Spray fused coating cracks

[Arturas]

Hi all,
I have some problems with cracking in sprayed / fused coating(NiCrBSi HRC 60). Steel Din 34CrNiMo6, roll diameter 400mm length 2000mm. After fusing there occur longitudinal cracking on the coating surface. One of crack is from the end of the roll where we start fusing and much more of them are from the end we finished fusing. Fusing was done with torch (fuel gas/oxygen). After spraying the temperature of the roll was approximately 70C . During fusing roll was built up around with firestone bricks walls, so quick cooling was max reduced. Of course after we finished fusing temerature where we started fusing was only 300-500C .
Maybe someone can explain why cracks could appear.
Many thanks
Arturas

[derek]

Spray/fused coating "HRC60 !!" on that big part "400 mm x 2000 mm" will have some crack during process and after process during cooling. In order to avoid this problem, you can use 3-4 torch to heat the part while processing, keep the part temperature at least 600C, and cool down the part as slow as possible (use thick fabric blanket to cover part). Hope you can solve this problem.

Good luck.

[Arturas]

Hello,
So you think that we have big difference of temperature from one and other end of the fused roll?
and if we will maintain heating with torches of fused part will help us to prevent cracking!
When we fuse 300mm diameter roll length 2000mm no cracks occur. So somehow it is related with expantion factor or with diameter. Please comment

Arturas

(01-26-2011 05:56 PM)derek Wrote:  Spray/fused coating "HRC60 !!" on that big part "400 mm x 2000 mm" will have some crack during process and after process during cooling. In order to avoid this problem, you can use 3-4 torch to heat the part while processing, keep the part temperature at least 600C, and cool down the part as slow as possible (use thick fabric blanket to cover part). Hope you can solve this problem.

Good luck.

[derek]

Because the heat expansion is different between substrate and coating, it will be very easy have crack on the surface of coating, the bigger part the easy crack occur. If I were you, I will spray the powder and use torch to heat the part up a while and starts fuse the coating. At the same time using other torch to keep the part temperature and then slow cool down.
The fuse sequence is also important. Maybe you can try not fuse from one end to other, from middle to end or two torch fuse at sam time.....

[Simon Kim]

I am quite agreed with Derek.
Maybe cooling gradient is different between dia 300mm and 400mm.
I have some experience for hard facing materials.
Rapid cooling is most often reason of the crack.
If you can adjust same temperature of 400mm roll surface temperature to 300mm roll, crack should not occured after then.
Post heating by torchs is good on coating surface for slow cooling.

I wish your success.

[Arturas]

Derek it is very important so I am trying to understand correctlly what you say. "I will spray the powder and use torch to heat the part up a while and starts fuse the coating" - here you want to say that during the spraying at the same time to use additionally torch for increasing temperature of the roll in order that max eliminate expansion factor of the substrate or you want to say that after spraying but before fusing the whole roll to heat up with torch???
Arturas

(01-26-2011 11:07 PM)derek Wrote:  Because the heat expansion is different between substrate and coating, it will be very easy have crack on the surface of coating, the bigger part the easy crack occur. If I were you, I will spray the powder and use torch to heat the part up a while and starts fuse the coating. At the same time using other torch to keep the part temperature and then slow cool down.
The fuse sequence is also important. Maybe you can try not fuse from one end to other, from middle to end or two torch fuse at sam time.....

[k09]

Hello Arturas,

Derek and Kin have already given you all the needed information on this. Although I donot have much experience doing this myself but just to be sure could you give me the following details.

1. Type of Roll for both Dia: Solid or Hollow?
2. You mentioned that the rolls were fused surrounded by firebrick. Any chance at all that the larger diameter could be causing any uneven heating conditions due to space constriction?

I have come across multi point preheating burners that could be employed for preheating. They would be more effective than single point burners.

Do Let us know if you were able to pin point the problem.

Regards

K09

[Arturas]

Hello,

1. That is solid roll
2."Any chance at all that the larger diameter could be causing any uneven heating conditions due to space constriction?" What here you wanted to say . Sorry please explain :-)
I am really intrested in multi point preheating burners so send me to email information about that. Thank you.
Arturas

(01-27-2011 09:51 AM)k09 Wrote:  Hello Arturas,

Derek and Kin have already given you all the needed information on this. Although I donot have much experience doing this myself but just to be sure could you give me the following details.

1. Type of Roll for both Dia: Solid or Hollow?
2. You mentioned that the rolls were fused surrounded by firebrick. Any chance at all that the larger diameter could be causing any uneven heating conditions due to space constriction?

I have come across multi point preheating burners that could be employed for preheating. They would be more effective than single point burners.

Do Let us know if you were able to pin point the problem.

Regards

K09

[Simon Kim]

Hi, Arturas
400mm diameter roll is bigger 78% than 300mm diameter roll.
So 400mm roll has bigger heat sink and needs more heat energy.
If your heating method is same as two rolls, bigger roll has lower temperature.

Simon

[k09]

I agree with Simon.. In terms of surface area there would be more area for heat dissipation and therefore faster cooling.

Secondly, solid rolls would require higher and rather continual heating to maintain temperature at higher level.

I am not sure if this is practical but if you are able to arrange burners below the roll along the entire length, only for pre-heating; it might give results.

Regards

K09

[derek]

OK, Let me make it clear if don't understand what I am talking about.
Grit blast the part => spray the part=> preheat the part => fuse the part at same time keep the temperature using torch (try to make the whole part same temperature) => finish fuse and post heat the part => heat the part try to slow down the cooling rate.

[Arturas]

Many many thanks
Arturas

[Arturas]

Hello K09 ,

My email arturasbabilius@gmail.com
I am looking forward to hear from you about burners

Arturas

[Gordon]

Hi Arturas

Busy thread Like all who have responded, the secret is temperature control and very slow cooling rates.

Quote:Of course after we finished fusing temperature where we started fusing was only 300-500C .

This is the clue, really all of the part should be kept above ~650C when fused, then very slowly cooled only once fusing is completed. Ideally, keeping part around 650C for about 1 - 2 hours before cooling should (with DIN 17200/34CrNiMo6 - 0.34%C, 1.5%Cr, 1.5%Ni, 0.2%Mo steel) allow for most of the steel to complete its transformation safely before cooling below this temperature. This just ensures that no low temperature transformation stresses add to the normal stresses induced by the cooling effect alone.

Provide adequate pre-heating so core does not sap heat from surface to rapidly after fusing. As suggested keep heating on pre-fused areas to stop temperature dropping below ~650C. A very slow cool, better still an isothermal arrest at about 650C for an hour or two before slow cooling.

[Stephen Booth]

Really interesting post. Good information. This is a very challenging application, and if the process fails, that is lot of work and materials to remove the cracked coating and apply a new coating.

Could you switch to plasma welding. Sure it will have some dilution, but it some ways it is easier to control the process than spray fuse, and you can focus your attention on controlling the cooling and thermal expansion. Probably also would not need so much preheat either.

[kschewe]

I would think the best way to fuse the coating would be in a vac furnace.

[Arturas]

No cracks!! Thank you for all info