UT inspection of 410 SS clad with 309

Date 07-19-2007 21:59

I don't UT stainless steel and I'm no expert, but I have read articles that list coarse grain boundaries in stainless steels producing reflectors that can be confused with those produced by typical weld discontinuities. In other words, the coarse grain boundaries act as reflectors.

Are you calibrating the UT equipment with a calibration block of the same base metal as that being test?

You haven't stated where the reflectors are located? Are you using a straight beam transduce to check the base metal in the through thickness direction and are you seeing any reflectors? Are you using an angle beam and if so, at what depth/distance are you seeing the reflector?

Al

By hogan

Date 07-19-2007 22:20

what is the joint configuration and the thickness? what is the accessability to both sides of the weld?

By Preco1 Date 07-20-2007 14:28

The joint configuration is a straight butt joint with 2 different thickesses of base material. One side is approximately .350" thick and the other is about .250" with .100" of 309SS PMIG welded on the bottom of that. The accessability is an issue. We are only able to get at it from one side since there are vanes welded in on the other side.

By hogan

Date 07-20-2007 14:37

if i'm understanding you, you have access to the 410 side only. can the you grind the weld flush. then ut it in the first leg, two directions. followed by 803056's suggestion of straight beam. this will take the interface out of the area of interest (sound path to indications).

By Preco1 Date 07-20-2007 14:25

I believe the cal. block is of the same material as the base metal of the part. The reflectors/indications are located about .180"-.200" down from the surface of the weld and are slightly off the centerline of the laser weld. In our UT process, we are not inspecting the weld in through the thickness direction. We inspect in a manner perpendicular to the thickness direction.

Thanks,
Jeremy

By dmilesdot (**)

Date 07-24-2007 14:45

The UT techs should be able to draw up a full size sketch using acutual thickness and by plotting the metal path to the indications. If their calibration is accurate, (dead nuts accurate), they should be able to tell you exactly if the indications are at the interface or are located elsewhere.

By CWI555

Date 07-25-2007 00:02 Edited 07-29-2007 05:05

DM welds are hard to UT, I believe your hitting on the correct idea. (edit) I had thought there may be a reflection problem, but the transmission for those two materials at shear velocities comes out a bit more than 98 percent. (4*z1*z2*100)/((z1+z2)sqrd) z= acoustic impedance = p(density g/cm3)*velocity.

That is assuming constants. Other possible causes would be grain structure and or heat affected zone. It is possible to run across a grain boundry that gives a localized area of dissimilar acoustic velocities. Since the acoustic impedance formula depends on density and velocity, your reflectance and transmission percentages can be skewed. This would be what Al was refering to. Given that potential material coefficients are proofed out, and you have cut and found nothing, it is very much likely the case at this time.

I've ran across that personally on several occassions. A good test for this would be to shoot a refracted longitudinal. If it is a grain structure issue, the difference between the compressional L wave, and the transverse Shear wave will show it'self. In otherwords, your apparent reflection will drop if not go away entirely when a longitudinal beam is passed through the same area. Refracted L wave transducers can acomplish this test with some ease.

For these same reasons, welds of most kinds, carbon, ss, etc that experience multiple repairs are subject to haz reflections and enlarged localized grain structure.
Your signal to noise ratio should be going up if thats the case.

Regards,
Gerald

By NDTIII

Date 07-28-2007 05:55

Gerald hit the nail on the head. S/S is a very coarse grain material. Especially S/S cladding which is like a casting. Shear waves will reflect, refract and attenuate quickly.
You should use refracted longitudinal waves. You calibraton block should be the same as your components.
The indications you are seeing could probably be from the shear waves refracting straight down in the cladding or could be from the veins that are attached to the ID. If you still see them with rfracted L-Waves, then that is what they are. Be careful using L-Waves though. Don't get confused and mistake the shear component with the L-Waves.
You should be able to verify them with a 0 degree probe.

By CWI555

Date 07-29-2007 05:38 Edited 07-29-2007 08:53

Good point, There are multiple components to a refracted L wave. creep, shear, direct, secondary creep, a round trip signal depending on geometric relationships, and sometimes surface depending on the ducer. In all cases, a model of the exam would be necessary before beginning the exam.

As a very very brief description of them:
(all effects depending on refracted angle)

Creep waves are often an ill defined wave form, depending on what side of the pond your on. They are a surface following longitudinal waves, with a max penetration depending on material of 12 to 16mm.

The shearwave component is in most refracted longitudinal ducers with varying angles depending on the direct signal angle.

Direct is the primary angle of the refracted longitudinal. In that it is a compressional waveform, it is ideal for large grain structures such as stainless steel, inconel, hastalloy etc.

Secondary creep, using a for instance, a 70 degree RL passing through a material the L wave hits the other side of the plate or tube in question and due to impingement angle (snells law) mode converts into a weak creep wave following the secondary surface and shear wave. It is very unreliable for detection of anything, and usually only of use in automated applications.

