There is always sensitization, or at least a bonding of C with Cr depending upon how you define sensitization. Even with the L Grades. Stuff just happens on the grain boundary. Its a microstructural superhighway. Its just a question of how much and whether its tolerable or not. With less carbon, as in the L Grades, and/or less time in the sensitization range there will be less bonding of C to Cr and consequently little to cause that specific type of corrosion concern, or be detectable in most tests available.

A few comments in addition to those already made, sensitivity in stainless is developed the more the weld joint is heated and the time that the weld joint is kept at about 600F and above, so yes it starts with the first weld bead and continues and grows as long as the joint is in the "sensitivity temperature range", the low carbon grades are better at narrowing the amount of sensitive zone. But what I read that I have a difference with is you said the welder was traveling real slow, fault #1, the other thing is the no filler metal was used on a butt joint ( fusion welding ), all butt joints will have "some" area at the face to "fill" in and the melted metal will fill the joint and it will be impossible the maintain "full base metal" cross section and cracking in the center of the joint is going to happen, it's all a matter of when, I hate to see a company that thinks they can save a few $$ and don't care enough about their customer or product to do it right. It's a classic example of a company that is in the business of welding and they little about it. But once the product is sold and they got their money, WHO CARES. Welcome to quality control mind set .........................