Journal for Research | Volume 02 | Issue 05 | July 2016 ISSN: 2395-7549
Investigation of Residual Stresses in Welding and its Effect on Mechanical Behavior of Stainless Steel 310 Mr. Umesh A. Khilare M.E. Scholar Department of Mechanical Engineering SAE, Kondhwa-Pune
Prof. S.B. Sollapur Professor Department of Mechanical Engineering SAE, Kondhwa-Pune
Prof. Shweta Wange Professor Department of Mechanical Engineering SAE, Kondhwa-Pune
Prof. Vikram Pawar Professor Department of Mechanical Engineering SAE, Kondhwa-Pune
Abstract AISI310 is widely used in the manufacture of heat exchangers, radiant tubes, muffles, furnace parts, food processing equipments. Welding is widely used fabrication method in such type of industrial applications; but unfortunately welding induces few problems such as residual stresses and induced distortions. Residual stresses have significant effect on performance of the weld joint subjected to loading. The AISI310 is welded using matched filler material. Thus the weld joint consist of two different materials having different behavior under tensile loading. In this study residual stresses are estimated by using Labeas and Diamantakos formula and values are assigned as an initial stress in FEM of weld joint. The weldment specimen model is subjected to tensile loading and effect of residual stress on local yielding is investigated. The ANSYS is used for this purpose. The response of weld joint to monotonically increasing tensile load is determined experimentally by conducting the transverse and longitudinal tension tests on UTM. The stress-strain behavior of the weld joint is studied vis a vis virgin 310 stainless steel alloy. Keywords: TIG Welding, FEA, AISI 310, Residual Stresses _______________________________________________________________________________________________________ I.
INTRODUCTION
Welding is defined as a process where two or more pieces of metal or thermoplastics are fastened together by use of heat and pressure. The process of applying heat softens the material and enables it to affix together as one in a joint area when an adequate amount of pressure is applied. The concept of welding first developed in the middle ages, though it did not form into the process of welding as it is today until the latest years of the 19th century. Before this, a process known as “forge welding� was the only means of joining two metal objects together. Forge welding consisted of using a flame to heat metal to extremely high temperatures and then hammering each piece together until they became one. This method was replaced around the time of the industrial revolution. Electric and gas flame heating methods proved to be much safer and faster for welders. Practically every material object that has made society what it is today, was created by welded construction tools or has been welded itself. Because of this, welders have a wide range of areas for employment many welders specialize in pipe welding or automobile welding while others specialize in machinery. The possibilities are endless for welders seeing as welding can be performed in a diverse range of locations, including underwater, though not all forms of welding are the same. Some forms of welding use gas, while others use electric and the newest forms involve use of a laser. The process of welding that is used depends on a variety of factors but the form and thickness of the material is usually the deciding factor for which method is most effective. Arc, Electro slag, Flux-Cored, Gas Metal-Arc, Gas Tungsten-Arc, Metal Inert Gas, Plasma Arc, Shielded-Metal Arc, Submerged Arc and Tungsten Inert Gas are the most widely used welding methods Welding can trace its historic development back to ancient times. The earliest examples come from the Bronze Age. Small gold circular boxes were made by pressure welding laps together. It is estimated that these boxes were made more than 2000years ago. During the Iron Age the Egyptians and people in the eastern Mediterranean area learned to weld pieces of iron together. Many tools were found which were made approximately 1000B.C. Welding is nothing but a material joining process in which two or more parts are mixed (or joined) at their contacting surfaces by suitable application of heat and/or pressure. The final part that is obtained by joining is known as 'weldment'. Weldment results in homogenous material and usually has same composition and characteristics as that of the two parts with which joining is done. Some welding processes require only heat while some require heat and pressure. In some other processes, external filler material is required to obtain coalescence (or mixing).Various metals as well as plastics can be joined by welding methods.
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Investigation of Residual Stresses in Welding and its Effect on Mechanical Behavior of Stainless Steel 310 (J4R/ Volume 02 / Issue 05 / 007)
Residual stresses in welded joints primarily develop due to differential weld thermal cycle (heating, peak temperature and cooling at the any moment during welding) experienced by the weld metal and region closed to fusion boundary i.e. heat affected zone. Type and magnitude of the residual stresses vary continuously during different stages of welding i.e. heating and cooling. During heating primarily compressive residual stress is developed in the region of base metal which is being heated for melting due to thermal expansion and the same (thermal expansion) is restricted by the low temperature surrounding base metal. After attaining a peak value compressive residual stress gradually decreases owing to softening of metal being heated. Compressive residual stress near the faying surfaces eventually reduces to zero as soon as melting starts and a reverse trend is observed during cooling stage of the welding. During cooling as metal starts to shrink, tensile residual stresses develop (only if shrinkage is not allowed either due to metallic continuity or constraint from job clamping) and their magnitude keeps on increasing until room temperature is attained. In general, greater is degree of constraint and elastic limit of melt higher will be the value of residual stresses. Residual stresses in-as welded structures may be up to the yield magnitude in tension; with post weld heat treatment, however they are generally relaxed to about 10-25% of this value. The magnitude and distribution of residual stresses introduced by welding are strongly influenced by geometry of structure. II. WELDING OF PLATES In the present work, TIG welding has been done for studying the effect of residual stresses on mechanical behaviour of butt welded joints of austenitic stainless steel i.e. stainless steel 310. For this purpose, AISI 310 stainless steel of 3 mm thickness has been used for square butt welding. In the present study, austenitic stainless steel material: AISI 310 has been selected. Welding set-up has been prepared, tested and made ready for doing welding. Each piece has the dimension 300mm x 150 mm x 3 mm. Butt joints are made by welding two such pieces. After welding, visual inspection of the steel plates has been done.
