×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
I'm investigating root cause for Fluorescent Magnetic Particle Inspection (MPI) rejections in 4340 Alloy Steel forgings.
The indication are linear that's why they are rejectable. There are no segregations, cracks or non-metalic inclusions in tested material. The microstructure is normal.
Some parts don't have any indications some parts have them sporadically in various places in various sizes. All linear indications are oriented in direction of grain flow. It is puzzling.
One of the rejected regions have been excised from the forging and aggressively etched. After etching you can see ridges (raised streaks) appear on the surface. The questionable areas apparently etched at slower rate comparing to surrounding areas. Part is approx. 12" dia machined tube. 1/4 wall. The whole part is about 40" long.
See attached photo of 2 different parts, one showing MPI indications (streaks), second showing parts section after etching showing ridges (raised streaks).
Please help explain where the streaks come from.
Is it non-homogenous material? (Vacuum arc re-melt)
Is it the forging process that creates those phenomena?
Is it something else?
Thank you.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Has the material been Q&T?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
What's Q&T?

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
Yes. The material has been quenched and tempered to 38 HRC.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
We didn't see MPI indications in additional inspection, after forging and prior to H/T. We see MPI indications after H/T. We can't identify any variables in H/T process.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Could it be a local microstructure difference because of some surface contamination in HT?
How much material do you have to remove for these to go away?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
Parts were clean going to H/T. MPI is done after H/T and machining off 0.125 inch. It doesn't go away, after removing 0.040. or even more.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
Some parts don't have significant streaks. some have a couple. some have many in various places. It is puzzling. What is it? Why it reacts to etching different that surrounding material?

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Could be delta ferrite. How are you magnetizing? Do you perform a full demag before hand?

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

You may need to section through one of the indications to check for evidence of quench cracks. Also, how was the 0.125 removed, by grinding or turning? You may also want to check your dies for evidence of marks that could be transferred to the part.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
Swall,

We are de-magnetizing prior to MPI. We magnetize using central conductor (head shots). We MPI to Boeing spec. HP6-5.
How do we tell if it is in fact delta ferrite? How to rest for it?
What are causes for delta ferrite?

Thank you.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
Dbooker630,

We did section a couple of these and we see no cracks, no segregation, no non-metalic inclusions, and the microstructure looks normal. WE are turning the parts. We are using state of the art carbide inserts made by Sandvik.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Have you tried re-heat treating any of these?
Do you normalize before hardening?
And perhaps a double temper.
I am leaning more toward a chemical segregation issue.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
EdStainless.

We did not try to re-heat treat. We are bound by frozen process. We could try to re-heat treat a sample.

We heat stock to 2000 deg F. (2350 max.) Then, Forge and air cool.

We do Normalize before hardening at 1650 deg F.
Temper at 1250 deg F.
Austenitize at 1525 deg F.
Quench in oil. 120 - 140 deg F.
Temper 1000 for 5 hrs, cool in still air.

How can we proof that this is a chemical segregation issue? Material supplier and another lab tested samples and reported normal microstructure.


RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

That would take some SEM work (BSE with element mapping).
Not too difficult

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Given that report I am more likely to question your MPI methods.
Are you sure that your MPI is the correct sensitivity (and not too high)?
If you clean these and have another tech re-test then are all of the indications the same size and in the same place?
There is nothing there.
End of story.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
We questioned the MPI method first. We lowered the amperage to achieve the lowest Gauss reading permitted by the spec. The indications do come back each time. they come back after we de-mag and re-mag.
They also come back after we remove 1mm (.039 inch) of stock, by turning. We had level 3 NDT technicians and Metallurgist look at those. NDT can't accept the parts. Metallurgists say there is nothing wrong with material.
What is it? Why etching reveals ridges?
Boeing will not accept parts, that did not pass MPI, and not knowing what that indication is. This maybe an "irrelevant MPI indication" but how to explain it? Many years, hundreds of parts, we did not see them, now more and more parts have them. 10% of part and increasing...

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
EdStainless,
By saying "there is nothing there" do you mean that is no "chemical segregation"?

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

No crack, no change in microstructure, no chemical segregation issue.
Honestly, if you have a technical contact at Boeing ask them to look over the package.
This can't be the first time that they have seen this.
And there is certainly nothing wrong with the material.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
For Flight Safety Part program any doubt is not acceptable. Boeing has also evaluated a sample - no defect found but there is no will accept without knowing what that indication is. Material will be scrapped, most likely. MPI can't accept parts assuming those are non-relevant indications if they look like possibly subsurface linear defects. Obviously, we can't evaluate without cutting the part up.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
If this was Delta Ferrite, as someone suggested, we should see it in the metallurgical report, correct?

