North American Steel Construction Conference
Taking Your Weld’s Temperature
A successful welded con-
nection requires properly moni-
important in highly restrained joints; and
it raises some steels above the tempera-
ture at which brittle fracture would occur
in fabrication. Additionally, preheat can
toring and sometimes adjusting the tem- be used to help ensure specific mechani-
perature of both base metal and weld cal properties, such as weld metal notch
toughness.
metal before, during, and after welding.
The heating and cooling of the weld When Should Preheat Be Used?
metal and its adjacent base metal will
affect the mechanical properties of the In determining whether or not to pre-
weld. Improper rates of heating and heat, the following should be considered:
cooling can result in weld cracking. To code requirements, section thickness,
measure and adjust the temperature base metal chemistry, restraint, ambient
effectively, the engineer must: calculate temperature, welding consumable hydro-
heat input (the energy transferred during gen content, and previous cracking prob-
the process of welding); determine lems. If a welding code must be fol-
whether or not to apply preheat; measure lowed, then the code generally will
and adjust the interpass temperature; and specify the minimum preheat tempera-
R. Scott decide whether—and how—postweld heat ture for a given base metal, welding
Funderburk treatment (PWHT) should be applied. process and section thickness. This mini-
mum value must be attained regardless of
is a the restraint or variation in base metal
Welding H
Fundamentals of Pre-Heat chemistry; however, the minimum value
Design Preheating involves heating the base may be increased if necessary.
Engineer metal, either in its entirety or just the When there are no codes governing
with region surrounding the joint, to a specific the welding, one must determine whether
The Lincoln desired temperature, called the preheat preheat is required, and if so, what pre-
temperature, prior to welding. Heating heat temperature will be appropriate. In
Electric may be continued during the welding
Company in general, preheat usually is not required
process, but frequently the heat from on low carbon steels less than 1” (25
Cleveland. welding is sufficient to maintain the mm) thick. However, as the chemistry,
This article desired temperature without a continua- diffusible hydrogen level of the weld
is excerpted tion of the external heat source. The metal, restraint or section thickness
interpass temperature, defined as the increases, the need for preheat also
from a base metal temperature at the time when
paper he is scheduled to present at increases.
welding is to be performed between the
the 2000 North American Steel first and last welding passes, cannot be What Preheat Temperature
Construction Conference in Las permitted to fall below the preheat tem- Is Required?
Vegas. perature. Interpass temperature is dis-
cussed more fully in the next section of Welding codes generally specify mini-
this paper. Preheating can produce mum values for the preheat temperature,
many beneficial effects; however, without which may or may not be adequate to
a working knowledge of the fundamen- prohibit cracking in every application.
tals involved, one risks wasting money, or For example, if a beam-to-column con-
even worse, degrading the integrity of the nection made of ASTM A572 Gr. 50
weldment. jumbo sections (thicknesses ranging from
4 to 5 in [100-125 mm]) is to be fabricat-
Why Preheat? ed with a low-hydrogen electrode, then a
There are four primary reasons to uti- minimum prequalified preheat of 225°F
lize preheat: it slows the cooling rate in (107°C) is required (AWS D1.1-96, Table
the weld metal and base metal, producing 3.2). However, for making butt splices in
a more ductile metallurgical structure jumbo sections, it is advisable to increase
with greater resistance to cracking; the the preheat temperature beyond the min-
slower cooling rate provides an opportu- imum prequalified level to that required
nity for hydrogen that may be present to by AISC for making butt splices in jumbo
diffuse out harmlessly, reducing the sections, namely 350°F (175°C) (AISC
potential for cracking; it reduces the LRFD J2.8). This conservative recom-
shrinkage stresses in the weld and adja- mendation acknowledges that the mini-
cent base metal, which is especially mum preheat requirements prescribed by
Modern Steel Construction / February 2000
AWS D1.1 may not be adequate for these How Is Preheat Applied? interpass temperatures will generally pro-
highly restrained connections. vide a finer grain structure and improved
The material thickness, size of the Charpy V notch toughness transition
When no welding code is specified, weldment and available heating equip- temperatures. However, when interpass
and the need for preheat has been estab- ment should be considered when choos- temperatures exceed approximately
lished, how does one determine an ing a method for applying preheat. For 500°F (260°C), this trend may be
appropriate preheat temperature? example, small production assemblies reversed. For example, the American
Consider an approach outlined in the may be heated most effectively in a fur- Welding Society (AWS) Position
American Welding Society’s Structural nace. However, large structural compo- Statement on the Northridge Earthquake
Welding Code, AWS D1.1, Annex XI: nents often require banks of heating recommends that the interpass tempera-
“Guideline on Alternative Methods for torches, electrical strip heaters, or induc- ture should not exceed 550°F (290°C)
Determining Preheat.” Two procedures tion or radiant heaters. when notch toughness is a requirement.