Round trip (using a 70 again), only exist when the primary component hits a reflector. if that reflector is in the central 2/3's of the wall thickness it can reflect the L wave back
hitting the back surface on the same side. It then mode converts into a shear component, and a secondary creep. The creep will not be received, but depending on the material, the refracted shear can make it's way back to the transducer and appear on the screen. Given that the L wave is faster than the shear wave, the time line will only reflect the L wave velocity. Adjustments for change in velocity for the shear leg have to be made and gated accordingly. (one down L wave, reflect back at L wave, then shear mode conversion completing the round trip) The shear component is sometimes called a head wave.

Surface or Rayleigh waves are potential as well and easily damped. Not usually a concern as they are easily eliminated.

In all cases, using Refracted L waves require a proper inspection plan, proper reference blocks (preferably FBH and notches) and modeling.

I'm glad you brought that up, looking at all those signals can be problematic at best for someone who's never used an RL.

Regards,
Gerald

By NDTIII

Date 07-29-2007 13:46

I thought of another question. Have you tried to plot the indications you see with shear waves? Although with the cladding that could be misleading.

By js55

Date 07-30-2007 18:47

May I ask, as non UT guy, can you UT another location away from the weld to see if the cladding in that location gives you the same reflection?
If it is the cladding layer, wouldn't it do the same thing everywhere?

By CWI555

Date 07-31-2007 01:54

(Assuming I understand the question properly) If a the material is 410 with 309 clad moving away from the weld will give a different internal geometry. Sound passing through the weld are will be passing through bi directional haz, (haz from clad haz from weld) from two axis. (which btw, that cross over area of this is another potential grain level reflector).
Then there is the weld itself, That will give a third dimension to it all. Then other geometric factors. In short it is unlikely going somewhere else will do much good.

By NDTIII

Date 07-31-2007 03:47 Edited 07-31-2007 04:07

Another thing we used to experience with S/S is beam redirection in the weld. A S/S/ weld has similar grain structure to a casting and will redirect the sound beam straight down into the root of the weld and give you an indication that appears to be in the HAZ but is actually nothing more than a root signal.
Have you tried plotting these indications to see where you think they may be showing up?

By CWI555

Date 07-31-2007 04:04

Another good point, and yet another reason to approach with an RL instead.

By js55

Date 07-31-2007 13:28

CWI555,
Maybe I'm misunderstanding, and perhaps there is something fundemental about UT I am ignorant of. And though I think what you say about the geometry is certainly not something I would argue since I know very little of UT, it seems to me the original issue is whether or not the cladding layer is causing a problem, and not anything asssociated with the weld geometry. Wouldn't the geometry of the caldding layer be the same throughout?
There will be no cladding layer at the joint root, though it will be immediately adjacent, and this is the assumed problem. It seems logical that if the caldding layer is interfering somehow it is doing so from a point outside of the actual weld itself. And if so why wouldn't farther out result in a similar response?
At this point, with due recognition of my ignorance of UT, I still think that a cladding influence can be logically eliminated or included depending upon the results.
If the problem is HAZ related as you conjecture, then its not the cladding. If it has anything to do with weld geometry,then its not the cladding.

By hogan

Date 07-31-2007 14:58

each time the sound passes through an interface of two different materials the angle that the sound is traveling is changed.

By jwright650 (*****)

Date 07-31-2007 15:09 Edited 07-31-2007 15:12

Snell's Law. http://en.wikipedia.org/wiki/Snell's_law

The difference in velocities between the two different materials is what hogan is talking about, it changes the refracted angles...then you won't know where your sound is.

By CWI555

Date 07-31-2007 22:26

I did not say that it was in fact the Haz, only exemplifying the multiple boundaries the sound travels through. As pointed out by hogan and John, snells law comes into play here, as well as Z=impedance (Z equals density(gm3) x velocity), potential Transmission coefficient problems ( T = (4*Z1*Z2) / (Z1 + Z2) squared) * 100, Frequency\wave length problems Especially true of higher frequency ducers for large grain structures.

Trying to proof the problem out by just going to a weld free area would be insufficient to prove one way or another if it was the cladding. The fact that there is no cladding area at the joint root will be problematic by itself. It is feasible that the sound is turning down at the cladding interface and that is assuming it could even make it through. The wave length formula is WL = velocity/frequency. As that formula shows, the Wave length will vary because velocity is a variable in the formula. Sound is faster in the Longitudinal mode with Shearwave being roughly half that of Longitudinal. The wavelength will be smaller for the shear, and when coupled with the transverse particle motion of a shear wave, gets into scattering and absorption issues/attenuation.
Longitudinal on the other hand is a compressional wave, and less susceptible to the deleterious effects on the pressure wave due to particle motion.
This is why an RL will always be better at 1st leg exams for large grain structure materials.

