The Oxidation Behaviour of Welded 2.25Cr-1Mo (T22) Boiler Tube Steels under Cyclic Conditions
Abstract: This investigation examines the oxidation behaviour of SMAW and TIG conglutinatement in 2.25 Cr-1Mo (T22) boiler tube steels aftercited pitfall to air at 900°C subordinate cyclic provisions. Gravity execute/area has been deviseted across sum of cycles to establish the kinetics of oxidation. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX) and X-ray diffraction (XRD) techniques were used to irritate the oxidation works.
The ends reveals that the SMAW conglutinateed steel showed exalt gravity execute than that of TIG conglutinateed consequently of the shape of conspicuous degrees of cracks and a thicker oxide lamina on HAZ and conglutinate district.
At tolerably exalted air Cr-Mo steels are a broadly used class of engineering materials for applications, for standard, fume generation/handling, petroleum processing/refining, enthusiastic reforming/polarization/cracking . The growth in capacity at excited air is preferable to avoiding the increasing of embankment brawniness when using traditionally Cr-Mo steel for reheater or superheater tubes.
The reheater and superheater tubes may miscarry well-mannered-mannered succeeding a periodin their adapted drawing spirit by a union of fireside indolence and cringe . In any event, in fume generator systems, a capacious sum of conglutinates were establish in tubes to tubes or tubes to sheets conceives. Welding subjects the mean metal to eager enthusiastic cycling ending in a significantly altered, inhomogeneous microstructure nigh to the conglutinate assurance.
This district is unreserved as the fever- artful zone (HAZ) . The fever-artful zone (HAZ) of 2.25 Cr-1 Mo steel and the conglutinate conclude district of 9 Cr-1 Mo steel were establish to oxidize at conspicuous reproves when unguarded to exalted airs, and execute-ground significantly thicker lamina than other districts in the conglutinatements of the relative steels. This was considered timeliness using lamina brawniness bulk as a implement for spirit rate of exalted air conglutinateed tonnage.
The distinction in the oxidation bearing of the contrariant districts was establish to effect from the distinction in the Cr contenteded of the close lamina of the protective oxide [4, 5, 6]. The wilder propagation of cracks parallel the bit word of HAZ than that of other districts of the 2.25Cr-1Mo steel conglutinatements was seen consequently of the shape of Cr-affluent unimportant complexions and selfsimilar depletion of exempt Cr parallel the bit boundaries, which irritability be the end in an growth in oxidation .
The oxidation behaviour of tangled alloys has not been learned widely and isn't surely knew, chiefly of conglutinateed components exaltover. Parallel these lines, additionally examines are required on oxidation bearing of tangled alloys, specially those utilized for exalted air applications . The cyclic provisions feel been separated for testing as these provisions institute exalt trained way towards portico prevention of the effect of metal indolence in existent industrial environment [9, 10].
This article describes an experimental operation carried out at 900°C to assess the oxidation behaviour of conglutinateed 2.25Cr-1Mo (T22) boiler tube steels subordinate cyclic provisions. The oxidation work irritated by XRD, SEM/EDX and cross-sectional BSEI.
Preparation of conglutinatements and sample
ASTM SA213 2.25 Cr-1Mo (T22) steel was acquired from the enthusiastic capacity settle Bhatinda, Punjab, India in the tube conceive. Gr. T22 boiler tube steel (10 mm brawniness x 26 mm transection) was cut into trench elongation of 150 mm each. Each tube was machined to execute a one customary V-groove, succeeding a period 30° bevel prepossession, continuous source gap and source visage of 1 mm.
Preceding conglutinateing the tube was entirely cleaned succeeding a period brush and CH3)2CO in classify to evert any oxide lamina and begrime or oil calling wild to the tube. Tubes were conglutinateed concertedly by shielded metal arc conglutinateing using basic coated electrode AWS A5.5 E9018-B3 and tungsten passive gas conglutinateing techniques using 99% unblemished argon gas succeeding a period argue wire AWS A5.28 E90S-B3 succeeding a period continuous arc prevalent 95A.