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Yes, residual delta ferrite would have been plain in the SEM work.
Have you looked at these using any other NDT methods? (UT)
Not that it helps these parts much but it might help inform what the cause is.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
Parts are UT per standard process and they all pass.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

You might ask your UT guys to look at them again, and to crank up the gain to see if they get any signal even just a bit more background noise.
I don't think that they will.
The chemistry, grain size, and hardness is uniform.
The only thing that I can think of is that somehow (maybe during a forge re-heat) areas got to hot and almost melted.
As they resolidified from a slushy state they developed a preferred grain texture.
This is why you can look at polished and etched samples under a microscope, different grain orientations etch at different rates.
Just fishing here.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Rafal,
There is currently discussion in AMS to allow the use of UT or MPI.
It looks like it is going to included.
Depending on the steel the UT will either be AMS2154 class A or AA.
And this will be for specs that also require micro-cleanliness.
This UT will be 100%
The ballot should be out very soon for the first spec.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
Ed,

Boeing will not entertain any other method of evaluation/acceptance for the forgings.
At least not without knowing exactly what the cause of indications is.
The goal is to find the root cause and eliminate it to prevent re-occurrence.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
Could this be "permeability variation" due to "Chemical banding", "chemical segregation"?
I understand that that the Met report doesn't show any problem.


"The only thing that I can think of is that somehow (maybe during a forge re-heat) areas got to hot and almost melted.
As they resolidified from a slushy state they developed a preferred grain texture."


How can we detect this "preferred grain texture"?

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Your reports don't show any chemical issues, so preferential orientation does become a suspect.
This can be checked with XRD or some specialized SEM methods.
I have never done it on an SEM but I have seen the figures generated.
You are measuring the orientation of each grain

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
Ed,

We will look into that.

Thank you very much!!

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

I am sure that you know but this could matter because magnetization is highly orientation dependent.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

question #1) was there any metal removal post heat treat prior tp MPI, MRB should rework parts remove less than .020 inch then re MPI.
question #2 ) so these appear to be surface imperfections. 4340 must be tempered within short time , don't remember the time but less than 5 hours after quench.
any one monitor the parts and source inspect during heat treat. also is the parent material being protected by copper, vacuum or inert atmosphere.
or is it heat treated in open furnace. look at the heat treat procedures and make sure it is being run to spec.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

I'll tag onto that, Carbon Potential control in the HT furnaces.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
Parts are rough machined prior to H/T. H/T is in "Air & products of combustion" gas furnace. Parts are quenched in oil. It is a fixed process, since hardness in a critical characteristic. Temper is done per spec.
The H/T plan says: "Quench to approx. 250 deg F part surface temperature". I don't see what delay is before part are tempered.
Temper is at 950-1050 F for 5 hrs +/- 30 minutes.

The MPI indications appear only on some parts in the same H/T lot and material heat lot. That's why it is so puzzling. We can't find any correlation between defected serial numbers and any process parameters.

The indications look like subsurface indications. That is why they are rejectable at MPI. After re-machining (skim cut) the indications are still there.

We are thinking Alloy segregation may be the cause.

Does carbon potential apply to "Air & products of combustion" atmosphere furnace?

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

It could depending on how much excess air you have, and how complete your combustion is, and how much variation there is.
We were lazy, our production furnaces were Nitrogen+methanol and set up for about 0.2C steels.
We ran everything from 0.10 to 0.40 but found that setting up in the middle kept things working.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

rafal USA
the recommended minimum stock removal after heat treat is .060 stock per surface minimum after heat treat, how much stock was removed. there will be surface inspections on forging surface. the bigger the diameter the increase in stock removal

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Are Your materials, forging/heat-treat/NDI-processes 100% per Boeing specs [IE: BMS, BAC, BSS, whatever]... or purely AMS specs... or some hybrid mix of Boeing & AMS?

Does Your rough-machining allow sufficient excess surface-material for post-heat-treat finish-machining to absolutely get rid of all HT surface decarburization?

How was Your raw forging stock provided: billet, bar, extruded/rolled-tube, forged tube, etc? How much hot-working/drawing do You actually accomplish?

