ThermalStrain,ResidualStresses and Distortion.pdf

7/24/2019 ThermalStrain,ResidualStresses and Distortion.pdf

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WELDING MECHANICSTHERMAL STRAINS, RESIDUAL STRESSES & DISTORTION

AP Dr. Samsudin Bani

Fabrication & Joining Section2013


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THERMAL STRAINS, RESIDUAL

STRESSES & DISTORTION


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THERMAL STRAIN

Thermal strain is a deformation of a material caused by

temperature change.

deformation = x (T)xL

where;

is coefficient of thermal expansion

(T) is change in Temp

L is original length

Strain = deformation / L(original)


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THERMAL STRESS

Thermal stress is related. Imagine the same scenario, but the

material is constrained and not allowed to expand.

The stress developed would be the same as taking the

expanded material and compressing it to fit within constraint.

Thermal Stress = x (T) x E

where E is young's modulus of elasticity.


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RESIDUAL STRESS

During welding a weldment undergoescomplex temperature changes that cause non-elastic strains.

After welding is completed these strainsremain in the welded plate causing distortionsand residual stress in the plate.

They need to be determined and allowed forin the calculation concerning the platestrength.


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RESIDUAL STRESS

Mechanism and distribution of residual stress

1. When welding is performed, the weld joint is exposed tovery high temperatures. As the yield point of metal fails athigher temperatures, the welded area yields due to

compression stress from the surrounding area. This is whydeformation occurs after welding.

2. Fig. 3.25 shows the mechanism of the above behavior. Inthis figure, W corresponds to the weld joint.

3. When only W is heated, B restrains W's expansion. As aresult, tensile stress occurs in B, and compression stress inW. These stresses, due to difference in temperature, arecalled thermal stresses.


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RESIDUAL STRESS

Mechanism and distribution of residual stress

3. With further heating of W, stress increases, W'syield point lowers, and at last W yields due tocompression stress (Fig. 3.25 (b)).

4. When W is cooled to room temperature, it is shorterthan before due to plastic deformation (Fig. 3.25(c)). In the case of an actual structure, W has

stiffeners affixed on both ends, tensile stressremains in W, and compression stress remains in B(Fig. 3.25 (d)).


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RESIDUAL STRESS

Fig. 1 shows a typical longitudinal residualstress distribution obtained theoretically ina plate when a weld bead is laid along the

center of the plate.

In the transverse direction of the plate, the

longitudinal stress is tensile over a smallregion at the weld and compressive overmost of the plate width.


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RESIDUAL STRESS

Fig 1. A typical residual stress distribution over the

transverse section of a plate


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RESIDUAL STRESS

The forces caused by the stresses must, of

course, balance. The maximum residual

stress occurs near the weld, is tensile and is

reported to be as high as the yield stress of

the material for any ordinary mild steel

(lowcarbon steel).


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Fig. 2 An idealised residual stress distribution over the

transverse section of a plate


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RESIDUAL STRESS

The actual residual stress distribution is oftenidealized as shown in the Fig. 2 where a width ofsteel is assumed to have yielded.

The width of the tension zone depends on thewelding parameters, the plate geometry and thethermal and mechanical properties of the platematerial.

Since residual stresses exist without external forces,the complete residual stress distribution is self-equilibrating.


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RESIDUAL STRESS

The value of compressive residual stress

depends on the width of the plate. The greater

width, the smaller is the compressive stress

required to balance the tension in the weld.

The width of the plate does influence the

general rise in temperature after welding andthis may influence the magnitude of the

residual stresses for a narrow plate.


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RESIDUAL STRESS

There are three stresses in the welded plate (Fig. 3):

1. those stresses parallel to the weld direction(longitudinal),

2. those perpendicular to the welding directionand in the plane of the plate (transverse)and

3. those in the direction of the thickness(through-thickness).


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RESIDUAL STRESS

Fig.3. Typical Residual Stress Distributions in Butt Weld


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RESIDUAL STRESS

The stresses may be approximately constant

through the thickness in a thin plate but

may vary considerably in a thicker plate for a

given heat input (Fig. 4).

