Welding lectures 9 10

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Casting Forming & WeldingCasting, Forming & Welding(ME31007)

Jinu Paul

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J u auDept. of Mechanical Engineering

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Course details: WeldingTopic 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 1

5 Physics of Welding Arc 1

6 Heat flow in welding 2

7 Design of weld joints 1

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

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

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

Heat flow in welds

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

ambient temp. of the work environment to above the liquidus temp. (possibly to boiling point and above for

hi h d it )some very high-energy-density processes)• The severity of this excursion in terms of the

– temp. reached – time taken to reach them– the time remain at them

completely determines the effects on structure (both

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completely determines the effects on structure (both microstructural for material changes and macrostructural for distortion)

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

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Thermal cycle characterization via thermocouples

Temp.

5Time

p

Thermal cycle- quasi-steady state • Thermocouples at various points along weld path• Approach of the heat source rapid rise in

temperature to a peak a very short hold at thattemperature to a peak a very short hold at that peak then a rapid drop in temperature once the source has passed by

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

• Quasi-steady state balance achieved between the t f i t d th t f l

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rate of energy input and the rate of energy loss or dissipation

• Quasi-steady state temperature isotherms surrounding a moving heat source remain steady and seem to move with the heat source (away from edges)

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Thermal cycle- quasi-steady state

S Fusion zone

HAZ

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Temperature isotherms surrounding a moving heat source remain steady and seem to move with the heat source

Time-Temperature curvesTp Time= ti

Temperature

8Time

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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) p ( mA mB mc)

decrease with distance from the weld line and occur at times (tmA, tmB, tmc) that increase. This allows the peak temperature, Tp to be plotted as a function of time

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

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

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p y g g gphenomenon explains – certain aspects of phase transformations that go on in the

heat-affected zone, – differential rates of thermal expansion/contraction that lead to

thermally induced stresses and, possibly, distortion

Spatial isotherms

Tp (Peak temperature)

Heating zone C liHeating zone Cooling zone

Cooling zone

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Heating zone

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The generalized heat flow equation

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

• The influencing parameters – the solidification rate of the weld metal, – the distribution of peak temperature in the HAZ – the cooling rates in the fusion and HAZ

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g– the distribution of heat between the fusion zone

and the heat-affected zone

• Requires mathematical formulation to quantify the influence of these parameters


Heat supplied + Heat generated = Heat consumed (for temp rise, melting) + Heatconsumed (for temp rise, melting) Heat transferred via conduction + Heat loss via convection & radiation

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x = coordinate in the direction of welding (mm)y = coordinate transverse to the welding direction (mm)y coordinate transverse to the welding 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 a

function of temperature(T) = density of the material (g/mm3) as a function of temp.C(T) = specific heat of the material (J/g-1 K-1) as a function of

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C(T) = specific heat of the material (J/g K ), as a function of temperature

Vx, Vy, and Vz = components of velocityQ = rate of any internal heat generation, (W/mm3)

The generalized heat flow equation• This general equation needs to be solved for one, two, or

three 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 in uniform cross sections remote from edges) in very thin weldments2 D l ti thi ld t i thi k ld t

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• 2-D solution thin weldments or in thicker weldments where the weld is full penetration and parallel-sided (as in EBW) to assess both longitudinal and transverse heat flow

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

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Weld geometry and dimensionalityof heat flow

(a)2-D heat flow for full-penetration

ld i thi l twelds in thin plates or sheets;

(b)2-D heat flow for full-penetration welds with parallel sides (as in EBW and some LBW)

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(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

from the heat source was uniform and moved with a 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)

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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).

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Assumption 2 Heat source is a point source, with all of the energy being deposited into thewith all of the energy being deposited into the weld at a single point– This assumption avoided complexities with density distribution of

the energy from different sources and restricted heat flow analysis to the heat-affected zone, beyond the fusion zone or weld pool boundary.

Assumption 3 The thermal properties (thermal conductivity k and product of the specific heat and

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conductivity, k, and product of the specific heat and 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 (Xi) where is the distance of the point heat (Xi), where is the distance of the point heat source from some fixed position along the x axis, depending on the velocity of welding, v

=x-vtwhere t is the time

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• This equation can be further simplified, in accordance with Rosenthal if a quasi stationaryaccordance with Rosenthal, if a quasi-stationary 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 q/v = a constant The result is

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q/v a constant. The result is

Rosenthal’s solution

• Rosenthal solved the simplified form of the heat flow equation above for both thin and thick plates in which the heat flow is basically 2-D and 3-D respectivelythe heat flow is basically 2 D and 3 D, respectively.

• For thin plates,

q = EI/v = heat input from the welding source (in J/m)

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q = EI/v = heat input from the welding source (in J/m)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

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• For the thick plate,

where d = depth of the weld (which for symmetrical ld i h lf f h ld id h i 2d)

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


• Above equations can each be written in a simpler form, giving the time-temperature distribution around a weldgiving the time temperature distribution around a weld when the position from the weld centerline is defined by a radial distance, r, where r2 = z2 + y2

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

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Dimensionless Weld Depth Vs Dimensionless Operating

ParameterB d R th l’ l ti f th i lifi d th• Based on Rosenthal’s solution of the simplified three-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.

