forged shaft flaws
forged shaft flaws
(OP)
What are folks' experiences with hot rolled 4140 shafting6 to 10 inch diameter being free of internal flaws?
I worked for a precision spindle company that made lots of steel shafts, usually from forged 8620, sometimes 4140 etc.
The only material flaw I recall was a seam or crack uncovered when a section of a fairly large (over 18 inch?) billet (Hot rolled?) was whittled down to a toothpick, relatively speaking.
My current employer has 6 foot long machine shafts made typically from annealed 4140, and includes an ultrasonic inspection, but would like to start skipping that step. I'm most concerned with surface flaws in radiuses, etc, so believe the finished shaft needs to be magnafluxed.
thanks
Dan T
I worked for a precision spindle company that made lots of steel shafts, usually from forged 8620, sometimes 4140 etc.
The only material flaw I recall was a seam or crack uncovered when a section of a fairly large (over 18 inch?) billet (Hot rolled?) was whittled down to a toothpick, relatively speaking.
My current employer has 6 foot long machine shafts made typically from annealed 4140, and includes an ultrasonic inspection, but would like to start skipping that step. I'm most concerned with surface flaws in radiuses, etc, so believe the finished shaft needs to be magnafluxed.
thanks
Dan T





RE: forged shaft flaws
I have not failed. I've just found 10,000 ways that won't work." — Thomas Edison
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RE: forged shaft flaws
RE: forged shaft flaws
if so I would recommend Mag & Demag at this point.
verify parts are good from the forging house.
a final mag of finished parts is mandatory.
RE: forged shaft flaws
If final parts are fully tested, which I assume they are, a simple cost analysis will give you the answer.
is intermediate NDT more expensive than additional costs associted with failure of NDT in final part (assuming same failure rate as you find now intermediately)???
RE: forged shaft flaws
Surface NDT as a minimum should be after all fabrication is completed. The soundness of the forging should be performed before machining in the event the forging must be rejected.
RE: forged shaft flaws
you are one of the most helpful on this board.
we all appreciate your Imput.
however in this case I will have to disagree in one point.
My experience both with mag & UT it is better to rough machine forgings then NDT test.
The rough machining removes all surface imperfections.
better results and no false indications.
In my line of work 99.9% of all parts for aircraft
are NDT after final machining?
RE: forged shaft flaws
Perhaps I was confused. I agree with you in that both volumetric examination and surface exams should be performed for critical components. I thought that was what I had stated in my second post, not the first post. Maybe what I should have added in the second post is that the sequencing of these exams is based on how your Level II or Level III examiner would like the surface prepared.
The point to all of the above is that eliminating volumetric examination needs to be carefully considered for rotating shafts. Simply relying on past vendor experience is NOT good enough. I always like the adage, trust but verify.
RE: forged shaft flaws
Thanks for the reply :>)
I agree.
RE: forged shaft flaws
We are puzzled as to why this was not detected before via UT. UT was performed at the foundry.
No way would I EVER accept a vendors assurances that rotors should be defect free and therefore skip volumetric examionation. Some vendors will ship you swiss cheese if they can talk you into accepting it.
That said, I'm wondering why volumetric examination failed us in this instance. It's looking like our delivery schedule is blown.
Best regards,
Tom McGuinness, PE
Turbosystems Engineering
www.turbosynthesis.com
RE: forged shaft flaws
I can give you one possibility having gone through a recent technical audit because we are purchasing a forging, not just a cast ingot, for a large turbine generator. The volumetric nondestructive testing for a large forging is usually conducted after forging and rough machining and is performed in zones.
For most reputable mills that supply rotor forgings for turbine or generator rotors, the UT is normally done using an automatic process. Some still use manual scans. The zones I referred to above are evaluated based on how critical the locations are relative to stress levels in service. My point to this is that there may have been different acceptance criteria depending on location in the rotor. I would review the procedure used by the foundry and their acceptance criteria for the entire rotor.
RE: forged shaft flaws
I've been reading of late about phased array UT and eddy current techniques. Do you have any suggestions as to the best NDE method for detecting core defects on rotor forgings? In this case the raw A470 Class 4 forging had a diameter of 9" in the bearing and seal areas and a finished diameter of 4". Today we are performing FPI to assess how many of the defects penetrate the surface.
Is there a good reference for acceptance criteria for rotor forgings? ASTM A470 limits linear defects to 0.125". In our case a few indications exceed this, but the entire shaft end is peppered with smaller defects. A photographic guide would be helpful.
Thank you for your suggestions.
RE: forged shaft flaws
rp
RE: forged shaft flaws
The best inspection method for rotor forgings is an automated UT system using phased array transducers. Nothing beats this system provided you have a qualified procedure, reference standard and certified examiners. For our rotor forging, we carefully evaluated the vendors NDT program and reviewed their acceptance criteria. The vendor also qualified their phased array procedure on actual portion of a rotor forging that contained defects. The acceptance criteria for critical zones in the rotor forging (after forging, heat treatment and rough machining) was smaller than 0.125". For the non-critical zones, 0.125" was used.
