Papers by Keyword: Sigma Phase

Abstract: In this study, we investigated the effect of Si and Mo on the sigma-phase precipitation in S32750 super duplex stainless steel slab. The activity for Mo with increasing Si and Mo was calculated by the Wagner formula, and the equilibrium solidification phase diagrams of S32750 duplex stainless steels with different Si and Mo contents were calculated using the thermo-calc software. The sigma phase precipitated mainly at ferrite/austenite phase boundaries and grew up towards the interior of ferrite phase in S32750 SDSS slab. The area fraction and size of the sigma phases significantly increased with increasing Mo content and Si content. Also, the increment in Mo and Si content affected the Mo concentration in sigma phase. The sample（Mo:3.4%,Si：0.3%） had a lowest sigma-phase area fraction of 2.84% and had lowest Mo content in σ phase.The calculation results showed that the increase of Mo and Si content increased the initial precipitation temperature and maximum precipitation amount of σ phase in S32750 SDSS equilibrium phase diagram. The activity of Mo also increased with increasing Si content and Mo content. That is, Mo and Si elements promoted the precipitation of σ phase.

Abstract: This paper is focused on evaluation of the plastic deformation effect, after the cold bending process for new austenitic material Super 304H designed for supercritical conditions. The aim of the project was to qualify the process of cold bending and then verify the properties of these bends in conditions approaching application. This paper presents the results of the plastic deformation effect on changes in the structure and mechanical properties at operating temperatures in corrosive environments coal combustion products and comparison to results before degradation. The tests were carried out on bends in two dimensions of tubes which are the most common sizes used in super-heaters for supercritical power plant boiler. The bends were produced by the method of cold bending on several bending radii in order to obtain information about the structure and behaviour of materials at various stages of deformation. To obtain reliable information tensile test, hardness test and metallographic examination were used on material from straight tubes of the bends and drawn parts before degradation and after degradation.

Abstract: The paper presents the results of the analysis of secondary phases formed during thermal exposure in creep resistant austenitic steels of SUPER 304H and HR3C types. These steels were worldwide used for construction of the superheaters (the heat exchangers) of supercritical and ultra-supercritical (USC) coal-fired power plants. In order to accelerate precipitation processes, the steels were isothermally aged at 675 °C for 20 000 h. The investigations of the precipitates were primarily focused on the occurrence of brittle phases. Changes in mechanical properties caused by occurrence of secondary phases were documented by tensile testing and measurement of the impact strength. For comparison reasons, the impact tests of the initial state of the steels were also performed. The results showed that long-term ageing had led to the significant decrease of the impact strength. Correlation between precipitation of secondary phase and measured values of mechanical properties is shown. The influence of brittle phases on long-term durability of the degraded steel regarding its insufficient impact strength has been discussed.

Abstract: Duplex Stainless steels (DSS) are biphasic austeno-ferritic steels in which the best combination of mechanical and corrosion resistance properties is achieved for almost equal volume fraction of the phases. These steels are classified according to their pitting corrosion resistance, assessed by the PREN index (Pitting Resistance Equivalent Number) which, although qualitatively, is widely employed as comparison. The present work is aimed to study the pitting resistance of four DSS grades (SAF 2101, 2304, 2205 and 2507) in the as-received condition and after isothermal aging in the critical range 750°C-900°C, to highlight the effect of secondary phases precipitation on the corrosion behavior. The materials were potentiodynamically tested in artificial seawater (pH7) at room temperature and the corresponding Critical Pitting Temperatures (CPT) were determined according to ASTM G150. Secondary phase precipitation mainly affected the lean duplex grades whereas the high-alloyed DSS were more stable even if large precipitation occurred.

Abstract: The fracture characteristics of two AISI 316 austenitic stainless steels, creep-tested at constant load and temperatures of 600 and 800°C, were studied by scanning electron microscopy. The morphological aspects of the fracture were analyzed and correlated to the ductility level attained under creep. In one of the 316 steels, a marked change from ductile to intergranular mode of fracture was observed in going from 600 to 800°C. It is proposed that the condition for crack nucleation at precipitates as well as grain boundary oxidation are responsible for these microstructural changes.

Abstract: The aim of this paper is to research the influence of a different heat treatment of duplex austenitic-ferritic stainless steel to a microstructure. First, the initial data for numerical simulation were obtained by tensile test. Numerical simulation serves to determine the state of the workpiece during open die forging. The second stage focused on the evaluation of the microstructure in state after dwell time at forging temperature (7-and 10-hour) and cooling (water, air). Metallographic analysis observed the influence on precipitation of secondary phase especially.

Abstract: Current trend in increasing steam parameters in ultra-supercritical (USC) boilers requires new materials not only for membrane walls, headers and pipelines but also for superheater and reheater tubes. Newly developed austentitic steels Super304H, TP347HFG and HR3C exhibit superior resistance in steam thanks to their fine-grained microstructure, especially in case of Super304H and TP347HFG. The paper presents the results of verification of properties of these steels tubes including creep resistance of the base metal and welded joints, which show promising level of long-term creep strength of the base metal and weld joints and these results are supplemented by some new knowledge about the development of the microstructure of these steels, especially sigma phase appearance.

Abstract: Superduplex stainless steel is an important class of stainless steels because it combines the benefits of ferrite and austenite phases, resulting in steels with better mechanical properties and corrosion resistance. However, a significant problem of this steel is the precipitation of deleterious phases during heat treatment. Among these precipitated phases, the most relevant is the sigma phase, because it causes higher loss of properties. The objective of this work therefore is to study the sigma phase precipitation in the superduplex stainless steel UNS S32520 when submitted to heat treatment of solubilization in three different temperatures (1050 C, 1150o C and 1250° C) and subsequently aged in the temperature of 850oC during 10 minutes, 30 minutes, 60 minutes, 3 hours and 10 hours, followed by water quenching. The results showed that as the solubilization temperature increases, there is a significant grain growth and an increase of the ferrite volumetric fraction, which delays the sigma phase precipitation in this superduplex stainless steel. Moreover, it can be verified that the hardness of the material is directly related to volumetric fraction of sigma present in the steel.

Abstract: This article was aimed at studying the influence of different aging times during the precipitation phase of the UNS S31803 (SAF 2205) duplex steel. To do so, all the test specimens were solution heat treated at 1050oC for 1 hour and quenched in water. After this step, the samples were aged at 850°C for periods of 10, 60 and 600 minutes and cooled in water. The aging heat treatments (TT) were performed in order to precipitate the sigma phase. Samples were subjected to microhardness and tensile tests so that the influence of the microstructural changes over the mechanical properties of the steel was assessed. Based on the microhardness and tensile test data, it was possible to verify a significant change over the mechanical properties of the UNS S31803 steel.

Abstract: Influence of ageing temperatures and ageing time intervals on the intermetallic phases of high Mo superaustenitic stainless steel (SASS) is studied. To form the intermetallic phases the alloy was aged for 1 hour and 10 hours at different temperatures in the range of 500-1000°C. Chi forms at temperatures up to 800°C. Sigma forms above 900 or 1000°C depending upon the ageing time. A property diagram is simulated using Thermo-calc software to corroborate the formation of chi and sigma phases as a function of ageing temperatures. EDS of sigma indicated its higher Mo content compared to the chi phase. Chi is enriched in chromium while sigma is enriched in Mo.