Normally you would just measure the velocity in a reference sample of known thickness. All modern flaw detectors and thickness gages have the capability to calibrate velocity when thickness or sound path length is known. Perhaps there is some special circumstance in this case that prevents that?
Be careful as the velocity will be different when changing wave modes. For example longitudinal waves travel around twice as fast as transverse waves in a given material. There are several reference charts floating around for longitudinal and shear velocities in commonly tested materials.
Use the formula c1/c2 = d1/d2.
Consider c1 as the velocity of steel= 5920m/s. d1 would be the thickness of the ss 316 sample that you want to test.
d2 would be the thickness you read on the A scan display if equipment is calibrated for 5920m/s velocity.
Now calculate mathematically the value of c2. This would be velocity of ss 316.
I'm with TJ on this one. I don't know why you want to do that in the first place, because there are many already accessible chartsthat give you those value. The other point I want to make, also along with what TJ said is that velocity will also change with Heat Treatment and material temperature. I've personnally experience this situation. You calibrate in the morning when the temperature inside the shop is 70F and find out on your cal out that you are as much as 2 mm off in the afternoon when the temperature has risen to 95F. If you still want that info, go to the link below. This is another great NDT site.
Michel, the suitability of simple tables to obtain acoustic velocities is not always reliable. There can be large variations, especially in metals where heat treatment and even rolling methods can cause significant differences even in the same alloy! I have seen up to 10% variation in carbon steels of the same alloy. Stainless steel is anisotropic and birerfringent. Therefore getting meaningful velocities will, at least for the shear mode, require you use a good scope (at least 100MHz sampling frequency) and be ready to use it in RF (not rectified) display to make sure you are identifying the correct wave (SH slow or SH fast) and phase. Ensure parallel surfaces of the test piece and use a good vernier caliper so you can get a repeatable thickness within about 10mircons. Since the material is anisotropic you may require mesurements in several directions. Guidance on this can be found in ASTM E-1961 (in the Appendix).
Totatlly agree with you. But as TJ and I said to Dave, I think really depends on what you are doing in the world of NDT. From is profile, Dave indicated that he is an inspector. From that point, I conclude that by doing a normal calibration on a test piece of a known thickness as Tom has mentionned, he should be all right. You see where I come from we have a saying that goes like this: "Don't try to be more Catholic than the Pope". Yes, we have to be as precise as possible, but 10 microns in a weld repair doesn't matter very much, specially when the welder will arc gouge it with a 5/16 rode.
Thanks very much for your help guys.
The Project I have is to measure the depth of a 4mm dia pore in a weld of 20mm thickness.
The cap has been removed to accomodate scanning.
Scanning has to be carried out with a Twin 2.5MHZ Compression Probe only.
Velocity measurements have to be made of the parent materials and weld to confirm accuracy as the depth of the pore is critcal.
The method I am using is the method which Tom mentioned.
I was keen to find out if anyone had other proven ideas as I dont have a great deal of experience with Ultrasonic Inspection of Stainless Steel Weldments.
You are right in following Tom's advice for this job. Many times even after meticulously calibrating my instrument I have found that my velocity was a little off compare to all the reference materials and charts I've read. And very seldom an inspector do really bother with velocity. What is important is that when using a specific calibration block, the signal amplitude of the backwall is at the proper place on your screen. Like TJ, Ed and myself said before, many factors will affect your velocity and in your case, since you are measuring a pore in a weld, the velocity of the base metal is differnet from the weld and the HAZ as well.
Of course my favorite way of determining velocity of a material which has a visible and measurable flaw (with calipers for instance) is to set your screen range then dial in your velocity until the signal reaches the known depth. Voila! Read your velocity.