Design and Metallurgy of Weld Joints (MEM-510)
Residual Stresses In Weld Joints Dr. Chaitanya Sharma 1-1
Residual Stresses In Weld Joints Lesson Objectives
In this chapter we shall discuss the following:
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Residual stresses (RS). Mechanism of RS development. Influence of RS on weld performance. Control of residual stresses Distortion and prevention
Keywords:
Learning Activities 1. Look up Keywords 2. View Slides; 3. Read Notes, 4. Listen to lecture
Residual stresses, transformation stress, thermal stress, quench stress, distortion, SCC, control of residual stress
What Are Residual Stresses? • Residual stresses are stresses that would exist in a body if all external loads were removed, normally due to non uniform temperature change during manufacturing process e.g. welding. • Residual stresses are locked-in stresses and develop primarily due to non-uniform volumetric change in metallic component irrespective of manufacturing processes. • They are sometimes called internal stresses. • Residual stresses can reduce the service life of a structure or even cause catastrophic failures.
Why Residual Stresses Are Important? • Residual stresses are of prime concern to the industries producing weld integrated structures around the globe because they have negative influence on weld integraty: Further they causes following: 1. Distortion & dimensional instability in welded structures; 2. Premature fracture/failure, 3. Significant reduction in fatigue strength and in-service performance of the welded structures. 4. Promote hydrogen-induced cracking, stress corrosion cracking and fatigue induced cracking. 5. Reduced Corrosion resistance, Promote Brittle fracture 6. Costly repairs and restoration of parts, equipment etc. 7. Residual stresses analysis helps in the estimation of reliability under real service conditions.
Causes Of Residual Stresses • Residual stresses are generated during most manufacturing processes involving material deformation, heat treatment, machining or processing operations that transform the shape or change the properties of a material. • They are originated from a number of sources and can be present in the unprocessed raw material, introduced during manufacturing or arise from inservice loading. The origin of residual stresses can be classified as: – Differential plastic flow; – Differential cooling rates; – Phase transformations with volume changes etc.
Types of Residual Stresses • Residual stresses can be defined as either macro or micro stresses and both may be present in a component at any one time. They can be classified as: • Type I: Macro residual stress that develop in the body of a component on a scale larger than the grain size of the material; • Type II: Micro residual stresses that vary on the scale of an individual grain; • Type III: Micro residual stresses that exist within a grain, essentially as a result of the presence of dislocations and other crystalline defects.
Development of Residual Stresses • The development of residual stresses can be explained by considering heating and cooling under constraint such as three bar arrangement shown in fig. • There are three identical bars connected to two rigid blocks.
Fig: Thermally induced stresses (a) During heating, (b) during cooling (c) residual stresses in weld
Development of Residual Stresses Continued… • All three bars are initially at room temperature. • The middle bar alone is heated up, but its thermal expansion is restrained by the side bars (Fig a). • Consequently, compressive stresses are produced in the middle bar, and they increase with increasing temperature until the yield stress in compression is reached. • When heating stops and the middle bar is allowed to cool off, its thermal contraction is restrained by the side bars (Fig b). • Consequently, the compressive stresses in the middle bar drop rapidly, change to tensile stresses, and increase with decreasing temperature until the yield stress in tension is reached. • Therefore, a residual tensile stress equal to the yield stress at room temperature is set up in the middle bar when it cools down to room temperature. • The residual stresses in the side bars are compressive stresses and equal to one-half of the tensile stress in the middle bar.
Residual Stresses In Weld metal • Roughly speaking, the weld metal and the adjacent base metal are analogous to the middle bar, and the areas farther away from the weld metal are analogous to the two side bars (Fig c). • This is because the expansion and contraction of the weld metal and the adjacent base metal are restrained by the areas farther away from the weld metal. • Consequently, after cooling to the room temperature, residual tensile stresses exist in the weld metal and the adjacent base metal, while residual compressive stresses exist in the areas farther away from the weld metal.
