Weldinglectures9!10!121030153810 Phpapp02

7/25/2019 Weldinglectures9!10!121030153810 Phpapp02

http://slidepdf.com/reader/full/weldinglectures910121030153810-phpapp02 1/25

10/5/2012

1

, (ME31007)

Jinu Paul

111

Dept. of

Mechanical

Engineering

1

Course details: Welding

Topic Hours

1. Introduction to welding science & technology 2-3

2 Welding Processes 4

3 Welding Energy sources & characteristics 1-2

4 Welding fluxes and coatings 15 Physics of Welding Arc 1

6 Heat flow in welding 2

7 Design of weld joints 1

2

8 Defects, Testing and inspection of weld joints 1-2

9 Metallurgical characteristics of welded joints, Weldabilityand welding of various metals and alloys 2

Total 19



10/5/2012

2

Welding Lecture ‐ 904 Oct 2012, Thursday, 8.30‐9.30 am

Heat flow in welds

333

The welding thermal cycle• Thermal excursion Weld temp. ranges from the

ambient temp. of the work environment to above theliquidus temp. (possibly to boiling point and above forsome very g -energy- ens y processes

• The severity of this excursion in terms of the

– temp. reached

– time taken to reach them

– the time remain at them

4

microstructural for material changes andmacrostructural for distortion)

• To quantify the thermal cycle mathematically, weneed temp. distribution in time and space co-ordinates



10/5/2012

3

Thermal cycle characterization via

thermocouples

Temp.

5

Time

Thermal cycle- quasi-steady state

• Thermocouples at various points along weld path

• Approach of the heat source rapid rise intem erature to a eak a ver short hold at that peak then a rapid drop in temperature once thesource has passed by

• A short time after the heat from the source beginsbeing deposited, the peak temperature & rest ofthe thermal cycle, reaches a quasi-steady state

• Quasi-steady state balance achieved between the

6

ra e o energy npu an e ra e o energy oss ordissipation

• Quasi-steady state temperature isothermssurrounding a moving heat source remain steady andseem to move with the heat source (away from edges)



10/5/2012

4

Thermal cycle- quasi-steady state

S Fusion

zone

HAZ

7

Temperature isotherms surrounding a moving heat source

remain steady and seem to move with the heat source

Time-Temperature curvesTp Time= ti

Temperature

8Time



10/5/2012

5

Time-Temperature curves• The peak temp. decrease with increasing distance from

the source, and more or less abruptly

• The maximum temperatures reached (TmA TmB, Tmc)decrease with distance from the weld line and occur attimes (tmA, tmB, tmc) that increase. This allows the peaktemperature, Tp to be plotted as a function of time

• Peak temp. separates the heating portion of the weldingthermal cycle from the cooling portion,

• At a time when points closest to a weld start cooling, thepoints farther away are still undergoing heating. This

9

phenomenon explains – certain aspects of phase transformations that go on in the

heat-affected zone,

– differential rates of thermal expansion/contraction that lead tothermally induced stresses and, possibly, distortion

Spatial isotherms

Tp (Peak temperature)

oo ng zone

Cooling zone

10

Heating zone



10/5/2012

6

The generalized heat flow equation

• Temp. distribution Controls microstructure,residual stresses and distortions, andchemical reactions (e.g., oxidation)

• The influencing parameters

– the solidification rate of the weld metal,

– the distribution of peak temperature in the HAZ

– the coolin rates in the fusion and HAZ

11

– the distribution of heat between the fusion zoneand the heat-affected zone

• Requires mathematical formulation toquantify the influence of these parameters


Heat suppl ied + Heat generated = Heat

consumed for tem rise meltin + Heat

transferred via conduction + Heat loss via

convection & radiation

12



10/5/2012

7


x = coordinate in the direction of welding (mm)

= coordinate transverse to the weldin direction mm

z = coordinate normal to weldment surface (mm)

T = temperature of the weldment, (K)

k(T) = thermal conductivity of the material (J/mm s-1K-1) as afunction of temperature

(T) = density of the material (g/mm3) as a function of temp.

