Super304H Austenitic Stainless Steel Research Articles

The study of the evolution of the microstructure and creep properties of Super 304H austenitic stainless steel after aging for up to 50,000 h

The study of the evolution of the microstructure and creep properties of Super 304H austenitic stainless steel after aging for up to 50,000 h

Microstructural evolution and stress relaxation cracking mechanism for Super304H austenitic stainless steel weld metal

• Stress relaxation cracking in Super304H austenitic stainless steel weld metal. • The pre-compressed CT technique combined with FEM analysis was successfully used. • The mechanism responsible for stress relaxation cracking formation was illustrated after short-term aging and long-term aging. The pre-compressed CT technique was used to quantitatively investigate the formation of stress relaxation cracks under different tensile residual stresses and aging time in Super304H austenitic stainless steel weld metal. The statistical results revealed that intergranular cracks could occur within 2000 h under 650 °C when the residual stress was applied with greater than 18 KN pre-compression force. Detailed grain interior and boundary analyses showed that the growth of intragranular Cu-rich particles could induce a strong grain interior, and the intergranular Nb(C, N) carbides were one of the causes to crack under short-term aging time. For long-term aging time conditions, the intergranular M 23 C 6 carbides were more susceptible to crack than intergranular Nb(C, N) carbides. Finally, the mechanism responsible for stress relaxation cracking formation was carefully illustrated for the weld metals after short-term aging and long-term aging, respectively.

Effect of Nb on the microstructural evolution and microhardness of Super304H austenitic stainless steel weld metals during long-term aging

Abstract The microstructural evolution and mechanical properties of Super304H austenitic stainless steel weld metals with Nb-free and 1.0 % Nb-additive were investigated during 650 °C long-term aging. Massive nanoscale M 23C6 particles distributed in the interdendritic regions of the Nb-free weld metal while most of the M 23C6 formed and coarsened on the phase interface between the primary Nb(C, N) phase and matrix in the Nb-bearing weld metal. Nano-sized Cu-rich particles dispersed inside the grains in both weld metals. All of these nano-sized particles gradually coarsened over time. The Cu-rich particles with high distribution density and nanometer size affect the microhardness of both weld metals most during long-term aging. The addition of Nb could increase the microhardness in the relatively short aging time, while the effect of Nb-addition on the microhardness in long-term aging is insignificant.

Implication of initial grain size on DRX mechanism and grain refinement in super-304H SS in a wide range of strain rates during large-strain hot deformation

In this work, we studied the effect of initial grain size on the hot compression characteristics of super-304H austenitic stainless steel in a range of strain rates (0.001–1 s−1) at a fixed temperature of 1223 K. Analysis of the flow curves reveals that the flow stress is inversely proportional to the average sub-grain diameter in both coarse and fine-grained specimens at higher strain (≥0.5) levels. Further, the fine-grained specimen following deformation at a low strain rate (0.001 s−1) reveals the occurrence of both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) mechanisms. In this condition, the CDRX is characterized by a progressive increase of boundary misorientation, while the DDRX is characterized by bulging and strain-induced boundary migration. In contrast, the CDRX characterized by the formation of microbands is majorly responsible for the grain refinement in the fine-grained specimen at higher strain rates (∼1 s−1) and in the coarse-grained specimen at all strain rates (0.001–1 s−1). The superposition of different DRX mechanisms leads to significant variations in grain refinement kinetics with strain rates and initial grain sizes. The findings of this investigation provide a foundation for the accurate control of the microstructures in the studied alloy with different initial grain sizes during the hot working at various strain rates.