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Changes In Temperature & Stresses During Welding
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The crosshatched area M–M’ is the region where plastic deformation occurs. Section A–A is ahead of the heat source and is not yet significantly affected by the heat input; the temperature change due to welding, ∆T, is essentially zero. Along section B–B intersecting the heat source, the temperature distribution is rather steep. Along section C–C at some distance behind the heat source, the temperature distribution becomes less steep and is eventually uniform. Along section D–D far away behind the heat source. Consider now the thermally induced stress along the longitudinal direction, σx . Since section A–A is not affected by the heat input, σx is zero. Along section B–B, σx is close to zero in the region underneath the heat source, since the weld pool does not have any strength to support any loads. In the regions somewhat away from the heat source, stresses are compressive (σx is negative) because the expansion of these areas is restrained by the surrounding metal of lower temperatures.
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Due to the low yield strength of the high-temperature metal in these areas, σx reaches the yield strength of the base metal at corresponding temperatures. In the areas farther away from the weld σx is tensile, and σx is balanced with compressive stresses in areas near the weld. Along section C–C the weld metal and the adjacent base metal have cooled and hence have a tendency to contract, thus producing tensile stresses (σx is positive). In the nearby areas σx is compressive. Finally, along section D–D the weld metal and the adjacent base metal have cooled and contracted further, thus producing higher tensile stresses in regions near the weld and compressive stresses in regions away from the weld. Since section D–D is well behind the heat source, the stress distribution does not change significantly beyond it, and this stress distribution is thus the residual stress distribution.
Residual Stress Distribution Transverse • Residual stress distribution across the weld shows tensile stress in the weld metal and the adjacent base metal and then goes compressive in the area further away from the weld metal.
Distribution Of The Longitudinal Residual Stress • According to Masubuchi and Martin , the distribution of the longitudinal residual stress σx can be approximated by the equation
• where σm is maximum residual stress, ~ = σy yield strength. • The parameter b is the width of the tension zone of σx as shown in fig.
Residual Stress Distribution Longitudinal • The distribution of the transverse residual stress σy along the length of the weld is shown in Fig b. • As shown, tensile stresses of relatively low magnitude are produced in the middle part of the weld, where thermal contraction in the transverse direction is restrained by the much cooler base metal farther away from the weld. • The tensile stresses in the middle part of the weld are balanced by compressive stresses at the ends of the weld. • If the lateral contraction of the joint is restrained by an external constraint approximately uniform tensile stresses are added along the weld as the reaction stress .
Residual Stresses In Welded Shapes
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Effect Of Temperature And Time On Stress Relief In Steel Welds PWHT is often used to reduce residual stresses. Fig shows the effect of temperature and time on stress relief in steel welds.
Thermal Treatments for Stress Relieving Weldments • The temperature ranges used for postweld heat treatment of various types of materials. • Other techniques such as preheat, peening, and vibration have also been used for stress relief. 1 - 19
Control Of Residual Stresses Minimizing the level of RS also requires a correct choice of the welding procedure, as illustrated below: • Tack welding: It is used to fix the position of two or more metal parts, which are to be joined by welding (Fig a). • Deposition of long welds: Welds of great length are usually deposited by serial welding of small section rather than by welding in one run (Fig b). • Multi-pass welding: In the case of multi-pass welding Lower the inter-pass temperature higher the residual stresses.
Fig: Tack welding Fig: Deposition of long welds Note: Number shows sequence of welding
Distortion • Residual stress due to heating and cooling of the HAZs and the contraction of the weld metal as it cools from a molten state to ambient temperature is an unavoidable feature of welded joints. • The stress deriving from this shrinkage results in distortion. This distortion may be localised, evenly distributed and acceptable or may render the entire structure unfit for its purpose. • In a ship’s hull, for instance, buckling of the hull lates can induce turbulence and increase drag; in piping it can restrict fluid flow; and in architectural applications it can be aesthetically unacceptable.
Distortion In Welds • As shown, angular distortion increases with workpiece thickness because of increasing amount of the weld metal and hence increasing solidiďŹ cation shrinkage and thermal contraction.
