03102 LIMITATIONS OF 17- 4 PH METALLURGICAL, MECHANICAL AND CORROSION ASPECTS

The use of 17-4 PH in sour service started long before the establishment of MR0175, and its coverage in this document has varied widely over the years. Thus, the real significance of the statement "according to NACE" had become less and less defined. In the context of the present debate on the general acceptability of 17-4 PH in sour service, an extensive literature survey of the very complex metallurgy of this grade has been carried out following a mechanical rupture in service. In addition, three different 17-4 PH materials have been studied in terms of microstructure, mechanical properties and susceptibility to sulfide stress cracking. It is shown that the content of residual ferrite and austenite in this nominally "martensitic" grade is a key issue for its mechanical properties. Regarding sulfide stress cracking resistance, the strong dependence upon the actual stress level has been confirmed. It is concluded that, with just the broad composition range of UNS 17400 and a simplified definition of the heat treatment, the reference to MR0175 cannot as such ensure fitness for purpose. It is then the responsibility of each supplier to qualify its design and manufacturing route. Key words : 17-4 PH, martensitic stainless steel, sustained load cracking, sulfide stress cracking, microstructure, mechanical properties, ferrite, austenite, martensite.