A hard spot is a localized area with increased hardness compared to the surrounding base metal. API 5L defines a hard spot as being larger than 2 inches in any direction with a hardness greater than 327 HBW1. Hard spots are believed to result from application of a local thermal cycle, such as localized quenching of the surface, to produce a microstructure that is different from (and harder than) the surrounding parent material. Hard spots have typically been observed in older vintage pipes that use plate as feedstock, mainly due to technological limitations and quality control issues in pre-1970 plate and pipe manufacturing methods, such as electric flash welding (EFW).
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