Knife-Line Attack in the HAZCr23C6 precipitate in HAZBand where
peak temperature is 800-1600FCan occur even in stabilized
gradesPeak temperature dissolves titanium carbidesCooling rate
doesnt allow them to form againStainless Steel
*Let us now start to investigate concerns in the true heat
affected zone.*Two major concerns occur in the heat affected zone
which effect weldability these are, a.) changes in structure as a
result of the thermal cycle experienced by the passage of the weld
and the resulting changes in mechanical properties coincident with
these structural changes, and b.) the occurrence of cold or delayed
cracking due to the absorption of hydrogen during welding.*First
lets review the thermal cycles experienced in the heat affected
zone as a result of the passage of the weld. The figure illustrated
here shows the temperature vs time curve at various distances from
the weld metal. Note that almost every thermal cycle imaginable
occurs over this short distance of the heat affected zone. Thus a
variety of structural and property variations are expected. *There
are two types of alloy systems which we will consider, those which
do not have an allotropic phase change during heating like copper,
and those which have an allotropic phase change on heating like
steel. We will first consider those materials which do not have an
allotropic phase change. The top schematic illustrates this type of
material. We will however consider that this material has been cold
worked (not the elongated cold worked grains present in the base
material (region A). The weld metal is represented by region C, and
the heat affected zone is region B.*Note that the heat of welding
has effected the structure of this material even though there are
no allotropic transformations. Recall that cold worked structures
undergo recover, recrystalization and grain growth when heated to
ever increasing temperatures. So it is in this material. As we
traverse from the cold worked elongated grains in the unaffected
base metal, we come to a region where the cold worked grains
undergo recovery and then shortly there after they recrystalize
into fine equiaxed new grains. Traversing still closer to the weld
region we note grain growth where the more favorably oriented
grains consume neighboring grains and grain growth occurs. The
grains within the weld epitaxially nucleate from the grains in the
heat affected zone at the fusion boundary, and grain growth
continues into the solidifying weld metal making very large
grains.*One of the factors that occur when cold worked grains
recrystalize and grain grow occurs we have already discussed, and
that is the material softens. Thus the heat affected zone and weld
metal will not hold the same strength level as the cold worked base
metal. Another consequence of increased grain size is perhaps
equally important and that is that the larger grains are more
brittle. A Charpy impact test is used to determine how much impact
energy a structure will absorb over various temperature ranges.
Note that the larger grain size material will become brittle and
not absorb much of an impact load even at temperatures around room
temperature and above.*A second way of strengthening materials
without allotropic phase changes is by precipitation strengthening.
(The first we just discussed was cold working). Recall that in
precipitation strengthening, the base metal is solutionized,
rapidly cooled and then aged at some moderately elevated
temperature to promote precipitate formation. There are two ways
that precipitation hardened material can be welded. One is to weld
on the full hard, that is the already aged base metal. The second
is to weld on material which has been solution annealed and rapidly
cooled, but not yet given the ageing heat treatment. In either
case, when welding, the heat affected zone will see some additional
time at temperature (varied temperature over the distance of the
HAZ) as illustrated above, and this will effect the aged or
overaged condition of the precipitates.*When welding on the already
aged (full hard) material, the unaffected base metal will have aged
precipitates that are just the right size for strengthening. The
heat affected zone, on the other hand, will experience some
additional heating. In the region farthest from the weld the heat
will be sufficient to overage the precipitates with the resulting
loss in strength. In regions closer to the weld, the heat will be
so excessive that the temperature will exceed the two phase region
and the single phase solutionizing region on the phase diagram will
be entered. Again, a loss in strength will occur, but this region
at least might be able to be re-aged to recover some strength.*Here
are presented hardness traverses of welds made in the pre-weld full
hard material. Note the softening as mentioned previously in the
as-welded condition. Note that heat input also has an effect on the
extent of softening in the as welded condition. In some cases, a
post-weld aging treatment can restore hardness in some of the
regions of this weld, but it never fully erases the effect of the
weld overaging.*On the other hand, welding precipitation hardened
material in the solution condition with a low heat input, only
slightly ages the material in the heat affected zone. Subsequent
post-weld ageing strengthens the entire weld region (only a slight
overaging occurs in the slightly ages regions from the weld). With
high heat input, however, the case is somewhat different as
moderate again occurs on welding and post-weld treatment only serve
to accentuate the overaging process. So care must be exercised when
establishing a welding procedure for welding the precipitation
hardened alloys.*Let us now turn our attention to the materials
which do have an allotropic phase change during heating. A typical
material like steel is ferrite at low temperatures and transforms
to austenite when heated. Each time the material goes through one
of these phase changes, new finer equiaxed grains grow starting
from the grain boundaries of the previous grains present. So in the
case of cold worked steels in the base metal, the elongated cold
worked grains will undergo recovery, recrystalization and grain
growth just as discussed above. But now the recrystallized grains
at higher temperature will undergo the allotropic phase change,
reducing the grain size again which then is followed by grain
growth at still higher temperature (nearer the weld). This
variation in grain structure is schematically shown in the lower
figure above.*This illustration shows the various regions in the
heat effected zone and what microstructure would be predicted as
related to the iron-carbon phase diagram. Note that at the far
extent of the weldment in the base metal, ferrite and cementite
arte expected. Closer to the weld some dual phase ferrite austenite
will occur at temperature of welding. Closer yet we would expect
single phase austenite, and then maybe some austenite of delta
ferrite and liquid mixtures until at the maximum temperature the
liquid phase would be present as the welding arc traverses. These
are the structures at temperature, but we now must consider what
happens during cooling.*We have already seen that the cooling rate
from welding can vary depending upon a number of weld variables.
