Residual Stresses and Distortion

Residual Stresses and Distortion

Lesson Objectives

When you finish this lesson you will understand:

the generation of residual stress and distortion in welds mechanisms to reduce residual stress

Learning Activities

Read Handbook pp 218-229View Slides; Read Notes, Listen to lectureDo on-line workbook

Keywords

Thermal Expansion Coefficient, Residual Stress, Distortion, Stress-Relief

Welding Design

Welding design involves consideration of strength requirements, cost, and service conditions

Mechanical & Physical propertiesJoint DesignWelding stress and distortion

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Covered

Earlier

We have been looking at the functions of designing weldments. We have looked at required mechanical and physical properties, joint designs which maintain integrity across the joint in either static or dynamic loading conditions, we have examined symbols which allow us to communicate our thinking about the weld joint. Before leaving the design considerations, however, we want to look at the residual stresses present in welds and some of the distortion problems which arise as a result of these stresses.

Lets begin by looking at sources of residual stresses and distortion. When materials undergo a change in temperature, they undergo a change in length or strain. In almost every case, heating a material causes expansion, cooling causes contraction. The thermal strain is related to the change in temperature by the equation above where alpha is the coefficient of thermal expansion. When a part is unconstrained, like the rocket above being warmed on one side by the sun, expansion occurs and the part distorts. Often, parts are constrained by surrounding material and the full amount of expansion or contraction can not occur. In these cases, residual stresses builds up within the part. When tensile residual stresses are present in one part of the material, counter balancing compressive stresses must also be present. The stresses around the weld bead or associated with grinding are good examples of these residual stresses distributions which are of interest t the welding engineer.

Residual Stress & Distortion in Welds

Heat flows from the weld area and causing the joint area to expand (See Previous Module)Thermal expansion and contraction due to welding can leave behind permanent stress and distortionHigher heat input welds are more prone to residual stress and distortion Increased restraint can decrease distortion but can result in higher residual stress

Residual Stress and Distortion

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The heat involved in welding produces thermal expansion. The stress associated with this expansion changes both during the course of the welding process, and after the weld is completed until the joint returns to ambient temperature. A pattern of permanent stress may be left in the joint; this is referred to as residual stress. Permanent strain left in the joint is referred to as distortion.

Processes that put large amounts of energy into the weld, such as the submerged arc process, are more likely to result in residual stress and distortion. Laser and electron beam welds, with their overall low heat input, tend not to result in residual stress or distortion problems.

In order to reduce distortion, the piece being joined may be fixed into position by clamping. This method will reduce distortion, but may increase the residual stress in the joint after welding.

Another method of reducing distortion is to weld the plates with a preset. If two plates tend to distort upwards by 5 after they are welded, then position the plates with a 5 downward preset before welding.

Lets look more closely at the generation of residual stresses in a part. If we first look at the samples above, and heat the middle bar of the free state sample, we would notice that the middle bare would expand, but there would be not stress built up in the outer bars as long as the expansion was not great enough to have the two faces on the middle bar touch. The case is not the same with the stress state sample. If the middle bar here were heated without heating the two outer bars, the middle bar would try to expand, but it would be constrained by the two outer bars. The middle bar would be under compression, while it forced the two outer bars into tension. In the next slide we will go through a heating and a cooling cycle of this middle bar. We will heat so much that the bars will not only experience tension and compression n the elastic region where we learned before that taking the load off causes the bars to go back to their original position, but we will heat to a point where plastic deformation occurs. It is also instructive to note before we begin this exercise that the three bars represent a model of a weldment where the heated middle bar is the weld and the cooler outer bars are the base metal on either side of the weld.

Starting at A where there are no stresses, we heat the middle bar, it expands and causes compressive stress in the middle bar as temperature increases. At point B, the stress in the bar exceeds the yield point at that temperature and plastic deformation begins. As temperature continues to increase the yield point at that temperature decreases causing the stress in the bar to follow path BC. At point C we begin to cool, and the bar begins to contract. At first this reduces the compressive stresses built up in the bar until the stress now equals zero (but the bar is still at about 1000F). With further cooling, the bar continues to contract but now it is putting itself into tension. This continues to point D where the tensile yield stress at that temperature is exceeded and further cooling and contraction causes tensile deformation until room temperature is reached and the tensile residual stresses of magnitude E are reached. What do you think was happening during this cycle in the two outer bars? The load in the opposite sense was divided between them. Can you draw the respective figure for the outside bar? In the final analysis, the center bar has residual tensile stress and the outside bars have residual compress when the sample has returned to room temperature. Lets see how this related to a weld.

