First, TWI found that incorrect heat treatment of the pipe had resulted in a brittle bainitic microstructure. Bainitic microstructures typically have lower toughness than quenched and tempered martensite which would be expected from correct heat treatment.
For the second part of the investigation, TWI's team travelled to the stockyard to carry out on-site metallography studies on pipes held in stock. All pipes were found to have satisfactory microstructures and did not exhibit brittle characteristics.
For the third phase of the work the operator asked TWI to investigate possible heat treatments which could produce microstructures with the required strength levels but poor toughness. TWI demonstrated that an excessively high austenitising temperature, followed by slow air cooling instead of quenching, resulted in a coarse bainitic structure. This was the only heat treatment route to produce a brittle microstructure within specification for hardness and strength.