7/24/2019 Preheating Involves Heating the Base Metal

1/3

Preheating involves heating the base metal, either in its entirety or just the region surroundingthe joint, to a specific desired temperature, called the preheat temperature, prior to welding.Heating may be continued during the welding process, but frequently the heat from welding issufficient to maintain the desired temperature without a continuation of the external heat source.The interpass temperature, defined as the base metal temperature between the first and lastwelding passes, cannot fall below the preheat temperature. Interpass temperature will not be

discussed further here. Preheating can produce many beneficial effects however, without awor!ing !nowledge of the fundamentals involved, one ris!s wasting money, or even worse,degrading the integrity of the weldment.

Why Preheat?There are four primary reasons to utili"e preheat# $%& it lowers the cooling rate in the weld metaland base metal, producing a more ductile metallurgical structure with greater resistant tocrac!ing $'& the slower cooling rate provides an opportunity for any hydrogen that may bepresent to diffuse out harmlessly without causing crac!ing $(& it reduces the shrin!age stressesin the weld and adjacent base metal, which is especially important in highly restrained joints and$)& it raises some steels above the temperature at which brittle fracture would occur infabrication. *dditionally, preheat can be used to help ensure specific mechanical properties,

such as notch toughness.

When Should Preheat be Used?In determining whether or not to preheat, the following array of factors should be considered#code requirements, section thic!ness, base metal chemistry, restraint, ambient temperature,filler metal hydrogen content and previous crac!ing problems. If a welding code must befollowed, then the code generally will specify the minimum preheat temperature for a given basemetal, welding process and section thic!ness. This minimum value must be attained regardlessof the restraint or variation in base metal chemistry however, the minimum value may beincreased if necessary. *n example is illustrated in the next section.

+hen there are no codes governing the welding, one must determine whether preheat is

required, and if so, what preheat temperature will be appropriate. In general, preheat usually isnot required on low carbon steels less than % in, $' mm& thic!. However, as the chemistry,diffusible hydrogen level of the weld metal, restraint or section thic!ness increases, the demandfor preheat also increases. There are several methods to determine the required preheattemperature for a given base metal and section thic!ness that will be discussed in the nextsection.

What Preheat Temperature is Required?+elding codes generally specify minimum values for the preheat temperature, which may ormay not be adequate to prohibit crac!ing in every application. -or example, if a beamtocolumnconnection is to be fabricated with a lowhydrogen electrode made of */T0 *1'2r3 and *(4

jumbo sections $thic!ness ranging from ) to in.&, then a minimum prequalified preheat of''5- $%3156& is required $*+/ 7%.%84, Table (.'&. However, for ma!ing butt splices in jumbosections, it is advisable to increase the preheat temperate beyond the minimum prequalifiedlevel to that required by *I/6 for ma!ing butt splices in jumbo sections, namely (35- $%156&$*I/6 9:-7 ;'.

7/24/2019 Preheating Involves Heating the Base Metal

2/3

+hen no welding codes are specified, and the need for preheat has been established, howdoes one determine an appropriate preheat temperature= *s a basis for discussion, consider

*+/ 7%.%84, *nnex >I# ?2uideline on *lternative 0ethods for 7etermining Preheat@@ whichpresents two procedures for establishing a preheat temperature developed primarily fromlaboratory crac!ing tests. These techniques are beneficial when the ris! of crac!ing is increaseddue to composition, restraint, hydrogen level or lower welding heat input.

The two methods outlined in *nnex >I of *+/ 7%.%84 are# $%& heat affected "one $H*A&hardness control and $'& hydrogen control. The H*A hardness control method, which isrestricted to fillet welds, is based on the assumption that crac!ing will not occur if the hardnessof the H*A is !ept below some critical value. This is achieved by controlling the cooling rate.The critical cooling rate for a given hardness can be related to the carbon equivalent of thesteel, which is defined as#

6B C 6 D $$0n D /i&E4& D $$6r D 0o D F&E& D $$Gi D 6u&E%&

-rom the critical cooling rate, a minimum preheat temperature can then be calculated.$lodgett@s paper entitled ?6alculating 6ooling :ates by 6omputer Programming@@ outlines acalculation procedure based on cooling rate, heat input, plate thic!ness, temperature at whichcooling rate is critical, preheat temperature, thermal conductivity and specific heat.& It should bepointed out, however, that ?although the method can be used to determine a preheat level, itsmain value is in determining the minimum heat input $and hence minimum weld si"e& thatprevents excessive hardening@@ $*nnex >I, paragraph (.),*+/ 7%.%84.&

The hydrogen control method is based on the assumption that crac!ing will not occur if theamount of hydrogen remaining in the joint after it has cooled down to about %'35- $356& doesnot exceed a critical value dependent on the composition of the steel and the restraint. Thisprocedure is extremely useful for high strength, lowalloy steels that have high hardenability.However, the calculated preheat may be too conservative for carbon steels.

The three basic steps of the hydrogen control method are# $%& 6alculate a compositionparameter similar to the carbon equivalent $'& 6alculate a susceptibility index as a function ofthe composition parameter and the filler metal diffusible hydrogen content and $(& 7eterminethe minimum preheat temperature from the restraint level, material thic!ness, and susceptibilityindex.

How is Preheat Applied?The material thic!ness, si"e of the weldment and available heating equipment should beconsidered when choosing a method for applying preheat. -or example, small productionassemblies may be heated most effectively in a furnace. However, large structural componentsoften require ban!s of heating torches, electrical strip heaters, or induction or radiant heaters.

* high level of accuracy generally is not required for preheating carbon steels. *lthough it isimportant that the wor! be heated to a minimum temperate, it is acceptable to exceed thattemperature by approximately %335- $)356&. However, this is not the case for quenched andtempered $JT& steels, since welding on overheated JT steels may be detrimental in the heataffected "one. Therefore, JT steels require that maximum and minimum preheat temperaturesbe established and closely followed.

7/24/2019 Preheating Involves Heating the Base Metal

3/3

+hen heating the joint to be welded, the *+/ 7%.% code requires that the minimum preheattemperature be established at a distance that is at least equal to the thic!ness of the thic!estmember, but not less than ( in. $1 mm& in all directions from the point of welding. To ensurethat the full material volume surrounding the joint is heated, it is recommended practice to heatthe side opposite of that which is to be welded and to measure the surface temperatureadjacent to the joint. -inally, the steel temperature should be chec!ed to verify that the minimum

preheat temperature has been established just prior to initiating the arc for each pass.

SummaryPreheat can prevent crac!ing andEor ensure specific mechanical properties such as notchtoughness.

Preheat must be used whenever applicable codes so specify when no codes apply to a givensituation, the welding engineer must determine whether or not preheat is needed, and whattemperature will be required for a given base metal and section thic!ness.

*nnex >I of *+/ 7%.%84 provides guidelines for alternative methods of determining properamounts of preheat# the H*A hardness control method, or the hydrogen control method.

Preheat may be applied in a furnace, or by using heating torches, electrical strip heaters, orinduction or radiant heaters. 6arbon steels do not require precise temperature accuracy, butinduction or radium heaters, maximum and minimum preheat temperatures must be followedclosely for quenched and tempered steels.

Bibliography*G/IE*+/ 7%.%84 /tructural +elding 6ode# /teel. The *merican +elding /ociety, %884.

ailey, G. +eldability of -erritic /teels. The +elding Institute, %88.

ailey, G. et al, +elding /teels +ithout Hydrogen 6rac!ing. The +elding Institute, %81(.

lodgett, 3. ?6alculating 6ooling :ates by 6omputer Programming,? +elding ;ournal. 0arch%8