The two most important are preheat and heat input. The cooling rate
is fastest when no preheat and low heat input are used to make the
weld. On the other hand, the cooling rate is slowest when high
preheat and high heat input are employed.*As we have learned
before, the cooing rate from austenite can effect the room
temperature structure as defined by the continuous cooling
transformation diagram. Rapid cooling results in non-equilibrium
hard brittle martensite. Slow cooling results in some higher
temperature transformation products such as bainite, ferrite and
pearlite which tend to be softer. Examining two welding procedures
here, one with no preheat (number 1) and the other with preheat
(number 2) we find some differences in structure. The no preheat
weld has a narrower HAZ and rapid cooling and the austenite
transforms to martensite on cooling giving a hard martensite peak
near the fusion line. The weld with preheat has a wider HAZ, a
slower cooling rate producing ferrite pearlite and bainite and the
fusion line peak is softer. There is also more outer HAZ region
grain growth and overaging so that the softening in the HAZ is
greater. Thus, once again, welding procedures have to be carefully
tailored for the material being welded.*As has been discussed, the
cooling rate in the HAZ of steel may be sufficiently high as to
produce martensite. In combination with hydrogen and stress, a
high-hardness martensitic microstructure can result in hydrogen
cracking. Hydrogen-induced cracking may be avoided by elimination
one of its three requisites: hydrogen, stress, high hardness
microstructure. Hydrogen can come from moisture; therefore,
electrodes and flux must be dry. Paint, oil, or heavy oxide layers
may also be sources of hydrogen.Hydrogen cracking can be detected
by ultrasonic methods; surface cracks may be detected visually or
with penetrant methods. Small cracking areas may be cut out and
repair welded. Extensive cracking may result in scrapped parts.*How
does the hydrogen get into the heat effected zone where the cold
cracking is often observed? Liquid metal can absorb more hydrogen
than solid austenite, and austenite more than ferrite. When welds
are made on wet material or with wet electrodes, the hydrogen is
absorbed into the liquid. As the liquid solidifies, if forces some
of the hydrogen which it is trying to get rid of into the
surrounding hot austenite. If there is still too much to be
absorbed even in a supersaturated solid, some hydrogen porosity may
form in the weld metal, a sure sign that poor procedures were
followed. *During cooling, the cooler material tries to push
hydrogen out while at the same time the solidifying weld metal
tries to push hydrogen out. Note that the large grained region of
the HAZ which just may have the hardest most susceptible
martensitic microstructure thus acquired hydrogen from both
directions and a supersaturated condition exists there.*The
hydrogen then slowly diffuses to any location where is can relieve
the stress of being stuck in the lattice in the supersaturated
condition. The hydrogen atoms are often carried by dislocation and
the preferred site for collection is often inclusions. At this
point, they can either weaken the surrounding structure or the
hydrogen atoms can recombine and form molecular hydrogen gas and
exert an internal pressure. As this pressure grows, the crack
slowly expands until a critical size is reached and catastrophic
failure occurs. This takes time at low temperature , thus the
common name of cold cracking or delayed cracking applies.*The time
after welding has an effect. As time proceeds, the hydrogen
diffuses away from the high concentration in the most critical
portion of the heat affected zone. If hydrogen diffuses away before
the critical crack length is reach, the weld has occurrence of some
microcracks but catastrophic failure does not occur. On the other
hand, if hydrogen diffusion is slower than that failure may occur.