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Almost certain the streak is a paramagnetic, or a very weak ferromagnetic phase. The beauty of magnetic particle inspection other NDT cannot replace.
Residual austenitic phase? which is non-ferromagnetic. new bainite ? which maybe less ferromagnetic ( than martensitic). Residual ferrite could be excluded since streaks did not show up b4 Q&T, also i think ferrite and martensitic show pretty similar magnetism.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
Following is explanation I got from a group of metallurgists from Canada:

"After reading through your last e-mail and reviewing the report you sent, I still believe it is a segregation issue. One thing that did come to mind was retained austenite in the microstructure. You can check with your NDT people and get there opinion on whether or not retained austenite could cause the indications you get during the MPI. Austenite is softer than martensite which could help explain the slight depression you get at the indications when machining/polishing the surface. Also, austenite should be more corrosion resistant than martensite which would help explain why you end up with raised ridges at the indication sites after your aggressive etching. Evaluation for retained austenite by optical microscopy isn’t that accurate, you are better off evaluating retained austenite by X-ray diffraction."

and:

"If it is retained austenite, likely what is occurring is sufficient segregation of austenite stabilizing elements (nickel, manganese, carbon) in the material to locally suppress the finish temperature for the martensite transformation so that austenite is still present in the material after quenching. "


It makes a lot of sense to me. Looks like several experts think this is likely a segregation issue that results in retained austenite.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

(OP)
Another photo of MPI indications.

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Then show it to me in your Ni, C, Mn element maps.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

I believe the indications are:

Lüders band
From Wikipedia, the free encyclopedia
Jump to navigationJump to search
Lüders bands, also known as slip bands or stretcher-strain marks, are localized bands of plastic deformation in metals experiencing tensile stresses, common to low-carbon steels and certain Al-Mg alloys.[1] First reported by Guillaume Piobert, and later by W. Lüders,[2] the mechanism that stimulates their appearance is known as dynamic strain aging, or the inhibition of dislocation motion by interstitial atoms (in steels, typically carbon and nitrogen), around which "atmospheres" or "zones" naturally congregate.


Figure 1: Measured strain distribution of a tensile test (shape memory alloy) during loading and unloading shows moving Lüders bands. The measurement was performed with a LIMESS Digital image correlation system.
As internal stresses tend to be highest at the shoulders of tensile test specimens, band formation is favored in those areas. However, the formation of Lüders bands depends primarily on the microscopic (i.e. average grain size and crystal structure, if applicable) and macroscopic geometries of the material. For example, a tensile-tested steel bar with a square cross-section tends to develop comparatively more bands than would a bar of identical composition having a circular cross-section.[3]

The formation of a Lüders band is preceded by a yield point and a drop in the flow stress. Then the band appears as a localized event of a single band between plastically deformed and undeformed material that moves with the constant cross head velocity. The Lüders Band usually starts at one end of the specimen and propagates toward the other end.[4] The visible front on the material usually makes a well-defined angle typically 50–55° from the specimen axis as it moves down the sample.[5] During the propagation of the band the nominal stress–strain curve is flat.[4] After the band has passed through the material the deformation proceeds uniformly with positive strain hardening. Sometimes Lüders band transition into the Portevin–Le Chatelier effect while changing the temperature or strain rate, this implies these are related phenomena [4] Lüders bands are known as a strain softening instability.[5]

If a sample is stretched beyond the range of the Lüder strain once, no Lüder strain occurs any more when the sample is deformed again, since the dislocations have already torn themselves away from the interstitial atoms. For this reason, deep drawing sheets are often cold rolled in advance to prevent the formation of stretcher-strain marks during the actual deep drawing process.[6] The formation of Lüder bands can occur again with a deformation over time, since the interstitial atoms accumulate by diffusing processes called precipitation hardening (or aging).


Just a thought.

Best regards - Al

RE: Non-relevant (MPI) indications, 4340 Steel alloy, Forged, H/T to 170ksi (38HRC)

Lüders bands creatively explain the bands, or so called grain flow. but two questions:
1. material was normalized and austenitized, can the bands survived at these high temps?
2. is Lüders bands non-magnetic such that MPI treats it as crack

Retained austenite could be from low cool rate, or quench delay (i see this often happens). hardenability of 4340 is pretty good, slight austenite stabilizing element segregation seems less likely to hold austenite. i highly doubt element mapping will reveal any obvious segregation. Trying to run a magnetic hysteresis loop to see if any difference on magnetization and permeability.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close