Super 304H Research Articles

Microstructure characterization and mechanical properties of austenitic Super 304H steel after operation

The paper presents the results of testing the microstructure and mechanical properties of austenitic Super 304H steel after approximately 31,000 h of operation at a temperature of 570 °C. The microstructure analysis showed that the utilization of the tested steel contributed to the precipitation of numerous M23C6 carbides and individual sigma phase particles at grain boundaries, while dispersive ε_Cu and MX particles was observed inside the grains. At the grain boundaries, the precipitates sometimes formed a continuous mesh. The presence of numerous secondary phases resulted in higher than standard strength properties while maintaining the required plastic properties.

Formation mechanism of large-sized CaO-SiO2-CaF2-Al2O3-MgO composite inclusions in super 304H stainless steel rods

Employing silicon deoxidation in the production of Super 304H stainless steel, large-sized CaO-SiO2-CaF2-Al2O3-MgO (CSFAM) composite inclusions are frequently observed in the rods, posing a substantial threat to product quality. This study delves into an analysis of the constituent phases of these large-sized inclusions, identifying the coexistence of liquid phase and spinel phase. Leveraging industrial experiments, systematic sampling of Super 304H stainless steel, and a comprehensive analysis involving dynamic and thermodynamic calculations, this study investigates the formation mechanism underlying such inclusions. The composition of inclusions in the sample before AOD tapping mirrors that of the slag, with high CaF2 content, indicating that they come from slag entrapment. Through kinetic calculations, it is indicated that the time of existence of the liquid film during the ascent of these inclusions is significantly longer than the collision time. This suggests that they are not easily removed and tend to persist in the steel. Despite LF refining, the predominant types of inclusions remain unchanged, and the decrease in temperature leads to the precipitation of solid-phase MgO within the inclusions. Solubility calculations for solid-phase MgO show a rising trend with temperature. Post-casting, MgO and Al2O3 content in inclusions increase, the solid-phase MgO region disappears, and in the subsequent cooling process, inclusions precipitate the stable spinel phase (MgAl2O4). Solubility calculations for solid-phase MgAl2O4 show a temperature-dependent increase. After the completion of casting, the steel ingot undergoes a deformation process through rolling, leading to the transformation of inclusions into large-sized, elongated, composite CSFAM inclusions.

Strengthening and Embrittling Mechanism of Super 304H Steel during Long-Term Aging at 650 °C.

Super 304H has been a crucial material for ultra-supercritical boilers. However, the relationship between microstructure evolution, strengthening mechanism, and embrittling behavior during long-term aging was lacking investigation. This investigation aimed to reveal the strengthening and embrittling mechanism from precipitates in Super 304H. The results showed that the hardness increment came from the grain boundary's M23C6 (GB's M23C6) and intragranular nano Cu-rich particles. After being aged for 5000 h, the GB's M23C6 and nano Cu-rich particles provided a hardness increment of approximately 10 HV and 30 HV, respectively. The impact toughness gradually decreased from 213 J/cm2 to 161 J/cm2 with the extending aging time. For the aged Super 304H, the GB's M23C6 provided a higher cracking source. In addition, the nano Cu-rich particle restricted the twin-induced plastic deformation of austenitic grain and depressed the absorbed energy from austenitic grain deformation.

Corrosion performance of austenitic super 304H in supercritical CO2 at different temperatures and O2 impurity

Corrosion performance of austenitic super 304H in supercritical CO2 at different temperatures and O2 impurity

Mechanical properties of Super 304H steel after long-term ageing at 650 and 700°C

Mechanical properties of Super 304H steel after long-term ageing at 650 and 700°C

Effect of Internal Pressure on Microstructural and Mechanical Properties of X10CrNiCuNb18-9-3 (SUPER 304H) Austenitic Stainless Steel

Despite the current tendencies towards decarbonization of the energy sector, it is still necessary to maintain the lifetime of existing coal-fired power plants. Austenitic stainless steels belong among materials used in energetics, in this case, SUPER 304H (X10CrNiCuNb18-9-3). This steel is used for the experiment that simulates real operating conditions, specifically high temperature (700°C) and constant inner pressure (25 MPa), which is induced by inserting a distilled water medium before sealing. This work deals with the evaluation of the microstructure and mechanical properties of the SUPER 304H specimens, both with and without inner pressure. All the specimens underwent the same long-term exposure in a form of laboratory annealing. The mechanical properties were evaluated by performing the tensile, impact and hardness tests and microstructure was characterized by light optical microscopy and scanning electron microscopy.

