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Investigation of fracture and determination of fracture toughness of modified 9Cr–1Mo steel weld metals using AE technique
Materials Science and Engineering A270 (1999) 260 – 266 www.elsevier.com/locate/msea
Investigation of fracture and determination of fracture toughness of modified 9Cr–1Mo steel weld metals using AE technique Xin Long a, Guangjun Cai b,*, Lars-Erik Svensson c a
Welding Laboratory, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, China b Department of Materials Engineering, Southwest Jiaotong Uni6ersity, Chengdu 610031, China c Centeral Laboratory, Esab AB, 412 77 Gothenburg, Sweden Received 15 November 1998; received in revised form 19 April 1999
Abstract Three-point bending test and acoustic emission technique are used to determine the fracture toughness and fracture process of three types of modified 9Cr–1Mo steel weld metals. Scanning electron microscopy is used to accomplish fractography analysis of fracture specimens. Microstructure of weld metals is investigated using optical microscopy and transmission electron microscopy. The fracture process and factors which affect fracture of the 9Cr – 1Mo steel weld metals in post-weld heat treated condition are studied. Experimental results show that the modified 9Cr – 1Mo steel weld metals fracture by a quasi-cleavage mechanism at ambient temperature. The microstructure of the weld metals is composed of mainly tempered martensite with M23C6 precipitates. In weld metals, microcracks nucleate from non-metallic inclusions. Fractures develop very quickly when cracks started to propagate. Comparatively, in weld metals with low strength, microcracks initiate at a low stress, but propagation of cracks is limited by plastic deformation. In weld metals with high strength, microcracks nucleate at high stresses, but cracks propagate very quickly and lead to almost immediate fracture of the specimens. As a result, weld metals with the low strength have a higher fracture toughness, while weld metals with higher strengths has a lower fracture toughness. © 1999 Elsevier Science S.A. All rights reserved. Keywords: 9Cr – 1Mo steel; Weld metal; Fracture toughness; Microstructure; Acoustic emission
1. Introduction Modified 9Cr– 1Mo heat resistant steels for high temperature applications have been used more and more in recent years, especially in the power industry, to fabricate turbines, pipes, etc. working at high temperatures [1–8]. In these applications, welding is one of most commonly used techniques. Therefore, weldability and quality of welding of 9Cr – 1Mo steels have attracted extensive attention during steel development [1 –4]. The behavior of a welded joint depends on properties of the base metal at welded condition (HAZ) and the weld metal. For the latter, because of the drastic metallurgical nature of welding microstructure,
* Corresponding author. Present address: Department of Materials Engineering, Southwest Jiaotong University, Chengdu 610031, PR China.
and consequently properties, are generally different from that of the base metal. The microstructure of weld metal and its relation with properties, is important to the development and application of modified 9Cr–1Mo steels. Strength and impact toughness of 9Cr–1Mo steels have be studied extensively [4,7–9]. However, not so much work has been reported on fracture toughness of 9Cr–1Mo steels due to the experimental difficulties. Because of the high alloying content, the 9Cr–1Mo steels have relatively high strength but low toughness. The toughness property of the steels has been one of the most important concerns in development and application of 9Cr–1Mo steels. In welded structures defects (including welding introduced inclusions) in the materials are generally inevitable. Fracture toughness is, therefore, an especially important property of 9Cr– 1Mo steel weld metals and is worthy of investigation.
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2. Experimental techniques
2.1. Preparation of the weld metals Shielded metal arc welding (SMAW) method was used to prepare the weld metals. Three types of 9Cr– 1Mo steel electrodes of 3.2 mm diameter were produced in Esab according to ISO 2560. The welding was carried out on plain carbon steel plates using welding parameters as: current 120 A, voltage 22 V, average welding speed 4 mm/s, and at a preheat and interpass temperature 200°C. The geometry of the welding plates is illustrated in Fig. 1. To avoid the dilution effect of the plain carbon steel plates on the weld metals, the joint surfaces were first deposited with a layer of the 9Cr–1Mo steel weld metals (buttered). After welding, the plates were immediately transferred to a furnace preset at 740°C and heated for 4 h as the post-weld heat treatment (PWHT). The chemical composition of the weld metals was analyzed and is given in Table 1. Hereafter, according to their composition, the weld metals are denoted as B1, B2 and B3. The weld metals have identical chromium and molybdenum contents, but vary in carbon and vanadium content. Comparatively, B1 has the lowest carbon and vanadium contents, and containing no niobium. Materials B2 and B3 are almost identical, except in B2 carbon content is higher and vanadium lower, than that in B3.
Fig. 1. Geometry of the welded joint and cut-out of the fracture toughness test specimens.
Fracture toughness is a comprehensive material property determined by the fracture mechanism, microstructure, etc., of the material. Investigations [1,2,7– 9] show that in general quench + tempered condition microstructure of modified 9Cr – 1Mo steels consists mainly of tempered martensite with precipitates. Some d-ferrite may also exist as remained high temperature transformed phase. Precipitates in the modified 9Cr– 1Mo steels are mainly M23C6 distributing on grain and martensite lath boundaries. Microstructure of 9Cr– 1Mo steel weld metals has also been studied [1,7] in post-weld heat treatment condition. Comparing with base metals, microstructure of 9Cr – 1Mo steel weld metals is obviously inhomogeneous. Depending on the heating temperature and composition of the weld metals, some martensite laths may change to polygonal ferrite by recrystallization. As the result of welding, non-metallic inclusions are present in the weld metals. Fracture mechanism and toughness of modified 9Cr– 1Mo steel weld metals are to a large extent determined by these microstructure features. The fracture toughness and fracture mechanisms of three 9Cr – 1Mo steel weld metals, and their relationship with microstructure are presented in the this paper.