Intergranular precipitation behavior and its influence on the stress relaxation cracking susceptibility of Super304H austenitic stainless steel weld metal during long-term aging

Stress relaxation cracking failure is widely reported in the materials used for thermal power plants, which seriously affected the safety of thermal power units under long-term service. In our study, the influence of intergranular precipitation behavior on the stress relaxation cracking susceptibility of Super304H austenitic stainless steel weld metal was investigated based on the pre-compressed CT technique. The 45° misorientation grain boundary has the lowest critical interfacial nucleation energy, followed by the grain boundaries away from the 45° misorientation. As a result, the coarse intergranular M23C6 carbides were more prone to precipitate at 45° misorientation grain boundary, which led to the lowest resistance to stress relaxation cracking susceptibility at 35 to 45° grain boundary. Further analysis indicated that intergranular cavities were easily initiated at the non-coherent M23C6/γ interface. Since the non-coherent interface with a low Cr concentration zone and high specific interfacial energy had a lower strength, the lattice distortion at the non-coherent interface could lead to higher strain concentration.

Creep lifetime prediction of virgin and service-exposed Super304H austenitic stainless steel boiler tubes based on hierarchical multiscale analysis and creep cavitation model

ABSTRACTCreep deformation of Super304H austenitic stainless steel boiler tubes in thermal power plants is evaluated using crystal elasto-viscoplastic finite element analysis (CEV-FEA). The material properties of Super304H at elevated temperatures are obtained from experiments and computations at different scales as follows: anisotropic elastic constants of Super304H at elevated temperatures (500–700°C) are obtained from molecular dynamics simulations; microstructures of virgin and service-exposed Super304H boiler tubes are observed using scanning electron microscopy; and macroscopic stress–strain and creep strain curves at 650°C are measured from tensile and miniature creep tests, respectively. Stress concentration evolution at a triple junction of grains featuring various combinations of crystallographic orientations in polycrystalline Super304H is evaluated using CEV-FEA during creep deformation. The remaining creep lifetime of polycrystalline Super304H austenitic stainless steel boiler tubes is estimated from the creep cavitation model, where the stress concentration at the triple junction is used as a representative stress driving creep cavitation.

An insight into oversaturated deformation-induced sigma precipitation in Super304H austenitic stainless steel

A new mechanism of the deformation-promoted precipitation of the sigma phase during aging at 650 °C in nanocrystallized Super304H austenitic stainless steel was proposed for its extremely early nucleation and subsequently fast growth. Unlike the reports of sigma phase nucleation at the recrystallizing grain boundaries in other deformed steels, the sigma phase was found to nucleate before recrystallization during aging in the over-saturated shot-peened Super304H steel at stress concentrations with high distortion energy (nanograin boundaries, twinning intersections and interfaces of dispersion strengthening phases like Nb(C,N)). The high-distortion areas favored the segregation of chromium and consequently facilitated the structure transformation from face-centered cubic austenite to topologically close-packed sigma phase. Sigma phase particles were found to grow slowly in the early stage of aging due to the residual compressive stress field in the deformed austenite matrix, and began to grow abnormally quickly to be 1–2 μm in size at the recrystallizing boundaries when recrystallization commences, due to both the release of residual compressive stress at the recrystallized region and the fast chromium diffusion at the deformed nanostructure region. To avoid the occurrence of sigma phases, the degree of surface deformation should be controlled to be lower than the deformation saturation value.

Hot corrosion behaviors of Super 304H austenitic stainless steel pre-coated in Na2SO4–25%NaCl mixture salt film

Hot corrosion rates of Super 304H austenitic stainless steel pre-coated in Na2SO4–25%NaCl mixture salt film in air at 650 °C and 750 °C have been investigated by corrosion kinetics curves. The corrosion products are examined by means of X-ray diffraction and scanning electron microscopy coupled with energy-dispersive spectrometer, and electron probe micro-analyzer to analyze the scale structure and element distribution of the cross section. The results indicate that the kinetics curves are similar and display parabolic growth, and the mass gain obviously increases with the increasing temperature. A two-layer oxide scale composed of Fe oxides containing CuFe2O4 and Cr2O3 forms on the surface of the alloy and easily exfoliates with extending corrosion time or with improving corrosion temperature. Moreover, a corrosion-affected zone with micropores and micro-cracks appears beneath the oxide scale. It is concluded that the selective oxidation occurs and a protective oxide scale forms at the early corrosion stage. The melt salts destroy the integrity of the oxide scale and accelerate hot corrosion of the alloy by the cyclical oxidation–chlorination during the further corrosion processing. In addition, internal sulfidation also contributes to the corrosion of the alloy.

