3. Residual Stresses

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Caren Lawson
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1 3. Residual Stresses

2 3. Residual Stresses 22 Figure 3.1 br-ei-3-1e.cdr weld Various Reasons of Residual Stress Development grinding disk less residual stresses, and it will never be stress-free! The emergence of residual stresses can be of very different nature, see three examples in Figure 3.1. Figure 3.2 details the causes of origin. In a produced workpiece, material-, production-, and wearcaused residual stresses are overlaying in such a way that a certain condition of residual stresses is created. Such a workpiece shows in service more or Figure 3.3 defines residual stresses of 1., 2., and 3. type. This grading is independent from the origin of the residual stresses. It is rather based on the three-dimensional ex of the stress conditions. Based on this definition, Figure 3.4 shows a typical distribution of residual stresses. Residual stresses, which build-up around dislocations and other lattice imperfections (σ III ), superimpose within a grain causing stresses of the 2 nd type and if spreading around several grains, bring out residual stresses of the 1 st type. The formation of residual stresses in a transition-free relevant material e.g. polyphase systems, non-metallic inclusions, grid defects forming e.g. thermal residual stresses br-ei-3-2e.cdr Figure 3.2 deforming residual stresses due to inhomogenuous deformationanisotropy Analysis of Residual Stress Development production separating residual stresses due to machining mechanical e.g. partial-plastic deformation of notched bars or close to inclusions, fatigue strain joining residual stresses due to welding wear thermal e.g. thermal residual stresses due to operational temperatur fields plating layer residual stresses Development of Residual Stresses chemical e.g. H-diffusion under electro-chemical corrosion changing material characteristics induction hardening, case hardening, nitriding

3 < < < 3. Residual Stresses 23 steel cylinder is shown in Figures 3.5. and 3.6. During water quenching of the homogeneous heated cylinder, the edge of the cylinder cools down faster than the core. Not before 1 seconds have elapsed is the temperature across the cylinder's cross section again General Definition of the Term Residual Stresses Residual stresses of the I. type are almost homogenuous across larger material areas (several grains). Internal forces related to residual stresses of I. type are in an equilibrium with view to any cross-sectional plane throughout the complete body. In addition, the internal torques related to the residual stresses with reference to each axis disappear. When interfering with force and torque equilibrium of bodies under residual stresses of the I. type, macroscopic dimension changes always develop. + - y I II III x Residual stresses of the II. type are almost homogenuous across small material areas (one grain or grain area). Internal forces and torques related to residual stresses of the II. type are in an equilibrium across a sufficient number of grains. When interfering with this equilibrium, macroscopic dimension changes may develop. x Residual stresses of the III. type are inhomogenuous across smallest material areas (some atomic distances). Internal forces and torques related to residual stresses of the III. type are in an equilibrium across small areas (sufficiently large part of a grain). When interfering with this equilibrium, macroscopic dimension changes do not develop. II III grain boundaries E = + II + III = residual stresses between several grains = residual stresses in a single grain = residual stresses in a point br-er3-3e.cdr br-er3-4e.cdr Definition of Residual Stresses Definition of Residual Stresses of I., II., and III. Type Figure 3.3 Figure 3.4 homogeneous. The left part of Figure 3.5 shows the T-tcurve of three different measurement points in the cylinder. Figure 3.6 shows the results of quenching on the stress condition in the cylinder. At the beginning of cooling, the cylinder edge starts shrinking faster than the core (upper figure). Through the stabilising effect of the cylinder core, Temperature 1 C mm diameter water cooling 1 edge 2 5 % radius 3 core M S s 1 4 Cooling time br-ei-3-5e.cdr Figure 3.5 Temperature 1 C Temperature in a Cylinder During Water Cooling s 1 s 15 s 2 s 25 s 35 s 45 s 5 s 1 s 53 s 2 68 s 1 28 s mm 17,5 1,5 3,5 3,5 1,5 17,5 Radius

4 3. Residual Stresses 24 tensile stress builds up at the edge areas while the core is exposed to stress. Resulting volume differences between core and edge are balanced by elastic and plastic deformations. When cooling is completed, edge and core are on the same temperature level, the plastically stretched edge now supports the unstressed core, so that stresses are present in the edge areas and tensile residual stresses in the core. 3 N/mm² E 2 D Volume differences between edge and core at start of cooling Compensation of volume differences by plastic deformation and stresses at start of cooling Stresses in the central rod 1-1 A C -2 B B' br-er3-6e.cdr Compensation of volume differences by plastic deformation and stresses at end of cooling -3 br-er3-7e.cdr 2 4 C 6 Temperature of the central rod Volume Changes During Cooling Residual Stress Development by Warming the Central Rod Figure 3.6 Figure 3.7 These changes are principally shown once again in Figure 3.7 with the 3-rod model. A warming of the middle rod causes at first an elastic expansion of the outer rods, the inner rod is exposed to stress (line A-B). Along the line B-C the rod is plastically deformed, because stresses have exceeded the yielding point. At point C, the cooling of the rod starts, it is exposed to tensile stress due to shrinking. Along the line D-E the rod is plastically deformed due to the influence of the counter members beeing in. At the point E the system has cooled down to its initial temperature. This point represents the remaining residual stress condition of this construction. If heating is stopped before point C is reached and cooled down to the initial temperature, then stress increase in the centre rod will be in parallel

