-
Preheating inWeldingTechnologyA Hot Topic-Detennination of the
I'Right Temperature
DipJ.-lng. Marcus von Buseh,Germanischer lloyd AG
-
34 nonstop 02/2010
Time and time again, welded structures arefound to exhibit
cracks, although the de-velopment mechanisms - and hence alsothe
possible countermeasures - have beenstudied widely and are now weil
understood.A very effective way of preventing cold cracksis to
preheat the weid area to higher tempera-tures in order to delay the
cooling ofthe weldedjoint; this promotes an increased release
ofhydrogen after the welding within a shortertime period than when
no preheating is ap-plied. Ifthe cooling time t8/5 (i.e. the time
tak-en to cool down from 800C to 500C) is longenough, excessive
hardening in the heat-af-fected zone can be avoided. Moreover,
preheat-ing can minimize the residual stress condition.Depending on
the welding conditions, "themechanical and technological
properties, inparticular the hardness and toughness of
theheat-affected zone [... ] can be influenced to agreater or
lesser degree" [1] through preheat-ing. If preheating is dispensed
with entirely oris inadequate in scope, a good way of ensuringthat
the joint wil! be free of cracks is neglected.Although preheating
is an effective means ofreliably preventing cold cracks, it is
frequentlyomitted or not carried out properly for reasonsof cost.
Ultimately, the decision on the necessityand level of preheating
falls within the responsi-bility of the manufacturer.
Today's standards provide the manufac-turer with a number of
methods of determin-ing the "right" preheating temperature with
aview to achieving a balance between techni-ca! necessity and
commercial interests. Theestablished determination procedures
in-clude methods A and B according to EN 1011-2 and the Pcm method
according to AWSDl.l.The following description explains, from a
prae-tical standpoint, how these methods can be ap-plied to
determining the preheating tempera-ture. The temperatures thus
obtained are thencompared for the same input parameters.
Fur-thermore, the possibilities of reducing the re-quired
preheating temperature are also indi-cated. Using shipbuilding as
an example, it isshown how many welding tasks make it neces-sary to
consider the topic of preheating. How-ever, the statements made in
this paper in re-spect of preheating do not apply exclusively
toshipbuilding; they rnay also be transferred to
other sectors.
...------------------------.,
Preheating -Where on the Ship?Ships are structures which are
subject to highlydynamic loading. As a result, it is crucial to
takefull account of crack initiation and crack propaga-tion. In
genera!, weid joints must be free of cracks.For example, this is
prescribed for ship structuresin the Rules for Classification and
Constructionissued by Germanischer Lloyd. This publicationalso
stipulates when and how much preheatingmust take place to prevent
the origination of coldcracks after welding:"The need to preheat
ferritic steels and the
preheating temperature dep end on a number offactors. These
include:
the chemica I composition of the base material(carbon
equivalent) and the weid metal,
the thickness of the workpiece and the typeof weid joint (two-
or three-dimensional heatflow),
the welding process and the welding param-eters (energy input
per unit length of weid),
the shrinkage and transformation stresses, and the diffusible
hydrogen content of the weid
metal." [2]
On the sh ip, there are components for whichone or more of these
parameters play a role.
-
Frequently, special attention must be given tothick-walled
components or matenals with hightendency towards hardness.
HuilThe huil of a seagoing vessel comprises, besidesthe keel,
bottom, outer shell and side structures,also the bow and stern
sections as well as decksand bulkheads. In are as of high dynamic
lead-ing, e.g. the sheer strake or hatch side coamingof a
containership, higher-tensile shipbuildingsteels with the GL
quality grades E36 and E40are used. These are fully killed and
fine-grain-treated steels with plate thicknesses rangingbetween 50
and 100 mmo some of which are
welded in single runs using high-performancemethods sueh as
electro-gas welding.
Stern Tube and Rudder HomThe drive shaft for the propeller runs
throughthe stem tube (see Fig. 2.1), while the rudderhom (Fig. 2.2)
serves to accommodate the Tud-der blade. In general. these
components consistof weldable cast steel with material
thicknessesof up to 400 mrn and are welded to the hull.
Main EngineThe foundation plat es, casing and frame of die-sel
engmes consist of cast steel or, in the case ofthe welded designs
occurring more frequentlynowadays, of norrnal-tensile steels with
platethicknesses of up to 300 mmo
PropellerShip propellers are made of cast copper alloys,e.g.
CUI-CU4, or of non-corrodlng, and usu-ally rnartensitic, cast steel
alloys, e.g. 12CrlNi,13Cr4Ni, 16CrSNi or 19CrllNi.
Steam BoilerHigh-temperature steels are used for construct-ing
steam boilers, e.g. 16Mo3, 13CrMo4-5 or11CrMo9-10 in accordance
with CR 12187.
