Band 62, Heft 5

In austenitic CrNi steels, in addition to austenite and δ-ferrite, undesirable components such as σ-phase or chromium carbides at grain boundaries can occur during metallographic examination. In order to distinguish these components reliably, it is often necessary to work with different etchants, which are described below [1]. In addition, optimal specimen preparation – manual, mechanical or electrolytic – is essential to obtain an optimal result and a reliable statement under the microscope. Information and suggestions are also given below.

GH5188 high-temperature alloy has excellent mechanical properties and oxidation resistance and is used in the aerospace field and other harsh working environments. Tension and creep are the main failure modes during component operation. Hence, this work systematically characterizes and discusses the microstructure and properties of the welded joints. The results indicate that the weld metal of GH5188 alloy is mainly composed of long columnar grains, equiaxed dendrites, and carbides in a stable microstructure. Welding heat-affected zone grains are dominated by austenitic grains. Carbides are mainly divided into the two types M 6 C and M 23 C 6 , widely distributed in the weld and heat-affected zone on the grain boundary. The microstructure after the tensile fracture was tested, and the main cause for the fracture of the joint was found to be carbide in the weld structure. The average ultimate tensile strength for the laser welded joints at room temperature is 936.7 MPa and the average elongation after fracture is 19.25 %. The welded joint sample was kept at a temperature of 927 °C with a fixed load of 76 MPa for 36 hours without fracture. In this work, the requirements of relevant enterprises could be met and theoretical support for practical applications could be provided.

In the present work, microstructural and mechanical characterization of electric resistance welded (ERW) low carbon steel specimens has been carried out. The tensile strength of the samples is 448 MPa and the ductility is 29.70 %. The microstructure revealed the presence of fine grains of pearlite and ferrite with large proportion of ferrite and small proportion of pearlite. It also revealed the presence of inclusion in the parent metal which is due to improper calcium treatment. Lack of fusion is also seen in one sample, which may be due to higher heat input. Energy dispersive spectroscopy revealed the presence of oxides in inclusion as well as lack of fusion region.

Recurring failures affected the impeller of a steam turbine-driven integrally geared centrifugal air compressor in an oil refinery. In both incidents, part of the main blade of the stage #1 impeller liberated. The first incident occurred three years ago, when the subject compressor tripped. The second event happened a year later and is the subject of this contribution, when again the subject compressor tripped from stage #1 impeller damage. The impeller failure was caused by the liberation of one main blade due to high cycle fatigue failure. The high cycle fatigue crack might have been initiated by bending loads from tip rubbing. Corrosion pits were found on the subject cracked blade and elsewhere on the impeller, despite its short time in service. Even though there is no direct evidence for crack initiation at corrosion pits, it is a distinct possibility that corrosion fatigue was the acting failure mechanism, i. e., that the main fatigue crack was induced at a corrosion pit acting as stress raiser. It follows that the cracking may or may not have been purely stress-driven. The electrically driven sister compressor did not fail. It is therefore perceived as possible that vibrations, induced by the steam turbine, are the underlying root cause of the failure or that the corrosive environment at the steam turbine-driven air compressor was somehow more aggressive.