Volume 60, Issue 9

A composite material was subjected to metallographic examination. It was manufactured by forging together different ferrous materials and nickel. This technique ranks among the Damascus techniques and is also used to manufacture so-called “krises”. A kris is a traditional dagger from Southeast Asia, particularly from Indonesia and Malaysia, manufactured by processing different iron and nickel alloys or meteorite iron containing nickel. As is typical for Damascus materials, widely varied patterns are obtained on the blades which may have different shapes. To manufacture a kris blade, sheets made of pure iron, C10 steel, meteorite iron, and nickel were forge-welded in preliminary tests. Samples were taken and subjected to metallographic examinations. Different layers can be recognized after polishing. Etchants typically used for steels (Nital, Klemm) cannot be used to reveal the nickel microstructure. The nickel grain boundaries can be revealed by ion etching. To improve the different ferrous material microstructures’ contrast, an additional heat tinting process was performed. The metallographic examination reveals that forge welding created a homogeneous joint between the individual layers.

Since 2013, the AG Fraktographie (Working Group (WG) Fractography) in the DVM/DGM Joint Committee “Elektronenmikroskopie in der Materialforschung” (Electron Microscopy in Materials Research) maintains a fractographic online database (“FractoDB”, [1, 2]) available to interested professionals. When it comes to identifying failure mechanisms and causes of damage, the analysis and evaluation of fracture surfaces and their characteristics constitute important aspects of the failure analysis. Cracks and fractures in real components can only be assessed if well-documented comparative fractures from laboratory tests are available – be it in samples or in comparison components. The WG Fractography therefore gathers image material, systematically carries out laboratory and round robin tests, and analyzes fractures from failure cases. From thus obtained data, datasets are compiled and made available via the database. Currently, a browsable inventory of more than 400 datasets with a total of more than 4500 images is available. It is organized in line with guideline VDI 3822 [3]. Other activities of the WG Fractography represented in the FractoDB include, among others, the development of a fractographic set of symbols [4, 5] and the analysis of fracture characteristics using machine learning [6]. This contribution reports on latest results and plans.

Failure investigations are mainly conducted for the purpose of clarifying the root causes for component failure. Possible aims of these investigations are to analyze the unexpected failure of a component during operation, to improve the material and to assess the quality of a product. Usually, ceramics can be used in similar ways as other structural materials. There are particularities, however, which need to be considered in failure analysis of ceramic components. This paper will therefore give a brief overview of the types of damage which may occur in ceramic materials, with the focus being on failures occurring at room temperature and medium temperatures (e. g., brittle fracture, contact stresses or stresses caused by rapid temperature changes).

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.