MT indication acceptance criteria vs inspector level rating 2

[Tmoose]

We need to improve a standard that requires mag particle inspection of a finished machined part.
Currently we say no indications over 1/16 inch allowed.
One shaft supplier objects, saying commercial grade shafting standards have no requirements regarding inclusions.
ASTM A576 and A29 are pretty vague.
A576 includes this-
"The bars shall be free of visible pipe and conditioned as necessary to remove injurious surface imperfections."

A29 slaps on the virtual handcuffs with this -
"The material shall be free of injurious defects and shall have a workmanlike finish."

API 687 - Rotor repair says this -
pretty darned small 0.015 inch long linear indications are always reportable
1/16" ( 0.063") long linear indications are the maximum acceptable on the loosest classification
1/32" ( 0.032") long linear indications are the maximum acceptable in critical areas.
crack like linear indications are always cause for rejection

http://www.everyspec.com/MIL-STD/MIL-STD-1800-1999/MIL-STD-1907_5732/

MIL STD 1907 says this -
no propagating discontinuities allowed
stringers up to 0.125" long are allowed almost anywhere
seams or laps up to 0.5 inch long are allowed on any unmachined surface

Is there a level of inspector certification qualified to determine if an indication is stringer inclusion or a lap, etc?

thanks

Dan T

 

[metengr]

Normally, ASNT Level II should be able. A Level III would.

 

[eyec]

what did you specify when you purchased the shaft; any acceptance criteria, Standard to meet, your own specification?

the vendor may just be wanting to increase the price of meeting your requirements. vendor standard and customer standard are some times cause for additional measures (costs).

 

[redpicker]

Consider for your acceptance criteria:
"All linear indications over 1/16" are to be reported. Cracks, laps, and seams are rejectable." This gives you a documented way of accepting the inclusion stringers that will meet your specification. If inclusion stringers are not acceptable for your product, you will have to use a higher quality steel (I would doubt that inclusion stringers are injurious for your product).

Are you dealing with new, freshly machined shafts, or inspection of used shafts? Inclusion stringers, which can produce an observable indicaion on a freshly machined surface, are not as detectable on "aged" surfaces. A lot depends on surface finish.

One of the problems is that the inspector is not going to be able to determine what causes the indication, only its location and orientation. I've seen PhD Metallurgists, after examination with optical and electron microscopes, argue whether or not a specific imperfection was a lap, seam, quench crack, or SC crack, so it is not realistic for a Level II inspector (with what, a HS diploma and a combined total of 48 hours worth of training?) to be able to accurately identify the source of any indication with just MT. A Level III should be able to opine on whether or not a specific MT procedure is capable of detecting these imperfections, but he won't have any better idea than the Level II (or the PhD's for that matter).

However, not many inspectors will argue whether a "crack, lap, or seam" is actually a crack, a lap, or a seam. You may get some that would argue that they can't tell a stringer from a seam, but I'd say those are likely ones that have not seen indications from stringers and/or seams.

rp

 

[gtaw]

One aerospace welding standard that I have worked with states that with regards to NDT, the acceptance criteria for the welded component shall be no more stringent than that of the base metal specification. Thus, if a linear indication of a specified length is acceptable for the base metal, the same holds true for the welded portion of the assembly. Likewise, if porosity of a certain size and frequency is acceptable in the base metal when examined by volumetric NDT, the same criteria applied to the welded joint.

It is not the inspector's responsibility to determine the cause of the indication and it is not usually part of their training. The Level II and the Level III should be able to identify the nature of the discontinuity that caused the indication, but destructive tests may be required to determine the extent of the discontinuity. Consider MT and PT; both methods are capable of detecting surface breaking linear discontinuities, i.e., crack, laps, cold shuts, etc., but neither test method can determine the depth of the discontinuity. The discontinuity is often unique to the manufacturing method and may result in similar indications associated with another manufacturing process. The inspector has to be provided with information regarding the material being inspected, the manufacturing method, service history, joint configuration, specific welding process used, etc. if the inspector is expected to provide the cause of the indication. Experience plays an important role in the inspector's ability to define the cause of the indication. If the inspector has limited experience working with the component being examined the discontinuity may be something he has not seen before.

A common mistake I have seen is the listing of "slag inclusion" on radiographic "reader sheets" when the welding process used was gas tungsten arc welding. Had the inspector viewing the film been told the welding process was GTAW and if the inspector was more familiar with the welding process he would have recognized slag inclusions are not possible with a welding process that does not employ a flux as part of the shielding mechanism.


Best regards - Al

 

[rustbuster]

NDT Technicians should always refer to indications as exactly that "linear indications". These methods cannot establish the reason they exist nor should a technician ever refer to an indication as a crack, lap, etc. as this requires metallographic examination.

 

[phighfill]

I am of the "Old School" (before politically correct linear and planar indications) where a crack was a crack. The MT indications go back to the particular codes that gives the criteria for recognizing and identifying the discontinuities. If you have an MT Level II individual that can not distinguish the difference between a very "straight" line of fusion and a "jagged" line that is representative of a crack then they need more training.