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Up Topic Welding Industry / Inspection & Qualification / NDT test for cladding materials
- - By wjsantamaria Date 02-23-2004 19:07

Somebody knows what is the best and realiable inspection method in order to know the remaining thickness of the cladding material ( SS 316), in the interior of a fractioning tower, the base material is ASTM 516, so my question is what is the best and realiable inspection method

I would appreciate your help .

Best regards

Javier S.
Parent - By thirdeye (***) Date 02-23-2004 21:50
Could you give us some more information? Do you you have access to the inside of the tower? Is the outside insulated or painted? What is the shell thickness? What was the original thickness of the cladding?
How was the caldding applied (weld build-up, or explosion bonding etc.)?
Was the cladded material inspected prior to being put into service?
Parent - - By NDTIII (***) Date 02-23-2004 22:38
UT, but you would need a SS 316 Calibration block. Preferably a step wedge that encompasses the thickness of the cladding. If you are inspecting from the unclad side, you would need a screen range to encompass the entire thickness of the vessel, including the ASTM 516 material. Not required if inspecting from the clad side. This should aslo be performed by an experienced technician.
Parent - By jwright650 (*****) Date 02-24-2004 00:58
Ultrasonic thickness gaging, provided you knew what the thickness is supposed to be orignally. Or UT w/ a straight beam like NTDIII mentioned to scan for disbonding or laminations of the material.
John Wright
Parent - - By NDTIII (***) Date 02-24-2004 01:29
The acoustic impedance between the 316 and 516 will be different thereby, giving you a back reflection at the interface between the 2 materials. Not only at disbonds. Once the interface is recognized you can compare the distance between the back wall of the cladding and the interface between the 2 materials. This is provided your calibration block has the same velocity as the cladding.
Parent - - By jwright650 (*****) Date 02-24-2004 10:23
NDTIII,
Thanks for reminding me about the acoustic impedance, I'll have to check my handy dandy velocity/impedance chart. I'm not used to seeing materials of different impedance cladded. In my line of work I don't get to see differences in velocities and impedances. You get used to seeing the same ole stuff day after day and you get rusty thinking through the process.
John Wright
Parent - - By NDTIII (***) Date 02-24-2004 12:09
That's OK John. Don't worry about it.

You can see the interface. between the 2 materials. It is not usually very high in amplitude, but is there, nonetheless. That's why I recommend using an experienced UT technician.

You have to know what signals to use. Now, I can't guarantee the accuracy, because SS316 wrought material (cal block) has a different velocity than SS cladding, which is basically a casting. Same as a weld. Velocities are dfferent everywhere and in all directions.

But it should not adversely affect a 0 degree L-wave at the proper frequency, and can be done as long as there is a reasonable tolerance for remaining thickness.

Ideally, a mockup with 316 cladding to 516 steel would be manufactured with the cladding applied in the same manner as the component to be examined with flat bottom holes or square notches in the cladding at various depths and used for calibration.
Parent - - By jwright650 (*****) Date 02-24-2004 12:59
NDTIII,
It does have a small reflected pressure (.6565%).
John Wright

PS. I was using values of 46.0 for (Z1)steel and 45.4 for the (Z2)SS316.
Z1-Z2 divided by Z1+Z2 = % of RP
How do you figure the Acoustic Impedance values if in a cast state as you described the weld material(cladding) being?
Parent - - By NDTIII (***) Date 02-24-2004 14:01
I don't know without knowing the acoustic inpedance of the cladding. That is all theoretical, anyway. You would have to deterine the nominal acoustic impedance of the materials if you wish to calculate it.

However, the acoustic impedances will differ to the point that, as I earlier stated, a mockup would be the ideal way to go.
Parent - By jwright650 (*****) Date 02-24-2004 14:11
NTDIII,
If I had to do it, I think I would want a mock-up to refer to. Good idea and hey thanks for discussing all this with me. My brain needs to be woke up every now and again.
We are having a meeting tomorrow, I'm hoping my company will hire an individual to take the safety duties off of me so I can get back into the shop and do more inspecting. I haven't looked at a weld in over a month now. I'm liable to forget how to turn that UT machine on. :)
John Wright
Parent - By thirdeye (***) Date 02-24-2004 14:40
Hey guys, take a look at this information. http://www.ndt.net/news/patents/1097/5661241.htm
B&W fine tuned this technique for measuring non-welded cladding thickness a few years ago (with the help of a few UT techs.....)

Up Topic Welding Industry / Inspection & Qualification / NDT test for cladding materials

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