THE HEAT AFFECTED ZONE

Nick Kostrikin

Liz Lehman

Objectives:

• Analyze the heat affected zone (HAZ) created by 3 types of welding and 2 different cooling rates

• Identify any changes in the properties of the material characteristic of each type of welding

• Identify any changes in the properties of the material based on the method of cooling used after welding

Parent metal:

• Low carbon steel ASTM 569• Easy to form and weld• Max carbon content of 0.15 wt%• 0.30 – 0.60 wt% Mg• Max phosphorous content of 0.04 wt%• 55,000 psi tensile strength • 30,000 psi yield strength• 30% elongation

TIG welding:

• Non-consumable tungsten electrode is used to create an arc

• Inert gas used to shield the weld zone from contaminants

• Temperature of electric arc exceeds 6500º F• The intense heat is focused on a very small area• The process is quick, clean, and free of slag and

sputter

A look at TIG welding…

MIG welding:

• A consumable wire is used to establish an arc and as a filler material in the weld zone

• Can be used with inert gas or flux cored filler wire to shield the weld zone from contaminates

• As with TIG welding, an intensely hot electric arc is created with the filler wire

• As with TIG welding, a minimum amount of material is necessary to produce a weld of maximum strength

MIG welding 101…

More MIG…

Gas welding:

• Brazing is a process of gas welding• Oxygen and acetylene are burned at correct proportions

to create a flame ranging from 5800º to 6300º F• An alloy of a lower melting temperature is used to join

the parts of the base metal• Brazing typically takes longer to weld than either TIG or

MIG• The flame is not as intense or focused as an electric arc• Because of the inherently lower tensile strength of brass,

a proportionately larger amount of brass must be used in the weld to provide sufficient strength

Ancient and modern forms of gas welding…

…alternative forms of gas welding

Getting the Right flame…

Procedure:

• Weld 2 sets of metal samples with TIG, MIG and BRAZING

• One set is to be AIR COOLED at room temperature• The other set is to be WATER QUENCHED• Conduct laboratory experimentation:

– Rockwell hardness measurements at 2mm increments.

– Fatigue testing by bending the sample at the joint 60º in both directions

• Collect and analyze data• Discussion of results

Examples of some TIG welds…

Our TIG welded samples…

70

75

80

85

90

0 10 20 30 40

Distance from the weld (mm)

Har

dn

ess

(HR

C)

Hardness plot of the TIG welded sample

Water quenched sample97 cycles to failure, crack at 10mm

Air cooled sample 116 cycles to failure, crack at 18mm

Our MIG welded samples…

50

60

70

80

90

0 10 20 30

Distance from the weld (mm)

Har

dn

ess

(HR

C)

Water quenched sample 72 cycles to failure, crack at 6mm Air cooled sample 85 cycles to failure, crack at 16mm

Hardness plot of the MIG welded sample

Our Brazed welded sample…

Air cooled sample6 cycles to failure, crack at welded joint

Water cooled sample4 cycles to failure, crack at welded joint

Inconclusive Results !!

60

65

70

75

80

85

0 5 10 15 20 25 30 35 40

Distance from the weld

Har

dne

ss (

HR

C)

TIG water quenched

TIG air cooled

MIG air cooled

MIG water cooled

Hardness comparison of TIG and MIG welding…

Effects of different types of welding:

• TIG welding created a very strong weld with good hardness and ductility

• MIG welding created a similarly strong weld with slightly greater hardness values, less ductility, and a smaller heat affected zone than TIG

• Gas welding with brass created a weld of insufficient strength hence its strength and ductility could not be compared to the two types of arc welding

Effects of different methods of cooling:

• Typically the grain structure adjacent to the weld has relatively lower hardness and greater ductility associated with a coarse grain size. Water quenching decreases the size of the grain structure, thus raising the hardness.

• The next zone consists of a band of finer grains at the critical temperature. This zone is relatively harder and less ductile than the first zone and is more prone to cracking. Water quenching tends to harden this zone and causes cracks to occur closer to the weld than air cooled samples.

• The third zone consists of a normal grain structure resembling those of the parent metal and is furthest from the weld.

THE END