Preheat of base metal before welding

[alvaro]

Could someone explain me why in some cases preheat of base metal is considered essential before welding and what preheat temperature is required?

[alesio]

The preheat reduces the cooling rate in the weldment (weld, HAZ and nearby base metal) and thus not only a more ductile metallurgical structure is produced (the more ductile metallurgical structure, the greater resistant to cracking) but also the lower cooling rate provides the hydrogen the opportunity to escape preventing possible hydrogen cracking. Moreover the preheat reduces the shrinkage stresses in the weldment as well as it ''brings'' some steels above a critical temperature at which brittle fracture could be happen during fabrication.

The necessity of preheat depends on chemical composition and thickness of base metal, the ambient temperature and the hydrogen content of filler metal. The preheat temperature specified by the code under which the welding is performed, i.e. you could find required preheat temperature on AWS D1.1, Table 3.2 or on ASME B31.3, Table 330.1.1 or on ASME VIII-1, Appendix R, etc.

[Logosweld]

Preheating involves heating the base metal, either in its entirety or just the region surrounding the joint, to a specific desired temperature, called the preheat temperature, prior to welding. Heating may be continued during the welding process, but frequently the heat from welding is sufficient to maintain the desired temperature without a continuation of the external heat source.

There are four primary reasons to utilize preheat:

(1) it lowers the cooling rate in the weld metal and base metal, producing a more ductile metallurgical structure with greater resistant to cracking
(2) the slower cooling rate provides an opportunity for any hydrogen that may be present to diffuse out harmlessly without causing cracking
(3) it reduces the shrinkage stresses in the weld and adjacent base metal, which is especially important in highly restrained joints and
(4) it raises some steels above the temperature at which brittle fracture would occur in fabrication. Additionally, preheat can be used to help ensure specific mechanical properties, such as notch toughness.

[Logosweld]

preheating might be effective as there might be variation in temperature of the welding metal which would be good for welding process or it would ease the process

[9074950730]

How to calculate preheat and interpass temperature of material like EN-10025 S235 J2G3?