Kenvlach
Yes, there is a chance, as post weld heat treatment is often written into a specification to avoid problems associated with sensitisation. Sometimes this is impractical due to the size of the vessel, but McGuire is right, if you use correct techniques, and specify materials that have very low carbon content or alloyed with a stabilisating substance such as Ti or Nb, then the risk is minimised. If the material is sensitised, and post-processing annealing is not specified,then an incorrect specification for passivation will be the least of your problems.
We are getting of the point here, UKCats original question related to corrosion potential on a machined flange face, due to pick-up or contamination from the cutting tool. Flanges for pressure vessels are normally made from annealed plate or forgings. The face of the flange is rarely heat affected, especially if they have been manufactured using laser or waterjet. Lubricants or coolants are used in the machining process, and many machine shops and others are not aware of the problems that can result from splashing them all over special alloys, and not correctly cleaning prior to further processing or putting the component into service.
Apart from the particle contamination issue mentioned earlier, lubricants contain a series of chemicals that are not only dangerous to humans, but also highly dangerous to stainless steels. Cutting fluid residue can also affect weld quality if the surface is not cleaned prior to weld deposition. Look at your Material Safety Data Sheets, and ask a corrosion engineer for a opinion.
To answer UKCats question, contamination of the flange surface will rarely come from contact with the cutting tool, it is more likely to be introduced by anything else that touches the surface, from marking pens, other processing operations including welding and grit blasting,through to contact with any specified gasket material or sealant. At least that is my experience down-under.
TJA