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On the fatigue behaviour of electron beam and gas tungsten arc weldments of 409M grade ferritic stainless steel
Materials and Design 35 (2012) 760–769
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On the fatigue behaviour of electron beam and gas tungsten arc weldments of 409M grade ferritic stainless steel A.K. Lakshminarayanan a,⇑, V. Balasubramanian a, G. Madhusudhan Reddy b a b
Centre for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India Metal Joining Group, Defence Metallurgical Research Laboratory (DMRL), Kanchanbagh 500 058, Hyderabad, India
a r t i c l e
i n f o
Article history: Received 17 June 2011 Accepted 10 October 2011 Available online 31 October 2011 Keywords: D. Welding E. Fatigue G. Fractography
a b s t r a c t Fatigue life and fatigue crack growth behaviour of the electron beam welded AISI 409M ferritic stainless steel joints in comparison with the gas tungsten arc welded joint and the base metal was studied. It is found that the joint fabricated by the electron beam welding process exhibited superior fatigue performance than that of the gas tungsten arc welded joint. Formation of a dual phase lath ferrite with fine martensitic microstructure, superior tensile properties and favourable residual stress field are the main reasons for the enhanced fatigue life and fatigue crack resistance of the electron beam welded joint. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ferritic stainless steels are the second most widely used group of stainless steels, due to their good corrosion resistance and lower production costs, as compared with austenitic stainless steels. Although these alloys have useful properties in the wrought condition, welding is known to reduce their toughness, ductility, and corrosion resistance, because of grain coarsening in the heat affected zone and fusion zone of the welded joints [1]. These difficulties have seriously hindered the use of this economical type of stainless steel for welded constructions. Nowadays, steel fabricators with modern production facilities are able to fabricate the modified 12 wt% Cr ferritic stainless steel with low carbon (<0.03%) and with low impurity levels, which improves the weldability and mechanical properties of such steels [2]. 409M grade ferritic stainless steels (FSSs) are widely used to construct coal wagons for transporting iron ore. The different parts made from 409M grade are box body (including the inner and outer walls, floor plate and wagon under-frame), vertical side stanchions and flab doors [3]. This steel is designed to transform partially to austenite on cooling, passing through the austenitic – ferritic phase field and improves the weldability and as-welded toughness by restricting the heat-affected zone (HAZ) grain growth [4]. Though the modified 12 wt% Cr ferritic stainless steels are having better weldability than conventionally used ferritic stainless ⇑ Corresponding author. Tel.: +91 4144 239734 (O), +91 9865431106 (M); fax: +91 4144 238080/238275. E-mail addresses: [email protected] (A.K. Lakshminarayanan), visvabalu@ yahoo.com, [email protected] (V. Balasubramanian), [email protected] (G. Madhusudhan Reddy). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.10.010
steels, the 409M grade steel still suffers from grain growth in the heat affected zone and fusion zone results in deterioration of the mechanical properties [5]. Due to the high energy density, electron beam welding (EBW) is attractive for ferritic stainless steel, because of the narrow width of the fusion zone and heat affected zone [6]. EBW is characterised by a short interaction time with intense energy density. As a result, the process has a lower heat input, which is expected to exercise greater control over grain coarsening and distortion [7]. The high joining rate, the deep and narrow weld, and the minimal heat-affected zone are basic advantages of EBW joints [8]. Microstructure and tensile properties of electron beam welded AISI 409M ferritic stainless steel joints were reported in our previous study [9]. As the fatigue failure is one of the prime concerns in structural design and the butt weld is a part of many structures, its evaluation and prediction of fatigue behaviour is very important to avoid catastrophic failure particularly in welded ferritic stainless steel structures. Few studies were reported on the fatigue behaviour of ferritic stainless steel weldments. Akita et al. [10], investigated fatigue crack resistance of gas tungsten arc welded AISI 444 ferritic stainless in both air and 3.5% NaCl aqueous solution. The crack growth rates within heat affected zone and the weld metal were lower compared to the base metal and the author reported that this is mainly due to the high closure level, changes in the residual stress, and hardness changes due to the welding. Taban et al. [11], evaluated the mechanical and metallurgical properties of laser beam welded (LBW) 12 wt% Cr modified ferritic stainless steel joints and it was found that the LBW joint showed superior fatigue strength compared to the base metal though the toughness of weld metal is much lower than that of the base metal. However, data on high-energy density process, namely, EBW of ferritic stainless
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A.K. Lakshminarayanan et al. / Materials and Design 35 (2012) 760–769 Table 1 Chemical composition of the base metal (spectrometry results). C
Cr
Ni
Nb
Cu
Si
Mn
P
S
N
Al
Co
Ti
Va
0.026
11.40
0.4
0.009
0.365
0.45
1.15
0.4
0.16
0.04
0.01
0.2
0.008
0.017
the process parameters used in this investigation are presented in Table 2. The fabricated joints are shown in Fig. 1.
Table 2 Welding conditions and process parameters. Parameters
EBW
GTAW
Current Voltage Shielding gas Gas flow rate (l/min) Gun to work distance Vacuum Welding speed (mm min1) Heat input (J/mm)
51 mA 55 kV – – 298 mm 104 bar 1000 143
120 A 20 V Argon 16 – – 80 1800
steel, are very scant. Moreover, details on fatigue life and fatigue crack growth properties of electron beam welded 409M grade ferritic stainless steels are not yet reported in the literature. The present paper reveals the results obtained from fatigue testing on the electron beam and gas tungsten arc welded AISI 409M ferritic stainless steel joints. The influence of weld metal microstructure, tensile property, and residual stress on fatigue behaviour is discussed on the basis of fatigue life, crack initiation, crack growth and fractographic analysis. 2. Experimental analysis 2.1. Base material The as-received base metal (BM) used in this study was 4 mm thick cold rolled, annealed and pickled AISI 409M grade ferritic stainless steel plates. Final annealing was performed after cold rolling at temperatures below the A1 (between 700 °C and 750 °C). The chemical composition of the base metal is presented in Table 1. 2.2. Welding conditions Trial runs were carried out to optimize the welding parameters so as to attain full penetration, defect free weld in a single pass. Square butt joints were prepared to fabricate GTAW and EBW joints without filler metal additions. The welding conditions and
2.3. Specimens used Hourglass specimens as per ASTM E468-08 [12] were prepared to the dimensions as shown in Fig. 2a, to evaluate the fatigue strength of the welded joints. A centre cracked tensile (CCT) specimen was used to evaluate the fatigue crack growth behaviour with the presence of a sharp notch, which was machined in the weld metal region (Fig. 2b) to the required length using the WEDM. Procedures prescribed by the ASTM E647-09 standard [13] were followed for the preparation and testing of the CCT specimens. 2.4. Procedures Microstructure of the welds was examined using optical and transmission electron microscopy (TEM). Thin foils with a thickness of about 1 mm were cross sectioned from the fusion zone of welded joints to prepare the TEM samples. After being mechanically ground to approximately 80 lm, the foils were further ground to a thickness of 15 lm by a dimple grinding machine. Then the electron transparent thin sections were electrolytically prepared by the twin-jet polisher. An electrolyte of 10% perchloric acid +90% ethanol solution (25 °C) was used during thinning. These thin foils were observed under a 200 kV potential using a Transmission Electron Microscope (Make: Philips, UK: Model: CM20). Fatigue experiments were conducted using a servo hydraulic, 100 kN fatigue testing machine (Make: Instron, UK; Model 8801). A frequency of 15 Hz and a constant amplitude uniaxial tensile loading condition (R = 0.1) was used uniformly for all the specimens. Before loading, the specimen surface near the notch was polished to facilitate the fatigue crack growth measurement. A travelling microscope with an accuracy of 0.01 mm was used to monitor the crack length. The specimen was loaded at a particular stress level (range), and following crack initiation from the tip of the machined notch, its subsequent propagation along the weld metal was recorded from initiation to the complete failure of the specimen. Similar crack growth experiments were conducted on
Fig. 1. Photographs of fabricated joints.
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(a) Hourglass fatigue (SN) specimen
(b) Centre Cracked Tensile (CCT) Specimen Note: W = 10 mm for GTAW joint and 3 mm for EBW joint Fig. 2. Dimensions of fatigue test specimens.
