Sigma Phase Formation and Embrittlement of Cast Iron-Chromium-Nickel (Fe-Cr-Ni) Alloys

Abstract

HK alloy is a member of the heat resistant cast alloy family (H-Series) steels. They are widely used in the petrochemical industry for components requiring enhanced high temperature properties. Microstructural changes occurring at high temperature clearly affects its mechanical properties. These properties have been shown in HK-40 steel subjected to high-temperature degradation and prone to the formation of sigma phase. The investigation carried out included metallurgical analysis, materials characterization and mechanical analysis. Metallurgical analysis included advanced metallography techniques to characterize its microstructure morphology and properties. Significant depletion of vital precipitates observed that definitely degraded its high temperature properties. Mechanical analysis included hardness profile, tensile testing of samples taken from the tree supports and tested in room temperature and in 800°C environments. Experimental results revealed that the structure of HK-40 affected by the formation of the high temperature brittle sigma-σ-phase. Nonetheless, mechanical properties did not suffer much at higher temperature.

An Investigation on the Cracking of Air Tubes of Rotary Kilns in a Sponge Iron Plant

“…In addition to the small and rounded carbides inside the austenite grains, coarser and plate or needlelike precipitates of sigma phases are observed within the grains. Similar microstructural features showing plate or needlelike precipitates of sigma phases have been observed by Babakr et al [2] and Łabanowski [7].…”

“…EDS analysis shows the presence of sigma phases along the grain boundary as well as within the grains. Sigma (r) phase is basically an ironchromium intermetallic compound that develops in austenitic stainless steels under prolonged exposure to high temperature (620-900°C) because of thermal instability [2][3][4]. The presence of sigma phase was confirmed by etching the sample with Groesbeck solution [5,6].…”

Comparative High Temperature Analysis of HVOF-Sprayed and Detonation Gun Sprayed Ni–20Cr Coating in Laboratory and Actual Boiler Environments

“…The role of the coating is pronounced during the transient period of erosion during which it saved the substrate steel from the enhanced E-C loss. The mass loss recorded after the initial transient cycles for both the coatings was meager, which indicates that the coatings could be useful to reduce the It has been reported that sigma (r) phase is hard and fragile, in addition, when forms, it consumes chromium and molybdenum present within the matrix [33], which leads to the depletion in these elements and thereby prevent formation of Cr 2 O 3 . XRD analysis of the exposed coated specimens tested in the given environments indicated that no brittle (sigma) phase was formed in the scales.…”

Evaluation of Embrittlement in Isochronal Aged Fe-Cr Alloys by Magnetic Hysteresis Loop Technique

“…For the alloys with a Cr content of more than 20%, there is a possibility of forming a σ phase above 550 o C, which also depends on the ageing time and impurity level in the alloy [8]. The formation of the σ phase is detrimental as it makes the material brittle and affects the creep and corrosion properties [9]. The 475 o C embrittlement arises in structural components during service whereas the σ phase embrittlement is produced during heat treatment, welding, or casting.…”