CREEP RESISTANCE OF WELD JOINT OF TUBE MADE OF P92 STEEL

Transcription

1 CREEP RESISTANCE OF WELD JOINT OF TUBE MADE OF P92 STEEL Josef KASL a, Dagmar JANDOVÁ a, Eva CHVOSTOVÁ a, Petr MARTÍNEK b a ŠKODA VÝZKUM s.r.o., Tylova 1/57, Plzeň, Česká republika, josef.kasl@skodavyzkum.cz b ZČU Plzeň, FS, KMM, Univerzitní 22, Plzeň Abstract One-side weld joint of W type was prepared from P92 type steel using GTAW & SMAW method. Creep testing of smooth cross-weld samples at temperatures ranging from 575 to 650 C and at stresses from 70 to 240 MPa was carried out. Ruptured samples were undergone fractographic analysis and extended metallographic investigation including scanning and transmission electron microscopy. After creep tests at temperatures above 575 C increase in size of secondary phases and cavities formation were evident. Voids were concentrated in fine prior austenite grain heat affected zones. In addition, sporadic occurrence of individual cavities was found out in the base material and the weld metal after tests at 625 and 650 C. During creep exposures at temperatures above 600 C Laves phase precipitated and their particles grew. Critical zone from the point of view of the creep failure is located in the fine prior austenite grain heat affected zone and in the overheated zone of the base material. Keywords: Steel P 92, weldments, creep test, microstructure, TEM 1. INTRODUCTION Turbines, boilers and steam piping belong to the most exposed parts of steam power plants. They have been operating under severe service conditions for several decades. Therefore high mechanical strength, good corrosion/oxidation resistance and high structural stability of materials used for their production are desired. Several new grades (P91, P92, P911) were developed in the last decades and they are currently used for producing various parts of high efficiency power plants. Grade P92 (X10CrWMoVNb9-2, ASME SA 335) is ferritic 9Cr W - 0.5Mo steel micro-alloyed with vanadium and niobium and with controlled boron and nitrogen contents. Due to excellent creep properties and high corrosion/oxidation resistance, which is equal to other high chromium ferritic steels, it is used for production of headers, boiler superheater and reheater tubes and main steam pipes for extremely severe steam conditions (temperatures exceeding 600 C and pressures of over 25 MPa) of advanced power plants [1]. Steel P92 reaches creep strength from 110 to 120 MPa at the temperature of 600 C for 105 hours. This steel contains only a small amount of nitrogen to reduce formation of boron nitrides. Low speed of coarsening of M 23 C 6 particles positively influences microstructure stability of this material. During tempering and/or creep exposure an intensive precipitation of Laves phase occurs, which leads to tungsten depletion of solid solution. However, it does not tend to more prominent decrease of creep strength. In spite of the fact that the amount of MX particles is rather low, only a moderate recovery as well as slow growth of subgrain occurred. It is well known that notwithstanding design for plant is based on creep strength of base materials, operating experience shows welds as the most critical parts of high temperature operating plants. So every producer of steam turbines needs to know the behaviour of weld joints at various parts of turbines, such as pipelines, casings, valves and so on during operation. Welded joints are commonly susceptible to fracture. Rupture is usually initiated in a specific region either during fabrication or during service as a result of structural heterogeneity of the weldment [2]. Since cyclic thermal and stress loading after each weld pass effects the

2 steel structure, great attention has to be paid to welding technologies and selection of convenient filler materials. This paper deals with the study of microstructure evaluation in the similar weld joints of P92 steel fabricated in industrial conditions after creep tests. 2. EXPERIMENTAL MATERIAL AND PROCEDURES Trial weld joints were prepared from segments of tubes of wrought ASME SA 335 Gr. P92 steel. Their outer diameter and wall thickness were 219 mm and 40 mm respectively. Normalising of the base material was done at 1,060 C and after cooling to room temperature tempering at 770 C for 2 hours was applied. Tubes were joined in both PC (longitudinal axis of tubes was vertical) and PF positions (longitudinal axis of tubes was horizontal). One-side welds of W type were carried out using manual welding; the root pass was done in internal protection by argon and filling passes using covered electrode (method according to EN ISO 4063, GTAW & SMAW according to ASME). Inductive heating with thermal insulation ensured a preheating temperature ranging from 200 to 250 C. The welding interpass temperature was kept below 300 C. The post weld heat treatment (PWHT) temperature of 760 C for 2 hours was applied. Thermanit 616 was used for root pass as well as for filler material. The chemical composition of the base metal and the consumable used is given in Table 1. Table 1. Chemical composition of base and filler materials (in wt. %) C Mn Si P S Cr Mo V W Ni Nb N B Al P92 - tube Thermanit Integrity and mechanical properties of weld joints have been evaluated according to the welding standards EN 288-2,3. All results were satisfactory. Selected mechanical properties of the base material as well as of the weld joints after PWHT are summarized in Table 2. Smooth cross-weld specimens with the length of 92 mm and the diameter of 8 mm were fabricated from the middle part (outside of the weld root) of the weld joint made in PC position. Creep tests to the rupture of these specimens were carried out at temperatures ranging from 575 C to 650 C and stresses from 70 MPa to 240 MPa. Fracture surfaces of ruptured samples were observed using scanning electron microscope (SEM). Then specimens were cut along their longitudinal axis. Macrostructure was revealed using Vilella- Bain s reagent and location of fracture in the weldment was specified. Hardness measurement along the specimen axis was performed. Microstructure on longitudinal sections was observed using light microscopy (LM) and scanning electron microscopy. The substructure was evaluated in transmission electron microscope (TEM). Both thin foils and extraction replicas were prepared from the selected most important parts of weld joint. The foils were thinned by a jet polishing in 6 percent solution of perchloric acid in methanol at -40 C. Energy dispersive X-ray microanalysis (EDX) and electron diffraction were used for the identification of secondary phases. Table 2. Mechanical properties of weld joints Weld position R p0,2 [MPa] R m [MPa] A [%] Z [%] Fracture location KV [J] HAZ WM HAZ WM - root PC BM 185,193,172 49,42,36 137,96,44 32,37,29 PF BM 94,185,179 19,16,22 200,182,181 33,33,37

