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Stress Relieving
Weld Defects
The heat of welding brings
about certain changes, both in the structure of
the steel being welded and in the weld metal.
Some of these changes occur during welding;
others occur when the metal is cooling.
This section describes
Temperature Differences,
Microstructural Changes, and
Hardness in a weld bead.
Heat from welding exposes
steel to a range of temperatures. The molten
weld metal--or weld puddle--reaches a
temperature of 3000°F (1649°C) or higher and
remains at high temperatures for a long time.
The steel adjacent to the weld is heated to near
its melting point, but remains at that high
temperature for a shorter time. Areas away from
the weld reach lower temperatures and remain at
those temperature for shorter periods of time. A
short distance from the weld, the temperature of
the plate may be only about 600°F (316°C).
Steel has upper and lower critical
temperatures--temperatures at which the
properties of that steel change. These critical
temperatures vary with each type and grade of
steel.
High temperatures change
the properties--or microstructure--of a
steel weld. Microstructural changes include the
following:
- Grain boundary
structure
- Hardness
- Strength properties
The length of time a weld
metal stays at its upper critical temperature
and the rate at which it cools affect its
microstructural changes. A weld puddle remains
at its steel's upper critical temperature for a
long time, producing a structure with large
grain size. Metal adjacent to the weld is at the
upper critical temperature for a very short
time, producing a slight decrease in grain size
and an increase in strength and hardness. Areas
away from the weld do not reach temperatures
high enough to change their microstructure or
base metal properties.
In multipass weld joints, each bead reheats
and refines the grain of the previously laid
bead. However, this refining is not likely to be
uniform throughout the joint.
Figure 13 shows a weld cross-section.
Figure 13: Weld Cross-Section
Figure 14 illustrates microstructural changes
and the temperatures at which they occur for
0.25% carbon steel plate.
Figure 14: Weld Bead Schematic
The rapid heating and
cooling of the steel results in the formation of
zones around the weld. Table 12 lists and
describes the four zones:
Table 12: Weld Bead Zones of Nonequilibrium
Microstructure
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Zone |
Definition |
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1. Composite zone |
Rapidly cooled cast
microstructure with a composition that
is a mixture of base metal and weld
metal
Coarse, large grain boundary structure
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2. Transition zones
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Rapidly cooled cast
microstructures of varying composition
between the base metal and the weld
metal
Metal that has been heated above its
upper critical temperature1526°F (830°C)
for 0.25% carbon steelbut has not melted
The zone of large grain growth most
prone to underbead cracking during and
after welding |
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3. Heat affected zone (HAZ) |
Region where the
steel has been heated above its lower
critical temperature1330°F (721°C) for
0.25% carbon steelto cause
microstructural changes
Zone in which grain refinement has taken
place |
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4. Unaffected base
metal |
Metal that has been
heated slightly, but not above the lower
critical temperature
Zone in which no structural changes have
occurred |
Hardness of a steel
indicates some of the properties of it. Hard
steels resist wear well but can crack easily.
Most steels used in process facilities should be
soft so that they do not crack.
Metallurgists measure the hardness of steel
using two methods:
Inspectors normally measure
process unit hardness in Brinell units--a
measure of the average hardness over many grains
of the metal. Refineries normally specify steel
to have a Brinell hardness under 200.
Vickers hardness measures the individual
grain hardness of the metal. Vickers tests are
used in a lab to qualify weld procedures and are
rarely used in the field.
This section covers
Phases in Steel Welds and
Phases of Equilibrium and Nonequilibrium.
Heating temperatures and
cooling rates produce microstructural changes in
the steel. The phases commonly found in steel
welds include the following:
- Martensite
- Austenite
- Ferrite
- Carbides
- Pearlite
- Bainite
Table 13 describes each
steel phase and its characteristics.
Table 13: Steel Phases
|
Steel Phase |
Crystal Structure |
Description |
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Martensite |
Body-centered
tetragonal |
Preheating the
steel prior to welding minimizes the
formation of martensite.
The formation of martensite in higher
alloys is unavoidable.
Post-weld heat treating (PWHT) softens,
or tempers, a martensitic
microstructure by converting it to an
aggregate of fine carbide particles in a
ferrite matrix.
Stainless steels which form martensitic
microstructures are called "martensitic"
stainless steels. Types of martensitic
stainless steels include 410, 416, and
420. |
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Austenite |
Face-centered cubic
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Carbon steel has
this crystal structure above 1342°F.
The carbon in the steel is uniformly
distributed.
Some stainless steelsthe "austenitic"
gradeshave this structure at all
temperatures because of their alloy
content. Types of austenitic stainless
steels include 304, 316, 321, 347, 310
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Ferrite |
Body-centered cubic
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Carbon steels at
temperatures below 1342°F consist of
ferrite and small amounts of iron
carbide. Ferrite is a very weak
material.
Stainless steels that maintain a
ferritic microstructure up to their
melting points are called "ferritic"
stainless steels. Types of ferritic
stainless steels include 405 and 409.
|
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Iron Carbides |
Varied |
Iron carbides are
intermetallic compounds containing
carbon with various compositions and
crystal structures.
The most common carbide found in steel
has the formula, Fe3C. Steels with
higher alloy content form carbides with
more complex chemical formulas. |
|
Pearlite |
Alternating layers
of ferrite and carbide |
Pearlite is an
aggregate of ferrite and carbide.
Microstructures in carbon steel
generally consist of pearlite and
ferrite. The amount of pearlite
increases with increasing carbon
content.
Most carbon steel weld procedures
promote the formation of
pearlite-ferrite microstructures in the
steel.
