Vilniaus Gediminas Technical University, J. Basanaviciaus str. 28, Vilnius, Lithuania

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1 Solid State Phenomena Online: ISSN: , Vol. 165, pp doi: / Trans Tech Publications, Switzerland Effects of Vibration energy Input on Stress Concentration in Weld and Heat-affected Zone of S355J2 steel Aurimas Jurčius 1a, Algirdas Vaclovas Valiulis 1b, Olegas Černašejus 1c 1 Vilniaus Gediminas Technical University, J. Basanaviciaus str. 28, Vilnius, Lithuania a aurimas.jurcius@montuotojas.lt, b algirdas.valiulis@vgtu.lt, c olecer@vgtu.lt Keywords: welded joints, residual stresses, vibratory stress relief, thermal treatment. Abstract. The existence of residual stresses induced by the welding process is an important reason of cracking and distortion in welded metal structures that may affect the fatigue life and dimensional stability significantly [1]. Heat treatment is one of the traditional methods to relieve the residual stresses. But it is often limited by the manufacturing condition and the size of the structures. In this paper a procedure called vibratory stress relief (VSR) is discussed. VSR is the process to reduce and re-distribute the internal residual stresses of welded structures by means of weldment mechanical vibration during welding. Parameters of VSR procedure are described in the paper. Residual stresses on weld bead are measured in three different specimens by X-ray diffraction method. Mechanical tests of welded specimens were also performed with purpose to evaluate VSR effect in weld metal and heat affected zone (HAZ). Introduction Existing methods for relieving residual stress from welds are: mechanical, heat and electromagnetic. The mechanical method may be performed by hammering or vibration. The heat method consists of heating the whole welded piece or each weld, one by one. The electromagnetic method uses the electromagnetic hammer technique [3]. In the heat treatment the part is heated until the yield point is reduced to less than the residual stress, which in turn causes local plastic distortion, decrease of the residual stress intensity and reduction of hardness. The vibration method introduces energy into the part by means of vibrations. For the stressed atomic structure there is no difference between the energy introduced through heat and the energy introduced through vibrations. The applied energy reorganizes the crystalline structure, relieving stress and stabilizing the piece, without distortion. This process is especially useful for stress relieving of big structures, for which the cost of treatment by electrical means would be high, and for parts with severe dimensional tolerances, in which heat treatment could cause distortions that would exceed them. The vibrators generally used have a frequency band of 0 to 100 Hz. They are connected to the structure, which should be supported on rubber blocks. Frequency is gradually increased until the first resonance is reached. This resonance is maintained for a specific period of time and then the frequency is increased again until the second resonance is reached and so on [4]. Sources of residual stress Residual stress in welds is produced by localized metal tensions occurring immediately after welding, which are [2]: - Contraction stress. This is the main source of residual stress. It takes place during the cooling of the welded areas, which have undergone non-uniform heating. - Stress due to higher surface cooling. When a weld cools down the surface cools faster than the inside, even if this cooling occurs in still air. The greater the thickness, the more stress is generated. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-06/03/16,20:44:50)

2 74 Mechatronic Systems and Materials: Materials Production Technologies - Stress due to phase transformation. It occurs due to the transformation of austenite (face centered cube, fcc) to ferrite (body centered cube, bcc), that causes an increase in volume to which the base metal is opposed. These three types of residual stress usually take place at the same time. Experience proves that the individual effects of each one can be linearly superposed. Effects of residual stress on welds Among others, the effects of residual stress on welded parts are the following [2]: - Residual stress magnifies the load that the weld has been designed to withstand. This can lead to the rupture of the material. - Reduction of stress corrosion resistance. The regions submitted to elastic tensile stress may suffer localized corrosion in aggressive environments. - Risk of cracks. All cracking mechanisms are affected by residual stress and distortion caused by localized heating. Stress relief by heat treatment Depending on the shape and size of the piece, heat treatment for residual stress relief can be carried out by heating the entire piece, or parts of it, in a furnace; or transforming it in a combustion chamber by installing a temporary burner into it, or treating the welds one by one by means of electric resistances. Heating by exothermic kits, which enjoyed some popularity in the sixties and seventies, was abandoned because it did not produce the expected results [3]. Stress relief by vibration energy Based on the weight of the piece, the VSR method introduces into it high amplitude and low frequency vibrations for a given period of time. This relieves residual stress without distortion or alteration of tensile strength, yield point or resistance to fatigue, and the static equilibrium is restored. The most efficient vibrations are the resonant ones, because in the resonance frequency vibrations stress is better distributed, if compared with sub-resonant frequency [4]. Low frequency vibrations carry high amplitude energy and are very efficient in the significant decrease of peak residual stress in metallic parts and welds. The equipment usually employed consists of a sturdy vibrator of variable speed, which is attached to the piece and an electronic control panel. Both are mounted into a portable cabinet. Also an accelerometer is installed that detects vibrations and transmits a signal to the control panel. The resonance point is then determined and displayed on a dial. If the vibrator is equipped with a recorder, a chart can also be obtained. The point of resonance is attained by varying the frequency of the vibrator until the proper one is reached. The average time required to reach the resonance frequency is two minutes. At this point, vibration is maintained for a given time, depending on the weight of the piece and its intended application. The time may range from ten minutes to an hour or more, but if it is exceeded, the piece will not suffer any damage due to fatigue or loss of tensile strength [9]. If structures are very big, long or have open spaces, it may be necessary to apply the procedure in several points. Some equipment performs the vibration process automatically. Vibration is maintained for 15 minutes, in a sequence of three different selected frequencies, each lasting five minutes. This setting is efficient to treat pieces weighing up to ten tons. For pieces weighing more than ten tons two consecutive 20 minute periods can be used, without the piece suffering any harm [4].