3 3. RESULTS The creep testing was carried out. Summary of samples, conditions of tests as well as obtained results are given in Table 3. Time to the rupture ranged from 100 to 17,000 hours. Creep rupture strength was evaluated using Larson-Miller parametric equation P = T * [C + log τ] where T represents temperature given in degree Kelvin, C is a specific constant for the given material (C = 36 for P92 steel) and τ means time to fracture in hours. Results of creep tests compared with the standardized creep rupture strength data of P92 steel [3] are graphically represented in Fig. 1. Up to 600 C the creep strength of the weld joint falls into the usually permitted scatter band ± 20 percent of the creep strength of the base material used for production of piping. For the testing temperature of 625 C the creep strength of the weld joint decreases below the scatter band for sample tested at stress below 80 MPa and lies on the boundary of the scatter band for sample tested at stress 90 MPa while for higher stresses falls into the scatter band. For the testing temperature of 650 C the creep strength of the weld joint decreases below the scatter band only for sample tested at the lowest stress 70 MPa. Differences between he creep strength of the weld joint and the creep strength of the base material are shown in Table 3. Table 3. List of samples. Sample Temp. [ C] Stress [MPa ] Duration [h] A [%] Z [%] σ BM - σ WM) /σ BM [%] Fracture Location Average hardness HV BM HAZ WM , BM/HAZ IC BM , HAZ FG/IC , HAZ FG/IC , HAZ FG , HAZ FG/IC , BM BM , HAZ FG , HAZ FG , HAZ FG , HAZ FG , HAZ IC , HAZ FG , HAZ FG , HAZ FG Fig 1. Creep rupture strength in dependency on Larson-Miller parameter; full line represents creep strength for base material, dashed line represents creep strength of base material minus 20 percent).

4 Fractographic analysis and observation of longitudinal sections of the ruptured cross-weld specimens show that samples tested at lower temperatures and higher stresses failed in the base material (BM) unaffected by welding while those tested at higher temperatures and lower stresses ruptured in the heat-affected zone (HAZ) of the base material (in the grain refined part or in the intercritically reheated part of the zone). Fracture locations of samples are given in Table 3. In the first above mentioned group ductile fracture occurred after very short durations of creep tests (up to two thousands hours). The fractures are transcrystalline ductile with considerable macroplastic deformation (elongation about 15%) and with the dimple morphology of the fracture surface. The second one includes samples ruptured by transgranular creep fracture. Longitudinal elongations of these specimens were usually a few percent. Individual small cracks formed of growing cavities joined and spread step by step across the sample. Finally the shear lips were formed on the sides of the test bar. Intercrystalline facets were not observed. Fracture surfaces of samples tested at temperatures above 625 C are covered with an oxide layer, so in some cases it is rather difficult to determine real fracture micro-mechanism. Particles of BN and fine (Al,Ca)-oxides with diameter about one micrometer were found on the fracture surfaces using ED microanalysis. Hardness profiles across the weld joints were determined on the longitudinal sections. Average values of hardness HV10 of the different parts of weld joins are summarized in Table 3. Before creep testing hardness of the BM was 226 and hardness of the weld metal (WM) 260. Local maxima in the coarse grained (CG) part of the HAZ near the fusion line and local minima in the fine (FG) grained or in the overheated part of the HAZ of HAZ were found. During creep testing hardness decreased in all regions of the weld joint already after short durations and hardness differences among individual areas become smaller. The biggest drop was found for sample tested at 650 C/100 MPa in both the WM (220 HV10) and in the BM (205). The minimum value of hardness about 190 HV10 was found in the FG HAZ. Microstructure of the BM, which was used for production of trial weld joint, corresponds to tempered martensite with a small amount of δ-ferrite. Isolated particles of δ-ferrite occurred rarely in central parts of metallograpfic sample in the cross section of the ring segments while rows of δ-ferrite particles were observed in the surface layers of rings (the region of top pass and root). Severely tempered martensitic structures were observed in all parts of weld joints the lath like structures in the BM, the WM and coarse austenite grain heat affected zone (CG HAZ), and the fine featureless structure in the fine prior austenite grain heat affected zone (FG HAZ). In the BM and also in the WM some particles of boron nitride were observed in size of a few micrometers. They usually originated at the surface of oxides aluminium oxides in the BM and silicon oxides in the WM. Comparing the chemical composition of experimental materials with the solubility diagram of BN in 9-11% Cr steels [4, 5] the presence of BN particles can be expected. Substructure of samples was investigated using TEM. The microstructure of tempered martensite consisted of ferrite laths divided into subgrains, coarse particles at grain and subgrain boundaries and fine intragranular precipitate. Some differences in dislocation density and distribution of precipitates were observed in individual zones. Microstructure in the FG HAZ exhibited a subgrain structure often of polygonal shape instead of typical martensitic lath-like structure observed in others parts of the weld joint. Post-weld heat treatment at 760 C resulted in precipitation of M 23 C 6 carbides especially at boundaries of prior austenite grains and ferrite laths and vanadium/niobium carbonitrides spread at boundaries and also within laths.

