tensile properties

[metalman]

Hi,
I am currently investigating a failed ductile iron casting.

During the investigation it appears that the casting has been reworked during manufacture to build up material on a dimensionally incorrect surface using some kind of metal spraying method. The metal had been sprayed onto an external surface that is under tension, generated by hoop stresses from a pressed in insert.

My knowledge of metal spraying is pretty sparse, but where I have come across these technologies it has not really been loaded in tension. I have seen bearings, piston rings and reworked cylinder heads, flanges, manifolds etc - generally surfaces that are loaded in compression.

My questions are -
1. Is a metal spraying rework a good idea in this instance (My gut feeling would be no)
2. Is there any data on tensile properties, elongation to failure, yield point etc on these type of layers. My ductile iron substrate has clearly elongated when loaded, but not yielded, whereas the spray coating did not seem able to and has fractured.

I do not know the coating type but do have metallographic analysis, including EDX available to me if I need to identify it further.

Thanks in advance.....Metalman.

[Gordon]

Hi Metalman

to the Surface Engineering Forum.

Just to be clear, has the substrate been stressed before or after coating?

In many cases thermal spray coatings tend to have residual surface tensile stresses mainly due to shrinkage during build-up. The amount is related to coating thickness, material and process. A few high velocity/lower temperature processes HVOF/HVAF/Cold-spray can reverse this trend however, producing compressive stresses.

Is there any data on tensile properties, elongation to failure, yield point etc on these type of layers.

Generally, not a lot. These properties in an unsupported/separated coating will be very much lower than your substrate. Thermal spray coatings in most cases do not add any significant strength to the part as a whole, they are used purely to alter surface properties. They are useful for dimensional recovery, but not for recovering any strength loss.

It does appear that you are working the part beyond the coatings capabilities. It would be useful to know coating type, thickness, substrate surface preparation etc. as this will be a factor.

Also, can the part be worked/stressed before coating?

[metalman]

Hi Gordon,
Thanks for your reponse.

I will try and clear a few things up. The casting was purchased from a foundry and failed when a mating part (that is an interference fit) was being pushed into it. It appears that the foundry added the coating to correct up a dimensional issue.
When the interference fit part was being pushed into it the casting the casting has been subject to a strain within the elastic limit of the base material, but the strain was obviously too much for the coating.

I have also had a chance to look at the coating. It is built up of 4 or 5 layers with an altenating composition. The first layer is aluminium/nickel, this layer is generally around 200-400 microns thick. The altenating layer is low alloy steel, the layers of this are generally thicker 600-800 microns.

Cheers,
Metalman

[Gordon]

Hi Metalman

1. Is a metal spraying rework a good idea in this instance (My gut feeling would be no)

I think I would have to agree. Although, I think spraying to restore dimension after insertion of interference fit part may be ok.