3 Solid State Phenomena Vol Two simple rules should be followed for all applications: a) support the piece in the best possible manner, isolating it from the floor or rigid structures, thus leaving it free to vibrate. b) the vibrator should be directly connected to the piece, in order to transfer the entire generated vibration energy. The method can be used on a wide range of ferrous and nonferrous metals, including carbon and stainless steel, cast iron, aluminum, titanium etc., in a large variety of shapes. Sizes can vary from small weldments, shafts and gears, to large welded and machined steel structures. However, it presents some limitations: it is not efficient for extruded, cold-worked and precipitation-hardened materials [10]. One of the most important benefits of the use of the VSR method is its capacity to relieve stress at any point of the manufacturing process, such as after machining, snagging, drilling or grinding. In welded parts, stress relief can be performed during welding, which is very useful to prevent concentration of residual stress that may cause warping of the piece. The method is especially compatible with MMA, MAG, MIG and TIG. With other welding processes some logistical problems may occur. Measuring the extent of stress relief Until recently there was no reliable method for the precise measurement of residual stress that were induced not only as a result of welding, but also from forging, cold drawing and other types of metal working. Now, with the use of x-ray diffractometry (XRD), the problem has been solved. In the past, the only way of checking if residual stress had been reduced to an acceptable level was by analogy with hardness. It is a well-known fact that materials become harder when submitted to stress. Experience acquired over the years, upon which the applicable standards are still based, demonstrated that if the hardness measured after stress relief had been performed was lower than a given empiric value, the treatment had been successful [10]. This condition was especially important if the weld was to be in contact with corrosive environments, as is the case of the chemical industry. It is well-known that, depending on the environment, metals exhibit less corrosion resistance if submitted to residual stress [11]. Until very recently, x-ray diffraction presented a serious operational problem. The equipment was too large to be taken to the point of use, and in many cases the weld could not be taken to the equipment. This situation has been overcome by the development of equipment small enough to be moved from one place to another. The applicable codes, however, continue to consider hardness as the parameter ruling the approval of stress relief, and this is why we have adopted it in our experimental research. Experimental research The experimental part of this study consisted in comparing the results of stress relief performed by heating (PWHT) and by vibration (VSR) on welds made from the same material, S355J2 (LST EN ) carbon steel, whose chemical and mechanical composition is given in Table 1 and Table 2. Three 12 mm thickness plates were welded with the weld located in the middle. The first piece was not submitted to any type of treatment, the second was submitted to heat treatment and the third to VSR. The electrode ELGA P48P was used for the first pass and ESAB OK for the rest. After the welding and before treatment the plates were machined to eliminate the weld reinforcement, i.e., to have a flush outer plate surface.

4 76 Mechatronic Systems and Materials: Materials Production Technologies Heat treatment Table 1. Chemical composition of steel specimens (manufacturer's data) Composition (mass), % Steel grade C Si Mn Cr Ni S P S355J Table 2. Mechanical properties of steel specimens (manufacturer's data) Tensile Yield Impact work Steel Elongation, strength, R grade m, strength, ISO-V KV, A MPa R e, MPa 5, % J/cm 2 S355J (20 o C) Heat treatment was carried out in the furnace according to Standard CR ISO/TR [5]. The heating took place at a maximum speed of 300 ºC / h until the soak temperature of 650 ºC was reached, at which point the plate was kept for 30 additional minutes. Then, the oven was switched off and allowed to cool with the plate in it. Vibratory stress relief The vibratory stress relief treatment was made according to the equipment instruction manual. The plate was firmly secured and the frequency of vibration was gradually increased until resonance was reached and maintained for ten minutes. Tensile test All tests were carried out according to guidelines of standard LST EN 876 (ISO 5178) [6]. Three specimens were cut, one from each plate, having the dimensions in conformance with the standard. The specimens were clamped in the MIRI-500K machine, taking them to rupture. The loading rate was 500 N/s. In all cases rupture occurred in the base metal and not in the welds or HAZ. Table 3 shows the information obtained from the tests. Impact test Table 3. Tensile test results Parameter o treatment VSR PWHT Yield strength, MPa Tensile strength R m, MPa Elongation, % The specimens for the impact test had the same dimensions as those of the tensile test with the addition of two lateral notches on the welding bead, as required by Standard LST EN 875 (ISO 9016) [7]. The three specimens were submitted to impact test in the Charpy machine, at room temperature, with a 30 kg hammer (Table 4). None of the specimens broke. Table 4. Impact test results Parameter o treatment VSR PWHT Absorbed energy, J