Specimen
Optical Micrographs
Transmission Electron Micrographs
(a)
(d)
(b)
(e)
Basemetal
GTAW
GB Martensite
EBW
Subgrains
(c) Fig. 3. Optical and transmission electron micrographs.
(f)
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a number of specimens at five different stress levels, and the experimental data were recorded. The fractured surface of the fatiguetested specimens were analysed using Scanning Electron Microscope (Make: JEOL, Japan; Model 6410LV) at higher magnification to study the fracture morphology to establish the nature of the fracture. Residual stress analyses were performed on top surface of the welded joints, weld metal region of SN specimens and at the crack tip of the CCT specimens in both perpendicular (rres, t) and parallel (rres, l) to the weld zone axis using X-ray stress analyser (Make: Stresstech OY, Finland; Model: XStress 3000) employing Cr Ka radiation. 3. Results 3.1. Microstructure and hardness Optical and TEM image of base metal (Fig. 3a and d) shows the characteristics of grain boundaries with ferrite grains and the presence of chromium carbides. Fusion zone of GTAW joint showed that the grain boundaries of widmanstatten ferrite were partially covered by the needle shape martensite, (Fig. 3b). Grain growth was observed in the fusion zone. Coarse lath ferrite structure with partially covered needle shape martensite and chromium carbides decorated the grain boundaries was observed in case of GTAW joint (Fig. 3e). Optical microstructure taken at the fusion zone of EBW joint is presented in Fig. 3c. It reveals the presence of multi directional ferrite with isolated martensite laths with various sizes. TEM image of FZ (Fig. 3f) clearly shows the presence of prior d ferrite grain boundary and the substructure and subgrain structure. The presence of chromium carbides and grain boundary martensite are also evidenced from Fig. 3f. This suggests that the equiaxed coarse grain microstructure of the base metal undergone a refinement during weld thermal cycle. The phases present in the weld metal such as ferrite, martensite and chromium carbides were confirmed through X-ray Diffraction (XRD) analysis and reported in our previous study [9]. The microhardness profile at the mid cross section of welded joints is presented in Fig. 4. The hardness of the as-received base metal is approximately 170 HV0.05. The hardness measurements indicate that the hardness values in both the weld metal and heat affected zone of the EBW joint are higher than the base metal. In case of GTAW joint, the weld metal exhibited slightly higher hard-
ness and HAZ exhibited lower hardness compared to the base metal. 3.2. Tensile properties During tensile testing, the transverse tensile specimens prepared from EBW and GTAW joints failed in base metal and heat affected zone respectively. Joint efficiency is the ratio between tensile strength of welded joint and tensile strength of the unwelded parent metal. EBW joints exhibited a joint efficiency of 100% respectively since the failure took place in the base metal region. However, GTAW joint exhibited a joint efficiency of 77%. Since the transverse tensile specimens prepared from EBW joints fractured in the base metal region, flat microtensile specimens were prepared to evaluate the tensile properties of the all weld metal of both the joints. In each condition, three tensile specimens were tested; the mean value of three results is presented in Table 3. Higher yield strength, higher tensile strength and lower fracture elongation were observed in case of EBW welds compared to the base metal. The yield strength, tensile strength and elongation of GTAW joint are lower than the base metal and EBW joint. After yielding the stress–strain relationship in the uniform deformation stage may be expressed by Holloman equation [14]. The yield ratio and strain hardening coefficient values for the base metal and the fusion zone of EBW and GTAW joints were evaluated and are presented in Table 3. 3.3. Residual stress Longitudinal and transverse residual stress components were measured on top surface of the EBW and GTAW joints as a function of distance from the weld centreline and are presented in Fig. 5. The surface residual stress (both transverse and longitudinal) perpendicular to the welding direction (at the weld centre) is of tensile in nature for both the joints. However the EBW joint showed lower residual stress compared to the GTAW joint. The fatigue specimens that were used are small in size relative to the welded joints. Redistribution and relaxation of residual stresses could occur in the specimens when they are machined from the welded joints and ground to the required dimensions. Hence, both the longitudinal and transverse residual stresses were measured at the weld metal region of SN specimen and at the crack tip of the CCT specimen and the results are presented in Table 4. The results indicate that a significant amount of residual stresses can still be present after sectioning and grinding the fatigue specimen extracted from the welded plate. These residual stresses will have significant effects on the fatigue initiation and propagation during the fatigue cycling. 3.4. Fatigue (SN) behaviour Specimens were tested at five different stress levels (125– 325 MPa). Four specimens were tested at each stress level and the mean value was used to analyze the results. SN curves (Fig. 6) were constructed for the base metal and the welded joints.
Table 3 Tensile properties of the base metal and electron beam welded AISI 409M FSS joints.
Fig. 4. Hardness profile at mid-cross section of EBW and GTAW joints.
Specimen
Yield strength (MPa)
Ultimate tensile strength (MPa)
Yield ratio
Strain hardening coefficient (n)
Elongation in 50 mm gauge length (%)
BM EBW GTAW
364 418 325
536 790 420
0.68 0.53 0.77
0.14 0.28 0.16
31 28 18
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(a) EBW Joint
(b) GTAW Fig. 5. Longitudinal and transverse residual stress profile.
Table 4 Residual stress summary. Joint
Magnitude of residual stress in weld metal (WM) region (MPa) Longitudinal
EBW GTAW
Transverse
As welded
Hourglass (SN) specimen
CCT specimen
As welded
Hourglass (SN) specimen
CCT specimen
+210 ± 14.4 +243 ± 12.4
+90 ± 10.2 +130 ± 8.0
+60 ± 6.7 +110 ± 10.5
+50 ± 5.6 +80 ± 8.8
20 ± 6.0 45 ± 10.0
50 ± 10.8 30 ± 12.4
The confidence and predicted interval of the fatigue strength values for the base metal and welded joints are presented in Fig. 7. For comparison, the stress corresponding to 2 106 cycles in the mean curve (Fig. 6a) was taken as fatigue strength of the welded joints and they are presented in Table 5. It is inferred that the EBW joint endured more number of cycles than the base metal and GTAW joint. The fatigue strength of unwelded AISI 409M grade
ferritic stainless steel is 185 MPa. But the fatigue strength of EBW joint is 195 MPa. This indicates that there is a 5% enhancement in fatigue strength values due to electron beam welding. However, GTAW joint shows the lowest fatigue strength of 150 MPa which is 19% lower than the base metal. The EBW joint exhibited 23% higher fatigue strength than the GTAW joint. Fatigue test results of welded joints are also compared with the fatigue design curves
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BM
Fig. 6. SN curves.
recommend by IIW [15]. It can be seen from Fig. 6, that fatigue curve of the both the joints fall above the FAT 112 line which is the highest limit for fusion welded steel. The effect of residual stress on fatigue strength is also presented in Fig. 6. The expression proposed by SWT (Smith, Watson and Topper method) [16] was used to account for the effect of residual stress on fatigue strength. Eq. (1) was used to find out the stress ratio to include the effect of residual stress [17].
R¼
rmin rmean þ rres Dr=2 ¼ rmax rmean þ rres þ Dr=2
GTAW ð1Þ
It is inferred that the fatigue strength values are lower compared to the base metal when the residual stress is accounted in SN curves. However, the EBW joint exhibited higher fatigue strength when compared to the GTAW joint due to the lower tensile residual stress in the weld metal region (Table 4). 3.5. Fatigue Crack Growth (FCG) behaviour The fatigue crack growth experiments were conducted at five different stress levels (Dr) of 75, 100, 125, 150, and 175 MPa under constant amplitude loading conditions (R = 0.1). The measured variation in crack length (2a) and the corresponding number of cycles (N) endured under the action of particular applied stress range are plotted as shown in Fig. 8, for the BM and welded joints. The fracture mechanics based Paris Power equation [18], was used to analyze the experimental results. The procedure prescribed in ASTM E647 standard was used to analyse the results. The crack growth rate da/dN for the propagation stage was calculated for the steady-state growth regime, at different intervals of crack length increment, against the associated number of cycles to propagation. The relationship between stress intensity factor (SIF) range and the corresponding crack growth rate in terms of best fit lines is shown in Fig. 9 for the BM and the welded joints. The data points plotted in the graph mostly correspond to the second stage of Paris sigmoidal relationship (106–103 mm/cycle). When the crack growth rate was around 103 mm/cycle, unstable crack growth occurred and the corresponding DK value was taken as critical SIF (DKcr). The value was not taken at the point of fracture, but was taken at the point of transition at which steady-state crack growth changes into unsteady state. Of the two joints, joint fabricated by
EBW Fig. 7. Confidence and predicted intervals for fatigue strength of welded joints.
