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Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks
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FUSION-8524; No. of Pages 5
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Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks Hun-Chea Jung a,∗ , Sa-Woong Kim a , Yun-Hee Lee b , Seung-Wook Baek c , Min-Su Ha a , Hee-Jin Shim a a
ITER Korea, National Fusion Research Institute, Daejeon, Republic of Korea Division of Convergence Technology, Korea Research Institute of Standard and Science (KRISS), Daejeon, Republic of Korea c Division of Industrial Metrology, Korea Research Institute of Standard and Science (KRISS), Daejeon, Republic of Korea b
h i g h l i g h t s • • • • •
PWHT for ITER blanket shield block should be performed for dimensional stability. Investigation of the effect of PWHT conditions on properties was performed. Instrumented indentation method for evaluation of properties was used. Residual stress and hardness decreased with increasing PWHT temperature. Optimization of PWHT conditions would be needed for satisfaction of requirement.
a r t i c l e
i n f o
Article history: Received 12 September 2015 Received in revised form 30 January 2016 Accepted 1 February 2016 Available online xxx Keywords: Shield block Welding Post weld heat treatment Heat treatment condition
a b s t r a c t The blanket shield block (SB) shall be required the tight tolerance because SB interfaces with many components, such as flexible support keypads, First Wall (FW) support contact surfaces, FW central bolt, electrical strap contact surfaces and attachment inserts for both FW and Vacuum Vessel (VV). In order to fulfil the tight tolerance requirement, stress relieving shall be performed for dimensional stability after cover welding operation. In this paper, effect of Post Weld Heat Treatment (PWHT) conditions, temperature and holding time, was investigated on the residual stress and hardness. The 316L Stainless Steel (SS) was prepared and welded by manual TIG welding by using filler material with 2.4 mm of diameter. Welded 316L SS plate was machined to prepare the specimen for PWHT. PWHT was implemented at 250, 300, 400 ◦ C for 2 and 3 h (400 ◦ C only) and residual stress after relaxation were determined. The evaluation of residual stress and hardness for each specimen was carried out by instrumented indentation technique. The residual stress and hardness were decreased with increasing the heat treatment temperature and holding time. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The main function of blanket SB is to provide the nuclear shielding to VV and external components and to absorb the radiation and particle heat fluxes from the plasma. The Full Scale Prototype (FSP) of blanket SB has been manufactured and tested in accordance with pre-qualification program by Korean Domestic Agency (KODA). And also, several R&D activities related to manufacturing of SB such as drilling, cover welding and slitting have been issued and implemented to resolve the tolerance issues [1–3].
∗ Corresponding author. E-mail address: [email protected] (H.-C. Jung).
The blanket SB, which located inside ITER main vacuum chamber and behind the FW, shall be required the tight tolerance because SB interfaces with many components, such as flexible support keypads, FW support contact surfaces, FW central bolt, electrical strap contact surfaces and attachment inserts for both FW and VV. In order to fulfil the tight tolerance requirement, the stress relieving for dimensional stability shall be performed after cover welding operation. The stress relieving can be done by various procedures like: optimal welding sequence, heat treatment or mechanically (i.e., shot peening, ultrasonic or vibratory treatment, etc.). For stress relieving with heat treatment, heat treatment conditions, such as temperature, holding time, heating and cooling rate are factors for determining the microstructure and mechanical properties. For dimensional stability of austenitic stainless steel using heat
http://dx.doi.org/10.1016/j.fusengdes.2016.02.007 0920-3796/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: H.-C. Jung, et al., Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.007
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ARTICLE IN PRESS
FUSION-8524; No. of Pages 5
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2
Indentaon load (L)
Pm
Loading S
Unloading
1
Indentaon depth (h)
hm
Fig. 1. Schematic picture of an indentation cycle.
