atigue Fracture of Engineering Materials Structures doi10.11114602695201001496X Assessment of creep rupture properties for dissimilar steels welded joints between T92 and HR3C YI GONG, JIAN CAO, 1 LI-NA Jl, 1 CHAO YANG, CHENG YAO, 1 ZHEN-GUO YANG, JUN WANG, I XIAO-MING LUO, 2 FU-MING GU, 2 AN-FANG QL SHANG-YUN YE and ZHENG-FEI HU4 I Department of Materials Science, Fudan University, Shangbai 200433, PR China, Shangbai Institute of Special equipment Inspection d Technical Research, Shanghai 200062, PR Cbina, Shangbai Boiler Works Ltd, Shangbai 200245, PR China, +School of Materials Science and Engineering, Tongji University, Shangbai 200092, PR Chim Received in final form 4 May 2010 ABSTRACT Dissimilar steels welded joints, between ferritic steel and austenitic stainless steel, are always encountered in high-temperature components in power plants. As two new grade ferritic steel and austenitic stainless steel, T92(9Cr0.5Mo2WVNb)and HR3 TP310HCbN), exhibit superior heat strength at elevated temperatures and are increas- ingly applied in ultra-supercritical (USC) plants around the world, a complete assessment of the properties for T92/HR3C dissimilar steels welded joints is urgently required. In this paper, metallographic microstructures across the joint were inspected by optical mi- roscope. Particularly, the creep rupture test was conducted on joints under different oad stresses at 625C to analyse creep strength and predict their service lives, while their fractograph were observed under scanning electron microscope. Additionally, finite element method was employed to investigate residual stress distribution of joints. Results showed that the joints were qualified under USC conditions, and T92 base material was commonly the weakest part of them Keywords creep rupture; dissimilar steels welded joints; finite element method; HR3C INTRODUCTION research. Consequently, heat-resistant steels, mainly re- Vith the worsening of global energy crisis an ferring to ferritic steels and austenitic stainless steels, have ronmental pollution, higher energy utilization and lower been pursued simultaneously since the emergence of USC CO2 emission are presently the two prior criteria for de- boilers for applications in boiler components, including signing fossil power plants, ' which will still serve as the superheater, reheater, header, turbine, steam piping and dominant energy form for at least 20 years. 2-As for fossil so on. Subsequently, for the sake of reliable services in power boilers, it is well known that the enhancement of USC boilers, a wealth of research has been carried out steam parameters can facilitate improvement in thermal on these heat-resistant steels as F12(X20CrMoV12. 1), efficiency, which can result in reduction in not only the fossil costs but also the COz, SO2 and NOx emission. vestigate characteristics of their base materials and perfor- Concretely, compared with supercritical(SC) boilers(24 mance deterioration after long-term services. 1-29 Among MPa, 565oC), modern ultra-supercritical (USC)boilers them, the T92(9Cr0 5Mo2WVNb), approximately simi- operating around 30 MPa and 600 C can lead to both lar to T9I but with a little modification in chemical com- an increase of 8-10% in thermal efficiency(from 37% positions for preferable high temperature properties than to 45-47%)and a reduction of 20-25% in COz emis- T91, and the HR3C (TP310HCbN) are the two typical representatives of new grade ferritic steels and austenitic Development of USC boilers with increasingly highe stainless steels for their superior heat strength above prospect in forthcoming USC boilers. However, appli- cations of T92 and HR3C are currently a bit constrained Correspondence:Z.-G.Yang.E-mail:zgyang@fudan.edu.cn due to the lack of sufficient literature and experience on @2010 Blackwell Publishing Ltd Fatigue Fract Engng Mater Struct 34, 83-96
