Fundamentals of Preheat by bjb17276

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									                Key Concepts in Welding Engineering
                by R. Scott Funderburk




Fundamentals of Preheat
Preheating involves heating the base
metal, either in its entirety or just the
region surrounding the joint, to a spe-
cific desired temperature, called the
preheat temperature, prior to welding.
Heating may be continued during the
welding process, but frequently the
heat from welding is sufficient to main-
tain the desired temperature without a
continuation of the external heat
source. The interpass temperature,
defined as the base metal tempera-
ture at the time when welding is to be
performed between the first and last
welding passes, cannot be permitted
to fall below the preheat temperature.
Interpass temperature will not be dis-
cussed further here; however, it will be    When Should Preheat                       restraint or section thickness increases,
                                                                                      the need for preheat also increases.
the subject of a future column.             Be Used?
Preheating can produce many benefi-
cial effects; however, without a working    In determining whether or not to pre-     What Preheat
                                            heat, the following should be consid-
knowledge of the fundamentals
                                            ered: code requirements, section          Temperature Is
involved, one risks wasting money, or
even worse, degrading the integrity of      thickness, base metal chemistry,          Required?
the weldment.                               restraint, ambient temperature, filler    Welding codes generally specify mini-
                                            metal hydrogen content and previous       mum values for the preheat tempera-
                                            cracking problems. If a welding code
Why Preheat?                                must be followed, then the code gen-
                                                                                      ture, which may or may not be
                                                                                      adequate to prohibit cracking in every
There are four primary reasons to uti-      erally will specify the minimum preheat   application. For example, if a beam-
lize preheat: (1) it slows the cooling      temperature for a given base metal,       to-column connection made of ASTM
rate in the weld metal and base metal,      welding process and section thick-        A572-Gr50 jumbo sections (thickness-
producing a more ductile metallurgical      ness. This minimum value must be          es ranging from 4 to 5 in [100-125
structure with greater resistance to        attained regardless of the restraint or   mm]) is to be fabricated with a low-
cracking; (2) the slower cooling rate       variation in base metal chemistry;        hydrogen electrode, then a minimum
provides an opportunity for hydrogen        however, the minimum value may be         prequalified preheat of 225°F (107°C)
that may be present to diffuse out          increased if necessary.                   is required (AWS D1.1-96, Table 3.2).
harmlessly, reducing the potential for                                                However, for making butt splices in
cracking; (3) it reduces the shrinkage      When there are no codes governing         jumbo sections, it is advisable to
stresses in the weld and adjacent           the welding, one must determine           increase the preheat temperature
base metal, which is especially impor-      whether preheat is required, and if so,   beyond the minimum prequalified level
tant in highly restrained joints; and (4)   what preheat temperature will be          to that required by AISC for making
it raises some steels above the tem-        appropriate. In general, preheat usu-     butt splices in jumbo sections, namely
perature at which brittle fracture would    ally is not required on low carbon        350°F (175°C) (AISC LRFD J2.8).
occur in fabrication. Additionally, pre-    steels less than 1 in (25 mm) thick.      This conservative recommendation
heat can be used to help ensure spe-        However, as the chemistry, diffusible     acknowledges that the minimum pre-
cific mechanical properties, such as        hydrogen level of the weld metal,         heat requirements prescribed by AWS
weld metal notch toughness.