The negative effect of high-intensity shot-peening on the intergranular corrosion behavior of the Super304H austenitic stainless steel

The influences of high-intensity shot-peening (SP) on the microstructure and intergranular corrosion susceptibility (IGCS) of the Super304H stainless steel during aging were investigated. The high-intensity SPed samples exhibited a quicker sensitization and desensitization process when aging at 650 °C, in comparison with the low-intensity SPed and coarse grained counterparts. With aging time increase from 24 h to 168 h, however, their IGCS increased again because the high energy stored inside the highly deformed surface promoted the recrystallization process and the formation of σ-phase was accelerated at the recrystallizing interfaces with low nucleation energy barrier and induced additional Cr-depleted zones.

Interpretation for the fast sigma phase precipitation in the high intensity shot peened nanocrystallined Super304H stainless steel

Surface shot peening (SP) with an intensity of 0.5 MPa/12 min plus 650 °C aging was performed on the Super304H austenitic stainless steel with an attempt to decrease its intergranular corrosion susceptibility more effectively. It was found that the local stress/strain concentration with higher energy at some typical sites caused by over saturated cold deformation induced the early nucleation of sigma phase and the enhanced chromium diffusion in nanocrystallized microstructure promoted its fast growth in the material. The critical shot peening time to trigger fast precipitation of sigma phase is between 8 and 12 min under 0.5 MPa for the Super304H steel. The occurrence of massive sigma phase during aging brought a positive move of the reactivation potential of chromium depleted zones around sigma phase compared to M23C6.

Investigating the effect of Cu-rich phase on the corrosion behavior of Super 304H austenitic stainless steel by TEM

The precipitation of a fine Cu-rich phase dispersedly distributed in the austenitic matrix can impart the novel properties of Super 304H SS. However, the contributions have not been well understood due to the difficulty encountered in obtaining precise experimental information on the corrosion events introduced by the nano-sized Cu-rich precipitates. In this work, we have applied TEM and clarified the dissolution occurred in Cu-rich phase and the dissolution-induced evolution in structure and chemistry on the surface. This work enabled us glean atomic scale insights on the effect of the Cu-rich phase on the corrosion behavior of the Super 304H SS.

Magnetism variance analysis of Super304H austenitic stainless steel after service

Abstract Magnetism variance of Super304H austenitic stainless steel superheater tubes after 36891h and 41000h service in the ultra-supercritical boilers was investigated by using optical microscope, scanning electron microscope, X-ray diffractometer, PPMS system (vibrating sample magnetometer, VSM), EBSD and electrochemical workstation. The results show that grain size of the 36891h serviced tube is extremely large with grade 2, and magnetization was detected in the tube, which was confirmed with a small amount of ferromagnetic phase distributing along some grain boundaries by EBSD observation, while the tube of 41000h service has a small grain size of grade 7–8 and no ferromagnetic phase or magnetization is present. The morphologies after DL-EPR test show that the intergranular corrosion susceptibility of the 41000h tube is higher, but the depth and width of the Cr-depleted zones at grain boundaries are significantly smaller than the 36981h tube. The precipitates in Super304H after service did not cause significant changes in its magnetic properties. Coarse grains, which led to reduction of the diffusion channels, made Cr difficult to diffuse from the matrix to Cr-depleted zones. As a result, the Cr content in the Cr-depleted zones maintained a lower level and resulted in the increase of Ms of the Cr-depleted zones, which induced martensitic transformation of Super304H tubes during the shut-down to room temperature of the boiler.