2.2. Fracture toughness testing It was found impractical to measure KIC of the post-welding heat treated 9Cr–1Mo steel weld metals directly, because in this condition the yield strength of the weld metals is not high enough to create the required plane–stress strain condition in the standard sized specimens. To satisfy the plane–stress strain criterion for fracture mechanics test, very large specimens will be required. Therefore, in the present work, instead of KIC, JIC of the weld metals was determined. Threepoint bending test was used to determine JIC values of the weld metals. The test was performed following ASTM E-813 standard. The measured JIC values of the weld metals were then transferred to KIC according to the fracture mechanics theory. The test was carried out at ambient temperature (about 25°C). Specimens used in the test were 20 × 40 mm in size and 160 mm in
Table 1 Chemical composition of the weld metals used in the investigation (in wt.%)
B1 B2 B3
C
Si
Mn
P
S
Cr
V
Mo
Ni
Nb
Cu
O
N
0.045 0.095 0.060
0.37 0.34 0.26
0.70 0.68 0.99
0.005 0.005 0.004
0.013 0.013 0.012
8.6 8.88 8.47
0.008 0.215 0.304
1.03 0.89 0.94
0.01 0.61 0.71
– 0.03 0.03
0.021 0.022 0.072
0.067 0.048 0.079
0.047 0.042 0.040
262
X. Long et al. / Materials Science and Engineering A270 (1999) 260–266
Fig. 2. Geometry of the three-point bending fracture toughness test specimens.
span. On each specimen a through-thickness crack (original crack) was prepared by high frequency fatigue technique at weld center along the welding direction, as shown in Fig. 2. The specimens were loaded at the rate of 0.5 mm/ min. It was found that, during loading, cracking of the specimens generally happened at a relatively high stress level, accompanying a loud noise, developed very quickly and resulted in complete failure of the specimens in a short time and over narrow loading intervals. Because of the lack of stable crack development stage, it was not possible to determine JIC of the materials using the conventional methods. Therefore, in the present experiment, acoustic emission (AE) technique was used to determine the initiation of cracking and monitor the development of cracks in the weld metals. Some work dealing with fracture toughness measurement of materials using the AE technique has been reported [10 – 13]. The AE technique is based on the fact that when a dynamic process which results in a quick energy release, such as cracking or fracture by a brittle mechanism, occurs in a material acoustic waves will be emitted. By detecting the acoustic emis-
Fig. 3. Schematic diagram of AE set and the three-point bending fracture toughness test set.
sion, the fracture process of the material could be detected and monitored. Fig. 3 schematically shows the experimental settings used in the present study. Two acoustic transducers are fixed on two sides of the specimen. During loading, AE signals are detected by the transducers (a small quantity of petroleum grease was used to bond the transducer on the specimen surface to reduce the signal loss). The signals are amplified by the amplifier, then transferred through a filter to filter out external acoustic noises, and recorded on an x–y recorder. The parameters of AE instrument for testing are set as: amplification 50 dB, threshold level of the filter 0.20 V, events interval 3 ms. An AE spectrum is composed of separate peaks. Each peak represents a step of crack development. Together with the AE spectrum, the load–loading point displacement curves are also recorded. To avoid misinterpreting the AE spectrum, the crack initiation point of each material is taken as the load at which the first strong AE peak occurred. JI values of the materials are calculated by the equation: JI =
Uc B(W− a)
(1)
where UC is the energy needed for the pre-prepared crack (original crack) to develop. In the experiment, UC is the area below the load–loading point displacement curve to the critical load. B is the specimen thickness, W is the specimen width and a the length of pre-crack (original crack), so W−a is the length of original ligament of the specimen. For each type of weld metal, five specimens are tested to get the average values of fracture toughness.
2.3. Fractography and microstructure in6estigation After fracture testing, fractography analysis was carried out to determine fracture mechanism of the weld metals. The fractography analysis was carried on fresh fractured surfaces of the tested specimens using scanning electron microscopy. Microstructure of the weld metals were investigated using optical microscopy (OM) and transmission electron microscopy (TEM). For OM observation, specimens were first cut from center of the weld metals and mechanically polished on a surface perpendicular to the welding direction, then etched in a solution of 50 ml HCl, 12.5 g CuCl4 in 100 ml ethanol. To prepare TEM specimens, slices of about 0.5 mm in thickness were cut from the weld metals perpendicular to the welding direction. The slices were ground to a thickness B0.1 mm and punched into 3 mm diameter disks. The disks were finally electropolished in 10% perchloric acid in methanol at a temperature B − 30°C and a potential about 80 V.
