Metallurgy of Welding
Welding Metallurgy
• Study of Welding Metallurgy is important because the
overall-mechanical properties of a weldments are
determined by the properties of individual microstructure
present in the weld deposit and the weld heat affected
zone. Major problem associated with welding fabrication is
the inability to obtain uniform mechanical properties
through out the weldments. Welding Metallurgy is
concerned with
1.
2.
3.
4.
Melting of electrode and base metal
Solidification of weld metal
Gas absorption
Slag metal reactions
Metallurgical Effects of Welding
1. Weld Metal
Weld Metal behaves like a casting with defects
and characteristics of a casting. Solidification
of metals is usually considered to be a
Nucleation and Growth process. Nucleation
involves creation of critical sized particles
(nuclei) and from which Growth will proceed.
Metallurgical Effects of Welding
2. Absorption of Gases by Weld
Gases such as Hydrogen gets dissolved in the
molten metal and may get trapped in the solid
metal if the cooling is rapid. These gases may get
retained in the microstructure or may form
bubbles to form porosity in the meld metal. The
gases may also react with the liquid metal or
with one another. The gases which often cause
trouble are Hydrogen, Nitrogen, Oxygen.
Metallurgical Effects of Welding
3. Slag Inclusions : Slag inclusions are frequently trapped
in fusion welds and there is difficulty of removing it.
4. Hot Cracking of Welds : Hot Cracking of welds occurs
at high temperatures. This may be due to the low
ductility of the base metal.
5. Heat Affected Zone (HAZ):
6. Corrosion of Welds : Depends on the composition of
base metal, process employed etc.
Welding Zones
If welded joint closely examined, the joint
consists of three distinct zones namely
1. Weld Metal Zone
2. Heat Affected Zone
3. Unaffected base metal or parent metal.
Weld Metal Zone
• Weld Metal is formed as well as the weld
metal solidifies from molten state. This is a
mixture of parent metal and electrode (or
filler metal). The ratio depends on welding
process used, type of joint and plate
thickness.
• Weld Zone resembles a cast metal.
Heat Affected Zone (HAZ)
• Adjacent to the Weld Metal Zone is the HAZ. HAZ is composed
of the parent metal that did not melt but was heated to a high
temperature for a sufficient period so that grain growth
occurred.
• Heat Affected Zone is that portion of base metal whose
mechanical properties and microstructure have been altered
by the heat of welding.
• The width of HAZ varies according the welding process and
technique.
• HAZ consists of three metallurgically distinguished regions.
Welding Zones
1. The Grain Growth Region :
• Grain Growth Region is immediately adjacent to Weld Metal Zone.
• In this zone, the metal his heated to a temperature well above the
Upper Critical Temperature(A3).
• This result in grain growth.
2. The Grain Refined Region :
• Adjacent to the Grain Growth Region is the Grain Refined Region.
• In this zone, the metal his heated to a temperature just above the
Upper Critical Temperature(A3).
• In this region finest grain structure exists.
3. The Transition Region :
• Adjacent to the Grain Refined Region is the Transition Region.
• In this zone, the metal his heated to a temperature between Upper
Critical Temperature(A3) and A1.
Unaffected base metal or parent metal.
• Outside the HAZ is the parent metal is the
parent metal that was not heated sufficiently
to change its microstructure.
Welding Stresses
• Residual stresses are stresses existing within the weldment
in the absence of external forces.
• They are in static equilibrium within the body and are
internally balanced.
Residual Stresses may be classified into two categories.
1. Macro stresses : They are large scale internal stresses
and may be developed by inhomogeneous plastic
deformation from external loading, by non-uniform
heating or by varied chemical diffusion.
2. Micro stresses : They are small scale internal stresses
and may be developed by heterogeneities due to
difference in elastic modulus, co-efficient of expansion etc.
Causes of Internal Stresses
1. Mechanical Residual Stresses Expansion and
contraction of heated metal due to the piece
being welded itself
2. Metallurgical Residual Stresses. Phase
transformation which takes place while cooling
3. Reaction Stresses. Expansion and contraction of
heated metal due to the other parts of structure
to which the piece being welded is attached
Effects of Thermal Stresses
• Residual Stresses can cause
1. Distortion of work-piece when welded.
2. Distortion of welded objects while machining.
3. Result in weld cracking.
4. Result in brittle fracture.
5. Result in lowered ductility.
6. Affect fatigue strength adversely.
7. Lower creep strength.
Methods for controlling Welding Stresses
• Welding stresses may be reduced (before the weld is made) by the
following methods.
1. Structure should be designed so that joints will have slowest residual
stresses. For example a double V groove butt joint instead of single V.
2. Since residual stresses are caused by thermal strains due to welding, a
reduction in amount of weld metal usually results in reduction of
residual stresses. For example use of a U-groove instead of V-grove
should result in reduction of amount of weld metal.
3. Peening will reduce residual stresses.
4. Cracks are great stress raisers. Crack tack welds should be chipped or
melted out before welding work.
Stress Relief Heat Treatment of
Weldments
1. Peening :
In Peening, outer fibres of the weld are elongated with the
help of hammer blows. When properly applied Peening
reduces residual stresses to a great extent. But Peening
reduces internal stresses of a low intensity. Peening also
reduces distortion. Peening should not be employed to the
first and last layers of weld. Excessive Peening will result in
cracking of weld.
2. Vibratory Stress Relief :
In this method, weld structures are subjected to vibrations to
relive residual stresses. The weld structure is placed on a
platform that vibrates. Up to 25%of residual stresses may be
relived by Vibratory Stresses Relief Treatment.
3.
4.
5.
Thermal Heat Treatment :
Thermal treatment of weld consists of heating the weld structure
uniformly to a suitable temperature for a predetermined period of
time followed by uniform cooling. Thermal Treatment is a better
substitute than Vibratory Stress relief.
Thermo-mechanical Treatment :
In this method, the weld is heated to set up another residual
stresses so as to counteract and thereby cancel the original
residual stresses due to welding. This is a low temperature
treatment when compared to Thermal Heat Treatment.
Reductions up to 60% of residual stresses have been reported by
this method.
Overstressing Treatment :
In this method, the weld is loaded above the yield stress of the
metal. When such pressure are removed, the residual stresses are
found to be largely disappeared.