Effect of Welding Process on Residual Stress Development in AISI 316 Thin Welds

Transcription

1 More info about this article: Effect of Welding Process on Residual Stress Development in AISI 316 Thin Welds S. Arun Kumar, S. Mahadevan, B.P.C. Rao, C.K. Mukhopadhyay and S. Murugan 1 Non Destructive Evaluation Division 1 Remote Handling, Irradiation Experiments and Robotics Division Metallurgy and Materials Group Indira Gandhi Centre for Atomic Research Kalpakkam, Tamil Nadu maharaja@igcar.gov.in Abstract Welding is an effectively used process for joining metals. Various types of welding processes are well established. Residual stress inevitably develops during inhomogeneous cooling of the weld pool. Higher tensile residual stresses present on the surfaces are detrimental to the components. Hence, it is essential to evaluate the surface residual stresses non-destructively. Among others, X-ray diffraction (XRD) is a widely used NDT technique. Using XRD technique it is possible to know the nature of residual stresses i.e., whether compressive or tensile as well as their absolute values. This paper presents the results of studies using three different welding processes. Three weld joints of AISI 316 stainless steel plates (thickness, 3 mm) have been prepared using tungsten inert gas welding (TIG), laser beam welding (LBW) and hybrid laser arc welding (HLAW) processes. The residual stresses present on the surface of the three weld joints have been evaluated non-destructively using XRD technique. The welding processes are evaluated based on the magnitudes of the residual stresses developed in the weld joints. Results will be presented in detail. Keywords: Welding, residual stress, X-ray diffraction, AISI 316 stainless steel. 1. Introduction Residual stresses can be defined as the stresses that are present inside a material after removal of external forces acting on the material.generally all manufacturing processes such as cutting, grinding, shot peening, welding, forging etc. cause invariably residual stresses. Residual stresses are vital in the structural integrity of operating components and their service life. Tensile residual stresses on the surface are detrimental in nature while the compressive stresses improve the fatigue strength and help in crack closure. Utmost care should be taken to welding related residual stress to avoid failure of the weld joints in a component. 433 Non-Destructive Evaluation 2016

2 Gas tungsten arc welding (GTAW), also known as tungsten inert gas welding, is an arc welding process that uses a non-consumable tungsten electrode in the presence inert shielding gas (argon or helium), and a filler metal to produce the weld. Laser beam welding (LBW) process uses monochromatic coherent laser light beam as heat sourcefor welding. When fine, focused laser beam is applied,very low heat input is given to material as compared to arc processes and this allows for narrow and deep welds. The hybrid laser arc welding (HLAW)[1]process is a variant of laser welding process where laser andelectric arc are coupled together and applied on the common molten zone. HLAW is found to have advantages such as increase in depth of penetration, aspect ratio, welding speed etc.when laser leads the arc, then the process is called laser leading hybrid welding. On the other hand when arcleads, it is known as arc leadinghybrid welding process.arc leading weldsare reported to have deeper penetration. Generally the magnitude of residual stress developed during welding depends on the heat input, weld volume, type of welding, electrode, weld groove design and base material properties. The information on residual stress developed in SS316 thin welds is scarce in literature [2,3,4]. SS316 has varied applications in pharmaceutical, marine, architectural and in the nuclear industry due to its corrosion and oxidation resistance.this paper reports a study on the development of residual stresses in the SS316 joints using LBW, GTAW and HLAW processes. 2. Experimental 2.1 Sample Details Two platesof dimension 75 mm x 75 mm x 3 mm were welded along the 75 mm side to get a weld joint of dimension 150mm x 75 mm x 3 mm. Three weld joints were prepared with the welding parameters listed intable 1.The chemical composition of the 316 SS is given in Table 2.Figure 1 shows the photograph of the weld joints and the linealong which the measurements were carried out. Solid state Neodymium- doped Yttrium Aluminum Garnet (Nd:YAG) rod was used as lasing medium for generation of laser beam with a wavelength of 1.0θ m for carrying out LψW and HLAW joints.the hybrid weld was prepared using an arc leading setup. The distance between the arc source and the laser source was 3 mm. Table 1. Welding parameters used for welding 3 mm thick 316 SS plate Parameters LBW GTAW HLAW Total Power (W) (350 W laser) Current (A) Voltage (V) Travel speed (mm/s) Bead Width Non-Destructive Evaluation 2016

3 Table 2. Chemical composition of the AISI type SS316 Elemen t Wt % C Cr Ni Mo Mn Si N S P Co V Fe Bal. Line of Measuremen (a) (b) (c) Residual stress measurements were carried out in the longitudinal direction of the weld Figure 1. The three SS weld joints prepared by (a) LBW (b) GTAW (c) HLAW joints at the weld centre (WC), heat affected zone (HAZ) and parent material along a line situated at the centre of the 75 mm side. 2.2 Residual Stress Measurements 435 Non-Destructive Evaluation 2016