= -1 -1

13

,

temperatureVx, Vy, and Vz = components of velocity

Q = rate of any internal heat generation, (W/mm3)


• This general equation needs to be solved for one, two, orthree dimensions depends on – Weld geometry,

– Whether the weld penetrates fully or partially

– Parallel sided or tapered, and

– Relative plate thickness• 1-D solution thin plate or sheet with a stationary source or

for welding under steady state (at constant speed and inuniform cross sections remote from edges) in very thinweldments

14

• - so u on n we men s or n c er we men swhere the weld is full penetration and parallel-sided (as inEBW) to assess both longitudinal and transverse heat flow

• 3-D solution thick weldment in which the weld is partialpenetration or non-parallel-sided (as is the case for mostsingle or multipass welds made with an arc source)



10/5/2012

8

Weld geometry and dimensionality

of heat flow(a)2-D heat flow for

full-penetration

we s n t n p ates

or sheets;

(b)2-D heat flow for

full-penetration

welds with parallel

sides (as in EBW

and some LBW)

15

(c)3-D heat flow for

partial penetration

welds in thick plate

(d)3D, condition for

near-full penetration

welds

• Rosenthal’s first critical assumption Energy input

from the heat source was uniform and moved with a

Rosenthal’s Simplified Approach

constant velocity v along the x-axis of a fixed

rectangular coordinate system

• The net heat input to the weld under theseconditions is

given by

Hnet = EI/v (J/m)

16

where is the the transfer efficiency of the process E

and I are the welding voltage (in V) and current (in A),

respectively, and v is the velocity of welding or travel

speed (in m/s).



10/5/2012

9


Assumption 2 Heat source is a point source,

weld at a single point – This assumption avoided complexities with density distribution of

the energy from different sources and restricted heat flowanalysis to the heat-affected zone, beyond the fusion zone orweld pool boundary.

Assumption 3 The thermal properties (thermal

17

, ,

density, Cp) of the material being welded are constants Assumption 4 Modify the coordinate system from a

fixed system to a moving system


• The moving coordinate system replace x with,

source from some fixed position along the x

axis, depending on the velocity of welding, v=x-vt

where t is the time

18



10/5/2012

10


• This equation can be further simplified, in

, -

temperature distribution exists.

• Temperature distribution around a point heat

source moving at constant velocity will settle

down to a steady form, such that dT/dt = 0, for

=

19

.

Rosenthal’s solution

• Rosenthal solved the simplified form of the heat flow

equation above for both thin and thick plates in which

- - , .

• For thin plates,

20

=

k = thermal conductivity (in J/m s-1 K-1)

= thermal diffusivity = k/pC, (in m/s)

R = (2 + y2 + z2)1/2, the distance from the heat source to a particular

fixed point (in m)

K0 = a Bessel function of the first kind, zero order



10/5/2012

11


• For the thick plate,

where d = depth of the weld (which for symmetrical

21

welds is half of the weld width, since w = 2d)


• Above equations can each be written in a simpler form,-

when the position from the weld centerline is defined by aradial distance, r, where r 2 = z2 + y2

• For the thin plate, the time-temperature distribution is

22



10/5/2012

12

Dimensionless Weld Depth Vs

Dimensionless OperatingParameter

• ase on osen a s so u on o e s mp e ree-

dimensional heat flow equation, Christiansen et al.

(1965) derived theoretical relationships between a weld

bead’s cross-sectional geometry and the welding

process operating conditions using dimensionless

parameters.

23

• e eore ca re a ons p e ween e mens on ess

weld width, D, and dimensionless operating parameter,n, is shown, where

Dimensionless weld depth (D) Vs

process operating parameter nChristiansen (1965)

24

• Width of the weld bead can

be determined (w = 2d)

• Width of heat-affected zone

can be determined



10/5/2012

13

Dimensionless Weld Depth Vs

Operating Parameter n

d = depth of penetration of the weld,

U = welding speed (m/s),

s = thermal diffusivity (k/C) of the base material (as a solid),

Q = rate of heat input to the workpiece (J/s),

Tm = melting point of the base material (the workpiece), and

To = temperature of the workpiece at the start of welding.

