Avoiding failures of very high pressure tubes: compromises between two damage mechanisms.

J,L. OTEGUI; H. KUNERT; J. BOOMAN; P. BARCIA; G. GUIDI

Bs. As. - Argentina

Congreso; SAM-CONAMET 2009; 2009

Sociedad Argentina de Materiales

This work addresses the influence of autofrettage in the occurrence of repeated leaks of thick wall curved steel tubes from a high pressure petrochemical reactor. These tubes are made of HP-4 steel, bent and heat treated, and then subjected to autofrettage. Experimental analyses of two recent failures are presented, which include fractographic, metallographic, chemical and mechanical tests. These leaks are the result of fatigue transgranular growth of cracks initiating from machining groves in the inner surface of the tubes, driven by pressure cycles during many years of normal operation. Experimental measurement of residual stresses helped verify that high temperature  ervice relieved most of the compressive residual stresses imposed by the autofrettage. Previous failures due to metallurgical susceptibility to intergranular cracking were found to be related to a phenomenon similar to Temper Embrittlement. Repeating autofrettage to mitigate fatigue failures would reproduce the deformation mechanisms that favored intergranular cracking of these tubes. It was recommended not to repeat this process as a means to recover old tubes, but to replace old tubes by new ones, made of a stronger material.