EBW process exhibited superior fatigue crack resistance compared to its counterpart. The EBW joint exhibited a critical SIF of p 32 MPa m which is 77% and 19% higher compared to the GTAW joint and the base metal. GTAW joint showed critical SIF of p 18 MPa m which is 33% lower compared to the base metal. 3.6. Fracture surface analysis The fatigue fracture surface appearance corresponding to crack initiation, crack propagation and the final failure regions of the base metal, EBW and GTAW joints (150 MPa alone), as observed
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A.K. Lakshminarayanan et al. / Materials and Design 35 (2012) 760–769 Table 5 Fatigue properties of base metal and welded joints. Joint type
Fatigue strength of the unnotched specimens at 2 106 cycles (MPa)
Fatigue strength of the notched specimens at 2 106 cycles (MPa)
Fatigue notch factor (Kf)
Notch sensitivity factor (q)
BM EBW GTAW
185 195 150
140 150 95
1.32 1.27 1.57
0.32 0.28 0.46
3.6.1. Initiation In both the base metal and the welded joints, the crack initiations sites are clearly visible (indicated by the arrow marks) and it can be observed from the fractographs that the fatigue cracks have initiated from multiple crack initiation sites (Fig. 10a, d and g) irrespective of welding processes used. Large number of crack initiation sites is visible in the fracture surface of the GTAW joint when compared to the EBW joint and the base metal.
Fig. 8. Fatigue crack growth curves.
3.6.2. Crack propagation Fig. 10b, e and h, reveal the fracture surface appearance of fatigue crack propagation (FCP) region (where steady state crack growth occurs) of the base metal, EBW and GTAW joints respectively. The fractograph of base metal shows the presence of striations in the facets itself (Fig. 10b). The striations are parallel and are at right angles to the local direction of crack propagation. Thus the arrows placed in the fractographs represent the crack propagation path. Fracture surface of EBW joints displays a progression of rather fine, but extremely irregular striations, separated here and there by secondary cracks (Fig. 10e). This is mainly due to the microlevel crack deviation by the dual phase lath ferrite with fibrous martensite microstructure of fusion zone. Fractograph of GTAW joint (Fig. 10h) shows the change from transgranular facets to dimples. The presence of secondary cracking and second phase particle (chromium carbides) accelerates the crack growth which led to larger spacing between the striation marks. 3.6.3. Final failure Fig. 10c, f and i, shows the fracture surface appearance of final failure region (FF) region (where unstable crack growth occurs) of the base metal and the welded joints. Even though unstable crack growth occurs in the final failure region, the final fracture is still in the ductile mode and it is evident from the presence of dimples. The fractograph of base metal presented in Fig. 10c reveals the structure of ductile mode failure with elongated dimples. The macro-level cracks in the transverse direction of the fatigue testing specimen also evidenced (i.e. splitting). The fracture surface of EBW joint (Fig. 10f) shows fibrous dimples along with the patches of fine dimples. GTAW joint fractograph shows the presences of many voids, which bear resemblance to conventional intergranular failure, are visible in addition with small area having fine dimples. The coarser facets are the characteristic final failure mode of GTAW joint is also evidenced from the fractograph (Fig 10i). Stair-step fracture surface indicative of Stage I fatigue fracture is also observed in Fig. 10i. 4. Discussion
Fig. 9. Relationship between FCGR and SIF range.
under the Scanning Electron Microscope (SEM) are displayed in Fig. 10. The fatigue crack initiation (FCI) region is corresponding to 1 mm from the tip of the machined notch; fatigue crack propagation (FCP) region is referred to 1–6 mm; final failure (FF) region is referred to 6 mm away from the crack initiation region.
The fatigue crack initiation behaviour, fatigue crack growth behaviour and fatigue strength of the ferritic stainless steel is influenced by the welding processes. Of the two joints, EBW joint offered higher fatigue life and crack growth resistance than the GTAW joint. In the CCT specimen, the notch is machined in the weld metal region of the joints by WEDM process to evaluate the crack growth behaviour under fatigue loading. The fatigue crack initiates from the tip of the machined notch and it grows in the weld metal region
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Crack Initiation
Crack Propagation
Final Failure
(a)
(b)
(c)
Base metal
EBW
(d)
(e)
(f)
(g)
(h)
(i)
Note: R: Rubbed Marks; SC – Secondary Cracks; P – Propagation Path Fig. 10. SEM fractographs of CCT specimen of welded joints.
until final failure takes place and hence the weld metal properties such as tensile property and impact toughness, microstructure and magnitude of the residual stress will have greater influence on fatigue performance of the joints. The effect of these properties on fatigue behaviour is discussed in the following sections. 4.1. Influence of microstructure Microstructure is known to affect fatigue strength, the crack initiation and propagation of the crack present in the weld metal region. EBW joint exhibited higher fatigue strength and fatigue crack growth resistance than the GTAW joint and base metal. This can be explained with the help of weld metal microstructure of the welded joints. Fine lath ferrite with fibrous martensite in the EBW joint is the most efficient in transferring load when compared to the coarse microstructure of the GTAW joint. This is mainly because, the load transfer takes place by shear action along the martensite/ferrite interface, and more interfacial energy is available, when the shape is fine continuous fibrous lath structure [19]. Coarse ferrite structure with discontinuous martensite structure normally results in inhomogeneous deformation and restricts initial plastic flow to a smaller fraction of the total volume of the ferrite matrix. Also, void growth occurs at a faster rate and with less plastic strain resulted in reduced ductility [20]. Early initiation of multiple cracks and faster propagation by linking up through the cracks in the coarse microstructure results in a lower fatigue life of the GTAW joints compared to their counterparts. An additional crack closure mechanism as explained by Sudhakar and Dwarakadasa [21], also contributes to the retardation of
crack growth. During fatigue loading, the martensite will be the load bearing phase, thus it shields the crack tip and thereby reduces the crack driving force at the crack tip. It is also shown that the martensite in a dual phase structure has the ability to constrain the plastic deformation of the ferrite phase [22]. This constraining effect is primarily a function of distance from the crack tip to the martensite. This distance causes the crack growth in the ferrite phase to decelerate as it approaches the martensite. The martensite with lower carbon content is tougher, and thereby effective in reducing the crack tip driving force by mechanisms of crack tip blunting and deflection [23]. As reported by Jang et al. [24], the fatigue crack mostly follows the directions of the dendrites along the ferrite phase, suggesting that the martensite/ferrite boundary is the preferred crack growth path. However, when the dendrite alignment is not favourable to the crack growth along the boundary, the crack grows across the boundary, resulting in a very wavy and tortuous crack path. The change in crack path results in the formation of secondary cracks. This is also one of the reasons for the reduced weld metal fatigue crack growth rate of EBW joints. It seems reasonable to believe that the reduction in crack growth rate of EBW joint is to a large extent caused by microstructural effects. 4.2. Influence of tensile properties The yield strength has a major influence on the fatigue life and fatigue crack behaviour of welded joints [25]. Higher yield strength and tensile strength of the electron beam welded ferritic stainless steel joint is greatly used to enhance the fatigue strength by delay-
