The purpose of preheat:-
Reduce the risk of hydrogen cracking
Reduce the hardness of the weld heat affected zone
Reduce shrinkage stresses during cooling and improve the distribution
of residual stresses.
If preheat is locally applied it must extend to at least 75mm from the
weld location and be preferably measured on the opposite face to the
one being welded.
Background To Preheating
When hydrogen diffusing from a solidified weld meets a hard
microstructure under a tensile stress a crack is likely!
Hydrogen cracking normally occurs in the heat affected zone where hard
microstructure is to be found, occasionally it can occur in weld
metal.
Hydrogen
This is a very searching gas that can be liberated by oil, grease,
rust etc. and water under the right conditions.
The greatest risk comes from hydrogen generated within the arc from
damp or contaminated welding consumables, mainly fluxes or electrode
coatings.
Contamination on the parent metal can also be a risk unless the heat
from the welding arc can drive it away. Moisture from condensation on
the parent metal will normally be driven off by the heat from the arc
before it can get into the weld pool.
Hydrogen in the atmosphere is unlikely to penetrate the arc envelope
unless welding is carried out in very damp and humid conditions.
A hydrogen crack can take anything from a few hours to 24 hours to
occur. After 24 hours cracking is still possible but less likely,
although there have been some reported cases of cracking at 72 hours.
It is therefore good practice to allow at least 48 hours before
carrying out any NDE.
Hydrogen will eventually disperse from the parent metal, within a few
days at room temperature or a few hours if held at around 200°C.
Hydrogen cracking is only possible at room temperature, this is why it
is also referred to as cold cracking
Parent Metal
A hydrogen crack requires a hard microstructure which is created by a
hardenable material subject to fast cooling from 800°C to 500°C.
Cooling can be slowed down by:-
applying preheat,
maintaining a high interpass temperature,
increasing welding power and reducing travel speed.
The heat sink caused by the parent metal thickness and the number of
available paths the heat can take to escape, also influence cooling
rate. (However once the heat sink reaches a certain size further
increases have a negligible effect on cooling rate.). This is why
when determining preheat the term combined thickness is used, for a
butt weld it is twice the thickness of the parent material and for a T
fillet weld three times the thickness.
The hardening of a carbon manganese steel/low alloy steel is
influenced primarily by carbon content and to a lesser extent other
constituents such as manganese, chrome, silicone etc.
The Carbon Equivalent is a formula used to express the harden-ability
of a particular alloy steel in terms of an equivalent plain carbon
steel. Several such formula exist, the one favoured for low alloy
steel is the IIW formula:
CEIIW = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
Current steel specification do not restrict or limit the Carbon
Equivalent and as most steel specs permit a wide range of composition
it is possible that one batch of steel may require pre-heat and
another may not.
Very low sulphur ( < 0.015%) will increase hardening and special
precautions are required when determining the minimum preheat level.
Additions of niobium also require special consideration.
For welds subject to high restraint more preheat is advisable
(suggest, Increase CE by 0.3 or go down one hydrogen scale).
References.
ASME B31.3 .and etc
EN1011 Part 2 (English version available from British Standards)
This standard is highly recommended as it gives details on this
preheat method and also includes methods covering fine grain and creep
resisting steels. It also includes practical guidance on the avoidance
of other cracking mechanisms. Much of the data contained in this
standard comes from TWI research tempered by practical experience from
industry. (It replaces BS5135)
On 4/17/10, asad azmi <azmiasadayub@gmail.com> wrote:
> Dear All,
>
>
> Please inform me about various methos by which Preheating can be achieved.
>
> Is it allowed to prheat the job using LPG Gas or oxy-acetylene gas.
>
> Is there any code reference/restritiction for methods of preheating.
>
> If possible please provide me literature on various methods of preheating.
>
>
> Thanks & Regards
>
> Asad Azmi
>
> --
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--
K.Muzip ur Rahuman.
QA/QC Coordinator.
BLNG
Adinin Works Engg.
Brunei
Mob;006737170703
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