5 An increase in size of M 23 C 6 particles, Laves phase particles precipitation and cavities formation were observed after creep tests at 575 C and lower stresses and at temperatures above 600 C. Growth occurred especially in the WM. Laves phase formed the largest observed particles (Fig. 2, 3). No important changes in size and distribution of fine precipitates were found out among sample after PWHT and samples after creep testing. A lot of cavities were indicated in FG HAZ, where fracture occurred. Some cavities were also present in the BM and the WM. Quantity and size of cavities increased with increasing temperature of creep testing. a b c d Fig 2. SEM micrographs of FG HAZ: sample after PWHT a) - secondary electron image, b) back-scattered electron image; sample tested at 625 C/80 MPa c) - secondary electron image, d) back-scattered electron image; new particles of Laves phase are visible in Fig d). Table 4. Particle sizes Quantitative evaluation of secondary phase particles in both the WM and the BM was performed for weld joint after PWHT and selected crept specimens using carbon extraction replicas. All particles observed were assigned to fine intragranular particles of a diameter from up to 40 nm or to coarse particles with a diameter higher than 40 nm. Coarse and fine particles were evaluated separately using image analysis software and type of phase was not taken into account. Results are given in table 4. After creep test at 575 C/240MPa/100hrs any changes in size and distribution of secondary phases were observed. After test at 600 C/160MPa/7,715hrs slight growth of coarse particles was found out. Significant increase was evident after test at 625 C/100MPa/6,732hrs. a b c d e f Fig 3. TEM micrographs of sample after PWHT - (a) BM, (b) FG HAZ, (c) WM and of sample tested at 625 C, 80 MPa - (d) BM, (e) FG HAZ, (f) WM.

6 4. CONCLUSIONS This paper deals with creep properties of the weld joint of pipilines fabricated from P92 steel using welding technology developed for industry praxis. Up to 600 C the creep strength of the weld joint falls into usually permitted scatter range ± 20 percent of the creep strength of the base material used for production of piping. At higher temperatures it decreases below this scatter range. Critical zone from the point of view of the creep failure is located in the fine prior austenite grain heat affected zone and in the overheated zone of the base material where cavitation failure was evident. Chemical composition of used steel and filler material lead to a formation of coarser particles of boron nitrides causing a decrease of ability of boron to inhibit coarsening of M 23 C 6 carbides. Coarsening of these particles together with precipitation and coarsening of new particles of Laves phase were observed after relatively short duration of creep tests at temperatures above 600 C. ACKNOWLEDGEMENTS This work was supported by Grant projects MSM and OC09041 from the Ministry of Education, Youth and Sports of the Czech Republic. LITERATURE [1] The T92/P92 Book. Vallourec & Mannesmann Tubes, edition [2] Mythili, R.- Thomas, P. V. Saroja, S. Vijayalakshmi, M. Raughunathan, V. S.: Microstructural modification due to reheating in multipass manual arc welds of 9Cr-1Mo steel. J. Nucl. Mater., 312, (2003). [3] Creep strength of steel X10CrWMoVNb 9-2 according the ECC data sheet. [4] Sakuraya, K - Okada, H. Abe, F.: Coarse Size BN type inclusions formed in boron bearing high Cr ferritic heat resistant steel. In Proc. of the 4 th International Conference of Advances in Materials Technology for Fossil Power Plants Hilton Head Island, SC, United States: ASM International. [5] Abe, F.: Effect of boron on creep deformation behaviour of 9Cr steel for USC boilers at 650 C. In Proc. of the 7 th International Charles Parsons Turbine Conference, Glasgow, 2007.