Fig. 1: Welded plates
Cutting of Tensile Test Specimen Now tensile test specimens are made by machining the welded samples by using laser cutting machine. Photographic view of a tensile test specimen is shown in Figure2.Tensile test specimens are tested on universal testing machine, and observations are made. A hydraulic jaw is used during testing for gripping the samples. All the related works that have been done by other researchers that are related to the current research problem should be summarized in this section. Times New Roman font with size 10 must be used in this section. Sub topic should be written as given below;
Fig. 2: Dimensions of the specimen prepared for tensile test
Estimation of Residual Stresses The residual stresses can be estimated using the formula proposed by Labeas and Diamantakos. The longitudinal residual stress in the weld region is estimated using equation (1). 1−[𝑥/𝑐𝑜]2 σy = σoy (0.5+z/t) (1) 1+[𝑥/𝑐𝑜]4
The transverse residual stress follows the following relationship σy = σoy (0.5+z/t)e-(x/d)2[1-12(y/L)2]
(2)
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Investigation of Residual Stresses in Welding and its Effect on Mechanical Behavior of Stainless Steel 310 (J4R/ Volume 02 / Issue 05 / 007)
Graphs for Calculated Residual Stresses
Fig. 3: Transverse residual stress distributio
Fig. 4: Longitudinal residual stress distributions
Analysis of Residual Stress The geometry of the plate with dimensions 300x300x300 is modeled in the ANSYS geometry the material properties are assigned to the model. The standard properties of stainless steel 310 are assigned to the plate. Transient thermal and static structural analysis is carried out for residual stresses.
Fig. 5: Longitudinal stress for welded plate
Fig. 6: Transverse residual stress for welded plate
Graphs for Simulated Residual Stresses
Fig. 7: Longitudinal residual stress distributions by software analysis Fig. 8: Transverse residual stress distributions by software analysis
III. TENSION TEST As mentioned previously, tensile specimens are machined in the desired orientation and according to the standards. The central portion (gage portion) of the length is usually of smaller cross section than the end portions. This ensures the failure to occur at a section where the stresses are not affected by the gripping device. The gage length is marked and elongation is measured between these markings during the test. Tensile test is carried out on Universal Testing Machine. True stress vs plastic strain graphs are plotted for all the specimens.
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Investigation of Residual Stresses in Welding and its Effect on Mechanical Behavior of Stainless Steel 310 (J4R/ Volume 02 / Issue 05 / 007)
Analysis of Tension Test The specimen is subjected to opposite side pull in order to simulate tension testing simulation. The transverse weldment, longitudinal weldment and virgin weldement was selected to carry tension test. The transverse and longitudinal test specimens are modelled by assigning respective residual stresses as initial stress. Response of this finite element model is simulated by applying monotonically increasing load iteratively with a time step of 0.001. Specimen geometry is modelled using 8-node hexahedral reduced integration continuum 3-d stress (C3D8R) Static (Elastic plastic) analysis is carried out. The stresses obtained in analysis are mentioned below also some snaps of analysis are inserted below in order to have better understanding of the system analysis.
Fig. 9: Stress results for Transverse weldment
Fig. 10: Stress plot for Longitudinal weldment
Fig. 11: Stress plot for virgin material
Stress Vs Strain Comparison for Transverse Virgin Tension Test Specimen
Fig. 12: Comparison of True stress vs plastic strain for transverse weldment specimen with virgin test specimen
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Investigation of Residual Stresses in Welding and its Effect on Mechanical Behavior of Stainless Steel 310 (J4R/ Volume 02 / Issue 05 / 007)
The Experimental and simulated stress-strain curves of stainless steel 310 longitudinal weldment specimen are compared with virgin alloy to quantify the effect of residual stresses. In case of longitudinal weldment specimen the stress-strain curve deviates from the curve of virgin alloy. The residual effect is seen at the start because during starting phase of tensile loading the residual stresses are dominant. Also the marginal deviation between stress-strain curve of longitudinal weldment specimen and virgin alloy is observed at the plastic strain of 0.1 IV. CONCLUSION It is seen that the residual stress in the transverse direction for butt welding of 3mm thick plates comes out to be maximum (117MPa) in the centre of weldment and in longitudinal direction comes out to be maximum (532Mpa)in the centre of weldment. Experimental and simulated stress-strain curves of stainless steel 310 longitudinal and transverse weldment specimen were compared with virgin alloy of stainless steel 310 to quantify the effect of residual stresses. Stress strain curves from finite element analysis agree with experimental results from tension test which validates the proposed model of assigning residual stresses as an initial stress. The finite element method is an efficient technique in analyzing residual stresses in welding processes. A 3-D finite element welding simulation was carried out assuming welding is carried out by Tungsten inert gas welding. The welding simulation was considered as a sequential coupled thermo-mechanical analysis and the element birth and death technique was employed for the simulation of filler metal deposition. REFERENCES [1] [2] [3] [4] [5] [6]
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