Evaluation on the hardness and microstructures of T91 reheater tubes after postweld heat treatment

Transcription

1 Accepted Manuscript Short communication Evaluation on the hardness and microstructures of T91 reheater tubes after postweld heat treatment M.Z. Hamzah, M.L. Ibrahim, Q.H. Chye, B. Ahmad, J.I. Inayat-Hussain, J. Purbolaksono PII: S (12) DOI: Reference: EFA 1829 To appear in: Engineering Failure Analysis Received Date: 25 August 2011 Accepted Date: 26 August 2012 Please cite this article as: Hamzah, M.Z., Ibrahim, M.L., Chye, Q.H., Ahmad, B., Inayat-Hussain, J.I., Purbolaksono, J., Evaluation on the hardness and microstructures of T91 reheater tubes after post-weld heat treatment, Engineering Failure Analysis (2012), doi: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

2 Evaluation on the hardness and microstructures of T91 reheater tubes after postweld heat treatment M.Z. Hamzah a, M.L. Ibrahim a, Q.H. Chye a, B. Ahmad a, J.I. Inayat-Hussain b, J. Purbolaksono c,1 a TNB Research Sdn Bhd, No. 1 Lorong Air Hitam, Kajang 43000, Malaysia b Department of Mechanical Engineering, Universiti Tenaga Nasional, Jalan IKRAM- UNITEN, Kajang 43000, Malaysia c Centre of Advanced Manufacturing and Materials Processing, Department of Engineering Design & Manufacture, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia Abstract Following visual inspections, hardness testing and microstructure evaluations for the investigation during the forced outage of a power plant in Malaysia, a number of T91 reheater (RH) tubes were discovered to have abnormal hardness and microstructures after the mandatory post-weld heat treatment (PWHT). The abnormal hardness and atypical microstructures of the affected tubes were due to exposure to the higher temperatures above AC1. Flattening tests were also carried out to evaluate the ductility of materials. Criteria formulated for further actions on the affected tubes were proposed in order to have safe and continued operations. Keywords: Reheater tube; Post-weld heat treatment; Hardness; Microstructure 1 Corresponding author: J. Purbolaksono; judha@um.edu.my; Phone:

3 1. Introduction Reheater and superheater tubes are vulnerable to high temperature upset condition, undergoing severe creep deformation or even final rupture. In general, boiler tubes in power plants have finite life because of prolonged exposure to high temperature, stress, aggressive environment, corrosive degradation, etc. Recently, several works on the failure investigation of reheater and superheater tubes of the power plants in Malaysia have been reported [1-6]. In order to have continued operations under higher temperatures and pressures for a long period of operation, uses of suitable boiler tube material in thermal power plants are required. In particular, a chromium molybdenum vanadium steel tube (SA213-T91) has been available in the market since two decades ago and was co-developed by the Combustion Engineering and the Oak Ridge National Laboratory in the late 1970s [7]. The SA213-T91 material has better creep and high temperature strengths than those of the widely used materials such as T11 and T22. In respect to the purpose of conducting PWHT, it may reduce residual stresses and tempers hardened microstructures. However, PWHT might also not positively benefit the overall properties of the weldment if not properly done and controlled. This paper in particular reports the findings of the investigation on hardness and microstructures of reheater tubes after the mandatory post-weld heat treatment (PWHT) during the forced outage of a power plant in Malaysia. 2. Visual inspection The reheater tube samples were taken from the affected site for visual inspection. Heavy darkish discoloration on the external surface shown in Fig. 1 may indicate that the tubes had been exposed to higher temperature above AC1. It is advisable to

4 conduct laboratory works covering heat treatments, hardness testing and microstructure examinations. 3. Laboratory works The reheater tube samples taken from the site were subjected to laboratory testing to evaluate the as-received conditions (subjected to the post-weld heat treatment) and the conditions after undergoing different heat treatment processes by microstructure examination and hardness testing. Similar testing to the unused tube sample was also carried out for reference. The heat treatment processes covered tempering and normalizing for different time and temperatures. Generally, the SA213-T91 tubes subjected to PWHT should conform to the following condition as: (a) (b) (c) (d) Surface should not have heavy darkish discoloration. Parent metal hardness is between 180 and 250 HV. Weld metal hardness is between 180 and 290 HV. Microstructure should contain fine martensite with copious precipitates. Hardness testing was performed to ensure whether or not the hardness ranges of all the tubes are within the specified requirement. Hardness readings were taken at different locations except those with heavy discoloration of a tube, and the average value was then determined. Hardness readings were performed on a laboratory micro-vickers machine with a load of 300 g. Microstructure was evaluated on prepared metallographic sections by an optical microscope at magnifications up to 500x. The microstructure of the unused tube sample is depicted in Fig. 2, showing fine martensite and copious precipitates of various sizes.

