Hard zones in large diameter pipe have been identified as the cause of the Kashagan pipelines failure, the replacement of which is estimated to cost $3.6B [1–4]. Some hard zone formation mechanisms have been known for years, but one mechanism is “new”, or more precisely, newly recognized. This mechanism can occur during the Thermomechanical Control Processing (TMCP) of plate and specifically during accelerated cooling. The hard microstructure is lath bainite and is isolated in thin (200–500 μm) surface regions. To distinguish the new mechanism from the others these surface regions are given the name local hard zones (LHZs).

There are currently no validated industry standard inspection methods that can be used to detect LHZs. This situation is challenging for an industry that relies on TMCP pipe for sour service applications. This paper explains the LHZ formation mechanism while a companion paper (Newbury, et al [59]) will describe TMCP monitoring techniques that can be used to mitigate the risk of LHZs.

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