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ing the fatigue crack initiation. The major reason for the improvement in fatigue strength also results from the higher work hardening rate of fine dual phase ferritic–martensitic structure. The size and the morphology of the hard phase in a dual phase material determine the material’s strengthening mechanism [26]. Due to the constraint exerted by the martensitic phase on the ferrite matrix, the strength of fusion zone was increased. When deformation is applied, the material in the vicinity of the martensite phase is prevented from deforming freely, resulting in an increase in the shear stresses required to produce further deformation. This type of ferritic–martensitic dual phase structure displays higher workhardening rate and higher flow strength than those of the single phase ferritic structure. Since the crack tip stress–strain field is influenced by the strain hardening exponent [27] of the material, the fracture at the crack tip process zone is affected and hence the crack propagation is delayed. In high strength weld metal, the plastic zone can easily extend into the parent material because the deformation and yielding occur in both fusion zone and the base metal. The stress relaxation can easily take place in the crack tip region. So more crack driving force is needed for crack extension and the fracture resistance of the higher strength weld metal is greater than the lower strength weld metal [28]. This is also one of reasons for the improved fatigue crack growth resistance of electron beam weld metal region compared to the weld metal of GTAW joint. 4.3. Influence of residual stress The residual stresses developed during the welding process can have a significant effect on the service performance of the welded material with respect to fatigue properties, and fatigue crack growth process. It is particularly important in high power density welding of thin plates which are highly susceptible to distortion and significantly deteriorate the fatigue behaviour [29]. In this study, tensile residual stress was observed in both the longitudinal (parallel to the crack growth) and transverse (perpendicular to the crack growth) directions irrespective of the welding processes used. The residual stress measurement was carried out mainly to understand the effect of heat input of welding processes and also to know the initial condition (stress state) of the test specimens of the two autogenous fusion welding processes used in this investigation to fabricate the AISI 409M grade FSS, the heat input involved in GTAW process is relatively higher compared EBW process. Hence the magnitude of tensile residual stress is higher in GTAW joint compared to EBW joint and this is also evident from the measured residual stress values presented in Table 5. Even though the compressive load acts on the specimen during grinding is uniform, the resultant residual stress values are different and this may be due to the differences existed in the original (as welded condition) tensile residual stress values. Lower tensile residual stress in the transverse direction joint resulted in improved fatigue life of EBW over the GTAW joint. Higher compressive residual stress perpendicular to the weld in CCT specimen acts beneficially in the enhancement of fatigue crack growth resistance of EBW joint compared to the GTAW joints. From the above discussion, it can be concluded that the enhanced fatigue life and improved fatigue crack growth resistance of EBW joint over the base metal is due to the synergetic effect of dual phase microstructure, superior tensile properties and favourable residual stress field of weld metal region. 5. Conclusions Fatigue life, crack initiation and growth rates in relation to microstructure, tensile properties and residual stresses were studied in
electron beam and gas tungsten arc welds of AISI 409M ferritic stainless steel. From this investigation, the following important conclusions were derived: The fatigue strength of the 409M ferritic stainless steel was improved by electron beam welding. The fatigue strength of EBW joint is 195 MPa at 2 106 cycles and the enhancement in fatigue strength value is approximately 19% compared to the GTAW joint. The fatigue strength of both the welded joints was lower than that of the base metal due the presence of tensile residual stress in the weld metal region of hourglass specimen. However, the EBW joint exhibited higher fatigue strength when compared to the GTAW joint due to the lower tensile residual stress field. p Higher critical SIF range (32 MPa m) was observed for the EBW joint than the GTAW joint. The enhancement in the critical SIF range was 77% compared to the GTAW joint. Superior tensile properties, dual phase lath ferritic–martensitic microstructure and favourable residual stress perpendicular to the crack growth are the main reason for the improved fatigue performance of the EBW joint compared to the GTAW joint.
Acknowledgements The authors wish to place their sincere thanks to the Department of Science and Technology (DST), Government of India for the financial support through Fast Track Scheme for Young Scientists R&D Project (SR/FTP/ETA043/2009) to carry out this investigation. In addition, the authors wish to record their sincere thanks to the referee’s for their constructive and valuable feedback. References [1] Amuda MOH, Mridha S. Grain refinement in ferritic stainless steel welds: the journey so far. Adv Mater Res 2010;83–86:1165–72. [2] Deleu E, Dhooge A, Taban E, Kaluc E. Possibilities and limitations to improve the weldability of low carbon 12Cr ferritic stainless steel for expanded industrial applications. Weld World 2009;53:R198–208. [3] 409M – Technical Data. Detail brochure. India: Steel Authority of India Limited (SAIL); 2004. p. 13–26. [4] Meyer AM. Interstitial diffusion from the weld metal into the high temperature heat affected zone in 11–12 percent chromium steel welded joints. MEng thesis. University of Pretoria, South Africa; 2000. [5] Taban E, Deleu E, Dhooge A, Kaluc E. Gas metal arc welding of modified X2CrNi12 ferritic stainless steel. Kovove Mater 2007;45:67–74. [6] Penasa M, rivela C. Application of the laser welding process to low thickness stainless steels. Weld Int 2003;17:947–57. [7] Yilbas BS, Sami M, Nickel J, Coban A, Said SAM. Introduction into the electron beam welding of austenitic 321-type stainless steel. J Mater Process Technol 1998;82:13–20. [8] Huang CA, Wang TH, Han WC, Lee CH. A study of the galvanic corrosion behavior of Inconel 718 after electron beam welding. Mater Chem Phys 2007;104:293–300. [9] Lakshminarayanan AK, Balasubramanian V, Madhusudhan Reddy G. Microstructure and mechanical properties of electron beam-welded AISI 409M-grade ferritic stainless steel. Int J Adv Manuf Technol 2011;55: 153–62. [10] Akita M, Nakajima M, Tokaji K, Shimizu T. Fatigue crack propagation of 444 stainless steel welded joints in air and in 3%NaCl aqueous solution. Mater Des 2006;27:92–9. [11] Taban E, Deleu E, Dhooge A, Kaluc E. Laser welding of modified 12% Cr stainless steel: strength, fatigue, toughness, microstructure and corrosion properties. Mater Des 2009;30:1193–200. [12] ASTM E468 – 08. Standard practice for verification of constant amplitude dynamic forces in an axial fatigue testing system. ASTM International; 2008. [13] ASTM E 647 – 05. Standard test method for measurement of fatigue crack growth rates. ASTM International; 2005. [14] Hollomon JH. Tensile deformation. Am Inst Mining Metall Eng Trans Iron Steel Div 1945;162:268–89. [15] Hobbacher AF. Recommendations for fatigue design of welded joints and components. Paris, France: International Institute of Welding; 2008. p. 41–2 [doc. XIII-2151r4-07/XV-1254r4-07] [16] Ince A, Glinka G. A modification of Morrow and Smith–Watson–Topper mean stress correction models. Fatigue Fract Eng Mater Struct 2011;00:1–14.
A.K. Lakshminarayanan et al. / Materials and Design 35 (2012) 760–769 [17] Nicholas T. High cycle fatigue – a mechanics of materials perspective. Elsevier Ltd.; 2006. p. 119. [18] Paris P, Erdogan F. A critical analysis of crack propagation laws. J Basic Eng Trans ASME 1963:528–34. [19] Bayram A, Uguz A, Ula M. Effects of microstructure and notches on the mechanical properties of dual-phase steels. Mater Char 1999;43: 259–69. [20] Mediratta SR, Ramaswamy V, Rama Rao P. Influence of ferrite–martensite microstructural morphology on the low cycle fatigue of a dual-phase steel. Int J Fatigue 1985;7(2):107–15. [21] Sudhakar KV, Dwarakadasa ES. A study on fatigue crack growth in dual phase martensitic steel in air environment. Bull Mater Sci 2000;23(3):193–9. [22] Wang ZG, Al SH. Fatigue of martensite–ferrite high strength low-alloy dual phase iron and steel. Res Inst Jpn 1999;39(8):747–59. [23] Deng RY, Ye ZJ. Fatigue crack growth rate in ferrite–martensite dual-phase steel. Theor Appl Fract Mech 1991;16:109–22.
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[24] Jang C, Cho PY, Kim M, Oh SJ, Yang JS. Effects of microstructure and residual stress on fatigue crack growth of stainless steel narrow gap welds. Mater Des 2010;31:1862–70. [25] Okayasu M, Sato K, Mizuno M, Hwang DY, Shin DH. Fatigue properties of ultrafine grained dual phase ferrite/martensite low carbon steel. Int J Fatigue 2008;30:1358–65. [26] Suzuki H, Mcevily AJE. Microstructural effects on fatigue crack growth in a low carbon steel. Metall Trans A Phys Metall 1979;10-A:475–81. [27] Laird C, Buchinger L. Hardening behavior in fatigue. Metall Mater Trans A 1985;16(12):2201–14. [28] Zhang H, Zhang Y, Li L, Ma X. Influence of weld mis-matching on fatigue crack growth behaviours of electron beam welded joints. Mater Sci Eng A 2002;334:141–6. [29] Ferro P, Zambon A, Bonollo F. Investigation of electron-beam welding in wrought Inconel 706—experimental and numerical analysis. Mater Sci Eng A 2005;392:94–105.