doi: 10.1111/j.1460-2695.2010.01496.x Assessment of creep rupture properties for dissimilar steels welded joints between T92 and HR3C YI GONG, 1 JIAN CAO, 1 LI-NA JI, 1 CHAO YANG, 1 CHENG YAO, 1 ZHEN-GUO YANG, 1 JUN WANG, 1 XIAO-MING LUO, 2 FU-MING GU, 2 AN-FANG QI, 3 SHANG-YUN YE 3 and ZHENG-FEI HU4 1Department of Materials Science, Fudan University, Shanghai 200433, PR China, 2Shanghai Institute of Special Equipment Inspection & Technical Research, Shanghai 200062, PR China, 3Shanghai Boiler Works Ltd., Shanghai 200245, PR China, 4School of Materials Science and Engineering, Tongji University, Shanghai 200092, PR China Received in final form 4 May 2010 ABSTRACT Dissimilar steels welded joints, between ferritic steel and austenitic stainless steel, are always encountered in high-temperature components in power plants. As two new grade ferritic steel and austenitic stainless steel, T92 (9Cr0.5Mo2WVNb) and HR3C (TP310HCbN), exhibit superior heat strength at elevated temperatures and are increasingly applied in ultra-supercritical (USC) plants around the world, a complete assessment of the properties for T92/HR3C dissimilar steels welded joints is urgently required. In this paper, metallographic microstructures across the joint were inspected by optical microscope. Particularly, the creep rupture test was conducted on joints under different load stresses at 625 ◦C to analyse creep strength and predict their service lives, while their fractograph were observed under scanning electron microscope. Additionally, finite element method was employed to investigate residual stress distribution of joints. Results showed that the joints were qualified under USC conditions, and T92 base material was commonly the weakest part of them. Keywords creep rupture; dissimilar steels welded joints; finite element method; HR3C; T92. INTRODUCTION With the worsening of global energy crisis and environmental pollution, higher energy utilization and lower CO2 emission are presently the two prior criteria for designing fossil power plants,1 which will still serve as the dominant energy form for at least 20 years.2–4 As for fossil power boilers, it is well known that the enhancement of steam parameters can facilitate improvement in thermal efficiency, which can result in reduction in not only the fossil costs but also the CO2, SO2 and NOx emission.5 Concretely, compared with supercritical (SC) boilers (24 MPa, 565 ◦C), modern ultra-supercritical (USC) boilers operating around 30 MPa and 600 ◦C can lead to both an increase of 8–10% in thermal efficiency (from 37% to 45–47%) and a reduction of 20–25% in CO2 emission.1,6–10 Development of USC boilers with increasingly higher steam parameters is an added incentive for boiler material Correspondence: Z.-G. Yang. E-mail: zgyang@fudan.edu.cn research. Consequently, heat-resistant steels, mainly referring to ferritic steels and austenitic stainless steels, have been pursued simultaneously since the emergence of USC boilers for applications in boiler components, including superheater, reheater, header, turbine, steam piping and so on. Subsequently, for the sake of reliable services in USC boilers, a wealth of research has been carried out on these heat-resistant steels as F12 (X20CrMoV12.1), T91 (9Cr1MoVNb), TP347H (18Cr10NiNb), etc. to investigate characteristics of their base materials and performance deterioration after long-term services.11–29 Among them, the T92 (9Cr0.5Mo2WVNb), approximately similar to T91 but with a little modification in chemical compositions for preferable high temperature properties than T91, and the HR3C (TP310HCbN) are the two typical representatives of new grade ferritic steels and austenitic stainless steels for their superior heat strength above 620 ◦C, and will certainly have a broad application prospect in forthcoming USC boilers. However, applications of T92 and HR3C are currently a bit constrained due to the lack of sufficient literature and experience on c 2010 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 34, 83–96 83 Fatigue & Fracture of Engineering Materials & Structures
