Super304H Steel Research Articles

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.

Analysis of the precipitated phase changes for Super304H heat resistant steel during shot time aging at 800°C

For analyzing the change trend of Super304H heat resistant steel steel precipitated phases, this article adopts the method of high temperature aging to simulate the material conditions during actual working environment. Super304H steel aged under 800 °C for short time 10 h, 20 and 30 h. And analyzed SEM morphology, precipitate phase statistics, composition changes of the original sample and the samples after aging. The results showed that the number of precipitated phase increased first, then decreased and then increased with the prolong of aging time, while the average size and area fraction of precipitated phase increased all the time. In the precipitated phase, Cr continued to increase while Ni decreased.

Study on Creep Rupture Strength of Friction Welded Super304H Steel Weld Joints

In this paper, the creep rupture strength for Super304H weld joints was evaluated by isothermal method. Service life prediction curves indicate that Super304H weld joints could guarante safety service for 105 hours at 625°C, which is in according with the Ultra-supercritical units steam conditions. When the stress is higher, the rupture mode of creep specimens was transcrystalline fracture, and results in forming dimples. When the stress is lower, the rupture mode of creep specimens was creep rupture, and results in cleavage rupture. So under the effect of long term low stress, creep cavities joined each other along the grain boundary.

Evaluation of intergranular corrosion susceptibility of Super304H steel after being aged at 600°C

The susceptibility to intergranular corrosion of as-received and 600°C aging-treated new austenite heat-resistance Super304H steel was investigated by double loop electrochemical potentiodynamic (EPR) test. Experimental results indicated that the as-received Super304H steel possesses lower tendency of intergranaular corrosion. With the increase of aging time, intergranular corrosion susceptibility of Super304H steel increases. Intergranular corrosion of Super304H steel occurs due to the precipitation of Cr23C6 along grain boundaries. At the initial stage of aging, the rapid precipitating behavior of Cr23C6 leads to the rapid increase of intergranlar corrosion susceptibility. The amount of Cr23C6 precipitates increases with the extension of aging time, and the intergranular corrosion susceptibility of Super304H steel increases accordingly. The extent of intergranular corrosion change is related to the precipitating rate of Cr23C6. ()

Preparation and characterization of slurry aluminide coating on Super304H boiler tube in combination with heat-treatment process

Aluminide coating, intended for application in ultra-supercritical boilers with a steam temperature of 650 °C, was deposited on the inner surface of Super304H steel tube in combination with its heat treatment. Relevant thermophysical, chemical, and mechanical properties of aluminized Super304H were characterized. The results indicated that the aluminide coating significantly improved steam oxidation resistance of Super304H by forming an extremely thin Al2O3 scale. This was superior to shot-peening treatment and use of steels with a higher Cr content. The aluminide coating showed very limited impact on the thermophysical properties of the matrix alloy due to its thin nature. The tensile strength and creep-rupture properties of Super304H were all within the allowable range for Super304H steel according to the ASME code. The high intrinsic strength of FeAl intermetallics slightly improved the yield strength of aluminized Super304H and microcracks relieved the stress in the coating, thus causing creep fracture on the uncoated side of samples. This research suggested that slurry aluminide coatings could be a viable option for application of Super304H steel in supercritical boilers with higher steam temperatures.

Precipitate evolution during the aging of Super304H steel and its influence on impact toughness

Super304H, which is an austenitic heat-resistant steel, was aged at 650 °C for up to 5000 h. Its microstructural evolution during aging was observed by scanning electron microscopy (SEM) equipped with electron backscatter diffraction (EBSD). The precipitate evolution in the aged specimens was analyzed by both X-ray diffraction (XRD) and transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) and selected area electron diffraction (SAED). In addition, the precipitation kinetics of M23C6 carbide was established. Hardness and impact tests were performed on the aged specimens. Finally, the relationship between the mechanical properties and microstructural evolution was revealed. After aging, Super304H steel exhibited the same fine grains as in the as-received condition with a Cu-rich phase, secondary Nb(C,N) precipitates inside the grains, and M23C6 carbides mainly at grain boundaries. The Cu-rich phase and the secondary Nb(C,N) precipitated in specimens aged for 500 h; this leads to a marked increase in hardness. Precipitation of the Z phase also occurred in the aged Super304H steel. M23C6 carbides precipitated rapidly during aging, with content approaching 90% of the saturated content in specimens aged for 5000 h. The reduction in impact toughness in Super304H steel was found to be associated with the precipitation of the M23C6 carbide, which is similar to that in the HR3C steel. However, the deterioration was much less significant as compared to HR3C steel because of the discontinuous distribution of carbides at grain boundaries due to the fine grain size.

Investigation on Oxidation Behavior of Super304H and HR3C Steel in High Temperature Steam from a 1000 MW Ultra-Supercritical Coal-Fired Boiler

Oxidation behavior of Super304H and HR3C steel in high temperature steam from an ultra-supercritical coal-fired boiler was investigated in this paper. The results showed that the steam oxidized surface of Super304H ware composed of Fe2O3, Cr2O3 and FeCr2O4, the oxide scale had a thickness of 50–70 μm. In addition, the steam oxidized surface of HR3C ware composed of Fe2O3, the oxide scale was about 20μm in thickness and contained few pitting. The oxidation product layer of the two samples could be divided into two layers, including outer layer enriched O element and Fe element, and inner layer enriched O element and Cr element. Furthermore, oxide scale spalling was observed on the surface of Super304H sample.