5 Meanwhile, the microstructures of the as-received sample subjected to PWHT and the samples following different heat treatment processes are shown in Fig. 3 and the descriptions of the microstructure and the average hardness reading of each sample are summarized in Table 1. The results presented in Table 1 indicate the following features as: - Overheated tube and weld metal subjected only to a subsequent tempering process would not be able to recover the full properties of T91 steel. - Overheated tube metal, subjected to re-normalizing and tempering at C cannot fully recover the full properties of T91 steel. However, weld metal subjected to this process can match or exceed properties of normal weld metal subjected to site PWHT. 4. Criteria formulated for further action An EPRI (Electricity Power Research Institute) document [8] provides specific recommendations for PWHT and the remedies available if the AC1 temperature is exceeded. This document is used as a guideline in making decisions for further action to be taken. The document stated that if a portion of the component is heated above the lower critical transformation temperature, one of the following actions shall be performed: - The components, in its entire part, must be normalized and tempered. - If the maximum holding temperature (775 o C) is exceeded, but does not exceed 800 o C, the weld metal shall be removed and replaced.

6 - The portion of the component heated above 800 o C and at least 75 mm on either side of the overheated zone must be removed and be normalized and tempered or replaced. - The allowable stress shall be that for Grade 9 material (SA213-T9 or equivalent product specification) at design temperature, provided that the portion of the component heated to a temperature greater than that allowed above is reheat treated within the specified temperature range ( o C). The maximum allowable stresses of SA213-T91 and SA213-T9 for different temperatures are presented in Table 2. The maximum allowable stress of SA213-T9 for a given temperature is significantly lower than that of SA213-T91. With regard to the future operations and referring to the EPRI document [8], the criteria for evaluating tubes subjected to PWHT may then be formulated for further actions as summarized in Table Flattening tests Flattening tests were performed to evaluate the ductility of the T91 materials for different heat treatments. Five samples of T91 tubes and their appearances after undergoing flattening test are shown in Fig. 4. Descriptions of the tube samples shown in Fig. 4 are as follows: - Tube A is a new reheater tube. - Tube B was presumably subjected to improper PWHT and no heat treatment was done. - Tube C was presumably subjected to improper PWHT and had heat treatments by normalizing at 1060 o C for 45 min and tempering at 750 o C for 2 h.

7 - Tube D was presumably subjected to improper PWHT and had heat treatments by normalizing at 1045 o C for 45 min and tempering at 750 o C for 2 h. - Tube E was presumably subjected to improper PWHT and had heat treatment by normalizing at 1060 o C for 45 min. It can be seen from Fig. 4 that Tubes A, C and D are shown to be fully flattened without cracking, showing ductile condition. Meanwhile, Tubes B and E were reported to have cracking and found to be not fully flattened, indicating brittle condition. Heat treatment by normalizing without tempering is not sufficient to soften the material. 6. Conclusions The abnormal hardness and atypical microstructures experienced by the affected tubes had been caused by improper PWHT after site welding. The PWHT procedures have to comply with a formal practical standard. Heat treatments by re-normalizing and tempering on the affected tubes cannot fully recover the properties as those of the original tube material. However, the heat treatment by normalizing at 1060 o C for 45 min and tempering at 780 o C for 2 h seemed able to recover the properties to be close to those of the original T91. Flattening test may also be used to indicate the ductility of materials. Criteria formulated on handling similar problems in the future were proposed in this paper. Acknowledgement The authors would like to thank Mrs. Norlia Berahim and members of the Materials Laboratory of TNB Research Sdn Bhd for laboratory works, and also thank Mr Abdul Rohim Saad and his team from TNB Remaco Repair Centre for continuous supports during completion of this project.

8 References [1] Purbolaksono J, Hong YW, Nor SSM, Othman H, Ahmad B. Evaluation on reheater tube failure. Eng Fail Anal 2009; 16(1): [2] Ahmad J, Purbolaksono J, Beng LC, Rashid AZ, Khinani A, Ali AA. Failure investigation on rear water wall tube of boiler. Eng Fail Anal 2009; 16(7): [3] Othman H, Purbolaksono J, Ahmad B. Failure investigation on deformed superheater tubes. Eng Fail Anal 2009; 16(1): [4] Ahmad J, Purbolaksono J, Beng LC. Failure analysis on high temperature superheater Inconel 800 tube. Eng Fail Anal 2010; 17(1): [5] Purbolaksono J, Ahmad J, Beng LC, Rashid AZ, Khinani A, Ali AA. Failure analysis on a primary superheater tube of a power plant. Eng Fail Anal 2009; 17(1): [6] Ahmad J, Purbolaksono J, Beng LC, Ahmad A, Failure evaluation on a highstrength alloy SA213-T91 superheater tube of a power generation. Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering 2010; 224 (4): [7] Sikka VK, Ward CT, Thomas KC. Modified 9Cr 1Mo steel an improved alloy for steam generator application. Ferritic steels for high-temperature applications. In Proceedings of the ASM International Conference on Production, fabrication properties, and applications of ferritic steels for high temperature applications, Metals Park, Ohio, 1983:

9 [8] Coleman K. Guidelines for welding creep strength-enhanced ferritic alloys, EPRI , California: Electric Power Research Institute, [9] ASME international electronic stress Table. Table 1A: The maximum allowable stress values for ferrous materials. Section II, Part D of The ASME boiler and pressure vessel code. Copy Right_ 1998 ASME international.