are presented for establishing a preheat
temperature. These techniques, devel- Preheating carbon steel to a precise
temperature generally is not required.
Why a Maximum?
oped primarily from laboratory cracking
tests, are beneficial when the risk of Although it is important that the work be It may be important to impose control
cracking is increased due to the chemical heated to a minimum temperature, it over the maximum interpass temperature
composition, a greater degree of restraint, usually is acceptable to exceed that tem- when certain mechanical weld metal
higher levels of hydrogen or lower weld- perature by approximately 100°F (40°C). properties are required. As described in
ing heat input. However, this is not the case for some the previous paragraph, weld metal notch
quenched and tempered (Q&T) steels toughness is one example. If a designer
The two methods outlined in Annex such as A514 or A517, since welding on expects a minimum strength level for a
XI of AWS D1.1 are: heat affected zone overheated Q&T steels may be detrimen- particular component that could experi-
(HAZ) hardness control and hydrogen tal in the heat affected zone. Therefore, ence extremely high interpass tempera-
control. The HAZ hardness control Q&T steels require that maximum and tures (i.e., due to its size or welding pro-
method, which is restricted to fillet welds, minimum preheat temperatures be estab- cedures), a maximum interpass
is based on the assumption that cracking lished and closely followed. Specific rec- temperature should be specified.
will not occur if the hardness of the HAZ ommendations should be obtained from Otherwise, the weld metal strength could
is kept below some critical value. This is the steel producer. be unacceptably low.
achieved by controlling the cooling rate.
The critical cooling rate for a given hard- When heating the joint to be welded, A maximum interpass temperature is
ness can be related to the carbon equiva- the AWS D1.1 code requires that the also necessary for quenched and tem-
lent of the steel, which is defined as: minimum preheat temperature be estab- pered (Q&T) steels, such as ASTM A514.
lished at a distance that is at least equal Due to the base metal heat treatment, it
to the thickness of the thickest member, is critical that the interpass temperature
Mn + Si Cr + Mo + V Ni + Cu
CE = C + + + but not less than 3 in (75 mm) in all be controlled within limits which will
6 5 15 directions from the point of welding. help provide adequate mechanical prop-
Finally, the interpass temperature should erties in the weld metal and the heat
From the critical cooling rate, a mini- be checked to verify that the minimum affected zone. Keep in mind, however,
mum preheat temperature can then be preheat temperature has been main- that maximum interpass temperature
calculated. AWS D1.1-96 states that tained just prior to initiating the arc for control is not always required. In fact,
“Although the method can be used to each pass. the AWS D1.1-98 Structural Welding
determine a preheat level, its main value Code – Steel does not impose such con-
is in determining the minimum heat trol.
input (and hence minimum weld size) Interpass Temperature
that prevents excessive hardening” “Interpass temperature” refers to the A Delicate Balance
(Annex XI, paragraph 3.4). temperature of the material in the weld
Particularly on sensitive base metals,
area immediately before the second and
The hydrogen control method is the minimum interpass temperature must
each subsequent pass of a multiple pass
based on the assumption that cracking be sufficient to prevent cracking, while
weld. In practice, the minimum interpass
will not occur if the amount of hydrogen the maximum interpass temperature
temperature is often equal to the mini-
remaining in the joint after it has cooled must be controlled to provide adequate
mum specified preheat temperature, but
down to about 120°F (50°C) does not mechanical properties. To maintain this
this is not required according to the defi-
exceed a critical value dependent on the balance, the following variables must also
nition.