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On the fatigue behaviour of electron beam and gas tungsten arc weldments of 409M grade ferritic stainless steel A.K. Lakshminarayanan a,⇑, V. Balasubramanian a, G. Madhusudhan Reddy b a b
Centre for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar 608 002, Tamil Nadu, India Metal Joining Group, Defence Metallurgical Research Laboratory (DMRL), Kanchanbagh 500 058, Hyderabad, India
a r t i c l e
i n f o
Article history: Received 17 June 2011 Accepted 10 October 2011 Available online 31 October 2011 Keywords: D. Welding E. Fatigue G. Fractography
a b s t r a c t Fatigue life and fatigue crack growth behaviour of the electron beam welded AISI 409M ferritic stainless steel joints in comparison with the gas tungsten arc welded joint and the base metal was studied. It is found that the joint fabricated by the electron beam welding process exhibited superior fatigue performance than that of the gas tungsten arc welded joint. Formation of a dual phase lath ferrite with fine martensitic microstructure, superior tensile properties and favourable residual stress field are the main reasons for the enhanced fatigue life and fatigue crack resistance of the electron beam welded joint. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ferritic stainless steels are the second most widely used group of stainless steels, due to their good corrosion resistance and lower production costs, as compared with austenitic stainless steels. Although these alloys have useful properties in the wrought condition, welding is known to reduce their toughness, ductility, and corrosion resistance, because of grain coarsening in the heat affected zone and fusion zone of the welded joints [1]. These difficulties have seriously hindered the use of this economical type of stainless steel for welded constructions. Nowadays, steel fabricators with modern production facilities are able to fabricate the modified 12 wt% Cr ferritic stainless steel with low carbon (<0.03%) and with low impurity levels, which improves the weldability and mechanical properties of such steels [2]. 409M grade ferritic stainless steels (FSSs) are widely used to construct coal wagons for transporting iron ore. The different parts made from 409M grade are box body (including the inner and outer walls, floor plate and wagon under-frame), vertical side stanchions and flab doors [3]. This steel is designed to transform partially to austenite on cooling, passing through the austenitic – ferritic phase field and improves the weldability and as-welded toughness by restricting the heat-affected zone (HAZ) grain growth [4]. Though the modified 12 wt% Cr ferritic stainless steels are having better weldability than conventionally used ferritic stainless ⇑ Corresponding author. Tel.: +91 4144 239734 (O), +91 9865431106 (M); fax: +91 4144 238080/238275. E-mail addresses: [email protected] (A.K. Lakshminarayanan), visvabalu@ yahoo.com, [email protected] (V. Balasubramanian), [email protected] (G. Madhusudhan Reddy). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.10.010
steels, the 409M grade steel still suffers from grain growth in the heat affected zone and fusion zone results in deterioration of the mechanical properties [5]. Due to the high energy density, electron beam welding (EBW) is attractive for ferritic stainless steel, because of the narrow width of the fusion zone and heat affected zone [6]. EBW is characterised by a short interaction time with intense energy density. As a result, the process has a lower heat input, which is expected to exercise greater control over grain coarsening and distortion [7]. The high joining rate, the deep and narrow weld, and the minimal heat-affected zone are basic advantages of EBW joints [8]. Microstructure and tensile properties of electron beam welded AISI 409M ferritic stainless steel joints were reported in our previous study [9]. As the fatigue failure is one of the prime concerns in structural design and the butt weld is a part of many structures, its evaluation and prediction of fatigue behaviour is very important to avoid catastrophic failure particularly in welded ferritic stainless steel structures. Few studies were reported on the fatigue behaviour of ferritic stainless steel weldments. Akita et al. [10], investigated fatigue crack resistance of gas tungsten arc welded AISI 444 ferritic stainless in both air and 3.5% NaCl aqueous solution. The crack growth rates within heat affected zone and the weld metal were lower compared to the base metal and the author reported that this is mainly due to the high closure level, changes in the residual stress, and hardness changes due to the welding. Taban et al. [11], evaluated the mechanical and metallurgical properties of laser beam welded (LBW) 12 wt% Cr modified ferritic stainless steel joints and it was found that the LBW joint showed superior fatigue strength compared to the base metal though the toughness of weld metal is much lower than that of the base metal. However, data on high-energy density process, namely, EBW of ferritic stainless
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A.K. Lakshminarayanan et al. / Materials and Design 35 (2012) 760–769 Table 1 Chemical composition of the base metal (spectrometry results). C
Cr
Ni
Nb
Cu
Si
Mn
P
S
N
Al
Co
Ti
Va
0.026
11.40
0.4
0.009
0.365
0.45
1.15
0.4
0.16
0.04
0.01
0.2
0.008
0.017
the process parameters used in this investigation are presented in Table 2. The fabricated joints are shown in Fig. 1.
Table 2 Welding conditions and process parameters. Parameters
EBW
GTAW
Current Voltage Shielding gas Gas flow rate (l/min) Gun to work distance Vacuum Welding speed (mm min1) Heat input (J/mm)
51 mA 55 kV – – 298 mm 104 bar 1000 143
120 A 20 V Argon 16 – – 80 1800
steel, are very scant. Moreover, details on fatigue life and fatigue crack growth properties of electron beam welded 409M grade ferritic stainless steels are not yet reported in the literature. The present paper reveals the results obtained from fatigue testing on the electron beam and gas tungsten arc welded AISI 409M ferritic stainless steel joints. The influence of weld metal microstructure, tensile property, and residual stress on fatigue behaviour is discussed on the basis of fatigue life, crack initiation, crack growth and fractographic analysis. 2. Experimental analysis 2.1. Base material The as-received base metal (BM) used in this study was 4 mm thick cold rolled, annealed and pickled AISI 409M grade ferritic stainless steel plates. Final annealing was performed after cold rolling at temperatures below the A1 (between 700 °C and 750 °C). The chemical composition of the base metal is presented in Table 1. 2.2. Welding conditions Trial runs were carried out to optimize the welding parameters so as to attain full penetration, defect free weld in a single pass. Square butt joints were prepared to fabricate GTAW and EBW joints without filler metal additions. The welding conditions and
2.3. Specimens used Hourglass specimens as per ASTM E468-08 [12] were prepared to the dimensions as shown in Fig. 2a, to evaluate the fatigue strength of the welded joints. A centre cracked tensile (CCT) specimen was used to evaluate the fatigue crack growth behaviour with the presence of a sharp notch, which was machined in the weld metal region (Fig. 2b) to the required length using the WEDM. Procedures prescribed by the ASTM E647-09 standard [13] were followed for the preparation and testing of the CCT specimens. 2.4. Procedures Microstructure of the welds was examined using optical and transmission electron microscopy (TEM). Thin foils with a thickness of about 1 mm were cross sectioned from the fusion zone of welded joints to prepare the TEM samples. After being mechanically ground to approximately 80 lm, the foils were further ground to a thickness of 15 lm by a dimple grinding machine. Then the electron transparent thin sections were electrolytically prepared by the twin-jet polisher. An electrolyte of 10% perchloric acid +90% ethanol solution (25 °C) was used during thinning. These thin foils were observed under a 200 kV potential using a Transmission Electron Microscope (Make: Philips, UK: Model: CM20). Fatigue experiments were conducted using a servo hydraulic, 100 kN fatigue testing machine (Make: Instron, UK; Model 8801). A frequency of 15 Hz and a constant amplitude uniaxial tensile loading condition (R = 0.1) was used uniformly for all the specimens. Before loading, the specimen surface near the notch was polished to facilitate the fatigue crack growth measurement. A travelling microscope with an accuracy of 0.01 mm was used to monitor the crack length. The specimen was loaded at a particular stress level (range), and following crack initiation from the tip of the machined notch, its subsequent propagation along the weld metal was recorded from initiation to the complete failure of the specimen. Similar crack growth experiments were conducted on
Fig. 1. Photographs of fabricated joints.
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(a) Hourglass fatigue (SN) specimen
(b) Centre Cracked Tensile (CCT) Specimen Note: W = 10 mm for GTAW joint and 3 mm for EBW joint Fig. 2. Dimensions of fatigue test specimens.
Specimen
Optical Micrographs
Transmission Electron Micrographs
(a)
(d)
(b)
(e)
Basemetal
GTAW
GB Martensite
EBW
Subgrains
(c) Fig. 3. Optical and transmission electron micrographs.
