AWS WHC1 07-2001 Residual Stress and Distortion.pdf

1、RESIDUALSTRESS AND DISTORTIONPrepared by theWelding Handbook Chapter Committee on Residual Stress and Distortion:K. Masubuchi, ChairMassachusetts Institute of TechnologyO. W. BlodgettThe Lincoln Electric CompanyS. MatsuiThe New Industry Research OrganizationC. O. RuudThe Pennsylvania State Universit

2、yC. L. TsaiThe Ohio State UniversityWelding Handbook Volume 1 Committee Member:T. D. HesseConsultantContentsIntroduction 2Fundamentals 2Nature and Causesof Residual Stress 4Effects ofResidual Stress 12Measurement ofResidual Stress 17Residual StressDistribution Patterns 22Effects of SpecimenSize and

3、Weight 26Effects of WeldingSequence 29Residual Stress inWelds Made withDifferent WeldingProcesses 30Weld Distortion 32Reducing orControlling ResidualStress and Distortion 55Conclusion 58Bibliography 58SupplementaryReading List 602 RESIDUAL STRESS AND DISTORTIONINTRODUCTIONThe types of residual stres

4、s that occur in welds andtheir respective distribution patterns are quite complex.This chapter presents an analysis of stress in single- andmultiple-pass welds and examines the various factors thatinteract to increase or decrease the magnitude of stress inwelds. As distortion in weldments is an impo

5、rtant factorin their serviceability, the procedures used to predict dis-tortion are also discussed here. In the final section, thevarious procedures used to reduce or control residualstress and distortion in welds are examined in detail.Since most information published on this subjectconcerns welds

6、produced with the arc welding pro-cesses, the discussions presented in this chapter almostexclusively address residual stress and distortion inwelds fabricated with these processes. A limited amountof information is presented on residual stress in spotwelded joints in titanium 8Al-1Mo-1V alloy.FUNDA

7、MENTALSA weldment undergoes localized heating during mostwelding processes; therefore, the temperature distribu-tion in the weldment is not uniform, and structural andmetallurgical changes take place as the weldingprogresses along a joint. Typically, the weld metal andthe heat-affected zone immediat

8、ely adjacent to the weldare at temperatures substantially above that of theunaffected base metal. As the weld pool solidifies andshrinks, it begins to exert stress on the surroundingweld metal and heat-affected zones. When the weldmetal first solidifies, it is hot and relatively weak; thus,it exerts

9、 little stress. As the weld cools to ambient tem-perature, however, the stress in the weld area increasesand eventually reaches the yield point of the base metaland the heat-affected zone.When a weld is made progressively, the portions ofthe weld that have already solidified resist the shrinkageof l

10、ater portions of the weld bead. Consequently, theportions welded first are strained in tension in a direc-tion longitudinal to the weld, that is, down the length ofthe weld bead, as shown in Figure 1.In the case of butt joints, little motion of the weld ispermitted in the transverse direction becaus

11、e of thepreparation of the weld joint and the stiffening effect ofunderlying passes. Because of shrinkage in the weld,transverse residual stress is also present, as shown inFigure 1. For fillet welds, the shrinkage stress is tensilealong the length and across the face of the weld, asshown in Figure

12、2.Residual stress in weldments can have two majoreffects. It can produce distortion or cause prematurefailure, or both. Distortion is caused when the heatedweld region contracts nonuniformly, causing shrinkagein one part of a weld to exert eccentric forces on theweld cross section. The weldment stra

13、ins elastically inresponse to this stress. Detectable distortion occurs as aresult of this nonuniform strain.RESIDUAL STRESSAND DISTORTIONCHAPTER 9Figure 1Longitudinal (L) and Transverse (T)Shrinkage Stress in a Butt Joint WeldRESIDUAL STRESS AND DISTORTION 3In butt joints in plate, this distortion

14、may appear asboth longitudinal and transverse shrinkage or contrac-tion. It may also appear as angular change (rotation)when the face of the weld shrinks more than the root.Angular change produces transverse bending in theplates along the weld length. These effects are illus-trated in Figure 3.Disto

15、rtion in fillet welds is similar to that whichoccurs in butt welds. Transverse and longitudinalshrinkage and angular distortion result from the unbal-anced nature of the stress present in these welds. As filletwelds are often used in combination with other welds inwelded structures, the specific res

16、ulting distortion maybe very complex. This behavior is shown in Figure 4.Distortion can be controlled by means of a numberof techniques. The most commonly used techniquescontrol the geometry of the welded joint either beforeor during welding. These techniques include (1) prepo-sitioning the workpiec

