CHAPTER 6. WELDING
Welding
is characterized by a very high peak temperature, above 1500°C, but retained
only for a very short time. Therefore, there will be no appreciable grain
growth unlike in the case of casting. The plate or pipe parts being joined act
like a huge sink of heat resulting in a drastic cooling of fused weld pool of
little volume. Such a rapid cooling rate is of the biggest concern during
welding because it has a potential of adversely affecting the microstructure of
not only the solidified weld metal but also the HAZ of the base metal.
6.1 Microstructures of Weld & HAZ
The microstructure of the weld metal
will be noticeably different from that of the base metal or HAZ because it
represents the as-solidified molten metal pool under rapid rate of cooling. See
fig. 6.1. Typical microstructure of the weld metal of low carbon and low alloy
steels consists of ferrite in different shapes and locations and bainite. They
are very fine-grained generally.
HAZ
is that portion of the base metal lying next to the fusion line of weld, which
had not melted but whose microstructure and hence mechanical properties have
been altered by the heat of welding. Near the fusion line the peak temperature
of HAZ can reach 1400°C. Grain growth and coarsening occur in this region. See
fig. 6.2.
Because of the relatively high cooling rate and large grain size,
acicular, rather than blocky, ferrite is formed at boundaries of large grains
of fine-pearlite or bainite. Due to coarsened grains, under very high cooling
rate, this region has the potential of getting transformed into martensite.
6.2 Residual Stress
6.2.1 Development
Let
us assume that during welding, a small band of base metal adjacent to fusion
zone reach an average temperature of 900°C and the rest of the base metal is at
an average temperature which is slightly more than the room temperature. The
heated band of base metal adjacent to the weld tries to expand, which is
restrained by the adjacent large mass of relatively cold portion of base metal.
This leads to a compressive stress induced in the heated band. Since a
temperature differential of 100-150°C is sufficient to induce stresses of such
a high magnitude as to exceed the yield strength of the steel, the heated band
soon starts flowing plastically during welding.
Now
after welding, the heated band begins to cool and shrink, which is again
restrained by the adjacent large mass of base metal. This leads to a tensile
stress induced in the heated band. The large thermal gradient during the
heating-cooling cycle of welding, thus, leads to the development of internal
stress, called the residual stress.
The
nature and distribution of the residual stress are complicated and difficult to
be evaluated. It exists along, across and through-thickness of the weld and
base metal, i.e., in a triaxial state. It is comprised of tensile and
compressive stresses in equilibrium. It is generally of the order of yield
strength of the material at its peak value.
6.2.2 Effects
Since all weld joints in the
as-welded condition will contain a system of internal tensile and compressive
stresses that balance each other, it may seem possible that stresses produced
due to externally applied loads, like hoop stress due to internal pressure
inside drum or piping, would add to the residual stress of like sign and would
quickly overstress the component, resulting in failure. If it is the case,
almost all welded joints will fail because, as we already noted, peak value of
residual stress will be of the order of yield strength. But it is not the case
because the stress system is in equilibrium. Therefore, stresses produced due
to the externally applied loading in tension cannot be additive to residual
tensile stresses until the balancing compressive stresses of the system in
equilibrium are overcome.
It
has been established through experiments that as the level of applied stress
(due to pressure, etc.) increases, the residual stress decreases due to local
yielding. Therefore, the effect of residual stress is significant only on
phenomena which occur under low applied stress i.e., at boiler components under
medium and low pressure and temperature such as brittle fracture and stress
corrosion cracking.
6.2.2.1 Brittle Fracture It occurs under
triaxial system of stresses. Under such a system, the yield strength of steel
increases and failure occurs in a brittle mode. Residual stresses, which are in
a triaxial state, and notch effects of weld defects may significantly lower the
toughness of the steel around the weld joints.
6.2.2.2 Stress Corrosion Cracking It is the result of the
combined effects of corrosion and tensile stress. Where this occurs, corrosion
alone would not have produced failure by cracking. Conversely, in the absence
of corrosion attack, the stresses present in the material would not have
produced failure.