5 Solid State Phenomena Vol X-ray diffraction test Stress measurements were made using a special X-ray diffractometer with a coordinate sensitive detector. The construction of the goniometric permits the use of this equipment on samples having considerable weight and offers the possibility of making some of the manipulations foreseen in the stress measurement technique [8]. Slit width for the incident X-ray beam was 0.5x2 mm for longitudinal stresses. A schematic diagram of the diffractometer is shown in Fig. 7. X-ray stress measurements were conducted by means of sin 2 ψ method [8], using Cr-K a radiation and (211) reflections. Measurement area HAZ Fusion zone Measuring points Fusion zone HAZ Fig. 1 Dimensions and image of a specimen Fig. 2 Scheme of the weld seam structure and points of stress measurement Experimental results of stress measurements using XRD on the front face of steel plate (Figs. 1-2) with linear weld seam are provided in Fig. 3. The residual stress state on the surface of the weld region for sample with linear weld seam is characterized by tensile stresses in the weld seam and by compressive stresses in the HAZ and the base metal near the HAZ. Fig. 3 Distribution of the residual stresses in as welded joint as determined by XRD measurement of the field perpendicular to the weld The experimental results presented in Fig. 3 do not contradict to the theory of introduction of residual stresses by welding. According to [8] the principal sources of residual stresses after welding are: shrinkage, quenching and phase transformation. Shrinkage causes the appearance of tensile stresses, whereas quenching and phase transformation induce tensile stresses at the weld seam. Tensile residual stresses on the surface of the weld seam indicate that they are caused mainly by quenching or phase transformation. Stress distribution presented in Fig. 3 is typical for residual stresses after welding. This means that the predominant cause of welded residual stresses is quenching.

6 78 Mechatronic Systems and Materials: Materials Production Technologies Summary First, it is noted that while the specimens that had been heat treated showed a decrease in tensile strength and an increase in elongation, as was to be expected, the vibration method practically does not alter those values, for it does not temper, normalize or anneal nor does it modify the mechanical properties of the material. Second, the energies absorbed in the impact test are essentially the same. None of the specimens broke, indicating that the welded material is ductile because it has a considerable amount of ferrite, as can be observed in the micrographs. The decrease in hardness resulting from both treatments was similar, indicating an effective reduction in residual stress. Thirdly, the obtained X-ray diffraction results of this study indicate that vibratory stress relief can indeed be used to reduce the residual stresses generated during welding process. References [1] D. Radaj: Welding residual stresses and distortion (DVS-Verl 2003), p [2] K. Masubuchi: Residual stresses and distortion. Metals Handbook, 10th ed., Vol 6, Welding, brazing, and soldering (ASM 1993) p [3] W. F. Hahn: Vibratory Residual Stress Relief and Modification in Metals to Conserve Resources and Prevent Pollution (Diss. Alfred University, 2002). [4] R. Claxton, A. Bentley: Vibratory Stress Relief Recent Developments. In: Metallurgia, Vol. 66, No 5, (1999), p 49, 51, 53. [5] CR ISO/TR 17663:2001 Welding. Guidelines for quality requirements for heat treatment in connection with welding and allied processes. [6] LST EN 876:1998 Destructive tests on welds in metallic materials - Longitudinal tensile test on weld metal in fusion welded joints. [7] LST EN 875:1998 Destructive tests on welds in metallic materials - Impact tests - Test specimen location, notch orientation and examination. [8] I. C Noyan, J.B. Cohen: Residual Stress, Measurement by Diffraction and Interpretation (New York 1997), p [9] A. Jurčius, A.V.Valiulis, V.Kumšlytis: Vibratory stress relieving it s advantages as an alternative to thermal treatment. Journal of Vibroengineering, Vol. 10, Iss. 1 (2008), p [10] A. Jurčius, A. V. Valiulis, V. Kumšlytis: Vibrational conditioning of metals is the alternative to classical thermal stress relief. Proc. of 13 th International conference Mechanika 2008 (Kaunas 2008), p [11] A. Jurčius, A.V.Valiulis: Reduce of material residual stresses using vibration energy. Proc. of 7 th International Conference Vibroengineering-2008 (Kaunas 2008), p

7 Mechatronic Systems and Materials: Materials Production Technologies / Effects of Vibration Energy Input on Stress Concentration in Weld and Heat-Affected Zone of S355J2 Steel /