I am searching for a test to periodically evaluate product that was welded by Plasma Transfer Arc (PTA). The primary concern is the ability of a brittle material (60 HRc) to remain intact while bonded to a more ductile substrate (30 HRc).
The product can be considered a shaft with hard material on the outside diameter. It is subject to torsion, heat and pressure.
Is there a suitable nondestructive test that could measure this trait? If not, is there a standard mechanical test available?
Thanks,
Don
Mr. Cummings,
I have one client with your same problem. They too have several of their parts which have a thermal spray deposit and experience lots of wear. They too have not found a practicle test method for this type of bonding.
They have used limited ultrasonic testing (UT) on critical parts, but the client found this method to be quite expensive due to the nature and geometry of the products. Penetrant was found to be the most practical method for surface discontinuities, but does not verify the integity of the bond of the substrate and the applied coating (that's where the UT was applied). One of my previous employers used flame sprayed coatings to repair impeller housings. These were hydro-tested (water), but it was an effective test method for leaks only. Mag-particle (MT) may be used if the coating is of a ferrous material and will detect discontinuities which are slightly subsurface, but the method will not verify bond strength.
The client finally resorted to a comprehensive quality program with specific QC hold points at each step of the operation and QA inspection of the suppliers by a third party.
By the way, these particular parts are for an aircraft application (non-powerplant). Good luck, and please post any resolve to your query.
I don't know what kind of bond strength you are looking for, but there is an instument called an "Elkometer" adhesion tester. It is usually used to test adhesion of paint, but I think it goes up to 1 or 2 thousand PSI? Details are real fuzzy as it was a while ago. You basically glue on an aluminum button with high strength glue and pull it off with this machine. So your surface has to be relatively flat and big enought to accomodate the legs of this tester (3 legs about 3" apart), and the surface had to be rough engough for the glue to stick to the paint or sprayed metal better than the paint or sprayed metal stuck to the substrate.
Hope this helps.
Since PTA is normally used for surfacing, I am assuming that your deposit is some type of wear resistant overlay. PTA is a welding process and, as is the case with all welding processes, a true metallurgical bond is formed. PTA is sometimes confused with Plasma Spray (a thermal spray process) in which the primary bond mechanism is mechanical.
The most reliable test method to evaluate any welding process for bonding to the substrate is ultrasonic testing. Although lack of bonding is not normally a problem with PTA, improper welding procedures can sometimes lead to defective overlays. Inspection by UT is not normally accomplished on an as-welded surface but usually requires a machined or ground surface.
PT can sometimes be used if the ends of the overlayed shaft are machined but the inspection is limited to any lack of fusion type defects open to the surface and will not give you an indication of the general quality of the overlay deposit in the middle of the component.
Thanks for everyone's input. Yes, this process is a welding process. Ultrasonic testing seems to be a good choice to indicate that the hardfacing has bonded to the base material.
However, I am also searching for something rather unusual. I need to determine that the hardfacing material is bonded within itself as well. My concern is that some bond certainly exists, but may be too weak under enduser conditions.
Would you consider any of the Nondestructive Testing Methods to be reliable in the case? If not, is there a standard destructive test in our industry (perhaps involving bending)?
Don,
Are you talking about thermal spraying using plasma arc transfer? If yes then you can find some info on the subject in AWS's "Thermal Spraying", the C2.18-93, and the C2.16-92 books. Depending on the application you can do anything from a chisel test to an adhesion test using pull-off dollys, similar to those used on paint coatings.
CHGuilford
Our process requires a true metallurgical bond (true weld instead of thermal spray).
It seems that I need a test that involves a significant amount of force.
I have to admit, you've confused me with your terminology:
1. What specific process is PTA?
2. You mention you require a "true metallurgical bond", could you please define this term?
3. Does the deposit penetrate the "substrate" or base metal?
4. Your first post states you are "welding" to a substrate - which is a term used for thermal spray operations. Welded joints have base metals which are joined, not substrates.
5. Does your process use electrodes, filler metal or a deposit of metallic (or non-metallic) material?
Non-coloquial terminology (nonstandard terms) makes it difficult to help with your post. This has been stated previously in the Forum by others. I am very curious about your process (specifics). This is why my confusion...
Have you considered a torsion test or 4-point bending test? I believe there are ASTM standards for these tests. Or how about applying the coating to a pipe of same material type as the shaft and hydrostatic test to failure of the coating? You should be able to calculate the failure stress from the pressure, and pipe dimensions.
For a nondestructive test, UT is probably your best bet. RT would be difficult to detect bond failures since it relies on density changes and the shot would be pretty much perpendicular to the shaft surface. Eddy current may be an option, but that's a bit beyond my knowledge of NDE.
Marty
Thanks everyone.
It seems that a comprehensive quality program is appropriate. I think I will continue searching for an appropriate mechanical test (perhaps the bend test).
Take care.
-Don
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