6.3 Cold Cracking
Martensitic
transformation and the resulting high hardness can lead to cracking in the weld
or HAZ, if the metal can not yield to relieve welding stresses. Such a cracking
is induced by the diffusible atomic hydrogen present in the weld / HAZ and
generally occurs at a temperature below 150°C immediately upon cooling or after
a period of several hours. This type of cracking is known as hydrogen cracking
or underbead cracking or cold cracking. The resulting cracking is
transgranular.
The
diffusion rate of hydrogen in steel at or near its melting point is very high.
Once in the weld metal, hydrogen atoms can diffuse rapidly into HAZ. The
absorbed hydrogen is rejected from steel during cooling and phase
transformations, whereupon it tends to concentrate at microstructural
dislocations and voids in the matrix. Molecular hydrogen forms in these voids
resulting in the development of pressure. This induces localized tensile
stresses, which may lead to cracking in harder weld / HAZ.
6.4 Hot Cracking
S
reacts with Fe to form FeS, which is soluble
in molten steel. The last trace of S-rich liquid between crystalline grains of
steel does not freeze until about 1000°C. The presence of these envelopes of
liquid FeS around the metal grains at so low a
temperature invariably results in intergranular hot cracking.
Mn
counteracts this evil by forming MnS, which unlike FeS,
does not dissolve in liquid steel. Therefore, MnS does not increase the
freezing range; in fact, it solidifies as globules in the liquid steel. P,
through similar mechanism, may induce hot cracking of steel. It is generally sufficient
to restrict the contents of S and P in the range of 0.03-0.05%.
6.5 Welding Carbon Steels and Cr-Mo
Steels
Using
a filler metal with a chemistry identical to that of the base metal may not
produce the desired results, because the microstructure of the weld metal is
entirely different from that of the base metal. For most carbon and low alloy
steels, rapid cooling rate involved in fusion welding results in a weld metal
that has higher strength and lower ductility and toughness than the base metal
when they are of the same chemistry. In addition, dilution of base metal, i.e.,
the melting of base metal and the subsequent mixing with the filler metal,
causes the final chemistry of the weld deposit to fall in between that of the
base and filler metals. Consequently, the filler metal often contains a lower C
level than the base metal. When required, strength of the weld metal is
improved not by increasing C content, but by adding alloying elements like Mn
and Mo.
Excellent
ductility and toughness can be achieved in weld metal if the microstructure is
essentially acicular ferrite with minimal bainite and no martensite and if the
grain size is as minimum as the welding conditions permit. Use of preheat helps
in reducing the cooling rate, thereby avoiding the hardening of weld metal and
HAZ. High heat input processes like SAW decreases the cooling rate but may
increase the grain size, whereas low heat input processes like SMAW result in
fine grained structure but increases the cooling rate. A WPS should be
qualified taking all these factors into account.
Hydrogen
cracking is usually not encountered while welding C-steels with C < 0.2% and
Mn < 1%. Use of low hydrogen electrodes is not essential for welding these
steels, especially when joint thickness is less than 1” and when joint
restraint is not severe. As C and Mn increase to about 0.3% and 1.4%
respectively, welds become susceptible to hydrogen cracking, because of
increased strength and hardening – welding with low hydrogen electrodes is
recommended.
Cr-Mo
steels will harden when quenched from austenizing temperatures and are
sensitive to hydrogen cracking. Appropriate preheat and low hydrogen electrodes
are to be used. Composition of filler metal should be nearly the same as that
of the base metal except for C, which is usually less than that of the base
metal.