(f)
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a number of specimens at five different stress levels, and the experimental data were recorded. The fractured surface of the fatiguetested specimens were analysed using Scanning Electron Microscope (Make: JEOL, Japan; Model 6410LV) at higher magnification to study the fracture morphology to establish the nature of the fracture. Residual stress analyses were performed on top surface of the welded joints, weld metal region of SN specimens and at the crack tip of the CCT specimens in both perpendicular (rres, t) and parallel (rres, l) to the weld zone axis using X-ray stress analyser (Make: Stresstech OY, Finland; Model: XStress 3000) employing Cr Ka radiation. 3. Results 3.1. Microstructure and hardness Optical and TEM image of base metal (Fig. 3a and d) shows the characteristics of grain boundaries with ferrite grains and the presence of chromium carbides. Fusion zone of GTAW joint showed that the grain boundaries of widmanstatten ferrite were partially covered by the needle shape martensite, (Fig. 3b). Grain growth was observed in the fusion zone. Coarse lath ferrite structure with partially covered needle shape martensite and chromium carbides decorated the grain boundaries was observed in case of GTAW joint (Fig. 3e). Optical microstructure taken at the fusion zone of EBW joint is presented in Fig. 3c. It reveals the presence of multi directional ferrite with isolated martensite laths with various sizes. TEM image of FZ (Fig. 3f) clearly shows the presence of prior d ferrite grain boundary and the substructure and subgrain structure. The presence of chromium carbides and grain boundary martensite are also evidenced from Fig. 3f. This suggests that the equiaxed coarse grain microstructure of the base metal undergone a refinement during weld thermal cycle. The phases present in the weld metal such as ferrite, martensite and chromium carbides were confirmed through X-ray Diffraction (XRD) analysis and reported in our previous study [9]. The microhardness profile at the mid cross section of welded joints is presented in Fig. 4. The hardness of the as-received base metal is approximately 170 HV0.05. The hardness measurements indicate that the hardness values in both the weld metal and heat affected zone of the EBW joint are higher than the base metal. In case of GTAW joint, the weld metal exhibited slightly higher hard-
ness and HAZ exhibited lower hardness compared to the base metal. 3.2. Tensile properties During tensile testing, the transverse tensile specimens prepared from EBW and GTAW joints failed in base metal and heat affected zone respectively. Joint efficiency is the ratio between tensile strength of welded joint and tensile strength of the unwelded parent metal. EBW joints exhibited a joint efficiency of 100% respectively since the failure took place in the base metal region. However, GTAW joint exhibited a joint efficiency of 77%. Since the transverse tensile specimens prepared from EBW joints fractured in the base metal region, flat microtensile specimens were prepared to evaluate the tensile properties of the all weld metal of both the joints. In each condition, three tensile specimens were tested; the mean value of three results is presented in Table 3. Higher yield strength, higher tensile strength and lower fracture elongation were observed in case of EBW welds compared to the base metal. The yield strength, tensile strength and elongation of GTAW joint are lower than the base metal and EBW joint. After yielding the stress–strain relationship in the uniform deformation stage may be expressed by Holloman equation [14]. The yield ratio and strain hardening coefficient values for the base metal and the fusion zone of EBW and GTAW joints were evaluated and are presented in Table 3. 3.3. Residual stress Longitudinal and transverse residual stress components were measured on top surface of the EBW and GTAW joints as a function of distance from the weld centreline and are presented in Fig. 5. The surface residual stress (both transverse and longitudinal) perpendicular to the welding direction (at the weld centre) is of tensile in nature for both the joints. However the EBW joint showed lower residual stress compared to the GTAW joint. The fatigue specimens that were used are small in size relative to the welded joints. Redistribution and relaxation of residual stresses could occur in the specimens when they are machined from the welded joints and ground to the required dimensions. Hence, both the longitudinal and transverse residual stresses were measured at the weld metal region of SN specimen and at the crack tip of the CCT specimen and the results are presented in Table 4. The results indicate that a significant amount of residual stresses can still be present after sectioning and grinding the fatigue specimen extracted from the welded plate. These residual stresses will have significant effects on the fatigue initiation and propagation during the fatigue cycling. 3.4. Fatigue (SN) behaviour Specimens were tested at five different stress levels (125– 325 MPa). Four specimens were tested at each stress level and the mean value was used to analyze the results. SN curves (Fig. 6) were constructed for the base metal and the welded joints.
Table 3 Tensile properties of the base metal and electron beam welded AISI 409M FSS joints.
Fig. 4. Hardness profile at mid-cross section of EBW and GTAW joints.
Specimen
Yield strength (MPa)
Ultimate tensile strength (MPa)
Yield ratio
Strain hardening coefficient (n)
Elongation in 50 mm gauge length (%)
BM EBW GTAW
364 418 325
536 790 420
0.68 0.53 0.77
0.14 0.28 0.16
31 28 18
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(a) EBW Joint
(b) GTAW Fig. 5. Longitudinal and transverse residual stress profile.
Table 4 Residual stress summary. Joint
Magnitude of residual stress in weld metal (WM) region (MPa) Longitudinal
EBW GTAW
Transverse
As welded
Hourglass (SN) specimen
CCT specimen
As welded
Hourglass (SN) specimen
CCT specimen
+210 ± 14.4 +243 ± 12.4
+90 ± 10.2 +130 ± 8.0
+60 ± 6.7 +110 ± 10.5
+50 ± 5.6 +80 ± 8.8
20 ± 6.0 45 ± 10.0
50 ± 10.8 30 ± 12.4
The confidence and predicted interval of the fatigue strength values for the base metal and welded joints are presented in Fig. 7. For comparison, the stress corresponding to 2 106 cycles in the mean curve (Fig. 6a) was taken as fatigue strength of the welded joints and they are presented in Table 5. It is inferred that the EBW joint endured more number of cycles than the base metal and GTAW joint. The fatigue strength of unwelded AISI 409M grade
ferritic stainless steel is 185 MPa. But the fatigue strength of EBW joint is 195 MPa. This indicates that there is a 5% enhancement in fatigue strength values due to electron beam welding. However, GTAW joint shows the lowest fatigue strength of 150 MPa which is 19% lower than the base metal. The EBW joint exhibited 23% higher fatigue strength than the GTAW joint. Fatigue test results of welded joints are also compared with the fatigue design curves
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BM
Fig. 6. SN curves.
recommend by IIW [15]. It can be seen from Fig. 6, that fatigue curve of the both the joints fall above the FAT 112 line which is the highest limit for fusion welded steel. The effect of residual stress on fatigue strength is also presented in Fig. 6. The expression proposed by SWT (Smith, Watson and Topper method) [16] was used to account for the effect of residual stress on fatigue strength. Eq. (1) was used to find out the stress ratio to include the effect of residual stress [17].
R¼
rmin rmean þ rres Dr=2 ¼ rmax rmean þ rres þ Dr=2
GTAW ð1Þ
It is inferred that the fatigue strength values are lower compared to the base metal when the residual stress is accounted in SN curves. However, the EBW joint exhibited higher fatigue strength when compared to the GTAW joint due to the lower tensile residual stress in the weld metal region (Table 4). 3.5. Fatigue Crack Growth (FCG) behaviour The fatigue crack growth experiments were conducted at five different stress levels (Dr) of 75, 100, 125, 150, and 175 MPa under constant amplitude loading conditions (R = 0.1). The measured variation in crack length (2a) and the corresponding number of cycles (N) endured under the action of particular applied stress range are plotted as shown in Fig. 8, for the BM and welded joints. The fracture mechanics based Paris Power equation [18], was used to analyze the experimental results. The procedure prescribed in ASTM E647 standard was used to analyse the results. The crack growth rate da/dN for the propagation stage was calculated for the steady-state growth regime, at different intervals of crack length increment, against the associated number of cycles to propagation. The relationship between stress intensity factor (SIF) range and the corresponding crack growth rate in terms of best fit lines is shown in Fig. 9 for the BM and the welded joints. The data points plotted in the graph mostly correspond to the second stage of Paris sigmoidal relationship (106–103 mm/cycle). When the crack growth rate was around 103 mm/cycle, unstable crack growth occurred and the corresponding DK value was taken as critical SIF (DKcr). The value was not taken at the point of fracture, but was taken at the point of transition at which steady-state crack growth changes into unsteady state. Of the two joints, joint fabricated by
EBW Fig. 7. Confidence and predicted intervals for fatigue strength of welded joints.
