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Yes William you can purchase pre-hardened ground steels. I used to buy 4142 pre-hardened precision ground stock which was still low enough Rockwell for normal machining. There are so many different types of metals available now that you can easily become confused trying to figure out which to use when. As David says “It isn’t which steel is better, it is which steel will be best suited to a particular application“. With normal watch and clock parts I normally use high carbon oil hardening steels and temper them to the hardness needed for the parts purpose. I have been experimenting a bit this past year with making parts with different types of metal though and may begin moving over to some more modern metals. S7 is possibly one of them. I have a piece of 1/8” s7 that I’ve wanted to try making some staffs out of due to it’s high impact properties to see how they polish out and hold up. In horology though most parts are not subjected to much shock (other than balance staff pivots) so wear rather than impact is usually my greatest concern. S7 machines fairly well but can be a little rough on HSS tools. If you drop your spindle speed down a little and maybe take an extra pass or so when cutting those threads you should be OK Tom.
You didn’t mention what the threaded part will be used for so you may have good reason for choosing S7. For those that would like to practice some single point threading for the first time I would suggest starting out with a piece of 6061 aluminum until you get a feel for it. Not necessary but is a lot easier on tool bits and cuts like butter when compared to some steels.
I have a couple of pdf files on my other laptop that covers when to use what metals that may be of help. I’ll dig them up a little later today and will put them up here for you.