17、es prior to welding so that thesubsequent weld distortion leaves them in the desiredfinal geometry and (2) restraining the workpieces sothey cannot distort during welding. Designing andwelding the joint so that weld deposits are balanced oneach side of the weld centerline is another useful tech-niqu

18、e. The selection of the welding process to be usedas well as the weld sequence can also influence distor-tion and residual stress.Residual stress and distortion affect the fracturebehavior of materials by contributing to buckling andbrittle fractures at low applied-stress levels. When resid-ual stre

19、ss and the accompanying distortion are present,buckling may occur at lower compressive loads thanwould otherwise be predicted. In tension, residual stressmay lead to high local stress in weld regions of lownotch toughness. This local stress may initiate brittlecracks that are propagated by any low o

20、verall stressthat is present. In addition, residual stress may contrib-ute to fatigue or corrosion failures.Residual stress may be reduced or eliminated by boththermal and mechanical means. During thermal stressrelief, the weldment is heated to a temperature at whichthe yield point of the metal is l

21、ow enough for plasticflow to occur and thus allow relaxation of stress. Themechanical properties of the weldment are usuallyaffected by thermal stress relief. For example, the brittlefracture resistance of many steel weldments is oftenimproved by thermal stress relief because residual stressin the w

22、eld is reduced and the heat-affected zones aretempered. The toughness of the heat-affected zones isimproved by this procedure. Mechanical stress-relieftreatments also reduce residual stress, but they do notsignificantly change the microstructure or hardness ofthe weld or heat-affected zone.Improving

23、 the reliability of welded metal structuresis of the utmost importance. During the design phase,engineers must consider the effects of residual stress anddistortion, the presence of discontinuities, the mechanicalFigure 2Longitudinal (L) and Transverse (T)Shrinkage Stress in a T-JointFigure 3Schemat

24、ic Representationof Distortion in a Butt JointFigure 4Schematic Representationof Distortion in a T-Joint4 RESIDUAL STRESS AND DISTORTIONproperties of the weldment, the requirements for non-destructive examination, and the total fabrication costs.The reduction of residual stress and distortion can be

25、achieved by means of a number of techniques, includingthe following:1. Choosing appropriate processes, procedures,welding sequence, and fixturing;2. Selecting optimal methods for stress relief andthe removal of distortions; and3. Using design details and materials to minimizethe effects of residual

26、stress and distortion.NATURE AND CAUSESOF RESIDUAL STRESSThe term residual stress refers to the stress that existsin a weldment after all external loads have beenremoved. Various terms have been used to describeresidual stress. These include internal stress, initialstress, inherent stress, reaction

27、stress, and locked-instress. However, the residual stress that occurs when astructure is subjected to nonuniform temperaturechange is usually termed thermal stress.Residual stress develops in metal structures duringthe various manufacturing stages for many reasons.During casting or mechanical workin

28、g (e.g., rolling,forging, or bending), stress may be produced in struc-tural components such as plates, bars, and sections. Itmay also occur during fabrication as a result of weld-ing, brazing, and thermal cutting operations.Heat treatments applied at various stages of manu-facture can also affect r

29、esidual stress. For example,quenching from elevated temperature can cause residualstress, whereas stress-relieving heat treatments canreduce it.MACROSCOPIC AND MICROSCOPIC RESIDUAL STRESSThe portions of a metal structure in which residualstress can be found vary greatly, ranging from large sec-tions

30、 of the structure to areas on the atomic scale.Examples of macroscopic residual stress are presentedin Figure 5. When a structure is heated by solar radia-tion on one side, thermal distortions and thermal stressare produced in the structure, as shown in Figure 5(A).The residual stress produced by we

31、lding is illustratedFigure 5(B). In this figure, it can be observed that thestress is confined to areas near the weld. Figure 5(C)depicts residual stress produced by grinding. In thiscase, the stress is highly localized in a thin layer near thesurface.Residual stress also occurs on a microscopic sca

32、le.For example, residual stress is produced in steels duringmartensitic transformation.1As this process takes placeat a low temperature, it results in the expansion of themetal.FORMATION OF RESIDUAL STRESSThe different types of residual stress are classifiedaccording to the mechanisms that produce t

33、hem,namely, structural mismatching and the uneven distri-bution of nonelastic strains, including plastic and ther-mal strains.Residual Stress Resulting from Structural MismatchFigure 6 illustrates a simple case in which residualstresses are produced when bars of different lengths areforcibly connect

34、ed. Figure 6(A) shows the system in thefree state. An opening exists between the two portionsof Bar Q, which is slightly shorter than Bars P and P.When these two portions are forcibly connected asshown in Figure 6(B), tensile residual stresses are pro-duced in Bar Q, while compressive residual stres