Comparison of Stress Relaxation Cracking Susceptibility of Austenitic Stainless Steels

Coal-fired power plants often have welded joints made up of 347H stainless steel. However, this alloy is known to fail because of stress relaxation cracking. Thus, quantitative evaluation methods are needed as screening measures. In this study, a Gleeble® thermomechanical simulator was implemented in 347H and Super 304H alloy heat- affected zone (HAZ) simulation and stress relaxation testing. In the case of 347H, carbide dissolution in the HAZ reduced the hardness value and promoted grain growth. Alternatively, the respective extent of precipitate dissolution and hardness reduction in the nitrogen-containing Super 304H was relatively small. The stress relaxation tests were performed at a temperature of 700°C (1292°F), which was maintained for up to 70 h. Consequently, all 347H specimens fractured within 32 h. Furthermore, the time to rupture substantially decreased as the strain was increased from 5 to 10% and then to 15%. Additionally, the hardness near the fractured surface increased, and the plastic deformation primarily occurred near the grain boundaries. Conversely, the Super 304H specimens did not fracture during the 70-h testing period, at which time their hardness distribution was observed to still be relatively uniform. These results demonstrate that the susceptibility of stress relaxation cracking can be quantitatively determined according to the material and strain.

Characteristics of Metallographic Preparation of Heat Exchanger Austenitic Stainless Steels

This paper describes a metallographic preparation of the heat-exchanger austenitic stainless steels, i.e., SUPER 304H, HR3C and Sanicro 25. The as-received materials delivered after the solution annealing are very resistant to the chemical etchants. Therefore, it is more efficient to optimize the parameters of the electrolytic etching in the acidic or basic electrolytes. After the materials’ operation under elevated temperatures (600–750 °C), the microstructure is easier to etch due to the reduction of corrosion resistance. For the following analyses, it is necessary to think of the etching purpose in advance. For the grain size determination, it is sufficient to use electrolytic etching in the oxalic acid solution or chemical swabbing in the combination of 15 ml HCl, 10 ml acetic acid and 5 ml HNO3. For the image analysis of the intermetallics, the previous methods can be too invasive and give skewed results. In many cases, this leads to the overetching and thus removal of the observed particles (intermetallics, carbides, etc.). It is suitable to use selective etching methods or only polishing for a more precise evaluation, e.g., electron microscopy.

Characterisation of the austenitic steels Super 304H and HR3C after air and steam oxidation

ABSTRACT This work aimed to evaluate the effect of a long-term pre-oxidation process in the air (5000 hours at 650 °C) on the steam oxidation resistance of Cr-rich steels with around 18 wt.% Cr (Super 304H) and around 25 wt.% Cr (HR3C) content exposed for the additional 5000 hours at 650 °C. The effect showed a high impact of a long-term pre-oxidation process and a high content of Cr in the metal matrix of the exposed steels. The steel with a lower Cr content showed spallation of the oxide scale under the oxidation process in the air, and only the steel with 25 wt.% Cr showed no spallation of the oxide scale in both conditions: air and steam. The post-exposure analysis in this work was carried out using Scanning Electron Microscopy (SEM) supported by Energy Dispersive X-ray Spectroscopy (EDS) using an X-Ray mapping function. Phase analysis was performed using an XRD diffractometer.

Microstructural characterisation of creep tested advanced austenitic stainless steel super 304H

ABSTRACT Microstructural characterisation was performed on the gauge section of two creep rupture samples manufactured from Super 304H; an advanced austenitic stainless steel that is used as superheater and reheater tubes in power plants. The samples had been exposed to uniaxial creep tests (Sample A: 700°C and 75Mpa, Sample B: 650°C and 120Mpa) where both exhibited low creep ductility. Six phases were identified: MX (Nb-rich), sigma, Cu-rich, M23C6, modified Z phase and Fe (BCC). Quantification of Nb-rich and Cu-rich particles revealed ~10% and ~16% more Nb-rich and Cu-rich particles respectively in Sample A versus Sample B. The amount of sigma and the average particle size for the Nb and Cu-rich particles was similar for both samples. There appears to be an association between the sigma phase and creep cavities as shown in 2D microstructural characterisation which could have contributed to the low creep ductility exhibited by these two creep samples.

