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Technical Discussions
Michael Plant
Michael Plant
00:36 Jul-29-1998
sound velocity variations in cast iron

I have recently completed a thickness survey on 52 paper machine dryer cans. These are basically hollow cast iron rolls with wall thicknesses ranging from 25-32 mm. When i tried comparing this survey to one carried out in 1990, I get crazy trends, with some cans gaining in thickness. The 1990 survey was done using a single velocity value while the recent survey was done by calibrating off each can (one point calibration for 6 readings per can, single crystal 2mHz probes. Not hartford wedge. ) I adjusted the 1990 values by the velocity ratio's for each can, but still get rogue trends.
I could not match up the measurement locations but the cans are resonably concentric and parallel. The variations are up to 6mm on 30mm. The cans clearly have not lost this much material.
Can anyone offer any explanation for this?
I suspect sound velocity variations within each casting. I found sound velocity variations between cans of 4484 and 5083 m/sec although i would not have expected to see this in a single casting . Is there any info on sound velocity variations in single castings and what would be typical variations?Could there be any other explanation?
Thanks in advance.

Mike


 
 Reply 
 
Tom Nelligan
Engineering,
retired, USA, Joined Nov 1998, 390

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
06:08 Jul-29-1998
Re: sound velocity variations in cast iron

: I have recently completed a thickness survey on 52 paper machine dryer cans. These are basically hollow cast iron rolls with wall thicknesses ranging from 25-32 mm. When i tried comparing this survey to one carried out in 1990, I get crazy trends, with some cans gaining in thickness. The 1990 survey was done using a single velocity value while the recent survey was done by calibrating off each can (one point calibration for 6 readings per can, single crystal 2mHz probes. Not hartford wedge. ) I adjusted the 1990 values by the velocity ratio's for each can, but still get rogue trends.
: I could not match up the measurement locations but the cans are resonably concentric and parallel. The variations are up to 6mm on 30mm. The cans clearly have not lost this much material.
: Can anyone offer any explanation for this?
: I suspect sound velocity variations within each casting. I found sound velocity variations between cans of 4484 and 5083 m/sec although i would not have expected to see this in asingle casting . Is there any info on sound velocity variations in single castings and what would be typical variations?Could there be any other explanation?
: Thanks in advance.

: Mike

The first thing I wonder about is whether all of the measurements, both the previous set and the ones you've just done, represent readings of valid backwall echoes. If you used a flaw detector or a thickness gage with a waveform display, this shouldn't be a problem, but if you used a gage without a waveform display it is conceivable that some readings represent grain scatter noise or other internal reflectors. This is especially true if instrument gain was set too high, or if setup was otherwise less than optimum. (The 2 MHz transducer frequency sounds reasonable for 30 mm of cast iron.) If the readings include both valid backwall echoes and occasional invalid scatter echoes, that would account for what you're seeing.

I have seen velocity variations of 10 per cent or more in large castings where the cooling rate (and hence grain size and/or nodularity) vary between thick and thin areas. Since your castings have a fairly uniform thickness, it would seem that they would have cooled uniformly, but it is still possible that graphite distribution is uneven.

Tom Nelligan
Senior Applications Engineer
Panametrics, Inc.


 
 Reply 
 
Michael Trinidad
Consultant,
LMATS Pty Ltd , Australia, Joined Jan 2003, 138

Michael Trinidad

Consultant,
LMATS Pty Ltd ,
Australia,
Joined Jan 2003
138
05:52 Aug-02-1998
Re: sound velocity variations in cast iron
: I have recently completed a thickness survey on 52 paper machine dryer cans. These are basically hollow cast iron rolls with wall thicknesses ranging from 25-32 mm. When i tried comparing this survey to one carried out in 1990, I get crazy trends, with some cans gaining in thickness. The 1990 survey was done using a single velocity value while the recent survey was done by calibrating off each can (one point calibration for 6 readings per can, single crystal 2mHz probes. Not hartford wedge. ) I adjusted the 1990 values by the velocity ratio's for each can, but still get rogue trends.
: I could not match up the measurement locations but the cans are resonably concentric and parallel. The variations are up to 6mm on 30mm. The cans clearly have not lost this much material.
: Can anyone offer any explanation for this?
: I suspect sound velocity variations within each casting. I found sound velocity variations between cans of 4484 and 5083 m/sec although i would not have expected to see this in a single casting . Is there any info on sound velocity variations in single castings and what would be typical variations?Could there be any other explanation?
: Thanks in advance.

: Mike

I have had the same experiance with cast iron and cast steel pump housings. The procedure was to calibrate the equipment using a web as a known thickness. Later when results were found to be dubious it was found the velocities in the casting differed substantially. A 10mm web used as a calibration point but could not be reproduced by measuring the adjacent identical web. Errors ranged from 2-5mm on a known 10mm thickness. I have also seen the same errors in bronze castings which must either be due to differentations in grain sizes or impurities in the metal.

kindest regards


Michael Trinidad


 
 Reply 
 
Udo Schlengermann
Consultant, -
Standards Consulting, Germany, Joined Nov 1998, 179

Udo Schlengermann

Consultant, -
Standards Consulting,
Germany,
Joined Nov 1998
179
09:32 Aug-03-1998
Re: sound velocity variations in cast iron

: I have recently completed a thickness survey on 52 paper machine dryer cans. These are basically hollow cast iron rolls with wall thicknesses ranging from 25-32 mm. When i tried comparing this survey to one carried out in 1990, I get crazy trends, with some cans gaining in thickness. The 1990 survey was done using a single velocity value while the recent survey was done by calibrating off each can (one point calibration for 6 readings per can, single crystal 2mHz probes. Not hartford wedge. ) I adjusted the 1990 values by the velocity ratio's for each can, but still get rogue trends.
: I could not match up the measurement locations but the cans are resonably concentric and parallel. The variations are up to 6mm on 30mm. The cans clearly have not lost this much material.
: Can anyone offer any explanation for this?
: I suspect sound velocity variations within each casting. I found sound velocity variations between cans of 4484 and 5083 m/sec although i would not have expected to see this in asingle casting . Is there any info on sound velocity variations in single castings and what would be typical variations?Could there be any other explanation?
: Thanks in advance.

: Mike

reply by Udo Schlengermann (uschlengermann@krautkramer.de)

As Tom Nelligan and Michael Trinidad said, depending on structure of iron castings variations of more than 10% of sound velocity may occur on the same casting.
To measure the real wall thickness you first have to measure the sound velocity on the spot where you want to determine the wall thickness.
This problem can be solved with the Krautkramer Auto-V-System, a combination of a special version of portable USN52 instrument, a special probe with 4 transducer elements, and a multiplexer program:
In a first step the velocity is determined by separate angle beam transmitter and receiver of known distance.
In a second step the time-of flight through the unknown wall is measured by a normal beam probe with separate transducers.
This time-of flight,together with the measured sound velocity at the same spot, is used to calculate the wall thickness.
Information on the Auto-V-System will follow directly to your e-mail address.

Kind regards

Udo Schlengermann


 
 Reply 
 

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