Influence of high temperature softening and solid solution on intergranular corrosion susceptibility of Super304H stainless steel

ABSTRACTThermos-mechanical processes were investigated to suppress the high intergranular corrosion sensitivity of Super304H austenitic stainless steel. The microstructure and grain boundary character distribution were characterised, respectively, by scanning electron microscope and electron backscatter diffraction analysis. The degree of sensitisation (DOS) was evaluated by the double-loop electrochemical polarisation reactivation test. The results show that the bulk primary Nb(C,N) particles were dissolved more into the matrix after high temperature softening at 1300°C for more than 30 min and the special boundaries (∑ ≤ 29) maintained at the same level as the as-received state after solid solution at 1150°C. The DOS was decreased after high-temperature softening and solid solution treatments, which was attributed mainly to the dissolution of primary bulk Nb(C,N) phase during high-temperature softening and the dispersive precipitation of the secondary fine Nb(C,N) phase during solid solution, which had effectively consumed carbon.

Effect of Surface Nanocrystallization on the Sensitization and Desensitization Behaviors of the Novel Super304H Stainless Steel

The newly developed Super304H austenitic stainless (SS) steel is being widely used in ultra supercritical fossil fired boilers because of its superior high temperature strength and oxidation resistance, but its high carbon content has increased greatly its intergranular corrosion susceptibility (IGCS). In the present paper, we have made an attempt to understand the sensitization and desensitization behaviors of the shot-peened Super304H SS during aging around service temperature. The shot peening on the Super304H SS resulted in its surface nanocrystallization accompanied with strain-induced αʹ-martensite. The strain-induced αʹ-martensite had good thermal stability at 600℃ and was blamed mainly for the significant increase of the degree of sensitization (DOS) of shot-peened Super304H SS when aged at 600℃ for different time. However, the DOS decreased greatly when aged at 650℃ for 24 h or 700℃ for 2 h, indicating the occurrence of a faster self-healing process of chromium depletion zones due to long range Cr diffusion enhancement by surface nanocrystallization and also the reverse transformation of αʹ-martensite to austenite at the same time. Figure 1

EBSD Analysis and Hot Tensile Properties of Pulsed Current Gas Tungsten Arc Welded Super 304h Austenitic Stainless Steel Joints

Super 304H austenitic stainless steel tubes containing 2.3 to 3 (%wt) of Cu is mainly used in superheaters and reheater of ultra-supercritical boilers. Welding high alloyed steels such as stainless steels can result in local variation of alloying elements within the weld metal and such segregation may affect the high temperature properties of the weld joint. It is preferable to control the solidification structure of the weld by altering the prevailing thermal gradients in the weld pool during welding. The effect of current pulsing on microstructure and hot tensile properties of filler added gas tungsten arc welding (GTAW) of super 304H is studied. Current pulsing is found to be beneficial in improving the hot tensile properties of super 304H GTAW joints at all test temperature, which is attributed to the grain refinement, increase in frequency of high angle grain boundaries, reduced segregation and finer carbonitrides precipitation.

Effect of Shot Peening on the Intergranular Corrosion Susceptibility of a Novel Super304H Austenitic Stainless Steel

The surface phase constituent of Super304H austenitic stainless steel, after shot peening and sensitization treatment at 600, 650, and 700 °C for 2 h, was characterized using x-ray diffraction method. The degree of sensitization (DOS) was investigated by means of double-loop electrochemical potentiokinetic reactivation (DL-EPR) test, and the morphology after DL-EPR test was observed by scanning electron microscopy (SEM). The results showed that nano-sized grains and strain-induced martensite together with compressive residual stress formed on the surface of Super304H steel after shot peening. Surface compressive residual stresses relaxed greatly after being sensitized at 600-700 °C for 2 h, and no systematic correlation was observed between the compressive residual stresses developed and the intergranular corrosion susceptibility (IGCS). Because of the occurrence of strain-induced martensite in the shot-peened specimens, their IGCS is much higher than that of the as-received specimen when being sensitized at 600-650 °C for 2 h. Besides, the DOS increased with the increasing of shot peening time and the content of strain-induced martensite. On the contrary, the IGCS of Super304H stainless steels subjected to shot peening was eliminated when being sensitized at 700 °C for 2 h because of the reverse transformation of strain-induced martensite and faster diffusion rate of Cr at higher temperature in ultrafine-grained austenite which had helped healing the chromium depletion zone in a very short time. In a word, shot peening promoted desensitization of Super304H steel in a time shorter than 2 h at higher temperature up to 700 °C.