25

, ,

Cross-sectional area of the weld bead can be determined

Can be applied to the heat-affected zone by simply substituting

TH for Tm where TH is the temperature of some relevant phase

transformation that could take place

Assignment -2

1) Find w and d for symmetrical weld bead as shown in

figure.

n e w o p ase rans on emp =

Material steel with Tm= 1510 CE=20 V

I= 200 A

Welding speed (v or U) =5 mm/s w

26

0

Arc efficiency =0.9

K=40 W/mK

C = 0.0044 J/mm3. C

t=5 mm

d



10/5/2012

14

Welding Lecture ‐ 1005 Oct 2012, Friday, 11.30‐12.30 am

Heat flow in welds

272727

Effect of welding parameters on heat

distribution

• The shape of the melt , size & heat distribution,

is a function of

1. Material (thermal conductivity, heat capacity,

density)

2. Welding speed, and

28

3. Welding power/energy density

4. Weldment plate thickness



10/5/2012

15

Effect of thermal conductivity (and

material property) on heat distribution

•

– tends to cause deposited heat to spread

– Smaller welds for a given heat input and

melting temperature

• For a given heat input, the lower the

29

melting point, the larger the weld

Material Thermal

diffusivity

=k/ C

(mm2/s)

Aluminium 84

Carbon steel 12

Effect of

Austenitic

steel

4

30

thermal

conductivity

(and material

property) on

heat distribution



10/5/2012

16

Effect of welding speed on

Shape of Fusion/HAZ

Velocit

y

0 Low Medium High Very high

Plan

view

Circle elliptical Elongated

ellipse

Tear

drop

Detached

tear drop

-

view

em -

spherical

ro a e

spheroidal

onga e

prolatespheroidal

teardrop

ear

drop

Effect of welding speed

32



10/5/2012

17

Tear drop formation at very high

velocityDetached tear drops at very high velocityContinuous tear drops

Weld directionWeld direction

Effect of welding speed

• For a stationary (spot) weld, the shape, is round, -

• Once the source is moved with constant velocity,

the weld pool and surrounding HAZ becomeelongated to an elliptical shape (plan view), andprolate spheroidal in 3-D

• With increased velocity, these zones become more

34

and more elliptical

• At some velocity (for each specific material), a teardrop shape forms, with a tail at the trailing end ofthe pool.



10/5/2012

18

Effect of welding speed• Increasing velocity elongates the teardrop

more and more, narrows the fusion and heat-affected zone overall melted volume constant

• Very high welding speeds the tail of theteardrop weld pool detaches isolate regionsof molten metal lead to shrinkage-inducedcracks alon the centerline of the weld

35

Efect of the thickness of a

weldment

36

•Thick weldment Small weld pool and heat-affected zone



10/5/2012

19

Effect of energy density

• Increased energy density increases theefficiency of melting, increases the amountof melting (especially in the depth direction) decreases the heat-affected zone.

• Shape of weld pool & HAZ will be distortedby any asymmetry around the joint.

• Asymmetry might be the result of the relative

37

. .,

elements as well as their relative thermalproperties (Tm, k & C)

Simplified Equations for Approximating

Welding Conditions

1) Peak Temperatures Predicting metallurgical

transformations meltin austenitization recr stallization of

cold-worked material, etc.) at a point in the solid material

near a weld requires some knowledge of the maximum

temperature reached at that specific location.

For a single-pass, full-penetration butt weld in a sheet or a

plate, the distribution of peak temperatures (Tp) in the base

material ad acent to the weld is iven b

38



10/5/2012

20

Peak Temperature

To = initial temperature of the weldment (K)

e = base of natural logarithms = 2.718

= density of the base material (g/mm3)

C = specific heat of the base material (J/g K- I)

=

39

y = 0 at the fusion zone boundary and where Tp = TmTm = melting (or liquidus) temperature of the material

being welded (K)

Hnet = EI/v (J/m)

Width of the Heat-Affected Zone• Peak temperature equation can be used to

calculate the width of the HAZ.