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

Preliminary Studies on Rare Elements Addition and Effect on Oxidation Behaviour of Pack Cementation Coatings Deposited on Variety of Steels at High Temperature

The aim of the paper was to investigate the air oxidation behaviour of pack aluminised steels exposed at 650 °C for 1000 h in static natural air atmosphere. The pack coatings were doped by rare elements such as gadolinium (Gd), cerium oxide (CeO2), and lanthanum (La) in order to enhance the corrosion resistance and plasticity of the deposited layers. In this work, the following steels were used: 16M, T91, VM12, Super 304H, and finally SANICRO25. The results indicated a much higher corrosion resistance in the coated 16M, T91, and VM12 steels; the steels with a higher Cr content than 16 wt % Cr indicated a better behaviour in the uncoated state than in the coated state. However, the observed difference in mass gain between the uncoated and the coated austenitic steels was not enormous. Furthermore, the addition of RE elements to the coating showed some effect in terms of coating thicknesses and differences in the layer structures. The materials prior to testing and after the exposure were investigated using XRD, the SEM X-ray maps with an EDS instrument were used for particular samples to evaluate the phase identifications, element concentrations, microstructure, and chemical composition.

Creep Deformation Property and Creep Life Evaluation of Super304H

Abstract Creep deformation behavior, creep strength property, and microstructural evolution during creep exposure were investigated on Super 304H steel for boiler tube. In the high stress and lower temperature regime, creep rupture strength of Super 304H steel is higher than that of SUS304H steel. The slope of stress versus time to rupture curve of Super 304H steel, however, becomes steeper with increase in creep exposure time and temperature, and the creep rupture strength of Super 304H steel becomes closer to that of SUS304H steel after the tens of thousands of hours at 700 °C and above. In the short-term, at 600 °C, creep rupture ductility increases with increase in creep rupture life. However, it tends to decrease after showing the maximum value and the creep rupture ductility decreases with increase in temperature. The complex shape of creep rate versus time curves, with two minima in creep rate, was observed at 600 °C. Several type precipitates of niobium carbonitride (Nb(C,N)), Z phase (NbCrN), and copper were observed in Super 304H steel, as well as M23C6 carbide and sigma phase observed in SUS304H steel. The change in slope of stress versus time to rupture curve is caused by disappearance of precipitation strengthening effect during creep exposure. Accuracy of creep rupture life evaluation was improved by stress range splitting method which takes into accounts of the change in slope of stress versus time to rupture curves was demonstrated.

Analysis of the relationship between changes in mechanical properties and precipitation in SUPER 304H steel during its exposure

Aim of the article is to provide properties comparison between initial state and two thermally exposed states of creep resistant steel SUPER 304H by usage of conventional and miniaturised tensile test specimens. Tests were performed at room temperature. Thermally exposed material was aged by 15 and 30 thousand hours at 675°C. This exposition which simulated accelerated operation exposition caused precipitation of intermetallic phases which affect mechanical properties like ultimate strength and elongation. The performed tests showed a decrease in elongation and ultimate strength between the first exposed state and initial state followed by partial increase of both tested values for the second exposed state. Sigma phase precipitation was confirmed for the exposed samples. The presence of the sigma phase also led to a change in the fracture morphology from full ductile (initial state) to combined ductile with the presence of brittle particle cleavage (exposed states). A comparison of the tests performed on classical and miniaturized test samples proved the possibilities of using miniaturized samples both for the basic material and for the exposed ones.

IMPACT OF LONG-TERM AGEING ON SIGMA PHASE PRECIPITATION PROCESS IN STEELS WITH AUSTENITIC MATRIX

Super 304H, HR3C and Sanicro 25 grade austenitic matrix steels are used in the construction of pressure components of boilers with supercritical operating parameters. The article presents the results of microstructure examination in delivery condition and after age-ing for up to 50,000 hours at 700°C. The microstructure exami-nation was performed using scanning and transmission electron microscopy. The precipitates were identified using transmission electron microscopy. In particular, the study analysed the v phase precipitation process and its dynamics depending on the ageing time. It has been shown that the intermetallic v phase plays a significant role in the loss of durability of the tested steel. It is related to its significant increase due to the influence of high temperature, and its coagulation and coalescence dynamics strongly depend on the ageing/operating temperature level. The qualitative and quan-titative identification of the intermetallic v phase precipitation pro-cess described in the study is important in the analysis of the loss of durability of the tested steels under creep conditions.

