forged shaft flaws

[Tmoose]

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

 

[metengr]

The abbreviations above are usually expressed as

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.

 

[tgmcg]

The following inspections were performed after finish machining;

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.

 

[redpicker]

Per the inspection reports you have, I would have to tell the customer that I couldn't accept them. You have a report verified by a (I'm assuming certified) Level III stating that the indications exceed the acceptable limits, so you really don't have the ability to overturn it.

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

 

[tgmcg]

Redpicker,

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.

 

[tgmcg]

It's curious how our inspector turned the rotor 180 degrees and found indications substantially larger than previously reported by the vendor during unwitnessed inspections.

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.

 

[Tmoose]

Sounds like it's time to add a "supplier to provide one week notice prior to UT so a representative from engineering or quality can make arrangements to witness the test" clause to the terms of the quote. And, to line up another supplier.

All of a sudden third party inspections sound like a good idea.

 

[tgmcg]

Tmoose,

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.

 

[tgmcg]

Despite having sound forgings on hand which the vendor advises could be machined in time for the unit shutdown, my client appears to be settling for fracture mechanics analysis for disposition and possible acceptance of said flaws. So much for their deep and proactive commitment to safety and loss prevention. Who believes that crap anyway when push comes to shove?

Sent my client a CYA letter today. Probably a futile attempt to avoid liability.

Might be time to start looking for a new gig.

 

[TVP]

tgmcg,

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.

 

[tgmcg]

Sorry for my last post expressing my frustration with the situation. What does one do when the client chooses to take on such an unnecessary risk?

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?

 

[redpicker]

It's tough to have your technical evaluation overidden by short-sighted commercial concerns. But, that is the way of business. You are paid for your opinion and they ignored it. This isn't the first and won't be the last time that happens.

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

 

[tgmcg]

Redpicker,

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.

 

[TVP]

 

[redpicker]

It's not so much as an appeal to a sense of greed as it is coming to grips with the workings of the marketplace. You might like to think that those with technical training make the decisions concerning technical issues, but in fact, business will always follow the golden rule.

He who has the gold makes the rules.

rp

 

[tgmcg]

I fully understand the working of the marketplace. But when it comes to decisions clearly affecting human safety, technical professionals do need to take a stand even if it means resigning from ones position or reporting the matter to a higher authority. Technical professionals yielding to the "suits" on high-consequence reliability issues leads to avoidable tragedy, such as the space shuttle Columbia disaster and countless aircraft accidents due to skipped inspections, etc, etc. It's far easier to yield to the suits on lesser issues such as buying model "A" versus "model "B" machine because of purely economic reasons. Fortunately, most decisions we engineers get overridden on in our career lay entirely within the economic sphere.

This is a highly important subject but is probably better discussed elsewhere on the forum.