Fig. 3. Tube furnace (LTF-15).
treatment, the maximum temperature and temperature difference in the component shall not exceed 425 ◦ C and 55 ◦ C, respectively [4,5]. In the welding of ITER blanket SB, Welding Procedure Qualification Test (WPQT) should be performed prior to the start of welding activity. In WPQT, the mechanical test, such as tensile, bend, hardness, etc., should be carried out and satisfied with requirement in accordance with related standard. Therefore, it is important to check the change of the residual stress as well as mechanical properties on the PWHT. The objective of this paper is to investigate the effect of PWHT conditions, temperature and holding time, on residual stress and hardness from a part of optimization of PWHT conditions. The 316L SS plate was prepared and welded by manual TIG welding by using filler material with 2.4 mm of diameter. The residual stress and hardness were measured by using instrumented indentation technique for investigation of correlation between PWHT conditions and properties.
on friction stir-welded API X80 steel. He reported that the instrumented indentation technique is a promising non-destructive residual stress measurement technique if there is sufficient distance between indentations. 2.1. Measurement of hardness The hardness was measured by using Depth Sensing Indentation (DSI) method introduced by Oliver and Pharr [8]. This is based on the mean contact pressure at the maximum indentation depth with the following expression [7–9]; H=
Pm Pm = AC f (hC )
(1)
where Pm is the peak load at the indenter displacement hm , AC is the projected contact area and function of geometry of indenter. For Vickers indenter, AC = 4 h2 c tan2 =24.5 h2 c (semi apical angle is 68◦ ). hc is the contact depth at the maximum load. The contact depth can be given as; Pm S
2. Instrumented indentation technique
hC = hm = ˛
Two types of methods have been applied for measurement of stress relieving; mechanical stress-relaxation method (e.g., holedrilling and saw-cutting) and physical-parameter analysis methods (X-ray diffraction, ultrasonic wave and neutron diffraction). Even if both methods have been applied generally, they have some difficulties to measurement of residual stress, which are destructive nature limits the wide application in industry for mechanical stress-relaxation and difficulty to separate intrinsic microstructural effects on the physical parameters from the effects of a residual stress for physical-parameter analysis methods [7]. The instrumented indentation technique is tool for measuring mechanical properties, such as hardness, young’s modulus, fracture toughness, only by analyzing the indentation load-displacement curve as shown in Fig. 1. Lee et al. [10] verified the validity of instrumented indentation method by comparing the energydispersive X-ray diffraction method for evaluation of residual stress
where S is the slope of the initial part of the unloading curve and ˛ is an indenter geometry constant, equal to 0.75 for Vickers indenters.
(2)
2.2. Measurement of residual stress If an arbitrary indentation state (ht , L0 ) is attained in an unstressed status and if the tensile or compressive in-plane stress, res , is applied to the loading state at the fixed penetration depth, ht , the indentation load, L0 , will be changed to a load, L, due to the change of surface penetration resistance. The load shift, L − L0 , due to the tensile or compressive stress application becomes a clue for stress quantification. The surface-normal deviator stress, D , is −2 res /3 by removing the hydrostatic stress 2 res /3 from the surface residual stress, res , and is added to the contact pressure. L − L0 is assumed to be the product of the selected deviator stress
Fig. 2. 316L SS plate and the joint design.
Please cite this article in press as: H.-C. Jung, et al., Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.007
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component and its corresponding contact area, AR C . Thus, an equation for the equi-biaxial residual stress is derived in terms of the indentation load and contact area as; res =
3(L0 − L) 2A2C
(3)
Here, L0 and L is the indentation load in relaxation and as-weld state, respectively and AR C is contact area in the tensile or compressive stress state as described Eq. (1). In order to determine the magnitude of actual biaxial stress, Eq. (3) developed as below [10]; res =
3(L0 − L) (1 + k)ARC
(4)
Here, k is stress ratio ( res,y / res,x ) and the Eq. (4) converges to Eq. (3), when a stress state approaches to the equi-biaxial state or k = 1.0. Several preliminary observations on the welded joints yielded a stress ratio of about 0.33, and this value was used in subsequent biaxial stress analyses [10,11]. 3. Experiment 3.1. Preparation of test specimen In this study, 316L stainless steel plate with 20 mm thickness was prepared. The 316L stainless steel plate with dimensions of 150 mm (w) × 400 mm (l) × 20 mm (t) was machined with single U groove, with groove angle of 12◦ and 1 mm root face as shown in Fig. 2. Narrow gap Tungsten Insert Gas (TIG) arc welding process was performed with filler metal with 2.4 mm diameter. The root gap was maintained 2 mm. The current and voltage for each pass were applied in the range of 100 ∼ 180 A and 10 ∼ 25 V, respectively. The inter-pass temperature between weld passes shall be maintained (or keep) 150 ◦ C temperature, in order to avoid hydrogen attack and crack susceptibility to the weld. After welding, test specimen with dimension of 50 mm (w) × 20 mm (h) × 5 mm (t) were machined from welded 316L stainless steel plate. 3.2. Post weld heat treatment PWHT process was implemented by using tube furnace (LTF-15, Lenton, UK) as shown in Fig. 3. Two types of test groups were prepared to investigate the effect the temperature and holding time on the residual stress and hardness. First, In order to investigate the effect of PWHT temperature on the residual stress, PWHT performed at 250 ◦ C, 300 ◦ C, 400 ◦ C for 2 and 3 h (400 ◦ C only) under Ar atmosphere. The heating and cooling rate of 1 ◦ C/min was applied. According to RCC-MR, the temperature difference in the components shall not be exceed 55 ◦ C [4]. In previous study [6], it was found out that the temperature control for below 40 ◦ C of temperature difference between center and edge of SB FSP, is possible with 1 ◦ C/min of heating and cooling rate. After PWHT, the surface of test specimen was etched for instrumented indentation test. 3.3. Instrumented indentation test In order to measure the residual stress and hardness, the instrumented indentation test by using commercial AIT-U system (Frontics Corp., Korea) with depth and load resolutions of 0.1 mand 5.6 gf was performed as show in Fig. 4. Hardness and residual stress was measured by using depth control of 100 m-. The indenter was applied Vickers-type. The distance between indentations was applied 2.5 mm, which is greater than 3 times the indent diameter (500 m).
