Heat Affected Zones Heat Affected Zones 22.pdf

Heat Affected Zones

1. THE HEAT AFFECTED ZONE

Nick Kostrikin
Liz Lehman

2. 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

3. 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

4. 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 6500° F
The intense heat is focused on a very small area
The process is quick, clean, and free of slag and
sputter

6. 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

9. Gas welding:

Brazing is a process of gas welding

Oxygen and acetylene are burned at correct proportions
to create a flame ranging from 5800° to 6300° 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

11. ...alternative forms of gas welding

12. Getting the Right flame...

Neutral flame
Oxygen is added until feather disapears.
Inner cone will be greenish-blue
will also be pointed, but not sharp.

13. 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 60°
in both directions

Collect and analyze data
Discussion of results

14. Examples of some TIG welds...

15. Our TIG welded samples...

16. Hardness plot of the TIG welded sample

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17. Our MIG welded samples...

18. Hardness plot of the MIG welded sample

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Distance from the weld (mm)

19. Our Brazed welded sample...

20. Air cooled sample

6 cycles to failure, crack at welded joint
Water cooled sample

4 cycles to failure, crack at welded joint

Inconclusive Results !!

21. Hardness comparison of TIG and MIG welding...

— TIG water quenched
TIG air cooled

MIG air cooled

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— MIG water cooled

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10 15 20 25 30 35
Distance from the weld

22. 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

23. 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.