WC Coating Craze Cracks

[demionkrishan]

Dear team,
What can be the real cause of WC craze Cracks?

Steps Below
1. Base material Pre grounded ( Used normal grinding wheel)
2. Grit blasted (Mesh size 20)
3. Applied Tungsten Carbide coating (WOKA 3652 500 microns)
4. Final Grinding (Diamond wheel)
5. Super Polishing

Please look at the attachment, I have carried out the process as usual. gave proper cooling.... I do not see any issues in process. Can anyone able to find the root cause? please leave your valid comments.

Regards,
Krishz    

[Vadim Verlotski]

Hi Krishz,
Such a crack pattern "crackle" arises when the layer is under high tensile stresses. These stresses form in a layer during cooling from temperature of the particles (1300-1500°C) to temperature of the substrate (100-200°C). By cooling the part you can not reduce these tensile stresses; only by means of small single layer thickness (quickly mouvement), these tensile stresses can be reduced.

Strangely enough, good cooling can even be counterproductive because compressive stresses form in the layer during the cooling of hot parts from coating temperature to room temperature, which partially or completely compensate for the abovementioned tensile stresses. In this sense, for coatings with low coefficients of thermal expansion (WC-Co and WC-Co-Cr), pre-heating the component to approx. 200-250°C is recomended (you should keep the part at this temperature throughout the coating process!).

In your case, I recommend you to do both at the same time: faster movement (higher number of revolutions) and adjustment (reduction) of the cooling. In the first point the rule is: "the higher the speed the better", but in the second action you should be careful - the part must not get hotter than 250°C.

Regards
Vadim

[plasmajet1990]

Hi,

I agree with Vadim's explanation.
Also you have to verify the diamond wheel it is possible that the diamond could be worn-out.

Regards,
Ion

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[Stephen Booth]

HI Demion

I also agree with Vadim, and Ion both have good information. If you search this site some you will see this information reconfirmed in many places. Typically you are seeking a very thin layer per pass with the WC and often too with the CrCNiCr material.

Diamond grinding belts might be a better bet, but I think the deep problem is speed and heat.

BTW looks like a mandrel? Its pretty common that several of the drilling contractors would use dye penetrante as their "go to inspection" tool.

(04-10-2018 06:14 PM)plasmajet1990 Wrote:  Hi,

I agree with Vadim's explanation.
Also you have to verify the diamond wheel it is possible that the diamond could be worn-out.

Regards,
Ion

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