Zhiyuan Liang

High Temperature Oxidation of Superalloys in Steam Environment

High temperature oxidation of superalloys in steam environment was studied. The experimental results showed that the oxidation resistance of the superalloys in steam environment was superior to heat resistant steel HR3C. Only the oxide scale formed on Super 304H steel was spalled at 800 °C. The oxide scales formed on superalloys Haynes 282 were composed of an external oxide layer of TiO2 and Cr2O3 and an internal oxide layer of Al2O3, TiO2 and Cr2O3. A uniform MnCr2O4 and Cr2O3 formed at the surface of superalloys Haynes 120 and Haynes 625. It was concluded that Cr and Ni improved the oxidation resistance in steam environment. The penetration depth of internal oxides was decided by the contents of Al and Ti. Finally, the oxidation resistance index of superalloys was comprehensively evaluated by the weight gain, oxide scale thickness and Cr depletion depth.

High temperature oxidation of Fe–Ni-base alloy HR120 and Ni-base alloy HAYNES 282 in steam

Abstract High temperature oxidation of Fe–Ni-base alloy HR120 and Ni-base alloy HAYNES 282 in steam at 800–1000 °C was investigated and compared. Results show that the oxidation kinetics of HAYNES 282 and HR120 followed a parabolic law. Temperature affected the stability of formed chromia, leading to the observation of (Fe, Cr)2O3, Fe2NiO4 on the surface of Fe–Ni-base alloy, which is not the case for Ni-base alloy. The strengthening element titanium accelerated the growth of chromia formed on the Ni-base alloy. It is concluded that the resistance of high temperature oxidation of HR120 is superior to HAYNES 282.

Influence of Aging treatment on the microstructure and mechanical properties of T92/Super 304H dissimilar metal welds

Abstract Influence of aging treatment on the microstructure and mechanical properties of T92/Super 304H dissimilar steel joints was investigated. The microstructure of T92/Super 304H dissimilar steel joints was characterized using optical microscopy, scanning electron microscopy and energy dispersive spectrometer. The results show that the tensile strength of dissimilar metal welds (DMWs) after 10,000 h aging treatment met the ASME T92 and Super 304H standards. Rupture positions were located in the T92 base metal because of the precipitates formed along the sub grain and prior grain boundaries. The tensile strength of DMWs initially increased with time up to 4000 h, then decreased between 4000 to 6000 h, and finally came to almost a constant value from 8000 to 10,000 h exposure. The decrease in the tensile strength resulted from the nucleation and growth of Laves phases at the sub-grains and prior austenitic grain boundaries. The low absorption of impact energy in the weld metal was related to the coarse grains and its grain orientation.

The Effect of Aging Heat Treatment on the Microstructure and Mechanical Properties of 10Cr20Ni25Mo1.5NbN Austenitic Steel

Abstract The effect of aging heat treatment on the microstructure and mechanical properties of 10Cr20Ni25Mo1.5NbN austenitic steel was investigated in this article. The microstructure was characterized by scanning electron microscopy, energy dispersive spectrometry and transmission electron microscopy. Results show that the microstructure of 10Cr20Ni25Mo1.5NbN austenitic is composed of austenite. This steel was strengthened by precipitates of secondary phases that were mainly M23C6 carbides and NbCrN nitrides. As aging treatment time increased, the tensile strength first rose (0–3,000 h) and then fell (3,000–5,000 h) due to the decrease of high density of dislocations. The impact absorbed energy decreased sharply, causing the sulfides to precipitate at the grain boundary. Therefore, the content of sulfur should be strictly controlled in the steelmaking process.

Analysis on high-temperature corrosion characteristic of 0Cr18Ni9NbCu3BN steel

Abstract The high-temperature corrosion characteristic of 0Cr18Ni9NbCu3BN steel was analysed in this paper. The samples were selected from a super-heater with service time of 34 696 h in an ultra-supercritical boiler in China. Scanning electron microscopy, energy spectrum analysis and energy dispersive X-ray spectroscopy were adopted to analyse the element constitute, morphology and phase structure of the corrosion scale. The results indicated that corrosion scale can be divided into three layers, and the middle layer of corrosion scale is enriched with Cr. The Cr-rich layer could not set an efficient barrier to prevent sulphur from penetrating to the matrix. It actually can prevent O from penetrating to metal base effectively. The main corrosion forms are high-temperature oxidation and sulphidation, while the sulphidation rate was greater than the oxidation rate. The sulphidation is the major type of corrosion attack. Oxidation and sulphidation interactively cause the corrosion progress consistently.

Aging Treatment on the Microstructures and Mechanical Properties of New Groove T92/Super 304H Dissimilar Steel Joints

Abstract The effect of aging treatment on the microstructures and mechanical properties of new groove T92/Super 304H dissimilar steel joints is studied in this paper. The experimental results show that the heat-affected zone (HAZ) of T92 is mainly composed of coarse-grained and fine-grained martensitic, whereas the microstructure of Super 304H HAZ and weld seam exhibit an austenitic structure. Aging treatment increases the nucleation and growth of second phase particles of the weld joints, especially at T92 side. The weld joints have a low tensile strength (<700 Mpa) and a high tensile strength (>700 Mpa) when the tensile fractures are located at weld seam and T92 base metal, respectively. With increasing aging time, the hardness and tensile strength of the weld joints initially decrease, then increase, and finally stabilize. Moreover, the weld joints have a maximum hardness value at the T92 heat-affected zone. At room temperature condition, the impact absorbed energy reaches the minimum value, which is related to the coarse grain containing equiaxed dendrites in the weld seam.

Studies on High Temperature Corrosion of Austenitic Heat‐Resistant Steels Super 304H/10Cr18Ni9Cu3NbN in Simulated and Real Combustion Gas Atmospheres

High temperature corrosion behavior of austenitic heat-resistant steel Super 304H/ 10Cr18Ni9Cu3NbN in simulated and real combustion gas conditions was investigated in this paper. An scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) was employed to characterize the morphology and cross-section of specimens cut from experimental tubes. Results show different corrosion resistance of Super 304H and 10Cr18Ni9Cu3NbN steels under a simulated condition. High temperature corrosion is a complicated process containing the molten complex alkali-iron tri-sulphate corrosion, high temperature oxidation and sulfuration. The corrosion rate might be controlled by sulfuration of metals. Finally, the exfoliation mechanism of high temperature scales was put forward which is mainly related to the temperature fluctuation and different thermal coefficients of expansion of multi-corrosion layers.

High temperature corrosion of typical positions in SUS 30432 tubing bend tested in simulated environment

Abstract The corrosion behaviour of typical positions in an SUS 30432 bend under simulated corrosive environments was investigated in this paper. A scanning electron microscope equipped with an energy dispersive spectrometer was used to characterise the surface and cross-section morphologies of specimens. Results show that the corrosion kinetics of all specimens were similar showing near parabolic law behaviour. The largest weight gain was located at the samples which were cut from the bottom of a bend. The corrosion scale consisted of an outer layer containing iron oxides and an inner layer containing Cr rich spinels and sulphides. The distances between grains in the samples from the bend were larger than that from the straight tube, which became smaller as the experimental time increased. It may be closely related to aggregation of dislocations near the grain boundaries after tube bending.