84 Y GoNG et al Table 1 Chemical compositions and heat treatment conditions of T92 and HR3C samples(wt%) Elements P s Si N 0.21 0.011.63 ASME 0.07-0.138.50- 0.30-0600.15-0.250.040.09≤0.40 0.30-0.60<0.020<0.010<0.500.03-0.07<0.041.50-2.000.001-0.006 SA-213 12400120.0010390.24 ASME 3.00-2700 0.20-0.601700-23.00≤2.000.030≤0.030≤1.500.15-0.35 TP310HC T92: 1050C x 20 min(normalizing)+760C x 60 min(tempering). HR3C: solution-treated at 1110.C minimum reep rupture performances of them and their welded (a) joints, let alone the dissimilar steels welded joints between them. Therefore, a thorough assessment of the compre hensive properties, particularly the creep properties of T92/HR3C dissimilar steels welded joints, seems pretty urgent. In this paper, besides various conventional mechanica tests including tensile test, bending test and hardness sur- ey, optical microscope(OM)was also applied to inspect the metallographic microstructures across the dissimilar steels welded joint between T92 and HR3C. Moreover, creep rupture test was particularly employed under dif- ferent load stresses at 625 oC to investigate the creep features of the joints, whose fractograph was then ob- served by using scanning electron microscope (SEM)as ell. Furthermore, the residual stress distribution of the element method(FEM), which was a tentative approach (b) to evaluate residual stress of the dissimilar steels welded joint between these two novel materials through the com the degradation curves of T92/HR3C dissimilar steels yelded joints were reported, but also the mechanism of voids initiating creep rupture was concretely discussed which may have critical significance in both service-life prediction and future heat-resistant steels preparation fo boiler components EXPERIMENTAL 20题 Tested materials were nominal T92 and hr3C heat- Fig. 1 Metallographic microstructures of tested base materials (a) resistant steels with scales of 480D x 8.4 mm thick 92,1500×(b)HR3C,200× and 48. D x 10.16 mm thick, respectively. Chemical compositions as well as heat treatment conditions of their crostructure of T92 sample is presented in Fig. la, which base materials are listed in Table 1, which are in accor- displays a typical tempered lath martensitic microstruc- dance with the requirements of ASME SA-213 T92 and ture. Similarly, metallographic microstructure of HR3C TP310HCbN specifications. Etched in agent of picric sample was also obtained after being etched in the agent acid(2, 4, 6-trinitrophenol)1. 25 g, HCI 20 ml, ethanol of CuSO4 4 g, HCl 20 ml and ethanol 20 ml for 20 s 10 ml and H2O 10 ml for 40 s, the metallographic mi- As is shown in Fig. 1b, HR3C presents a fine-grained @2010 Blackwell Publishing Ltd Fatigue Fract Engng Mater Struct 34, 83-96
84 Y. GONG et al. Table 1 Chemical compositions and heat treatment conditions of T92 and HR3C samples (wt%) Elements C Cr Mo V Nb Ni Mn P S Si N Al W B T92 Sample 0.11 8.76 0.36 0.21 0.059 0.25 0.46 0.016 0.002 0.39 0.044 0.01 1.63 0.0033 ASME 0.07–0.13 8.50–9.50 0.30–0.60 0.15–0.25 0.04–0.09 ≤0.40 0.30–0.60 ≤0.020 ≤0.010 ≤0.50 0.03–0.07 ≤0.04 1.50–2.00 0.001–0.006 SA-213 T92 HR3C 0.06 24.63 / / 0.49 20.29 1.24 0.012 0.001 0.39 0.24 / / / Sample ASME ≤0.10 23.00–27.00 / / 0.20–0.60 17.00–23.00 ≤2.00 ≤0.030 ≤0.030 ≤1.50 0.15–0.35 / / / SA-213 TP310HCbN Heat treatment conditions: T92: 1050 ◦C × 20 min (normalizing) + 760 ◦C × 60 min (tempering). HR3C: solution-treated at 1110 ◦C minimum. creep rupture performances of them and their welded joints, let alone the dissimilar steels welded joints between them. Therefore, a thorough assessment of the comprehensive properties, particularly the creep properties of T92/HR3C dissimilar steels welded joints, seems pretty urgent. In this paper, besides various conventional mechanical tests including tensile test, bending test and hardness survey, optical microscope (OM) was also applied to inspect the metallographic microstructures across the dissimilar steels welded joint between T92 and HR3C. Moreover, creep rupture test was particularly employed under different load stresses at 625 ◦C to investigate the creep features of the joints, whose fractograph was then observed by using scanning electron microscope (SEM) as well. Furthermore, the residual stress distribution of the welded joint after welding was calculated by using finite element method (FEM), which was a tentative approach to evaluate residual stress of the dissimilar steels welded joint between these two novel materials through the computational simulation method. Finally, based on the analysis