Defects from steelmaking and forging, such as pipe, bursts and large inclusions, sit inside the bar, where neither magnetic particle nor penetrant inspection can see them. In a large-diameter stud carrying a heavy load, an internal flaw is a weak section that no surface check will find. Ultrasonic testing looks right through the bar while it is still plain, so a flawed bar is found before it is threaded into a stud.
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model dataRadial, straight-beam, longitudinal-wave contact testing as ASTM E114 and A388 describe. Bolting over 2 in., examined before threading; a search unit of 1 in² or less at 2 or 2¼ MHz; the back-wall echo set to 80% of full screen height; rejection for an indication over 20%, or a back-wall echo of 40% or less: the ASTM A962 supplementary requirement. Bar and transducer drawn to scale; the flaw and its echo heights are for illustration. The pulse is shown 200,000 times slower, from a sound speed in steel of about 5.9 km/s.
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