling. However, if, even with slow cooling, tensile stresses destroy the coating, then the problem may not be solved at all with this powder material, which is very hard and does not tolerate tensile stresses well. If I were you, I would try to repeat the manufacture of a part with a less brittle coating of the NiCrBSi system, for example, Höganäs 76-W-50 or Höganäs 72-W-40 (the hardness of 74-W-60 is 58HRC, the hardness of 76-W-50 is 50HRC,  and 72-W-40 is only 37HRC hard).

Dear team,

we doing the thermal spraying induction fusing of the NiCrBSi powder for the plunger .

we facing the porosity issue. and some cracking issue . If i go more than 1020°C then small pin hole was created after machining.

1) Please suggest me how the control the porosity.
2) what is the fusing temp for the induction fusing .
3)how to eliminate the sagging issue.
4) what is the prefect temp range in the induction fusing of the NiCrBsi powder / Hogan's 74-w-60.
5) we doing the steel grit blasting #14 and surface roughness before spraying is the 11micro -mm
6) coating thickness is 1.00mm .
7)induction preheating temp is 150°C
8)spraying torch is metco 6P-2
9) Induction fusing Rpm -80 and travel speed is 6mm/sec
10) total length of the shaft is 500mm
11) induction fuse 1st pass temp is 400°C 2nd pass 600°C 3rd pass 750°C 4th pass 875°C 5th pass 975°C.
12) slow cool in the air .

please suggest us the reason for the pinhole and porosity .

Regards,
Lavkesh


Find

[img=1000x1000]PHOTO-2022-07-26-11-06-26[/img]


Attached Files
.pdf   PHOTO-2022-07-26-11-06-26.pdf (Size: 94.13 KB / Downloads: 111)
Reply
07-27-2022, 11:59 AM,
#35
RE: 16C-NS cracking issue
Hi Lavkesh, glad to see you posted here.
Reply
07-27-2022, 01:01 PM,
#36
RE: 16C-NS cracking issue
(07-27-2022, 11:59 AM)Stephen Booth Wrote: Hi Lavkesh, glad to see you posted here.

Powder hoganas 74-W-60
Surface preparation #14 steel grit
compressed air 3.5bar blasting air pressure
Spray gun is Metco 6P-2
powder hardness is 60 Hrc
base material is AISI 1035 and coating thickness is 1.00mm .

we doing manual blasting on shaft and result is good.
we are using the automatic blasting machine and this machine don't have the grit classifier.
so we don't get the result .
Reply
08-13-2022, 07:34 AM,
#37
RE: 16C-NS cracking issue
Dear Lavkesh.
I have been spraying and fusing self-fluxing NiCrBSi alloys for quite a long time and I clearly understood one thing: surface preparation for spraying is the most important factor in the quality of the coating. Spraying and fusing errors also affect the quality of the coating, but their influence is much less than the influence of surface preparation. In your case, I see exactly the problem with insufficient surface preparation.
What requirements must be observed in order to achieve optimal surface preparation? I will simply list these requirements, and you compare them with what is happening in your production.

1. The surface of the part must be cleaned of all contaminants and thoroughly degreased (water or non-aqueous detergents).
2. Before sandblasting, the surface of the part should be heated to a temperature of 80-120°C.
3. Sandblasting abrasive must be fresh (dust-free), clean, dry and never contaminated with oil. To ensure the latter requirement, it is necessary to install two fine oil filters in series on the compressed air pipeline.
4. The abrasive needs to be changed very often.
5. After sandblasting, the part must not cool below 50°C. It must either be sprayed immediately or stored in a heating cabinet.
6. Cooling air for spraying must be free of water and oil.
Reply




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