RE: forged shaft flaws
Please advise What type of steel & heat treat?
Parts where ground?
RE: forged shaft flaws
RE: forged shaft flaws
According to our inspector who witnessed the FPI inspection of the two rotors, "the shaft journal and seal areas looked like the Milky Way". In several areas, the FPI results lined up with the MPI results. Some surface indications exceed 0.500" in length. Despite this, the vendor continues to advocate a fracture mechanics analysis of the shaft as a possible means for determining shaft suitability for service...wishful thinking I suspect. The vendor would love to ship these rotors, and the customer would very much like to take posession of the machine prior to the scheduled shutdown.
In view of the unusually large number of confirmed surface defects, I have to wonder at what concentration of subcritical defects does fracture mechanics analysis cease to be a valid method for determining shaft integrity? I am far from convinced that fracture mechanics analysis is an appropriate tool for this situation. Any thoughts/comments?
Best regards,
Tom McGuinness, PE
Turbosystems Engineering
www.turbosynthesis.com
RE: forged shaft flaws
According to our inspector who witnessed the FPI inspection of the two rotors, "the shaft journal and seal areas looked like the Milky Way". In several areas, the FPI results lined up with the MPI results. Some surface indications exceed 0.500" in length. Despite this, the vendor continues to advocate a fracture mechanics analysis of the shaft as a means for determining shaft suitability for service...wishful thinking I suspect. The vendor is desperate to ship, and the customer is desperate to take posession of the machine prior to the scheduled shutdown.
In view of the unusually large number of confirmed surface defects, I have to wonder at what concentration of subcritical defects does fracture mechanics analysis cease to be a valid method for determining shaft integrity? I am far from convinced that FMA is an appropriate tool for this situation. Any thoughts/comments?
RE: forged shaft flaws
I agree. I would not recommend you accept the new forgings under stated NDT results. I deal with large steam turbine rotors and having to disposition a new forging with this density of defects including your statement is unacceptable. If I were the Purchaser, I would not accept the rotors. My questions is this, was there an acceptance criteria for the new forging? I have done numerous audits and for the large turbine OEM's, most have very conservative acceptance criterion for new forgings with no disposition of flaws using fracture mechanics. If the forgings are found to contain rejectable defects by volumetric UT, they are rejected.
RE: forged shaft flaws
FPI-Fluorescent Particle Inspection?
MPI-Magnetic Particle Inspection-OK this one I get
FMA-Fracture Mechanics Analysis?
Understand that a mag indication is not a defect unless it exceeds the acceptance criteria. What is the accpetance criteria for the shafts you are inspecting? If the indications exceeds those, they are defects and must be removed or the shaft must be rejected.
It sure sounds as if what you are seeing are non-metallic inclusion stringers. It is rare that these are detected with dry powder MPI, but with wet flourescent MPI, they can be seen. Even with wet MPI, it is rare that such indications would exceed 0.125 inch, but I've seen it. To detect such imperfections with a volumetric UT would be very rare, unless they are much larger than you are describing.
If these are non-metallic inclusions, and they are oriented parallel to the direction of principal stress, they may not be all that detrimental. I have accepted such imperfections in motor output shafts (4-6 inch diameter) with no problems reported. I'll admit that the reasons I accpeted them included that I had no justifiable reason to reject them; the manufacturing specification was not written very well and required wet mag to inspect for forging defects, quench cracks, etc..., which these definately were not. I will add that after seeing them, and talking to the mill that produced the steel, I started requiring AMS 2300 cleanliness testing on the bar stock used (aka "Aircraft Quality") because I wasn't entirely comfortable with accepting them. You might consider a dye penetrant inspection to convince yourself there is not any depth to these indications. This may or may not help you with your decision.
I don't think I'd put much weight behind a fracture mechanics analysis, since the big concern these would cause me would be initiation points for fatigue cracks, which fracture mechanics would not consider.
The big question that will need to be answered, of course, is who is going to pay for them? Unless the vendor can be shown why they don't conform to the specs he was given, he is going to want to get paid. Of course, the customer would be better off to scrap them (eating the cost) if they are going to fail in service, but he won't be happy about it.
rp
RE: forged shaft flaws
MPI - magnetic particle inspection, which can be dry powder or wet fluorescent if specified.
FPI - fluorescent penetrant inspection - like Liquid PT except slightly more sensitive.
RE: forged shaft flaws
1. Wet Fluorescent Magenetic Particle Examination
2. Water Wash Fluorescent Penetrant Examination
The specified shaft material is ASTM A470 Class 4 forged steel. I am told that the ASTM standard itself establishes a maximum linear defect size of 0.125". The vendors own acceptance criteria is 0.125" maximum defect size.