composition of the steel and the restraint. be considered: time between passes, base
This procedure is extremely useful for Why Is Interpass metal thickness, preheat temperature,
high strength, low-alloy steels that have ambient conditions, heat transfer charac-
Temperature Important?
high hardenability. However, the calcu- teristics, and heat input from welding.
lated preheat may be somewhat conserv- Interpass temperature is just as impor-
ative for carbon steels. The three basic For example, weldments with smaller
tant as, if not more important than, pre-
steps of the hydrogen control method cross-sectional areas naturally tend to
heat temperature, with regard to the
are: (1) calculate the composition para- “accumulate” interpass temperature: as
mechanical and microstructural proper-
meter; (2) calculate a susceptibility index the welding operation continues, the
ties of weldments. For instance, the yield
as a function of the composition parame- temperature of the part increases. As a
and ultimate tensile strengths of the weld
ter and the filler metal diffusible hydro- general rule, if the cross-sectional area is
metal are both a function of the interpass
gen content; and (3) determine the mini- less than 20 in² (130 cm²), then the
temperature. High values of interpass
mum preheat temperature from the interpass temperature will tend to
temperature tend to reduce the weld
restraint level, material thickness, and increase with each sequential weld pass if
metal strength. Additionally, higher
susceptibility index. normal production rates are maintained.
Modern Steel Construction / February 2000
However, if the cross-sectional area is Steel and the AWS D1.5 Bridge Welding
greater than 40 in² (260 cm²), then the Code require that the interpass tempera-
interpass temperature generally decreases ture be maintained “for a distance at
throughout the welding sequence unless least equal to the thickness of the thick-
an external heat source is applied. est welded part (but not less than 3 in [75
mm]) in all directions from the point of
How Is Interpass Temperature welding.” This makes sense, and is con-
Measured and Controlled? servative when controlling the minimum
interpass temperature. However, if maxi-
One accepted method of controlling mum interpass temperature is also to be
the interpass temperature is to use two controlled, then the actual interpass tem-
temperature indicating crayons. A sur- perature in the adjacent base metal may
face applied temperature indicating cray- significantly exceed the maximum speci-
on (often referred to by the trade name Figure 1. Criteria for hydrogen
fied interpass temperature. If this is the induced cracking (HIC).
Tempilstik) melts when the material to situation, it is more appropriate to mea-
which it is applied reaches the crayon’s sure the temperature 1 in (25 mm) away
melting temperature. The crayons are from the weld toe.
available in a variety of melting tempera-
potential for hydrogen induced cracking
tures, and each individual crayon is In other cases, specific industries have
(HIC). For HIC to occur, the following
labeled with its approximate melting adopted self-imposed regulations. For
variables must be present (see Figure 1):
point. One temperature indicating cray- example, in the ship building industry,
a sensitive microstructure, a sufficient
on is typically used to measure both the the interpass temperature is typically
level of hydrogen, or a high level of stress
minimum specified preheat temperature maintained 1 in (25 mm) away from the
(e.g., as a result of highly constrained
and the minimum specified interpass weld toe and within the first foot (300
connections). In structural steels, hydro-
temperature, while the second is a higher mm) of its start. In this particular case,
gen embrittlement occurs at tempera-
temperature crayon used to measure the the preheat is applied from the back side
tures close to the ambient temperature.
maximum specified interpass tempera- of the joint so as to completely “soak” the
Therefore, it is possible to avoid cracking
ture (if required). base metal.
in a susceptible microstructure by diffus-
The welder first heats the joint to be Although there is some debate as to ing hydrogen from the welded area
welded and checks the base metal tem- where the interpass temperature should before it cools. After welding has been
perature at the code-designated location be measured, most experts agree that it completed, the steel must not be allowed
(see “How is Preheat Applied?”) by must be maintained for some reasonable to cool to room temperature; instead, it
marking the base metal with the first distance away from the welded joint. should be immediately heated from the
temperature-indicating crayon. When Since this decision may greatly influence interpass temperature to the post heat
the minimum specified preheat tempera- the fabrication cost, a reasonable and temperature and held at this temperature
ture is reached (when the first crayon practical location must be determined. for some minimum amount of time.
