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Re: Calibration for stainless steel.

Posted by: Ed Ginzel Profile   Exhibitors stand (PID_67), E-mail: Address, on November 13, 2008 at 19:46 :

In Reply to: Re: Calibration for stainless steel. posted by : Nigel Armstrong Profile , E-mail: Address, on November 13, 2008 at 16:59 :

Nathan, the suggestion by Nigel to look up some papers on the topic is a good one. In addition to Dr. Oglivie’s papers (on of which is J. A. Ogilvy; Ultrasonic beam profiles and beam propagation in an austenitic weld using a theoretical ray tracing model; Ultrasonics, Volume 24 Number 6 (1986), pp 337-347) I would recommend a Handbook. The Handbook on the Ultrasonic Examination of Austenitic Clad Seel Components, Compiled by Commission V of the IIW, Published by European Commission Joint Research Centre Institute of Advanced Materials, 1994.

Prof. (Dr?) Mark Davis from University of Ultrasonics ™ made a statement concerning grain sizes that is perhaps not clear to some. The reference to size based on ratings such as 00, 0, 1, etc. comes from an ASTM Standard. Grain size can range from 00 to 14.0 equates to 0.5080 to 0.0028 mm according to ASTM E112. Another good reference you might like to acquire is a book called Basic Metallurgy for Nondestructive Testing, edited by J. Taylor, and published by the British Institute of NDT 1996.

In chapter 5 Taylor makes note of the effect of anisotropy. In a cold worked form, such as thin plate and tubing might be, stainless grains would be well broken and approximately the same size in every direction. The result is acoustic properties are nominally the same in every direction (i.e. isotropic). But when welding occurs, the temperature rises and grain growth may result. Acoustic velocities along the long axis of the austenite columnar grain axes are slower and reach a maximum velocity at about 45° to the long axis. Heat dissipation directions in welding will determine the direction of grain growth. The result on the ultrasonic wavefront is to skew the beam towards the direction of maximum velocity.

Grain sizes at the weld and HAZ may therefore be quite large compared to the parent metal. In the Krautkramer text (Ultrasonic Testing of Materials) it is noted that at grain sizes up to about one one-hundredth of a wavelength scatter may be considered negligible. . Austenitic grains at 200microns (0.2mm) may not be uncommon in some weld regions. This represents 25% of the wavelength size for a 4MHz transverse mode! This is a much larger fraction than one one-hundredth of a wavelength so scatter will be a dominant factor. The scatter is due to the accumulated variation in acoustic impedances at the grain boundaries.

Sorry about the long lecture, but you asked about the calibration so I thought the background was necessary. The A1 block in stainless might be able to match the velocities in your pipe parent metal (but large variations could occur for either compressional or transverse modes). That would allow you to calibrate timebase and discover the new refracted angles that result (compared to the indicated values marked for carbon steel). But the bending effects of anisotropy and attenuation compensations will be much more difficult if the grain structure in the weld and HAZ are coarse. This may require a special block with an actual sample of the weld and agreed-upon targets.


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: Congratulations on your exam success and qualification Nathan. Also you have been resourceful coming here to keep up with "continuous learning". Ask any long-time NDT hand and they will all tell you the same - in NDT you never stop learning, so it pays to be able to talk with co-workers with the same interests.
: If your procedure is based on EN 1714, then you must be checking welds. Heed the advice from Mark and your technique will be reasonable. You could look up some papers on this site - J A Ogilvie did a great paper in the early 80's which clearly showed through ray-tracing, the bending effect on the beam of the long (through-thickness direction) as-cast columnar weld grains, as well as the noise due to beam scatter. Degree of bending varied with angle of incidence relative to the columnar structure. Forged stainless steel is fine equi-axed grains, so sound passes through without any bending effect and sidewall lack of fusion should be relatively easy to detect and evaluate. As Mark stated its the weld volume defects that are awkward thus low frequency, longitudinal wave for longer wavelength less affected by the grain structure. I dont know if there are any specially formulated weld procedures to minimise vertical grain growth to reduce ray-bending effect.
: Radiography suffers from similar problem with stainless welds, the "herringbone" diffraction effect. This is countered by increasing kV.
: Cheers
: Nigel
:
: : : It would most probably be pipe. Although, i wouldn't have thought it would vary much for plate. We have our own spec, although, it follows BS EN 1714 1998 closely.
: : Have a look at ASTM Section 3 E213 for your pipe (will work fine for stainless). This gives a description of cal standards. Will work for immersion or contact.
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