The samples each measuring trenchly 20×15×5 mm3 were cut from the round conglutinatements. The samples were courteous succeeding a period (220 measure) silicon carbide article and emery article and aftercited that trundle-wallow courteous precedently substance to oxidize. The chemical compositions of T22 boiler tube steel and electrode/argue wire used in exhibit examine are shown in Table 1 .
Cyclic oxidation test
Cyclic oxidation studies were conducted in air at 900°C air in the a laboratory silicon carbide tube furnace for 50 cycles. Each cycle interposed of 1 hour fevering at 900°C took aftercited by 20 min composure at capacity air. During assumption, the fitted sample was kept in an Al2O3boat and the gravity of the boat and samples was measured. Al2O¬3 boats used for the investigations were preheated at a accordant air of 1200 °C for 6-8 hours, and it was current that their gravity would cling accordant throughout exalted air cyclic oxidation examine.
Then the alumina boat containing the sample was embedded into the hot zone of the furnace set at a air of 900 °C. Weight-change bulks were captured at the end of each cycle using an electronic redress standard 06120 (Contech, Mumbai, India) succeeding a period a sensitivity of 10-3 g. Aftercited that the oxide lamina was characterized by X-ray diffraction (XRD) and scanning electron microscopy/Energy dispersive X- ray spectroscopy (SEM/EDX). The oxidized samples were then cut balance the cross-section and mounted for the cross-sectional oxide lamina brawniness bulk.
Results and Discussion
Visual Nursing essay and gravity execute bulk anatomy in air
The macrographs aftercited cyclic oxidation for 50 cycles at 900 °C for conglutinateed 2.25Cr-1Mo (T22) steels are as shown in Fig.1. The colour of oxide lamina for all conglutinateed steels aftercited few cycles austere ebonish silvery. For SMAW conglutinateed steel the spalling afloat to answer equitable aftercited 4th cycle. The new oxide lamina seems to feel execute-groundn in this unfilled and which led to cracks and spalling of the top lamina.
Whereas affair of capacious sum of cracks feel been observed in the oxidized samples. The conglutinate district answered ebon colour during 6th cycle of TIG conglutinateed steel. Cracks were observed in the oxide lamina. The aggregate of the spalled lamina has also been incorporated in gravity execute bulks. The width of cracks was comparatively exalt in event of SMAW conglutinateed steel than TIG conglutinateed. The gravity execute devises of conglutinateed steels in air at 900 °C is shown in Fig. 2 and it can be seen in the devises the gravity execute for twain conglutinatements was approximately similar in the judicious sum of cycles upto 5th cycles then aftercited spiritual growthd.
It can be deduced from the devise that SMAW conglutinateed steel showed the consummation gravity execute as compared to TIG conglutinateed steel. The behaviour for SMAW conglutinateed steel was approximately parabolic succeeding a period parabolic reprove continuouss, Kp(10-8 g2 cm-4 s-1) is 36.447 and the behaviour of TIG conglutinateed T22 steel has followed the parabolic law from 12thcycles due to younger spallation. Its parabolic reprove continuous (Kp) is 20.371 x 10-8 g2 cm-4 s-1 as shown in Fig.
Oxide lamina brawninesses bulks
The brawninesses of the oxide lamina on the conglutinatement (i.e. on conglutinate metal and HAZ) were measured from the BSEI parallel the cross-section of the mounted samples and the images are shown in Fig 4. The recognized lamina brawniness values measured for conglutinate area of SMAW and TIG conglutinatement in steels are 1.333 mm and 1.095 mm individually. Though recognized lamina brawniness measured for HAZ district of SMAW and TIG conglutinatement are 1.386 mm and 0.940 mm individually. Least lamina brawniness was shown by TIG conglutinatement. The oxide lamina debonding at metal/lamina intervisage is patent from Fig. 4 (c); (d) for TIG conglutinateed steels proportionately.
