Microstructure Changes on T92 Steel after Accelerated Ageing Process at 654ºC

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1 Journal Of Industrial Engineering Research ISSN Journal home page: (2): 1-5 RSEARCH ARTICLE Microstructure Changes on T92 Steel after Accelerated Ageing Process at 654ºC 1Suraya M.Nadzir, 1 Ng G.P, 2 Badrol Ahmad 1 TNB Research Sdn Bhd, Material Engineering Group, Kajang, Selangor Malaysia. 2 TNB Berhad, Generation, 59200, Bangsa, Kuala Lumpur, Malaysia. Address For Correspondence: Suraya M.Nadzir, TNB Research Sdn Bhd, Material Engineering Group, Kajang, Selangor Malaysia. Received 3 June 2016; accepted 10 September 2016; published 30 September 2016 A B S T R A C T Grade T92 steel was introduced as material construction for component that operates in superheater condition for supercritical boiler technology. Microstructure of this steel is complex as compared to low alloy steel to provide higher creep strength property. This study was conducted to examine the changes in microstructural of T92 steel after accelerated ageing process at 654 C and their effect on creep behaviour. Exposure of T92 samples at varying duration shows microstructural changes which include coarsening of precipitates, formation of Laves Phases and development of equaixed grain. These microstructural changes of T92 steel after the accelerated ageing process indicate the initial stage of creep strength reduction. Key words: T92, microstructure changes, development of precipitate, creep strength INTRODUCTION T92 steel is mainly use as construction material for superheater component in new boiler technology such as supercritical boiler. This steel was introduced to power industry since 1990 s. However, application of this steel in Malaysia is relatively new. The important aspect of T92 service performance relates to creep behaviour because this steel is operated in creep regime. Understanding of creep behaviour of these steel is essential in order to assess creep damage progressing over service exposed duration. Grade T92 steel is a modification from previous steel grade 9Cr-1Mo-V (T/P91). The important improvement made for this steel is higher creep strength as compare to T91. This improvement is mainly achieved by addition of 0.2% Vanadium (V), Nb, 0.05 N, 1.8% W, and reduction of Mo between 1 to 0.5%. The essential difference in chemical composition of between T91 and T92 steels is tungsten (W) content which is important for the solid solution strengthening of the matrix. However, studies show that this element increases the tendency for Laves phase formation. Laves phase is one of the undesirable phases that forms in T92 steel which coarsen rapidly and reduce the precipitation strengthening to increases the possibility of creep failure at certain temperatures following a period of service exposure. This study was conducted to understand the creep development in T92 steel which manufactured for boiler application. This study examined changes in microstructural of T92 steel after accelerated thermal ageing process at 654 C and their effect on creep behaviour. Experimental: Material Characterization: Open Access Journal Published BY IWNEST Publication 2016 IWNEST Publisher All rights reserved This work is licensed under the Creative Commons Attribution International License (CC BY). To Cite This Article: Suraya M.Nadzir, Ng G.P, Ahmad., Microstructure Changes on T92 Steel after Accelerated Ageing Process at 654ºC. Journal of Industrial Engineering Research, 2(2): 1-5, 2016

2 2 Suraya M.Nadzir et al., 2016 Material characterization was conducted using x-ray spectrometer technique and result will be compared to steel grade ASTM A213-T92 that coded in ASME Section II Part D as shown in Table 1. Table 1: Chemical Composition T92 steel Chemical composition wt. % C Mn P S Si Cr Mo V W Ni Nb Al Fe ASME code range for T max max steel Bal. Accelerated Thermal Ageing Process: The as-received T92 tube samples with outer diameter of 40mm and thickness of 10mm were thermally aged to simulate service induced degradation equivalent to 100,000 hours service exposed at 580 C. The thermal ageing process was conducted for durations of 500 and 1000 hours at 654 C in box furnace. The heating and cooling rate was controlled at 250ºC/hours and 150ºC/hour respectively. 2 tube samples were place in the furnace and they were attached with thermocouple type K to monitor in-line steel temperature. The first (T91- A5) and second (T91-A10) samples were taken out from furnace after 500 and 1000 hours of heating respectively. Sample details are shown in Table 2. The accelerated thermal ageing condition of 580ºC for 500 and 1000 hours was determined using parametric Larson-Miller Parameter (LMP) relationship, LMP = (T) (Log 10 t r + C) x 10-3 ( C = 20 ) [1]. In this study, the effect of stress was negligible because creep property is depends heavily on thermal as compared to stress. Table 2: T92 Accelerated Aged Samples Sample ID Ageing Duration (hours) Equivalent to service exposed at 580ºC (hours) T92- A ,000 T92- A ,000 Microstructure Examination: The aged samples were subjected to microstructure examination using Optical Microscope (OM), Scanning Force Microscope (SEM) and Electron Backscatter Diffraction (EBSD). RESULT AND DISCUSSION Chemical composition of as-received T92 sample shows that the sample is consistent with steel grade ASTM213-T92. In view of steel user, the performance of this steel is acceptable and its creep performance also expected to be in the recommended range. Chemical compositions of T92 sample is shown in Table 3. Table 3: Chemical Composition for T92 sample Chemical composition C Mn P S Si Cr Mo V W Ni Nb Al Fe wt. % T92 sample < <0.002 Bal. ASME code range for T max max steel Bal. Visual observation on accelerated thermal aged samples did not show any abnormalities or significant changes on the sample surface. No signed of overheating or burn mark was observed and this shows that the samples were exposed to even temperature throughout the accelerated thermal ageing process. In general, microstructure of T92 steel (for boiler application) is tempered martensite with very fine precipitates at grain boundary and in grain interior. Changes in microstructure for this steel can be determined by; i) Changes in precipitates condition include distribution, type and size and ii) Changes of grain size and orientation. Observation on microstructure images that were captured using OM and SEM techniques show minor changes which is coarsening of precipitates at Prior Austenite Grain Boundary (PAGB) and sub-grain (martensite lath). This is based on the observation of thicker grain boundary and sub-grain lines on microstructural image for sample 1000hours aged as compared to other samples as shown in Figure 1. The tempered martensite structure is consider complex structure and is it not easy to interpret using OM images.

