Chapter 3B Harasawa

8/18/2019 Chapter 3B Harasawa

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Edited by the courtesy of textbooks published by The Japan Welding Society forcertification of welding coordination personnel in The Japan Welding EngineeringSociety standard ( WES 8103) , and AWS Welding Handbook Vol.1

WELDING TECHNOLOGY SEMINAR 200 9by H.HARASAWA ( Technical Advisor )

3B. DESIGN

(For Welding Engineer)

CONTENTS

3.1. GENERAL CONSIDERATIONS

3.2 PROPERTIES OF METALS3.2.1 Mechanical Properties of Weld Joints

Static strength, Fatigue strength, Fracture toughness, Creep rupture3.2.2 Physical Properties

Thermal expansion, Thermal conductivity, Melting point, Specific heat etc.3.2.3 Corrosion Properties

3.3 RESIDUAL STRESSES AND DISTORTION3.3.1 Residual Stresses3.3.2 Distortion

3.4 DESIGN PROGRAM

3.5 WELD JOINT GEOMETRY AND WELDING SYMBOLS

3.6 CALCULATION OF WELD STRENGTH


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3.3. RESIDUAL STRESSES AND DISTORTION

3.3.1 Residual Stresses

(1) Characteristics

1) The effects are normally accounted forin the design rules for the application.

2) In frame structures with residual stress,precautions should be considered especiallyfor the reduction of fatigue strength and/or

the risk of stress corrosion crack .

3) In vessel structures with heavy plate thickness,PWHT ( Post Weld Heat Treatment )

is dominantly applied to eliminate residual stress .

(2) Mechanism and distribution of residual stressWhen welding is performed, the weld joint is exposed to very

high temperatures. As the yield point of metal fails at highertemperatures, the welded area yields due to compression stressfrom the surrounding area . This is why deformation occurs afterwelding. Fig. 3.8 shows the mechanism of the above behavior.In this figure, W corresponds to the weld joint .

When only W is heated, B restrains W's expansion . As a result,tensile stress occurs in B, and compression stress in W. Thesestresses, due to difference in temperature, are called thermalstresses. With further heating of W, stress increases, W's yieldpoint lowers, and at last W yields due to compression stress(Fig. 3.8 (b) ). When W is cooled to room temperature, it isshorter than before due to plastic deformation (Fig. 3.8 (c) ). Inthe case of an actual structure, W has stiffeners affixed on bothends, tensile stress remains in W, and compression stressremains in B ( Fig. 3.8 (d) )


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Stiffener

Stiffener

Yield by compressionW is fixed to stiffener bypulling up

Original Only W is heated.( W yields bycompression)

W is cut fromstiffener andcooled.

W is fixed to stiffener again.

Fig3. Mechanism of Residual Stress in Weld Joint

Residual stress -characteristics-• Mechanism

Deformation in free condition is arrestedby RESIDUAL STRESS. Large R.S. along welding line

Small R.S. transverse to welding line• Characteristics

RESIDUAL STRESS is induced by internal force.

NO RESIDUAL STRESS at free edge. All R.S. in body are balanced

Nothing to give a reaction


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Fig.3. Typical Residual Stress Distributions in Butt Weld

(3) Influence of residual stress on mechanical propertiesof weld

A weld joint in a ductile material (such as mild or low alloysteel) can withstand considerable plastic deformation before

breaking.

When static tensile force acts on a weld joint, the local area(with residual stress) yields.The residual stress is eliminated just prior to the yield of the

total joint, so the residual stress does not affect the breakingstress of the weld joint.


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1) Influence of residual stress on fatigue strengthIt is not easy to determine the exact influence of residual

stress on fatigue strength, as various other influences suchas metallurgical unevenness, weld defects, andreinforcement coexist with residual stress in the weld joint.

According to sophisticated tests, as tensile residual stressraises mean stress levels, fatigue strength lowers .

It is thought that residual stress has little influence on low-cycle fatigue strength, as residual stress is minimized by therepetition of high stress before the occurrence of a fatigue

crack .

However, when a sharp notch exists in a weld structure,experiments show that fatigue cracks initiate at far lowertensile stress than yield stress.

2) Influence of residual str ess on br ittle fr acturesIf a sharp notch exists in a structure, even a ductile material

such as mild steel can be broken below designed stresslevels by a brittle fracture at a low temperature.