results, not only the creep rupture performances and the degradation curves of T92/HR3C dissimilar steels welded joints were reported, but also the mechanism of voids initiating creep rupture was concretely discussed, which may have critical significance in both service-life prediction and future heat-resistant steels preparation for boiler components. EXPERIMENTAL Tested materials were nominal T92 and HR3C heatresistant steels with scales of 48O D × 8.4 mm thick and 48.26O D × 10.16 mm thick, respectively. Chemical compositions as well as heat treatment conditions of their base materials are listed in Table 1, which are in accordance with the requirements of ASME SA-213 T92 and TP310HCbN specifications.30 Etched in agent of picric acid (2, 4, 6-trinitrophenol) 1.25 g, HCl 20 ml, ethanol 10 ml and H2O 10 ml for 40 s, the metallographic miFig. 1 Metallographic microstructures of tested base materials (a) T92, 1500× (b) HR3C, 200×. crostructure of T92 sample is presented in Fig. 1a, which displays a typical tempered lath martensitic microstructure. Similarly, metallographic microstructure of HR3C sample was also obtained after being etched in the agent of CuSO4 4 g, HCl 20 ml and ethanol 20 ml for 20 s. As is shown in Fig. 1b, HR3C presents a fine-grained c 2010 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 34, 83–96
CREEP PROPERTIES OF T92/HR3C WELDED JOINTS 85 Table 2 Chemical compositions of welding wire ERNiCr-3(wt%) P Si Cu Ni Ti ERNiCr-3 welding wire 00302901.300.0040.0010.040.0172.50.3120.0 Nb240 ASME SFA-5.14(AWS) ERNICI-3≤0.102.5-3.5≤3.0≤0.030≤0.015≤0.50≤0.50≥67.0≤0.7518.00-22.002.0-3.0 Table 3 Tensile test results of t92/HR3C dissimilar steels welded joints Tensile strength 50 Sample No (os, MPa) Rupture position 707 unit: m T92 base material Fig 2 Dimension of creep rupture test specimen T92 specification ≥620 HR3C specification 2655 austenitic microstructure with average grain size of about 7, which conforms with the requirement that the Table 4 Bending test results of T92/HR3C dissimilar steels grain size of TP310HCbN must be coarser than 7(in- welded joints cluding 7)in ASME specification. 30 The T92/HR3C dissimilar steels welded joints were Bending sty yle Sample No. Test condition welded by means of gas tungsten arc welding(GTAW) Face bending 1 D=4T,a=180° Qualified ith pure argon gas(Ar) as the shielding gas and Aws D=3T,a=50F ERNiCr-3(corresponding to INCONEL 82/182)as the Back bending D=4T, a= 180 Qualified welding wire, whose chemical compositions are listed in D=3T,a=50 Table 2. Subsequently, the welded joints were subjected to the post weld heat treatment (PWHT) at 760-770oc D denotes the bending diameter, 'T' denotes the material thickness; for 2 h to eliminate the residual stress ar denotes the bending angle. A variety of mechanical tests for the welded joints were then successively carried out. Tensile test, bending test RESULTS AND DISCUSSION and hardness survey were performed at room temperature Mechanical tests results according to the AsTME8-04, E290-97a(2004)and E92 82(2003)e2 standards, respectively. Also, metallographic As is clear from Table 3, the T92/HR3C dissimilar steels microstructure across the welded joint, especially in the welded joints exhibit qualified tensile strength, and the two heat-affected zones(HAz)and the weld seam, was in- T92 base material part is their weakest region under load spected under LEICA DMLM OM. In accordance with stresses.Table 4 reveals the sign that the welded joints also ASTME139-06 standard, the creep rupture test was con- present eligible toughness. In addition, no cracks were ducted at 625 oC under load stresses of 110, 120, 130, founded on the bended surfaces. Hardness survey results 140, 150, 160 and 180 MPa, respectively. Figure 2 shows are displayed in Fig 3, which indicates that the T92 base the round bar configuration of the creep rupture speci- material and weld seam are the two low-hardness parts men with the size of 10 mm diameter for base materials and 50 mm gage length for welded joint. After the creep Metallographic microstructure inspection rupture test, micro morphologies of the cross-sections of the ruptured samples were observed