Th th ti l l ti hi b t th di i l

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• The theoretical relationship between the dimensionless weld width, D, and dimensionless operating parameter, n, is shown, where

Dimensionless weld depth (D) Vs process operating parameter n

Christiansen (1965)

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• Width of the weld bead can be determined (w = 2d)• Width of heat-affected zone can be determined

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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. For a symmetrical weld bead the width of the weld bead w = 2d

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For a symmetrical weld bead, the width of the weld bead w = 2d, 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 -21) Find w and d for symmetrical weld bead as shown in

figure.2) Fi d th idth f HAZ ( h t iti t 730 C)2) Find the width of HAZ (phase transition temp = 730 C)

Material steel with Tm= 1510 CE=20 VI= 200 AWelding speed (v or U) =5 mm/sT = 25 C

w

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T0= 25 CArc efficiency =0.9K=40 W/mKC = 0.0044 J/mm3. Ct=5 mm

d

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

Heat flow in welds

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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

3 W ldi / d i

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3. Welding power/energy density

4. Weldment plate thickness

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Effect of thermal conductivity (and material property) on heat distribution

• Increasing thermal conductivityIncreasing thermal conductivity – 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

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melting point, the larger the weld

Material Thermal diffusivity =k/C(mm2/s)

Aluminium 84

Carbon steel 12

Effect of th l

Austenitic steel

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thermal conductivity (and material property) on heat distribution

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Effect of welding speed on Shape of Fusion/HAZ

Increasing velocity

Velocity

0 Low Medium High Very high

Plan view

Circle elliptical Elongated ellipse

Tear drop

Detached tear drop

3 D H i P l t El t d 3D 3D t

Increasing velocity

3-D view

Hemi-spherical

Prolate spheroidal

Elongated prolate

spheroidal

3D tear drop

3D tear drop

Effect of welding speed

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Tear drop formation at very high velocity

Detached tear drops at very high velocityContinuous tear drops

Weld directionWeld direction


• For a stationary (spot) weld, the shape, is round (plan view) and approximately hemispherical in 3 D(plan view), and approximately hemispherical in 3-D

• Once the source is moved with constant velocity, the weld pool and surrounding HAZ become elongated to an elliptical shape (plan view), and prolate spheroidal in 3-D

• With increased velocity, these zones become more

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y,and more elliptical

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

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• 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 the teardrop weld pool detaches isolate regions of molten metal lead to shrinkage-induced cracks along the centerline of the weld

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cracks along the centerline of the weld

Efect of the thickness of a weldment

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•Thick weldment Small weld pool and heat-affected zone

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Effect of energy density

• Increased energy density increases the efficiency of melting, increases the amount y gof melting (especially in the depth direction) decreases the heat-affected zone.

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

• Asymmetry might be the result of the relative thermal mass (e g thickness) of the joint

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thermal mass (e.g., thickness) of the joint elements as well as their relative thermal properties (Tm, k & C)

Simplified Equations for Approximating Welding Conditions

1) Peak Temperatures Predicting metallurgical transformations (melting, austenitization, recrystallization oftransformations (melting, austenitization, recrystallization 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 adjacent to the weld is given by

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material adjacent to the weld is given by

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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)h = thickness of the base material (mm)

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h = thickness of the base material (mm)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 Tp Tre or Taup re au

• The width of the HAZ is determined by the value of y that yields a Tp equal to the pertinent transformation temperature (recrystallization temperature, austenitizing temperature, etc.).

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

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Tp =Tm

• (Remember the assumption in Rosenthal’s solution of the generalized equation of heat flow, Heat was deposited at a point, and there was no melted region, but just a HAZ)

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Assignment 3- Part AA single full penetration weld pass is made on steel using the following parameters.T = 1510 C E=20 V I= 200 A Welding speed (v or U) =5

a) Calculate the peak temperatures at distances of 1.5 and 3.0 mm from the weld fusion boundaryb) Calculate the width of HAZ if the recrystallization temperature

Tm= 1510 C, E=20 V, I= 200 A, Welding speed (v or U) =5 mm/s, T0= 25 C, Arc efficiency =0.9, C = 0.0044 J/mm3. C, t=5 mm, Hnet= 720 J/mm

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is 730 Cc) 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 have a profound effect on its microstructurehave a profound effect on its microstructure, properties, and response to post weld heat treatment.

• The solidification time, St, in seconds, is given by

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Solidification rate• L is the latent heat of fusion (J/mm3), • k is the thermal conductivity of the base material y

(J/m s-1 K-1), • , C, Tm and To are as before • St is the time (s) elapsed from the beginning to

the end of solidification.• The solidification rate, which is derived from the

lidifi ti ti h l d t i th t f

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solidification time, helps determine the nature of the growth mode (with temperature gradient) and the 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– the homogeneity the distribution and form of the phases and– the homogeneity, the distribution and form of the phases and

constituents in the microstructure, of both the FZ and the HAZ for diffusion controlled transformations

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

• If cooling rates are too high in certain steels hard, untempered martensite embrittling the weld adds susceptibility to embrittlement by hydrogen.

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p y y y g• By calculating the cooling rate, it is possible to decide

whether undesirable microstructures are likely to result.• Preheat To reduce the cooling rate. Cooling rate is

primarily calculated to determine the need for preheat.

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Cooling Rates-Thick plates

• For a single pass butt weld (equal thickness),

For thick plates,

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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)C f f ( / 1)

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C = specific heat of the base material (J/g K-1)pC = volumetric specific heat (J/mm3 K-1)

Increasing the initial temperature, T0 or applying preheat, decreases the cooling rate

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Relative plate thickness factor

≤ 0.75 Thin plate equation is valid 0.75 Thick plate equation is valid

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Assignment 3- Part B

Find the best welding speed to be used for welding g p g6 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

f i 6 C/ t t t f 550 C

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performance is 6 C/s at a temperature of 550 C.

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Assignment 3- Part B-Solution

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Assignment 3- Part B-Solution

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Welding lectures 9 10