Fig. 4. Indentation machine (AIT-U system).
4. Results Fig. 5 shows the experimental indentation curves from the test specimen at 0 mm (Fig. 5(a)) and 2 mm (Fig. 5(b)) of distance from weld of center. All tests clearly indicated the indentation curves without indenter drops due to voids or other defects. The residual stress at 0 mm (weld region) and 2 mm (near HAZ) has positive (tensile) and negative sign (compressive), respectively. During the welding, non-uniform heat distributions and plastic deformations occur on the material due to introduced high heat input to the material. Residual stresses induced by shrinkage of the molten region are usually tensile. The compressive residual stress is dominant in the HAZ where the temperature exceeds the critical values [12]. Fig. 6 shows the effect of PWHT temperature and holding time on the residual stress, which is calculated by Eq. (4). The residual stress decreased with increasing the PWHT temperature and holding time. In weld region, the tensile residual stress of test specimen (400 ◦ C, 2 h) is about 750 MPa. Away from the weld region, the tensile residual stress was decreased and observed the compressive residual stress. In order to verify the reliability of indentation hardness, Vickers hardness test with load of 2 kgf was performed for specimen before heat treatment. Fig. 7 shows the measured indentation hardness and Vickers hardness results for specimen before heat treatment. The indentation hardness is underestimated compared with Vickers hardness. Oliver [8] reports that for Eq. (2) based on the contact area under load, it may deviate from the traditional hardness measured from the area of the residual hardness impression if there is significant elastic recovery during unloading. Sawa [13] investigated the correlation between indentation and Vickers hardness by
Please cite this article in press as: H.-C. Jung, et al., Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.007
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Fig. 5. Experimental indentation curves from the test specimen at 0 mm (a) and 2 mm (b) of distance from weld center.
Fig. 6. Effect of PWHT temperature and holding time on residual stress relaxation.
using several materials, which have different hardness. He explains that indentation hardness and Vickers hardness have relationship using a coefficient C as below; HIT = CHV
(5)
Fig. 7. Relationship between indentation hardness and Vickers Hardness.
Also, he reports that this equation can apply to the materials other than extremely low hardness materials by using 1.25 as
Please cite this article in press as: H.-C. Jung, et al., Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.007
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The follow conclusions are obtained. (1) The residual stress decreased with increasing the PWHT temperature and holding time. Although the indentation hardness decreased with increasing the PWHT temperature and holding time, the additional studies to reduce the hardness after PWHT should be needed for satisfaction of requirement in further work. Acknowledgments This research was supported by the Ministry of Science, ICT and Future Planning of Republic of Korea under the Korean ITER project contract (2007-2006983). ITER is a Nuclear Facility INB-17. Fig. 8. Effect of PWHT temperature and holding time on indentation hardness.