Pearlite is found in the soft, ductile
steels used for pressure vessels. |
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Bainite |
Unorganized mix of
austenite and carbides |
Bainite is the
microstructure that forms in many steels
when austenite transforms during cooling
in the temperature range of 600°F1000°F.
Unlike pearlite, these carbides are not
organized in layers.
Several forms of bainite with different
morphologies exist.
Some forms of bainite are relatively
hard. Weld procedures for carbon steel
are generally designed to avoid the
formation of bainite.
In higher alloys where the formation of
bainite cannot be avoided through
preheating, post-weld heat treating is
used to soften the bainite formed during
welding. |
Austenite, ferrite,
pearlite, and carbides are equilibrium phases
which are softer and less prone to cracking.
Martensite and bainite are nonequilibrium
phases. They contain residual stresses making
them harder and prone to cracking.
Weld procedures are designed to minimize the
formation of martensite and other hard,
nonequilibrium phases. If the high alloy content
of the steel restricts the use of preheating and
other weld procedures designed to minimize the
formation of martensite, the weld is given a
post-weld heat treatment to soften or "temper"
the hard phases formed during welding.
The hardness of the welded
steel and the microstructures that form are
affected by three basic factors:
- Heating temperatures
- Cooling times
- Base metal composition
(amount of alloying)
Two types of diagrams
illustrate the interrelationship of these
factors:
- Equilibrium phase
diagrams illustrate the relationship
between temperature, steel composition, and
the equilibrium phases present in steel.
- Continuous cooling
transformation (CCT) diagrams illustrate
the relationship between temperature,
cooling time, phases that form in steel, and
hardness of the steel.
This section covers
Equilibrium Phase Diagrams,
Continuous Cooling Transformation Diagrams,
and
Effects of Alloying.
Equilibrium phase diagrams
show the phases of equilibrium that develop as a
result of the relationship between temperature
and steel composition. Metallurgists use these
diagrams and Continuous Cooling Transformation (CCT)
diagrams to make one of two determinations:
- How to make the steel
- What to expect when
that steel is welded
Figure 15 shows the effect
of welding on a carbon steel pipe containing
only 0.20% carbon.
Figure 15: Iron-Iron Carbide Equilibrium Phase
Diagram
Welding this carbon steel
pipe causes a portion of it to liquefy. As the
liquid steel cools slowly, some high-temperature
solid phase--called delta--will form.
This phase and the remaining liquid transforms
to austenite. In austenite, the carbon in the
steel is uniformly distributed in the crystal
structure.
If the liquid steel cools rapidly, martensite
develops. The carbon is evenly distributed
throughout the martensitic crystal structure--as
it was in the austenite. However, it no longer
fits well. This highly strained crystal lattice
is generally hard and brittle.
The Continuous Cooling
Transformation (CCT) diagram performs two
functions:
- It illustrates the
relationship between temperature, cooling
time, and the phases that form in a steel of
a specific composition.
- It shows cooling
curves and the hardness that results from
cooling the steel at various rates.
CCT diagrams exist for all
types of steel. Alloy steel diagrams are
significantly different than those of carbon
steel.
Figure 16 shows a CCT diagram for steel
alloyed only with carbon and manganese.
Figure 16: CCT Diagram for Steel Alloyed with
Carbon and Manganese
Cooling this medium carbon
steel at various rates results in only marginal
changes in hardness. Only very high cooling
rates cause bainite or martensite to form.
Hardness is a measurement of how much
martensite, bainite, or both were formed during
the welding process. Under most conditions,
welding this particular carbon steel will not
produce a hard heat affected zone. However,
thick pieces of this steel will have cooling
rates high enough to form hard phases.
For carbon steel, the hardness must be below
200 Brinell. Most welds above 200 Brinell are
prone to cracking. Performing a PWHT reduces the
residual stresses measured by the hardness.
Curve 1 in Figure 17 shows the effects of
cooling carbon steel from its lower critical
temperature to 900°F in approximately 8 seconds
and to ambient temperature in 90 seconds, for a
total cooling time of approximately 100 seconds.
Curve 2 in Figure 17 shows the effects of slower
cooling times on a thin piece of carbon steel.
Curve 1 shows a predicted hardness of 265
Brinell, indicating that a PWHT is required.
Lack of preheat or thick steel may have caused
the rapid cooling. Curve 2 shows that welding a
thinner plate of the same material causes the
heat to be absorbed more slowly into the base
metal. This causes the cooling time to
significantly increase and produces a hardness
of only 190 Brinell.
Figure 17: CCT Diagram Comparing Rapidly Cooled
and Slowly Cooled Carbon Steel
An increase in alloy
content moves CCT diagrams to the right, making
the formation of harder phases more likely. It
takes longer for phases such as soft pearlite
and ferrite structures to form during cooling.
Steel hardenability is the effect of alloy
content on a steel as it cools from
austenitizing temperatures.
Figure 18 shows the CCT diagram for a steel
which has about the same composition as the
previous carbon steel except for the addition of
0.53% molybdenum.
This relatively small molybdenum addition
significantly changes the microstructures and
increases hardness levels. Because alloy content
affects steel characteristics, different steel
grades require different weld procedures and
PWHT.
Figure 18: CCT Diagram for Steel Alloyed with
Carbon, Manganese, and Molybdenum
Figure 19 compares the
effects of short and long cooling times for
alloyed steel.
Figure 19: CCT Diagram for Alloyed Steel
Comparing Short and Long Cooling Times
Curve 1 shows that cooling
this weld in 100 seconds yields a Vickers
hardness of approximately 620. Curve 2 shows
that the weld requires a five-hour cooling time
to reach an acceptable hardness below 200. Only
PWHT can control the cool-down and produce the
desired hardness.
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