A set of four high pressure heat exchangers at the Typpi Oy ammonia plant in Oulu, Finland failed on the 19 March 1970 (see Fig. 1). The plant had been restarted after a two-week shut-down and had been running at the process pressure of 230 bar (58% of the hydrotest pressure) for about an hour when the head chambers of the water coolers fractured suddenly. Pieces of two of the head chambers (A and B) were thrown up to 250m and the third chamber (C) showed extensive cracking around the nozzles. The fourth cooler (D) appeared undamaged.
Personnel working near the heat exchangers did not hear or see any signs of leakage prior to the failure and the records showed the process conditions to be normal. The inlet temperature of the effluent was +10◦C and its outlet temperature +3◦C. The ambient temperature was 3◦C below zero.
The head chambers were forged slightly oversized from a creep resisting Ni-Mo-V steel and normalised at 920◦C. Following rough machining to close to the final dimensions (outside diameter 1090mm, length 1100mm with thickness 85-150mm), the forgings were heat treated at 950◦C for four hours, oil quenched, tempered at 675◦C and finally air cooled. Following visual and ultrasonic inspection, the final machining was carried out.
A mild steel overlay was deposited on the tube-plate face using manual metal arc (MMA) welding. The last weld metal to be deposited was around the circumference of the tube-plate, where it joined the chamber barrel. Following welding of other attachments to the chamber (except the tubes), the chamber was given a post-weld heat treatment at 560-580◦C. The weld overlay around the circumference of the tube-plate was then skimmed and the whole area inspected with a dye penetrant prior to the attachment of the tubes. Final inspection by ultrasonics and dye penetrant testing was carried out before leak testing and hydrostatic testing of the assembled water cooler.
Causes of Failure
Investigation of the failure concentrated on head chambers A and B which fractured completely around their circumferences, in a brittle manner (see Fig. 2). The origin of the fracture in chamber A was in the heat affected zone (HAZ) of a nozzle attachment weld but was not associated with a defect. In chamber B, the fracture started from a small oxidised crack in the toe of the weld overlay around the circumference of the tube-plate. From consideration of the deformation of the plant and positions of the broken pieces, it was determined that chamber B failed first.
Tests on the material of the forged chamber showed that the chemical composition was within specification. The results of Charpy V-notch impact tests were, however, much lower than those shown on the certification test records (average of 12J at 0◦C compared to 80-180J at the same temperature). This was determined to be due to the much slower cooling rate from the hardening temperature (950◦C) of the massive head chamber compared to the test material ring used for the original Charpy tests.
Metallographic studies of the chamber steel revealed an upper bainite microstructure instead of the desired tempered martensite. Upper bainite microstructures typically exhibit good strength but poor ductility. The nil-ductility transition (NDT) temperature of the head chamber material (as measured by the Pellini drop weight test) was +20◦C, confirming its low toughness. Regions of high hardness were found in weld HAZs which had not been appreciably softened by the post-weld heat-treatment (PWHT). The PWHT was also shown to have been insufficient to cause stress relaxation. The evidence indicated that the small original defect was formed during PWHT due to an excessive heating rate for a steel of this composition. Then during the year between the hydrotest and the commission of the water coolers, the defect extended by a stress-corrosion mechanism in the presence of high hardnesses and residual stresses. Finally, while in operation, the already low toughness material was embrittled by hydrogen from the process environment so creating the critical conditions for brittle fracture.
This failure illustrated the importance of acceptance tests being made on material typical of the structure and of correct post-weld heat treatment conditions being specified. It also demonstrated how the benefit of a hydrotest at a higher pressure than the working pressure could be removed by crack extension in service.
This is a case history taken from Report 632/1998 . For further case histories, Industrial Members may consult the full report.
Professional & WJS members and non-members of TWI can obtain further case histories by reading the following article:-
Six case histories of pressure vessel failures
Engineering Failure Analysis, vol 3, no 3. 1996. pp.157-170.