Excert from the latest inspection report;
"Under Wet Fluorescent Magnetic Particle we could repeat the indication mapped out Tuesday night. I had the rotor turned 180 degrees and performed the Mag Particle once again. A number of additional indications noted. These indications also looked at by "John Doe" Level III and agreed with the findings. Some of the linear indications are up to .500 long.
The next step was to perform the Water Wash Fluorescent Penetrant. The results on the governor end looks like the Milky Way many dots on the surfaces. Some of these patterns of dots are in line with each other. The dot pattern is in the same direction as the indications noted with the Mag Particle inspection. In my opinion these are the starting and/or ending of the linear indications noted on wet mag. If you look close on the mag indications you can see a bright dot and then a fuzzy line or tail running from the dot. I also did note one surface linear indication about .140 long. These two rotors do have surface indications."
The operating loss resulting from delayed shipment of these machines is enormous, thus the reluctance by the end-user to reject them without rock solid technical justification. The case for rejection looks pretty darn solid to me, but my neck isn't on the line.
RE: forged shaft flaws
If the vendor could show that the indications are irrelevant, it might be different, but the fact that they were revealed as exceeding the acceptance criteria using two NDT methods, there really isn't any other option.
I still am of the opinion, however, that it is quite possible that UT inspection of the rough machined forging would not reveal these types of imperfections. Steel cleanliness and forging practices are what will control these.
rp
RE: forged shaft flaws
The above-mentioned Level III is an employee of the vendor. Can you say "conflict of interest"?
The vendor has located a rough forging we can use from another job of theirs. I've suggested that before machining any other areas of the forging that they rough machine the journal and seal areas to within 0.25" of finish ID and then perform the same NDE as above. This should allow us to quickly verify the condition of the forging.
I completely agree with you that quality cannot be inspected into a forging.
RE: forged shaft flaws
In recent months we've found several cracks in various types of turbomachinery components merely by examining areas where the vendor pressured us not to look. It's gotten to the point that if the vendor suggest there's no justification for looking at some area of the rotor, that's exactly where I want to look.
So many coincidences.
RE: forged shaft flaws
All of a sudden third party inspections sound like a good idea.
RE: forged shaft flaws
Whatever you do, be sure to maintain a competitive environment for suppliers of key components, thereby allowing you to re-allocate sourcing in response to vendor performance with minimum delay. Do not depend on your suppliers to provide adequate source inspection for their subs. Wherever possible, never do business with any supplier whose QA department reports to production instead of the President/CEO. If a vendors internal processes appear incapable of recycling glaringly defective components for re-evaluation/scrap without your having to ride them every step of the way, run away as fast as you can.
Independent 3rd party inspection is indispensable these days. Even with vendors who are ISO 9000 certified....dare I say, especially with some vendors who are ISO 9000 certified. The relationship we have with some vendors is downright Orwellian.
Good luck and Caveat Emptor.
RE: forged shaft flaws
Sent my client a CYA letter today. Probably a futile attempt to avoid liability.
Might be time to start looking for a new gig.
RE: forged shaft flaws
Your line about where Quality reports to in the corporate structure really resonates with me: if Quality reports to Production/Manufacturing/Operations instead of to a separate Quality department, you can be assured that meeting customer requirements is not the top priority. Quality should never be beholden to Operations.
RE: forged shaft flaws
The turbine we're buying is to replace an existing machine that's been in service since 1985 and has never been opened. We could have performed an internal re-rate of the existing machine, but opted to replace since we have no idea what we'll find in there after 25 years of service. So now they want to install a machine with 0.500" cracks in the rotor and perhaps go for another 25 years with no plans or reason to expect there will be any follow-up inspection to monitor those cracks. They will run the machine till it fails. How does one continue work for a client like this when the expectation is for more of same?
RE: forged shaft flaws
I wouldn't be too terribly concerned about the shaft failing. You are on record saying the part is rejectable. I believe that is the correct opinion as it did not meet the inspection requirements.
They decided to use the part based on an analysis that determined that the imperfections revealed by the inspections would not be injurious. Maybe this analysis is correct, maybe not. In any case, if the experience an early failure, they won't be asking you why you accepted it. If they get lucky and it does last 25 years, well, they made a bigger profit and, therefore, can be expected to pay their technical people more. Win/win.
rp
RE: forged shaft flaws
Thank you for your words of wisdom. Much appreciated. I need to cut back on the caffeine. :)
In situations like this, the penalty for being wrong can be serious property damage, personal injury or worse. It's tough to be philosophical about it. But your effort to appeal to my sense of greed has found some traction. :)
RE: forged shaft flaws
RE: forged shaft flaws
He who has the gold makes the rules.
rp
RE: forged shaft flaws
This is a highly important subject but is probably better discussed elsewhere on the forum.