In that case, there will be lower bending

occurring in a thin plate than in a thicker

plate


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RESIDUAL STRESS

Fig. 4


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RESIDUAL STRESS

Masubuchi reported that the magnitude anddistribution of the residual stresses in a weldmentwere found to be affected by;

1. The temperature distribution in the weldment.2. The thermal expansion characteristic of the material.

3. The mechanical properties of the material at elevatedtemperature.

He added that the weld length had little effect on themaximum stress in either the weld area or the areasfurther away from the weld.


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RESIDUAL STRESS

(1) Characteristics

i. The effects are normally accounted for in thedesign rules for the application.

ii. In frame structures with residual stress,precautions should be considered especially for thereduction of fatigue strength and/or the risk ofstress corrosion crack.

iii. In vessel structures with heavy plate thickness,PWHT (Post Weld Heat Treatment ) is dominantlyapplied to eliminate residual stress.


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RESIDUAL STRESS

Fig. 5


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RESIDUAL STRESS

1) Influence of residual stress on fatigue strength

i. It is not easy to determine the exact influence

of residual stress on fatigue strength, as variousother influences such as metallurgicalunevenness, weld defects, and reinforcementcoexist with residual stress in the weld joint.

ii. According to sophisticated tests, as tensileresidual stress raises mean stress levels, fatiguestrength lowers.


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RESIDUAL STRESS

1) Influence of residual stress on fatigue strength

i. It is thought that residual stress has littleinfluence on low-cycle fatigue strength, as

residual stress is minimized by the repetition ofhigh stress before the occurrence of a fatiguecrack.

ii. However, when a sharp notch exists in a weldstructure, experiments show that fatigue cracksinitiate at far lower tensile stress than yieldstress.


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RESIDUAL STRESS

2) Influence of residual stress on brittle fractures

i. If a sharp notch exists in a structure, even a

ductile material such as mild steel can bebroken below designed stress levels by abrittle fracture at a low temperature.

ii. When tensile residual stress coexists with anotch, the structure may be broken at farlower stress levels than normal.


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RESIDUAL STRESS

2) Influence of residual stress on brittle fractures

i. According to experiments, the transition temperature

of a structure with residual stress (as weldedcondition) is higher than that for a structure without

residual stress.

ii. Post weld heat treatment (PWHT) can remove

residual stress and is effective in preventing

brittle fractures.


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RESIDUAL STRESS3) Influence of residual stress on buckling and stress

corrosion

i. Residual stress often causes buckling of a structure.

ii. Stress corrosion occurs when, under tensile stressconditions, a crack appears and widens in thepresence of materials such as H2S and water,alkali, nitrate, coal gas, liquid ammonia, etc.

iii. In general, cold working and the existence oftensile stress promotes stress corrosion.


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RESIDUAL STRESS(4) Removal of residual stress

1) Post weld heat treatment and methods

i. There are mechanical treatments as well as heat treatmentsto remove residual stress.

ii. Post weld heat treatment (PWHT) is the most important ofthese. The yield point of a metal falls at high temperatures.When a metal under stress (belowthe yield point) is subjectedto high temperatures, plastic deformation occurs in the metalto reduce the stress.

Both tensile residual stress and compression residual stresscoexist in a weld structure, and both residual stresses can beeliminated by maintaining the structure at a hightemperature.


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RESIDUAL STRESS

(4) Removal of residual stress

1) Post weld heat treatment and methods

i. Typically, a structure of mild steel is placed into afurnace, heated to 540-600for 1 hour per 25mmof structure thickness, then slowly cooled. In thecase of a structure of 2.25Cr- 1Mo steel, heating iscarded out at 680for 1 hour per 25mm ofstructure thickness.

ii. When a structure is too large to be placed in afurnace, local heat treatment is applied along theweld joint. Heating temperature should be belowtempering temperature in the case of a structureof quenched and tempered steel.


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RESIDUAL STRESS

(5) Effects of PWHT

i. The effects of PWHT are not only removal of residualstress but also the softening of the HAZ, improvement of

elongation, discharging of diffused hydrogen along theweld joint, recovery of notch toughness, and control ofdistortion.

ii. In the case of any steel structure in which toughness at

the weld joint is deemed insufficient and wherethickness is more than 38mm (1 and 1/2inches), PWHT isusually required. PWHT is often used to control stresscorrosion.