by using PHILIPS As is well known, a dissimilar steels welded joint is often KL30FEG SEM(Eindhoven, The Netherlands). approximately divided into five regions, i. e. base material Residual stress analysis of the welded joint after weld- A, HAZ of A, weld seam, HAZ of B and base material ing was carried out by means of finite element analysis B. In this paper, the metallographic microstructures of software ANSYS 10.0( Canonsburg Pennsylvania, USA). the five regions across the T92/HR3C dissimilar steels The FEM analysis result could clearly reflect the resid- welded joint were inspected under Om ual stress distribution of the T92/HR3C dissimilar steels Figure 4a presents the metallographic microstructure of welded joint in a convenient way, which may also theo- T92 base material after welding, in which no signs of dif- retically supplement the results of mechanical and creep ferential are detected when compared with that of ori inal material in Fig. la. However, in the HAZ of T92 @2010 Blackwell Publishing Ltd Fatigue Fract Engng Mater Struct 34, 83-96
C R E E P P RO P E RTI E S O F T 9 2/H R 3 C W E LD ED JOINT S 85 Table 2 Chemical compositions of welding wire ERNiCr-3 (wt%) Elements C Mn Fe P S Si Cu Ni Ti Cr Nb+Ta ERNiCr-3 welding wire 0.030 2.90 1.30 0.004 0.001 0.04 0.01 72.5 0.31 20.0 Nb 2.40 ASME SFA-5.14 (AWS) ERNiCr-3 ≤0.10 2.5–3.5 ≤3.0 ≤0.030 ≤0.015 ≤0.50 ≤0.50 ≥67.0 ≤0.75 18.00–22.00 2.0–3.0 Fig. 2 Dimension of creep rupture test specimen. austenitic microstructure with average grain size of about 7, which conforms with the requirement that the grain size of TP310HCbN must be coarser than 7 (including 7) in ASME specification.30 The T92/HR3C dissimilar steels welded joints were welded by means of gas tungsten arc welding (GTAW) with pure argon gas (Ar) as the shielding gas and AWS ERNiCr-3 (corresponding to INCONEL 82/182) as the welding wire, whose chemical compositions are listed in Table 2.31 Subsequently, the welded joints were subjected to the post weld heat treatment (PWHT) at 760–770 ◦C for 2 h to eliminate the residual stress. A variety of mechanical tests for the welded joints were then successively carried out. Tensile test, bending test and hardness survey were performed at room temperature according to the ASTM E8-04, E290-97a(2004) and E92- 82(2003)e2 standards, respectively. Also, metallographic microstructure across the welded joint, especially in the two heat-affected zones (HAZ) and the weld seam, was inspected under LEICA DMLM OM. In accordance with ASTM E139-06 standard, the creep rupture test was conducted at 625 ◦C under load stresses of 110, 120, 130, 140, 150, 160 and 180 MPa, respectively. Figure 2 shows the round bar configuration of the creep rupture specimen with the size of 10 mm diameter for base materials and 50 mm gage length for welded joint. After the creep rupture test, micro morphologies of the cross-sections of the ruptured samples were observed by using PHILIPS XL30FEG SEM (Eindhoven, The Netherlands). Residual stress analysis of the welded joint after welding was carried out by means of finite element analysis software ANSYS 10.0 (Canonsburg Pennsylvania, USA). The FEM analysis result could clearly reflect the residual stress distribution of the T92/HR3C dissimilar steels welded joint in a convenient way, which may also theoretically supplement the results of mechanical and creep tests. Table 3 Tensile test results of T92/HR3C dissimilar steels welded joints Tensile strength Sample No. (σs, MPa) Rupture position 1 707 T92 base material 2 699 T92 base material T92 specification ≥620 / HR3C specification ≥655 / Table 4 Bending test results of T92/HR3C dissimilar steels welded joints Bending style Sample No. Test condition Result Face bending 1 D = 4T, α = 180◦ Qualified 2 D = 3T, α = 50◦ Back bending 1 D = 4T, α = 180◦ Qualified 2 D = 3T, α = 50◦ ‘D’ denotes the bending diameter; ‘T’ denotes the material thickness; ‘α’ denotes the bending angle. RESULTS AND DISCUSSION Mechanical tests results As is clear from Table 3, the T92/HR3C dissimilar steels welded joints exhibit qualified tensile strength, and the T92 base material part is their weakest region under load stresses. Table 