spray and fuse substrate problems

[pakalano]

Good afternoon all - I was hoping to find some help with a failure issue we are having. The part is a rotating shaft that was typically chromed in the past. The customer wanted to gain more wear resistance so we opted for a spray and fuse nickel/chrome. The sustrate is a low carbon steel. History showed the part only failing from wearing out. Since using a spray and fuse process (same exact substrate) the part is actually shearing off under the same load conditions. There is indications of lateral cracking though the coating; the shaft was twisting under load and the coating did not. Actually the results of a lab test showed the cracks actually penetrated the substrate slightly.
Has anyone had experience of a metelurgical change in low carbon steel that would shear just from the changes occuring during spray and fuse? The parts were air cooled, not insulated, but there were no apparant cracks before the part was put in service.
Any help would be appreciated.

[Gordon]

Hi Pakalano

More detailed information on the coating, the steel substrate and spray and fuse operation may help.

Alternatively, you could try HVOF/HVAF WC/CoCr coatings as a replacement for hard chromium plate. Advantage here would be that you will not be changing properties of the substrate.

Fusing typically will involve temperatures above 1025 C, so some metallurgical changes in steel substrates will be inevitable to some extent. The steels composition and previous metallurgical condition will be important.

[pakalano]

Hello Gordon,
Thanks for your response. The substrate material is 1018 cold rolled. The coating used is a Eutectic 23045 spray and fuse coating. It is applied with the part hanging on a vertical positioner and as it is rotating lowers into a pit in the ground (its a long piece). Once sprayed, the part is induction fused at 1910F. The part is 1.4" in diameter. Once the fusing process is complete, the part is cooled in ambient air (20deg C)
I agree with the idea of HVOF applied tungsten. I have been trying to convince the customer of just this. From my perspective it is 100% the way to go, but I must explore all avenues of the spray and fuse issue as that is what I've been hired to do. The plan is to spray the same coating and not fuse it; even with the porosity left on the part we still will have the wear properties needed and we can apply a sealer if required.
I am going to try and obtain a material report from the foundry for this material. then I can report what its exact makeup is and hopefully that will help.
Brad

Hi Pakalano

More detailed information on the coating, the steel substrate and spray and fuse operation may help.

Alternatively, you could try HVOF/HVAF WC/CoCr coatings as a replacement for hard chromium plate. Advantage here would be that you will not be changing properties of the substrate.

Fusing typically will involve temperatures above 1025 C, so some metallurgical changes in steel substrates will be inevitable to some extent. The steels composition and previous metallurgical condition will be important.