3 3 Suraya M.Nadzir et al., 2016 Microstructure images of T92 samples captured using OM Microstructure images of T92 samples captured using SEM As-Received 500hrs 1000hrs As-Received 500hrs 1000hrs Transverse Longitudinal Fig. 1: Microstructure images of T92 samples Formation of precipitates in T92 steel is mainly for precipitation strengthening mechanism which relates to creep strengthening. Typical types of precipitates that form during heat treatment process for steel grade 9 to 12% Cr are known as M 23 C 6, Nb(C,N), M 2 X and MX [2]. These precipitates formed from the alloying elements such as Vanadium, Tungsten, Molybdenum and Niobium [2, 3, and 4]. However, over service exposed duration, new type of precipitates such as Laves Phase also can be found in T92 sample. The appearance of Laves Phase in T92 aged samples shows that it has been exposed to creep degradation. Types of precipitates in T92 samples were detected using EDS techniques and results shown in Table 2. Table 2: Types of precipitates in T92 samples Sample As-received (T92-A0) Aged 500hrs (T92-A5) Aged 1000hrs (T92-A10) Type of precipitates M 23C 6, and MX M 23C 6 (Cr,W), MX(Nb,/V) and Fe 2(Mo,W) laves phase M 23C 6, (Cr, W), MX(Nb,/V) and Fe 2(Mo,W) laves phase Microstructure changes also examined using EBSD techniques and comparison was made between asreceived, accelerated aged sample and creep tested samples. The microstructures of aged samples show equaiexed structure which is not similar to as-received sample. Creep strengthening of the aged samples may reduce because the structure arrangement is similar to creep tested samples structure as shown in Figure 2. Observation made on as-received, aged and creep tested samples are: i. Clear Martensitic lath can be seen in as-received sample. ii. No distinct martensitic lath was observed in aged and creep tested samples. iii. Grain size of aged samples is generally smaller that creep tested sample. iv. The low angle grain boundary was found reduced on aged and creep tested samples as compared to asreceived sample.

4 4 Suraya M.Nadzir et al., 2016 Microstructure images of T92 samples capture using EBSD As-Received 500hrs 1000hrs Creep tested samples ( 650ºC/120MPa) Accelerated thermal aged samples Fig. 2: EBSD images on T92 samples (As-received, Aged and Creep tested samples) Measurements of grain size, low angle grain boundary and lath width were conducted using separate software. Results show that the aged samples have smaller grain size, low percentage of LAGB and bigger lath width as compared to as-received samples as shown in Table 3. This results support the microstructure images that showing equaiexed structure arrangement on aged samples. Table 2: Changes on grain condition Measurement on As-received 500hours 1000hours Grain size (µm) Low Angle Grain boundary (LAGB) (%) Lath width (µm) Conclusion: Through this study, changes of T92 microstructure after accelerated thermal ageing process at 645ºC were review and analysed. Results show; 1) Precipitates at grain boundary and in the grain interior are growth, 2) Formation of Laves Phase after after 500hours thermally aged, and iii) Microstructure change from clear appearance of martensite lath to unclear and to equaixed grain structure after 500hours thermally aged. These findings indicate the T92 steel will experience significant creep degradation after 50,000hours in service exposed at 580 ºC. Creep is permanent degradation which occurs slowly due to operating temperature and stress condition. The development of creep has to be monitored to increase component reliability. Exposure to higher temperature and or stress will accelerate the creep degradation process. REFERENCES [1] Abson, D.J. and J.S. Rothwell, Mechanical properties of high productivity weldments in the advanced 9% chromium steel-fb2. The Welding Institute UK. [2] Fujio Abe, Precipitate design for creep strengthening of 9% Cr tempered martensitic steel for untrasupercritical power plant. Heat Resistant Design Group, Structural Metals Center, National Institute for Materials Science (NIMS) Japan. [3] F.Abe, M Teneike, K. Sawada. Sowasa, Alloy Design Of Creep Resistant 9Cr Steel Using A Dispersion Of Nano-Sized Carbonitrides. International journal of pressure vessel and piping 84.

5 5 Suraya M.Nadzir et al., 2016 [4] Abson, D.J. and J.S. Rothwell, 2010.Mechanical Properties Of High Productivity Weldments In The Advanced 9% Chromium Steel-FB2. The Welding Institute, UK. [5] Jones, R.L., Development Of Two-Layer Deposition Techniques For The Manual Metal Arc Repair Welding Of Thick C-Mn Steel Plate Without Postweld Heat Treatment. The Welding Institute, UK.