EBW process exhibited superior fatigue crack resistance compared to its counterpart. The EBW joint exhibited a critical SIF of p 32 MPa m which is 77% and 19% higher compared to the GTAW joint and the base metal. GTAW joint showed critical SIF of p 18 MPa m which is 33% lower compared to the base metal. 3.6. Fracture surface analysis The fatigue fracture surface appearance corresponding to crack initiation, crack propagation and the final failure regions of the base metal, EBW and GTAW joints (150 MPa alone), as observed
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A.K. Lakshminarayanan et al. / Materials and Design 35 (2012) 760–769 Table 5 Fatigue properties of base metal and welded joints. Joint type
Fatigue strength of the unnotched specimens at 2 106 cycles (MPa)
Fatigue strength of the notched specimens at 2 106 cycles (MPa)
Fatigue notch factor (Kf)
Notch sensitivity factor (q)
BM EBW GTAW
185 195 150
140 150 95
1.32 1.27 1.57
0.32 0.28 0.46
3.6.1. Initiation In both the base metal and the welded joints, the crack initiations sites are clearly visible (indicated by the arrow marks) and it can be observed from the fractographs that the fatigue cracks have initiated from multiple crack initiation sites (Fig. 10a, d and g) irrespective of welding processes used. Large number of crack initiation sites is visible in the fracture surface of the GTAW joint when compared to the EBW joint and the base metal.
Fig. 8. Fatigue crack growth curves.
3.6.2. Crack propagation Fig. 10b, e and h, reveal the fracture surface appearance of fatigue crack propagation (FCP) region (where steady state crack growth occurs) of the base metal, EBW and GTAW joints respectively. The fractograph of base metal shows the presence of striations in the facets itself (Fig. 10b). The striations are parallel and are at right angles to the local direction of crack propagation. Thus the arrows placed in the fractographs represent the crack propagation path. Fracture surface of EBW joints displays a progression of rather fine, but extremely irregular striations, separated here and there by secondary cracks (Fig. 10e). This is mainly due to the microlevel crack deviation by the dual phase lath ferrite with fibrous martensite microstructure of fusion zone. Fractograph of GTAW joint (Fig. 10h) shows the change from transgranular facets to dimples. The presence of secondary cracking and second phase particle (chromium carbides) accelerates the crack growth which led to larger spacing between the striation marks. 3.6.3. Final failure Fig. 10c, f and i, shows the fracture surface appearance of final failure region (FF) region (where unstable crack growth occurs) of the base metal and the welded joints. Even though unstable crack growth occurs in the final failure region, the final fracture is still in the ductile mode and it is evident from the presence of dimples. The fractograph of base metal presented in Fig. 10c reveals the structure of ductile mode failure with elongated dimples. The macro-level cracks in the transverse direction of the fatigue testing specimen also evidenced (i.e. splitting). The fracture surface of EBW joint (Fig. 10f) shows fibrous dimples along with the patches of fine dimples. GTAW joint fractograph shows the presences of many voids, which bear resemblance to conventional intergranular failure, are visible in addition with small area having fine dimples. The coarser facets are the characteristic final failure mode of GTAW joint is also evidenced from the fractograph (Fig 10i). Stair-step fracture surface indicative of Stage I fatigue fracture is also observed in Fig. 10i. 4. Discussion
Fig. 9. Relationship between FCGR and SIF range.
under the Scanning Electron Microscope (SEM) are displayed in Fig. 10. The fatigue crack initiation (FCI) region is corresponding to 1 mm from the tip of the machined notch; fatigue crack propagation (FCP) region is referred to 1–6 mm; final failure (FF) region is referred to 6 mm away from the crack initiation region.
The fatigue crack initiation behaviour, fatigue crack growth behaviour and fatigue strength of the ferritic stainless steel is influenced by the welding processes. Of the two joints, EBW joint offered higher fatigue life and crack growth resistance than the GTAW joint. In the CCT specimen, the notch is machined in the weld metal region of the joints by WEDM process to evaluate the crack growth behaviour under fatigue loading. The fatigue crack initiates from the tip of the machined notch and it grows in the weld metal region
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Crack Initiation
Crack Propagation
Final Failure
(a)
(b)
(c)
Base metal
EBW
(d)
(e)
(f)
(g)
(h)
(i)
Note: R: Rubbed Marks; SC – Secondary Cracks; P – Propagation Path Fig. 10. SEM fractographs of CCT specimen of welded joints.
until final failure takes place and hence the weld metal properties such as tensile property and impact toughness, microstructure and magnitude of the residual stress will have greater influence on fatigue performance of the joints. The effect of these properties on fatigue behaviour is discussed in the following sections. 4.1. Influence of microstructure Microstructure is known to affect fatigue strength, the crack initiation and propagation of the crack present in the weld metal region. EBW joint exhibited higher fatigue strength and fatigue crack growth resistance than the GTAW joint and base metal. This can be explained with the help of weld metal microstructure of the welded joints. Fine lath ferrite with fibrous martensite in the EBW joint is the most efficient in transferring load when compared to the coarse microstructure of the GTAW joint. This is mainly because, the load transfer takes place by shear action along the martensite/ferrite interface, and more interfacial energy is available, when the shape is fine continuous fibrous lath structure [19]. Coarse ferrite structure with discontinuous martensite structure normally results in inhomogeneous deformation and restricts initial plastic flow to a smaller fraction of the total volume of the ferrite matrix. Also, void growth occurs at a faster rate and with less plastic strain resulted in reduced ductility [20]. Early initiation of multiple cracks and faster propagation by linking up through the cracks in the coarse microstructure results in a lower fatigue life of the GTAW joints compared to their counterparts. An additional crack closure mechanism as explained by Sudhakar and Dwarakadasa [21], also contributes to the retardation of
crack growth. During fatigue loading, the martensite will be the load bearing phase, thus it shields the crack tip and thereby reduces the crack driving force at the crack tip. It is also shown that the martensite in a dual phase structure has the ability to constrain the plastic deformation of the ferrite phase [22]. This constraining effect is primarily a function of distance from the crack tip to the martensite. This distance causes the crack growth in the ferrite phase to decelerate as it approaches the martensite. The martensite with lower carbon content is tougher, and thereby effective in reducing the crack tip driving force by mechanisms of crack tip blunting and deflection [23]. As reported by Jang et al. [24], the fatigue crack mostly follows the directions of the dendrites along the ferrite phase, suggesting that the martensite/ferrite boundary is the preferred crack growth path. However, when the dendrite alignment is not favourable to the crack growth along the boundary, the crack grows across the boundary, resulting in a very wavy and tortuous crack path. The change in crack path results in the formation of secondary cracks. This is also one of the reasons for the reduced weld metal fatigue crack growth rate of EBW joints. It seems reasonable to believe that the reduction in crack growth rate of EBW joint is to a large extent caused by microstructural effects. 4.2. Influence of tensile properties The yield strength has a major influence on the fatigue life and fatigue crack behaviour of welded joints [25]. Higher yield strength and tensile strength of the electron beam welded ferritic stainless steel joint is greatly used to enhance the fatigue strength by delay-