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3. Results and discussion
3.1. Determination of JIC by AE As the loading increased, the loading-point displacement increases linearly, corresponding to the elastic deformation of the specimens. In many cases, some weak AE peaks occur when the specimen is still in elastic state. These weak peaks may be caused by decohesion or cracking of inclusions in the weld metals. When the loading increased to a certain level, a strong AE peak occurs accompanied by obvious crack propagation. After this, cracks develop rather quickly until the specimens fracture completely. During this process, more AE signals are recorded. In many cases, pop-in occurs during loading. Pop-in always produces a strong AE peak. Fig. 4 is curves of load – loading point displacement with AE spectra of the weld metals. From the curves of load – loading point displacement, it can be seen that, less plastic deformation occurs in B2 and B3 (Fig. 4b–c). When the cracks start to propagate in these two weld metals, they develop very quickly and result in complete fracture of the specimens in a short time and narrow loading interval. There is hardly a stable crack-development stage in these materials during loading. In material B1, however, more plastic deformation occurs and cracks develop relatively slowly. Indicated by the AE spectra, the first strong AE peak (which represents the crack initiation) occurs at a relatively larger material deformation and higher stress (Fig. 4a). After the onset of crack propagation, the stress in the material still increases. It means that there is a stable crack-development stage during loading of B1. Comparatively, AE in material B1 is less frequent and the AE peaks have similar strength. In B2 and B3, however, AE is more frequent and there is generally an obvious strong emission ahead of the series of AE. Taking the load corresponding to the onset of the prepared crack propagating as the critical load, the determined J-integral by Eq. (1) is JI. In the present test, because AE technique is sensitive to crack propagation, and the load determined by AE spectrum is conservative compared to the conventional fracture mechanics test, at the stage of onset of crack propagation (determined by AE), it is reasonable to believe that the tip of the initial crack is in the plane strain condition, so the measured JI is JIC. The determined values of the weld metals are given in Table 2. Inserting the determined JIC values into the following criteria 25JIC B\ sy
(2)
25 JIC W − a\ s
(3)
Fig. 4. Curves of the load – loading point displacement with the AS spectra of the weld metals a)B1, b)B2 and c)B3.
where sy is the yield strength of the 9Cr–1Mo steel weld metals at the testing temperature. Inserting the parameters of specimen size, the measured JIC and yield strengths of the materials (which were measured as 364, 545 and 565 MPa for weld metals B1, B2 and B3, respectively [1]), these criteria are satisfied, so that the measured JIC values are valid for the materials. JIC has the following relation with KIC: JIC =
(1-62)K 2IC E
(4)
where 6 is Poisson ratio and E Young’s modulus of the materials. Taking 6= 0.3 and E= 2.1× 105 MPa, KIC values of the materials are calculated from JIC and are
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Table 2 The results of fracture toughness measurement of the weld metals Exp. No.
JIC (MPa m)
JIC (MPa m) (average)
KIC (MPa m1/2)
B1-2 B1-2 B1-4 B1-5 B1-7
16.4 17.8 27.2 20.1 13.3
19.0 19.0 19.0 19.0 19.0
62 62 62 62 62
B2-1 B2-2 B2-3 B2-4 B2-5
4.7 8.6 10.1 15.6 4.7
8.7 8.7 8.7 8.7 8.7
43 43 43 43 43
B3-1 B3-4 B3-5 B3-6 B3-7
5.0 8.1 8.0 9.4 7.5
7.4 7.4 7.4 7.4 7.4
39 39 39 39 39
also given in Table 2. It can be seen from Table 2, as the general case with fracture toughness measurement of weld metals, the JIC values vary in a quite large range. This is believed to be because of the inhomogeneity in the materials.
3.2. Fractography and micrography of the weld metals Fig. 5 is a SEM micrograph of the fracture surface of a tested specimen. It can be seen that the fracture mechanism of the material is a mixture of brittle transgranular cleavage and ductile fibrous fracture. According to fractography analysis, all the materials fractured by the same mechanism, but the fraction of cleavage and fibrous fracture varies. Comparatively, material B1 has the highest fraction of ductile fibrous area on the fracture surface, consistent with more plastic deformation and the highest fracture toughness. Materials B2
Fig. 5. SEM micrograph of the fracture surface of tested specimen (weld metal B3).
Fig. 6. SEM micrograph of the fracture surface showing cleavage crack nucleated from a non-metallic inclusion (weld metal B1).
and B3 have identical fracture surfaces, correspondingly, their fracture toughness is at a same low level. SEM observation shows that the cleavage cracks nucleate on non-metallic inclusions in the weld metals (Fig. 6), then develop to the surrounding matrix. The cleavage facets have various sizes. In the 9% Cr steels, the stability of austenite is increased by the high content Cr and austenite will transform to full martensite even by air cooling (in a not very thick piece). After each welding pass, when the weld metals cooled down below Ms temperature (which were measured to be 390°C for B1, 380°C for B2 and 380°C for B3 [14]), the weld metals will transform to martensite. The PWHT process is, therefore, the process of martensite tempering. During this process, the dislocation density in the quenched martensite decreases, and alloying elements will react with carbon and precipitate out as carbides. OM observation shows that the microstructure of the weld metals is composed of mainly lath martensite. The columnar feature of the microstructure could be recognized. In material B1, because of its relative low alloy content some martensite has recrystallized and transformed to polygonal ferrite during PWHT. These polygonal ferrite distributes as bands between different welding beads (Fig. 7). Under TEM observation, the tempered martensite is laths with carbide precipitates on boundaries. The dislocation density in the lath martensite is not high. The precipitates are predominantly M23C6, M23C6 particles also distributed on the primary austenite grain boundaries (Fig. 8). The size of M23C6 varies in a wide range, from 20 to 150 nm in diameter. Besides M23C6, MC precipitates must exist in the weld metals, at least in B2 and B3 since they have considerable quantity of vanadium and carbon. Actually, vanadium rich MC precipitates were found in material B3 [1]. Unfortunately, perhaps because of its ultra fine size, no MC precipitates were found during the present experiment.