• Define T Tre or Tau

• The width of the HAZ is determined by the value ofy that yields a Tp equal to the pertinent

transformation temperature (recrystallizationtemperature, austenitizing temperature, etc.).

• Equation cannot be used to estimate the width ofthe fusion zone, since it becomes unsolvable when

40

p = m

• (Remember the assumption in Rosenthal’s solutionof the generalized equation of heat flow, Heatwas deposited at a point, and there was no meltedregion, but just a HAZ)



10/5/2012

21

Assignment 3- Part A

A single full penetration weld pass is made on steel using the

following parameters.

= = = =

a) Calculate the peak temperatures at distances of 1.5 and 3.0

mm from the weld fusion boundary

b) Calculate the width of HAZ if the recrystallization temperature

m , , ,

mm/s, T0= 25 C, Arc efficiency =0.9, C = 0.0044 J/mm3. C,

t=5 mm, Hnet= 720 J/mm

41

is 730 C

c) Find the influence on the width of HAZ if a preheated sample

is used (Assume preheat temp =200 C)

d) Find the influence on the width of HAZ if the net energy is

increased by 10%

Solidification rate

• The rate at which weld metal solidifies can

,

properties, and response to post weld heat

treatment.• The solidification time, St, in seconds, is

given by

42



10/5/2012

22

Solidification rate• L is the latent heat of fusion (J/mm3),

• k is the thermal conductivity of the base material(J/m s-1 K-1),

• , C, Tm and To are as before

• St is the time (s) elapsed from the beginning tothe end of solidification.

• The solidification rate, which is derived from the

43

so ca on me, e ps e erm ne e na ure othe growth mode (with temperature gradient) andthe size of the grains

(will be discussed in detail on a separate lecture)

Cooling Rates

• The rate of cooling influences – the coarseness or fineness of the resulting structure

– ,constituents in the microstructure, of both the FZ and the HAZ fordiffusion controlled transformations

• Cooling rate determines which transformation (phase) willoccur and, thus, which phases or constituents will result.

• If cooling rates are too high in certain steels hard,untempered martensite embrittling the weld addssusce tibilit to embrittlement b h dro en.

44

• By calculating the cooling rate, it is possible to decidewhether undesirable microstructures are likely to result.

• Preheat To reduce the cooling rate. Cooling rate isprimarily calculated to determine the need for preheat.



10/5/2012

23

Cooling Rates-Thick plates• For a single pass butt weld (equal thickness),

For thick plates,

45

R = cooling rate at the weld center line (K/s)

k = thermal conductivity of the material (J/mm s-1 K-1 )

T0 = initial plate temperature (K)

Tc = temperature at which the cooling rate is calculated (K)

Cooling Rates-Thin plates

For thin plates,

h = thickness of the base material (mm)

p = density of the base material (g/mm3)

46

C = specific heat of the base material (J/g K- )

pC = volumetric specific heat (J/mm3 K-1)

Increasing the initial temperature, T0 or applying preheat,

decreases the cooling rate



10/5/2012

24

Relative plate thickness factor

≤ 0.75 Thin plate equation is valid

0.75 Thick plate equation is valid

47

Assignment 3- Part B

Find the best welding speed to be used for welding

6 mm steel plates with an ambient temperature of

30 C with the welding transformer set at 25 V and

current passing is 300 A. The arc efficiency is 0.9

and possible travel speeds ranges from 5-10 mm/s.

The limiting cooling rate for satisfactory

48

per ormance s s a a empera ure o .



10/5/2012

Assignment 3- Part B-Solution

49

Assignment 3- Part B-Solution

50

Weldinglectures9!10!121030153810 Phpapp02

Documents

Transcript of Weldinglectures9!10!121030153810 Phpapp02