                                         Welding Innovation Vol. XIV, No. 2, 1997
D1.1 may not be adequate for these            The three basic steps of the hydrogen     full material volume surrounding the
highly restrained connections.                control method are: (1) Calculate a       joint is heated, it is recommended
                                              composition parameter similar to the      practice to heat the side opposite of
When no welding code is specified,            carbon equivalent; (2) Calculate a sus-   that which is to be welded and to mea-
and the need for preheat has been             ceptibility index as a function of the    sure the surface temperature adjacent
established, how does one determine           composition parameter and the filler      to the joint. Finally, the interpass tem-
an appropriate preheat temperature?           metal diffusible hydrogen content; and    perature should be checked to verify
Consider AWS D1.1-96, Annex XI:               (3) Determine the minimum preheat         that the minimum preheat temperature
“Guideline on Alternative Methods for         temperature from the restraint level,     has been maintained just prior to initi-
Determining Preheat” which presents           material thickness, and susceptibility    ating the arc for each pass.
two procedures for establishing a pre-        index.
heat temperature developed primarily                                                    Summary
from laboratory cracking tests. These         How Is Preheat                            • Preheat can minimize cracking
techniques are beneficial when the
risk of cracking is increased due to the      Applied?                                    and/or ensure specific mechanical
chemical composition, a greater               The material thickness, size of the         properties such as notch toughness.
degree of restraint, higher levels of         weldment and available heating equip-     • Preheat must be used whenever
hydrogen or lower welding heat input.         ment should be considered when              applicable codes so specify; when
                                              choosing a method for applying pre-         no codes apply to a given situation,
The two methods outlined in Annex XI          heat. For example, small production         the welding engineer must deter-
of AWS D1.1-96 are: (1) heat affected         assemblies may be heated most effec-        mine whether or not preheat is
zone (HAZ) hardness control and (2)                                                       needed, and what temperature will
hydrogen control. The HAZ hardness                                                        be required for a given base metal
control method, which is restricted to         Preheat must be used                       and section thickness.
fillet welds, is based on the assump-           whenever applicable                     • Annex XI of AWS D1.1-96 provides
                                                                                          guidelines for alternative methods of
tion that cracking will not occur if the
hardness of the HAZ is kept below
                                                 codes so specify...                      determining proper amounts of pre-
some critical value. This is achieved                                                     heat: the HAZ hardness control
by controlling the cooling rate. The          tively in a furnace. However, large         method, or the hydrogen control
critical cooling rate for a given hard-       structural components often require         method.
ness can be related to the carbon             banks of heating torches, electrical      • Preheat may be applied in a fur-
equivalent of the steel, which is             strip heaters, or induction or radiant      nace, or by using heating torches,
defined as:                                   heaters.                                    electrical strip heaters, or induction
                                                                                          or radiant heaters. Carbon steels do
          (Mn + Si) (Cr + Mo + V) (Ni + Cu)
CE=C +             +             +            Preheating carbon steel to a precise        not require precise temperature
             6            5           15
                                              temperature generally is not required.      accuracy, but maximum and mini-
From the critical cooling rate, a mini-       Although it is important that the work      mum preheat temperatures must be
mum preheat temperature can then be           be heated to a minimum temperature,         followed closely for quenched and
calculated. AWS D1.1-96 states that           it usually is acceptable to exceed that     tempered steels.
“Although the method can be used to           temperature by approximately 100°F
determine a preheat level, its main           (40°C). However, this is not the case
value is in determining the minimum           for some quenched and tempered            For Further Reading...
heat input (and hence minimum weld            (Q&T) steels such as A514 or A517,        ANSI/AWS D1.1-96 Structural Welding Code: Steel.
size) that prevents excessive harden-         since welding on overheated Q&T              The American Welding Society, 1996.
                                                                                        Bailey, N. Weldability of Ferritic Steels. The Welding
ing” (Annex XI, paragraph 3.4).               steels may be detrimental in the heat        Institute, 1995.
                                              affected zone. Therefore, Q&T steels      Bailey, N. et al. Welding Steels Without Hydrogen
                                                                                           Cracking. The Welding Institute, 1973.
The hydrogen control method is based          require that maximum and minimum          Graville, B.A. The Principles of Cold Cracking Control
on the assumption that cracking will not      preheat temperatures be established          in Welds. Dominion Bridge Company, Ltd., 1975.
                                                                                        Stout, R.D. and Doty, W.D., Weldability of Steels.
occur if the amount of hydrogen               and closely followed.                        Welding Research Council, 1971.
remaining in the joint after it has cooled                                              The Procedure Handbook of Arc Welding. The James
down to about 120°F (50°C) does not           When heating the joint to be welded,         F. Lincoln Arc Welding Foundation, 1994.

exceed a critical value dependent on          the AWS D1.1 code requires that the
the composition of the steel and the          minimum preheat temperature be
restraint. This procedure is extremely        established at a distance that is at
useful for high strength, low-alloy steels    least equal to the thickness of the
that have high hardenability. However,        thickest member, but not less than
the calculated preheat may be some-           3 in (75 mm) in all directions from the
what conservative for carbon steels.          point of welding. To ensure that the

                                          Welding Innovation Vol. XIV, No. 2, 1997

								
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