When tensile residual stress coexists with a notch as shownin Fig. 3.6 , the structure may be broken at far lower stress

levels than normal. According to experiments, the transition temperature of astructure with residual stress (as welded condition) is higherthan that for a structure without residual stress.

Post weld heat treatment (PWHT) can remove residualstress and is effective in preventing brittle fractures.


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3) Influence of r esidual str ess on buckling and str esscorrosionResidual stress often causes buckling of a structure.

Stress corrosion occurs when, under tensile stressconditions, a crack appears and widens in the presenceof materials such as H2S and water, alkali, nitrate, coalgas, liquid ammonia, etc .

In general, cold working and the existence of tensilestress promotes stress corrosion.

(4) Removal of residual stress1) Post w eld heat treatment and methodsThere are mechanical treatments as well as heat treatments to remove

residual stress.Post weld heat treatment ( PWHT ) is the most important of these. The

yield point of a metal falls at high temperatures. When a metal understress (below the yield point) is subjected to high temperatures, plasticdeformation occurs in the metal to reduce the stress. Both tensileresidual stress and compression residual stress coexist in a weldstructure, and both residual stresses can be eliminated by maintainingthe structure at a high temperature.

Typically, a structure of mild steel is placed into a furnace, heated to540-600 for 1 hour per 25mm of structure thickness, then slowlycooled . In the case of a structure of 2.25Cr- 1Mo steel , heating iscarded out at 680 for 1 hour per 25mm of structure thickness. When astructure is too large to be placed in a furnace, local heat treatment isapplied along the weld joint. Heating temperature should be belowtempering temperature in the case of a structure of quenched andtempered steel.


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(2) Effect s of PWHTThe effects of PWHT are not only removal of residual

stress but also the softening of the HAZ , improvement ofelongation , discharging of diffused hydrogen along theweld joint, recovery of notch toughness , and control ofdistortion .

In the case of any steel structure in which toughness at theweld joint is deemed insufficient and where thickness ismore than 38mm (1 and 1/2inches), PWHT is usually

required . PWHT is often used to control stress corrosion .

3.3.2 Distortion

A weld structure may become distorted due to localexpansion and shrinkage along the weld line as shown inFig. 3.10

The effects are also accountedin the design rules for the application.

Distortion such as angular distortion acts as SCF ( StressConcentration Factor )

closely related to the initiation of fatigue crackand/or brittle fracture.


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(1) Typical weld distor tion

1) Transverse shrinkageTransverse shrinkage of butt weld joint S can be

approximated as follows:

S = 0.18L (mm)L: breadth of weld groove

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

2) Rotative distortionDuring manual welding, a weld groove may contract and the

root 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.

3) Angular distor tion Angular distortion is a result of asymmetric heating onthe face and reverse side of a plate.

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

Angular distortion worsens with increasing heat inputuntil a maximum distortion appears at a certain heatinput. Beyond a certain heat input, distortion is muchless because both sides of the plate are heated.


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(a) Transverse shrinkage (b) Longitudinal shrinkage (c) Longitudinal bending

(e) Rotative dist ortion

Direction ofwelding

Direction ofwelding

(d) Angular distortion (f) Buckling

In-plane distortion

Out-of-plane distortion


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(2) Prevention of distort ion

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

Weld distortion can be controlled by adjusting thesefactors. Good weld sequence, good deposition sequence,and proper use of restraint jigs can reduce distortionsignificantly.

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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3.4 DESIGN PROGRAM

3.4.1 Determination of Load Conditions

(1) Torque : shaft, revolving part

(2) Forces on members: dead weight of parts

(3) Maximum load on members : crane hoist, shovel,lift track, material handling equipment

(4) Maximum strength : cables

(5) Shear force : pins

(6) Frequency of applied load : cranes, railway bridges,vibrating machines/ equipment


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3.4.2 Major Design Factors

(1) Strength and stiffness(2) Safety factor (3)Good appearance(4)Deep, symmetrical sections to resist bending(5)Welding the end of beams rigidly(6)Rigidity with welded stiffeners to minimize the weight of

material(7)Tubular sections or diagonal bracing for torsion loading(8)Standard rolled sections, plate, and bar for economy

and availability

(9) Accessibility for maintenance(10) Standard, commercially available components

specified by index tables, way units, heads, and columns

3.4.3 Designing The Weldment

(1) General Points for effective design

1)Easy handling of materials, inexpensive tooling, andaccessibility of the joints

