Transcript Document

Materials Behavior
Basic Regions of a Weld
 Fusion Zone: area that
is completely melted
 Heat-Affected Zone:
portion of the base metal
not melted but whose
mechanical
properties
and microstructure were
affected by the heat of
the joining process
 Base Metal
Fusion Weld
Zone
Fig : Characteristics
of a typical
fusion weld zone
in oxyfuel gas
and arc welding.
Composite Zone Concerns
Grain Structure
(a)
(b)
Fig : Grain structure in (a) a deep weld (b) a shallow weld. Note
that the grains in the solidified weld metal are perpendicular
to the surface of the base metal. In a good weld, the
solidification line at the center in the deep weld shown in (a)
has grain migration, which develops uniform strength in the
weld bead.
Solidification of
Weld metal
• Solidification begins with formation of columnar
grains which is similar to casting
• Grains relatively long and form parallel to the
heat flow
• Grain structure and size depend on the specific
alloy
• Weld metal has a cast structure because it has
cooled slowly, it has grain structure
• Results depends on alloys ,composition and
thermal cycling to which the joint is subjected.
• Pre-heating is important for metals having high
thermal conductivity
Weld Beads
(a)
(b)
Fig : (a) Weld bead (on a cold-rolled nickel strip) produced by a
laser beam. (b) Microhardness profile across the weld bead.
Note the lower hardness of the weld bead compared to the
base metal.
Heat affected Zone
•
Heat effected zone is within the metal itself
• Properties depend on:
•
Rate of heat input and cooling
•
Temperature to which the zone was raised
•
Original grain size ,Grain orientation , Degree of prior
cold work
• The strength and hardness depend on:
•
how original strength and hardness of the base metal
was developed prior to the welding.
•
Heat applied during welding which Recrystallises
elongated grains of cold worked base metal.
Weld Quality
•
Welding discontinuities can be caused by inadequate or
careless application
•
The major discontinuities that affect weld quality are
•
Porosity
•
Slag Inclusions
•
Incomplete fusion and penetration
•
Weld profile
•
Cracks
•
Lamellar tears
•
Surface damage
•
Residual stresses
Cracks
•
Cracks occur in various directions and various locations
Factors causing cracks:
•
Temperature gradients that cause thermal stresses in the
weld zone
•
Variations in the composition of the weld zone.
•
Embrittlement of grain boundaries
•
Inability Of the weld metal to contract during cooling
Cracks
Fig : Types of cracks (in welded joints) caused by thermal stresses
that develop during solidification and contraction of the weld bead
and the surrounding structure. (a) Crater cracks (b) Various types
of cracks in butt and T joints.
Cracks
• Cracks are classified as Hot or Cold.
• Hot cracks – Occur at elevated temperatures
• Cold cracks – Occur after solidification
• Basic crack prevention measures :
1.Change the joint design ,to minimize stresses
from the shrinkage during cooling
2.Change the parameters,
sequence of welding process
procedures,
3.Preheat the components to be welded
4.Avoid rapid cooling of the welded components
the
Cracks in Weld Beads
Fig : Crack in a weld bead,
due to the fact that the
two components were
not allowed to contract
after the weld was
completed.
Perils of Welding Free-Machining Steels

Solidification cracking
due to impurity
elements


Sulfur, phosphorus,
boron
Impurity segregation at
weld centerline creates
low ductility area

Combines with shrinkage
stress to cause cracking
Manganese Can Prevent Solidification
Cracking


Manganese combines with sulfur to form MnS
particles
Use a filler metal with higher manganese to absorb
sulfur
ER70S-3
0.06 - 0.15
0.9 - 1.4
0.45 - 0.7
0.02
0.035
0.05
Composition ER70-6
carbon
0.07-0.15
manganese 1.4 - 1.85
silicon
0.8 - 1.15
phosphorus
0.02
sulfur
0.035
copper
0.5
Residual Stresses:
• Caused because of localized heating and cooling
during welding, expansion and contraction of the
weld area causes residual stresses in the work
piece.
• Effects:
• 1.Distortion,Warping and buckling of welded parts
• 2.Stress corrosion cracking
• 3.Reduced fatigue life
Distortion after Welding
Fig : Distortion of parts after welding : (a) butt joints; (b) fillet
welds. Distortion is caused by differential thermal expansion
and contraction of different parts of the welded assembly.
Stress relieving of welds :
• Preheating reduces problems caused by
heating the base metal or the parts to be
welded
•
Heating can be done electrically, in furnace or by OAW
torch and for thin surfaces by radiant lamp or hot air
blast.
• Some other methods of stress relieving :
Peening, hammering or surface rolling