The purpose of preheat:-
1. Reduce the risk of hydrogen cracking
2. Reduce the hardness of the weld heat affected zone
3. 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.
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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.
HydrogenThis 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 MetalA 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:-
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 (
The purpose of preheat:-
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