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Analysis of Residual Stress

It is generally known that temperatures during welding areusually very high and in the vicinity of a weldment. As suchthe region near the weld yields and deforms plastically.

On cooling, solidification and contraction take place but thecontraction is restrained by the adjacent cooler base plateresulting in the formation of a shrinkage force, (Fs) at theweld.

Because of the restraint against the shrinkage, the weld isstressed in tension. A stress field is thereby generated inthe plate which contains balanced zones of compressiveand tensile stresses (Figure 5).


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In Figure above, the area ABBA represents thelongitudinal tensile force (the shrinkage force, Fslocked into the weld. White et al. in analysis the stressfield in a welded plate suggested that it is better to

consider a force given by the full area ACCA in thefigure and called this Tendon Force, FT.

This is because they believed that it is the tendon force

which is a function purely of the welding parametersand therefore should be used to predict the residualstress.


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They then developed a simple relationship between the tendonforce and the welding parameters as follows;

(3.14)

Where, FT = Tendon force (kN); Q = A V, heat input (W)and v = weldingspeed (mms-1). H is a dimensionless constant and depend on the type ofwelding processes and the arc efficiency ()[44]. White at el. Proposed avalue of HT= 0.22 for the MIG welding process when considering the valueof = 80%.

However, the concept of a shrinkage force (FS) was used byKamtekar in assessing the residual stresses in thin steel plates. Hebegan by developing a relationship between the shrinkage force (FS)and the welding parameters as follow;


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(3.15)

Because it has been shown that only a proportion of the totalheat input (governed by the arc efficiency ) is actuallytransferred to the plate, the arc efficiency n has to be allowed

for in the calculation of the shrinkage force.

A relationship between Hs and the heat input per unit lengthwas developed. From the relationship Hswas derived where, theshrinkage force (inEq. 3.15) was theoretically estimated.

In determining the value of Hs, several tests were conducted andthe theoretically and experimental value of Hswere compared.From the comparison Hswas found to be 0.12 (for= 80%:MIGwelding)


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Because of yielding, a narrow band of plastic region of width ycforms about the weld (as shown in Figure 5) along the length of theplate. The extent of the plastic region depends on the weldingparameters, the yield stress (o) of the plate material and the platethickness and they are expressed as follows;

(3.16)

where, Fsis the shrinkage force (kN), yc is the width of plastic zone aboutthe weld (mm);

o

is the yieid stress (Nmm-2) and d is the plate thickness(mm).


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Outside the plastic region (onthe assumptionthat the rest of the section remains elastic andthe stress in uniform through the thickness)the stresses arc compressive and the meancompressive stress (A) is expresses as;

(3.17)

where W is the width of the plate (mm).


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Distortion

Because of solidification shrinkage

and thermal contraction of the weld

metal during welding, the workpiecehas a tendency to distort.


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Types of Distortion


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Type of Distortion


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Distortion(1) Typical weld distortion

1) Transverse shrinkage

Transverse shrinkage of butt weld joint S can be approximatedas follows:

S = 0.18L (mm)

where L: breadth of weld groove

In other words, the larger the root gap, groove angle, ordeposited metal, the more the transverse shrinkage.

2) Rotativedistortion

During manual welding, a weld groove may contract and theroot gap may narrow due to low heat input and slow welding.Conversely, during submerged arc welding, the root gap maywiden due to excessive heat input and fast welding.


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Distortion3) Angular distortion

i. Angular distortion is a result of asymmetric heatingon the face and reverse side of a plate.

ii. This occurs more frequently with multi- pass weldingusing a V- groove than when welding using a doubleV- groove.

iii. Angular distortion worsens with increasing heatinput until a maximum distortion appears at acertain heat input. Beyond a certain heat input,distortion is much less because both sides of theplate are heated.


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Welding distortion

In a bead-on-plate weld, there are two possible type of distortion. Theyare;

1. Longitudinal bending (Figure 7)

2. Transverse angular change or wrap-up () about the weld line(Figure 8).

When a weld is laid on a plate surface it does not coincide with thecentroidal axis of the plate; therefore longitudinal shrinkage of the weldinduces bending moments causing the plate to bed longitudinally.