6.6 Welding Austenitic Stainless Steels
6.6.1 δ-ferrite in Weld Metal
Alloys that are fully austenitic in
the wrought form often exhibit a two phase austenite-δ-ferrite weld metal
microstructure. The presence of δ-ferrite in austenitic stainless steel weld
metal is known to be beneficial in reducing the tendency for weld
solidification cracking. Filler metal high in C and Ni like ER310 produce fully
austenitic weld metal. Though this may exhibit acceptable cracking resistance,
the inherent resistance of ferrite-containing weld metals has been repeatedly
shown to be superior. As a result, many of the commonly used austenitic
stainless steel filler materials like ER308, ER309, ER347, etc., are formulated
so that the as-deposited weld metal contains 2-10% δ-ferrite.
6.6.2 Sensitization – HAZ
Because
heating and cooling in the HAZ are relatively rapid, regions of HAZ heated to
peak temperatures below about 650°C are immune to sensitization. In addition,
the HAZ region immediately adjacent to the fusion boundary also is resistant
because C solubility is high at peak temperatures achieved and cooling through
the sensitization range is generally rapid enough to suppress carbide
precipitation. This leaves a narrow region in the HAZ, somewhat removed from
the fusion boundary, that will be heated into the sensitization temperature
range. If the time within this temperature range is sufficient, intergranular
carbides will nucleate along the grain boundaries.
Low
weld heat input and low interpass temperature increase the cooling rate,
thereby reducing the time in the sensitization temperature range. Otherwise,
resolution anneal or, use of L-grades or stabilized grades will have to be
resorted to for solving this problem.
6.6.3 Sensitization – Weld
Weld metals containing even small amounts of δ-ferrite are not generally as susceptible to such severe sensitization as wrought material of similar composition. In ferrite-containing weld metal, carbide precipitation occurs more uniformly throughout the structure at the γ-δ interphase boundaries. Because δ-phase has a relatively high Cr content (in the order of 25% in a 18Cr-8Ni alloy), Cr depletion due to carbide formation is not so significant, and there is not a continuous path through the weld along the δ-ferrite boundaries. Consequently, the weld metal is not generally susceptible to intergranular corrosion. Fully austenitic weld metals e.g. welds produced by ER310, however, may be susceptible to intergranular corrosion when exposed to severely corrosive environments.
6.7 Dissimilar Weld of Cr-Mo ferritic
& Cr-Ni austenitic steels
Experience with Dissimilar Metal
Welds, DMW, involving ferritic steels and austenitic stainless steels and
welded with type 309 filler, has shown a significant number of failures in time
much less than the expected service life. The majority of such transition joint
failures occur in ferritic HAZ adjacent to the weld interface.
Cr
in steel has a greater affinity for C than Fe. When a C-steel or low alloy
steel is welded with a stainless steel filler containing a significant amount
of Cr, C will diffuse from base metal into the weld metal at temperatures above
about 425°C. The extent of diffusion is a function of temperature and exposure
time. It increases rapidly at 600°C and above. C migration can take place
during PWHT or during service above 425°C. The result is a region in HAZ on the
ferritic steel side that is denuded of C. This region is very soft and weak in
creep.
During cyclic temperature service,
HAZ of base metals will be subjected to varying shear stresses because of the
large difference in the coefficients of thermal expansion of ferritic and
austenitic steels. When type 309 filler is used all the thermal mismatch will
be forced on the weaker ferritic HAZ resulting in the failure. On the other
hand, if a Cr-containing Ni-based filler is used the thermal mismatch will be
forced on the stronger austenitic stainless steel side of the joint. The reason
for this is that Ni-based filler metal has a coefficient of thermal expansion that
matches that of Cr-Mo steel.
It
has been observed that, for DMWs in superheater coils put in service with metal
temperatures around 600°C and high pressures, by selecting a Ni-based filler,
life of the joints are extended by five times as compared to type 309 filler
joints. ERNiCr-3 and ENiCrFe-2 or ENiCrFe-3 are recommended. However, for
service metal temperatures less than 475°C, type 309 filler is adequate for
DMWs.
Research
sponsored by Electric Power Research Institute of U.S. suggests the following
guidelines for DMWs for substantial improvement in the expected life:
·
Use Ni-based fillers for all
DMWs.
·
Locate DMW at as low a
temperature as possible.