mark melts), the first welding pass can One foot away from the joint is probably Although various code and service
commence. Immediately before the sec- excessive, while a tenth of an inch, or on requirements can dictate a variety of
ond and subsequent passes, the minimum the weld itself, is probably excessively temperatures and hold times, 450°F
and maximum (if specified) interpass conservative. However, one inch from (230°C) is a common post heating tem-
temperature should be checked in the the weld toe seems appropriate. perature to be maintained for 1 hour per
proper location. The lower temperature inch (25 mm) of thickness.
crayon should melt, indicating that the
Postweld Heat Treatment The need for post heating assumes a
temperature of the base metal is greater
potential hydrogen cracking problem
than the melting temperature of the cray- Postweld heat treatment (PWHT),
exists due to a sensitive base metal
on, while the higher temperature crayon defined as any heat treatment after weld-
microstructure, high levels of hydrogen,
should not melt, indicating that the base ing, is often used to improve the proper-
and/or high stresses, and is not necessary
metal temperature is not above the maxi- ties of a weldment. In concept, PWHT
for most applications. It may, however,
mum interpass temperature. can encompass many different potential
be a code requirement. For example,
treatments; however, in steel fabrication,
If the lower temperature crayon does post heating is often required for critical
the two most common procedures used
not melt, additional heat should be repairs, such as those defined under the
are post heating and stress relieving.
applied to the joint until the crayon mark Fracture Control Plan (FCP) for
on the base metal melts. And if the When is PWHT Required? Nonredundant Members of the
upper temperature crayon melts, the AASHTO/AWS D1.5 Bridge Welding
joint should be allowed to slowly cool in The need for PWHT is driven by code Code. The FCP provision is 450 to
the ambient air until the upper tempera- and application requirements, as well as 600°F (230 to 315°C) for “not less than
ture crayon no longer melts, while the the service environment. In general, one hour for each inch (25 mm) of weld
lower temperature crayon does melt. when PWHT is required, the goal is to thickness, or two hours, whichever is
Then the next welding pass can begin. increase the resistance to brittle fracture less.” When it is essential that nothing go
and relaxing residual stresses. Other wrong, post heating can be used as
Where Should Interpass desired results from PWHT may include “insurance” against hydrogen cracking.
Temperature Be Measured? hardness reduction, and material strength However, when the causes of hydrogen
enhancements. cracking are not present, post heating is
There are both codes and industry not necessary, and unjustifiable costs may
standards that specify where the interpass Post Heating result if it is done.
temperature is to be checked. Both the
AWS D1.1-98 Structural Welding Code – Post heating is used to minimize the
Modern Steel Construction / February 2000
Stress Relief cracking, where the component fractures where,
during the heating process, can also
Stress relief heat treatment is used to occur. Thus, the specific application and H = heat input (kJ/in or kJ/mm)
reduce the stresses that remain locked in steel must be considered when determin- E = arc voltage (volts)
a structure as a consequence of manufac- ing the need, the temperature and time I = current (amps)
turing processes. There are many of treatment if applied, and other details S = travel speed (in/min or mm/min)
sources of residual stresses, and those regarding PWHT. This equation is useful for comparing
due to welding are of a magnitude rough-
The filler metal composition is also different welding procedures for a given
ly equal to the yield strength of the base
important. After heat treatment, the welding process. However, heat input is
material. Uniformly heating a structure
properties of the deposited weld can be not necessarily applicable for comparing
to a sufficiently high temperature, but
considerably different than the as-welded different processes (e.g., SMAW and
below the lower transformation tempera-
properties. For example, an E7018 GMAW), unless additional data are avail-
ture range, and then uniformly cooling it,
deposit may have a tensile strength of 75 able such as the heat transfer efficiency
can relax these residual stresses. Carbon
ksi (500 MPa) in the as-welded condi- (Linnert, 1994).
steels are typically held at 1,100 to
1,250°F (600 to 675°C) for 1 hour per tion. However, after stress relieving, it How is Heat Input Measured?
inch (25 mm) of thickness. may have a tensile strength of only 65 ksi
(450 MPa). Therefore, the stress relieved Heat input can not be measured
One commonly overlooked detail is properties of the weld metal, as well as directly. It can, however, be calculated
that after welding, the component must the base metal, should be evaluated. from the measured values of arc voltage,
be allowed to cool to room temperature current and travel speed.