Thereby creating longitudinal bending stresses in the plate. Because ofthis bending, there is a vertical displacement or out-of-straightness(Figure7) of the plate along the weld direction; this is referred to as alongitudinal distortion ( is usually very small).


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Welding distortion

Fig 7


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Welding distortion

Fig. 8


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Welding distortion

The transverse shrinkage as the relative inward movementof points D D (Fig. 9) at the mid-thickness located oneither side of a butt weld joint. This shrinkage andtherefore the angular distortion are functions of the weldparameters, plate geometry and the material properties.

The magnitude of . Is highly sensitive to the degree ofpenetration (depth of weld) and the angular restraintpresent during welding. If the weld does not fully penetratethe plate thickness, then the unmelted part of the plate

below the molten pool will resist shrinkage and therefore,resulting in a lower .


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Welding distortion

When the heat input increases. However, when

the heat input is high enough to cause yielding

throughout the thickness then the transverseshrinkage becomes more or less uniform through

the thickness resulting in a lower .

The relationship of the angle and the heat input

described above is thus shown in Fig. 10.


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Welding distortion

Fig. 9


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Welding distortion

Fig. 10: the angle versus the heat input in a given plate geometry


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Welding distortion

The other type of distortion is the wrap-up

()of the plate about the weld line (Figure

8). is the result of the non-uniform

temperature distribution through the platethickness because this distribution will

result in the formation of non-uniform

plastic strains so that the shrinkage will verythrough the plate thickness.


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Welding distortion

In principle, the amount of angular distortion of a weldedplate would depend on the plate geometry (especially theplate thickness), the weld heat input and the degree ofrestraint.

For a bead-on-plate weld when the plate is not restrainedand free to move, the angular distortion increases with heatinput. However, as the heat input increases yielding increaseand tends to become uniform through the thickness.

The transverse shrinkage then would be constant through

the thickness and as such the angular distortion is expectedto decrease.


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WeldingDistortion

1. The longitudinal distortions (hence bending

stresses) and compressive residual stressesincrease significantly when the plate is narrow.

2. The longitudinal distortion (hence bending stress)initially increases with heat input per unit length,but decreases as the heat input large.

3. The shrinkage force (F) can be estimated using Eq.3.15 and consequently can be used in a rough

estimate of the width of the yield zone and theresidual stresses.

4. The Hs factor is Eq. 3.15 was found experimentallyto be between 0.11 and 0.16.


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WeldingDistortion5. It can be estimated that the angle in 8, 15 and

25mm thick plates reached its maximum at ofbetween 70 and 100kNmm . Beyond this rangethe angle decreased significantly in the 8mmthick plate but remained approximately

constant in the 15 or 25mm thick plates.

6. For a given heat input per unit area (throughthethickness), , before is maximum, the angulardistortion in a thin plate is greater than in athicker plate.

7. The plate width has negligible effect on theangular distortion.


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Welding distortion

Prevention of distortion

Factors which affect weld distortion include heat input,preheating temperature, thickness of plate, type of weld groove,restraint, weld sequence or deposition sequence, and welding

process.

Weld distortion can be controlled by adjusting these factors.

i. Good weld sequence, good deposition sequence, and proper

use of restraint jigs can reduce distortion significantly.

ii. Pre-strain can also minimize distortion. Distortion can berepaired with roller or press machines, or by local heatingand cooling (spot or line heating).


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Analysis of Distortion

A simple analytical model was then

developed, based on a simply supported

beam with the shrinkage force (Fs) applied at

the welded edges as the external force. Fromthe model, the vertical distortion (Figure 11) is

given by;


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Fig. 11


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Where

(Fs is the shrinkage force); L = distance

between the neutral axis of the beam and the

weld L = length of beam at the mid-length (i.e.

L / 2) the equation becomes;


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To predict the angular distortion ( ), the

empirical relationship was developed:

in degrees.

Q = heat input

V = welding speed

d = plate thickness


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Welding distortion

i. Design consideration

ii. Materials consideration

iii. Preheating

iv. Welding procedurev. Welding sequence

vi. PWHT

vii. Natural ageing

viii. Peening

ix. Vibratory stress relieving


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THE END

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