Welding Heat Treatment

Don't weld it before you all know.

Most likely yes, but that depends on the established WPS, PQ. Establish them, including WQ before you touch the main job. This you must be well aware of.

The real problem that you have when welding high carbon steels is the formation of very hard crystalline structures called Martensite in the vicinity of the weld as it cools.

Preheating slows down the cooling as the weld progresses, and so reduces the temperature gradient and should be carried out. Post heating furthers that process if the Carbon content is particularly high (over 0.45%). This minimises the formation of the Martensite and improves ductility of the weld.

If you fail to carry this procedure out, there is every chance that the weld will fail especially in heavy section steels as they will draw the heat from the weld much faster than thinner sections.

If you know the carbon content, then you can generally know what pre and post heat is required - ie. - up to about 0.3% Carbon, preheating is optional but preferable - 0.3 to 0.45% preheat to 200°C, - above 0.45% increase preheat to max 400°C

Post heat should be applied before the weld cools to around 350°C.