Super304H Steel Research Articles

Nonlinear ultrasonic characterization of intergranular corrosion damage in super 304H steel tube

Purpose – Super304H steel is a new fine-grained austenitic heat-resistant stainless steel developed in recent years, and it is widely used in high temperature section superheater and reheater tubes of ultra-supercritical thermal power units’ boiler. Currently intergranular corrosion (IGC) has occurred in a few austenitic stainless steel tubes in ultra-supercritical units and led to boiler leakage. The purpose of this paper is to find a nondestructive method to quickly and easily detect IGC of austenitic stainless steel tube. Design/methodology/approach – This paper uses the nonlinear characteristics of ultrasonic propagation in steel tube to detect the IGC of Super304H tube. Findings – The experimental results show that the nonlinear coefficient generally increases sensitively with the degree of IGC; hence, the nonlinear coefficient can be used to assess IGC degree of tubes, and the nonlinear coefficient measurement method is repeatable for the same tube. Research limitations/implications – A theory of how IGC would affect the ultrasonic signals and lead to a nonlinear response needs further research. Practical implications – A nondestructive method to quickly and easily detect IGC is provided. Social implications – Using ultrasonic nonlinear coefficient to assess IGC degree of tubes is a new try. Originality/value – This paper provides a new way to test IGC.

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.

Observation of phase transformation and grain growth phenomena in aluminised Super 304H during steam oxidation

Electron backscattered diffraction (EBSD) maps acquired in the diffusion zone of aluminised Super 304H steel samples showed that a phase change and associated grain growth phenomena had occurred during high temperature aging in flowing steam environments. This phase transformation could have a significant influence on reducing the oxidation resistance and mechanical stability of the aluminised layer at high temperatures. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy and EBSD mapping provides a valuable insight into microstructural changes occurring in coated alloy systems during high temperature aging processes.

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.

Influence of chemical composition on intergranular corrosion susceptibility of novel Super304H austenitic heat-resistant steel

In order to investigate the influence of chemical compositions on the intergranular corrosion (IGC) resistance of the Super304H steel, the chemical compositions and IGC levels of as received Super304H tubes were analysed and evaluated respectively by spectral analysis, scanning electron microscopy and immersion test in H2SO4–CuSO4–Cu chip solution. The results show that there exist four IGC levels in all the tubes. In condition of similar C content or similar Nb content, the IGC susceptibility is revealed to change randomly with the increasing Nb content or C content, respectively. Further more, when C content is more than 0·10%, Super304H steel will be easily sensitised to IGC, while almost all the tubes without IGC susceptibility have Nb content close to 0·60% of upper Nb limit. As to the influence of Nb/C mass ratio on IGC resistance, the sample with Nb/C ratio less than 5·00 will be easily sensitised and the sample with Nb/C ratio more than 8·15 will have no IGC susceptibility, but the IGC levels of tubes with Nb/C ratio between 5·00 and 8·15 incline to fluctuate randomly. The high susceptibility of some samples in this Nb/C region is attributed to the improper heat treatment process.

Analysis on the Structure and Mechanism of Cracking and Spalling for Super304H Steel Oxide Films in High-Temperature Steam

In this paper, by means of Scanning Electronic Microscope (SEM), X-ray diffraction analyzer (XRD) and Cross hatch scanning analysis, the Structure and mechanism of cracking and spalling for Super304H steel oxide films in high temperature steam were investigated. With the oxidation proceeding, the surface of Super304H steel specimens is covered by a large amount of tiny holes, which causes the occurrence of an outer oxide layer. The Super304H oxide film generally has a double-layer structure: the outer layer mainly for iron oxide Fe3O4, also being covered by a small amount of Fe2O3, and the inner layer with the FeCr2O4 spinel phase as primary component. Because of the inhomogeneity of oxide distribution, large amounts of smaller gaps appear in the Super304H steel oxide films. Due to the differences among the outer layer, the inner layer and steel matrix in thermal expansion coefficient, when the steam temperature around the specimen changes periodically, the stress variation arise in the Super304H steel and oxide films, which causes the cracking and spalling of oxide films.