Residual Stress Pattern

Residual stress is present across an unrestrained butt weld after coolingTension near weld beadCompression away from the weld beadRestraint can affect this stress state

Compression

Tension

Residual Stress and Distortion

Note that the black center line and the two blue outside lines represents the previous model. As a weld cools, it attempts to contract more than the base metal, since the base metal was not heated to as high a temperature. As the weld shrinks, it is restrained by the surrounding base metal. Thus, after welding, a state of residual tension is produced in the weld. The base metal near the weld is in compression, which balances out the tension to yield a net force of zero on an unrestrained plate. The presence of restraint can affect this stress pattern.

Let us now examine the development of the residual stress profile as the weld bead passes. The diagram above plots both the temperature profile at each cross section as well as the expected residual stress distribution profile. At section AA in front of the weld, there is not yet any temperature change and therefore the stress is zero. At section BB through the molten weld nugget there is a large temperature profile. This leads to compressive stresses around the nugget (the liquid however can not support a stress) and balancing tensile stresses in the base metal. At section cc where there is still some temperature distribution,however, because of thermal diffusion the profile is spreading and the peak is lowering. The stress profile shows some tensile stresses developing at the weld centerline and compressive stresses in the base metal. Finally at DD when the weld has cooled to room temperature , the stress profile examine before emerges.

Since the residual stress is not only two dimensional but rather three dimensional, there must be a stress distribution built up in the longitudinal weld direction as well. And indeed there is as represented by the diagram presented above. These stress distributions will remain at the high levels noted here as long the the plate remains fixed and unable to move to accommodate the stress or unless some other stress relieving treatment is performed.

If the weld with residual stress distribution is then put into service and loaded with additional stress, there may be some modification of the residual stress distribution. With the application of a tensile load, the original residual stress distribution will increase in tensile load. Some yielding may occur at the higher stress regions (Curve b or c). If the load is then removed, a new residual stress curve reflecting the localized yielding results (curve e or f).

Turn to the person sitting next to you and discuss (1 min.):

We have seen from the metallurgy module that dislocation tangles and pile-ups occurring above the yield strength of materials cause the higher stresses. We have seen here that by a temperature cycle that varies across the plate we have regions that yield and residual stresses occur. We have seen that imposing operating stresses above yield cause changes in the residual stress distribution by movement of the dislocations. How else then might we get rid of some of these dislocations to remove these residual stresses?

Types of Distortion

Residual Stress and Distortion

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Distortion

Transverse Shrinkage

Rotational Distortion

Longitudinal Bending

Buckling

Longitudinal Shrinkage

Angular Change

Residual Stress and Distortion

Transverse shrinkage results in a decreased plate width after welding.

Longitudinal shrinkage causes the plate to bow inward in the vicinity of the ends of the weld.

Angular distortion changes the alignment of the plates from their original placement prior to welding. In the example above, the previously flat base of the fillet weld has rotated towards the vertical member.

Rotational distortion becomes a factor when a long section has to be welded. In this case, tack welds are generally used to hold a section in place. For a cylindrical weld, e.g., a pipe or storage tank, block welding is used. Welding is accomplished in sections, alternating from one side to another to balance distortion.

Longitudinal bending results in the bowing of the flat base plate of a long fillet weld.

Buckling is perhaps the most difficult type of distortion to correct. Essentially, the material adopts a sinusoidal wave pattern in response to welding stress. This distortion is most often seen in the welding of thin plate or panel material.

Eliminating Distortion

Preset members to counteract distortionFixtures to clamp workpiece in placeRestraint reduces distortion but increases residual stressStress-relief heat treatment

Angular distortion after welding

Preset members before welding

Residual Stress and Distortion

In order to reduce distortion, the piece being joined may clamped into position. This method will reduce distortion, but may increase the residual stress in the joint after welding.

Another method of reducing distortion is to weld the plates with a preset. If two plates typically distort upwards by 5 after they are welded, then position the plates with a 5 downward preset before welding.

Heat treatment after welding can be used to relieve residual stress. In steels, stress relief is accomplished in the 1100-1200F temperature range.

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