References coefficient C. It can be confirmed that the recalculated indentation hardness by Eq. (5) is coincided with Vickers hardness. Fig. 8 shows the effect of PWHT conditions on the indentation hardness. The indentation hardness of each specimen was calculated by Eqs. (2) and (5). The indentation hardness decreased with increasing temperature and holding time. In the welding region, the indentation hardness of test specimen (400 ◦ C, 2 h) is about 2.2 GPa, which is about 91% of indentation hardness for test specimen before PWHT. According to related standard [14], the permitted maximum hardness of heat treated material is about 77 ∼ 85% of maximum hardness for non-heat treated material. In PWHT, the microstructure and mechanical properties of welded components are depended on the PWHT conditions, such as temperature, holding time, heating and cooling rate, etc. Especially, in cooling process in which SB is cooled down to room temperature, the shrinkage produces residual stress since the cover plate is fixed at both ends. Therefore, it needs to reduce the heating and cooling rate for satisfaction of requirement and the additional studies should be needed for satisfaction of requirement in further work. 6. Conclusions In this paper, effect of PWHT conditions, temperature and holding time, was investigated on the residual stress and hardness. The 316L stainless steel was prepared and welded by manual TIG welding. The evaluation of residual stress and hardness for each specimen was carried out by instrumented indentation technique for investigation of correlation between PWHT conditions and properties.
[1] D.H. Kim, et al., Status of design and R&D for ITER blanket in Korea, Fusion Sci. Technol. 64 (2013) 126–130. [2] S.W. Kim, et al., Manufacturing and testing of full scale prototype for ITER blanket shield block, Fusion Eng. Des. 93 (2015) 69–75. [3] M.S. Ha, et al., Current status of final design and R&D for ITER blanket shield blocks in Korea, Nucl. Fusion 55 (2015). [4] RCC-MR, Design and Construction Rules for Mechanical Components of Nuclear Installations, afcen, 2007. [5] T. Maysa, et al., Microstructure and intergranular corrosion of the austenitic stainless steel 1.4970, J. Nucl. Mater. 358 (2006) 40–46. [6] S.W. Kim et al. Verification of dimensional stability on ITER blanket shield block after stress relieving International Symposium on Fusion Nuclear Technology 12. [7] J.I. Jang, Estimation of residual stress by instrumented indentation: a review, J. Ceram. Process. Res. 10 (3) (2009) 391–400. [8] W.C. Oliver, G.M. Pharr, Measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodolog, J. Mater. Res. 19 (1) (2004) 3–20. [9] J. Gubicza, et al., A new method for hardness determination from depth sensing indentation tests, J. Mater. Res. 11 (1996) 2964–2967. [10] Y. Lee, et al., Using the instrumented indentation technique for stress characterization of friction stir-welded API X80 stee, Philos. Mag. 86 (33–35) (2006) 5497–5504. [11] C.A. Charitidis, et al., Residual stress and deformation mechanism of friction stir welded aluminum alloys by nanoindentation, Mate. Sci. Eng. A 540 (2012) 226–234. [12] N.S. Rossini, Method of measuring residual stress in components, Mater. Des. 35 (2012) 572–588. [13] T. Sawa, Correlation between nanoindentation test result and vickers hardness, IMEKO 2010 TC3 TC5 and TC22Conferences 2016. [14] CEN, Specification and qualification of welding procedures for metallic materials-Welding procedure test − part 1: Arc and gas welding of steels and arc welding of nickel and nickel alloys 2004 Edition.
Please cite this article in press as: H.-C. Jung, et al., Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.007
ARTICLE IN PRESS
FUSION-8524; No. of Pages 5
Fusion Engineering and Design xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks Hun-Chea Jung a,∗ , Sa-Woong Kim a , Yun-Hee Lee b , Seung-Wook Baek c , Min-Su Ha a , Hee-Jin Shim a a
ITER Korea, National Fusion Research Institute, Daejeon, Republic of Korea Division of Convergence Technology, Korea Research Institute of Standard and Science (KRISS), Daejeon, Republic of Korea c Division of Industrial Metrology, Korea Research Institute of Standard and Science (KRISS), Daejeon, Republic of Korea b
h i g h l i g h t s • • • • •
PWHT for ITER blanket shield block should be performed for dimensional stability. Investigation of the effect of PWHT conditions on properties was performed. Instrumented indentation method for evaluation of properties was used. Residual stress and hardness decreased with increasing PWHT temperature. Optimization of PWHT conditions would be needed for satisfaction of requirement.