4 reveals the sign that the welded joints also present eligible toughness. In addition, no cracks were founded on the bended surfaces. Hardness survey results are displayed in Fig. 3, which indicates that the T92 base material and weld seam are the two low-hardness parts. Metallographic microstructure inspection As is well known, a dissimilar steels welded joint is often approximately divided into five regions, i.e. base material A, HAZ of A, weld seam, HAZ of B and base material B. In this paper, the metallographic microstructures of the five regions across the T92/HR3C dissimilar steels welded joint were inspected under OM. Figure 4a presents the metallographic microstructure of T92 base material after welding, in which no signs of differential are detected when compared with that of original material in Fig. 1a. However, in the HAZ of T92, c 2010 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 34, 83–96
Y. GoNG et al T92 T92 HAZ weld seam HR3C HAZ HR3C 270 它230 Fig, 3 Hardness distribution Distance(mm T92/HR3C dissimilar steels welded joints obvious sorbitic microstructure with coarsened laths load stresses is in accordance with the results in the lit could be observed in Fig. 4b. The width of the sorbite erature. 23 Together with the tensile test results, it can lath is nearly two times that of martensite lath, which may be concluded that the T92 base material part is actually result in an increase of hardness in HAZ of T92 as well the weakest region of the t92/HR3C dissimilar steels a decrease of toughness in this region simultaneously. welded joint under relatively higher stresses. Neverthe The mechanism can be explained that coarser laths may less, compared with the research data of Falat et al. 4 the block the growth of cracks under stresses, and eventu- rupture time of our T92/HR3C dissimilar steels welded ally lead to brittle rupture in this region Near the weld joints under 120 MPa at 625C actually even exceeds seam,a distinct boundary between HAZ of T92 and weld the counterpart value of P92/p92(P"denotes the word seam can be observed in Fig. 4c. Figure 4d shows the pipe, which contains the same chemical compositions of stripe-shaped austenitic microstructure of the weld seam, T, i.e. tube, but with a larger diameter and thickness) whose stripe width has already reached around 20 um. similar steels welded joint, 1174 h. Correspondingly, like Fig. 4c, Fig. 4e gives the boundary According to the classic theory about creep rupture test, between weld seam and HAZ of HR3C. Average grain a double logarithmic relationship between load stress o size of the austenite in HAZ of Hr3C is about 6(Fig. and rupture time t, i. e Igo versus lgt, can be expresse 40, and carbides as M23 C6 have precipitated at the grain in Eq.(1), in which the letters A and b both denote the boundaries. Compared with the average grain size of 7 material parameters of the tested samples. Accordingly HR3C may also lead to increase in hardness in this region. fitting result ofi plot of lgo versus lgt and the linear in HR3C base material, the coarsened grains in HAZ of Fig. 5 presents the Fig 4g is the metallographic microstructure of HR3C status too. Collectively, the schematic diagram as well gt=lgA_blgo base material, which shows no changes with its original (1) as the metallographic microstructures distribution across the T92/HR3C dissimilar steels welded joint is displayed hus, the threshold steam stress for service of the in Fig. 4h. T92/HR3C dissimilar steels welded joints exposed at 625C, which can be determined by extrapolation of the fitted line to 10 h, is 61.89 MPa. However, in practi Creep rupture test results al applications, some other unpredicted factors may also affect the creep strength of welded joints. Hence, safety Table 5 lists the creep rupture test results under load coefficient n is always adopted to modify the predicted stresses ranging from 110 to 180 MPa at 625C. This threshold stress obtained from linear extrapolation Con phenomenon that rupture positions vary with change in sequently, permitted stress [o] can be expressed as Eq (2) @2010 Blackwell Publishing Ltd Fatigue Fract Engng Mater Struct 34, 83-96