Oxidation Behavior of Austenitic Steels in Supercritical Water Containing Dissolved Oxygen

Oxidation tests of austenitic steel Super304H and HR3C were conducted at 550-600 °C in supercritical water under 25 MPa with the dissolved oxygen content. The oxidation rate of Super304H rapidly increased with increasing temperature, while the weight change of HR3C at 600 °C is slightly larger than that at 550 °C. A double-layer oxide scale developed on Super304H and HR3C steel, which was made up of an Fe-rich outer layer and a Cr-rich inner layer. The criterion contents of Cr were calculated for Super304H and HR3C. The effect of temperature and the Cr content on oxidation rate and oxide composition were discussed.

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.

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.

Post-weld Heat Treatment and Groove Angles Affect the Mechanical Properties of T92/Super 304H Dissimilar Steel Weld Joints

AbstractThe microstructures and mechanical properties of dissimilar weld joints between T92 and Super 304H steels were investigated. Dissimilar weld joints with four groove angles were constructed using gas tungsten arc welding. The results showed that post-weld heat treatment improved the mechanical properties of the dissimilar weld joints. The optimal groove angle for T92/Super 304H dissimilar weld joints was found to be 20°, considering mechanical properties. Furthermore, the transformation from equiaxed dendrites to columnar dendrites was observed in the weld metal. Epitaxial growth and delta ferrites were found around the fusion line between the Super 304H and the weld metal.

Aspekty strukturalne różnorodnych złączy spawanych ze stali Super 304H i T91

The paper presents the results of macro- and microscopic tests of heterogeneous welded joints made of Super 304H steel and T91 steel. The examined welded joints were made with the 141 (TIG) method in the PF welding position, using a Metrode EPRI P87 wire as the additional material. The tests were performed on the joint after welding and after heat treatment which consisted in the stress relief annealing at the temperature of 760 °C and soaking time of 2 hours. The investigated joints were marked respectively: S1 (the joint with- out heat treatment) and S2 (the joint after heat treatment). The process of welding, as well as the heat treatment in the area of the heat-affected zone (HAZ) in the case of Super 304H steel contributed to the precipitation processes mostly on the boundaries of grains. In the HAZ of T91 steel these processes caused the development of a degraded martensitic structure with numerous precipitates of diverse morphology.

Oxidation behaviour of Super 304H stainless steel in supercritical water

ABSTRACTOxidation of Super 304H steel in supercritical water at 600°C and 25 MPa was investigated, for the oxidation time of up to 1000 h. The oxidation kinetics approximately followed a parabolic law. The composition and microstructure of the oxide were investigated using the SEM equipped with EDS, EBSD and XRD. The phase compositions of the oxide evolve with increasing time. Cr2O3 was present at the initial stage and not detected while Fe2O3 appeared at longer time. EBSD indicated that continuous Cr2O3 formed at the interface of oxide and matrix for HR3C at 600°C for 40 h but only scattered Cr2O3 can be observed for 200 h. After the formation of a thin Fe–Cr–Ni oxide film on the Super 304H steel surface, the Fe-rich outer nodular oxide begins to grow and forms a continuous layer after 1000 h. The growth process of Super304H steel in supercritical water was discussed.

Microstructure and phases of deposited metal of SUPER304H steel under high temperature Persistent stress

In order to investigate the high temperature rupture property of deposited metal of SUPER304H steel, the high temperature tensile test was carried out, and the microstructure transformation of deposited metal of SUPER304H steel under high temperature persistent stress were studied. Most of the solidification subgrain boundaries dissolve. The effect of the high temperature enduring on the microstructure is not obvious. Temperature and time are the main factors that influence the change of microstructure. Under the action of high temperature stress, the corrosion resistance of austenite decreases significantly due to the occurrence of chromium deficiency. With the persistent stress of 200 MPa, the precipitated phase of deposited metal is Nb (C, N), M23C6, NbCrN phase, and a certain amount of alpha phase is precipitated in the deposited metal with a persistent stress of 78 MPa. The precipitation of M23C6 phase is the main reason for the decrease of the corrosion resistance, especially the decrease of the corrosion resistance.