X. Long et al. / Materials Science and Engineering A270 (1999) 260–266
Fig. 7. Optical micrograph showing polygonal ferrite in weld metal B1.
Non-metallic inclusions of low density could be found by TEM observation. The non-metallic inclusions are generally round, about 1 mm in diameter.
3.3. Fracture process of the weld metals As mentioned earlier, AE signals are emitted when cracks initiate or develop in the materials. As it is shown in the experiment, the modified 9Cr – 1Mo steel weld metals fracture by a mixture mechanism of brittle cleavages and some ductile dimpling. In this case, cracks form due to plastic deformation of the material. As a result of plastic deformation, strength of the material near the crack tip increases due to work hardening. When the strength increased to cleavage stress of the material, the cracks propagate by the cleavage mechanism. In weld metals, non-metallic inclusions are generally sites of crack nucleation. When deformations, either plastic or elastic, accumulate to certain quantity non-metallic inclusions crack or decohere with the
Fig. 8. M23C6 precipitates on the primary austenite grain boundary (weld metal B2).
265
matrix. As the stress and deformation in the material increase, the cracked inclusions develop into microcracks. Development of the micro-cracks is dependent on stresses and plastic deformation of the material. Plastic deformation releases stresses in the material. For material B1, because of its relative low strength, more plastic deformation occurs. So that cracks develop at higher external loads, which are needed to accumulate enough stresses in the crack tip areas. The same situation occurs with every step of crack propagation. As a result, for material B1, the first strong AE occurred at a higher external load, and the frequency of AE was low. In this case, there was a ‘stable’ crack-development stage. The fraction of ductile fibrous fracture is also higher as the result of more plastic deformation. (Quantitative analysis on crack propagation will be done in another paper). For materials B2 and B3, because of the high strength, little plastic deformation could occur. So that, cracks developed very quickly as soon as the stress increased to the critical level. Consistently, the frequency of AE was high representing the quick crack propagation by cleavage. As a result, B1 has a higher fracture toughness, and B2 and B3 have lower fracture toughness.
4. Conclusions Microstructure of the modified 9Cr–1Mo steel weld metals in the post welding heat treated condition is composed of tempered martensite and precipitates, mainly M23C6. Fracture toughness and fracture process of the 9Cr–1Mo steel weld metals are determined by AE technique. The fracture mechanism of modified 9Cr–1Mo steel weld metals is a mixture of ductile fibrous and brittle cleavage. The fraction of brittle cleavage and ductile fibrous, as well as fracture behavior, varies with composition of the weld metals. Comparatively, in the weld metal with low alloy content and strength (material B1), onset of crack propagation occurs at a higher stress level and cracks develop slowly due to plastic deformation. There is a stable crack-development stage with the material. In the weld metals of higher strength, onset of crack propagation occurs at a low stress level. Cracks develop very quickly and result to complete fracture of the specimens in a very short time. There is hardly any stable crack-developing stage during loading. As the result, the weld metal of the low strength has a higher fracture toughness, and the weld metals of high strengths have lower fracture toughness. The experiment showed that AE technique can be used to determine fracture toughness and monitor fracture process of materials which could not be investigated by other means.
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Acknowledgements The experiment was done in the Welding Laboratory, Department of Materials Engineering, Southwest Jiaotong University, Chengdu, China. The project was supported by funds from Chinese National Education Committee and Esab AB Group.
References [1] Cai Guang-Jun, Microstructure and Mechanical Properties of High Chromium Steel Weld Metals, Department of Physics, Chalmers University of Technology, Sweden, 1994 Ph.D. thesis. [2] P. Greenfield, A Review of the Properties of 9–12% Cr Steel for Use as HP/IP Rotors in Advanced Steam Turbines, Committee of the European Communities, Office for Official Publications of the European Communities, Luxembourg, 1989.
.
[3] V.K. Sikka, in: A.K. Khare (Ed.), Ferritic Steels for HighTemperature Applications, ASM, New York, 1981, pp. 65–68. [4] G. Guntry, T91 Book, France, 1990. [5] W. Bendick, M. Ring, Steel Res. 67 (1996) 328 – 385. [6] J. Hald, Steel Res. 67 (1996) 369 – 374. [7] G.W. Cuninhm, in: A.K. Khare (Ed.), Ferritic Steels for High-temperature Applications, ASM, New York, 1981, pp. 3 – 6. [8] S.J. Sanderson, in: A.K. Khare (Ed.), Ferritic Steels for Hightemperature Applications, ASM, New York, 1981, pp. 85–100. [9] G.-J. Cai, Metall. Trans. 28A (1997) 1417 – 1428. [10] C.S. Camerin, J.M.A. Rebello, S.D. Soares, NDT E Int. 25 (1992) 127 – 133. [11] Y. Blanchette, J.I. Dickson, M.N. Bassin, Eng. Fract. Mech. 17 (1982) 227 – 234. [12] V.I. Ivanov, NDT Int. 17 (1984) 323 – 328. [13] K. Shimomura, T. Shoji, H. Takahashi, J. Testing Evaluation 15 (1987) 257 – 264. [14] Internal Report on microstructure transfromation of 9Cr1Mo Steel weld, Esab AB, 1998.