Numerical Analysis on Heat Transfer and Oxidation Characteristics of High-Temperature Steam Pipes in Supercritical Units

In this paper, for Super304H steel, the growth law of oxide films and the heat transfer characteristics between the pipe and the working fluid were investigated by using numerical software ANSYS, which simulated comprehensively the effects of pipe size, flow rates of steam, flue gas temperature, and steam temperature on the formation and thickness of the oxide film. A bigger pipe wall thickness, a smaller steam flow rate or a higher flue gas temperature will lead the faster growth of the oxide film thickness, the heat flux density through the wall being smaller and the wall temperature being higher. With increases in steam temperature and thickness of the oxide film, the heat flux through the wall decreases with a small amplitude, and the average temperature of tube walls increases slightly.

Research on a Surface Shot Peeling Process for Increasing the Anti-Oxidation Property of Super304H Steel in High-Temperature Steam

At present, the superheater and reheater pipes in supercritical and ultra-supercritical boilers have encountered with widespread problems of high temperature steam oxidation, seriously affecting the operation safety and economy of the power generation unit. After several tests and contrastive analysis on the main chemical composition and microstructure of specimens before and after shot peening treatment, this paper developed a set of detailed surface shot peening process with specific process parameters, and finds that for the Super304H oxide film, the relative content of Cr element increases by shot peening treatment, with the relative Cr content of the treated specimen being 2.65% higher than that of the untreated, displaying that the anti-oxidation properties of high chromium austenitic steels in high temperature steam is effectively increased.

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.

Intergranular Corrosion of Weld Metal of Super Type 304H Steel During 650°C Aging

The intergranular corrosion of the weld metal of super Type 304H (UNS S30409) stainless steel during aging at 650°C was investigated using electrochemical potentiokinetic reactivation (EPR). Experimental results showed that the as-welded weld joint possesses a good corrosion-resistant property, but the formation of chromium-depleted zones caused by precipitation of Cr23C6 between cellular dendrites increased the reactivation ratio (Ra) when the weld metal was aged at 650°C for a prolonged time, indicating the acceleration of intergranular corrosion with increased aging time. Nevertheless, it was found that Cr23C6 showed the morphology with isolated particles after being aged for 500 h and Ra became stable.

Calculation and Analysis of Microstructure of Austenitic Steel for Supercritical Unit

Microstructure and the precipitated high temperature ferrite δ phase of an austenitic steel 10Cr18Ni9NbCu3BN tube was investigated. It reveals that segregation during solidification process results in the precipitation of high temperature ferrite. The calculated amount of δ phase was consistent with our XRD analysis. With decreasing solution treatment temperature, Nb-containing phase will be refined and the amount of δ phase as well as process-cost can be reduced. Because of the highest performance/cost ratio, austenitic super 304H steel is applied as pressure component under supercritical conditions. It was originally developed by Sumitomo Metal Industries, Ltd and Mitsubishi. Based on 304H, Super304H has lowered the upper limit of Mn, but added Nb, N and Cu. Elements Nb and N can form stable NbN,Nb(C,N)-phase, so as to refine the grain size and result in precipitation-hardening. Cu can form coherent segregation phase which also has the hardening effect, decreases the hardening rates in the cold-working process and improves the plastic formation of steel. In this kind of steel, the main strengthening phases are copper-rich phase, MCs. The alloying effect of elements Nb,N and Cu can increase allowable stress and service life of the steel under the working temperature[1]. In this paper, experimental and theoretical analysis was carried out in order to develop new 10Cr18Ni9NbCu3BN steel tube. In accordance with ASME code case 2328-1, the contents of steel 10Cr18Ni9NbCu3BN were listed as follows in Table 1.

Microstructure and Corrosion Resistant Property Research of Super 304H and TP347H Heat-Resistant Steel Welding Joint

Microstructure of Super 304H and TP347H austenitic heat resistance steel welding joint by handwork argon tungsten-arc welding and electro-chemical corrosion property of weld metal and two base materials were investigated by SEM and M273 constant potential rectifier. The results shows that microstructure of the weld metalwere austenitic matrix + δ ferrite; In 5% HCl Solution, corrosion resistant capabilities were super 304H base material＞ weld metal＞TP347H base material; in 9.8% H2SO4 Solution, corrosion resistant capabilities were Super 304H base material＞TP347H base material＞ weld metal; Super 304H and TP347H austenitic heat resistance steel welding joint has better corrosion resistant property in HCl solution than in H2SO4 solution; Inter-crystalline corrosion results were consistence to results electro-chemical corrosion experiment in 9.8% H2SO4 solution.