10 Table 1. Descriptions of the microstructures and the average hardness reading of each sample Condition/ heat treatment - Original/ unused parent metal; - No heat treatment. - Parent metal with PWHT; - No heat treatment. - Weld metal with PWHT; - No heat treatment - Parent metal with PWHT; - Tempering at 750 o C for 2 h. - Weld metal with PWHT; - Tempering at 750 o C for 2 h. - Parent metal with PWHT; - Normalizing at 1060 o C for 45 min; - Tempering at 750 o C for 2 h. - Weld metal with PWHT; - Normalizing at 1060 o C for 45 min; - Tempering at 750 o C for 2 h. - Parent metal with PWHT; - Normalizing at 1060 o C for 45 min; - Tempering at 780 o C for 2 h. - Weld metal with PWHT; - Normalizing at 1060 o C for 45 min; - Tempering at 780 o C for 2 h. Condition of microstructures Fine martensite and copious precipitates of various sizes (Fig. 2) Medium grained structures (mostly martensite) and few visible precipitates (Fig. 3a) Coarse grained structures, mainly non-tempered martensite, and no visible precipitates (Fig. 3b) Medium grained structures with tempered martensite and moderate precipitates (Fig. 3c) Coarse grained structures, mainly tempered martensite, and some precipitates (Fig. 3d) Medium grained structures with localized coarseness, tempered martensite and moderate precipitates (Fig. 3e) Very coarse grained structures, mainly tempered martensite, and some precipitates (Fig. 3f) Medium grained structures, tempered martensite, and copious precipitates that is finer than the original parent (Fig. 3g) Coarse grained structures, tempered martensite, and moderate precipitates (Fig. 3h) Hardness, HV Remarks Good microstructure; Hardness within the required range. Atypical microstructure; Hardness exceeding the required range. Atypical microstructure; Hardness exceeding the required range. Microstructure and creep strength are not as optimum as the origin; Hardness fairly within the required range. Microstructure is not optimum as that of the origin; Hardness within the required range. Creep strength may be below than that of weld metal made on site. Microstructure is not optimum as that of the origin; Hardness within the required range; Creep strength is inferior to the normal T91. Microstructure is not as optimum as the origin; Hardness within the required range. Creep strength is considered to be equal to that of weld metal made on site. Microstructure is not optimum but hardness is within required levels; The 780 o C tempering temperature has induced more precipitation than that at 750 o C; Creep strength is considered to be nearly the same as normal T91. Microstructure is close to optimum for weld metal; Hardness is within required levels; Creep strength is likely to be superior to T91 weld metal made at site.

11 Table 2. The maximum allowable stress value for different temperatures [9]. Temperature ( o C) SA213-T91 (MPa) SA213-T9 (MPa)

12 Table 3. Some proposed criteria for further actions following PWHT. Category Condition Cause Action 1 Parent HV within the specified range; Weld HV > 290 HV Temperature for PWHT is far below AC1 I 2 Parent HV >250 HV; Weld HV unknown Temperature for PWHT > AC1 II or III 3 Parent HV < 180 HV; Weld HV <180 HV Temperature for PWHT is very close to AC1 II, III or IV 4 Heavy discoloration Temperature for PWHT > AC1 II or III 5 Parent HV > 250 HV; Weld HV > 250 HV Temperature for PWHT > AC1 II 6 Parent HV > 250 HV; Weld HV > 250 HV Temperature for PWHT > AC1 II or III Action I: Action II: Action III: Action IV: Re-PWHT and accept as normal Cut out the affected part and replace with a new section Cut out the affected part and re-heat treated (normalizing and tempering) in an external furnace Downgrade the allowable stress to that of T9 and no other action needed except to monitor condition in 10,000 h

13 Fig. 1. The reheater tube samples taken from the affected site. Fig. 2. Microstructure of the original tube sample: (a) Magnification 200x and (b) Magnification 500x.

14 Fig. 3. Microstructures of the as-received samples subjected to PWHT and different heat treatment processes: (a) Parent metal with no heat treatment; (b) Weld metal with no heat treatment; (c) Parent metal with tempering at 750 o C for 2 h; (d) Weld metal with tempering at 750 o C for 2 h; (e) Parent metal with normalizing at 1060 o C for 45 min and tempering at 750 o C for 2 h; (f) Weld metal with normalizing at 1060 o C for 45 min and tempering at 750 o C for 2 h; (g) Parent metal with normalizing at 1060 o C for 45 min and tempering at 780 o C for 2 h; (h) Weld metal with normalizing at 1060 o C for 45 min and tempering at 780 o C for 2 h.

15 Fig. 4. Five samples of T91 tubes and their appearances after undergoing flattening tests.

16 Research Highlights Post-weld heat treatments on SA213-T91 boiler tubes. Recovering the specified hardness and microstructure through heat treatments. Criteria for further actions on evaluating PWHT of tubes are proposed.