5 3. Residual Stresses 25 with the elastic areas. Starting with point B, the same residual stress condition is present as in a case of heating up to a temperature above 6 C. Figure 3.8 divides the development of residual stresses in welded seams in three different mechanisms. Shrinking stresses: these are stresses formed through uniform cooling of the seam. Caused by expansion restriction of the colder areas at the edge of the weld and base material, tensile stresses develop along and crosswise to the seam. Quenching stresses: If cooling is not homogenous, the surface of the weld cools down faster than the core areas. If the high-temperature limit of elasticity is exceeded due to buildup stress differences, stresses will be present at the weld surface after cooling. In contrast, the core shows tensile stresses in cold condition (see also Figure 3.6). Transition stresses: Transitions in the ferrite and perlite stage cause normally only residual stresses, because within this temperature range the yield strength of the steel is so low that generated stresses can be undone by plastic deformations. This is not the case with transitions in the Bainite and martensite stage. A transition of the austenite causes an increase in volume (transition cfc in cbc, the cfc lattice has a higher density, additional volume increase through lattice deformation). In the case of a homogenous transition, the weld will consequently unfold stresses. If the transition of the edge areas happens earlier than the transition of the slower cooling core, plastic deformations of the core area may be present similar to quenching (see above: quenching stresses). In this case, the weld surface will show tensile stresses after cooling. Generally these mechanisms cannot be separated accurately from each other, thus the residual stress condition of a weld will represent an overlap of the cases as shown in the 4 th figure. This overlap of the different mechanisms makes a forecast of the remaining residual stress condition difficult. -x 1. Shrinking stresses + +y -x +x 3. Transformation stresses + +y inhomogenuous transformation -x +x +y -y +x 2. Quenching stresses 4. Overlap options of case 1., 2. and y -x +x Stress Distributions and Superpositions Perpendicular to Welded Joint -x - -y homogenuous transformation - -y - -y br-er3-8.cdr Figure 3.8 +x

6 3. Residual Stresses 26 A B weldpool Seam area of plastic deformations C D br-er3-9e.cdr Figure 3.9 M x A B C D M' Temperature distribution 1. cut A-A T ~ 2. cutt B-B 3. cut C-C 4. cut D-D T = Stress distribution X Formation of Residual Stresses Caused by Welding Heat stress-free residual stresses Figure 3.9 shows the building-up of residual stresses crosswise to a welded seam in analogy to the 3-rod model of Figure 3.7. This figure considers only shrinking residual stresses. Before application of welding heat, the seam area is stress-free (cut A-A). At the weldpool the highest temperature of the welding cycle can be found (cut B-B), metal is liquid. At this point, there are no residual stresses, because molten metal cannot transmit forces at the weldpool. Areas close to the joint expand through welding heat but are supported by areas which are not so close to the seam. Thus, areas close to the joint show compression stress, areas away from the joint tensile stress. In cut C-C the already solidified weld metal starts to shrink and is supported by areas close to the seam, the weld metal shows tensile stresses, the adjacent areas compression stresses. In cut D-D is the temperature completely balanced, a residual stress condition is recognised as shown in the lower right figure. Figure 3.1 shows how much residual stresses are influenced by constraining effects of adjacent material. The resulting stress in the presented case is calculated according to Hooke: σ = ε E 3115 mm 15 mm 13 a a material S235JR (St 37) 1. a = 1 mm = 8 N/mm² 2. a = 15 mm = 53 N/mm² 3. a = 2 mm = 4 N/mm² 4. a = 25 mm = 3 N/mm² 5. a = 3 mm = 27 N/mm² br-er3-1e.cdr Elongation ε is calculated as l/a ( l is the length change due to shrinking). With con- Shrinking Stresses in a Firmly Clamped Plate Figure 3.1

