Intergranular Corrosion in X39CrMo17-1 – A Review

Abstract

Abstract The aim of this contribution is to review cases of repeated snap ring failures in large gas turbine engine service due to intergranular attack, or intergranular corrosion, IGC, and to shed some light on metallurgical aspects of the wet corrosion resistance of the subject high-alloy martensitic stainless chromium steel X39CrMo17-1, DIN 1.4122. It is important to note that only snap rings, a.k.a. retaining rings for bores or Seeger rings, were affected by those failures, namely fractures resulting from intergranular attack (IGA, Kornzerfall). It was determined that the metallurgical root cause of failure was sensitization, resulting from secondary chromium carbide precipitation at grain boundaries (GB), and the ensuing chromium depletion in narrow bands adjacent to the GB, which rendered the matrix there sensitive to selective corrosion (the chromium content in these narrow seams, measuring only a few 10 nm in width, drops below the corrosion resistance threshold of ca. 10.5 %). Snap rings, as the name suggests, require a certain “snappiness” to serve their purpose. It is because of this requirement that all snap rings that experienced failures in the field were heat treated per DIN EN 10088-2, providing for a low tempering temperature of max. 250 °C, leading to a relatively high hardness of ca. 580 HV 10, providing said “snappiness”. It turned out that the microstructure resulting from this heat treatment (H/T) is sensitized, rendering the material sensitive to IGA. To the knowledge of the authors, no other components made of this martensitic stainless chromium steel are given this H/T, but rather a high-temperature tempering of 650 °C–750 °C per DIN EN 10088-3, producing a lower hardness in the order of ca. 300 HV 10 and largely a lack of “snappiness”, the latter being not required by those non-snap ring applications. Metallurgically, it became clear that high-temperature tempering per DIN EN 10088-3 eliminates sensitization by “backfill” diffusion of chromium from the non-depleted matrix into formerly depleted regions next to GB. It was therefore decided in the authors’ organization to eliminate X39CrMo17-1 per DIN EN 10088-2 as a possible material selection. The metallurgical investigations described in this contribution corroborate and support this decision.