·
Make the welds in vertical
rather than horizontal runs of tubing to minimize bending stresses that may
contribute to the overall stress.
·
Make the DMWs in the shop. The
weld should be smoothly blended to the base metal with no undercut or other
abrupt section change that would act as a stress raiser.
CHAPTER 7. STRESS RELIEVING
Stress
relieving, SR, is the most frequently employed heat treatment process in a
boiler fabricating industry. It consists of uniformly heating the component to
a specified temperature below LCT, soaking it there for a definite time and
then cooling it uniformly in the furnace until at least 400°C and then in air.
During SR, all the following events occur: relieving of internal stresses,
tempering and recrystallization. But the extent to which each of these events
occurs rather depends on the initial state of the steel before SR, i.e., as
welded, as hardened or as cold worked.
7.1 Relieving of Internal Stresses
Welding causes the development
of large internal stresses of the order
of yield strength of the base metal. This system of residual stresses is
believed to cause brittle fracture and stress corrosion cracking in service.
Room temperature yield strength of
the base metal provides an excellent estimate of the level of localized
residual stress that can be present in a weldment. To relieve the residual
stress requires that the component be heated to a temperature where its yield
strength approaches a value that corresponds to an acceptable level of stress.
Holding at this temperature can further reduce the residual stress through
further yielding due to creep. Uniform cooling after SR is mandatory if these
levels of residual stress are to be maintained.
7.1.1 Code
Requirements
Most of the parameters of SR heat
treatment are dictated by the code requirements so that there will be little
flexibility for the fabricator. For an example, for carbon steels, IBR requires a minimum SR temperature of 600°C and a soaking time @ 2½ minutes
per mm of thickness.
BS 1113 gives detailed requirements of other
parameters. Uniform heating and cooling are required during SR to avoid
distortion of the component, possible cracking and re-introduction of internal
stresses. Between 400°C and soaking temperature BS 1113 requires that the heating / cooling rate be
at least 5500°C / hr divided by the thickness in mm; this rate should not be
faster than 220°C / hr and need not be slower than 55°C / hr.
7.1.2 Local
SR
With local heating as opposed to
furnace heating, no matter how long the holding time, there is always a
temperature gradient through the thickness. The thicker the section, the
greater will be this gradient and higher will be the stresses produced by it.
If possible, this gradient should be measured by fixing thermocouple on the
inside surface of weld; this gradient
should be controlled such that the stresses produced do not exceed the yield
strength of the base metal. AWS (ref #11) recommends a temperature differential
of 66°C through thickness. Assuming a linear thermal expansion coefficient of
15 x 10-6 mm per mm per °C for 600 – 750°C and
Young’s Modulus of 200 GPa at room temperature, this temperature differential
corresponds to a stress = 15 x 10-6 x 66 x 200 x 109 Pa = 200 MPa, well below the yield strength of
most of the steels used in boiler industry.
Longitudinal and circumferential
temperature gradients for a cylindrical component under local SR should also be
uniform and within the limit. To ensure this BS 1113 requires that the
temperature at a distance of at least 2.5 x sqrt (r x t), where r = internal radius of
cylindrical component and t = its thickness, from the weld center-line should
be at least half of the peak temperature reached at the weld.
By arranging sufficient width of
band of heating elements and insulation all these temperature gradients can be
controlled so that stresses will not be re-introduced.
7.1.3 Carbon
Steel
IBR requires an SR
temperature of at least 600°C for carbon steels but it permits a lower
temperature up to 550°C for critical components like drum, provided soaking
time is correspondingly increased. Though the short-term yield strength at
elevated temperature is, for all steels, independent of soaking time, for carbon steel, due to its
poor creep properties, the percentage stress relief increases as the soaking
time is increased at a given SR temperature, especially after about 500°C. ASME
Sec.I requires a soaking time @ 2 hrs per inch thickness at 565°C whereas it
requires a soaking time @ 1 hr per inch thickness at 593°C. Due to good creep
properties, for Cr-Mo and stainless steels this phenomenon does not occur.