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ing the fatigue crack initiation. The major reason for the improvement in fatigue strength also results from the higher work hardening rate of fine dual phase ferritic–martensitic structure. The size and the morphology of the hard phase in a dual phase material determine the material’s strengthening mechanism [26]. Due to the constraint exerted by the martensitic phase on the ferrite matrix, the strength of fusion zone was increased. When deformation is applied, the material in the vicinity of the martensite phase is prevented from deforming freely, resulting in an increase in the shear stresses required to produce further deformation. This type of ferritic–martensitic dual phase structure displays higher workhardening rate and higher flow strength than those of the single phase ferritic structure. Since the crack tip stress–strain field is influenced by the strain hardening exponent [27] of the material, the fracture at the crack tip process zone is affected and hence the crack propagation is delayed. In high strength weld metal, the plastic zone can easily extend into the parent material because the deformation and yielding occur in both fusion zone and the base metal. The stress relaxation can easily take place in the crack tip region. So more crack driving force is needed for crack extension and the fracture resistance of the higher strength weld metal is greater than the lower strength weld metal [28]. This is also one of reasons for the improved fatigue crack growth resistance of electron beam weld metal region compared to the weld metal of GTAW joint. 4.3. Influence of residual stress The residual stresses developed during the welding process can have a significant effect on the service performance of the welded material with respect to fatigue properties, and fatigue crack growth process. It is particularly important in high power density welding of thin plates which are highly susceptible to distortion and significantly deteriorate the fatigue behaviour [29]. In this study, tensile residual stress was observed in both the longitudinal (parallel to the crack growth) and transverse (perpendicular to the crack growth) directions irrespective of the welding processes used. The residual stress measurement was carried out mainly to understand the effect of heat input of welding processes and also to know the initial condition (stress state) of the test specimens of the two autogenous fusion welding processes used in this investigation to fabricate the AISI 409M grade FSS, the heat input involved in GTAW process is relatively higher compared EBW process. Hence the magnitude of tensile residual stress is higher in GTAW joint compared to EBW joint and this is also evident from the measured residual stress values presented in Table 5. Even though the compressive load acts on the specimen during grinding is uniform, the resultant residual stress values are different and this may be due to the differences existed in the original (as welded condition) tensile residual stress values. Lower tensile residual stress in the transverse direction joint resulted in improved fatigue life of EBW over the GTAW joint. Higher compressive residual stress perpendicular to the weld in CCT specimen acts beneficially in the enhancement of fatigue crack growth resistance of EBW joint compared to the GTAW joints. From the above discussion, it can be concluded that the enhanced fatigue life and improved fatigue crack growth resistance of EBW joint over the base metal is due to the synergetic effect of dual phase microstructure, superior tensile properties and favourable residual stress field of weld metal region. 5. Conclusions Fatigue life, crack initiation and growth rates in relation to microstructure, tensile properties and residual stresses were studied in
electron beam and gas tungsten arc welds of AISI 409M ferritic stainless steel. From this investigation, the following important conclusions were derived: The fatigue strength of the 409M ferritic stainless steel was improved by electron beam welding. The fatigue strength of EBW joint is 195 MPa at 2 106 cycles and the enhancement in fatigue strength value is approximately 19% compared to the GTAW joint. The fatigue strength of both the welded joints was lower than that of the base metal due the presence of tensile residual stress in the weld metal region of hourglass specimen. However, the EBW joint exhibited higher fatigue strength when compared to the GTAW joint due to the lower tensile residual stress field. p Higher critical SIF range (32 MPa m) was observed for the EBW joint than the GTAW joint. The enhancement in the critical SIF range was 77% compared to the GTAW joint. Superior tensile properties, dual phase lath ferritic–martensitic microstructure and favourable residual stress perpendicular to the crack growth are the main reason for the improved fatigue performance of the EBW joint compared to the GTAW joint.
Acknowledgements The authors wish to place their sincere thanks to the Department of Science and Technology (DST), Government of India for the financial support through Fast Track Scheme for Young Scientists R&D Project (SR/FTP/ETA043/2009) to carry out this investigation. In addition, the authors wish to record their sincere thanks to the referee’s for their constructive and valuable feedback. References [1] Amuda MOH, Mridha S. Grain refinement in ferritic stainless steel welds: the journey so far. Adv Mater Res 2010;83–86:1165–72. [2] Deleu E, Dhooge A, Taban E, Kaluc E. Possibilities and limitations to improve the weldability of low carbon 12Cr ferritic stainless steel for expanded industrial applications. Weld World 2009;53:R198–208. [3] 409M – Technical Data. Detail brochure. India: Steel Authority of India Limited (SAIL); 2004. p. 13–26. [4] Meyer AM. Interstitial diffusion from the weld metal into the high temperature heat affected zone in 11–12 percent chromium steel welded joints. MEng thesis. University of Pretoria, South Africa; 2000. [5] Taban E, Deleu E, Dhooge A, Kaluc E. Gas metal arc welding of modified X2CrNi12 ferritic stainless steel. Kovove Mater 2007;45:67–74. [6] Penasa M, rivela C. Application of the laser welding process to low thickness stainless steels. Weld Int 2003;17:947–57. [7] Yilbas BS, Sami M, Nickel J, Coban A, Said SAM. Introduction into the electron beam welding of austenitic 321-type stainless steel. J Mater Process Technol 1998;82:13–20. [8] Huang CA, Wang TH, Han WC, Lee CH. A study of the galvanic corrosion behavior of Inconel 718 after electron beam welding. Mater Chem Phys 2007;104:293–300. [9] Lakshminarayanan AK, Balasubramanian V, Madhusudhan Reddy G. Microstructure and mechanical properties of electron beam-welded AISI 409M-grade ferritic stainless steel. Int J Adv Manuf Technol 2011;55: 153–62. [10] Akita M, Nakajima M, Tokaji K, Shimizu T. Fatigue crack propagation of 444 stainless steel welded joints in air and in 3%NaCl aqueous solution. Mater Des 2006;27:92–9. [11] Taban E, Deleu E, Dhooge A, Kaluc E. Laser welding of modified 12% Cr stainless steel: strength, fatigue, toughness, microstructure and corrosion properties. Mater Des 2009;30:1193–200. [12] ASTM E468 – 08. Standard practice for verification of constant amplitude dynamic forces in an axial fatigue testing system. ASTM International; 2008. [13] ASTM E 647 – 05. Standard test method for measurement of fatigue crack growth rates. ASTM International; 2005. [14] Hollomon JH. Tensile deformation. Am Inst Mining Metall Eng Trans Iron Steel Div 1945;162:268–89. [15] Hobbacher AF. Recommendations for fatigue design of welded joints and components. Paris, France: International Institute of Welding; 2008. p. 41–2 [doc. XIII-2151r4-07/XV-1254r4-07] [16] Ince A, Glinka G. A modification of Morrow and Smith–Watson–Topper mean stress correction models. Fatigue Fract Eng Mater Struct 2011;00:1–14.
A.K. Lakshminarayanan et al. / Materials and Design 35 (2012) 760–769 [17] Nicholas T. High cycle fatigue – a mechanics of materials perspective. Elsevier Ltd.; 2006. p. 119. [18] Paris P, Erdogan F. A critical analysis of crack propagation laws. J Basic Eng Trans ASME 1963:528–34. [19] Bayram A, Uguz A, Ula M. Effects of microstructure and notches on the mechanical properties of dual-phase steels. Mater Char 1999;43: 259–69. [20] Mediratta SR, Ramaswamy V, Rama Rao P. Influence of ferrite–martensite microstructural morphology on the low cycle fatigue of a dual-phase steel. Int J Fatigue 1985;7(2):107–15. [21] Sudhakar KV, Dwarakadasa ES. A study on fatigue crack growth in dual phase martensitic steel in air environment. Bull Mater Sci 2000;23(3):193–9. [22] Wang ZG, Al SH. Fatigue of martensite–ferrite high strength low-alloy dual phase iron and steel. Res Inst Jpn 1999;39(8):747–59. [23] Deng RY, Ye ZJ. Fatigue crack growth rate in ferrite–martensite dual-phase steel. Theor Appl Fract Mech 1991;16:109–22.
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[24] Jang C, Cho PY, Kim M, Oh SJ, Yang JS. Effects of microstructure and residual stress on fatigue crack growth of stainless steel narrow gap welds. Mater Des 2010;31:1862–70. [25] Okayasu M, Sato K, Mizuno M, Hwang DY, Shin DH. Fatigue properties of ultrafine grained dual phase ferrite/martensite low carbon steel. Int J Fatigue 2008;30:1358–65. [26] Suzuki H, Mcevily AJE. Microstructural effects on fatigue crack growth in a low carbon steel. Metall Trans A Phys Metall 1979;10-A:475–81. [27] Laird C, Buchinger L. Hardening behavior in fatigue. Metall Mater Trans A 1985;16(12):2201–14. [28] Zhang H, Zhang Y, Li L, Ma X. Influence of weld mis-matching on fatigue crack growth behaviours of electron beam welded joints. Mater Sci Eng A 2002;334:141–6. [29] Ferro P, Zambon A, Bonollo F. Investigation of electron-beam welding in wrought Inconel 706—experimental and numerical analysis. Mater Sci Eng A 2005;392:94–105.