a r t i c l e
i n f o
Article history: Received 12 September 2015 Received in revised form 30 January 2016 Accepted 1 February 2016 Available online xxx Keywords: Shield block Welding Post weld heat treatment Heat treatment condition
a b s t r a c t The blanket shield block (SB) shall be required the tight tolerance because SB interfaces with many components, such as flexible support keypads, First Wall (FW) support contact surfaces, FW central bolt, electrical strap contact surfaces and attachment inserts for both FW and Vacuum Vessel (VV). In order to fulfil the tight tolerance requirement, stress relieving shall be performed for dimensional stability after cover welding operation. In this paper, effect of Post Weld Heat Treatment (PWHT) conditions, temperature and holding time, was investigated on the residual stress and hardness. The 316L Stainless Steel (SS) was prepared and welded by manual TIG welding by using filler material with 2.4 mm of diameter. Welded 316L SS plate was machined to prepare the specimen for PWHT. PWHT was implemented at 250, 300, 400 ◦ C for 2 and 3 h (400 ◦ C only) and residual stress after relaxation were determined. The evaluation of residual stress and hardness for each specimen was carried out by instrumented indentation technique. The residual stress and hardness were decreased with increasing the heat treatment temperature and holding time. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The main function of blanket SB is to provide the nuclear shielding to VV and external components and to absorb the radiation and particle heat fluxes from the plasma. The Full Scale Prototype (FSP) of blanket SB has been manufactured and tested in accordance with pre-qualification program by Korean Domestic Agency (KODA). And also, several R&D activities related to manufacturing of SB such as drilling, cover welding and slitting have been issued and implemented to resolve the tolerance issues [1–3].
∗ Corresponding author. E-mail address: [email protected] (H.-C. Jung).
The blanket SB, which located inside ITER main vacuum chamber and behind the FW, shall be required the tight tolerance because SB interfaces with many components, such as flexible support keypads, FW support contact surfaces, FW central bolt, electrical strap contact surfaces and attachment inserts for both FW and VV. In order to fulfil the tight tolerance requirement, the stress relieving for dimensional stability shall be performed after cover welding operation. The stress relieving can be done by various procedures like: optimal welding sequence, heat treatment or mechanically (i.e., shot peening, ultrasonic or vibratory treatment, etc.). For stress relieving with heat treatment, heat treatment conditions, such as temperature, holding time, heating and cooling rate are factors for determining the microstructure and mechanical properties. For dimensional stability of austenitic stainless steel using heat
http://dx.doi.org/10.1016/j.fusengdes.2016.02.007 0920-3796/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: H.-C. Jung, et al., Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.007
G Model
ARTICLE IN PRESS
FUSION-8524; No. of Pages 5
H.-C. Jung et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
2
Indentaon load (L)
Pm
Loading S
Unloading
1
Indentaon depth (h)
hm
Fig. 1. Schematic picture of an indentation cycle.
Fig. 3. Tube furnace (LTF-15).
treatment, the maximum temperature and temperature difference in the component shall not exceed 425 ◦ C and 55 ◦ C, respectively [4,5]. In the welding of ITER blanket SB, Welding Procedure Qualification Test (WPQT) should be performed prior to the start of welding activity. In WPQT, the mechanical test, such as tensile, bend, hardness, etc., should be carried out and satisfied with requirement in accordance with related standard. Therefore, it is important to check the change of the residual stress as well as mechanical properties on the PWHT. The objective of this paper is to investigate the effect of PWHT conditions, temperature and holding time, on residual stress and hardness from a part of optimization of PWHT conditions. The 316L SS plate was prepared and welded by manual TIG welding by using filler material with 2.4 mm of diameter. The residual stress and hardness were measured by using instrumented indentation technique for investigation of correlation between PWHT conditions and properties.
on friction stir-welded API X80 steel. He reported that the instrumented indentation technique is a promising non-destructive residual stress measurement technique if there is sufficient distance between indentations. 2.1. Measurement of hardness The hardness was measured by using Depth Sensing Indentation (DSI) method introduced by Oliver and Pharr [8]. This is based on the mean contact pressure at the maximum indentation depth with the following expression [7–9]; H=
Pm Pm = AC f (hC )
(1)
where Pm is the peak load at the indenter displacement hm , AC is the projected contact area and function of geometry of indenter. For Vickers indenter, AC = 4 h2 c tan2 =24.5 h2 c (semi apical angle is 68◦ ). hc is the contact depth at the maximum load. The contact depth can be given as; Pm S
2. Instrumented indentation technique
hC = hm = ˛
Two types of methods have been applied for measurement of stress relieving; mechanical stress-relaxation method (e.g., holedrilling and saw-cutting) and physical-parameter analysis methods (X-ray diffraction, ultrasonic wave and neutron diffraction). Even if both methods have been applied generally, they have some difficulties to measurement of residual stress, which are destructive nature limits the wide application in industry for mechanical stress-relaxation and difficulty to separate intrinsic microstructural effects on the physical parameters from the effects of a residual stress for physical-parameter analysis methods [7]. The instrumented indentation technique is tool for measuring mechanical properties, such as hardness, young’s modulus, fracture toughness, only by analyzing the indentation load-displacement curve as shown in Fig. 1. Lee et al. [10] verified the validity of instrumented indentation method by comparing the energydispersive X-ray diffraction method for evaluation of residual stress
where S is the slope of the initial part of the unloading curve and ˛ is an indenter geometry constant, equal to 0.75 for Vickers indenters.
