I have noticed in D1.1-2000 there are not references to preheat in plasma cutting, yet we are referenced by engineering to preheat any material above 3/4" by using the Section 3 requirements of preheat for welding.

I have not worked anywhere that there was any preheat requirements for plasma cutting before now. Does anyone have any information to share on this?

You can look in the AISC Steel Construction Manual for some direction with regards to the need to preheat thick sections prior to thermal cutting. The Thirteenth Edition addresses the subject in Section M-2, page 145.

Your engineering department may be taking a conservative approach based on past experience. Some undesirable microstructures can result along the cut edge as a result of rapid cooling. The problem is exacerbated as the carbon equivalency increases and the thickness of the base metal increases. Members that will be hot dipped galvanized are a concern because of cracking issues related to the carbon equivalence and the potential for hard microstructures that can result when high strength low alloy steels are thermally cut.

Best regards - Al

Thanks for the info