Damage characterization of an ASTM A 213 grade 91 tube after 116.000 hr of service in a reforming plant (original) (raw)
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This study deals with microstructural evolution, mechanical properties and oxidation/carburization behaviour of an ASTM A213 T91 super-heater tube after long term service in a power plant ; approximately 157.000 hours (about 18 years) in subcritical conditions (542 • C and 174 bar). This work provides good experience data to better evaluate the performances of this material, it can be referential for readers who are interested in martensitic heat-resistant steels, because the major previous studies were conducted in laboratories focusing on short creep experiences and oxidation behaviour. Our investigations suggest remarkable changes in the core and the walls of the tube. In the bulk material, the recovery of the tempered martensitic matrix and the coarsening of the M 23 C 6 carbides were observed, but the mechanical properties and the creep strength are still good. However, carburization/oxidation represents a major life time limiting factor by affecting about 20% of the tube thickness. The mechanisms of carburization, oxidation and microstructure evolution are here discussed.
Mechanical properties and phases evolution in T91 steel during long-term high-temperature exposure
Engineering Failure Analysis, 2020
Scanning and transmission electron microscopy was used to characterize quantity, pattern, size, and distribution of microstructure and precipitated phases in T91 steel exposed to high temperature in the subcritical unit superheater of a specific power plant. The materials were mechanically and electrochemically tested using a universal testing machine and an electrochemical workstation. The structure and performance of the original material and after the long-term service time were compared. The hardness and strength of T91 steel initially increased before they decreased during long-term service. It took 4000 h for the T91 steel to transform from the original to the service state, forming (Cr, Fe, V, Mo) 23 C 6 multi-component mixed phase, which delayed or inhibited the growth of the M 23 C 6-type carbides during the long-term high-temperature service. The MX-type carbide formation would pin dislocations and increase the strength. When the service time was between 4000 and 130,000 h, the carbide coarsening was obvious, and the effect of solid solution strengthening was reduced, the dislocation density was decreased, reducing the material's strength below the original state level. Electrochemical tests showed that longer service time degraded corrosion resistance of the T91 steel.
steel research international, 2013
P91 steel has been widely used in power generation industry. Generally, P91 creep resisting steel is produced by normalizing and tempering. The normalizing temperature is in the range of 1040-10808C and tempering temperature is in the range of 750-7808C. The microstructure after tempering is tempered martensite with precipitates of carbides, M23C6, and vanadium/niobium rich carbo-nitride of the type MX (M ¼ V or Nb and X ¼ C or N). The presence of carbide precipitates improves creep rupture strength due to precipitation hardening. The carbide coarsening and microstructure degradation during service will result in deterioration in creep strength. It is important to understand the microstructural evolution of P91 steel during long-term operation. In this work, the steels in the virgin condition (normalized and tempered), service exposed (9 years at 6008C) and post service exposed re-normalized and tempered in an attempt to restore the original microstructure were characterized. A range of microscopy techniques, predominantly TEM, were applied to understand the effect of these thermal histories on the microstructure of the materials.
Micro
ASTM A213 T91 steel is widely used in power plants and petrochemical industry for long-term service components. Due to its high resistance to creep, thermomechanical fatigue and corrosion, the use of grade 91 steel allows usual plant service parameters to be raised up to ultra-supercritical conditions (600 °C, 300 bar) so that performances are remarkably increased. The strongest factors that affect performances are the time of exposure, the temperature and the applied stress: such parameters can dramatically decrease the service life of a plant component. The improved mechanical properties of grade 91 are strictly related to its specific microstructure: a tempered martensite matrix with fine precipitates embedded in. Two typologies of secondary phases are present: M23C6 carbides (where M = Cr/Fe/Mo/Mn) and finely dispersed MX-type carbonitrides (where M = V/Nb and X = C/N). This study is focused on the microstructure evolution of grade 91 steel under creep conditions. First, three s...
Analysis of changes in lattice parameter of a grade 91 steel during thermal ageing at 550 °C
Engineering Failure Analysis Volume 97, March 2019, Pages 43-52, 2019
Microstructural changes in the T91 steel (also known as the modified 9Cr1Mo steel), largely used in thermal and nuclear power plants as well as petrochemical factories, have been studied, after isothermal ageing (at 550 °C) in laboratory, for different durations up to 7000 h (about 10 months of exposure). In-depth analysis about chemical and structural changes in the metallic substrate and secondary carbides (M23C6) were conducted using SEM, EDX and X-ray diffraction instrumentations. Results reveal a progressive restoration of the tempered martensitic matrix with holding time. Then, a continuous increase in size of secondary carbides was noticed until stabilization after >5000 h of ageing. Kinetic parameters of matrix lattice were determined using Johnson-Mehl-Avrami (JMA) model. Natural logarithm of the relative lattice parameter of the matrix varies linearly with that of ageing time. Thus, the Avrami exponent n and activation energy E were established. Diffusion of chromium and molybdenum from matrix into carbides lattice was proved which explains their continuous coarsening. That has a harmful effect on the long term stability of the microstructure. The time-temperature dependence of this transformation could be then a good indicator to assess the resistance of T91 steel to heat exposure.