(2)
2.2. Measurement of residual stress If an arbitrary indentation state (ht , L0 ) is attained in an unstressed status and if the tensile or compressive in-plane stress, res , is applied to the loading state at the fixed penetration depth, ht , the indentation load, L0 , will be changed to a load, L, due to the change of surface penetration resistance. The load shift, L − L0 , due to the tensile or compressive stress application becomes a clue for stress quantification. The surface-normal deviator stress, D , is −2 res /3 by removing the hydrostatic stress 2 res /3 from the surface residual stress, res , and is added to the contact pressure. L − L0 is assumed to be the product of the selected deviator stress
Fig. 2. 316L SS plate and the joint design.
Please cite this article in press as: H.-C. Jung, et al., Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.007
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component and its corresponding contact area, AR C . Thus, an equation for the equi-biaxial residual stress is derived in terms of the indentation load and contact area as; res =
3(L0 − L) 2A2C
(3)
Here, L0 and L is the indentation load in relaxation and as-weld state, respectively and AR C is contact area in the tensile or compressive stress state as described Eq. (1). In order to determine the magnitude of actual biaxial stress, Eq. (3) developed as below [10]; res =
3(L0 − L) (1 + k)ARC
(4)
Here, k is stress ratio ( res,y / res,x ) and the Eq. (4) converges to Eq. (3), when a stress state approaches to the equi-biaxial state or k = 1.0. Several preliminary observations on the welded joints yielded a stress ratio of about 0.33, and this value was used in subsequent biaxial stress analyses [10,11]. 3. Experiment 3.1. Preparation of test specimen In this study, 316L stainless steel plate with 20 mm thickness was prepared. The 316L stainless steel plate with dimensions of 150 mm (w) × 400 mm (l) × 20 mm (t) was machined with single U groove, with groove angle of 12◦ and 1 mm root face as shown in Fig. 2. Narrow gap Tungsten Insert Gas (TIG) arc welding process was performed with filler metal with 2.4 mm diameter. The root gap was maintained 2 mm. The current and voltage for each pass were applied in the range of 100 ∼ 180 A and 10 ∼ 25 V, respectively. The inter-pass temperature between weld passes shall be maintained (or keep) 150 ◦ C temperature, in order to avoid hydrogen attack and crack susceptibility to the weld. After welding, test specimen with dimension of 50 mm (w) × 20 mm (h) × 5 mm (t) were machined from welded 316L stainless steel plate. 3.2. Post weld heat treatment PWHT process was implemented by using tube furnace (LTF-15, Lenton, UK) as shown in Fig. 3. Two types of test groups were prepared to investigate the effect the temperature and holding time on the residual stress and hardness. First, In order to investigate the effect of PWHT temperature on the residual stress, PWHT performed at 250 ◦ C, 300 ◦ C, 400 ◦ C for 2 and 3 h (400 ◦ C only) under Ar atmosphere. The heating and cooling rate of 1 ◦ C/min was applied. According to RCC-MR, the temperature difference in the components shall not be exceed 55 ◦ C [4]. In previous study [6], it was found out that the temperature control for below 40 ◦ C of temperature difference between center and edge of SB FSP, is possible with 1 ◦ C/min of heating and cooling rate. After PWHT, the surface of test specimen was etched for instrumented indentation test. 3.3. Instrumented indentation test In order to measure the residual stress and hardness, the instrumented indentation test by using commercial AIT-U system (Frontics Corp., Korea) with depth and load resolutions of 0.1 mand 5.6 gf was performed as show in Fig. 4. Hardness and residual stress was measured by using depth control of 100 m-. The indenter was applied Vickers-type. The distance between indentations was applied 2.5 mm, which is greater than 3 times the indent diameter (500 m).
