Papers by Keyword: Duplex Steel

Abstract: The understanding of crack growth and therefore failure of components in geothermal energy production could lower maintenance costs tremendously. During in-situ corrosion fatigue experiments of high alloyed steel in geothermal brine the electrochemical potential was measured as well as temperature, frequency and pH of the surrounding geothermal brine. The decline of the electrochemical potential is in good agreement with estimated increasing linear crack propagation velocity. During initial crack propagation the electrochemical potential is in good agreement with the stress amplitude applied revealing that a steeper decline of potential indicates faster crack propagation. However, towards the end of propagation, this potential becomes independent of the stress amplitude.

Abstract: IIn a geothermal environment, cathodic protection is employed to improve resistance against corrosion fatigue. However, during the cathodic reactions under applied potential, hydrogen is generated and assimilated, leading to a reduced lifetime expectancy of high-alloyed steels. The corrosion fatigue mechanism of a standard duplex stainless steel X2CrNiMoN22-5-3 (1.4462) specimen loaded with hydrogen was studied in a corrosion chamber specifically designed for the purpose, surrounded by the electrolyte of the Northern German Basin at 369 K. The microstructural reactions resulting in hydrogen incorporation significantly decrease the number of cycles to failure of the specimen. This reduction is attributed to hydrogen enhancing crack propagation and causing early failure, primarily due to the deterioration of the mechanical properties of the ferritic phase rather than corrosion reactions or corrosive degradation.

Abstract: Standard duplex stainless steel X2CrNiMoN22-5-3 is resistant to both, corrosion and mechanical stress, but corrosion fatigue (CF) lowers the lifetime expectancy in a geothermal environment such as the Northern German Basin. Laboratory experiments used the in-situ electrolyte at 369 K in a specifically designed corrosion chamber applying rotation bending cyclic load to failure. CF behaviour was compared to pure push-pull load. Corrosion kinetics are independent of the load applied. Failure is initiated by pits resulting in mechanical degradation. Increased thickness of the passivating layer surrounding pits enhances degradation or delamination. Also, sharp notches located along the pit edge increase notch effects and stress concentration consequently leading to fast crack propagation and early failure.

Abstract: In geothermal environment corrosion fatigue lowers the lifetime expectancy of high alloyed steels. Therefore, cathodic protection enhances corrosion resistance although hydrogen is produced during the cathodic reactions under applied potential. Corrosion fatigue of standard duplex stainless steel X2CrNiMoN22-5-3, 1.4462 specimen loaded with hydrogen was investigated in a specifically designed corrosion chamber surrounded by the Northern German Basin electrolyte at 369 K. The reactions of the microstructure associated with hydrogen incorporation lowers the number of cycles to failure of specimen significantly at various stress amplitudes, for example at σ_a = 275 MPa and U_SHE = -36 mV by 80%. Hydrogen enhances crack propagation and early failure due to the degradation of the mechanical properties of the ferritic phase.

Abstract: This work aims to optimize the process parameters for laser based DED manufacturing of a water jet impeller with critical requirements to material and mechanical properties. The concerned material is 2205 duplex stainless steel with balanced ferrite-austenite microstructure. The optimization approach of the processing parameters relies on the assessment of their effect on microstructure and mechanical properties versus the requirements set by the maritime industry. The work aims to achieve an as-built microstructure with the required ferrite-austenite balance and mechanical properties, without the need for post-processing heat treatment. The work particularly focuses on the influence of the deposition speed of the DED process. The results show that duplex stainless steel with a 50-50 ferrite-austenite balance can be achieved directly from the DED process. A high deposition speed produced fine-grained microstructure resulting in a high tensile strength and toughness, well above the set requirements. However, it reduced the ductility, represented by tensile elongation due to the formation of welding defects. Reducing the deposition speed by 20% eliminated the welding defects but resulted in the formation of a distinct microstructure with coarse grains, elongated in the deposited layer. This microstructure improved the tensile elongation, but strongly reduced the toughness, represented by Charpy V impact energy values. The coarse grains in the deposited layer facilitated a fast fracture propagation initiated by the placement of the Charpy V notch. However, the presented results demonstrate the great potential for manufacturing duplex stainless steels by DED, where a suitable microstructure for optimal mechanical performance can be obtained by narrowing the optimization windows on the process parameters.

Abstract: The cold-rolled Fe-8Mn-6Al-0.4C duplex low-density steel was annealed at different conditions to obtain ferrite + austenite duplex microstructure. The excellent mechanical properties (i.e., elongation of 52%, tensile strength of 785 MPa, and a product of tensile strength and elongation of 40.9 GPa·%) have been obtained by adjusting the volume fraction and the stability of austenite. The microstructure of the experimental steels was analyzed by scanning electron microscopy (SEM) and electron back-scatter diffraction (EBSD), and the volume fraction of austenite was estimated by the X-ray diffraction (XRD). The results show that the distribution of austenite grain size is inhomogeneous, and that the mechanical stability of austenite is mainly affected by the alloying partitioning and the variation of grain size during the annealing process. The increase of elongation is attributed to the degradation in mechanical stability of austenite, which can efficiently promote an occurrence of transformation induced plasticity (TRIP) effect.

Abstract: Duplex steels are an important group of materials which are generally used for applications where resistance to corrosion, or high strength and creep resistance at elevated temperatures, are required. They are used extensively in nuclear plant. Although ultrasonic inspection methods have been routinely used in industry for some three decades, it is well known that cast or welded austenitic components can be difficult, or even impossible, to examine ultrasonically. Development of ultrasonic techniques is therefore in progress in several countries to provide improvements which are being sought on safety and economic grounds.The aim of this paper is a brief description of the relevant metallurgical characteristics given before a consideration of the physical properties of the weld metal and the current theoretical models used to describe ultrasound propagation in it. The paper deals with the steps taken to improve the capabilities of ultrasonic inspection and includes a discussion of the problems of flaw location and sizing.

Abstract: This study addressed the evolution of microstructure across some duplex stainless steel joints welded by laser beam without and with applying additional materials. The chemical composition and the thickness of the parent material modify the welded joints profile also. Similar technological parameter results different macrostructure and ferrite content at different type of additional material. Pitting corrosion resistance were also studied.

Abstract: The aim of this work was to analyze the performance of joints made by TIG (Tungsten Inert Gas) welding process in austenitic and duplex stainless steels with special regards to their corrosion resistance. Three different types of stainless steel were butt welded with TIG method. Ferric-chloride test and electrochemical treatments revealed how does the TIG process affects the corrosion resistance depending upon the alloy used for welding the joint. This work focuses on the weldability of the 2304, 2404 and 304 type stainless steel heterogeneous welds.

Abstract: Residual stress measurements were carried out on duplex steel samples using X-ray diffraction technique. Directional residual stress was investigated on the surface of the heat effected zone of joints. Spatial residual stress distribution were examined in the ferrite and austenite phases separately, using different radiation-ray source. The different mechanical properties of each phases were taken into account during the stress calculations. Noticeable stress gradient was observed between ferrite and austenite phases.