Cranes and Laad Suspension DevicesFor crane components,
higher-tensile ship-building steels and high-tensile fine-grained
~
FIGURE 2.1. Stern tube. rlGIJRE 2,2. Rudder hom, F1GURE3.
Submarin2 U32 of the German Navy.
nonstep 02/2010 35
-
FIGURE4.Qualitative influences
on the preheatingtemperature.
36 nonstop 02/2010
Preheating temperature isreduced
.'Influencing factor
,.l;
Low alloying element content
Preheating temperature isincreased
Chemical composition of the base material Higher alloying
elementcontent
Low
Thin
High
Low
High
Hydrogen content of the weid metal
Thickness of the workpiece or component
Heat input during welding
Residual stress condition
Ambient or workpiece temperature (heatdissipation)
High
Thick
Low
High
Low
~ structural steels are used, offering yieldstrengths of up to
960 MPa.
Pressure VesselsThe pressure hullof a submarine (Fig. 3)
con-sists of quenched and tempered structuraI steelsmeeting the
German Naval Standard, such asthe grades HY80or HYIOO.
Determining the 11 Right"TemperatureAs already mentioned in the
introduetion. themanufacturer of a welded structure is facedwith
the challenge of producing a componentthat is crack-free - or at
least crack-resistant- at acceptable cost. The necessary
preheat-ing temperature depends on the complexity ofthe component,
the welding process used, themagnitude of the component's residual
stressesand the ambient temperature. Fig. 4 shows thequalitative
influence of various factors on thepreheating temperature
level.
If a quantitative estimation of the preheat-ing temperature is
needed, a range of conceptsare available for diverse applications.
The guide-lines of Germanischer Lloyd as an example forshipbuilding
as wel! as the DIN 18800-7 stand-ard for steel construction in
Germany providethe manufacturer with an according choice ofmethods
of determining the preheating temper-ature. Both rulebooks,
however, give prefereneeto a particular method.
Gl Rules for Classification and Construction:
"The operating temperature to be maintained
(minimum preheating temperature and maxi-mum interpass
temperature) for (hull) structur-al steels may be determined in
accordance withEN 1011-2."
DIN 18800-7:"The required preheat tempera-tures are available in
SEW088."
As a matter of principle, this means the man-ufacturer is
permitted to use competing meth-ods to optimize the preheating
temperature inhis best interests. Verification that a certain
pre-heating temperature will yield the intended out-come (lower
hardness and therefore a reducedcrack susceptibility) can be
provided by meansof e.g. a welding procedure test. The followingare
possible methods of determining the mini-mum preheating
temperature:
DIN EN 1011-2:2001 - Annex C, Method A(derived trom the British
Standard) graphicaldetermination
DIN EN 1011-2:2001 - Annex e, Method B(based on SEW88)
computational determina-tion
AWS D1.1 - Annex XI (American method) com-putationalltabular
determination
JIS B 8285:2003 (Japanese method on the ba-sis of the carbon
equivalent eEN).
Fig. 5 compares the concepts according to EN1011-2 and AWSDl.l.
The Iapariese method willnot be considered in this paper.
The basis of all methods is the determina-tion of a carbon
equivalent. Since other alloy-ing elements besides carbon also
promote coldcracking, carbon equivalents are often usedto estimate
the crack sensitivity. CaIculationof the carbon equivalent is
defined by numer-ous formulae, in which the various alloying
ele-
-
ENo1011~2MeltiodA o' - .'0': ~ ,--
, ~~' ~~ _. "'- J ,~
-
FIGURE 8.Diagram from
EN 1011-2.
FIGURE 9.Maximum alloy
contentsfor CET.
38 nonstop 02/2010
200 ,----'!-r--r--r.....,---.,-r-r--.----.180
--+--+-+--i--l--I-+-+---j
Minimum preheating temperature in oe160 - --I---j I I I I I
I
E 150 I 125 100 I 75 50 20 0.~ 140 -- H-i 1--1~ 120 -i-125(
+---without-~ I. I.~ 100 I .......,..-+--,I~.,...~.--;S I~
801-----j-~-~~~+S/~~c:zs
~ : ~---.-:.:~-::~I----.-j-I- --_ --=-I !O~-~-~~-~-~-~~~
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Heat input in kJ/mm
Ta be used lor carbon equivalent not exceeding
~ done by adding together all the plate thick-nesses of the
affected joint:
tcombined = t1 + t2 + t3For directly opposed twin fillet weids
that aredeposited simultaneously, the combined thick-ness
determined in this way must be halved.After that, the heat input is
calculated as followsfrom the are voltage, the welding current
am-perage and the rate oftravel:
Q = k- U[V] . I [A] . welding time [sec]= -..kLlength of weid
[mm] . 1000 mm
where k is a correctioin factor for the weldingprocess. For
submerged are welding, k is set to1. For MAGand flux-cored are
welding, k = 0.8.In the case of multi-wire welding, the heat in-put
must be calculated as the sum of the re-sults obtained for each
single wire, using theindividual current and voltage parameters.
