welding lecture 6

8/9/2019 Welding Lecture 6

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Weld thermal cycle

An intense, moving heat source is applied to the material whichcreates a weld pool. A large part of the applied heat flowsthrough the parent material on each side of the joint.

It is useful to predict the form of temperature gradient aboutthis heat source to understand: width and penetration depth of aweld as a function of heat input, microstructural change in theHAZ and residual stresses.


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Rosenthals solution to heat flow equations: A point heatsource moving at a constant speed

vt x

t T

z T

yT

xT

=

=

+

+

222

2

2

2

2

T= temperature, K

t=time

L= thermal conductivity


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For a given welding process, weld geometry and material type, thecooling time through the range 800 500 oC ( t8-5 ) is constant anddepends on heat input.


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Thermal cycles experienced by indicated locations in the heat-affected zone of an arc weld made with 100,000 joules/in. for in.steel plate at room temperature


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Effect of energy input and preheat temperature on the peak temperature distribution in in. steel plate welded at roomtemperature with manual shielded metal-arc welding


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Effect of initial plate temperature on thermal cycles in heat-affectedzone of welds in in. steel plate using covered electrodes showingcomparison for 100,000 joules/in. and 47,000 joules/in. energy inputs


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Effect of energy input on temperature isotherms at surface of in.steel plate. Upper portion 100,000 joules/in., 24 V, 208 A,3 in./min; lower portion 50,000 joules/in., 24 V, 208 A, 6 in./min


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Effect of plate thickness on the thermal cycles in the weld heat-affected zone of manual shielded metal-arc welds in steel plate.Energy input 47,000 joules/in. Initial plate temperature 80 OF(80 OC).


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Effect of plate thickness and heat input on the time to cool from800 oC to 500 oC ( t8-5 ). Lines: calculated values, points:experimentally measured values.


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Calculated T-t plot during welding of 30 mm steel plate


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WeldingConditions

Energy Input(J/mm)

PreheatTemperature

(OC)

Cooling rate at 650 OC

For Butt Welds in in. Plate

(oC/s)

For FilletWelds(oC/s)

2,0002,0002,000

2012020520

205

1175

443420

4,0004,000

41.7

105


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Nomogram for calculatingthe cooling time in CO 2 /O2 arc welding.


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Schematic diagram of a heat affected zone (HAZ) of a single pass HSLAsteel weld with the Fe-C phase diagram


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allotriomorphic ferrite

w widmansttten ferrite

a acicular ferrite


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1: Grain boundary ferrite, 2: polygonal ferrite

3: side plate ferrite, Widmansttten ferrite 4:acicular ferrite

Typical microstructure in low carbon, low alloy steel weldments


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5: bainite

Typical microstructure in low carbon, low alloy steel weldments


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Typical microstructural constituents of weld metal: C-Mn Steel


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Weld metal microstructure showing fine acicular ferrite : HSLA 80 Steel


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Impact energy,J

Impact strength at 15 oC of SAW metal of TS 785 MPa


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a: fusion zone

b: grain growth zone

Various region in the HAZ of aMMA-welded normalised fine-

grained pearlitic steel


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c: grain refined zone

d: partially transformedzone and zone of spheroidised carbide

e: Parent metal


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Region of partial transformation in HAZ of C-steel


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HAZ microstructure in HSLA80 steel showing polygonaland acicular ferrite

HSLA 80 steel: Base metal


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Formation of crack in HAZ

Conditions for cracking:

Hardness above a critical level (>350 VHN is crack sensitive)Presence of tensile stressesPresence of hydrogen (cold cracking or delayed cracking)


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Hardness depends on cooling rate (heat input, thickness) andcomposition (carbon equivalent).


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CE < 0.35 No risk of cracking except at high CR (thick section)0.35 < CE < 0.55 preheat to avoid cracking (costly)

CE > 0.55 both preheating and postheating to avoid cracking


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Hydrogen enhances risk of cracking (cold cracking or delayedcracking). Can occur at 100 oC to 200 oC

Hydrogen cracks originating in the HAZ(note, the type of cracks shown would not beexpected to form in the same weldment)

Crack along the coarse grainstructure in the HAZ


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Causes

There are three factors which combine to cause cracking:

hydrogen generated by the welding processa hard brittle structure which is susceptible to crackingtensile stresses acting on the welded joint

Cracking usually occurs at temperatures at or near normal ambient. It iscaused by the diffusion of hydrogen to the highly stressed, hardened part of the weldment.


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Graville Diagram


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welding lecture 6

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