Elevated temperature post weld treatment will allow fast hydrogen
diffusion and may help in the reduction of cold cracking.*The above
diagram summarizes the discussions about delayed cracking. The red
regions are crack sensitive regions while the blue represents the
safe region. Materials with high hardenabilty will promote the
formation of martensite, and materials with high carbon content
will produce a harder martensite. Increases in heat input and
preheat and stress reliving practices increases the safety against
hydrogen delayed cracking. And the decrease in hydrogen in the
welding process likewise increases the safety region.*If the
cooling rate is slowed sufficiently, martensite is not formed. The
application of preheat before welding slows the cooling rate after
welding. Preheat, as the name implies, involves heating up the
plate to be welded to a specified temperature prior to welding.
Thus, after welding, the temperature differential between the weld
and the surrounding plate is less. This acts to slow the cooling
rate and avoid the formation of martensite. By reducing the
temperature differential between the weld and the surrounding
plate, preheat also helps to reduce shrinkage stress and
distortion. Since steels are susceptible to hydrogen cracking, the
preheat also provides energy for hydrogen to escape from the metal.
Hydrogen is introduced into the metal from several sources:
moisture in the shielding gas or flux, degreasing agents that were
not properly removed prior to welding, moisture in the air.*If the
cooling rate is slowed sufficiently, martensite is not formed. The
application of preheat before welding slows the cooling rate after
welding. Preheat, as the name implies, involves heating up the
plate to be welded to a specified temperature prior to welding.
Thus the temperature differential between the weld and the
surrounding plate is less. This acts to slow the cooling rate and
avoid the formation of martensite. Preheat has the added benefit of
reducing residual stress and distortion by reducing the temperature
differential between the weld and the surrounding plate.In a
manufacturing operation, the time, equipment, and energy costs
associated with preheat detract from the overall productivity of
the welding operation. Also, in confined spaces, high preheat
temperatures, as high as 500F for some steels, are a major source
of discomfort for the welder. Nonetheless, based on composition and
other factors, preheat is required for many steels. Ensuring its
proper application and control can be a daunting task; however, the
alternative to proper preheat is clear - scrapping parts after
welding. *Which formula to use? There are a number of carbon
equivalent formulae and charts and graphs. Check to ensure that you
are using a method suited to your steel. Software packages are also
available that examine joint geometry, process variables, and steel
composition to compute the proper preheat and postweld heat
treatment. Procedure selection for avoiding hydrogen cracking is
often made based on the chemical composition of a material.
Unfortunately, records that contain chemical composition
information for older, existing structures, such as pipelines and
piping systems, are often difficult or impossible to locate. In
these cases, estimates of chemical composition can be made based on
the maximum allowable limits of the specification to which the
materials were produced. This usually results in an overestimation
of the tendency for unacceptably high hardness levels to result and
can, therefore, be restrictively over-conservative.The chemical
composition of an in-service component can be determined by removal
of samples for laboratory analysis, provided that care is taken in
removing the samples and that they are properly analyzed. For
example, a flat area can be machined on the pipe provided that the
remaining wall thickness is greater than the specified nominal wall
thickness minus the pipe mill tolerance. The amount of material
required for direct spectrographic analysis of remelted machining
chips is 10 to 20 grams. Care should be taken to prevent oxidation
(i.e., overheating) of the machining chips. Low-cost portable
milling machines that are used primarily to cut keyways in shafts
can be used for this purpose.
*If martensite is produced in the HAZ, its poor mechanical
properties can be remedied through a post-weld heat treatment.
Heating the martensite to an elevated temperature, but not high
enough to change it back to austenite, allows some of the carbon to
form iron carbide. This process is referred to as tempering. It
reduces the hardness and increases the ductility of the martensite.
