LTT
LTT
LTT
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The development of compressive residual stresses it also acts like an austenite stabilizer when added
within the weld metal by lowering the Ms temperature, together with Ni as the case in austenitic stainless steels.
thus, leads to beneficial effects on fatigue life [5-7, 9, Since C is not desired for improved toughness as well
10] and distortion [11]. Although compressive stresses as Mn, the MS temperature can be lowered by adding Ni
are developed within weld metal by lowering the M S and Cr to the filler wire with small amounts of other
temperature with the use of low transformation alloying additions, e.g. Mo. The MS temperature can
temperature welding consumables which leads to approximately be calculated using the Equation 1 [13].
benefial effects on fatigue strength and distortion the
fracture toughness may deteriorate. However, the MS = 561 – 474C – 33Mn – 17Cr – 17Ni – 21Mo (oC) (1)
fracture toughness of the recently developed LTT filler
alloy has been found satisfactory [12]. This equation provides reasonably good agreement also
with minor variations of the chemical composition.
2. Martensitic transformation start (MS) Thus, the temperature at which martensitic
transfomation occurs in welding of structural steels can
temperature be controlled by tailoring the chemical composition of
Martensite is of the greatest technological importance in the filler wire, i.e. the low M S temperature [5]. A fully
steels where it can lead to an outstanding combination martensitic structure within the fusion zone can be
of strength (> 3500 MPa) and toughness (> 200 developed if both the MS and MF temperatures are
MPa.m1/2). The temperature at which martensitic above the room temperature, whereas a small amount of
transformation, frequently also called a shear or retained austenite remains provided that the MF
displacive transformation, takes place depends on the temperature lies below the room temperature.
chemical composition of the steel.
Austenite stabilizing elements, such as C, Mn, Ni and
N, retard the transition from austenite to martensite. Out
of all the carbon content strongly affects the martensite
start (MS) temperature. Although Cr is a alpha stabilizer
Table 1.MS temperatures and chemical compositions of some conventional and newly developed LTT filler wires
Mf
Ms temp.,
Main alloying additions in filler wire (wt.%) temp.,
Filler wire (oC) Ref.
(oC)
C Ni Cr Mn Si Mo
Conventional wires
MGS-63B
0.03 -- 0.42 1.09 0.50 0.29 500 -- 5-7
OK
Autrod
0.10 -- -- 1.10 0.70 -- 500 -- 10
12.51
Mn1 -- 0.08 12.3 1.76 -- -- 533 400 16
Cr1 -- 0.10 10.5 0.09 -- -- 537 486 16
LTT wires
10Cr-10Ni 0.025 10.0 10.0 0.70 0.32 0.13 180 -- 5-7
OK
Tubrod
0.01 6.7 12.5 1.80 0.40 2.50 200 -- 10
15.55
-- 0.02 10.14 9.76 0.19 0.39 0.17 205 <0 17
-- 0.076 6.13 6.14 0.55 0.43 0.10 380 130 17
0.04 6.0 8.0 0.70 0.40 -- 281 -- 15
0.04 8.0 10.0 0.70 0.40 -- 213 15
0.04 10.0 10.0 0.70 0.40 -- 179 15
0.04 12.0 10.0 0.70 0.40 -- 145 -- 15
0.04 8.0 8.0 0.70 1.60 -- 247 -- 15
0.04 10.0 10.0 0.70 1.60 -- 179 -- 15
0.03 10.0 10.0 0.70 0.32 -- 180 -- 15
Ni1 -- 3.41 11.0 0.20 -- -- 293 147 16
Ni3 -- 9.99 12.0 0.28 -- -- 170 135 16
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63rd Annual Assembly & International Conference of the International Institute of Welding
11-17 July 2010, Istanbul, Turkey
AWST-10/35
Recently, several studies have been conducted to welded joints due to the expansion of martensite, which
produce low transformation temperature (LTT) filler is hindered by carbon atoms. Classical X-ray
wires in order to decrease the tensile resiadual stresses diffraction (using X-ray radiation) as well as diffraction
developed in the weld zone, even to produce methods using high energy synchroton radiation can be
compressional residual stresses [5-7, 9, 10]. Table 1 used for resial stress measurements of weldments [15].
