Either type of thermometer works fine as long as it is calibrated correctly. The only advantage to the infrared type is that it is non-contact and you can take accurate temperatures at a distance.
I have a couple 5 gallon buckets of various spring steel I have collected over the years. I have saved flat springs from everything from old clocks to heavy cable retrievers to truck leaf springs. I also did my electrical apprenticeship in a steel mill, and would often watch for when the cold rolling mills were running different grades of steel that might come in handy in the future. I would shear strips of different grades and thicknesses, and tag it with information from the Met-Lab analysis so I'd know what I had when I went to use it.
A few years ago, I needed to make a top lever spring for a Lefever, so I found some spring stock of suitable thickness in my little inventory. I noted that it was very hard right off the mill, so I decided to anneal it before bending and shaping my springs. I cut about a dozen short strips, as I was going to make several spares since broken top lever springs are somewhat common in Lefevers, and put them inside a short length of 2" pipe threaded and capped on both ends with a small vent hole drilled in one cap. I wrapped the spring stock in paper so it would burn off the oxygen to prevent scale formation, and put it on the log grate of my fireplace and cooked it dull red for several hours, then buried it in the hot coals before going to bed so it could slowly cool. I expected it to be dead soft the next day, but it still snapped like a pretzel when I tried to bend it. This is why I told Ken that it doesn't always go exactly by the book when you use different steels. You know you have a good spring when you compress it fully and it doesn't either snap or take a set after hardening and tempering. If it works, then you make notes. Life would be simpler if you could always use the exact same steel, but not as interesting.