86 Y. GONG et al. Fig. 3 Hardness distribution across the T92/HR3C dissimilar steels welded joints. obvious sorbitic microstructure with coarsened laths could be observed in Fig. 4b. The width of the sorbite lath is nearly two times that of martensite lath, which may result in an increase of hardness in HAZ of T92 as well as a decrease of toughness in this region simultaneously. The mechanism can be explained that coarser laths may block the growth of cracks under stresses, and eventually lead to brittle rupture in this region. Near the weld seam, a distinct boundary between HAZ of T92 and weld seam can be observed in Fig. 4c. Figure 4d shows the stripe-shaped austenitic microstructure of the weld seam, whose stripe width has already reached around 20 μm. Correspondingly, like Fig. 4c, Fig. 4e gives the boundary between weld seam and HAZ of HR3C. Average grain size of the austenite in HAZ of HR3C is about 6 (Fig. 4f), and carbides as M23C6 have precipitated at the grain boundaries. Compared with the average grain size of 7 in HR3C base material, the coarsened grains in HAZ of HR3C may also lead to increase in hardness in this region. Fig. 4g is the metallographic microstructure of HR3C base material, which shows no changes with its original status too. Collectively, the schematic diagram as well as the metallographic microstructures distribution across the T92/HR3C dissimilar steels welded joint is displayed in Fig. 4h. Creep rupture test results Table 5 lists the creep rupture test results under load stresses ranging from 110 to 180 MPa at 625 ◦C. This phenomenon that rupture positions vary with change in load stresses is in accordance with the results in the literature.23 Together with the tensile test results, it can be concluded that the T92 base material part is actually the weakest region of the T92/HR3C dissimilar steels welded joint under relatively higher stresses. Nevertheless, compared with the research data of Falat et al., 24 the rupture time of our T92/HR3C dissimilar steels welded joints under 120 MPa at 625 ◦C actually even exceeds the counterpart value of P92/P92 (‘P’ denotes the word ‘pipe’, which contains the same chemical compositions of ‘T’, i.e. ‘tube’, but with a larger diameter and thickness) similar steels welded joint, 1174 h. According to the classic theory about creep rupture test, a double logarithmic relationship between load stress σ and rupture time t, i.e. lgσ versus lgt, can be expressed in Eq. (1), in which the letters A and B both denote the material parameters of the tested samples. Accordingly, Fig. 5 presents the plot of lgσ versus lgt and the linear fitting result of it. lg t = lg A − B lg σ. (1) Thus, the threshold steam stress for service of the T92/HR3C dissimilar steels welded joints exposed at 625 ◦C, which can be determined by extrapolation of the fitted line to 105 h, is 61.89 MPa. However, in practical applications, some other unpredicted factors may also affect the creep strength of welded joints. Hence, safety coefficient n is always adopted to modify the predicted threshold stress obtained from linear extrapolation. Consequently, permitted stress [σ] can be expressed as Eq. (2), c 2010 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 34, 83–96
CREEP PROPERTIES OF T92/HR3C WELDED JOINTS 87 (b) (c) 少了m austenite T92 HAZ HR3C HAZ 0 1cm 2 Fig. 4 Metallographic microstructures of the five different regions across the welded joint(a)T92 base material, (b) HAZ of T92, (c) boundary between HAZ of T92 and weld seam, (d)weld seam, (e) boundary between weld seam and HAZ of HR3C,(D) HAZ of HR3C, (g) HR3C base material, (h)schematic diagram of welded joint @2010 Blackwell Publishing Ltd Fatigue Fract Engng Mater Struct 34, 83-96
C R E E P P RO P E RTI E S O F T 9 2/H R 3 C W E LD ED JOINT S 87 Fig. 4 Metallographic microstructures of the five different regions across the welded joint (a) T92 base material, (b) HAZ of T92, (c) boundary between HAZ of T92 and weld seam, (d) weld seam, (e) boundary between weld seam and HAZ of HR3C, (f) HAZ of HR3C, (g) HR3C base material, (h) schematic diagram of welded joint. c 2010 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 34, 83–96