That is no easy feat or challenge. Possible but takes extensive metallurgy skills as well as experience to do successfully. I cant say for sure, as I only know the principles, not the specifics from my experience in manufacturing, But typically most sprokets and gears have a soft inside and very hard surface of which the depth of this hardening varies greatly depending on a lot of variables.
In aerospace manufacturing we did this to very rigid and consistent stds, and as long as you followed the tech materials you got a consistent result. But in Non aerospace apps,, quality and consistency varied a great deal.
BSA had exemplary mettalurgy to a point in their day because of their other business interests, but by todays standards its night and day difference.
Bt for comparison, for many years on Nortons, and to a lesser degree other makes it was common to machine or grind down 530 sprockets to take a 520 chain, But sensibly now many vendors are offering CS sprockets in a 520 width so you dont hear about it as often, But the initial issue was wildly different results depending on who did the work. Some ended up with a sprocket soft as butter.
My wife deals with this daily as "Logistics lead" in scheduling machine jobs, vendors and suppliers, customer and which machine group she sends out each job to. Some materials its better to make it in a softer material and the post manuf process to specs, Some materials its better to start with a tougher metal, only a very experienced machinist or the engineers who set up the jobs can say, In manufacturing it also depends on material availability and timelines.
Here is what a typical supplier has to say about EN36:
Applications:
As a carburising steel grade it is suitable for roller and ball bearings of extra light section, aeroplane and motor crankshafts requiring hard surfaces for roller paths, connecting rods with case-hardened ends, as well as highly stressed gudgeon pins, gears and certain types of collets
Carburising / Case Hardening:
Case hardening will give a hard wear resisting surface and a tough core strength EN36. Surfaces must be free from dirt and grease, and of low sulphur content. Case hardening can be carried out in salt baths with selected content depending on the type of salt and the depth of penetration required. Other considerations during the heat treatment process include the type of furnace, quenching medium and work piece transfer facilities. Time, temperatures and salt selection will be advised by your heat treatment provider.
Softening:
If it is necessary to soften the EN36 for machining between the carburising and subsequent heat treatment stages, heat to 640-650°C. Soak at this temperature for 1 to 2 hours and cool slowly in the furnace or in air.
Heat Treatment:
Heat treatment temperatures, including rate of heating, cooling and soaking times will vary due to factors such as the shape and size of each EN36 steel component. Other considerations during the heat treatment process include the type of furnace, quenching medium and work piece transfer facilities. Please consult your heat treatment provider for full guidance on heat treatment of EN36 case hardening steel.
