As JHCC said John Deere advertises their blades as spring steel. They also advertise they are heat treated to 40-45 Rockwell. https://t.jdparts.deere.com/partsmkt/document/english/pmac/4382_fb_Lawn_Mower_Blades.htm

I don't know their specific heat treat process to get to that hardness level, but from the data sheets I've looked at it would be something like a 900 degree F temper to bring the steel back down to 40-45 after hardening. Unless you have a good heat treat kiln or are good at judging temp by color with a torch (I use a toaster oven...), I'd say just sharpen them trying not to overheat them too bad. Given where they are already tempered at, they have plenty of wiggle on temp. I've sharpened John Deere blades multiple times. It's almost frustrating that they take resharpening as well as they do because I like to use the worn out ones for knives  

4 hours ago, Marc1 said:

I see, just looked it up, it's a pull behind slasher. Yes, those blades sure take a beating. Are the blades expensive? 

Pretty much anything trees (1-2 inches diameter) weeds mostly.

if you are using it as a slasher, and if blades are truly spring steel, I would leave them alone. If you are the inquisitive type and want to experiment and have the time to do so and cut in an area where there are no innocent bystanders, you could take one blade, mark it with a grinder in the back edge, and attempt to harden it, write down the process and temperatures used and then compare the performance with the other blades. In time you may get good at it and get just the right hardness ... or you just buy another set

Ehh... overhardened blades on something whirling that fast smacking 2 inch saplings would make me purdy nervous. I don't know for sure if the John Deere blades for that equipment is also spring steel, but I would theorize yes. Brushog blades, which seems to be more the alley of your equipment, are 5160 right around that 40 Rockwell also. Unless you're confident you can get the hardness back out, I'd vote sharpen. Ok actually I vote buy new ones and forge the old ones into knives  

On a different note, if an over hardened blade brakes and digs a hole in the body of the deck, you can test the hardness ... ding= not very hard, point poking out= harder ... only a hole left and blade sailing straight through = much harder ...

23 minutes ago, lanternnate said:

Ok actually I vote buy new ones and forge the old ones into knives  

That gets my vote!

Jeff ... forgot to comment on sharpening mower blades. When I did use to sharpen them, I first did so following the chamfer from the manufacturer, that was one face flat, the one facing the ground and the opposite tapering down. This way the edge was closest to the ground. When this shape in theory gives a better cut, in practice with irregular ground and stones in the mix, the edge receives the most damage. After a while I decided to change this and started to grind the taper the opposite way, taper up and edge on the top face. This way the edge lasted longer. I was very surprised when years later I bought a set of blades that came ground this way from the manufacturer. They had copied my idea !!