Super304H Austenitic Steel Research Articles

Investigation of creep degradation in a Super304H austenitic steel and its nonlinear ultrasonic assessment method

ABSTRACT This paper reports the degradation behaviour of Super304H during the creep process. The failure mechanism of this Super304H is the typical intergranular damage, the creep voids and micro-crack are mainly concentrated on the GBs and triple point GBs. Meanwhile, there is a lot of precipitates inside void chain, those particles are the M23C6 phase, which indicate that the coarsening and growth of M23C6 phase are detrimental to the microstructural property of Super304H. Besides, the GND density of Super304H increased remarkably at the creep acceleration period, and the dislocation structure also changed from the pattern of straight-line to the zigzag. In addition, the nonlinear ultrasonic measurement results showed that there is a general upward trend of nonlinear parameter β’ of Super304H during creep. The increase of the nonlinearity effect in Super304H could be due to several factors such as the precipitates precipitation, dislocation density/structure change, and the microdefects formation.

Structural Stability of the SUPER304H Steel Used in Energetics.

This paper describes the influence of technological treatments (i.e., bending or welding) on the structural stability of SUPER304H austenitic steel used in reheaters and superheaters in fossil fuel power plants. Although the worldwide trend is transitioning to green power sources, the lifetime of existing power plants has to be prolonged until the transition is complete. Experimental material was tested in as-received state (straight tubes), bends, and homogeneous weld joints. Part of the specimens was solution-annealed after the technological operation. Afterwards, all the samples were thermally aged in furnace (650, 675 and 700 °C) for 7560–20,000 h. For comparison, bent specimens were placed at experimental sites on an operating powerplant for 10,000+ h. The long-term aging causes the formation of Cr-based carbides on the grain boundaries along with the Fe-Cr sigma phase. Combination of elevated temperature and residual stress accelerates formation of the sigma phase. This can be prevented by solution-annealing after bending. Mechanical properties were evaluated by Vickers hardness and tensile tests. The microstructure was observed using light optical microscopy (LOM) and scanning electron microscopy (SEM) with the energy-dispersive X-ray detector (EDXS). Electron backscatter diffraction (EBSD) and X-ray powder diffraction (XRPD) were used to characterize the brittle phases.

Creep degradation assessment of a Super304H austenitic steel by using misorientation‐derived parameters (electron backscatter diffraction)

AbstractThis paper reports the degradation assessment of Super304H during the creep by using misorientation‐derived parameters (EBSD). The results manifested that with increasing creep time, the change of misorientation parameters (i.e., Kernel Average Misorientation [KAM], grain reference orientation deviation [GROD], Grain Orientation Spread [GOS], and geometrically necessary dislocation [GND] density) of Super304H was significant. Meanwhile, the above parameters presented a non‐monotonic variation trend, which decreased slightly at first and then increased at the later. In addition, the increase amplitude of GROD and GOS was larger than the KAM and GND density, which manifest that the GROD and GOS are more suited to the creep degradation assessment of Super304H.

Effects of Friction Time on microstructures and mechanical properties of friction welded T92/Super304H dissimilar steel weld joints

In this paper, the T92 martensitic steel and the Super304H austenitic steel were welded by continuous drive friction welding technique. The friction welding joints exhibited good mechanical and metallurgical properties. The influence of friction time on the microstructure and mechanical properties of T92/Super304H dissimilar steel joints was investigated. With an increase in the friction time, the grains in the welding zone and the heat affected zone didn’t grow up and the number of second-phase particles precipitates increased. The tensile fracture location was in the Super304H base metal and didn’t change. With an increase in the friction time, the tensile strength didn’t change, and the impact toughness decreased.

Effects of Friction Speed on Microstructures and Mechanical Properties of Friction Welded Super304H Welding Joints

The Super304H austenitic steel were welding though friction welding technique. The Super304H joints showed excellent microstructure and mechanical properties. The effects for friction speed on the microstructure and mechanical properties of Super304H welding joints was investigated. With an increase in the friction speed, the grains didn’t grow up and second-phase particles precipitates increased in welding joints. The tensile fracture location was in the Super304H heat affected zone. When the friction speed was increased, the tensile strength increased and impact toughness was decreased.

