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9.13.2 Preheat
Preheating is carried out in the welding of HSLA steel mainly for the prevention of hydrogen induced
cracking. As is generally accepted, the hydrogen cracking is controlled by three factors: (i) hydrogen content,
(ii) residual tensile stress, and (iii) sensitivity of microstructure to the cracking. The preheat reduces the
cooling rate of the weld thermal cycle, and lowers the hydrogen content of the weld metal by increasing the
released hydrogen content during the cooling process of the weld thermal cycle. Since the residual stress
(ii) increases with the base metal strength, a higher preheat temperature is required for the welding of the
high strength steel. It exposes the welder, however, to severe working conditions and raises the cost for the
welding process, which in some cases leads to insufficient work or careless mistakes that may cause serious
trouble or accidents afterwards. In order to avoid these, counter measures to factors (i), (ii), and (iii) listed
above are necessary.
TIG and MIG welding have the advantage of low hydrogen content compared with those processes using
flux, though they are not suitable for high welding heat inputs.
9.13.3 Residual stress
As for the residual stress, it should be noted that the γ → α transformation of the steel weld metal is
accompanied with dilatation sufficient to relieve a significant part of the tensile stress generated during
the cooling process of the weld thermal cycle, provided that it occurs at temperatures close to room
temperatures. The effect of the weld metal with a low transformation temperature on the reduction of
the tensile residual stress was already proved by Shiga et al. Its effect on the hydrogen induced cracking,
however, remains unclear. In particular, the retained austenite, which is usually introduced into steels after
a transformation at lower temperatures, will have important effects; i.e. the retained austenite has higher
solubility and lower diffusivity of hydrogen than ferrite and martensite, and so can act as a trapping site of
hydrogen, which reduces the mobility of hydrogen in the weld. This effect can contribute to the prevention
of hydrogen induced cracking by retarding the hydrogen accumulation at spots where the residual tensile
stress is concentrated. It is, however, also pointed out that the retained austenite may have a detrimental
effect on the hydrogen induced cracking, if it ejects hydrogen when transforming to martensite by stress
assisted transformation or subzero treatment. Thus, better understanding of the effect of retained austenite
on the behaviour of hydrogen is necessary in order to utilise the weld metal with a low transformation
temperature for the steel with high strength.
Since the microstructure consisting of bainite and martensite mentioned above also involves the greater
amount of retained austenite than those of the acicular ferrite, the effect of the retained austenite on the
hydrogen behaviour must be taken into account for the development of the weld metal for the steel of more
than 780 MPa classes in general. Several authors reported that the hydrogen induced cracking occurred in
the weld metal in the arc-welded joint of the HSLA steel of more than 780 MPa classes, while it occurred in
the HAZ in those of the steels of 580 MPa class or less. Only little information, however, is available about
the mechanism or controlling factor of the hydrogen induced cracking in the weld metal, which is a subject
to be investigated further.
It was already reported that the reduction in welding residual stress by the use of the weld metal with
low transformation temperature could improve the weld fatigue strength for the steel of less than
580 MPa classes. We can also expect an improvement of fracture toughness by the retained austenite
present abundantly in the weld metal with low transformation temperature. It is known well that the
retained austenite with a suitable chemical composition undergoes the stress-induced transformation at
the ambient temperature. This transformation is accompanied with transformation-induced plasticity and
relieves the stress concentration at the crack tip, increasing the fracture toughness. Thus one can expect
various beneficial effects of the weld metal with low transformation temperature on the mechanical
properties of the weld. In addition to the adverse effect on the hydrogen induced cracking as suggested
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