I've heat treated small parts, and small knife blades, made of air hardening steel by simply heating with a torch until a magnet doesn't stick, then maintaining that heat until I got bored/impatient, then let air cool. No rejects to date. Drawing the hardness afterwards is the tricky part - polish to get a clear view of the steel's surface then re-heat with the torch as evenly as possible until it reaches brownish-blue, stop.

Crude? Yes, but for small parts I've had great luck.

Was air hardening steel the best choice for an object like a hammer that needs to withstand repeated shocks?

Now that I can do. I have a set of small torches. As far as material the A1 was what I had without ordering some material. Shipping has gone crazy. I ordered a 1 inch dia x 1/8 key cutter and a 2 inch dia x.020 wide jewelers saw from MSC. the cost was around 70.00 and the shipping was 20.00.



JMH

Addendum: I've left small knife blades made as per my protocols outlined above un-drawn, in other words merely heated and air cooled. In as such hard-as-hell but brittle condition they hold an edge marvelously but don't you dare drop it or try to bend it or pry with it. The trade off is acceptable to me, but admittedly not to the world in general.

A1 tool steel is a high-performance air-hardening tool steel widely used in industrial and manufacturing applications. Its balanced combination of toughness, wear resistance, and dimensional stability makes it ideal for a variety of demanding tasks.

The above from a tool steel web site.

I've personally machined and ground A1 in making trim dies and D2 (a even harder to machine and grind tool steel) for punches.

The material came from MSC. It is indeed A1 material. I made a set of set triggers a sear and a hammer for my Ballard build because all that stuff is unobtainable or at least I couldn't find any original or aftermarket no castings nothing. Going to finish dry fitting everything then do the torch method and hope for the best. If it breaks I'll make it again. I am good at that. I made the trigger plate 5 times before I got the geometry right.


JMH

MSC used to have a heat treat spec sheet with their steel, I went to their site and searched for A1 steel but none came up.  And, didn't find heat treat specs with the other steels anymore either.   

It's always hard to treat correctly, without the right specs.  Heat up, quench, and draw is probably as good as you'll get.