In carbon steels, a hard spot is the result of non-uniform and localized quenching during manufacturing. Hard spots have a grain phase and microstructure called martensite which is harder than the bainite and ferrite normally present in the steel. While a harder microstructure means a higher tensile strength, it also means a decrease in ductility and an increase in brittleness. With the tendency of carbon steel pipelines to develop stress corrosion cracking (SCC) when operating in a susceptible environment, their integrity in the presence of a brittle area is being put to the test. Cracks developing inside a hard spot represent a major threat for pipeline operators around the world.

So, how could steel manufacturers in the 1950s make sure that their produced pipes did not include some of those metallurgical anomalies? Short answer is, they could not. This resulted in major hard spots left undetected and still present to this day in buried pipelines. Fortunately, with today’s technologies, a first screening with in-line inspection (ILI) tools employing magnetic flux leakage (MFL) can identify areas with greater metallurgical hardness along the pipeline. Once found, direct assessment by way of integrity digs accommodates further investigation to assess more accurately the hard spots.

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