[1]Depending on the relevant hydrogen scale (e.g.C - see Fig. 7)
and the carbon equivalent (e.g.0.43 - see Fig. 8), the appropriate
diagram ischosen from EN 1011-2.
The necessary preheating temperature is ob-tained from the
chosen diagram by reading off
-max.o.71
\v
\
P48P,'>.
BV, CL,GL, LR,
FIGURE 10.Ascertainingthe hydrogencontent from '.1the packag-
AC/De
ing label(manufac- SSSSS)SSS 350-400 "C, 2h
turer: ELGA).
the preheating line immediately above or to theleft of the
intersection of the lines for heat inputand combined thickness.
EN 1011-2 Method BThis method is applicable to the are weld-ing
of steels belonging to groups 1 to 4 ac-cording to CR ISO 15608.
The basis of thismethod is provided by comprehensive ex-aminations
of the cold cracking behaviourof steels during welding, bath
through spe-cial cold cracking tests and welded joint tests.As for
Method A, the carbon equivalent mustfirst be determined. However, a
different for-mula is used here:
\
CET= C + Mn + Mo + Cr+Cu + Ni10 20 40
which applies to the alloying contents stated inFig. 9.
By analogy to Method A, the hydrogen con-tent of the welding
consumable in ml/ 100 g ofweid metal is also required; this can be
taken di-rectly from the datasheet or by reading off thestandard
designation on the packaging (see Fig.10).
-
The heat input is determined exactly as forMethod A. The
calculation of the prehearingtemperature eau then be carried out
according10 the following formula:
TrC] ;;:;697 eET+160
.tanh(3~)+62HDo,35+(53CET-32)O-328Thisrelationship applies to
steels with a yield pointof up to 1,000 MPa andeET:::0.2% to
0.5%.plate thickness d = 10 mm to 90 mm,hydrogen content HD :::
1m/11009 to 20 ml/100 g,heat input Q = 0.5 kJ/mmto 4.0 kJ/m.
Procedure According to AWS D1.1 -Pem MethadHere toa, a carbon
equivalent is calculated asthe input quantity. However, the formula
forPcm involves more elements than for CE or eET.For example, it
also considers the element bo-ron, which is a fine-gram agentthat
ensures nu-clei at high temperatures but at the same timehas a
powerful hardening effect. Baron is given 5times the weighting of
carbon in the calculation.The Pcm method is especiaJly suitable for
shortcooling times and for root welding appJications.
Pan=C+~+Mn+ CU+M+ Cr+Mo+V +5.830 20 20 60 20 15 10
The susceptibility index describing the sensi-tivity towards
hydrogen-Induced cracks arisingfrom the hydrogen content of the
welding con-sumable ean then be determined (see Fig. 11).
Finally, and here this approach differs fromthe methods
described above, the preheatingtemperature is determined as a
function of the
Susceptibility Index _ .Hl = 5 mll100 9 weid metalH2 = 10
ml/1009 weid metalH3 = 30 mll100 9 weid metal(aU consumablesnot
indudedin Hl or H2)
A
B
C
residu al stress level. A distinction is made be-tween three
levels of the stress condition:
11 low - simple fillet and butt weids with suffi-dent
possibility of shrinkage
11 medium - weids with limited possibility ofshrinkage that are
already connected to largercomponents
11 high - walds without any possibility of shrink-age (e.g. very
thck plates or rep air weids).
lf the plate thickness (for the thickest plate ofthe joint) and
the hydrogen seale are known, theprehearing temperature can be
taken from thetable in Fig. 12.
The heat introduced by the welding process isnot considered by
the Pcm metlied.
Comparison of the MethodsAn example (Fig. 13) will be used to
show whatresuJts are obtained by the three methods forthe sarne
input quantities. A higher-tensileshipbuildtng steel of the grade
GL-E36 (similarto S355 NL according to DIN EN 10025-3) witha
thickness of 50 mm is chosen as the material.
Various specimens are to be butt-welded withdifferent heat
inputs (l or 2.5 kJ/mm) and dif-ferentwelding consumables (hydrogen
content:5 or 10 mi/IOO g weld rnetal). The Iadle analy-sis yields
the corresponding carbon equivalents(Fig. 14). By considering the
plate thicknesses(Fig. 15), the prehearing ternperatures are
de-termined according to the different methods(Fig. 16).