Fig. 4. Indentation machine (AIT-U system).
4. Results Fig. 5 shows the experimental indentation curves from the test specimen at 0 mm (Fig. 5(a)) and 2 mm (Fig. 5(b)) of distance from weld of center. All tests clearly indicated the indentation curves without indenter drops due to voids or other defects. The residual stress at 0 mm (weld region) and 2 mm (near HAZ) has positive (tensile) and negative sign (compressive), respectively. During the welding, non-uniform heat distributions and plastic deformations occur on the material due to introduced high heat input to the material. Residual stresses induced by shrinkage of the molten region are usually tensile. The compressive residual stress is dominant in the HAZ where the temperature exceeds the critical values [12]. Fig. 6 shows the effect of PWHT temperature and holding time on the residual stress, which is calculated by Eq. (4). The residual stress decreased with increasing the PWHT temperature and holding time. In weld region, the tensile residual stress of test specimen (400 ◦ C, 2 h) is about 750 MPa. Away from the weld region, the tensile residual stress was decreased and observed the compressive residual stress. In order to verify the reliability of indentation hardness, Vickers hardness test with load of 2 kgf was performed for specimen before heat treatment. Fig. 7 shows the measured indentation hardness and Vickers hardness results for specimen before heat treatment. The indentation hardness is underestimated compared with Vickers hardness. Oliver [8] reports that for Eq. (2) based on the contact area under load, it may deviate from the traditional hardness measured from the area of the residual hardness impression if there is significant elastic recovery during unloading. Sawa [13] investigated the correlation between indentation and Vickers hardness by
Please cite this article in press as: H.-C. Jung, et al., Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.007
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Fig. 5. Experimental indentation curves from the test specimen at 0 mm (a) and 2 mm (b) of distance from weld center.
Fig. 6. Effect of PWHT temperature and holding time on residual stress relaxation.
using several materials, which have different hardness. He explains that indentation hardness and Vickers hardness have relationship using a coefficient C as below; HIT = CHV
(5)
Fig. 7. Relationship between indentation hardness and Vickers Hardness.
Also, he reports that this equation can apply to the materials other than extremely low hardness materials by using 1.25 as
Please cite this article in press as: H.-C. Jung, et al., Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.007
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The follow conclusions are obtained. (1) The residual stress decreased with increasing the PWHT temperature and holding time. Although the indentation hardness decreased with increasing the PWHT temperature and holding time, the additional studies to reduce the hardness after PWHT should be needed for satisfaction of requirement in further work. Acknowledgments This research was supported by the Ministry of Science, ICT and Future Planning of Republic of Korea under the Korean ITER project contract (2007-2006983). ITER is a Nuclear Facility INB-17. Fig. 8. Effect of PWHT temperature and holding time on indentation hardness.
References coefficient C. It can be confirmed that the recalculated indentation hardness by Eq. (5) is coincided with Vickers hardness. Fig. 8 shows the effect of PWHT conditions on the indentation hardness. The indentation hardness of each specimen was calculated by Eqs. (2) and (5). The indentation hardness decreased with increasing temperature and holding time. In the welding region, the indentation hardness of test specimen (400 ◦ C, 2 h) is about 2.2 GPa, which is about 91% of indentation hardness for test specimen before PWHT. According to related standard [14], the permitted maximum hardness of heat treated material is about 77 ∼ 85% of maximum hardness for non-heat treated material. In PWHT, the microstructure and mechanical properties of welded components are depended on the PWHT conditions, such as temperature, holding time, heating and cooling rate, etc. Especially, in cooling process in which SB is cooled down to room temperature, the shrinkage produces residual stress since the cover plate is fixed at both ends. Therefore, it needs to reduce the heating and cooling rate for satisfaction of requirement and the additional studies should be needed for satisfaction of requirement in further work. 6. Conclusions In this paper, effect of PWHT conditions, temperature and holding time, was investigated on the residual stress and hardness. The 316L stainless steel was prepared and welded by manual TIG welding. The evaluation of residual stress and hardness for each specimen was carried out by instrumented indentation technique for investigation of correlation between PWHT conditions and properties.
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Please cite this article in press as: H.-C. Jung, et al., Investigation of effect of post weld heat treatment conditions on residual stress for ITER blanket shield blocks, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.02.007