gives the MS temperatures and chemical compositions
of some conventional and newly developed LTT filler Recent studies have revealed that compressional
wires. residual stresses within the weld region can be obtained
by using LTT filler wires. Figure 4. shows how
However, the chemical composition of the fusion zone compressional residual stresses are produced within the
is a mixture of those of the filler wire and the steel weld zone using LTT filler wires whereas tensile
plates welded. This dilution should be taken into residual stresses are formed if conventional filler wires
account for the actual MS temperature of the weld with higher MS temperatures are employed. This can be
metal. The transformation temperatures can be explained as follows. The martensitic transformation
determined during welding by temperature occurs at the latest stage of the cooling so the thermal
measurement techniques, such as the SS-DTA method contraction effect due to cooling is lower than the
(Sensor Differential Thermal Analysis) [14, 15]. In this volumetric expansion due to the phase transformation if
technique, type C (W-Re) thermocouples should be the MS temperature is sufficiently low. That is the
immersed into the liquid weld metal, instead of type K thermal contraction effect is minimized. If the
(Ni-CrNi) in order to avoid the reaction, and the martensitic transformation ends well above the room
measured cooling curve is then plotted. The deviations temperature it insures the maximum possible formation
in the curve reveal the start and end of solid state of martensite whereas some retained austenite remains
transformations [15]. The MS temperatures determined if the MF temperature is below the room temperature.
during welding of S690Q steel plates using LTT-wires Figure 4.a. shows difference of dilatation in cooling
were quite different than those of the LTT wires used between conventional and LTT filler wires [5, 6, 10,
due to the dilution effect [15]. 18].
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The lower transformation temperature of filler weld
metal results in considerably lower residual stresses as
shown clearly in Figure 5. Moreover, LTT filler wires
may introduce significant longitudinal compressive
residual stresses within the fusion zone [20]. Similarly,
numerous recent works conducted to investigate the
effect of MS temperature of the filler wire on the
residual stresses developed within the weld region of
high strength steels [1, 8, 15, 17, 18, 20, 21] also
indicate that the amount of distortion developed may be
reduced and even compressive residual stresses can be
induced with the use of these newly developed LTT
filler materials.
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63rd Annual Assembly & International Conference of the International Institute of Welding
11-17 July 2010, Istanbul, Turkey
AWST-10/35
Figure 5. Comparison of longitudinal stresses measured in the joints produced using conventional and LTT wires by neutron diffraction [21]
5. Cold cracking
Cold cracks are defects that results from the
contamination of microstructure by hydrogen and
occurs between -50 0C and 150 0C. This type of
cracking can be seen after weeks or even months after
Figure 6. Comparison of fatigue performance of steel welds welding [28]. Such cracking is associated with the
produced with conventional and LTT welding consumables [18]. combined presence of three factors; susceptible
microstructure, diffusible hydrogen and residual stress
The fatigue crack growth properties of joints welded [23]. Result of the diffusion of elemental hydrogen is
with low transformation temperature welding wire are that after migration to dislocations, hydrogen forms
also shown in Figure 7. with the fatigue crack growth pockets and creates pressure to expand the defect and
behavior of conventional welded joints indicated by thus results in a crack. For cracking, conductive
bands. The plot for the joints welded with low microstructure to crack growth is also required. This
transformation temperature welding wire moves to the type of crack is mostly seen as transgranular [29]. The
right side of these bands, indicating a higher fatigue toughness of the alloy will be improved as a result of
performance. The fatigue threshold of joint which is presence of stable dispersed austenite films in low
welded with LTT is about twice that for conventional carbon martensitic stainless steels. Considering
welded joints [5]. cracking phenomena, as retained austenite is present
near a propogating crack, concentrated strain field at the
crack tip induces transformation into martensite. This
transformation will absorb energy and acts like a
toughening mechanism. Volumetric expansion resulted
from the martensitic transformation, would lead to close
the crack and relieve stresses at crack tip. This
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mechanism will absorb the strain energy and limit crack
extension. Also, austenite has higher solubility for
hydrogen than martensite, and thus it will absorb
hydrogen from martensite. This will, in turn, help to
lower hydrogen concentration in martensite [30].
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63rd Annual Assembly & International Conference of the International Institute of Welding
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AWST-10/35
treatment such as PWHT or shot peening. Large scale consumables’, Engineering Failure Analysis, 2009, 16, 2186-
2194
fatigue testing identical to real life applications is still
[11] H.K.D.H. Bhadeshia, ‘Possible effects of stress on steel weld
lacking. The use of LTT consumables with sufficiently microstructures’, Mathematical Modelling of Weld
low MS temperatures also avoids cold-cracking. Phenomena-II, Institute of Materials, 1995, 71-119
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presence of retained austenite) and residual stresses are
U.K., 2007
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[14] B.T. Alexandrov and J.C. Lippold, ‘Methodology for in-situ
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Figure 10. Effect of MS temperature, and thus microstructure, on effects on ferritic weld residual stresses with X-rays and
the dilatation strain and crack ratio obtained in Y-groove weld neutrons’, Metal. Mater. Trans. A, 2008, 39A, 3070-3078
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high strength steel welds’, Science and Technology of
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