Investigation of fracture and determination of fracture toughness of modified 9Cr–1Mo steel weld metals using AE technique Xin Long a, Guangjun Cai b,*, Lars-Erik Svensson c a
Welding Laboratory, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, China b Department of Materials Engineering, Southwest Jiaotong Uni6ersity, Chengdu 610031, China c Centeral Laboratory, Esab AB, 412 77 Gothenburg, Sweden Received 15 November 1998; received in revised form 19 April 1999
Abstract Three-point bending test and acoustic emission technique are used to determine the fracture toughness and fracture process of three types of modified 9Cr–1Mo steel weld metals. Scanning electron microscopy is used to accomplish fractography analysis of fracture specimens. Microstructure of weld metals is investigated using optical microscopy and transmission electron microscopy. The fracture process and factors which affect fracture of the 9Cr – 1Mo steel weld metals in post-weld heat treated condition are studied. Experimental results show that the modified 9Cr – 1Mo steel weld metals fracture by a quasi-cleavage mechanism at ambient temperature. The microstructure of the weld metals is composed of mainly tempered martensite with M23C6 precipitates. In weld metals, microcracks nucleate from non-metallic inclusions. Fractures develop very quickly when cracks started to propagate. Comparatively, in weld metals with low strength, microcracks initiate at a low stress, but propagation of cracks is limited by plastic deformation. In weld metals with high strength, microcracks nucleate at high stresses, but cracks propagate very quickly and lead to almost immediate fracture of the specimens. As a result, weld metals with the low strength have a higher fracture toughness, while weld metals with higher strengths has a lower fracture toughness. © 1999 Elsevier Science S.A. All rights reserved. Keywords: 9Cr – 1Mo steel; Weld metal; Fracture toughness; Microstructure; Acoustic emission
1. Introduction Modified 9Cr– 1Mo heat resistant steels for high temperature applications have been used more and more in recent years, especially in the power industry, to fabricate turbines, pipes, etc. working at high temperatures [1–8]. In these applications, welding is one of most commonly used techniques. Therefore, weldability and quality of welding of 9Cr – 1Mo steels have attracted extensive attention during steel development [1 –4]. The behavior of a welded joint depends on properties of the base metal at welded condition (HAZ) and the weld metal. For the latter, because of the drastic metallurgical nature of welding microstructure,
* Corresponding author. Present address: Department of Materials Engineering, Southwest Jiaotong University, Chengdu 610031, PR China.
and consequently properties, are generally different from that of the base metal. The microstructure of weld metal and its relation with properties, is important to the development and application of modified 9Cr–1Mo steels. Strength and impact toughness of 9Cr–1Mo steels have be studied extensively [4,7–9]. However, not so much work has been reported on fracture toughness of 9Cr–1Mo steels due to the experimental difficulties. Because of the high alloying content, the 9Cr–1Mo steels have relatively high strength but low toughness. The toughness property of the steels has been one of the most important concerns in development and application of 9Cr–1Mo steels. In welded structures defects (including welding introduced inclusions) in the materials are generally inevitable. Fracture toughness is, therefore, an especially important property of 9Cr– 1Mo steel weld metals and is worthy of investigation.
0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 9 9 ) 0 0 1 9 0 - 2
X. Long et al. / Materials Science and Engineering A270 (1999) 260–266
261
2. Experimental techniques
2.1. Preparation of the weld metals Shielded metal arc welding (SMAW) method was used to prepare the weld metals. Three types of 9Cr– 1Mo steel electrodes of 3.2 mm diameter were produced in Esab according to ISO 2560. The welding was carried out on plain carbon steel plates using welding parameters as: current 120 A, voltage 22 V, average welding speed 4 mm/s, and at a preheat and interpass temperature 200°C. The geometry of the welding plates is illustrated in Fig. 1. To avoid the dilution effect of the plain carbon steel plates on the weld metals, the joint surfaces were first deposited with a layer of the 9Cr–1Mo steel weld metals (buttered). After welding, the plates were immediately transferred to a furnace preset at 740°C and heated for 4 h as the post-weld heat treatment (PWHT). The chemical composition of the weld metals was analyzed and is given in Table 1. Hereafter, according to their composition, the weld metals are denoted as B1, B2 and B3. The weld metals have identical chromium and molybdenum contents, but vary in carbon and vanadium content. Comparatively, B1 has the lowest carbon and vanadium contents, and containing no niobium. Materials B2 and B3 are almost identical, except in B2 carbon content is higher and vanadium lower, than that in B3.
Fig. 1. Geometry of the welded joint and cut-out of the fracture toughness test specimens.
Fracture toughness is a comprehensive material property determined by the fracture mechanism, microstructure, etc., of the material. Investigations [1,2,7– 9] show that in general quench + tempered condition microstructure of modified 9Cr – 1Mo steels consists mainly of tempered martensite with precipitates. Some d-ferrite may also exist as remained high temperature transformed phase. Precipitates in the modified 9Cr– 1Mo steels are mainly M23C6 distributing on grain and martensite lath boundaries. Microstructure of 9Cr– 1Mo steel weld metals has also been studied [1,7] in post-weld heat treatment condition. Comparing with base metals, microstructure of 9Cr – 1Mo steel weld metals is obviously inhomogeneous. Depending on the heating temperature and composition of the weld metals, some martensite laths may change to polygonal ferrite by recrystallization. As the result of welding, non-metallic inclusions are present in the weld metals. Fracture mechanism and toughness of modified 9Cr– 1Mo steel weld metals are to a large extent determined by these microstructure features. The fracture toughness and fracture mechanisms of three 9Cr – 1Mo steel weld metals, and their relationship with microstructure are presented in the this paper.
