NDT Acceptance Criteria for Bars and Forgings

[mechowl]

Good Afternoon,

I'm looking for standards that can be referenced to define NDT acceptance criteria for the following:

Rotating Shaft (Critical Component)
Material 4140
Test Specified - Ultrasonic Test and Liquid Penetrant Test

ASTM A788 (Standard Specification for Steel Forgings, General Requirements) does list criteria for UT, MPI, and LPT, however specifically states "Because of differences in manufacture, hot rolled, or hot rolled and cold finished bars (semi-finished or finished), billets or blooms are not considered to be forgings."

If the shaft was to be machined out of hot or colded rolled round bar, would this standard for acceptance criteria still be applicable or appropriate to apply? If not, are there any other ASTM standards that outline Classes of acceptance for bar (for UT, MPI, and LPT)? I have found an EN BS standard for UT of bars, but nothing from ASTM.

With regards to the A788 standard itself, what is the order of most stringent to least stringent UT level? I assume BR, DA, and then S.

Any comments with regards to what level's listed for acceptance are realistic?

Thanks

 

[metengr]

You need an acceptance standard based on service duty and not necessarily fabrication acceptance. This is why material specification will typically require an examination method but they will not provide acceptance criteria because this is end user, application specific.

You have one of three choices - either develop your own acceptance criteria based on type and magnitude of service stresses and acceptable flaw size using proper design margin, or try to find an international standard for rotating equipment or try to locate AWS D14.6, Specification for Welding of Rotating Elements of Equipment. The acceptance criteria for welds will be as stringent as required for the base material.

 

[mechowl]

Unfortunately I am not aware of any acceptance standards based on service duties for shafting. I will try to obtain a copy of AWS D14.6 to see what is listed, but I find it very hard to believe there are no articles or similar acceptance criteria on machine shafting. I wouldn't even know where to start as far as developing my own accepance criteria (however I would like to learn). If there are any other tips or suggestions they are much appreciated.

At this point I am going to call out UT and DPI - Class 3 for 100% of the shaft in accordance with BS EN 10308 and 10228. This is of course in absence of finding other relevant criteria.

Thanks

 

[metengr]

mechowl;
The relatively easy solution is to have no linear indications permitted on the shaft surface using wet fluorescent MT or Liquid Penetrant testing – this would a one type of acceptance criteria.

Ultrasonic testing of forged shaft material is more complex because now you need to establish some threshold reporting criteria because with UT you are using shear wave to look for internal forging defects or inclusions. With a solid bore shaft I would use the UT shear wave method to ensure no planar internal defects - cracks or bursts.

 

[redpicker]

Relevant I have a problem with the "no linear indications permitted" as an acceptance criterion for MT (or PT, for that matter). With MT, any accumulation of magnetic particles is defined as an indication, and a "linear indication" is one whose major axis is at least three times its minor axis. That means, any place where three particles line up on the surface is rejectable, an impossible requirement to ask of an inspector. With PT, because of bleeding, a linear imperfection can produce a non-linear indication, so accepting non-linear indications with PT may not be a wise choice.

A much better criterion for MT would be "no indications over 1/8" in length". I would need to consult with a Level III in PT to come up with an appropriate criterion for that method. Having a minimum length gives the inspector the ability to ignore the small stuff, most of which are non-relevant indications.

For UT, "UT shear wave method to ensure no planer internal defect - cracks or bursts" is not an acceptance criteria, it is an inspection requirement.

The short answer is that you really need to consult with a Level III inspector to determine what inspection methods and procedures are appropriate to be able to locate the imperfections that you are trying to detect.

rp

 

[metengr]

I was just looking through my copy of AWS D14.6 and I believe this will address your NDT examination procedure and acceptance criteria concerns.

 

[mechowl]

metengr

I looked in my library and it appears I don't have a copy of D14.6. By the sounds of it I should consider purchasing a copy of it and will consider that tomorrow.

redpicker

Is there any relevance to my past post about bs en 10308 and 10228 Class III acceptance?

THanks

 

[metengr]

By the way, using AWS D14.6, the acceptance for linear indications using wet fluorescent MT or Liquid PT defined as length more than 3X the width is 1/64" or less. Use a proper specification for rotating equipment, it is all there.