Paper: Effect of aging process on steam oxidation behaviour of austenitic steels Super 304H and HR3C

The aim of this work was to evaluate effect of aging process on steam oxidation resistance of Cr rich steels with around 18 wt % (Super 304H) and around 25 wt % (HR3C). The samples are still under exposure, however the results after 100, 200, 500 and 1000 h are available (the whole test is planned for 10 000 h). The effect of aging process on steam oxidation after first 1000 h showed the formation of a thinner oxide scale in contrast to the samples exposed in steam atmosphere only. Further, it was found, fine grain structure determine lower mass gain of the sample exposed in steam oxidation process, but in air oxidation process mass gain is mainly governed by Cr content in metal matrix. The post exposed analyses in this work were carried out using Scanning Electron Microscopy (SEM) supported by Energy Dispersive X-ray Spectroscopy (EDS) using X-Ray mapping function.

Optimizing the welding parameters for Straight tube butt welding of dissimilar metals

Straight Tube Butt Welding is a process in which straight tubes are welded. Tubes with a 63.5 mm as outer diameter and 4 mm wall thickness are used as the base material in the following combination: Super 304H and T91. The filler material ERNiCR Fe-7A is selected based on the properties of two metals with 0.8 mm outer diameter. The study is to find out the near input parameters in gas metal arc welding of tempered martensitic steel T91 and austenitic steel super 304H with nickel-based filler wires. Trials are conducted with these alloy steel tubes by varying the input process parameters. All input parameters are taken and optimized by using Taguchi’s method analysis and their optimized values are taken for their welding. High tensile strength is obtained from their optimized values, and the specimen is tested for the tensile and micro-hardness tests of welded joints.

Analysis of the power boiler superheater strain-and-stress state under creep conditions

ABSTRACTThe paper presents a procedure for assessing the condition of the current structure of boiler tubes. First, a modified version of the Garofalo equation is proposed to model the strain-and-stress state under creep conditions in Super304H steel. In parallel with the creep model development, the microstructure is examined using SEM and TEM techniques. The Super 304H steel structure is tested for a new material and for the superheater material after 100,000 h of operation. According to the presented description of the tested steel degradation, it is possible to operate the material for another 100,000 h if the design conditions are met. The results show that the conventional approach based on European standards does not exclude the superheater safe operation. However, a more detailed analysis, taking account of additional loads from the pipeline and using the proposed creep equation, proves that the conditions for the superheater safety are not satisfied.

Corrosion and Carburization Behavior of Heat-Resistant Steels in a High-Temperature Supercritical Carbon Dioxide Environment

The corrosion behavior of heat-resistant steels, including T91, VM12, Super 304H, and Sanicro 25, in high-temperature supercritical carbon dioxide at 650 °C and 15 MPa for 1000 h was investigated. X-ray diffraction, Raman spectroscopy, and glow-discharge optical emission spectrometry were employed to characterize the corrosion products. The results showed that the weight gain of the investigated steels is in the order of T91 > VM12 > Super 304H > Sanicro 25. Compared with the dual layers of Fe3O4 and FexCr3−xO4 on T91 and VM12, the chromia-rich oxide scales on Super 304H and Sanicro 25 enhanced their corrosion resistance. Internal carburization zones were found underneath the oxide scales, indicating the presence of oxidation and carburization reactions in the supercritical carbon dioxide. Moreover, the total carbon uptake into the T91 and VM12 steels approximately followed a parabolic law with experimental time but followed a linear law for the austenitic heat-resistant steel Sanicro 25. A corrosion mechanism that couples oxidation and carburization is summarized and quantified for oxidation and carburization to determine the dominant reaction.

Effect of creep lifetime on geometric optimization of boiler tubes for thermal power plants

ABSTRACTIn this study, thermomechanical and creep analyses of boiler tubes are performed and a procedure for cost-based optimization of boiler tube geometries is presented. First, analytical thermal convection and conduction heat transfer under ultra-supercritical (USC) and advanced ultra-supercritical (A-USC) conditions as well as numerical evolution of creep stresses in the boiler tube are obtained. Subsequently, the results are used to calculate the thermal efficiency and creep lifetime of austenitic stainless steel Super 304H and Ni-based alloy Nimonic alloy 80A. The geometric parameters of the boiler tube are then optimized to minimize the total cost related to the boiler tubes including capital investment and operational costs. Comparing with the conventional approach which assumes a fixed creep lifetime for simplicity, the present results also showed that creep lifetime assessment is important and should be included in the optimization. This optimization procedure can also be applied to other boiler tube materials under various operating conditions of thermal power plants.