before stress relieving. If the weldment is Electrodes containing chromium and
not allowed to cool, the residual stresses molybdenum, such as E8018-B2 and In determining the arc voltage (E), the
never get “locked” into place. The resid- E9018-B3, are classified according to the voltage should be measured as close to
ual stresses must be established in order AWS A5.5 filler metal specification in the the arc as possible, as opposed to the
to relieve them. stress relieved condition. The E8018-B2 value displayed on the welding machine
classification, for example, has a required voltmeter. Measuring the voltage across
Stress relieving offers several benefits. tensile strength of 80 ksi (550 MPa) min- the arc provides the actual voltage drop
For example, when a component with imum after stress relieving at 1,275°F across the welding arc. The welding
high residual stresses is machined, the (690°C) for 1 hour. In the as-welded machine voltmeter reading is always
material tends to move during the metal condition, however, the tensile strength higher than the arc voltage due to the
removal operation as the stresses are may be as high as 120 ksi (825 MPa). resistance of the welding cables (see
redistributed. After stress relieving, how- Figure 3). The machine voltage, there-
ever, greater dimensional stability is For specific PWHT recommendations, fore, can be used only for approximate
maintained during machining, providing one should consult the filler metal manu- calculations and, in the case of significant
for increased dimensional reliability. facturer and/or the steel producer. voltage drops, may lead to heat input cal-
In addition, the potential for stress culation errors.
corrosion cracking is reduced, and the What is Heat Input? The welding current (I) is measured
metallurgical structure can be improved In arc welding, energy is transferred with either an inductance meter (tong
through stress relieving. The steel from the welding electrode to the base meter) or a shunt with appropriate
becomes softer and more ductile through metal by an electric arc. When the metering equipment. The current is
the precipitation of iron carbide at tem- welder starts the arc, both the base metal never fixed with respect to time, especial-
peratures associated with stress relieving. and the filler metal are melted to create ly on a microsecond level. With SMAW,
Finally, the chances for hydrogen the weld. This melting is possible the current is also a function of the arc
induced cracking (HIC) are reduced, because a sufficient amount of power length, which is dependent on the
although this benefit should not be the (energy transferred per unit time) and welder’s skill. Therefore, the current
only reason for stress relieving. At the energy density is supplied to the elec- used in the heat input calculations
elevated temperatures associated with trode. should be the average value.
stress relieving, hydrogen often will Heat input is a relative measure of the The travel speed (S) is the forward
migrate from the weld metal and the heat energy transferred per unit length of velocity of the arc measured in either
affected zone. However, as discussed weld. It is an important characteristic inches per minute or millimeters per
previously, HIC can be minimized by because, like preheat and interpass tem- minute. Only the forward progress con-
heating at temperatures lower than stress perature, it influences the cooling rate, tributes to the travel speed. If a weaving
relieving temperatures, resulting in lower which may affect the mechanical proper- technique is used, only the forward speed
PWHT costs. ties and metallurgical structure of the counts, not the oscillation rate. For verti-
weld and the HAZ (see Figure 2). Heat cal welding, the upward or downward
Other Considerations
input is typically calculated as the ratio of speed of the arc is used. The travel
When determining whether or not to the power (i.e., voltage x current) to the speed must be in terms of minutes and
implement a postweld heat treatment, the velocity of the heat source (i.e., the arc) not seconds for the dimensions to bal-
alloying system and previous heat treat- as follows: ance in the heat input equation.
ment of the base metal must be consid-
When the travel speed is measured,
ered. The properties of quenched and 60 EI
tempered alloy steels, for instance, can be H= the arc should be established for an
adversely affected by PWHT if the tem- 1000 S amount of time that will produce an
accurate average speed. A continuous
perature exceeds the tempering tempera-
welding time of 30 seconds is suggested.
ture of the base metal. Stress relief
If this is not possible for the production
heat input. This intuitively makes sense,
because as more energy is supplied to the
arc, more filler metal and base metal will
be melted per unit length, resulting in a
larger weld bead. If a welder makes one
weld with a fast travel speed and another
with a slow travel speed, keeping current
and voltage the same for both, then the
weld made at the slower travel speed will
be larger than the faster one. The fol-
lowing equation is an approximation for
the fillet weld leg size based on heat
input (Miller, 1998):
Figure 2. Heat input influences cooling rate. H
ω=
500
where,
ω = fillet weld leg size (in)
H = heat input (kJ/in)
Although the precise relationship
between heat input and fillet weld size
also depends on other variables, includ-
ing the process and polarity, this equa-
tion is a helpful tool, especially in creat-
ing and reviewing welding procedures.