2.2. Fracture toughness testing It was found impractical to measure KIC of the post-welding heat treated 9Cr–1Mo steel weld metals directly, because in this condition the yield strength of the weld metals is not high enough to create the required plane–stress strain condition in the standard sized specimens. To satisfy the plane–stress strain criterion for fracture mechanics test, very large specimens will be required. Therefore, in the present work, instead of KIC, JIC of the weld metals was determined. Threepoint bending test was used to determine JIC values of the weld metals. The test was performed following ASTM E-813 standard. The measured JIC values of the weld metals were then transferred to KIC according to the fracture mechanics theory. The test was carried out at ambient temperature (about 25°C). Specimens used in the test were 20 × 40 mm in size and 160 mm in
Table 1 Chemical composition of the weld metals used in the investigation (in wt.%)
B1 B2 B3
C
Si
Mn
P
S
Cr
V
Mo
Ni
Nb
Cu
O
N
0.045 0.095 0.060
0.37 0.34 0.26
0.70 0.68 0.99
0.005 0.005 0.004
0.013 0.013 0.012
8.6 8.88 8.47
0.008 0.215 0.304
1.03 0.89 0.94
0.01 0.61 0.71
– 0.03 0.03
0.021 0.022 0.072
0.067 0.048 0.079
0.047 0.042 0.040
262
X. Long et al. / Materials Science and Engineering A270 (1999) 260–266
Fig. 2. Geometry of the three-point bending fracture toughness test specimens.
span. On each specimen a through-thickness crack (original crack) was prepared by high frequency fatigue technique at weld center along the welding direction, as shown in Fig. 2. The specimens were loaded at the rate of 0.5 mm/ min. It was found that, during loading, cracking of the specimens generally happened at a relatively high stress level, accompanying a loud noise, developed very quickly and resulted in complete failure of the specimens in a short time and over narrow loading intervals. Because of the lack of stable crack development stage, it was not possible to determine JIC of the materials using the conventional methods. Therefore, in the present experiment, acoustic emission (AE) technique was used to determine the initiation of cracking and monitor the development of cracks in the weld metals. Some work dealing with fracture toughness measurement of materials using the AE technique has been reported [10 – 13]. The AE technique is based on the fact that when a dynamic process which results in a quick energy release, such as cracking or fracture by a brittle mechanism, occurs in a material acoustic waves will be emitted. By detecting the acoustic emis-
Fig. 3. Schematic diagram of AE set and the three-point bending fracture toughness test set.
sion, the fracture process of the material could be detected and monitored. Fig. 3 schematically shows the experimental settings used in the present study. Two acoustic transducers are fixed on two sides of the specimen. During loading, AE signals are detected by the transducers (a small quantity of petroleum grease was used to bond the transducer on the specimen surface to reduce the signal loss). The signals are amplified by the amplifier, then transferred through a filter to filter out external acoustic noises, and recorded on an x–y recorder. The parameters of AE instrument for testing are set as: amplification 50 dB, threshold level of the filter 0.20 V, events interval 3 ms. An AE spectrum is composed of separate peaks. Each peak represents a step of crack development. Together with the AE spectrum, the load–loading point displacement curves are also recorded. To avoid misinterpreting the AE spectrum, the crack initiation point of each material is taken as the load at which the first strong AE peak occurred. JI values of the materials are calculated by the equation: JI =
Uc B(W− a)
(1)
where UC is the energy needed for the pre-prepared crack (original crack) to develop. In the experiment, UC is the area below the load–loading point displacement curve to the critical load. B is the specimen thickness, W is the specimen width and a the length of pre-crack (original crack), so W−a is the length of original ligament of the specimen. For each type of weld metal, five specimens are tested to get the average values of fracture toughness.
2.3. Fractography and microstructure in6estigation After fracture testing, fractography analysis was carried out to determine fracture mechanism of the weld metals. The fractography analysis was carried on fresh fractured surfaces of the tested specimens using scanning electron microscopy. Microstructure of the weld metals were investigated using optical microscopy (OM) and transmission electron microscopy (TEM). For OM observation, specimens were first cut from center of the weld metals and mechanically polished on a surface perpendicular to the welding direction, then etched in a solution of 50 ml HCl, 12.5 g CuCl4 in 100 ml ethanol. To prepare TEM specimens, slices of about 0.5 mm in thickness were cut from the weld metals perpendicular to the welding direction. The slices were ground to a thickness B0.1 mm and punched into 3 mm diameter disks. The disks were finally electropolished in 10% perchloric acid in methanol at a temperature B − 30°C and a potential about 80 V.
