I think you are asking how to attach a Pt wire to carbon steel for a short time at 800 C /1472 F. I expect a plain carbon steel weld, like MIG, would do it. It may not melt the Pt during the few seconds of welding ; the Pt would almost be mechanically attached by the weld deposit . Keeping in mind that Pt thermocouples are routinely used to measure molten steel temperatures above 3200 F. Because your steel is "cold rolled" , not heat-treated , strength properties of the weld HAZ will not change significantly; trading cold rolled ( strength ?) for a fast normalize. PS - "solidus" has nothing to do with tempering steel.