2)Check with the shop for ideas to cost savings

3)Establish realistic tolerances based on end use andsuitability for service

4)Minimize the number of pieces to reduce assembly time andthe amount of welding


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(2)Part Preparation

Select appropriate method for available material andequipment and the relative cost

Method: Thermal cuttingShearingSawingBlankingNibblingMachining

Back weld preparation: Air carbon arc gougingOxygen gougingChipping

(3)FormingFactors to choose forming method:

Base metal compositionPart thicknessOver-all dimensionsProduction volume

TolerancesCost

Cold forming reduces the ductility and increase the yieldstrength of metals .

Generally, the relevant standard provides maximum coldforming allowances.

For example, Section VIII of the ASME Boiler and PressureVessel Code requires under certain circumstances coldforming that results in extreme fiber elongation over 5 % inplates must be stress relieved.


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(4) Weld Joint Design

Should be selected primarily on the basis of loadrequirement.

However, variables in design and layout cansubstantially affect costs.

The application of general rules is mandatory.

(5) Size and Amount of Weld

Over design is a common error,as is over-welding in production.

Control of weld size begins with design,but it must be maintained

during the assembly and welding operations.


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Basic guides for control the size and amount of welds:

1) Adequate but minimum size and length should be specified.Oversize welds:

causes excessive distortion and higher residual stressincreases cost

The size of fillet weld is importantbecause the amount of weld required increases

as the square of the weld size increases.

2) Continuous fillet weld is preferable to intermittent fillet weld.It is less costly,

and there are fewer weld terminationsthat are potential sites of discontinuities.

3) Intermittent fillet weld is usedwhen static load conditions do not require continuous weld.

This weld should not be used under cyclic loading conditions.

4) To derive maximum advantage of automatic welding ,it may be better to use one continuous weld

rather than several short welds

5) The weld should be placed in the section of least thickness,and the weld size should be based on the load of that section.

6) Welding of stiffeners and diaphragms should be limited to that required tocarry the load,

and should be based on expected out-of-plane distortion of the supportedcomponents

under service loads as well as during shipment and handling.

7) The amount of welding should be kept to a minimum to limit distortion andinternal stresses and, thus, the need and cost for stress-relieving andstraightening.


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(6) Subassemblies

In visualizing assembly procedures, the weldment intosubassemblies in several ways

could offer the greatest cost savings .

The following are advantages of subassemblies:1) Several subassemblies can be

worked on simultaneously .2) Better access for welding can be provided,

and automatic welding may be permitted.

3) Distortion may be easier to control .4) Large size welds may be deposited under

lesser restraint with minimizing residual stressesin the completed weldment.

5) Machining of subassemblies to close tolerances can bedone before final assembly.

If necessary, stress relief can be performed before finalassembly.

6) Chamber compartment can be leak testedand painted before final assembly.

7) In-process inspection and repair is facilitated.

8) Handling costs may be much lower .

When possible, it is desirable to construct the weldment fromstandard sections,

so that the welding of each can be balancedabout the neutral axis.


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3.4.4 Welding Procedures

The following guidelines can be effective in weldment design:

(1) Flat position in welding is preferable for Quality, Cost, and Delivery.

(2) Joint design requiring welding only from one side should beconsidered

to avoid manipulation or overhead welding.

(3) Backing strips increase the speed of welding

for the first pass in groove weld.

(4) The use of low hydrogen electrodes or welding processeseliminates or reduces preheat requirements.

(5) Reinforcement of a weld is generally unnecessary toobtain a full- strength joint.

(6) With T-joints in thick plate, lamellar tearing should beavoided using a material

with improved mechanical properties forthrough-thickness directions.

(7) Joints in thick sections should be welded underconditions of least restraint ;

for example, prior to installation of stiffeners.

(8) Sequencing of fit up, fixturing, and welding is importantfor box members made of plates,

because correction of distortionafter completion of welding is virtually impossible.


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3.4.5. Laminations and Lamellar Tearing

(1) Consideration of the problem of lamellar tearing mustinclude design aspects

and welding procedures that are consistentwith the through-thickness properties

of the base material .(2) The designer should specify material properties, and NDE

procedures such as UTfor the receiving inspection of material

and for the critical welds after fabrication.