Fig: Distortion in butt Welds of AA 5083 with thickness 6.4-38 mm 1 - 22
Types of distortion? Distortion is caused by differential thermal expansion and contraction of different regions of the welded assembly. Distortion occurs in six main forms: 1. Longitudinal shrinkage 2. Transverse shrinkage 3. Angular distortion 4. Bowing and dishing 5. Buckling 6. Twisting
Explanations To Different Forms of Distortion • • • • • •
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Contraction of the weld area on cooling results in both transverse and longitudinal shrinkage. Non-uniform contraction (through thickness) produces angular distortion in addition to longitudinal and transverse shrinkage. For example, in a single V butt weld, the first weld run produces longitudinal and transverse shrinkage and rotation. Upward angular distortion usually occurs when the weld is made from the top of the workpiece alone The second run causes the plates to rotate using the first weld deposit as a fulcrum. Hence, balanced welding in a double side V butt joint can be used to produce uniform contraction and prevent angular distortion. Similarly, in a single side fillet weld, non-uniform contraction produces angular distortion of the upstanding leg. Double side fillet welds can therefore be used to control distortion in the upstanding fillet but because the weld is only deposited on one side of the base plate, angular distortion will now be produced in the plate.
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Longitudinal bowing in welded plates happens when the weld centre is not coincident with the neutral axis of the section so that longitudinal shrinkage in the welds bends the section into a curved shape. Clad plate tends to bow in two directions due to longitudinal and transverse shrinkage of the cladding; this produces a dished shape. Dishing is also produced in stiffened plating. Plates usually dish inwards between the stiffeners, because of angular distortion at the stiffener attachment welds (see main photograph). In plating, long range compressive stresses can cause elastic buckling in thin plates, resulting in dishing, bowing or rippling. Distortion due to elastic buckling is unstable: if you attempt to flatten a buckled plate, it will probably 'snap' through and dish out in the opposite direction. Twisting in a box section is caused by shear deformation at the corner joints. This is caused by unequal longitudinal thermal expansion of the abutting edges. Increasing the number of tack welds to prevent shear deformation often reduces the amount of twisting.
Distortion of a welded structure
Fig: Distortion of a welded structure.
Factor Affecting Distortion • The amount of distortion is affected by: 1. Heat input from the welding process, 2. Welding sequence, 3. Joint design, 4. Degree of fixing/clamping i.e. restraining of plates 5. Stresses in the parent metal and 6. Physical characteristics of parent metal .
How To Control Distortion? Distortion may be minimized by following measures: 1. Weld on or very close to the neutral axis. 2. Balance welds about the neutral axis of the component. 3. Use lowest heat input process and welding parameters, 4. Use the fewest number of weld passes to fill the joint. 5. On long welds, weld from the centre towards the ends.
6. Use a ‘back-step’ sequence• 7. Break the construction down into sub-assemblies, 8. Preset the components. 9. Use automatic welding. 10. Use a planned welding sequence. 11. Use adequate tack welds. 12. Ensure that the joint fitup is accurate. 13. Do not over-weld. 14. Use jigs and fixtures
Measure To Reduce Distortion
Measure To Reduce Distortion
Methods For Controlling Distortion
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Rectification of distortion • If the measures discussed earlier are not effective, remedial measures to rectify the distortion will be necessary. • These may be based upon those used for steel but great care needs to be exercised if such techniques are used. • The most effective methods are those that use some form of mechanical working or stretching as these will not significantly affect the mechanical properties of the base materials. • Longitudinal bow in welded beams should preferably be done cold by pressing, and buckled plate may be pressed flat. • As a last resort, local spot or line heating may be used to heatshrink items that have been distorted by the welding of, for instance, stiffeners. The high thermal conductivity of aluminium
means that local heating with an oxy-gas torch is not very effective. If this technique is to be used then electric induction heating is the most effective method of introducing sufficient heat
Rectification Techniques For Distortion As a last resort, local spot or line heating may be used to heat-shrink items that have been distorted by welding of, for instance, stiffeners
Line
Wedge
Spot heating
How much shall I allow for weld shrinkage? • It is almost impossible to predict accurately the amount of shrinking. Nevertheless, a 'rule of thumb' has been composed based on the size of the weld deposit. When welding steel, the following allowances should be made to cover shrinkage at the assembly stage. • Transverse Shrinkage • Fillet Welds 0.8mm per weld where the leg length does not exceed 3/4 plate thickness • Butt weld 1.5 to 3mm per weld for 60° V joint, depending on number of runs • Longitudinal Shrinkage • Fillet Welds 0.8mm per 3m of weld • Butt Welds 3mm per 3m of weld • Increasing the leg length of fillet welds, in particular, increases shrinkage.