Precipitation Kinetics of M23C6 Carbides in the Super304H Austenitic Heat-Resistant Steel

The precipitation kinetics of M23C6 carbides in Super304H and TP304H steels were investigated using the selective-etching method, SEM backscattered electron images and Image-Pro-Plus 6.0 software. Precipitation–temperature–time (PTT) diagrams of M23C6 carbides in the as-received Super304H (fine grains), coarsened Super304H (coarse grains) and TP304H (coarse grains) steels all show the typical C-shaped character with nose temperature range from 800 to 850 °C. Compared with the TP304H steel, the same trend is found of the PTT curve of M23C6 carbides for both kinds of Super304H steels, but their start lines move to the right and finish lines to the left. The preferential formation of Nb(C,N) phase at grain boundaries in the Super304H steels inhibited the nucleation of M23C6 carbides in the early stage of precipitation, causing the right shift of the start line of PTT curve. The main reason for the left shift of the finish line of the two Super304H steels was the quicker growing and coarsening rate of M23C6 in the later precipitation stage due to their higher C content than in TP304H. For the difference in PPT curves between the two grain sizes of the Super304H steel, the lower diffusion rate of atoms in the coarse-grained Super304H steel may explain its righter finish line than the fine-grained counterpart, while the reason for its lefter start line is due to the higher solute segregation along coarse-grained boundaries.

Microstructure and properties of deposited metal for SUPER304H steel

The microstructure and properties of Super304H weld joints were developed and studied. The Creq and Nieq composition points of the deposited metal were all located in the austenite region. The quantity and size of the Nb-phase in the deposited metal with 0.28wt.% Nb were lower than tnat in the other two deposited metals. The phases were mainly γ and Nb(C, N). The content of Nb(C, N) phases in the deposited metal with the 0.6 wt.% Nb content was approximately 2.2 times of the deposited metal with the 0.28 wt.% Nb content. The Nb-phases in the deposited metal with 0.28 wt.% Nb content were lower in quantity, weaker in the strengthening effect, of lower yield strength and lower in size than that the deposited metal with the 0.6 wt.% Nb content.

Microstructure and property evolutions of a novel Super304H steel during high temperature creeping

ABSTRACTHigh-temperature creep tests of a novel Super304H steel under 650 °C/195 MPa were conducted and the evolutions of microstructure and property with creep time of the material were investigated by using optical microscope, scanning electron microscope, micro Vickers hardness tester and electrochemical workstation. The results show that M23C6 carbides precipitated along grain boundaries of austenite matrix in a chain distribution and then got coarsened with the increase of creep time. Creep cavities started to form near the surface when the steel was crept for 2500 h. Afterward creep cavities increased, developed, interconnected and finally formed micro cracks along grain boundaries till fracture at the time of 4578 h. The hardness of the steel increased dramatically at the early stage of creeping and reached a high level at 500 h, and then kept a stable state at the succedent stage till fracture. Intergranular corrosion susceptibility of the steel increased first and then declined gradually, indicating the occurrence of sensitization – desensitization process of the steel during creeping.

The Microstructure Stability of Super304H Steel Aging at 750°C

The microstructure stability and the effect of microstructure evolution on the properties of Super304H steel aged at 750°C for different time were investigated. The results showed that the microstructure of the Super304H steel after aging at 750°C consists of primary matrix and large amount of precipitated phases. The main precipitates were Cu-rich phase, MX phase and M23C6 carbides in austenitic matrix during aging. The M23C6 carbides precipitate preferentially at grain boundaries, and then coarsening remarkably with increasing aging time. The MX phase was stable during the whole aging process. The Cu-rich particle is smaller than 10 nm at the initial aging stage, and then coarsen with increasing aging time. The strength and hardness increase obviously at the initial stage due to the interaction of Cu-rich phase, MX phase and M23C6 carbides precipitation strength, in which Cu-rich phase strengthen action is the dominant. Coarsening M23C6 carbides weaken the strength during aging.

Microstructural Evolution of Super304H Steel upon Long-Term Aging

The Nb-stabilized and Cu-strengthened austenitic stainless steel Super304H was aged at 600~700°C up to 20,000 hrs. Scanning electron microscope (SEM), and transmission electron microscope (TEM) were used to study the effect of aging on the microstructural evolution of the specimen, with focus on the precipitation behavior in relation to the temperature and time. Only NbCs in size range 2 μm ~ 50 nm were observed to be scarcely distributed in matrix in the as-received specimen. Upon aging, precipitation of σ-phase (~5 μm) and Cr-rich M23C6 (~1 μm) along grain boundary, and nanosized Cu precipitates (~65 nm) in the grain interior were formed. The size and fraction of the σ-phase and M23C6 increased with the increase of aging temperature and/or aging time, with higher sensitivity to the temperature. The size of Cu precipitates was relatively stable, while the fraction and number density increased with the aging temperature/time. The microhardness upon aging increased with increase of aging temperature/time, due to the precipitation of the nanosized Cu precipitates.

The Effect of σ-Phase Formation on Long-Term Durability of Welding Joints in SUPER 304H Steels

The Effect of σ-Phase Formation on Long-Term Durability of Welding Joints in SUPER 304H Steels J. Horvatha,b,∗, P. Kral and J. Janovec CTU in Prague, Faculty of Mechanical Engineering, Department of Materials Engineering, Karlovo naměsti 13, 121 35 Prague, Czech Republic UJP PRAHA a.s., Nad Kaminkou 1345, 156 00 Prague, Czech Republic Institute of Physics of Materials, ASCR v.v.i., Žižkova 22, 616 62 Brno, Czech Republic