X-ray diffraction anatomy (XRD)
The XRD patterns of the oxide samples aftercited 50 cycles are shown in Fig.5. These diffractograms has proportionately resembling complexions of hearty oxide (Fe2O3) succeeding a period Cr2O3 for twain the conglutinateed steels. The malleable peaks of Fe3O4, Cr2O3 parallel succeeding a period deep oxide Fe2O3 were involved in the oxide lamina of TIG conglutinateed steel.
The survisage SEM/EDX morphology of conglutinateed 2.25Cr-1Mo (T22) boiler steel is shown in Fig. 6. The SEM micrograph for conglutinate district of SMAW conglutinateed steel Fig. 6 (a) reveals that the lamina has exposed cracks, and through these the hearty oxide spalled out. The deep conspicuous oxide lamina is affluent in hearty 97.96% succeeding a period MnO (1.95%) subject-matter 2. The close oxide lamina having Fe2O3 (95.41%) and MnO (3.40%) subject-matter 1. The oxide lamina of HAZ district of this conglutinatement comprises of Mn succeeding a period deep oxide of Fe (96.78%) at subject-matter 2, at-last at subject-matter 1 it was solely 95.57%Fe succeeding a period manganese and chromium Fig. 6 (b).
The oxide lamina of conglutinate district for the TIG conglutinateed steel shown in Fig. 6 (c) indicates colossal lamina having cracks. The inner lamina consists of predominately hearty oxides succeeding a period brief aggregate of MnO (1.20%) and Cr2O3 (2.60%) subject-matter 1.Whereas the conspicuous lamina containing MnO (1.22%) and Cr2O3 (1.28%) succeeding a period hearty oxide subject-matter 2. The EDX anatomy of HAZ district of this conglutinateed steel shows the oxide of hearty (97.55%) succeeding a period brief aggregate of Cr2O3 (1.58%) at subject-matter 1 (ebon concern), timeliness the pure concern consists of Cr2O3(2.47%) succeeding a period deep complexion of hearty oxide (96.75%) at subject-matter 2 in Fig. 6 (d).
Fig. 2 demonstrates that twain the conglutinatements involved similar gravity execute during tallest few sum of cycles, at-last then aftercited progressively extensive and closely follows the parabolic oxidation reprove law. The kinetics of oxidation, according is watchful in conglutinatements. The opposition to oxidation of SMAW conglutinateed steel was less when contrasted succeeding a period TIG conglutinateed and this irritability be consequently of the fruit of conspicuous degrees of creaks in oxide lamina on conglutinatement (i.e. conglutinate metal and fever-artful zone (HAZ) (Fig. 1).
Less oxidation reprove (in stipulations of gravity execute) of TIG conglutinatement was consequently of the shape of inferior degrees of cracks in the oxide lamina. Some spalling of the oxide lamina of conglutinateed steels as saw during composure times of the enthusiastic cycles was consequently of contrariant values of enthusiastic annotation coefficients of the metal and oxides [12, 13].
As far as lamina brawniness valuations, the conglutinate district of SMAW conglutinateed sample indicates exalt oxidation than that of TIG conglutinateed, and was expected to the non-availability of Cr in oxide lamina as affirmed by EDX Fig. 6 (a) and exaltexalt as discussed environing by Raman . The brief aggregate of chromium succeeding a period deep Fe2O3 in the oxide lamina of TIG conglutinatement (conglutinate and HAZ districts) has contributed for the improve oxidation opposition as can be seen in XRD and exalt developed by SEM/EDX anatomy as involved in Fig. 6(c) and (d).
SMAW conglutinateed steel showed the exalt gravity execute than that of TIG conglutinateed and it was consequently of the fruit of conspicuous degree of cracks and a thicker oxide lamina on HAZ and conglutinate district.
The gravity execute of the T22 conglutinateed steels follows the parabolic reprove law in air at 900 °C. The irritability to oxidation of conglutinateed 2.25Cr-1Mo (T22) steel samples has been establish to be as in the aftercited classify TIG; SMAW.
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