7 Force 3. Residual Stresses 27 stant joint volume will shrinking and l always have the same value. Thus the elongation ε depends only on the value a. The smaller the a is chosen, the higher are the resulting stresses. Effects of transition on cooling can be estimated from Figure Here curves of temperature- and length-changes of ferritic and austenitic steels are drawn. It is clear that a ferritic lattice has a higher volume than an austenitic lattice at the same temperature. A steel which transforms from austenite to one of the ferrite types increases its volume at the critical point. This sudden rise in volume can be up to 3% in the case of martensite formation. welding sample 3 x 1 x 3 (7,14) groove angle 6, depth 4,5 mm Longitudinal expansion l ferritic steel austenitic steel 8 firm clamping 1 thermo couples to calculator force sensor links N C mild steel with transformation Temperature elektrode heat affected zone 2 force Temperature [ C] -2 temperature s Time br-er3-11e.cdr br-er3-12e.cdr Longitudinal Expansion of Various Steels Force Measurement During Cooling of a Weld Figure 3.11 Figure 3.12 To record the effects of this behaviour on the stress condition of the weld, sample welds are carried out in the test device outlined in Figure Thermo couples measure the T-t curve at the weld seam, a force sensor records the force which tries to bend the samples. The lower picture shows the results of such a test. The temperature behaviour at the fusionline as well as the force necessary to hold the sample over the time is plotted.

8 3. Residual Stresses 28 In the temperature range above 6 C the force sensor registers a tensile force which is caused by the shrinking of the austenite. Between 6 and 4 C a large drop in force can be seen, which is caused by the transition of the austenite. The repeated increase of the force is based on further shrinking of the ferrite. With the help of TTT diagrams of base material and welding consumable, the transition temperatures and/or temperature areas for the individual zones of the welded joint can be determined. With these data and with the course of temperature it can be clearly determined in which part of the curve the force drop is caused by the transition of the welding consumable and in steel consumable electrode sample shape (V-groove, 6 ) type of welding position of the HAZ residual stress distribution L br-ei-3-13e.cdr Figure 3.13 austenitic S69QL (StE 7) S69QL (StE 7) austenitic austenitic high-strength surface weld surface weld Influence of Material Combination on Residual Stress Distribution in a Weld surface weld Angle change 14 % f = 1 f = 3 f = 7 f = 13 br-er3-14e.cdr Influence of Welding Sequence on Angle Distortion 5 42' 2 8' 1 51' a = 5 a = 7 a = 9 a = 12,5 which part by transition in the heat affected zone (HAZ). These results can be used to determine the longitudinal residual stresses transversal to the joint, as shown in Figure During welding of austenitic transition-free materials only tensile residual stresses are caused in the welded area according to Figure 3.8. If an austenitic electrode is welded to a StE 7, transitions occur in the area of the heat affected zone which lead to a decrease of tensile stresses. If a high-strength electrode which has a martensitic transition, is welded to a StE 7, then there will be residual stresses in the weld metal and tensile residual stresses in the HAZ. Figure 3.14

3. Residual Stresses 29 If parts to be welded are not fixed, the shrinking of the weld will cause an angular distortion of the workpieces, Figure 3.14.

9 3. Residual Stresses 29 If parts to be welded are not fixed, the shrinking of the weld will cause an angular distortion of the workpieces, Figure If the workpieces can shrink unrestricted in this way, the remaining residual stresses will be much lower than in case with firm clamping. Methods to determine residual stresses can be divided into destructive, non-destructive, and conditionally destructive methods. The borehole and ring core method can be considered as conditionally destructive, Figures 3.15 and In both cases, present residual stresses are released through partial material removal and the resulting deformations are then measured by wire strain gauges. An essential advantage of the borehole method is the very small material removal, the diameter of the br-ei-3-15e.cdr Figure 3.15 section workpiece borehole is only 1 to 5 mm, the bore depth is 1- to 2-times the borehole diameter. The disadvantage here is that only surface elongations can be measured, thus the results are limited residual stresses in the surface area of the workpiece. WSG Residual Stress Determination Using Bore Hole Procedure c plan a With the ring core method, a crown milling cutter is used to mill a ring groove around a three-axes wire strain gauge. The core is released from the force effects and stress-relieved. At the time when the resilience of the core is measured, the detection of the residual stress distribution b Figure 3.16

10 3. Residual Stresses 3 across the depth is also possible. Both methods are limited in their suitability for measuring welding residual stresses, because steep strain gradients in the HAZ may cause wrong measurements. The table in Figure 3.17 shows a survey of measurement methods for residual stresses and what causes residual stresses to be picked-up when using one of the respective methods. Figure 3.17 Figure 3.18 shows a survey of the completely destructive procedures of residual stress recognition. cutting in layers f f y x cutting-in z f assumption of stress distribution biaxial any uniaxial locally different linear, tensile residual stresses on top, down stresses measured variable bending deflection f curves reduced curves tear f y z zy residual stresses partial residual stress relief by z drilling T 45 L tripleaxial independent of smple length,, L T R length change circumference change L T L T R slitting.46f uniaxial linear symmetrically with reference to rod axis tear f partial residual stress relief by z br-ei-3-18e.cdr Destructive Methods for Determination of Residual Stresses Figure 3.18

9. Welding Defects 109

9. Welding Defects 9. Welding Defects 109 Figures 9.1 to 9.4 give a rough survey about the classification of welding defects to DIN 8524. This standard does not classify existing welding defects according