Although the strength may be somewhat reduced, the toughness
increases. Post-weld heat treatment also helps to reduce any
residual stress left behind from the welding process.Some
compositions of steel were designed to always form martensite on
cooling in order to take advantage of its high strength. These
steels are generally postweld heat treated in order to increase the
ductility of the martensite.*If martensite is produced in the HAZ,
its poor mechanical properties can be remedied through a post-weld
heat treatment. Heating the martensite to an elevated temperature,
but not high enough to transform it back to austenite, allows some
of the carbon to form iron carbide. This process is referred to as
tempering. It reduces the hardness and increases the ductility of
the martensite. Although the strength may be somewhat reduced, the
toughness increases. Post-weld heat treatment also helps to reduce
any residual stress left behind from the welding process.As with
the preheat process, the time, equipment, and energy costs
associated with postweld heat treatment detract from the overall
productivity of the welding operation. Temperbead or controlled
deposition welding sequences have been designed that are
self-tempering. In these processes, the heat from subsequent
welding passes tempers the martensite produced by prior passes.
Such processes have been used successfully for many years,
particularly for weld repair. These processes do require special
welder training and have limited use with some of the more
hardenable materials (higher alloyed steels such as 2.25Cr-1Mo)
when it comes to meeting code requirements for harsh service
environments.*Lets end our discussion of weld problems by
considering difficulties which may occur in the base metal.
Ordinarily, one would not consider that there could be any problems
occurring in the base metal as it does not see the effect of any
weld heat. But let us not forget that the weld solidification does
introduce some stresses into the weldment because of the
solidification and variable expansion and contraction of heated
materials.*In the processing of steel, sulfur combines with
manganese to form MnS inclusions in the ingot. When the ingot is
rolled, these inclusions elongate into what are referred to as
stringers. The strength of the steel in the direction transverse to
these stringers is reduced. The stress produced by welding can
cause cracks if the weld is made in the rolling direction, parallel
to the stringers. Small regions can be cut out and replaced with
weld metal. Large regions may result in scrapping the part. *This
illustrates how the rolled out inclusions (mainly MnS) can de-bond
from the base metal matrix and under the action of short transverse
(through thickness) stresses they can actually link to form a
stepped like fracture. Improving cleanliness of the steel during
steel processing, and improving through thickness properties by
steel making processed line calcium or rare earth treatment which
produces inclusions which to not roll out a long stringer during
plate processing can help. Also laying a weld bead on top of the
plate which has lower strength and improved ductility before
welding the attachment can help by letting the weld bead take the
shrinkage stresses rather than transmitting them into the base
plate.*In a multi-pass weld, the heating and cooling cycles of one
pass are superimposed upon those of previous passes. Portions of
previous passes are heated high enough to form austenite again, and
upon cooling this austenite once again can transform to ferrite and
pearlite or to martensite. Some portions of previous weld passes
will not transform to austenite but will be tempered by the heat
from subsequent passes. All in all, this leads to a rather
complicated structure in multi-pass welds.*In a multi-pass weld,
the heating and cooling cycles of one pass are superimposed upon
those of previous passes. Portions of previous passes are heated
high enough to form austenite again, and upon cooling this
austenite once again can transform to ferrite and pearlite or to
martensite. Some portions of previous weld passes will not
transform to austenite but will be tempered by the heat from
subsequent passes. All in all, this leads to a rather complicated
structure in multi-pass welds.*Steels containing molybdenum or
vanadium resist creep at elevated-temperature. These steels, along
with thick sections of high-strength, low-alloy steels, are subject
to reheat cracking in combination with residual stress and low
creep-ductility in the HAZ. During postweld heat treatment, cracks
form along the grain boundaries in the HAZ, particularly in the
coarse-grained region near the fusion line. Defects at the weld toe
can promote reheat cracking; therefore, grinding or peening the
weld toe can help prevent this cracking. The cracked area must be
heat treated to restore ductility prior to repair. Then it can be
cut out beyond the ends of the cracks and rewelded.*A discrete band
in the heat affected zone of the austenitic stainless steel welds
experiences peak temperatures in the 800-1600F temperature range
associated with sensitization. Chromium carbide precipitation in
this region can lower the chromium content near the grain
boundaries to less than 12%, thereby causing
sensitization.Stabilized grades can also suffer from knife-line
attack. Elevated temperatures in the heat-affected zone can
dissolve titanium and niobium carbides. The fast cooling rates in
the welded joint do not allow these carbides to reform. This leaves
excess free carbon, which can then form chromium carbides.
![Welding Metallurgy Pc[1].Ppt 2](https://img.dokumen.tips/doc/110x75/55cf9b8b550346d033a67c09/welding-metallurgy-pc1ppt-2.jpg)
![Welding Metallurgy[1]](https://img.dokumen.tips/doc/110x75/55cf8e70550346703b9232af/welding-metallurgy1.jpg)