7.1.4
Austenitic stainless steel
Due to their high yield strength at
elevated temperatures and good creep properties, austenitic stainless steels
have to be heated to about 900°C to attain adequate stress relief. Quenching or
other rapid cooling will usually reintroduce the residual stresses. SR is
necessary only when the component is, in service, subjected to environments
conducive to stress corrosion cracking. Selection of an optimum SR treatment is
difficult because heat treatments that provide adequate SR can impair the corrosion
resistance of austenitic stainless steel
and heat treatments that are not harmful to corrosion resistance may not
provide adequate SR. To avoid specifying a heat treatment that might prove
harmful, ASME Sec.I and ANSI B31.1 neither require nor prohibit stress relief
of austenitic stainless steels.
When a component is fabricated out
of carbon- or Cr-Mo steel and austenitic stainless steel, heat treatment of the
component should be carefully planned. Let us see a case study to illustrate
this point. Dozing nozzle of size OD 31.8 mm x 4 mm thick conforming to SA 213 TP304H was
telescopically socket-welded to a carbon steel stub of a power plant drum. The
drum was given an SR treatment at 610°C for 540 minutes.
To assess the damage to the
corrosion resistance property of stainless steel, intergranular corrosion test
as per A262 was carried out with a sample test piece subjected to a similar SR.
Oxalic acid etch test as per Practice A clearly revealed chromium carbide
precipitation along austenite grain boundaries. See fig. 7.1. Further
flattening test after exposure to sulphuric acid – copper sulphate solution, as
per Practice E, resulted in several cracks on the inside and outside surfaces.
Obviously the nozzle is unfit to carry chemical solution in service. Solution
to this problem is either to use stabilized grades like TP347 or to weld TP304H
nozzle after SR of drum is completed.
7.2 Tempering
– SR of Cr-Mo steels
During SR the harder microstructure constituents like martensite and
other carbides in the weld and HAZ get tempered into softer carbides. This is
of special importance in the case of Cr-Mo steels. SR improves the toughness
and ductility of weld and HAZ of these air-hardening steels so that these
weldments can withstand the hoop and expansion stresses in service without
cracking. To illustrate this point AWS (ref #11) gives the following data:
All-weld tensile strength of a 0.08C-2¼Cr-1Mo weld was measured to be as high
as 900 MPa with its ductility as low as 10% in the as-welded condition but
after SR at 700°C for 1 hr, its tensile strength dropped to below 650 MPa and
ductility rose to 20%.
For Cr-Mo steels the effect of
secondary hardening during SR ends before 600°C. From 600°C onwards until LCT,
the coalescence of carbides proceeds, resulting in a progressive drop in
hardness. Long-term creep properties of Cr-Mo steels get impaired due to this
coalescence of alloy carbides. Therefore, an SR temperature near LCT will
impair its creep resistance but an SR temperature near 600°C will result in
high and unacceptable hardness of weld
and HAZ which may lead to cracking in service. In addition, if the SR
temperature exceeds the tempering temperature of base metal raw material, again
mechanical properties including creep resistance of base metal might be
adversely affected after SR. Most of the Cr-Mo pipe and tube manufacturers
select tempering temperatures above 700°C so that an SR temperature below 700°C
may safely be selected, provided adequate ductility of weld and HAZ is
obtained.
The above discussion explains the
wide range of temperatures given for SR of Cr-Mo steels by
IBR (for an example, for 2¼Cr-1Mo steel, IBR recommends a range of 650 - 750°C) with a tag, “This wide range is necessary because of the marked dependence
of the mechanical properties of these steels on the tempering temperature. In
production a definite temperature with a tolerance of ± 20°C
would be selected to ensure that the mechanical properties upon which
the design was based are in fact achieved.”
7.3
Recrystallization – SR of Cold Bends
Cold bending / forming operation on
pipes and tubes induces elongation and distortion of grains of the steel,
leading to loss in ductility. During SR these distorted grains recrystallize
and gain equiaxial shape so that the original ductility of the steel is
restored.
