Welder here. Aluminum oxide inhalation is correlated with an increased risk for alzheimers, not cancer. Hexavalent chromium is a carcinogen, and that comes from heating up stainless steels. So you are actively replacing a relatively non toxic oxide with the potential for an actually toxic carcinogenic gaseous metal, assuming your pocket knife is some sort of stainless steel (statistically very likely).
As a welder, much of my work straddles the line between art and mass production. I’ve made many, many beautiful welds that will never been seen by another person for at least the next 20 years, if ever. Some of the best that come to mind are stainless steel welds on industrial equipment that get buried under paint or insulation. I spent 3 years welding parts for US battleships and Navy cruisers as well as the occasional weird airforce part. Most of those welds will never been seen by living people after leaving my old shop.