3.5 WELD JOINT GEOMETRY AND WELDING SYMBOLS3.5.1 Types of weld jo ints

Fig. 3.11 Butt joint

Fig. 3.12 T joint and cruciform joint

Fig. 3.13 Corner joint Fig. 3.14 Lap joint

Fig. 3.15 Spliced joint Fig. 3.16 Flare joint


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3.5.2 Weld grooves

3.5.3 Fillet weld joint s


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R: Root opening, A: Groove angle,S: Size of weld joint (depth of groove, leg length, etc.),L: Weld length, n: number of weld, P: Pitch of weld

3.6 Welding symbols

V

J

U

Flare

Fillet

Welding symbols -AWS, JIS-

Square

Note

Penetration depth (AWS)

260 o

G

Field weld flag

Point togroove face

10(15)

Groove depth, if PJP (AWS)

Rootopening

Groove angle

Finishing

Arrow side

Other side

• Non-symmetric groove

Arrow sideOther side

Arrow side

Other side

Point togroove face

Reference linein grooved plate side

Bevel


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FilletSize=6mm

Machinefinishing

45o

10

All aroundfield weld,Size=7mm

7

All aroundfield weld

045 oM

106

Fillet weldat other side

Bevel grooveat arrow side,PJPPoint to

groove face

Welding symbols -Examples-

Surrounding with circlemeans PJP in JIS

Grooved butt joint of tubeV groove with

depth=10mmangle=60 oroot gap=2mm

Melt through (back bead)

UT from outside after weld

45 o

12

6

22

2

60 o

45 o

60 o2

612

Symmetric informationshould be above ref. line

Reference line should belaid in the grooved plate side

Welding symbols -Examples-

260 o

10

UT


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r=6

Detail note islaid in the tail

13

134

40 o

40 o

r=6

40o

013

Welding symbols -Examples-Symmetric informationshould be above ref. line

Reference line should belaid in the grooved plate side

Location of welding symbol


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Writing welding symbol

for a beveled groove

Auxiliary symbols for NDT etc.


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Example of groove weld (1)



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Example of fillet weld symbols (1)



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Symbols for non-destructive testing

Division Symbol NoteRadiographictest

General RT The category “ General” isthe case where only themethod of each test such asthe radiographic test and thelike is s hown and the contentis not given.

An appr opri ate mark ing canbe used, as required, fortests not represented by thesymbols show n here.(Example)Leak test LTVisual test VT

Eddy current test ET

Double wallphotographing

RT-W

Ultrasonictest

General UT

Normal beam method UT-N

Angle beam method UT-A

Magneticparticletesting

General MT

Fl uo res cen ce d et ec ti ng MT-F

Penetranttest

General PT

Fl uo res cen ce d et ec ti ng PT-F

Non-fluorescencedetecting

PT-D

Whole test These symbols shall besuffixed to each symbol oftest.Spot test (sampling test)

3.6 Calculation of weld strength

Any weld joint in a structure should furnish enough strengthto withstand any anticipated static or dynamic load.When PWHT is specified, the base metal and weld materials

should be chosen carefully to ensure required structuralstrength.

Structures for public use should be built carefully inaccordance with applicable rules, provisions, and/orspecifications.

Considerations to ease of welding, the possibility of futurenon- destructive tests, and future repair operations arealways necessary, and step-by-step communication betweendesigners and production/welding engineers is also essential.


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∑=

al

Pσ

∑=

al

Pτ

∑ al

(1) Load and st ress on weld joint

a

a

a

a

PJP : a = [groove depth]( a -XX in specific cases )

Leg S a

Safety side design

Assured minimum throat

LegS 1 a

Leg

S2

a= 0.7S

Theoretical throat -examples-

S 2


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Throat thickness of full penetration groove joint

a) Butt joint b) Butt welding of different thickness plate c) T joint

Throat thickness of fillet weld(Fillet weld with equal leg length)


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832

6060 o

60 o

But joint with partial penetration

20

10100 (10X20)

100

30 o

T joint with partial penetration

4010

45 ob

t

Practice -effective cross sectional area -Determine the effective cross sectional area of the jointsas follow;

(a)(b)

(c) (d)

• Fillet size : S

t1

t2 S

Design Guide Line for Steel Bridge(Japanese Society of Civil Engineering)t1 S 2 t2 and S 6mm