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3. Results and discussion
3.1. Determination of JIC by AE As the loading increased, the loading-point displacement increases linearly, corresponding to the elastic deformation of the specimens. In many cases, some weak AE peaks occur when the specimen is still in elastic state. These weak peaks may be caused by decohesion or cracking of inclusions in the weld metals. When the loading increased to a certain level, a strong AE peak occurs accompanied by obvious crack propagation. After this, cracks develop rather quickly until the specimens fracture completely. During this process, more AE signals are recorded. In many cases, pop-in occurs during loading. Pop-in always produces a strong AE peak. Fig. 4 is curves of load – loading point displacement with AE spectra of the weld metals. From the curves of load – loading point displacement, it can be seen that, less plastic deformation occurs in B2 and B3 (Fig. 4b–c). When the cracks start to propagate in these two weld metals, they develop very quickly and result in complete fracture of the specimens in a short time and narrow loading interval. There is hardly a stable crack-development stage in these materials during loading. In material B1, however, more plastic deformation occurs and cracks develop relatively slowly. Indicated by the AE spectra, the first strong AE peak (which represents the crack initiation) occurs at a relatively larger material deformation and higher stress (Fig. 4a). After the onset of crack propagation, the stress in the material still increases. It means that there is a stable crack-development stage during loading of B1. Comparatively, AE in material B1 is less frequent and the AE peaks have similar strength. In B2 and B3, however, AE is more frequent and there is generally an obvious strong emission ahead of the series of AE. Taking the load corresponding to the onset of the prepared crack propagating as the critical load, the determined J-integral by Eq. (1) is JI. In the present test, because AE technique is sensitive to crack propagation, and the load determined by AE spectrum is conservative compared to the conventional fracture mechanics test, at the stage of onset of crack propagation (determined by AE), it is reasonable to believe that the tip of the initial crack is in the plane strain condition, so the measured JI is JIC. The determined values of the weld metals are given in Table 2. Inserting the determined JIC values into the following criteria 25JIC B\ sy
(2)
25 JIC W − a\ s
(3)
Fig. 4. Curves of the load – loading point displacement with the AS spectra of the weld metals a)B1, b)B2 and c)B3.
where sy is the yield strength of the 9Cr–1Mo steel weld metals at the testing temperature. Inserting the parameters of specimen size, the measured JIC and yield strengths of the materials (which were measured as 364, 545 and 565 MPa for weld metals B1, B2 and B3, respectively [1]), these criteria are satisfied, so that the measured JIC values are valid for the materials. JIC has the following relation with KIC: JIC =
(1-62)K 2IC E
(4)
where 6 is Poisson ratio and E Young’s modulus of the materials. Taking 6= 0.3 and E= 2.1× 105 MPa, KIC values of the materials are calculated from JIC and are
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Table 2 The results of fracture toughness measurement of the weld metals Exp. No.
JIC (MPa m)
JIC (MPa m) (average)
KIC (MPa m1/2)
B1-2 B1-2 B1-4 B1-5 B1-7
16.4 17.8 27.2 20.1 13.3
19.0 19.0 19.0 19.0 19.0
62 62 62 62 62
B2-1 B2-2 B2-3 B2-4 B2-5
4.7 8.6 10.1 15.6 4.7
8.7 8.7 8.7 8.7 8.7
43 43 43 43 43
B3-1 B3-4 B3-5 B3-6 B3-7
5.0 8.1 8.0 9.4 7.5
7.4 7.4 7.4 7.4 7.4
39 39 39 39 39
also given in Table 2. It can be seen from Table 2, as the general case with fracture toughness measurement of weld metals, the JIC values vary in a quite large range. This is believed to be because of the inhomogeneity in the materials.
3.2. Fractography and micrography of the weld metals Fig. 5 is a SEM micrograph of the fracture surface of a tested specimen. It can be seen that the fracture mechanism of the material is a mixture of brittle transgranular cleavage and ductile fibrous fracture. According to fractography analysis, all the materials fractured by the same mechanism, but the fraction of cleavage and fibrous fracture varies. Comparatively, material B1 has the highest fraction of ductile fibrous area on the fracture surface, consistent with more plastic deformation and the highest fracture toughness. Materials B2
Fig. 5. SEM micrograph of the fracture surface of tested specimen (weld metal B3).
Fig. 6. SEM micrograph of the fracture surface showing cleavage crack nucleated from a non-metallic inclusion (weld metal B1).
and B3 have identical fracture surfaces, correspondingly, their fracture toughness is at a same low level. SEM observation shows that the cleavage cracks nucleate on non-metallic inclusions in the weld metals (Fig. 6), then develop to the surrounding matrix. The cleavage facets have various sizes. In the 9% Cr steels, the stability of austenite is increased by the high content Cr and austenite will transform to full martensite even by air cooling (in a not very thick piece). After each welding pass, when the weld metals cooled down below Ms temperature (which were measured to be 390°C for B1, 380°C for B2 and 380°C for B3 [14]), the weld metals will transform to martensite. The PWHT process is, therefore, the process of martensite tempering. During this process, the dislocation density in the quenched martensite decreases, and alloying elements will react with carbon and precipitate out as carbides. OM observation shows that the microstructure of the weld metals is composed of mainly lath martensite. The columnar feature of the microstructure could be recognized. In material B1, because of its relative low alloy content some martensite has recrystallized and transformed to polygonal ferrite during PWHT. These polygonal ferrite distributes as bands between different welding beads (Fig. 7). Under TEM observation, the tempered martensite is laths with carbide precipitates on boundaries. The dislocation density in the lath martensite is not high. The precipitates are predominantly M23C6, M23C6 particles also distributed on the primary austenite grain boundaries (Fig. 8). The size of M23C6 varies in a wide range, from 20 to 150 nm in diameter. Besides M23C6, MC precipitates must exist in the weld metals, at least in B2 and B3 since they have considerable quantity of vanadium and carbon. Actually, vanadium rich MC precipitates were found in material B3 [1]. Unfortunately, perhaps because of its ultra fine size, no MC precipitates were found during the present experiment.