For example, if a minimum fillet weld
size is specified, then the corresponding
minimum heat input can be determined
and controlled.
Cooling Rate is a Function of
Heat Input
The effect of heat input on cooling
rate is similar to that of the preheat tem-
perature. As either the heat input or the
preheat temperature increases, the rate
Figure 3. The arc voltage is always lower than the machine voltage due to the of cooling decreases for a given base
resistance of the welding cables. metal thickness. These two variables
interact with others such as material
thickness, specific heat, density, and ther-
joint (e.g., short welds), a test weld should With SMAW, the resistance of the mal conductivity to influence the cooling
be run on a mock-up joint that will pro- electrode changes as it is melted, which rate. The following proportionality func-
vide a sufficient length to determine the results in a voltage change. The temper- tion shows this relationship between pre-
travel speed. The travel speed accuracy ature of the electrode also increases heat temperature, heat input and cooling
with manual or semi-automatic welding while its length is reduced during weld- rate:
is dependent on the welder. However, ing, both of which influence the overall
with automatic welding, the speed is set resistance. Average values are used in 1
on the motor controlled travel carriage. this case as well. R∝
To H
Transient Values The transient nature of these factors is
usually not considered when calculating where,
For processes in which the voltage heat input, and the averages are adequate R = cooling rate (oF/sec or oC/sec)
and current vary significantly with time, for procedure qualification or simple To = preheat temperature (oF or oC)
such as short-circuiting GMAW, the aver- comparison of welding procedures. H = heat input (kJ/in or kJ/mm)
age values of these variables are used in However, for scientific experimentation
calculating the heat input. For example, of cooling rate and heat input a more The cooling rate is a primary factor
with GMAW-pulsed arc, the current is accurate analysis procedure may be that determines the final metallurgical
pulsed at a specified frequency from a required, including instantaneously mon- structure of the weld and heat affected
minimum value (background current) to itoring the voltage, current and travel zone (HAZ), and is especially important
the maximum value (peak current). The speed to calculate the actual heat input. with heat-treated steels. When welding
average value between the maximum and quenched and tempered steels, for exam-
minimum current and voltage will pro- Weld Size Is Related to Heat Input ple, slow cooling rates (resulting from
vide an approximate heat input value for extremely high heat inputs) can soften
these welding processes. The cross-sectional area of a weld is the material adjacent to the weld, reduc-
generally proportional to the amount of ing the load-carrying capacity of the con-
nection.
Modern Steel Construction / February 2000
Table 1: How material properties are affected by increasing heat input for SMAW required by Table 5.8.” (AWS D1.1-98,
para. C5.14).
Property* Change
Quenched and Tempered Steels
Yield Strength under 30%
When quenched and tempered steels
Tensile Strength under 10% (e.g., A514 and A517) are to be welded,
the heat input, as well as minimum pre-
Percent Elongation over 10% heat and maximum interpass tempera-
Notch Toughness over 10% for 15
under 50% for 50
(AWS D1.1-98, para. 5.7). If high heat
Hardness under 10% input welding is used, the HAZ can be
*SMAW with heat input range of 15 to 110 kJ/in.
significantly weakened due to high tem-
peratures and slower cooling rates.