Design Guide Line for Steel Structure

(Architectural Institution of Japan)t1 6mm ;

t1 S 1.3 t2 and S 4

AWS D1.1t2= ~ 6.4mm S=3mm t1t2= 6.4 ~ 12.7mm S=5mm t1t2= 12.7 ~ 19mm S=6mm t1t2= 19mm ~ S=8mm t1

(2)Required weld size for fil let joint


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Face joint Appliedforce P Applied

force P

Load AreaStress=

Allowable stress Effective area Allowable load

(3) Allowable stress in basic design-normal stress and shear stress-

Internalforce P

Section A 2Section A 1Normal stress =

P A 1

Shear stressτ = P

A 2

Appliedforce P

Appliedforce P

Internal force P

σ d σ a σ s

Safety Factor =σ sσ a

CJP

Reference strength σ s

Allowable stress σ aDesign stress σ d

Strain

Stress

σ ys

σ uts

Allowable stress σ a

Allowable stress τ a

Reference strength σ s σ ys, 0.7 σ uts σ ysσ ys

1.5σ ys

1.5 3FilletPJP

σ ys

1.7σ ys

1.7 3

Al lowable st ress

Design Guide Linefor Steel Bridge

Design Guide Linefor Steel Structure


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0.707 x 4 x L x 4 (path) x 100(N/mm 2) = 100 x 10 3 (N)

Weld length

Throat of fillet weld Allowable stress

4

L

A

A A-A section

4

Allowable σ a ?

10 5

0.707 x 16 x 100L= =89 (mm)

Example -basic joint design - A steel plate is inserted into the slit of a tube. Both of the plate and thetube are jointed with fillet weld. This joint must be tolerant of 100 kNtensile loading. Determine the required weld length. Suppose that theyield strength ( σ ys) of the material are 346 N/mm 2. Safety factor shouldbe taken as 2.

Reference strength σ s = σ ys = 346 N/mm 2

σ s

2346

2= =173 N/mm 2σ a =

σ a 1731.73

= =100 N/mm 2τ a = 3

25mm

PP

Steel plate(7mm)

100mm

The steel piece is welded on the steel plate of 7mm thickness shown in thefigure below. The minimum size of the fillet weld should be referred toDesign Guide Line for Steel Structure (Architectural Institution of Japan) .Estimate the endurable tensile load P. Suppose that the yield strength ( σ ys)of the material are 173 N/mm 2. Safety factor should be taken as 2. Nobending moment can be assumed in the weld.

Practice -basic joint design -


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25mm

PP

Steel plate(7mm)

100mm

The steel piece is welded on the steel plate of 7mm thickness shown in thefigure below. The minimum size of the fillet weld should be referred toDesign Guide Line for Steel Structure (Architectural Institution of Japan) .Estimate the endurable tensile load P. Suppose that the yield strength ( σ ys)of the material are 173 N/mm 2. Safety factor should be taken as 2. Nobending moment can be assumed in the weld.

Practice -answer -

• Allowable stress σ a =173/2=86.5 (N/mm 2)

• Fillet weld size

1.3 t 2 S t 11.3 25 = 6.5 S 7

S=6.5(mm)Throat (mm)

6.5 X 0.707 X 2 (25+100) X 50 =57 (kN)

Length(mm)

τ a (N/mm 2)

86.51.73

= =50 (N/mm 2)τ a = 3

σ a

Design the plate thickness of the butt joint shown in (b) in order to assurethe same joint strength with the fillet weld joint shown in (a). Suppose thesame steel in both joints.

8

10

20

P

P

t

P

P

(a)Depth=100mm

Practice -basic joint design -

(b)


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Design the plate thickness of the butt joint shown in (b) in order to assurethe same joint strength with the fillet weld joint shown in (a). Suppose thesame steel in both joints.

8

10

20

P

P

t

P

P

(a)Depth=100mm

Practice -answer -

(b)

Allowable stress : σ a

P a=0.707 X 8 X 2 X 100 X σ a X 3

1Weld length(mm)

Throat (mm) Allowable stress

Fillet weld is designed

by shear stress in any loading

P b=t X 100 X σ a

Coming from “P a= P b”

0.707 X 8 X 2

3t = =6.5 (mm)

Endurable strength in (a); P a?

Endurable strength in (b); P b?

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Chapter 3B Harasawa