7.3.1 Cold
Forming vs Hot Forming
A temperature 56°C below LCT may be taken as the demarcation
line between cold and hot forming operations, in line with ANSI B31.1.
If hot forming is performed with a
starting temperature above around 980°C, grain growth may take place, adversely
affecting the mechanical properties of the steel. A post forming heat
treatment, PFHT, of normalizing for carbon steel and normalizing and tempering
for Cr-Mo steel will be required. If hot
forming is performed within the temperature range between 980°C and UCT, there
will be no need for any PFHT for carbon steel and tempering / SR will be
adequate for Cr-Mo steels because this range represents the normalizing range.
If
hot forming is performed with an ending temperature below UCT and above LCT, a PFHT of
normalizing for carbon steel and normalizing and tempering for Cr-Mo
steel is desirable.
Cold formed components need only a
tempering / SR heat treatment.
Codes require PFHT only when the
distortion of grains is considered severe enough.
In the light of the above discussion
readers are requested to ponder over the following extract from SA 234:
“Hot-formed carbon steel fittings upon which the final forming operation is
completed at a temperature above 620°C and below 980°C need not be heat treated
provided they are cooled in still air. Hot-formed carbon steel fittings
finished at temperatures in excess of 980°C shall subsequently be annealed,
normalized, or normalized and tempered. Hot-forged fittings NPS 4 and smaller
need not be heat treated. Cold-formed carbon steel fittings upon which the
final forming operation is completed at a temperature below 620°C, shall be
normalized, or shall be stress relieved at 595 to 690°C.”
Now readers are requested to verify
the following interpretation of the above provisions of the Standard:
· 620°C is taken as the demarcation
temperature between cold forming and hot forming.
· Grain growth is assumed to take place above
980°C.
· Grain growth may be ignored for fittings of size NPS 4 and smaller.
· An SR at 600°C or a sub-critical anneal up
to 690°C is acceptable for cold-formed fittings.
Readers are, however, cautioned not to extend the
above interpretation of a specific portion of SA 234 to other raw material
specifications or to forming during fabrication.
7.3.2 Pipe
Bending
IBR, in line with ANSI B31.1,
requires, for carbon steel, SR for cold forming and normalizing for hot forming
performed outside the normalizing range, only if thickness of the pipe exceeds
¾”. For Cr-Mo steels IBR requires tempering for cold forming and normalizing
& tempering or tempering ( it should be interpreted as tempering for
forming in the normalizing range, and
normalizing & tempering for forming performed outside the
normalizing range ), only if either NPS exceeds 4” or thickness of the pipe
exceeds ½”.
7.3.3 Tube
Bending
BS 1113 deals with post bending heat
treatment requirements for tube bending in detail. For cold bending it requires
tempering only when R/D < 1.3 or thinning exceeds 25%. It suggests
an expanded range of 1100 - 850°C for standard hot bending temperature range.
When hot bending is performed in this standard range, BS 1113 requires
tempering for 2¼Cr-1Mo steel and no heat treatment for other steels. When hot
bending is not performed in this standard range, it requires normalizing &
tempering for 2¼Cr-1Mo steel and normalizing for other steels. Soaking time for
normalizing / tempering shall be at least 30 minutes.
Cold-worked regions of austenitic
stainless steels are preferentially attacked by stress corrosion in corrosive
environments. For cold-worked austenitic stainless steels carbide precipitation
may occur at temperatures as low as 425°C. Therefore, it is desirable to give
solution anneal for austenitic stainless steel after bending. BS 1113 requires
solution treatment if hot forming is performed outside the range 1100 - 900°C.
Solution anneal may be given in a temperature range of 1000 - 1100°C for at
least 10 minutes.
Stabilized grades like 321 or 347 do
not require water quenching or even other accelerated cooling from annealing
temperature to prevent subsequent intergranular corrosion. Air cooling is
generally adequate.
----- end of Part - 4/4 ------
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