When I was an engineering student and before getting my degree, I spent three months in a seamless pipe mill. This was called "Factory practice" and was compulsory to get the degree. Well, this happened 45 years ago.
The pipes were first hot rolled in the so called "pilgrim mill" and then calibrated in the so called "calibrating mill", from which they came out with the proper dimensions (diameter and thickness).
Then they were visual inspected. Highly experienced inspectors observed the pipe surface and detected any imperfection, including cracks. If necessary, they used a magnifying glass. When the imperfection (including cracks) was detected, they filed a small portion of it with a tiny triangular file. This was done to visually estimate the imperfection depth. If it was deeper than that allowed by the standard (usually 12.5% of the pipe thickness) the pipe was discarded. If it was less deep than that, the pipe surface was ground with a grinding wheel, taking care to make a smooth and tapered grind.
How did they know that the depth of the imperfection was within standard? That was the first question I asked in those days. The answer: "That's why they're highly experienced inspectors!"
Giovanni S. Crisi
Sao Paulo - Brazil
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