steel quality industry "standards" 4

[Tmoose]

A steel shaft supplier has made some comments regarding a mag particle requirement in one of our internal standards.

".........Unfortunately, 1/16" is better than the "cleanest" steel that's made. Below is the typical indication lengths for various steel qualities.

Commercial Quality - there's no linear indication requirement
AQ (aircraft quality) - 1" - 1.5" max indication
VAR (vacuum arc remelt) - 1/4" max indication (this is the cleanest steel)

No steel is perfect and all of it will have some indications in it. Our mill has committed to 1/8" max indication without having to specify a cleaner grade of steel such as AQ or VAR. .............."

Is this "commercial quality" something that exists in some definition or even standard somewhere?

thanks

Dan T

 

[Maui]

The details regarding the test method and equipment, the nature and orientation of the flaw, and the material grade will have a decided impact on their capability to detect flaws below a certain size range. For the successful detection of a discontinuity, there must be a field of sufficient strength oriented in a generally favorable direction to produce strong leakage fields. For maximum detectability, the field set up in the part should be at right angles to the length of a suspected discontinuity. This is especially true if the discontinuity is small and fine. The characteristics of a discontinuity that enhance its detectability are:

1.) Its depth is at right angles to the surface
2.) Its width at the surface is small, so that the air gap it creates is small
3.) Its length at the surface is large with respect to its width
4.) It is comparatively deep in proportion to the width of its surface opening.

Many incipient fatigue cracks and fine grinding cracks are less than 0.025 mm (0.001 in.) deep and have surface openings of perhaps one-tenth that or less. Such cracks are readily located using wet-method magnetic particle inspection. The depth of the crack has a pronounced effect on its detectability; the deeper the crack, the stronger the indication for a given level of magnetization. This is because the stronger leakage flux causes greater distortion of the field in the part.

Maui

http://www.EngineeringMetallurgy.com

 

[Tmoose]

Hi Maui,

thanks for the info.

My question should have been clearer that the supplier's description of acceptable indications for various classes of steel ( Commercial, Aircraft, and VAR ) is new to me.

Their suggestion that ANY sized indication is acceptable in "Commercial" steel was downright surprising. Heck, when grading hardwood beams " Splits shall not exceed in the aggregate in inches in length twice the surface measure of the piece..........."

Dan T

 

[redpicker]

The problem here is the difference between an "indication" and a "defect" or "rejectable imperfection".

What your supplier is referring to is maximum acceptable lengths of inclusion stringers in various inspection grades.

What is called "commercial quality" does not have any requirements for inclusion rating with MPI, which is what he means when he says there is no linear indication length requirement. This is common for "air melt" steels. There are inclusion rating standards for air melt steels (astm E45, for example), they just do not use MPI as the acceptance criteria. There is a quality of air melt steels qualified with MPI under AMS 2304, known as "para-premium". The limits are similar to those for AQ, but a little looser. This is not very popular since applications that are suitable for air melt steels generally do not require this level of cleanliness.

The next, "Aircraft Quality", is qualified per AMS2301, has the inclusion content qualified by MPI. These are the limits he has quoted. The specific procedure is in the standard, but basically a sample is machined from a sample billet and inspected. Not only is the maximum length of any indication revealed, but a maximum frequency (number of indications) is also specified. It is important to note that these indications are only from inclusion stringers, not laps, seams, or other cracks in the material. The standard specifies specific processing to eliminate those from the analysis. It is generally understood that vacuum degassing is required to meet this quality.

Finally, is the VAR/VIM-VAR quality, or AMS2300. These are re-melt steels where the secondary melting is done to remove more of the inclusions. This makes the material very expensive as it has to be processed twice, both times under very controlled conditions. This is generally reserved for high-strength critical applications (such as high-pressure valves) where the cost of the material can be justified.

As mentioned above, these indications are only applicable to non-metallic inclusions. The discontinuities Maui describes (grinding cracks, fatigue cracks) are considerably more harmful than inclusions because they have residual stresses and/or fatigue damage associated with them that is not easily detectable with MPI (or other NDE methods), which is why such small indications are considered rejectable; these type of imperfections should not be accepted if discovered as it will always be cheaper to scrap the part than pay for the results of a failed part.

What is needed, in my opinion, is a declaration of the types of imperfections that is intended to be rejected by the inspection. While there are applications where inclusion stringers are detrimental, those are the applications where the manufacturing standards require the inclusion testing qualification testing. And, these inspection qualification standards do not imply that these are the minimum length of indications that can be rejected (as your supplier seems to indicate). Aircraft parts are manufactured and inspected all the time to 1/16" maximum length acceptance criteria. But, the inspection criteria is limited to specific orientation, depth, or location of the indication and not only the length.

For example, in your shaft example, longitudinal inclusion stringers generally do not result in initiating failures, particularly if they are not very deep, but transverse indications can be very detrimental. Likewise, longitudinal laps or seams probably should not be accepted (adequate machining allowance should be given to remove surface imperfections). Transverse imperfections, on the other hand, can be disastrous, even if shallow.

While I am not sure what your internal standards are calling for, if they are rejecting material for longitudinal stringers, then they probably have the result of trying to inspect quality into the product (meaning, trying to produce to a higher quality level than the processes are capable of). While your supplier is correct in that longitudinal inclusion stringers cannot be avoided if you are using air-melt steels, the implication that the material may contain cracks of unlimited length is not correct.

rp

 

[Tmoose]

thanks for all your input.

I'm familiar with automotive Mag particle inspection "standards" that permitted some level of longitudinal indications (or maybe even ignore them altogether)when inspecting new or used connecting rods, so would be on board with suggesting a similar revision to our standard, with wording differentiating between inclusions and "other" types of indications, and especially transverse versus longitudinal "indications" if the stress group says that is appropriate for loading and service of our various sized machines.

The original authors ( a well meaning PE, and an experienced and generally very practical PhD Engineer/Metalurgist) are not readily available for comment to determine the source and history of the mag particle inspection, and some other parts of the standard that are also problematic today, 4 years after the std was issued.

thanks,

Dan T

 

[redpicker]

TmooseI think you are on the right track, but a few more things to think about if you pursue this. Most NDT inspectors will balk at the idea of trying to differentiate between an indication produced by an inclusion and one produced by a crack. To them, they look the same. All they know is length, location, and orientation. They can probe for depth, but that is about it. This means you have to come up with an acceptance criteria that will accept non-injurious imperfections and reject the injurious ones. The only times I've personally had to deal with this problem, I had the inspectors go ahead and reject the parts and I'd evaluate them on a case-by-case basis. I had to do this becasue I wasn't comfortable with telling the inspectors to accept all longitudinal indications under 1.5 inches (which was really my only other option). I don't recall any than I did not eventually accpet. While probalby not your specific application, these were high-strength (150 KSI yield) shafts seeing high torsional and bending loads and I never heard of any service failures resulting from this.

Good Luck.

rp