35、ses areproduced in Bars P and P. If the cross-sectional areas ofP, P, and Q are equal, the absolute values of the stressesin Q are twice those present in P and P. The entire sys-tem becomes slightly shorter after the two portions ofbar Q are forcibly connected.Satoh, Matsui, and Machida2used an expe

36、rimentalsystem similar to that shown in Figure 6 to study themechanisms leading to the formation of residualstresses. Figure 7 shows the experimental system used.Two round bars were used to restrain the movement ofthe round bar specimen shown in the middle. Theround bar specimen, which was set in th

37、e rigid frame,was subjected to a thermal cycle simulating the weldingthermal cycle. The specimen was first heated using ahigh-frequency induction device. The specimen wasthen naturally air-cooled or control-cooled using astream of argon gas. A load cell attached to the speci-men was used to measure

38、thermal stresses developed inthe specimen. The thermal cycle was measured bymeans of thermocouples.1. Martensitic transformation in steel is described in Chapter 4, Vol. 1of the Welding Handbook, 9th ed., Miami: American Welding Society.2. Satoh, K., S. Matsui, and T. Machida, 1966, Thermal Stresses

39、Developed in High-Strength Steels Subjected to Thermal Cycles Simu-lating Weld Heat-Affected Zone, Journal of Japan Welding Society35(9): 780788 (in Japanese).RESIDUAL STRESS AND DISTORTION 5Figure 5Macroscopic Residual Stresses on Various Scales: (A) Thermal Distortion Due toSolar Heating; (B) Resi

40、dual Stress Due to Welding; and (C) Residual Stress Due to GrindingFigure 6Residual Stress Produced When Bars of Different LengthsAre Forcibly Connected: (A) Free State and (B) Stressed State6 RESIDUAL STRESS AND DISTORTIONExperiments were performed on two types of steel:(1) a low-carbon steel and (

41、2) a Japanese HT-70 steel,which is very similar to the U.S. HY-80 steel. TheHT-70 steel is a low-alloy, high-strength steel with theminimum tensile strength of 99.6 ksi (70 kg/mm2or686 MPa), while the HY-80 steel has the minimumyield strength of 80 ksi (56.2 kg/mm2or 552 MPa). Thelatter is widely us

42、ed for the pressurized hulls of U.S.submarines.The experimental results obtained in the investiga-tion of the low-carbon steel and HY-80 steel specimensare shown in Figures 8(A) and Figure 8(B), respectively.Figure 8(A) illustrates the changes in stresses thatoccurred in the low-carbon steel bar dur

43、ing the heatingand cooling cycles. While the middle bar specimen wasbeing heated, compressive stresses were produced in thespecimen because the expansion of the heated middlebar was restrained by the rigid frame.As the temperature of the middle bar specimenincreased, the compressive stresses in the

44、bar increased,as shown by Line AB. As the temperature of the middlebar specimen reached approximately 600F (300C),the stress reached the yield stress of 34 ksi (230 MPa),as indicated by Point B.As the temperature was further increased beyondPoint B, the compressive stresses in the middle bar spec-im

45、en were limited to the yield stress, which decreasedwith temperature, as shown by Curve BC. When thetemperature of the middle bar specimen approximated1380F (750C), the compressive stress in the middlebar became almost zero. The heating was terminatedat Point D when the temperature reached 2570F(141

46、0C).As the temperature of the middle bar specimen fellbelow 2570F (1410C), the bar started to shrink, butthe tensile stresses remained very low until the temper-ature decreased to approximately 1100F (600C), indi-cated by Point E. The stress increased rather rapidlywhen the temperature decreased bel

47、ow 1100F (600C).The stress reached the yield stress in tension at 400F(204C), as indicated by Point F.As the temperature decreased further, the magnitudeof the stress increased only slightly to the final value ofapproximately 60 ksi (406 MPa). Line X shows themanner in which the stresses increased d

48、uring air cool-ing, while Line Y shows how the stresses increased dur-ing rapid cooling. It can be observed that the differencein the cooling method had little effect on the amount ofresidual stresses that remained in the specimen after theheating and cooling cycle.Figure 8(B) shows the results of s

49、imilar experimentsperformed on the high-strength steel, HY-80. Althoughthe results shown in Figure 8(B) are similar to those pre-sented in Figure 8(A), several differences can be noted.Regarding Point B, at which the compressive stressreached the maximum, the HY-80 specimen showed themaximum stress of 51 ksi (406 MPa) as compared to34 ksi (234 MPa) for the low-carbon steel specimen.This difference is due to the differing strengths of thesematerials at elevated temperatures. A unique feature ofthe HY-80 specimen is the significant drop in tensileresidual stresses during