A metallurgical investigation on and off site revealed the cause of an unexpected failure in an offshore drill pipe. A major North Sea operator experienced a fracture in one of its offshore drill pipes during routine testing. As the pipes were reportedly drawn from new stock, and met API specifications, the operator asked TWI to determine the cause of the failure.
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.
More importantly, neither inadequate tempering of martensite nor incorrect cooling following normal austenitising, produced brittle structures capable of passing QC checks on strength and hardness. Thus, the peculiar heating condition of the fractured pipe had occurred during the forging process, following which, no remedial heat treatment had been performed.
Fig. 1. Tempered martensite. Material with this microstructure has an optimum combination of component strength and toughness
Fig. 2. Bainite with coarse prior austenite grain size. Material is strong but brittle - an unacceptable microstructure which could result from incorrect heat treatment