The benefit of reducing the available hydro-gen is clearly
recognizable for both of the meth-ods according to EN 1011-2. The
preheating tem-perature is reduced considerably. The same ef-fect
could also he expected ofthe Pcm meth- ~
, '-
Pem < 0.28 Prm < 0.33 Pcm,< 0.38
B
D
E
E
F
oE
f
cco
FIGURE 11. Tabletor determiningJ the index tor
ti: sU5ceptibility to~.~, hydrogen-induced-- cracks according
to
AW D1.1.
nonstop02l261G39
-
FIGURE 12.lablefor determiningthe minimum
preheating tem-perature according
toAWS Dl.l.
FIGURE 13.Ladleanalysis of ship-
building steel gradeGL-E36.
FIGURE 14.Comparison of thecarbon equivalents.
40 non stop 02/2010
Restraint Ilevel
low
-
,. '. ft:Heat EN 1011-2input Method A1.0 kJ/ 500( 129(
5 ml/ mm
100 9 2.5 kJ/ No 106( 1500(mm preheating
1.0 kJ/ 125( 159( 1500(10 ml/ mm
100 9 2.5 kJ/ Nopreheating
136( 1500(mm
ture exhibits a linear dependency on the carboncontent of the
base material (see Fig. 17).A de-crease in the CETby 0.01 % reduces
the requiredtemperature by as much as 7.5K!
Through the steel order, the manufacturertherefore can exert the
largest influence on thepreheating temperature that is needed
duringproduction. By changing the material procure-ment from
normalized plate to thermo-rne-chanically rolled steel grades, the
CETcan easi-ly be reduced by 0.05% and thus the preheat-ing
temperature by approx. 40 oe. Iudging byFig. 16,Methad Aaccording
to EN 1011-2mustbe preferred for the ehosen example from
theviewpoint of the welding contractor, since it de-mands the
lowest preheating temperature. De-pending on the conditions at
hand, it mayalsobe possible to reduce the temperature further
byapplying the measures deseribed above.
SummaryPreheating is an effeetive way of preventingthe
occurrenee of eold cracks. Owing to their
250
200
:,; 150.sEi 100
,.!'"
50
00.15 0.55
FIGURE 17. Preheating temperature as a function of thecarbon
equivalent according to EN 1011-2.
FIGURE 16.- Comparison of the
preheating temperatures.
hardenabIe material or large dimensions, manyeomponents suffer
the risk of cold craeking.This was demonstrated by means of
examplesfrom the shipbuilding industry. A number ofstandardized
methods with different approach-es are available to determine the
preheatingtemperature. In methods A and B accordingto EN 1011-2,
the heat input during welding istaken into account, whereas this is
neglected inthe Pcm method aecording to AWSDl.l. Herethe estimation
of the preheating temperatureis conducted in relation to the
residual stresscondition. The various methods may be usedin
competition with each other. The results ob-tained can then be
verified before the start ofproduetion, e.g. with the aid of
welding proce-dure tests. For economie reasons, the goal willbe to
keep the preheating temperature as lowas possible. Through the
welding process (heatinput), the welding consumable (hydrogen
sup-ply) and the chemical composition of the ma-terial (carbon
equivalent), welding productioncontractors have several effective
ways of influ-encing the preheating temperature.
Literature[11 DIN EN 1011-2
[2] Germanischer Lloyd, Rules for Classification and
Construc-tion, 11- Materials and Welding, 3 - Welding (Edition
1999)
[3] Schmidt, Zwtz, Br, Schulze - Execution of Steel
Structur-
es I Notes on DIN 18800-7 [in German]
[4] Technical Report 1967, IIW Doe. IX-535-67
[5] Uwer, D. und H. Hhne - Characterization of the cold
cracking behaviour of steels during weid ing. SchweiBen
und Schneiden 43 (1991), No. 4, p. 195-199 [in German]
and IIW Doe. IX-1630-91
[6]lto, Y. und K. Bessyo - Weldability Formula of High
Strength Steels, Related to Heat-Affected Zone Crack ing.
Sumitomo Search, 1 (1969), H. 5, p. 59-70, and IIW Doe.
IX-631-69 (1969), 1-18
Secondary Literature[7] Jahrbuch SchweiBtechnik 1988 - 4.8
Welding Production -
Preheating [in German]
[8] TWI website - Job Knowledge for Weiders - Defects I
Hy-drogen Cracks in Steel
[9] Kobelco Welding Today - Cold Cracks: Causes and Cures[10]
linde Sonderdruck 41/01 - Preheating by Flame in the
Welding Workshop [in German]
nonstop 0212010 41