Effects of Friction Pressure on Microstructures and Mechanical Properties of Friction Welded T92/Super304H Dissimilar Steel Weld Joints

In this paper, the T92 martensitic steel and the Super304H austenitic steel were welded by continuous drive friction welding technique. The friction welding joints exhibited good mechanical and metallurgical properties. The influence of friction pressure on the microstructures and mechanical properties of T92/Super304H dissimilar steel joints was investigated. With an increase in the friction pressure, the grains in the welding zone and the heat affected zone didn’t grow up and the number of second-phase particles precipitates increased. The Vickers hardness increased and the impact toughness decreased, the tensile fracture and fracture location didn’t change.

Effects of Friction Pressure on Microstructures and Mechanical properties of friction welded Super304H austenitic welding joints

In this paper, the Super304H austenitic steel were welded by continuous drive friction welding technique. The friction welding joints exhibited good mechanical and metallurgical properties. The influence of friction pressure on the microstructure and mechanical properties of Super304H welding joints was investigated. With an increase in the friction pressure, the grains in the welding zone and the heat affected zone didn’t grow up and the number of the precipitated phases increased. The fracture location of tensile strength samples was in the heat affected zone. With an increase in the friction pressure, the Vickers hardness and the tensile strength increased.

Ordering of Primary Carbonitrides in an Austenitic Steel Revealed by Transmission Electron Microscopy and Atom Probe Tomography.

We reveal for the first time an ordering phenomenon of a type of carbonitrides in a Super304H austenitic steel via the techniques of transmission electron microscopy and atom probe tomography. Solution-treated Super304H austenitic steel samples containing a few primary carbonitrides were aged at 923 K for 5000 h. The results show that the primary carbonitrides in the Super304H steel are non-stoichiometric compounds enriched in Nb and with a NaCl-type structure. The crystal structure of the Nb-rich carbonitrides is in a disordered state in the solution-treated steel, while in an ordered state in the aged steel. This observation suggests the occurrence of a disorder–order structural transition in the primary carbonitrides during long-term aging. We found that such a disorder–order structural transition is accompanied by carbon diffusion from the primary carbonitrides into the austenitic matrix. The ordering phenomenon is discussed based on the long-range ordering of structure vacancies in the non-metal lattice of the superstructure, providing new insights for the understanding of ordering in non-stoichiometric compounds.

Steady-State Creep Behavior of Super304H Austenitic Steel at Elevated Temperatures

Creep behavior of Super304H austenitic steel has been investigated at elevated temperatures of 923–973 K and at applied stress of 190–210 MPa. The results show that the apparent stress exponent and activation energy in the creep deformation range from 16.2 to 27.4 and from 602.1 to 769.3 kJ/mol at different temperatures, respectively. These high values imply the presence of a threshold stress due to an interaction between the dislocations and Cu-rich precipitates during creep deformation. The creep mechanism is associated with the dislocation climbing governed by the matrix lattice diffusion. The origin of the threshold stress is mainly attributed to the coherency strain induced in the matrix by Cu-rich precipitates. The theoretically estimated threshold stresses from Cu-rich precipitates agree reasonably with the experimental results.

Properties, structure and creep resistance of austenitic steel Super 304H

Abstract This paper presents selected material characteristics of Super 304H austenitic steel used for elements of boilers with super- and ultra-critical steam parameters. In particular, abridged and long-term creep tests with and without elongation measurement during testing and investigations of microstructural changes due to long-term impact of temperature and stress were carried out. The practical aspect of the use of creep test and structure investigation results in forecasting the durability of materials operating under creep conditions was presented. The characteristics of steels with regard to creep tests and structure investigations developed in this paper are used for estimation of changes in functional properties of the material of elements operating under creep conditions.

Coarsening and Hardening Behaviors of Cu-Rich Precipitates in Super304H Austenitic Steel

The coarsening and hardening behaviors of Cu-rich precipitates in Super304H austenitic steel aged at 923 K, 973 K, and 1023 K (650 °C, 700 °C, and 750 °C), respectively, have been investigated through measuring the particle size by transmission electron microscopy and microhardness. The results showed the Cu-rich precipitates have a cubic-to-cubic crystallographic relationship and coherent interface with the austenitic matrix during long-time aging, and that the coarsening behavior of the Cu-rich particles can be predicted with the help of the Lifshitz–Slyozov–Wagner theory. The activation energy for coarsening of the Cu-rich precipitates was evaluated to be 212 ± 3 kJ/mol. The coarsening of Cu-rich precipitates is controlled mainly by the volume diffusion of copper atoms in the austenitic matrix. The contributions to the maximum microhardness occurring at different aging temperatures from precipitation strengthening range from about 17 to 25 pct. The strengthening of the Cu-rich precipitates arises mainly from the coherency strain and partially from stacking-fault strengthening.