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Fig. 7. Optical micrograph showing polygonal ferrite in weld metal B1.
Non-metallic inclusions of low density could be found by TEM observation. The non-metallic inclusions are generally round, about 1 mm in diameter.
3.3. Fracture process of the weld metals As mentioned earlier, AE signals are emitted when cracks initiate or develop in the materials. As it is shown in the experiment, the modified 9Cr – 1Mo steel weld metals fracture by a mixture mechanism of brittle cleavages and some ductile dimpling. In this case, cracks form due to plastic deformation of the material. As a result of plastic deformation, strength of the material near the crack tip increases due to work hardening. When the strength increased to cleavage stress of the material, the cracks propagate by the cleavage mechanism. In weld metals, non-metallic inclusions are generally sites of crack nucleation. When deformations, either plastic or elastic, accumulate to certain quantity non-metallic inclusions crack or decohere with the
Fig. 8. M23C6 precipitates on the primary austenite grain boundary (weld metal B2).
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matrix. As the stress and deformation in the material increase, the cracked inclusions develop into microcracks. Development of the micro-cracks is dependent on stresses and plastic deformation of the material. Plastic deformation releases stresses in the material. For material B1, because of its relative low strength, more plastic deformation occurs. So that cracks develop at higher external loads, which are needed to accumulate enough stresses in the crack tip areas. The same situation occurs with every step of crack propagation. As a result, for material B1, the first strong AE occurred at a higher external load, and the frequency of AE was low. In this case, there was a ‘stable’ crack-development stage. The fraction of ductile fibrous fracture is also higher as the result of more plastic deformation. (Quantitative analysis on crack propagation will be done in another paper). For materials B2 and B3, because of the high strength, little plastic deformation could occur. So that, cracks developed very quickly as soon as the stress increased to the critical level. Consistently, the frequency of AE was high representing the quick crack propagation by cleavage. As a result, B1 has a higher fracture toughness, and B2 and B3 have lower fracture toughness.
4. Conclusions Microstructure of the modified 9Cr–1Mo steel weld metals in the post welding heat treated condition is composed of tempered martensite and precipitates, mainly M23C6. Fracture toughness and fracture process of the 9Cr–1Mo steel weld metals are determined by AE technique. The fracture mechanism of modified 9Cr–1Mo steel weld metals is a mixture of ductile fibrous and brittle cleavage. The fraction of brittle cleavage and ductile fibrous, as well as fracture behavior, varies with composition of the weld metals. Comparatively, in the weld metal with low alloy content and strength (material B1), onset of crack propagation occurs at a higher stress level and cracks develop slowly due to plastic deformation. There is a stable crack-development stage with the material. In the weld metals of higher strength, onset of crack propagation occurs at a low stress level. Cracks develop very quickly and result to complete fracture of the specimens in a very short time. There is hardly any stable crack-developing stage during loading. As the result, the weld metal of the low strength has a higher fracture toughness, and the weld metals of high strengths have lower fracture toughness. The experiment showed that AE technique can be used to determine fracture toughness and monitor fracture process of materials which could not be investigated by other means.
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Acknowledgements The experiment was done in the Welding Laboratory, Department of Materials Engineering, Southwest Jiaotong University, Chengdu, China. The project was supported by funds from Chinese National Education Committee and Esab AB Group.
References [1] Cai Guang-Jun, Microstructure and Mechanical Properties of High Chromium Steel Weld Metals, Department of Physics, Chalmers University of Technology, Sweden, 1994 Ph.D. thesis. [2] P. Greenfield, A Review of the Properties of 9–12% Cr Steel for Use as HP/IP Rotors in Advanced Steam Turbines, Committee of the European Communities, Office for Official Publications of the European Communities, Luxembourg, 1989.
.
[3] V.K. Sikka, in: A.K. Khare (Ed.), Ferritic Steels for HighTemperature Applications, ASM, New York, 1981, pp. 65–68. [4] G. Guntry, T91 Book, France, 1990. [5] W. Bendick, M. Ring, Steel Res. 67 (1996) 328 – 385. [6] J. Hald, Steel Res. 67 (1996) 369 – 374. [7] G.W. Cuninhm, in: A.K. Khare (Ed.), Ferritic Steels for High-temperature Applications, ASM, New York, 1981, pp. 3 – 6. [8] S.J. Sanderson, in: A.K. Khare (Ed.), Ferritic Steels for Hightemperature Applications, ASM, New York, 1981, pp. 85–100. [9] G.-J. Cai, Metall. Trans. 28A (1997) 1417 – 1428. [10] C.S. Camerin, J.M.A. Rebello, S.D. Soares, NDT E Int. 25 (1992) 127 – 133. [11] Y. Blanchette, J.I. Dickson, M.N. Bassin, Eng. Fract. Mech. 17 (1982) 227 – 234. [12] V.I. Ivanov, NDT Int. 17 (1984) 323 – 328. [13] K. Shimomura, T. Shoji, H. Takahashi, J. Testing Evaluation 15 (1987) 257 – 264. [14] Internal Report on microstructure transfromation of 9Cr1Mo Steel weld, Esab AB, 1998.