Effect of filler addition on solidification behaviour and hot tensile properties of GTA-welded tube joints of Super 304H austenitic stainless steel

Super 304H austenitic stainless steel tubes containing 2.3 to 3 (wt.%) of Cu are mainly used in superheaters and reheaters of ultra super critical boilers due their excellent corrosion and oxidation resistance. Cu addition causes precipitation strengthening effect by fine Cu-rich precipitates which evolve during creep conditions and results in increased creep strength. The microstructural evolution of stainless steels during welding significantly affects the material properties. In this work, solidification mode microstructure and hot tensile properties of autogenous and filler-added Super 304H gas tungsten arc (GTA) welded tube joints were correlated. Autogenous welds of Super 304H solidified as austenite with 1.57 % delta ferrite and intercellular (Fe,Cr)23(C,B)6 borocarbides. In filler-added welds, the higher Ni equivalent and addition of carbide stabilizing elements (Nb,Mo) eliminated δ ferrite and segregation of B as borocarbides. The filler-added welds exhibited superior tensile strength than autogenous welds at both room and high temperature.

B22-P-04Effect of Shot Peening on the Microstructural Evolution of a Super304H Austenitic Stainless Steel

B22-P-04Effect of Shot Peening on the Microstructural Evolution of a Super304H Austenitic Stainless Steel

Stress corrosion cracking behaviour of gas tungsten arc welded super austenitic stainless steel joints

Abstract Super 304H austenitic stainless steel with 3% of copper posses excellent creep strength and corrosion resistance, which is mainly used in heat exchanger tubing of the boiler. Heat exchangers are used in nuclear power plants and marine vehicles which are intended to operate in chloride rich offshore environment. Chloride stress corrosion cracking is the most likely life limiting failure with austenitic stainless steel tubing. Welding may worsen the stress corrosion cracking susceptibility of the material. Stress corrosion cracking susceptibility of Super 304H parent metal and gas tungsten arc (GTA) welded joints were studied by constant load tests in 45% boiling MgCl 2 solution. Stress corrosion cracking resistance of Super 304H stainless steel was deteriorated by GTA welding due to the formation of susceptible microstructure in the HAZ of the weld joint and the residual stresses. The mechanism of cracking was found to be anodic path cracking, with transgranular nature of crack propagation. Linear relationships were derived to predict the time to failure by extrapolating the rate of steady state elongation.

Effects of Boron on Microstructure and Metastable Pitting Corrosion Behavior of Super304H Austenitic Stainless Steel

The effects of boron on the microstructure and the pitting corrosion behavior, especially the metastable pitting, were investigated in Super304H austenitic stainless steels with 0–130 ppm boron solution treated at 1100°C. The microstructures were analyzed using scanning electron microscopy, transmission electron microscopy with energy dispersive spectroscopy and secondary ion mass spectroscopy. Meanwhile, the pitting corrosion behavior was evaluated by potentiodynamic and potentiostatic polarization tests in 3.5% NaCl solution at 25°C. Microstructural characterizations showed that boron addition favored the precipitation of Cr-rich boride (M2B), which resulted in an asymmetric Cr-depleted zone around M2B and reduced the chromium content in the austenite matrix. The Cr-depleted zones could act as extra sites for pit initiation, along with the interfaces between the austenite matrix and non-metallic inclusions. The potentiodynamic polarization test results indicated that boron addition decreased the pitting potential. The potentiostatic test results revealed that boron addition promoted the initiation frequency and growth rate of metastable pits, and increased the probability of stable pitting.