Precipitation of Nanosized MX at Coherent Cu-Rich Phases in Super304H Austenitic Steel

The present investigation of transmission electron microscopy reports the precipitation of nanosized and cubical-shaped incoherent Nb-rich MX at the coherent Cu-rich phases in the austenitic matrix of the Super304H steel. In addition, the nanosized Nb-rich MX phases were often observed to precipitate on dislocations during creep. It is concluded that the dense incoherent Nb-rich MX and coherent Cu-rich precipitates with a nanosized diameter contribute excellent creep resistance in the steel.

Tensile yield behavior and precipitation strengthening mechanism in Super304H steel

Tensile yield behaviors of the solution- and aging-treated Super304H austenitic steel have been investigated at temperatures from 300 to 923K. The results showed that the contribution to yield stress from precipitation strengthening of Cu-rich phases is 17–23%. The higher thermal activation volume and energy imply an interaction between dislocation and Cu-rich precipitates. The precipitation strengthening mainly arises from coherency strain and partially from stacking fault strengthening. The calculated shear stress from precipitation strengthening agrees reasonably with the experimental result.

High Temperature Materials for Nuclear Fast Fission and Fusion Reactors and Advanced Fossil Power Plants

Development of materials plays a crucial role in the economic feasibility of fast nuclear fission and fusion power plant. In order to meet this objective, one of the methods is to extend the fuel burnup and decreasing doubling time. The burnup is largely limited by the void swelling and creep resistances of the fuel cladding and wrapping materials. India's 500 MWe Prototype Fast Breeder Reactor (PFBR) is in advanced stage of construction. The major structural materials chosen for PFBR with MOX fuel are alloy D9 as fuel clad and wrapper material, 316LN austenitic stainless steel for reactor components and piping and modified 9Cr-1Mo steel for steam generator. In order to improve the burnup further, titanium, phosphorous and silicon contents in alloy D9 have been optimized for better swelling and creep resistances to develop modified version of alloy D9 as IFAC-1. Creep resistance of inherently void swelling resistance 9Cr-ferritic steel has been improved with the dispersion of nano-size yttria to develop oxide dispersion strengthened (ODS) steel clad tube with long- term creep strength, similar to D9, for increasing the fuel burnup. Development of modified 9Cr-1Mo steel clad tube and 9Cr-1Mo steel wrapper for future metallic fuel reactors being developed for reducing the doubling time are in progress. Extensive studies on resistance of this new generation core materials to void swelling are also under progress along with material development. Improved versions of 316LN stainless steel with nitrogen content of about 0.14 wt.% having higher creep strength to increase the life of fast reactor and modified 9Cr-1Mo steel with reduced nitrogen content and controlled addition of boron to improve type IV cracking resistance for steam generator are other developments. India's participation in ITER programme necessitates the development of India-specific RAFM steel for Test Blanket Module (TBM). A comprehensive research programme is being carried out to develop India-specific 9Cr-W-Ta RAFM steel with the optimization of tungsten and tantalum contents for better combination of strength and toughness. Based of the extensive mechanical tests including impact, tensile, creep and fatigue on four heats of RAFM steels having tungsten in the range 1 – 2 wt. % and tantalum in the range 0.06 -.014 wt., the RAFM steel having 1.4 wt. % tungsten with 0.06 wt. % tantalum is found to possess better combination of strength and toughness. This steel is considered as India-specific RAFM steel and TBM is being manufactured by this RAFM steel. To limit the emission of green house gases, a research and development programme has been initiated to develop advanced ultra super critical fossil fuel fired thermal power plants working at temperature of around 973 K and pressure of 300 bar. High temperature creep strength super 304H austenitic steel and Inconel 617 superalloy tubes are indigenously developed for this purpose.