Failure analysis of tube-to-tubesheet welds in cracked gas heat exchangers.

FAZZINI, P.; LUIS JOSE OTEGUI

Eng. Failure Analysis

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2004 vol. 11 p. 903 - 913

The causes of cracking in the closing circumferencial tube to tube sheet welds of five cracked gas?steam heat exchangers in a petrochemical plant were studied, in order to identify the mechanisms that caused the failures, to define responsibilities, and to evaluate possible corrective actions. From samples extracted from the tubesheets with and without reported failures, it was possible to analyze the morphology of the defects initiating the fractures, their interaction with the microstructural characteristic, and the effects of possible problems related to materials, fabrication, welding, maintenance or operation. Crack propagation was due to a mechanism of microstructural enbrittlement. High hardness in the interface between the weld and the tube base metal was found, 5 Rockwell C points higher in the failed cold tubesheets than in the non-failed hot tubesheets analyzed. The microstructure of the cracked tubesheets did not correspond to the quenched and tempered steel specified by the manufacturer. Microstructures and chemical composition of hot and cold tubesheets were identified. Discrepancies with manufacturer?s specifications were verified, defects in the in-field PWHT were found to be the main reason for the failures.