However, the AWS code requirements do
not universally apply to all quenched and
How Does Heat Input Affect correlation between heat input and tempered steels. For example, with
Mechanical Properties? mechanical properties was established for ASTM A913 Grades 60 or 65, which are
submerged arc welding (SAW) with typi- quenched and self-tempered, the heat
Significantly varying the heat input cal highway bridge fabrication heat input input limitations of AWS D1.1 paragraph
typically will affect the material proper- levels of 50 to 90 kJ/in (Medlock, 1998). 5.7 do not apply (AWS D1.1-98, Table
ties in the weld. The following table In this case, the test results did show 3.1 and 3.2, footnote 9 and 4, respective-
shows how the listed properties change varying properties; however, no discern- ly).
with increasing heat input. An arrow able trends were established.
pointed up designates that the property AWS D1.5 Bridge Welding Code
increases as heat input increases. An Welding Code Requirements
arrow pointed down designates that the The AWS D1.5-96 Bridge Welding
As discussed previously, heat input Code has provisions for heat input in two
property decreases as heat input increas-
can affect the mechanical properties and areas: procedure qualification and frac-
es. Next to the arrow is the approximate
metallurgical structure in the weld and ture critical nonredundant members.
amount that a particular property
HAZ of weldments. The AWS D1.1
changed from the minimum to maximum
Structural Welding Code — Steel controls There are three different methods for
value of heat input tested.
heat input in three areas: (1) qualified qualifying procedures in AWS D1.5: the
Other than notch toughness, all of the Welding Procedure Specifications, (2) Maximum Heat Input Method, the
mechanical properties show a monotonic minimum fillet weld sizes (an indirect Maximum-Minimum Heat Input Method,
relationship to heat input, that is, the method of controlling heat input) and (3) and the Production Procedure Method.
mechanical property only increases or quenched and tempered steels. For the Maximum Heat Input Method,
decreases with increasing heat input. the heat input must be between 60% and
When heat input control is a contract 100% of the value from the Procedure
Notch toughness, however, increases
requirement, and if the procedure used Qualification Record (PQR) used to qual-
slightly and then drops significantly as
in production has a corresponding heat ify the WPS (AWS D1.5-96, para. 5.12.1).
heat input increases. The change in
input that is 10% or greater than that With the Maximum-Minimum Heat
notch toughness is not just tied to the
recorded in the Procedure Qualification Input Method, the heat input must fall
heat input, but is also significantly influ-
Record (PQR), then the qualified WPS between that of the two required qualifi-
enced by the weld bead size. As the bead
must be requalified (AWS D1.1-98, Table cation tests. If the Production Procedure
size increases, which corresponds to a
4.5, item 18). This is primarily due to Method is used, the heat input can only
higher heat input, the notch toughness
concerns regarding the potential alter- deviate from the PQR by the following:
tends to decrease. In multiple-pass
ation of the weld metal and HAZ an increase of up to 10% or a decrease
welds, a portion of the previous weld pass
mechanical properties. not greater than 30% (AWS D1.5, Table
is refined, and the toughness improved,
as the heat from each pass tempers the The code also controls the heat input 5.3, item 17).
weld metal below it. If the beads are by limiting the minimum size of fillet
smaller, more grain refinement occurs, welds (AWS D1.1-98, Table 5.8). Fracture Critical Nonredundant
resulting in better notch toughness, all According to the Commentary, “For non- Members
other factors being even. low-hydrogen processes, the Chapter 12 of AWS D1.5 applies to
minimum size specified is intended to fracture critical nonredundant members
Tests have been conducted with
ensure sufficient heat input to reduce the (FCMs). The minimum preheat tempera-
SMAW electrodes and procedures that
possibility of cracking in either the heat- ture for a FCM is selected based on the
provided heat inputs varying from 15
affected zone or weld metal” (AWS D1.1- heat input, material grade and thickness,
kJ/in (0.6 kJ/mm) to 110 kJ/in (4.3
98, para. C5.14). For multiple-pass fillet and filler metal diffusible hydrogen con-
kJ/mm) (Evans, 1997). This represents a
welds, the Commentary includes the fol- tent (AWS D1.5, Tables 12.3, 12.4
very large heat input range, which
lowing: and12.5). Although the focus in chapter
encompasses most applications of SMAW.
“Should fillet weld sizes greater than 12 of AWS D1.5 is the minimum preheat
If the changes in heat input are rela- temperature, the heat input value is an
the minimum sizes be required for these
tively small, as opposed to those of the equally controlling variable.
thicknesses, then each individual pass of
previous table, then the mechanical
multiple-pass welds must represent the
properties may not be significantly
same heat input per inch of weld length
changed. In another study, no significant
as provided by the minimum fillet size