4 Residual stress measurements were carried out using a portable X-ray stress analyzer (Rigaku Strainflex MSF-βM). This instrument has a back reflection type goniometer with a β scan range from 140 to 170 having parallel beam geometry. The XRD intensity profilesat each of the locations where stress measurements are to be carried were obtained at various angles and were corrected for background and absorption. The peak β values are obtained at various angles by suitable peak fitting algorithms. From the slope of sin 2 vs peak β or d plot, the residual E 1 d ( )( hkl ) ( ) (1) 2 1 d0 sin stress is determined based on equation (1) which is based on a plane stress model [5,6].A Young s modulus of 196 GPa for SS was used as the material property to estimate the residual stress values of the specimen. At all the identified locations of the measurements, two sets of measurements were carried out and the values reported are averaged stress values based on the combined analysis of the change in the peak location with varying. The experimental parameters were optimized by taking a test scan on the parent material. The residual stress measurements were carried out along the longitudinal direction on the welded plate across the weld joint using the experimental parameters listed in Table 3. It is to be noted that the measurements were carried out on the surface of all the joints in the as-welded condition. Table 3. Experimental parameters used for stress measurement by XRD technique 3. Results & Discussions Operating Voltage 30 kv Current 7 ma X-Radiation Chromium K Wavelength Å ψ range 15.5 to range Step Width 0.1 Dwell Time 3 s Aperture 2 mm Filters No Plane (311) The measured residual stresses at various locations on the SS weld joints are given in Table 4.As the bead width of the laser and hybrid joint were between 2 and 3 mmand HAZ werecloser, measurements were carrid out with a gap of 2mm from the weld centre. Figure 2 shows the residual stress profile across the weld centre. 436 Non-Destructive Evaluation 2016

5 All the weld joints showed typical M stress profile across the weld.the measured experiment values which are reported were in good agreement with the published literature[4, 5]. In the LBW joint, at WC a tensile stress of ~220 MPa was observed. A peak tensile stress of ~300 MPa was observed at 5 mm from the WC. The bead width of the LBW was least when compared with other joints. Table 4. Residual stress along the longitudinal direction Sl. No. Location from weld centre (mm) RS values (MPa) LBW GTAW HLAW ±10 140±25 124± ±21 473±22 258± ±25 267±35 187± ±38 204±43 164± ±32-267± ±17 415±32 262± ±21 232±22 187±20 In the GTAW joint, at WC a tensile stress of ~200 MPa was observed. A peak tensile stress of ~450 MPa was observed on either side. The peak stress observed above the yield stress of SS316 at 5 mm may be due to the change in the material properties locally and or the experiments were carried out in the as welded condition. The stress gradient close to the weld zone was highest in this joint.the stress affected zone was broader as the bead width of the 437 Non-Destructive Evaluation 2016

6 GTAW joint was larger. Figure 2. Residual stress profiles across the WC of the three weld joints. In the HLAW joint, a tensile stress of ~165 MPa was observed at WC. A peak tensile stress of ~267 MPa was observed. The stress gradients across the weld centre were similar in the LBW and HLAW weld joints. Mild texture observed due to the intensity variations at the weld centre has contributed to the larger error at the weld centre. The minor asymmetric residual stress profiles observed in LBW and HLAW joints are attributed to smaller bead width and closer measurements averaged over 2 mm. 4. Conclusion The three weld joints showed typical M type stress profile and residual stresses were found to be tensile within a region of 10 mm from weld centre. In the LBW and HLAW joints, the stress gradients are lesser and hence, the stress affected zone is limited to 5 mm. Both laser and hybrid welding stress profiles are nearly closer to each other. The weld joint prepared by GTAW process showed higher peak tensile stress values as compared to the other two welding processes. The stress affected extended upto 7mm. The large heat input used and wider bead width are found to result in higher peak stresses and extended stress affected zone respectively. Among the three weld joints, the hybrid weld joint was found to have least tensile stress at the weld centre and lower stress gradient.addition of heat source along with the laser would have caused an annealing effect hence, reduction in stress.in this limited study, as the stress induced by the hybrid welding process is lower, hybrid welding process may be preferred for welding of thin stainless steels. References [1] A. M. Chelladurai, K. A. Gopal, S. Murugan, S. Venugopal, and T. Jayakumar, 2014 Development of pulsed laser-gtaw hybrid welding, in International Institute of Welding International Welding Congress (IC 2014), New Delhi [2] J.R.Chukkan, M. Vasudevan, S. Muthukumaran, R.R. Kumar, N. Chandrasekhar,2015. Simulation of laser butt welding of AISI 316L stainless steel sheet using various heat sources and experimental validation. J. Mater. Process. Technol.219, Non-Destructive Evaluation 2016

7 [3] D. Deng, H. Murakawa, Prediction of welding distortion and residual stress in a thin plate butt-welded joint. Comput. Mater. Sci. 43, [4] K.C. Ganesh, M. Vasudevan, K.R. Balasubramanian, N. Chandrasekhar, S. Mahadevan, P. Vasantharaja, T. Jayakumar, Modeling, Prediction and Validation of Thermal Cycles, Residual Stresses and Distortion in Type 316 LN Stainless Steel Weld Joint made by TIG Welding Process. Procedia Engineering, 86 (2014) [5] Baldev Raj, T. Jayakumar, S. Mahadevan, 2009, X-ray Diffraction Based Residual Stress Measurements for Assessment of Fatigue Damage and Rejuvenation Process for Undercarriages of Aircrafts, J. Nondestruct Eval. (2009) 28, [6] S. Arun Kumar, S. Mahadevan, Manoj Kumar Raja, B. Purnachandra Rao, Shaju K. Albert and S. Murugan, 2015, Comparison of welding induced residual stresses in austenitic and ferritic steel weld joints, NDE-2015, Hyderabad, P Non-Destructive Evaluation 2016