The Formation of Submicrometer Scale Grains in a Super304H Steel during Multiple Compressions at 700°C

The deformation behavior and the microstructure evolution in a 304-type austenitic stainless steel were studied in multiple forging tests at temperature of 700°C. The flow stresses increased to its maximum value with straining to about 1 and, then, slightly decreased resulting in a steady state deformation behavior at strains above 3. The structural changes were characterized by the development of a spatial net of deformation subboundaries, the misorientations of which increased to the values typical of conventional grain boundaries. The number of ultrafine grains increased with straining, leading to development of submicrocrystalline structure. The fraction of submicrocrystalline structure composed of ultrafine grains with an average size of about 300 nm exceeded 0.7 after straining to 2.

Effect of Cu Content on Aging Precipitation Behaviors of Cu-Rich Phase in Fe-Cr-Ni Alloy

The precipitation characteristics and effect on strengthening mechanism of Cu-rich phases during short-time and long-time aging for Super 304H steels with different Cu content were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the size of Cu-rich phase particles increases, the interspace of Cu-rich phase particles decreases and the density of Cu-rich phases increases with the increase of Cu content during short-time aging (approximately 800 h) at 650 °C for Super 304H steels. During long-time aging (more than 2000 h) at 650 °C, Cu-rich phase precipitates sufficiently and the strengthening effect of Cu-rich phase is preferable in Super 304H steel containing Cu of 4%. The strengthening effect of Cu-rich phase in Super 304H steels containing Cu of 2.2% or 5% is weaker than that with Cu of 4% during long-time aging (more than 2000 h).

Hot deformation behavior of Super304H austenitic heat resistant steel

The hot compression tests of Super304H austenitic heat resistant steel were carried out at 800–1200°C and 0.005–5 s−1 using a Gleeble 3500 thermal-mechanical simulator, and its deformation behavior was analyzed. The results show that the flow stress of Super304H steel decreases with the decrease of strain rate and the increase of deformation temperature; the hot deformation activation energy of the steel is 485 kJ/mol. The hot deformation equation and the relationship between the peak stress and the deformation temperature and strain rate is obtained. The softening caused by deformation heating cannot be neglected when both the deformation temperature and strain rate are higher.

Microstructure Evolution of a Novel Super304H Steel Aged at High Temperatures

The microstructure evolution of a novel Super304H stainless steel aged at different temperatures was investigated using various analysis methods. The results reveal that the microstructure of the Super304H steel after aging at 973–1623 K consists of the primary γ matrix and a small amount of precipitated phases. Grain size of γ-matrix shows a slow increase when the aging temperature is lower than 1373 K and it increases quickly when the aging temperature is beyond 1423 K. The lattice parameter of γ-matrix varies at different aging temperatures, and this variation corresponds to the phases precipitated or redissolved. The variation of grain size and precipitated phases can affect the performance of the steel when exposing to steam oxidation and be subject to creep at high temperatures.

Processing maps and hot workability of Super304H austenitic heat-resistant stainless steel

Hot-compression and hot-tension experiments were conducted on a Gleeble3500 thermal–mechanical simulator, and the microstructure diagram of Super304H austenitic heat-resistant stainless steel as well as its thermoplastic diagram under different deformation conditions was obtained. Based on the dynamic material model theories and Prasad instability criterion, its hot processing maps were developed. The deformation mechanisms of different regions in the maps were analyzed, and corresponding microstructures were investigated. The results show that for Super304H steel, the critical Z value, below which the complete dynamic recrystallization may occur, is about 4.23 × 10 20. During the hot tension at 0.1 and 1 s −1, the total elongation and reduction in area increased with the increase of deformation temperature ranging from 800 to 1100 °C. As for the hot working of Super304H austenitic heat-resistant stainless steel, large-strain deformation can be carried out at 1100 °C and at the strain rate above 0.5 s −1.