I am referring to steam valve used on steam pipelines to regulate the flow of steam to whatever use it is being put; throttling steam flow tends to cut the seats and/or the discs, so occasionally, they need to be reseated, the disc can be replaced if it is hard fiber or if stellite, it can be lapped or ground, the seat, however needs to be lapped in place, if stellite, if it is a bronze seat there are similar cutters for both angled and flat seats.