Surface preparation and its effect on fatigue life and bond strength

[WMW]

We are involved in the repair of bearing surfaces on large rotating shafting subjected to reverse bending stress. We have a customer that is extremely concerned about the fatigue life of their equipment. The concern is that grit blasting the bearing surface prior to metallzing will adversely effect the fatigue life of the shaft. Does anyone know of a proven, adequate surface prepation method that can be used under a twin wire arc or HVOF coating? We are looking for a balance between enhancing fatigue life properties and the bond strength of the coating. Or is there any information available about the effects of a different metallized coatings on the fatigue strength of the part?

[Gordon]

Hi WMW

to the Surface Engineering Forum.

Sorry for late response, I think your thread was over looked because you placed it in the wrong forum section. I will move it to surface engineering, where hopefully it should gain more attention.

I can only respond to question somewhat generically in saying thermal spray coatings are said to have little effect on fatigue strength in themselves. Grit blast preparation is said to give a slightly negative effect fatigue strength. I think it may be dangerous to take generalisations and apply to specific applications and conditions. I have seen a number of papers on the subject over the years, some conflicting, but I think this down specific conditions of materials, processing and testing.

I have come across heavy shot peening of surfaces prior to light grit blasting, supposedly to compensate for fatigue loss effects of grit blasting.

Is there a history of fatigue failure with this component or is your customer being overly concerned?

[Viking]

We are involved in the repair of bearing surfaces on large rotating shafting subjected to reverse bending stress. We have a customer that is extremely concerned about the fatigue life of their equipment. The concern is that grit blasting the bearing surface prior to metallzing will adversely effect the fatigue life of the shaft.

This responce based entirely on observation (over decades),
and has no educated basis.

I have been suspisious for years of grit-balsting not necessarily always
being the best prep for a number of build-up tecniques.

Once I got involved in tigging my own repair build-ups
on tool-steels, I noticed that some blasting of some regions
on some tools gives poor wetting, or blocks it entirely in corners
and tight regions as I try to get the corners to collapse and
receive filler.

I did some test blasts at angles from very near paralell to
the tig-target area, to perpendicular.
Also nozzel up close to far off.
When tigging blasted regions where close nozzle or perpendicular
or both were used, I'm convinced that blast media has been
imbedded into the tool-steel.

This could interfere with any bond no?.
But as for fatigue.......on large shafting,
professional (uneducated) "opinion" is
very unlikely. I have seen all manner of failures.
And nothing to suggest or support that concern.
Viking.

[Gordon]

Hi WMW and Viking

I have been suspisious for years of grit-balsting not necessarily always
being the best prep for a number of build-up tecniques.

When tigging blasted regions where close nozzle or perpendicular
or both were used, I'm convinced that blast media has been
imbedded into the tool-steel.

This could interfere with any bond no?.

From the point of view of thermal spray coatings and no doubt other coating processes that predominantly rely on mechanical keying for bonding, grit blasting is extremely important. Processes involving metallurgical bonding such as welding could be very different though.

[drramc]

Hi,
The grit blasting supposed to produce compressive residual stress on the surface of the component.Hence grit blasting as such should improve the fatigue life to an extent depending on the quantum of residual stress generated.Hence I believe that there should not be any loss of fatigue strength by grit blasting.
ram chattopadhyay