Microstructural Degradation in Power Plant Steels and Life Assessment of Power Plant Components
Procedia Engineering, 2013
Extensive creep testing was carried out on 1Cr1Mo¼ tempered) and aged condition. Both the steels exhibit the temperature range between 813 and 873 K (540°C higher temperature ranges. Casting steel showed wedge of forged steel, the voids were elliptical and flat which deformation and creep ductility of the two steels investi tertiary stage. Based on detailed microstructural investi damage mechanisms such as structural transformation, applied for remaining life assessment of the two steels. slightly on aging; both for rotor forging and casing Mo 2 C carbides and coagulation of others resulting in aging times was due to recovery in ferrite, gradual de and transitional character of precipitated carbides. Cre assessment calculations have also been carried out using
High-Temperature Degradation and Protection of Ferritic and Austenitic Steels in Steam Generators
Journal of Materials Engineering and Performance, 1998
The useful life of superheaters and reheaters of power stations which use heavy fuel oil is shortened and their continuous service is inhibited by corrosion (fireside) and creep-type problems. The increase of corrosion attack on boilers is caused by the presence of fuel ash deposits containing mainly vanadium, sodium, and sulfur which form low-melting-point compounds. The tubes are exposed to the action of high stresses and high temperatures, producing the so-called "creep damage." In this work, two kinds of results are reported: lab and field studies using a 2.25Cr-1Mo steel. The laboratory work was in turn divided into two parts. In the first, the steel was exposed to the action of natural ash deposits in oxidant atmospheres at 600 °C for 24 h. In the second part, tensile specimens were creep tested in Na 2 SO 4 , V 2 O 5 , and their mixture over a temperature range of 580 to 620 °C. In the field work, components of a power station were coated with different types of nickel-and iron-base coatings containing chromium, Fe-Cr, and Fe-Si using the powder flame spraying technique. After testing, the coated tubes were analyzed using electron microscopy. The results showed that all the coating systems had good corrosion resistance, especially those containing silicon or chromium.
Corrosion of a T91 steel tube, used in subcritical conditions in an oil power plant for 157,000 hours was characterized 7 mainly through SEM, TEM, EDX and DRX analyses. Severe oxidation and carburization took place in both the outer (boiler) 8 and inner (steam) wall sides. The nature and morphology of the oxide scale multi-layer structure (hematite, magnetite and spinel) 9 depended on the environment exposure. Specific attention was given to the internal oxidation zone (IOZ) at the oxide/metal 10 interface. Diffusion of chromium during the oxidation process was determined, and was proposed to be responsible for the 11 continuous advancement of oxidation to the core of material, which eventually gave rise to the spinel. Finally, in the bulk 12 material, coarsening of the secondary carbides (M 23 C 6) was the main form of microstructural evolution. 13
The objective of this work is to characterize the evolution of the microstructure of the ASTM A213 T91 steel, used in power plants of electricity generation, for high temperatures up to 650° C, to optimize the performances of the installations and to reduce the pollutant emissions. The high temperature and pressure inside the boilers promote the microstructural evolution of the martensitic matrix of the T91 by increasing the size of the precipitates by the time exposure. The growth of these precipitates is explained by the absorption of the matrix elements, which will constitue in long term a favorable site for the nucleation of the microcavities; that triggers the creep phenomenon. An experience has been conducted in the aim of understanding the evolution of the microstructure ofthe steel at a temperature of 550° C. So far, four samples were removed from the furnance for time intervals of: 0, 260, 760 and 1260 hours. Examination by scanning electron microscopy of the martensitic matrix shows a slight increase of the sizes of precipitated M23C6 and stability for MX precipitates.
International Journal of Pressure Vessels and Piping, 2010
This paper presents results on the evolution of microstructure (both matrix and precipitates) of an ASME Grade 91 steel that has been creep tested for 113,431h at 600°C under a load of 80MPa. The microstructure was investigated using transmission electron microscopy (TEM) and revealed chromium rich M 23 C 6 carbides, MX-type precipitates, Laves phases and modified Z-phases. Only a small amount of modified Z-phase was found. In order to quantify coarsening of precipitates and growth of new phases during creep, the size distributions of the identified precipitates were determined by analysis of TEM images. In addition to this, the size distribution of Laves phases was determined by image analysis of scanning electron micrographs. Substructure modifications and creep damage were investigated on cross section of the creep specimen using Electron Backscatter Diffraction and Scanning Electron Microscopy.