Without having the actual piece in question in my hands, I can't tell you for sure what the problem is, but I would lay even money on it's a combination of events aggravated by trying to use a shearwave probe.

If you have a scope available to do an experiment with, you can try putting a 5mhz shearwave probe on an inconel 625 plate or hastalloy, and watch what happens. (flange forgings work well for this to). Then put a 5mhz straight beam probe on to the same material, and make note of the differences in the base line hash. Repeat the same with A36.

With the right combination of equipment, training, and background, it's not uncommon to pick out intergrannular stress corrosion cracking in Inconel and other large grain materials and welds. I've done as has many others. DM welds in nukes to be exact (stretching memory), A600/309butter/308 filler/316 stainless. Mapping that alone was an all day affair.
it's only good for the first leg at that.

By NDTIII

Date 08-01-2007 04:04 Edited 08-01-2007 04:16

The original question was "Has anyone else experienced faulty UT indications due to the different material types? If so, what was done to remedy the situation?" Actually we need more information. He didn't state where the transducer was located when he saw the indications. Where are the veins located and what is their orientation? From what side of the weld are you seeing the indications? Is it the clad or unclad side. Your description says only one side is clad. Have you tried polotting these indicaitons to see where thay are located?
If you are seeing it from one side and not the other side you can pretty much rule out planar flaws (almost). If you are seeing it from the unclad side it is most likely the interface of the cladding. It is difficult to tell without actually having your hand on the transducer. I am having a hard time understanding why one side is clad and the other is not.

Of course when sound passes from one medium to another you will have reflection, refraction and mode conversion. The angle change is directly proportional to the velocities of the two materials and the mode of propogation and the angle of reflection is equal to the angle of incidence. If the velocities are the same there will be no change. If the velocity increases so will the angle and so forth. But when I say there is reflection and refraction, I mean the sound will reflect out and not back to the search unit unless it strikes something that will make it reflect back to the search unit like a flaw.

Then again, if nothing shows up with PT after excavation, it's possible that you are removing the defect when you excavate.

By js55

Date 08-01-2007 13:27

"If you are seeing it from one side and not the other side you can pretty much rule out planar flaws (almost). If you are seeing it from the unclad side it is most likely the interface of the cladding."
This is actually quite similar to the point I was trying to make. The fact that a process of logical elimination may be valuable in troubleshooting the problem. And also something a layman such as myself can understand and apply considering all the tech talk is beyond me.
NDTIII just did a much better job of expressing than I did. And proposed perhaps a more promising method of applying it.

By CWI555

Date 08-02-2007 02:47

I don't mean to be a smart ass here, but failure to see a planar from one side is by no means a sign that one is not present. Angle of incidence is very important to picking up a planar flaw. If you look at this video I've uploaded http://www.youtube.com/watch?v=oZR846TSQds You can see an example of the importance of angle of incidence using an inspection mirror, a flash light, and video camera. The camera is approximately 90 degrees minus a few degrees. It starts with the mirror turned flat, in line with the flashlight both laid down on a counter top in line with each other. As this example begins, only ambient light can be seen at this point, as the mirror is turned up and the camera lense/mirror incident angle gets to within 30 degrees of 45 out to each other, you will note an increase in the light intensity. As it gets within 20 degrees a definite increase of intensity is noted, and at about 45 you can see the direct reflection from bulb of the flash light. At that point from the cameras point of view, the most light is being reflected.
Going on past that point angling in towards the flash light, you will note that when the mirror gets within 20-25 degrees of the plane of the flash light, a reflection from the mirror can be seen on top of the flashlight rim. When it gets to the same plane you can see a direct reflection.
An ultrasonic wave will behave the same way. You have to be impinging on the planar at an angle that reflects enough energy back to the transducer to register on the scope. If the planar flaw is outside approximately 20 degrees from the angle of energy incidence, it is not likely to be seen and more likely to redirect the sound downwards via reflection and refraction. this means that from the opposite side of what is considered nearest to perpendicular to the planar, it could barely be seen if it all.
Even in that, the sound could also be turned down by a haz interaction, a weld fusion zone (transmission/reflection coefficient) interaction. and many other modes.
I have to disagree that a planar is eliminated (even almost).

A secondary creep wave could have also been set up at the cladding layer, or the back side of the weld. This could mode convert the sound to an L wave, and assuming initial exam in shear, would throw off where the actual flaw is if there is one. This is because the differences in velocities. If your machine thinks the reflection coming back is in shear wave mode and the machine is calibrated in shear, then the time of flight will be off because of the conversion to L wave which is roughly twice as fast as the T wave.

One thing that is absolutely true, everything is conjecture without the exact details of the exam. Everything is academic until exact details are known.

Respectfully,
Gerald

By NDTIII

Date 08-02-2007 04:03

That's OK Gerald. You're absolutely right. That's why I said (almost).